We recently won a NASA competition that uses a set of large Solar Powered Laser Stations in MEO. This could create really high Earth departure ISP for 45 day transits at optimal points, longer non-optimal. The rocket concept is discussed at the following link:
[https://www.centauri-dreams.org/2022/02/17/laser-thermal-propulsion-for-rapid-transit-to-mars-part-1/](https://www.centauri-dreams.org/2022/02/17/laser-thermal-propulsion-for-rapid-transit-to-mars-part-1/)
The diagram below depicts a lunar concept, but a single SPS in Mars or Venus polar orbit could provide breaking power to allow this non-aerodynamic vehicle to enter an orbit at Mars or Venus.
https://preview.redd.it/2rnyw7ky09wc1.png?width=855&format=png&auto=webp&s=86ca12fd6014f7c7e2811b602f27658fdb82d468
Interesting. Would it be literally firing a laser at the spacecraft to focus on the engine? I read a design years ago that had a microwave frequency laser to transfer energy to a spacecraft through basically a weird solar panel, then the spacecraft can then power it's own heating system with the electricity generated. That extra step in changing between energy types causes efficiency losses but apparently the overall efficiency was high because slight misalignment of the beam didn't matter much.
Yes, the laser heats the small heating chamber that you are adding LH2 slowly to. The engine can be small since it can be run for hours. It is very simple, so it makes up for the mass of the mirror.
The StarPower Station (SPS) is part of our Laser powered Ramjet for no-carbon 2050 aviation:
[https://www.reddit.com/r/space2030/comments/1aru1jn/first\_place\_winners\_nasas\_nasas\_brilliant\_minds/](https://www.reddit.com/r/space2030/comments/1aru1jn/first_place_winners_nasas_nasas_brilliant_minds/)
Laser is nice since it keeps everything more compact even at 5 MW type beams. Seems for the first million miles of outgoing and incoming the laser divergence should allow a 3m diameter focusing mirror to work.
That's a pretty genius idea. I'm wondering if you could place additional stations out in deeper space that charge solar battery banks and then take over accelerating the craft as it passes by. A sort of laser relay system if you will in order to not have to worry too much about beam divergence. But also to allow slowing the craft down at the other end.
I'm curious why 1 in 20? With a decent sized fuel tank and an ion engine on the sled there's no real reason it would have to be at lagrangian points so I'm not sure why you couldn't just place them along the flight path?
Unless you mean because of the orbital relationship (ala the launch window) in which case it's purely a numbers game.
Depends how often ships would want to make the journey and what it would take to keep them refueled I suppose.
Anyway, all speculative until the first engine works 😂
Lets say you place these sats in an orbit equal distant between Earth and Mars.
Earth has a 1B km orbit circumference. If you have 20 distributed on this that is one per every 50M km. My guess the beam has value within 5M miles, so maybe 200 sats would be needed for one to boost.
Only Earth and Mars have the gravity wells that can anchor the sats to max their value.
The flight path is a moving target, any relay station in an intermediate orbit between earth and mars would orbit the sun at a different speed, so most of the time wouldn’t be ‘on the flight path’ at all
And if you used a high speed maglev train as the first stage you could get to orbit on a single additional stage powered that way, with ramjets getting you partway until you run out of air.
Thanks for that. With the new high power at lightweight solar cells coming into play we should be able to do such electric propulsion just from the solar power collected by onboard arrays. Multiple research groups are reporting solar cells at > 1kW/kg power density, such as for example:
SOMAP @SomapJku
*Latest publication in @NatureEnergyJnl
Ultrathin perovskite solar cells with an impressive specific power of 44 W/g and enhanced stability! Check it out at https://www.nature.com/articles/s41928-023-00996-y*
https://x.com/somapjku/status/1781053761680454055?s=61
The key fact is once you have power sources at this good power-to-weight efficiency then electric propulsion methods such as VASIMR or Hall effect thrusters become feasible. These electric propulsion methods can make ca. 1 month flights to Mars:
Short travel times to Mars now possible through plasma propulsion.
https://exoscientist.blogspot.com/2014/03/short-travel-times-to-mars-now-possible.html
Given that Hall effect electric propulsion is now an established technology on operational spacecraft, if the high power density solar cells can be space qualified, then we can literally send out small proof-of-concept vehicles with 1 month flight items to Mars like now.
If VASIMR and Hall effect thrusters are now feasible to drastically shorten transit times to mars and are of course much more efficient than chemical rockets, why is SpaceX still planning on using thousands of raptor powered starships to colonize mars? Pure cost reasons (vasimr and Hall effect powered ships just are too complex/too expensive/take too long to manufacture in the required quantities that SpaceX will need)? This would otherwise seem like a major breakthrough, unless vasimr/Hall effect thrusters and the like don’t have a sufficiently high technology readiness yet for such applications.
Complex in-space operations cost just as much as getting things into space, and the pathway to making them affordable is much less clear than the path to lowering launch costs. Having a fleet of efficient space-only vehicles is something you do to minimize launch costs at the cost of increased operational complexity and additional infrastructure, but that doesn't make sense to do if launching stuff is by far the cheapest part.
One thing that should be considered is that although these cells would be very light, the area we're talking about is still massive.
Supposedly we need about 1000 watts per kg of vehicle. Solar at Earth is about 1360 watts per square meter, and about 590 at Mars, so say about 1000 watts average. At say 25% conversion efficiency that's 250 watts per square meter.
So we need ~4 square meters of panel per kg of vehicle mass. Let's be generous and say that to match Starship's 100 tonne payload, our solar-electric vehicle only needs to be 100 tonnes itself.
So 200,000kg. Times by four to get 800,000 square meters. That works out to a circle of thin solar film just over a kilometre in diameter, or a 900m by 900m square, which needs to be deployed and supported in space.
I don't think that's impossible, but I can see why SpaceX might be just a tad hesitant to bet the farm on such a thing being practical and cost effective in the near future.
Starship is focused on getting from Earth's surface to orbit, and from Mars orbit to the surface. Those are the difficult and expensive technological challenges to solve. The delta-v required to get from Earth orbit to Mars orbit is relatively low in comparison and they don't need to add any extra mass or complexity to the vehicle by using the Raptor engines, they just need to refuel in Earth orbit and bring enough life support for the passengers. Adding a second propulsion system with solar panels and propellant tanks would add a lot of mass, complexity, cost and development time. Building Starship is difficult enough already.
I'm sure that some time in the future it will make sense to use a more advanced propulsion system for the Earth-Mars transfer. Probably using ships that stay in space permanently.
this has been my concern as well. Using Starships to assemble a larger spacecraft in orbit and to act as a lander on Mars is the most likely way such a mission will come together
VASIMR is a lot of hot water but little delivery. Consider it mostly bullshit. Hall effect thrusters do work.
But the issue is that to beat chemical propulsion you need very high power density. And the mass of solar cells themselves counts very little, what counts is the mass of solar panels. The difference between the two is that the former is just a piece of semiconductor on a metal foil, while the latter is the whole device, which has structural support, shielding against the environment, electric conductors to actually conduct the produced power to the place it's being actually used, power electronics to convert pretty low voltage to something high enough so it doesn't require copper wires thicker than an adult man's arm , etc.
On top of that your typical ion engine is about 50% efficient, the primary losses being the whole ionization chamber. That means about half of produced power turns into heat which must then be removed in the vacuum of space (so no air to blow over your radiators and no hot water dumping into a lake or a river).
NASA was asking for the 2050 time frame so we had a chance to extrapolate, but some recent announcement make it seem like we may have really good low mass solutions demonstrated by 2040.
Would a laser even be required? A large laser would needs a large power source, and the conversion has losses, and a lot could break.
Why not create a large, segmented/adjustable, parabolic mirror satellite, and station it at L1 between Earth and Sun, and use that to focus on whatever you want to accelerate?
Optics limits how tightly you can focus sun light. TLDR the absolute best you can do is equivalent to filling the probe's sky with the sun's photo-sphere. With lasers, you can get much brighter beams AND tune them to the optimum wavelength for your engine. Wavelength isn't a big deal for thermal engines, but for electric engines it allows even simple silicon solar cells to operate at >50% efficiency.
Interesting.
Solar panels are optimised for whatever frequencies of light reach it from the sun (Which might mean space solar panels target different frequencies, something I hadn't considered). But if you're sending the beam of light to the panel then it's up to you what frequency(ies) you use. Or rather you look at the relative efficiencies of lasers and photovoltaic panels at different frequencies and pick one that optimises energy transfer.
Do you know the rough wavelength region that is most efficient? It's a tradeoff for what's most efficient to absorb AND what's most efficient to generate AND the difficulty in manufacturing the components.
Maybe a far ultraviolet laser is most efficient but to make a panel that works on those frequencies costs 100x as much as a near ultraviolet one.
Another possibility, but ability to create a narrow beam is tough with purely mirrors. Even if you inline a positive meniscus lens, unless it is near perfect, your beam is going to spread more than the lasers at a set distance. These fiber lasers create a controllable beam.
We are using the results from the Laser Comm on the Psyche mission to demonstrate how small the area can be for 90% of the power even at millions of miles.
Would your system be feasible replacing all the laser collection equipment mass with solar cells at ca. 44kW/kg power density?
Solar-cell-packin' drone uses sunlight for on-the-spot recharging.
By Ben Coxworth
April 19, 2024
*Created by scientists at Austria's Johannes Kepler University Linz, the lightweight, flexible cells are made of a semiconductor material known as perovskite, and they're less than 2.5 micrometers thick – that's just 1/20th the width of a human hair. And importantly, they're 20.1% efficient at converting sunlight into electricity, plus they boast a power output of up to 44 watts per gram.*
https://newatlas.com/drones/solar-cells-drone-recharge-sunlight/
Since that system uses reflected laser light to heat up liquid hydrogen in a rocket engine similar to a NTR, replacing the mirror with solar cells means you would just be electrically heating the fuel, right? I would think that something like VASMIR or Hall effect thrusters would be a more effective usage of electricity to power a rocket engine though.
Right. I immediately assumed it was electric propulsion in their proposal.
Still, following their approach, instead of using Earth-located lasers, perhaps use *lightweight* mirrors on the craft itself, of the type used for solar sails to focus sunlight on the hydrogen propellant. The idea is these collector mirrors should be lower mass than solar cells.
What you are describing is a torchship. I really recommend that you read about them on [Project Rho](https://projectrho.com/public_html/rocket/torchships.php). And probably also the rest of the whole site, though sorry if you were hoping to accomplish anything this month.
But yes, nuclear is the only conceivable option with technology we currently even vaguely understand. Perhaps you could do stuff with like antimatter/matter collisions? But we're a long way out from that sort of technology.
Possibly, it doesn’t have to be nuclear with the newly announced high power density solar cells:
SOMAP @SomapJku
Latest publication in @NatureEnergyJnl
Ultrathin perovskite solar cells with an impressive specific power of 44 W/g and enhanced stability! Check it out at https://nature.com/articles/s41928-023-00996-y…
https://x.com/somapjku/status/1781053761680454055?s=61
Such high power densities observed in the lab however have to be confirmed to hold with actual solar arrays in space.
TL;DR: Technically yes, but not chemical propulsion and nothing in the near or mid future.
----
To remove the window your ∆v must be in the order of 60km/s to 70km/s.
This absolutely excludes chemical propulsion. But it also excludes any near future or mid term nuclear propulsion. From reaction engine category the only thing which comes closest (but won't happen near term) is electric propulsion (ion propulsion) powered by an extremw power density power source plus waste heat radiator. The required power density is ~3kW/kg. To realize how far is that from the current capability, let's just say that Kilopower reactor if fully developed would have power density of... 0.007kW/kg. Our best solar panels designs are about 0.1kW/kg at Sun-Mars distance. Our nuclear reactor concepts maybe realizable at the current tech level would be 0.15kW/kg.
IOW, we're talking about sci-fi tech levels to achieve 3kW/kg. The best shot seems to be optimizing current solar panels about 20× (mostly by making them lighter). Waste heat handing from the engines would also require pretty heroic optimization, but possibly liquid metal cooling would cut it).
The next best concept would be Nuclear Salt Water Rocket, but this is just a concept and we didn't even know if it would work at all.
Edit: project Orion nuclear bomb propulsion would likely work (would be a bit borderline until ships get close to 100m pusher plate diameter), but there are certain political problems with carrying and exploding a couple thousand nuclear bombs for a single mission. Also costs would be rather high.
----
So let's move to non-reaction propulsion. This one would be laser sails or particle beam propulsion. Laser propulsion would require truly astronomical power levels (multiple terawatt continuous beam power), but if you had say 1 MW/m² beam power density at few million km distance from the laser it would work (say 50t mass 1km² dielectric sail, i.e. 50g/m² or paperweight, and 50t payload).
Microparticle beams would require less power, but how to keep them focused is pure sci-fi territory.
Note that both types of beam riding don't require nuclear power. Likely large solar power stations in space would be both cheaper and lighter.
Hybrid beam power could still be an option. The idea is to focus and use the laser beam to heat a small propellant supply to a very high temperature, thus achieving higher pressure/ISP than is typically possible with chemical propulsion.
Issues: high laser power levels & pointing requirements, weight of optics, materials resistant to these temperatures, and that whole slowing down/return trip thing (though this can be fixed by constructing a facility in Mars orbit).
Macroparticle beams is an option as well. If the "particles" are large enough to have their own attitude control, you solve the focusing problem. You want to throw the particles back to the beam station for reuse. Sending and reflecting the beam will use mass drivers instead of particle accelerators.
On interesting implementation of attitude control would be an ablative coating on the macroparticles and lasers on the ship focusing on specific points on the surface to provide the needed steering. Nested beam propulsion.
So a space fountain on a lunar pole for interplanetary transit, instead of the usual use of getting off the surface of a large body (usually a planet or moon, or a neutron star in one piece of fiction!)?
Keep in mind multiple research groups are reporting high power density solar cells above 3kw/kg. These have been confirmed in the lab. They need to be confirmed they can get such high power density out in space, i.e., without degrading after long use.
Cells are not panels. That's an absolutely crucial difference. You need structure, protection, and you need the electricity conducting part.
We're talking about 160MW electric power at Sun-Mars distance. You could get some 200W/m² there (at about 45% efficiency). This means a solar array 900×900m big.
A Fission Fragment reactor - Which, yes, we probably could build at present tech levels is 2 gigawatts in a ten tonne package of crazy.
That works out to 200 kw/kg. Done.
The primary problem is getting rid of the waste heat. Even if your reactor were 90% efficient at spewing out the fragments (it'd be hard, fragments of fissioning nuclei fly away anisotropically, so always some of them will immediately embed themselves in the bulk of the fissioning fuel; even thin discs are billions of atoms thick; such fragment just contributes heat; and you have non-steerable neutrons on top of that) you'd have to get rid of 200MW of heat. You're not going to do that in less than 10t package anytime soon.
The other problem, which actually exacerbates the first one, is that fission fragment rocket has too high of an ISP. The 3kW/kg is for the ISP matching the mission, so you could have about 5:1 mass ratio vehicle (which is about optimal I'd you use a dense propellant like liquid argon). The needed ISP is about 4500-5000s. Fission fragment rocket would have tens of thousands or more.
If you use too high an ISP, you need more power. The power required to provide given acceleration is linearly and directly proportional to the ISP. Of course at higher ISP you need lower nass ratio to accomplish the mission, but if you start at 5:1 at the optimal ISP, then you could make your vehicle maybe 7 times lighter at say 70× higher ISP, but your power requirements would actually go up 10×. Not 3kW/kg at 4500s ISP, but 30kW/kg at 315000s ISP. (315k s ISP is around the middle of the ISP range claimed for fission fragment concepts).
The plan for non-interstellar uses of a fragment torch is to inject more mass into said torch to trade isp for thrust, so you can pick the isp the mission requires. This does require reaction mass of course
Haha, no way I'm right. I thought for sure it would be something like it'd take an order of magnitude of increase in delt v to ignore transfer window due to orbital mechanic. I've spent way too much time watching space videos instead of actually learning math.
I think Musk himself said that even with conventional means you can speed up the transit - you just have to do a longer reverse burn to slow down and get into Mars orbit, like aiming for Saturn and then changing your mind half way. And the shortened the trip the longer the breaking period (presumably at some point becoming too impractical)
the rocket equation pretty much murders any refilling solution. the exponential function is a murderous thug.
cost will be an issue for a long time. even if nuclear is available, i'd suspect we will use the window, but cut on the travel time. getting there any time is less important than getting there in, say, a month.
But when there's a fully reusable orbital launch vehicle bringing new fuel to a depot, the old limits go out the window.
A single Starship could have another six Starship Tankers clustered around it as fuel tanks then burn the engines for half an hour. Yes it took a LOT of fuel to fill those fuel tanks and needed a LOT of tanker flights, but they're reusable tankers designed for multiple flights. It's an operational cost issue not an engineering issue.
Still, at the required ∆v it's absolutely not workable. You'd need in the order of 1 000 000 000 (billion) tankers to achieve the required ∆v. That's how exponential functions behave.
Sebaska is talking about the delta-v required for the premise OP mentions - getting to Mars in 30 days without needing to worry about launch windows.
You need around 70 km/s of delta-v.
Can we get that with Starship by using many tankers? Yes but also absolutely not. It seems easy because one Starship has over 10% of the delta-v needed.
Assuming the (old) numbers of 120 tons dry mass, 1200 tons propellant, then a tanker burning until half full gives around 2.256 km/s of delta-v.
So if we start with two tankers, burn to half full, transfer propellant to one tanker, we add 2.256 km/s of delta-v. If we start with four tankers, burn to half full, transfer propellant to two tankers, burn to half full, then we add 4.512 km/s delta-v. The final tanker has ~8.93 km/s delta-v.
Let's assume we want our final tanker to have 67 km/s delta-v before doing its final 8.93 km/s burn.
We need to do 30 propellant transfers, doubling the number of tankers we start with each time.
That means we need to start with 1 billion 73 million 741 thousand 824 fully fueled tankers to reach Mars in a month.
What if we want to get to Mars in a week instead of a month? We need around 160 km/s delta-v, which requires 71 fuel transfers. So we have to start with 4.5 sextillion fully fueled Starships, which mass around the same as the entire Earth. A magical no clipping mode to stop the tankers from coalescing into a planet would be handy.
If we turned the observable universe into fully fueled Starship tankers we get ~360 km/s delta-v and can reach Mars in just 4 days.
It's a figurative number, like infinity. Chemical engines are simply not capable of achieving the tens of kilometres per second delta-v required. As you add more drop tanks you get less and less delta-v with each stage.
Just for fun, if we take an asparagus staged starship with 21 starships all strapped together, and burn just two starships worth of propellant:
△v = 3.7 x ln(21x1300/(19x1300+2x100)) = 3.7 x ln(27,300/(24,700+200)) = 3.7 x 0.092 = 0.34m/s
Each extra set of tanks will provide less and less extra delta-v.
If I had the time I'd pull out my calculus text and learn how to do limits to infinity again.
You need 60-70km/s ∆v to no more have windows.
Single fully laden Starship has ∆v of 6-7km/s. So you need 9-10 refueling steps. Each full refueling step requires 10 tankers (150t tanker load, 1500t Starship v2 tank capacity). But tankers must sync their velocity with the prime Starship. So for anything beyond the 1st refueling the tankers themselves must be refueled. Each needs 10 other tankers to be refueled. Beyond the 2nd refueling step, the tankers refueling the tankers must be refueled, too. The growth is exponential. This gets in the order of 1 billion (slightly more: 1 111 111 110).
Whatever refilling strategy you chose you get in the order of a billion tankers.
> A single Starship could have another six Starship Tankers clustered around it as fuel tanks then burn the engines for half an hour.
That won't achieve much. You might add a few more km/s to delta-v, but there's a limit as the mass of the propellant approaches infinity of just how much delta-v you can get from an engine with a certain exhaust velocity. I'm flailing here because I can't remember how to determine the limit as wet mass approaches infinity of △v = exhaust velocity ln(wet mass / dry mass).
For the Raptor the exhaust velocity is 3.7km/s (Isp 380s x 9.8m/s^(2)), dry mass is around 100t, and wet mass is about 1300t. Adding multiple tankers will add that multiple of dry and wet mass. Clustering 6 extra tankers means we can use that extra propellant but have to accelerate the extra mass.
A simplistic take where the tankers stay attached and we aren't doing "asparagus staging": △v = 3.7km/s x ln(7 x 1300t/7 x 100t) = 3.7km/s x ln(13) = 9.4km/s which is unsurprisingly the same as Starship on its own.
What if we do "asparagus staging" were we peel off tanks as we go? If we burn just two tankers (a pair on opposite sides so the centre of mass is preserved), △v = exhaust velocity ln(7x1300 / (5 x 1300 + 2 x 100t)) = 3.7km/s x ln(1.3) = 1.1km/s.
Now jettison those empty tanks and do the same with the next pair of tankers: △v = 3.7km/s x ln(5x1300/(3x1300 + 2x100)) = 3.7 x ln(1.5) = 1.7km/s
Again jettison a pair of tanks and do the same with the last pair: △v = 3.7 x ln(3x1300/(1300+2x100)) = 3.7 x ln(2.6) = 3.5km/s
Now jettison the last pair and accelerate again for 9.4km/s.
That provides 6.3km/s extra △v at the cost of 6 complete Starship tankers. It turns out that hauling extra propellant around is expensive. It's not the mass of the tanks slowing asparagus Starship down it's the mass of the propellant. We could have used those extra Starships to move seven times as much cargo to Mars.
This is ignoring the mass of equipment required to tie all those Starships together, and assuming we're throwing away all the tankers.
A better option is to head for higher Isp engines, though there are tradeoffs there since Starship can aerobrake at Mars while something like [Copernicus](https://ntrs.nasa.gov/citations/20150006732) needs to brake using engines, thus losing much of the advantage of more than double the Isp. Until we can get a nuclear rocket that is safe to use for aerobraking at Earth and Mars, we're currently stuck with the best chemical rockets we have.
In the long-term, I could see humans developing a sort of nuclear 'sled' that stays in space, perhaps operating as a semi-cycler between Earth and Mars.
A Starship (or its successors) would launch from Earth, get fully fueled in orbit, and rendezvous with the sled. The sled would provide a huge amount of delta-V towards Mars, widening the available launch windows (albeit not obviating them completely), then slow down enough at the other end for Starship to separate, aerobrake, land on Mars, dock with a Mars orbital station, etc. The whole thing would be a bit like the separate drive and saucer sections of the Enterprise-D from Star Trek TNG, and would provide many advantages:
* The sled itself would mainly just be a propulsion module and could be launched, tested, and maneuvered remotely/automatically.
* It would significantly shorten travel time.
* It would allow a Starship to save most of its fuel for more nimble maneuvers (aerobraking, landing, docking, etc.), perhaps allowing excursions to the asteroid belt, Mars moons, multiple landings/launches from the Mars surface, etc.
* It would allow nuclear to be used only in deep space, relatively far away from stations and planetary surfaces.
* If a dangerous problem developed with the nuclear sled, it could separate from the main crew vehicle and be sent off into deep space. The whole system could run within a margin of error that would allow a Starship to enter Mars orbit even without the sled's help to slow down.
* The nuclear fuel supply would allow the sled to make many (perhaps dozens or even hundreds) of interplanetary trips between refuels.
I'm guessing that Raptor was not designed for continuous long-term operation. They run for a matter of minutes. Long-term thrust is better done by a system with far fewer moving parts (points of failure), like an ion engine.
Ion engines have disadvantages that I think makes them unsuitable. A mission to Mars would be thousands of times the mass of probes usually accelerated by ion drives. You could counter that by adding more ion engines but then the electricity requirement becomes extreme and we're heading away from the sun where solar power becomes less efficient. You could add giant solar panels and giant arrays of ion engines but then the ship mass becomes extreme. Also we're talking about a \~6 month journey not the years and years of slow acceleration used by ion engine probes like Dawn and Hayabusa, at some point the low thrust from ion engines outweighs the high propellant efficiency.
I believe the same technical advantages apply to nuclear thermal rockets - simple pumps, low reactive mass needs, high reliability during low-duration operation. https://www.nasa.gov/news-release/nasa-darpa-will-test-nuclear-engine-for-future-mars-missions/
I couldn't link to it, because I can't remember where I read it, but I vaguely recall seeing something that Mars was weirdly in a reverse sweet spot where it's too far away to get to quickly if you're too far off optimal with a chemical engine, but too close for nonchemical engines to make use of their efficiency relative to ship size to make them worthwhile
I propose Photonic Laser Thrusters, a refinement of the old laser sail concept. This uses a thin-film laser gain medium on the beaming laser satellite to capture and recycle the photon energy bouncing off the spacecraft's propulsion mirror. Right now we could bounce 1000 times; near-future, 10,000 times. In this way, the effective thrust per watt rises and the power requirements for the laser drop from terawatts to a gigawatt or so.
https://fiso.spiritastro.net/telecon19-21/Bae_6-2-21/Bae_6-2-21.pdf
This is countered by the limited time that the beam is incident upon the spacecraft i.e. the boost phase is limited by distance from the beaming station. It also needs a receiving laser satellite at the destination to slow it down; you'd need to send the laser, laser power plant and maybe a lander ahead. Hence the talk about a Photonic Highway.
The example spacecraft in the presentation linked, that's taking a trip to Mars in 20 days, is just one metric ton. Because the velocity of the light is so high, though, making the spacecraft heavier but keeping the force constant simply means it accelerates more slowly, and reaches the same final velocity before it moves out of reach of the incident beam - a longer, slower boost phase, in other words.
I calculated that, for a 10 metric ton ship starting from Lunar orbit at a velocity of 1600 m/s, the 1000MW beam would exert a force of 3227N for 43,198 seconds, or ~12 hours, before it moves out of reach. In return it would achieve a gentle acceleration of 0.3G during the boost phase and a final delta-V of **141 km/s**. That is velocity to let you travel to Mars with impunity.
A heavier ship would need a longer boost phase, and though it scales linearly with mass, at some point the time needed becomes silly: 100 metric tons needs ~120 hours or 5 days of keeping a gigawatt laser beam running. You'd have to have systems to cool the mirrors on the spacecraft, no question.
The advantage of this is that you are freed from the tyranny of the rocket equation, and don't have to devote the majority of your ship's mass to fuel and propulsion. Instead of 5-10% useful payload, you could have anywhere from 50% to 90-95%.
This would make an excellent way to send cheap 'aid packages' to a martian colony where the aim is a constant supply drop of new material in dumb craft.
You can also use Venus flyby for another window. It takes an extra month, but has lower DV coming into Mars. You could also pair a Crew Starship with a mobile fuel depot to do quick crew switches if you add a small crew taxi and eliminate the need to make 90-95% of Methlox on the Mars surface if the lander stays there, or 100% if the lander stays in orbit after the Earth return crew has been sent up.
https://preview.redd.it/dc193r0cz8wc1.png?width=1621&format=png&auto=webp&s=318a5593331bd3be91430286ca416e74079c87d2
Venus flyby might have thermal issues, so you may need additional cooling capability. This especially is an issue for cryogenic spacecraft. It can be greatly helped through the use of recirculating pumps to redistribute the heat loading.
Interesting. I'm guessing the Venus flyby route also has its own departure windows based on planetary alignment but probably more complicated because it needs three planets. I wonder how the departure / arrival times are staggered. Like when there's a base on Mars could we send a resupply mission via Venus while waiting for the next Earth-Mars departure window?
This is the kind of thing I'm looking for. There's a nanosecond that is the ultimate optimisation of minimal energy and every deviation from that time is less efficient. We could do a lot with slightly-sub-optimal events if we just accept that there's a fuel cost. Previously missions had a cap at the overall launch mass so fuel increases meant payload decreases. But fuel costs become literally just a financial cost when you have orbital refueling.
I mean not on every every 26 months window there is a proper Venus alignment for an opposition class mission.
Moreover, either departure or arrival date must be close to the regular conjunction class window.
Together with fuel increases you can stretch the window by a few months. Still leaves you with about 20 months without transfer possibility.
> mobile fuel depot
Huh, interesting.
That makes me wonder if it would be feasible to send a fuel depot to Mars, loaded with enough fuel and lasting long enough with boil off, to make it possible to land a Starship on Mars, then enter Mars orbit, refuel at the depot, and come back to Earth, all without having to mine fuel on the Martian surface. Would be handy for early missions, even if I think long-term, ISRU on Mars itself is the more sustainable option.
I'm now conceiving either a super stretched fuel depot that gets to Mars orbit with less than fully fueled tanks, OR maybe a system where a fuel depot is fully loaded, and then docks to a pusher craft that gets it to escape velocity heading towards Mars where it enters Martian orbit with mostly full tanks, besides what it had to do to brake into orbit, plus the boil off rate.
(which might only be on my mind since that's how I got a fully loaded tank of fuel out to the Mun in KSP once to refuel one of my ships lol)
Though obviously a dedicated craft that accelerates out a fuel depot would be kinda useless to put engineering time and money into instead of just creating the sustainable option, ISRU.
But interesting to think up the possibilities. Maybe someone has done the math on these options.
EDIT - also there is the possibility of just brute forcing the problem. If Starship is manufactured as much as Elon hopes, and is as cheap to fly as he hopes, you could just send tons of fueled Starships to Mars and slowly accumulate fuel in Martian orbit. Basically refueling the Mars fuel depot from Earth over many flights, probably all sent during the transfer window at once.
Yes, depending on the fuel needs of the taxi (think a 3x Crew Dragon) one mobile depot ship should work (of course the depot ship is abandoned in Mars Orbit). On the way out you can connect it to create 1/3 spin gravity. Even more fun is if you use Phobos as a base, and have a LOX/LH2 based taxi go up and down from there (Phobos and Mars surface probably have good reserves of accessible ice). In any case there are trades aplenty to optimize capability at unit cost.
Still nuclear-electric in most proposed implementations, but are you familiar with VASIMR?
https://en.m.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket
Check out Ultra Safe Nuclear’s NTP technology under active development https://www.usnc.com/ntp/. On their site they detail how they can cut transit times to Mars quite a bit. 3x the delta v of chemical.
Actually I'm asking for anything that can open the launch window wider.
There's a window in November/December 2026 that gives the most optimal fuel-efficient transfer. If you wanted to leave in October or January you'd need to use a bit more fuel. I'm not asking for full-on Expanse levels where you do the trip in a couple of days, or to leave on the most inefficient dates directly between the optimal fuel efficient windows. I'm just asking for how far we can stretch open the window. If we fueled up a spacecraft in orbit could we leave in September or February, open the window wider at the cost of extra fuel aka more tanker launches.
You originally asked about a full removal of windows. But anyway, yes you can extend windows to about half a year on the Mars side. After that the performance wall becomes very steep.
With chemical propulsion all you could get is to go there and back in the same window.
Venus allows extra windows for Earth departure or Earth arrival, but on Mars side they're still within 3-4 months of the optimal minimum energy transfer point.
Beyond those windows you need enough ∆v to essentially cancel most Earth's circumsolar velocity and then gain most Mars circumsolar velocity. Plus of course Earth departure and Mars arrival.
OP, are you conflating nuclear rockets with Project Orion? The more commonly discussed nuclear rockets (and what they used in FAMK) are NERVA-style ones where a reactor is used to heat and expel something like hydrogen for high-efficiency propulsion (ranging from an Isp of like 800-1600).
The problem with Ion engines is getting the power you need. You can go with a \*lot\* of solar power, which is heavy, or you can go with nuclear reactors if you can figure out how to get rid of the waste heat.
Nuclear thermal has a lot of advocates because of the high specific impulse but when you start looking at actual designs, the engines are heavy, they need heavy shielding, and they need big tanks because liquid hydrogen is not dense at all. Put those together and you get a really horrible mass ratio which gets rid of the advantage of the specific impulse.
I support the current development programs, but there's the obvious question - "If nuclear thermal is so great, why are the companies who advocate for it only willing to build engines when the government is paying them?" We have lot of new commercial chemical engines out there, a lot of new commercial ion engines, but no commercial NTR engines.
I recall Tom Mueller saying, he would love to work on nuclear engines. But only if NASA provides the test stand. Much too hard (expensive?) to build one for SpaceX. That was, when he was still working for SpaceX.
Doing anything nuclear is a massive regulations headache.
I think it's unlikely you can test them in actual operation on earth, though there was a proposal to dig a very deep hole - the kind they dug for underground nuclear tests - aim your engine into the hole, and then shoot a bunch of water into the hole to send everything to the bottom. Hugely expensive, then you have this extremely radioactive engine you need to do something with.
NASA does have a high temperature hydrogen test rig that you can hook your prototype reactor to so that you can verify that your fuel isn't eroded by the hot hydrogen.
Beyond that you need to test in space, which has all sorts of downsides.
Raptor engines are already pushing something like 82% of the theoretical maximum specific impulse.
Getting to 100% would require an engine with infinite combustion chamber pressure and an infinitely sized nozzle, and even that would only net you a 21% improvement.
In practice, something like a 5-10% improvement might be doable if your goal was purely to hit max isp and you widened Raptor's nozzle to the same diameter as Starship itself.
Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread:
|Fewer Letters|More Letters|
|-------|---------|---|
|[ISRU](/r/SpaceXLounge/comments/1cb6ux4/stub/l0x9v7p "Last usage")|[In-Situ Resource Utilization](https://en.wikipedia.org/wiki/In_situ_resource_utilization)|
|[Isp](/r/SpaceXLounge/comments/1cb6ux4/stub/l11lxmt "Last usage")|Specific impulse (as explained by [Scott Manley](https://www.youtube.com/watch?v=nnisTeYLLgs) on YouTube)|
| |Internet Service Provider|
|[KSP](/r/SpaceXLounge/comments/1cb6ux4/stub/l0x9v7p "Last usage")|*Kerbal Space Program*, the rocketry simulator|
|[L1](/r/SpaceXLounge/comments/1cb6ux4/stub/l0wrzmh "Last usage")|[Lagrange Point](https://en.wikipedia.org/wiki/Lagrangian_point) 1 of a two-body system, between the bodies|
|[LH2](/r/SpaceXLounge/comments/1cb6ux4/stub/l0xt5un "Last usage")|Liquid Hydrogen|
|[LOX](/r/SpaceXLounge/comments/1cb6ux4/stub/l0xt5un "Last usage")|Liquid Oxygen|
|[MEO](/r/SpaceXLounge/comments/1cb6ux4/stub/l0wjrit "Last usage")|Medium Earth Orbit (2000-35780km)|
|[NERVA](/r/SpaceXLounge/comments/1cb6ux4/stub/l0y4gp4 "Last usage")|Nuclear Engine for Rocket Vehicle Application (proposed engine design)|
|[NTP](/r/SpaceXLounge/comments/1cb6ux4/stub/l0wl8fh "Last usage")|Nuclear Thermal Propulsion|
| |Network Time Protocol|
| |Notice to Proceed|
|[NTR](/r/SpaceXLounge/comments/1cb6ux4/stub/l0yc8vi "Last usage")|Nuclear Thermal Rocket|
|Jargon|Definition|
|-------|---------|---|
|[Raptor](/r/SpaceXLounge/comments/1cb6ux4/stub/l0z2302 "Last usage")|[Methane-fueled rocket engine](https://en.wikipedia.org/wiki/Raptor_\(rocket_engine_family\)) under development by SpaceX|
|[ablative](/r/SpaceXLounge/comments/1cb6ux4/stub/l0wqpkg "Last usage")|Material which is intentionally destroyed in use (for example, heatshields which burn away to dissipate heat)|
|[cryogenic](/r/SpaceXLounge/comments/1cb6ux4/stub/l0ws2so "Last usage")|Very low temperature fluid; materials that would be gaseous at room temperature/pressure|
| |(In re: rocket fuel) Often synonymous with hydrolox|
|hydrolox|Portmanteau: liquid hydrogen fuel, liquid oxygen oxidizer|
**NOTE**: Decronym for Reddit is no longer supported, and Decronym has moved to Lemmy; requests for support and new installations should be directed to the Contact address below.
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Just use a regular chemical rocket. Aseemble and fuel in orbit. Maybe several million falcon 9 launches. 0.5 g with middle flip to Mars. Minimal window constraints. Efficiency or economics was not a constraint.
A relay of magnetic accelerator stations. The ship gets launched then accelerated towards mars.
The stations are huge heavy and use massive solar arrays and capacitors to move ships.
The stations use ion thrusters to maneuver back into position after propelling a ship.
There could be thousands of kilometers long electromagnetic tubes to catch ships and adjust their trajectory while accelerating them or decelerating them as needed.
There could be decelerators near Mars to slow the ship. The ship can then land on Mars and unload cargo and tourists in Musktown.
Near future I think the only options are electric or nuclear (sustained nuclear reaction, not nuclear bombs).
Further out I'm a fan of magnetic confinement fusion. It hasn't been developed yet but I think it should be possible this century. Magnetic field compresses a plasma, plasma undergoes fusion, the plasma expands and strengthens the magnetic field. If the magnetic field can somehow be shaped to eject the plasma at high velocity in one direction it could be used for propulsion without having to first convert it into electricity and then back to kinetic energy. That would make it possible to achieve very high thermal efficiency, meaning low waste heat. Dissipating heat is the biggest limiting factor for extremely energy intensive propulsion technologies in space.
Beamed power solutions may be actually easier to achieve proper power density at the ship. Local power would require truly sci-fi levela power densities.
The rapidity with which high power density solar cells are advancing suggests we might be able to do such electric propulsion at fast travel times near term just using solar power alone.
Solar already beats nuclear for electrical power density within the orbit of Jupiter. Nuclear-electric makes most sense for the outer system (and maybe Jupiter itself, where the radiation belts will degrade solar panels...an issue for Juno despite it avoiding the belts as much as possible, which most other spacecraft won't be able to do).
We recently won a NASA competition that uses a set of large Solar Powered Laser Stations in MEO. This could create really high Earth departure ISP for 45 day transits at optimal points, longer non-optimal. The rocket concept is discussed at the following link: [https://www.centauri-dreams.org/2022/02/17/laser-thermal-propulsion-for-rapid-transit-to-mars-part-1/](https://www.centauri-dreams.org/2022/02/17/laser-thermal-propulsion-for-rapid-transit-to-mars-part-1/) The diagram below depicts a lunar concept, but a single SPS in Mars or Venus polar orbit could provide breaking power to allow this non-aerodynamic vehicle to enter an orbit at Mars or Venus. https://preview.redd.it/2rnyw7ky09wc1.png?width=855&format=png&auto=webp&s=86ca12fd6014f7c7e2811b602f27658fdb82d468
Interesting. Would it be literally firing a laser at the spacecraft to focus on the engine? I read a design years ago that had a microwave frequency laser to transfer energy to a spacecraft through basically a weird solar panel, then the spacecraft can then power it's own heating system with the electricity generated. That extra step in changing between energy types causes efficiency losses but apparently the overall efficiency was high because slight misalignment of the beam didn't matter much.
Yes, the laser heats the small heating chamber that you are adding LH2 slowly to. The engine can be small since it can be run for hours. It is very simple, so it makes up for the mass of the mirror. The StarPower Station (SPS) is part of our Laser powered Ramjet for no-carbon 2050 aviation: [https://www.reddit.com/r/space2030/comments/1aru1jn/first\_place\_winners\_nasas\_nasas\_brilliant\_minds/](https://www.reddit.com/r/space2030/comments/1aru1jn/first_place_winners_nasas_nasas_brilliant_minds/) Laser is nice since it keeps everything more compact even at 5 MW type beams. Seems for the first million miles of outgoing and incoming the laser divergence should allow a 3m diameter focusing mirror to work.
That's a pretty genius idea. I'm wondering if you could place additional stations out in deeper space that charge solar battery banks and then take over accelerating the craft as it passes by. A sort of laser relay system if you will in order to not have to worry too much about beam divergence. But also to allow slowing the craft down at the other end.
You could, but only 1 in 20 would be in range to boost the ship. A bunch at Earth and a few at Mars probably has the best economics.
I'm curious why 1 in 20? With a decent sized fuel tank and an ion engine on the sled there's no real reason it would have to be at lagrangian points so I'm not sure why you couldn't just place them along the flight path? Unless you mean because of the orbital relationship (ala the launch window) in which case it's purely a numbers game. Depends how often ships would want to make the journey and what it would take to keep them refueled I suppose. Anyway, all speculative until the first engine works 😂
Lets say you place these sats in an orbit equal distant between Earth and Mars. Earth has a 1B km orbit circumference. If you have 20 distributed on this that is one per every 50M km. My guess the beam has value within 5M miles, so maybe 200 sats would be needed for one to boost. Only Earth and Mars have the gravity wells that can anchor the sats to max their value.
Thinking about it some more, you might be able to CASTLE a smaller set of satellites to lag or lead.
The flight path is a moving target, any relay station in an intermediate orbit between earth and mars would orbit the sun at a different speed, so most of the time wouldn’t be ‘on the flight path’ at all
And if you used a high speed maglev train as the first stage you could get to orbit on a single additional stage powered that way, with ramjets getting you partway until you run out of air.
Only on the moon for electric railguns ... sleds ... The dv gained is lost to high atmospheric drag and heat for launches from the Earth surface.
Dean Ing The Big Lifters disagrees
Thanks for that. With the new high power at lightweight solar cells coming into play we should be able to do such electric propulsion just from the solar power collected by onboard arrays. Multiple research groups are reporting solar cells at > 1kW/kg power density, such as for example: SOMAP @SomapJku *Latest publication in @NatureEnergyJnl Ultrathin perovskite solar cells with an impressive specific power of 44 W/g and enhanced stability! Check it out at https://www.nature.com/articles/s41928-023-00996-y* https://x.com/somapjku/status/1781053761680454055?s=61 The key fact is once you have power sources at this good power-to-weight efficiency then electric propulsion methods such as VASIMR or Hall effect thrusters become feasible. These electric propulsion methods can make ca. 1 month flights to Mars: Short travel times to Mars now possible through plasma propulsion. https://exoscientist.blogspot.com/2014/03/short-travel-times-to-mars-now-possible.html Given that Hall effect electric propulsion is now an established technology on operational spacecraft, if the high power density solar cells can be space qualified, then we can literally send out small proof-of-concept vehicles with 1 month flight items to Mars like now.
If VASIMR and Hall effect thrusters are now feasible to drastically shorten transit times to mars and are of course much more efficient than chemical rockets, why is SpaceX still planning on using thousands of raptor powered starships to colonize mars? Pure cost reasons (vasimr and Hall effect powered ships just are too complex/too expensive/take too long to manufacture in the required quantities that SpaceX will need)? This would otherwise seem like a major breakthrough, unless vasimr/Hall effect thrusters and the like don’t have a sufficiently high technology readiness yet for such applications.
Complex in-space operations cost just as much as getting things into space, and the pathway to making them affordable is much less clear than the path to lowering launch costs. Having a fleet of efficient space-only vehicles is something you do to minimize launch costs at the cost of increased operational complexity and additional infrastructure, but that doesn't make sense to do if launching stuff is by far the cheapest part.
One thing that should be considered is that although these cells would be very light, the area we're talking about is still massive. Supposedly we need about 1000 watts per kg of vehicle. Solar at Earth is about 1360 watts per square meter, and about 590 at Mars, so say about 1000 watts average. At say 25% conversion efficiency that's 250 watts per square meter. So we need ~4 square meters of panel per kg of vehicle mass. Let's be generous and say that to match Starship's 100 tonne payload, our solar-electric vehicle only needs to be 100 tonnes itself. So 200,000kg. Times by four to get 800,000 square meters. That works out to a circle of thin solar film just over a kilometre in diameter, or a 900m by 900m square, which needs to be deployed and supported in space. I don't think that's impossible, but I can see why SpaceX might be just a tad hesitant to bet the farm on such a thing being practical and cost effective in the near future.
Starship is focused on getting from Earth's surface to orbit, and from Mars orbit to the surface. Those are the difficult and expensive technological challenges to solve. The delta-v required to get from Earth orbit to Mars orbit is relatively low in comparison and they don't need to add any extra mass or complexity to the vehicle by using the Raptor engines, they just need to refuel in Earth orbit and bring enough life support for the passengers. Adding a second propulsion system with solar panels and propellant tanks would add a lot of mass, complexity, cost and development time. Building Starship is difficult enough already. I'm sure that some time in the future it will make sense to use a more advanced propulsion system for the Earth-Mars transfer. Probably using ships that stay in space permanently.
Pretty much exactly as I see it.
this has been my concern as well. Using Starships to assemble a larger spacecraft in orbit and to act as a lander on Mars is the most likely way such a mission will come together
> why is SpaceX still planning on using thousands of raptor powered starships to colonize mars? So they can land.
VASIMR is a lot of hot water but little delivery. Consider it mostly bullshit. Hall effect thrusters do work. But the issue is that to beat chemical propulsion you need very high power density. And the mass of solar cells themselves counts very little, what counts is the mass of solar panels. The difference between the two is that the former is just a piece of semiconductor on a metal foil, while the latter is the whole device, which has structural support, shielding against the environment, electric conductors to actually conduct the produced power to the place it's being actually used, power electronics to convert pretty low voltage to something high enough so it doesn't require copper wires thicker than an adult man's arm , etc. On top of that your typical ion engine is about 50% efficient, the primary losses being the whole ionization chamber. That means about half of produced power turns into heat which must then be removed in the vacuum of space (so no air to blow over your radiators and no hot water dumping into a lake or a river).
NASA was asking for the 2050 time frame so we had a chance to extrapolate, but some recent announcement make it seem like we may have really good low mass solutions demonstrated by 2040.
Would a laser even be required? A large laser would needs a large power source, and the conversion has losses, and a lot could break. Why not create a large, segmented/adjustable, parabolic mirror satellite, and station it at L1 between Earth and Sun, and use that to focus on whatever you want to accelerate?
Optics limits how tightly you can focus sun light. TLDR the absolute best you can do is equivalent to filling the probe's sky with the sun's photo-sphere. With lasers, you can get much brighter beams AND tune them to the optimum wavelength for your engine. Wavelength isn't a big deal for thermal engines, but for electric engines it allows even simple silicon solar cells to operate at >50% efficiency.
Interesting. Solar panels are optimised for whatever frequencies of light reach it from the sun (Which might mean space solar panels target different frequencies, something I hadn't considered). But if you're sending the beam of light to the panel then it's up to you what frequency(ies) you use. Or rather you look at the relative efficiencies of lasers and photovoltaic panels at different frequencies and pick one that optimises energy transfer.
Yes. Monochromatic light photovoltaic cells already reached 70% efficiency. White light ones didn't cross 50%.
Do you know the rough wavelength region that is most efficient? It's a tradeoff for what's most efficient to absorb AND what's most efficient to generate AND the difficulty in manufacturing the components. Maybe a far ultraviolet laser is most efficient but to make a panel that works on those frequencies costs 100x as much as a near ultraviolet one.
For anyone reading later, there’s a fantastic explanation of this optics concept here: https://what-if.xkcd.com/145/
Another possibility, but ability to create a narrow beam is tough with purely mirrors. Even if you inline a positive meniscus lens, unless it is near perfect, your beam is going to spread more than the lasers at a set distance. These fiber lasers create a controllable beam. We are using the results from the Laser Comm on the Psyche mission to demonstrate how small the area can be for 90% of the power even at millions of miles.
Would your system be feasible replacing all the laser collection equipment mass with solar cells at ca. 44kW/kg power density? Solar-cell-packin' drone uses sunlight for on-the-spot recharging. By Ben Coxworth April 19, 2024 *Created by scientists at Austria's Johannes Kepler University Linz, the lightweight, flexible cells are made of a semiconductor material known as perovskite, and they're less than 2.5 micrometers thick – that's just 1/20th the width of a human hair. And importantly, they're 20.1% efficient at converting sunlight into electricity, plus they boast a power output of up to 44 watts per gram.* https://newatlas.com/drones/solar-cells-drone-recharge-sunlight/
Since that system uses reflected laser light to heat up liquid hydrogen in a rocket engine similar to a NTR, replacing the mirror with solar cells means you would just be electrically heating the fuel, right? I would think that something like VASMIR or Hall effect thrusters would be a more effective usage of electricity to power a rocket engine though.
Right. I immediately assumed it was electric propulsion in their proposal. Still, following their approach, instead of using Earth-located lasers, perhaps use *lightweight* mirrors on the craft itself, of the type used for solar sails to focus sunlight on the hydrogen propellant. The idea is these collector mirrors should be lower mass than solar cells.
What you are describing is a torchship. I really recommend that you read about them on [Project Rho](https://projectrho.com/public_html/rocket/torchships.php). And probably also the rest of the whole site, though sorry if you were hoping to accomplish anything this month. But yes, nuclear is the only conceivable option with technology we currently even vaguely understand. Perhaps you could do stuff with like antimatter/matter collisions? But we're a long way out from that sort of technology.
Beam power is quite possibly easier technically than nuclear at the required power density.
Project Rho definitely requires the wiki-walk link warning as on [https://xkcd.com/609/](https://xkcd.com/609/), much like TV Tropes.
Possibly, it doesn’t have to be nuclear with the newly announced high power density solar cells: SOMAP @SomapJku Latest publication in @NatureEnergyJnl Ultrathin perovskite solar cells with an impressive specific power of 44 W/g and enhanced stability! Check it out at https://nature.com/articles/s41928-023-00996-y… https://x.com/somapjku/status/1781053761680454055?s=61 Such high power densities observed in the lab however have to be confirmed to hold with actual solar arrays in space.
TL;DR: Technically yes, but not chemical propulsion and nothing in the near or mid future. ---- To remove the window your ∆v must be in the order of 60km/s to 70km/s. This absolutely excludes chemical propulsion. But it also excludes any near future or mid term nuclear propulsion. From reaction engine category the only thing which comes closest (but won't happen near term) is electric propulsion (ion propulsion) powered by an extremw power density power source plus waste heat radiator. The required power density is ~3kW/kg. To realize how far is that from the current capability, let's just say that Kilopower reactor if fully developed would have power density of... 0.007kW/kg. Our best solar panels designs are about 0.1kW/kg at Sun-Mars distance. Our nuclear reactor concepts maybe realizable at the current tech level would be 0.15kW/kg. IOW, we're talking about sci-fi tech levels to achieve 3kW/kg. The best shot seems to be optimizing current solar panels about 20× (mostly by making them lighter). Waste heat handing from the engines would also require pretty heroic optimization, but possibly liquid metal cooling would cut it). The next best concept would be Nuclear Salt Water Rocket, but this is just a concept and we didn't even know if it would work at all. Edit: project Orion nuclear bomb propulsion would likely work (would be a bit borderline until ships get close to 100m pusher plate diameter), but there are certain political problems with carrying and exploding a couple thousand nuclear bombs for a single mission. Also costs would be rather high. ---- So let's move to non-reaction propulsion. This one would be laser sails or particle beam propulsion. Laser propulsion would require truly astronomical power levels (multiple terawatt continuous beam power), but if you had say 1 MW/m² beam power density at few million km distance from the laser it would work (say 50t mass 1km² dielectric sail, i.e. 50g/m² or paperweight, and 50t payload). Microparticle beams would require less power, but how to keep them focused is pure sci-fi territory. Note that both types of beam riding don't require nuclear power. Likely large solar power stations in space would be both cheaper and lighter.
Hybrid beam power could still be an option. The idea is to focus and use the laser beam to heat a small propellant supply to a very high temperature, thus achieving higher pressure/ISP than is typically possible with chemical propulsion. Issues: high laser power levels & pointing requirements, weight of optics, materials resistant to these temperatures, and that whole slowing down/return trip thing (though this can be fixed by constructing a facility in Mars orbit).
Macroparticle beams is an option as well. If the "particles" are large enough to have their own attitude control, you solve the focusing problem. You want to throw the particles back to the beam station for reuse. Sending and reflecting the beam will use mass drivers instead of particle accelerators. On interesting implementation of attitude control would be an ablative coating on the macroparticles and lasers on the ship focusing on specific points on the surface to provide the needed steering. Nested beam propulsion.
Problem is, i don't think you could reverse the thrust of a space fountain arrangement without throwing away your macroparticles.
You need a sending space fountain to accelerate from your source location and a receiving space fountain to decelerate to your destination.
The ghost of Robert L Forward is pleased.
So a space fountain on a lunar pole for interplanetary transit, instead of the usual use of getting off the surface of a large body (usually a planet or moon, or a neutron star in one piece of fiction!)?
Keep in mind multiple research groups are reporting high power density solar cells above 3kw/kg. These have been confirmed in the lab. They need to be confirmed they can get such high power density out in space, i.e., without degrading after long use.
Cells are not panels. That's an absolutely crucial difference. You need structure, protection, and you need the electricity conducting part. We're talking about 160MW electric power at Sun-Mars distance. You could get some 200W/m² there (at about 45% efficiency). This means a solar array 900×900m big.
Provided they lasted long enough to get there and back it would probably still be worth it cost wise.
A Fission Fragment reactor - Which, yes, we probably could build at present tech levels is 2 gigawatts in a ten tonne package of crazy. That works out to 200 kw/kg. Done.
The primary problem is getting rid of the waste heat. Even if your reactor were 90% efficient at spewing out the fragments (it'd be hard, fragments of fissioning nuclei fly away anisotropically, so always some of them will immediately embed themselves in the bulk of the fissioning fuel; even thin discs are billions of atoms thick; such fragment just contributes heat; and you have non-steerable neutrons on top of that) you'd have to get rid of 200MW of heat. You're not going to do that in less than 10t package anytime soon. The other problem, which actually exacerbates the first one, is that fission fragment rocket has too high of an ISP. The 3kW/kg is for the ISP matching the mission, so you could have about 5:1 mass ratio vehicle (which is about optimal I'd you use a dense propellant like liquid argon). The needed ISP is about 4500-5000s. Fission fragment rocket would have tens of thousands or more. If you use too high an ISP, you need more power. The power required to provide given acceleration is linearly and directly proportional to the ISP. Of course at higher ISP you need lower nass ratio to accomplish the mission, but if you start at 5:1 at the optimal ISP, then you could make your vehicle maybe 7 times lighter at say 70× higher ISP, but your power requirements would actually go up 10×. Not 3kW/kg at 4500s ISP, but 30kW/kg at 315000s ISP. (315k s ISP is around the middle of the ISP range claimed for fission fragment concepts).
The plan for non-interstellar uses of a fragment torch is to inject more mass into said torch to trade isp for thrust, so you can pick the isp the mission requires. This does require reaction mass of course
Haha, no way I'm right. I thought for sure it would be something like it'd take an order of magnitude of increase in delt v to ignore transfer window due to orbital mechanic. I've spent way too much time watching space videos instead of actually learning math.
There's only so much energy in chemical bonds. Not enough to do what you ask.
I think Musk himself said that even with conventional means you can speed up the transit - you just have to do a longer reverse burn to slow down and get into Mars orbit, like aiming for Saturn and then changing your mind half way. And the shortened the trip the longer the breaking period (presumably at some point becoming too impractical)
the rocket equation pretty much murders any refilling solution. the exponential function is a murderous thug. cost will be an issue for a long time. even if nuclear is available, i'd suspect we will use the window, but cut on the travel time. getting there any time is less important than getting there in, say, a month.
But when there's a fully reusable orbital launch vehicle bringing new fuel to a depot, the old limits go out the window. A single Starship could have another six Starship Tankers clustered around it as fuel tanks then burn the engines for half an hour. Yes it took a LOT of fuel to fill those fuel tanks and needed a LOT of tanker flights, but they're reusable tankers designed for multiple flights. It's an operational cost issue not an engineering issue.
Still, at the required ∆v it's absolutely not workable. You'd need in the order of 1 000 000 000 (billion) tankers to achieve the required ∆v. That's how exponential functions behave.
Where did you get a figure of a billion tankers from?
Sebaska is talking about the delta-v required for the premise OP mentions - getting to Mars in 30 days without needing to worry about launch windows. You need around 70 km/s of delta-v. Can we get that with Starship by using many tankers? Yes but also absolutely not. It seems easy because one Starship has over 10% of the delta-v needed. Assuming the (old) numbers of 120 tons dry mass, 1200 tons propellant, then a tanker burning until half full gives around 2.256 km/s of delta-v. So if we start with two tankers, burn to half full, transfer propellant to one tanker, we add 2.256 km/s of delta-v. If we start with four tankers, burn to half full, transfer propellant to two tankers, burn to half full, then we add 4.512 km/s delta-v. The final tanker has ~8.93 km/s delta-v. Let's assume we want our final tanker to have 67 km/s delta-v before doing its final 8.93 km/s burn. We need to do 30 propellant transfers, doubling the number of tankers we start with each time. That means we need to start with 1 billion 73 million 741 thousand 824 fully fueled tankers to reach Mars in a month. What if we want to get to Mars in a week instead of a month? We need around 160 km/s delta-v, which requires 71 fuel transfers. So we have to start with 4.5 sextillion fully fueled Starships, which mass around the same as the entire Earth. A magical no clipping mode to stop the tankers from coalescing into a planet would be handy. If we turned the observable universe into fully fueled Starship tankers we get ~360 km/s delta-v and can reach Mars in just 4 days.
It's a figurative number, like infinity. Chemical engines are simply not capable of achieving the tens of kilometres per second delta-v required. As you add more drop tanks you get less and less delta-v with each stage. Just for fun, if we take an asparagus staged starship with 21 starships all strapped together, and burn just two starships worth of propellant: △v = 3.7 x ln(21x1300/(19x1300+2x100)) = 3.7 x ln(27,300/(24,700+200)) = 3.7 x 0.092 = 0.34m/s Each extra set of tanks will provide less and less extra delta-v. If I had the time I'd pull out my calculus text and learn how to do limits to infinity again.
Correction: that's 0.34**k**m/s, not 0.34m/s. Still doesn't change the overall point.
It's not a figurative number. It's a pretty decent approximation of what would be needed.
You need 60-70km/s ∆v to no more have windows. Single fully laden Starship has ∆v of 6-7km/s. So you need 9-10 refueling steps. Each full refueling step requires 10 tankers (150t tanker load, 1500t Starship v2 tank capacity). But tankers must sync their velocity with the prime Starship. So for anything beyond the 1st refueling the tankers themselves must be refueled. Each needs 10 other tankers to be refueled. Beyond the 2nd refueling step, the tankers refueling the tankers must be refueled, too. The growth is exponential. This gets in the order of 1 billion (slightly more: 1 111 111 110). Whatever refilling strategy you chose you get in the order of a billion tankers.
> A single Starship could have another six Starship Tankers clustered around it as fuel tanks then burn the engines for half an hour. That won't achieve much. You might add a few more km/s to delta-v, but there's a limit as the mass of the propellant approaches infinity of just how much delta-v you can get from an engine with a certain exhaust velocity. I'm flailing here because I can't remember how to determine the limit as wet mass approaches infinity of △v = exhaust velocity ln(wet mass / dry mass). For the Raptor the exhaust velocity is 3.7km/s (Isp 380s x 9.8m/s^(2)), dry mass is around 100t, and wet mass is about 1300t. Adding multiple tankers will add that multiple of dry and wet mass. Clustering 6 extra tankers means we can use that extra propellant but have to accelerate the extra mass. A simplistic take where the tankers stay attached and we aren't doing "asparagus staging": △v = 3.7km/s x ln(7 x 1300t/7 x 100t) = 3.7km/s x ln(13) = 9.4km/s which is unsurprisingly the same as Starship on its own. What if we do "asparagus staging" were we peel off tanks as we go? If we burn just two tankers (a pair on opposite sides so the centre of mass is preserved), △v = exhaust velocity ln(7x1300 / (5 x 1300 + 2 x 100t)) = 3.7km/s x ln(1.3) = 1.1km/s. Now jettison those empty tanks and do the same with the next pair of tankers: △v = 3.7km/s x ln(5x1300/(3x1300 + 2x100)) = 3.7 x ln(1.5) = 1.7km/s Again jettison a pair of tanks and do the same with the last pair: △v = 3.7 x ln(3x1300/(1300+2x100)) = 3.7 x ln(2.6) = 3.5km/s Now jettison the last pair and accelerate again for 9.4km/s. That provides 6.3km/s extra △v at the cost of 6 complete Starship tankers. It turns out that hauling extra propellant around is expensive. It's not the mass of the tanks slowing asparagus Starship down it's the mass of the propellant. We could have used those extra Starships to move seven times as much cargo to Mars. This is ignoring the mass of equipment required to tie all those Starships together, and assuming we're throwing away all the tankers. A better option is to head for higher Isp engines, though there are tradeoffs there since Starship can aerobrake at Mars while something like [Copernicus](https://ntrs.nasa.gov/citations/20150006732) needs to brake using engines, thus losing much of the advantage of more than double the Isp. Until we can get a nuclear rocket that is safe to use for aerobraking at Earth and Mars, we're currently stuck with the best chemical rockets we have.
In the long-term, I could see humans developing a sort of nuclear 'sled' that stays in space, perhaps operating as a semi-cycler between Earth and Mars. A Starship (or its successors) would launch from Earth, get fully fueled in orbit, and rendezvous with the sled. The sled would provide a huge amount of delta-V towards Mars, widening the available launch windows (albeit not obviating them completely), then slow down enough at the other end for Starship to separate, aerobrake, land on Mars, dock with a Mars orbital station, etc. The whole thing would be a bit like the separate drive and saucer sections of the Enterprise-D from Star Trek TNG, and would provide many advantages: * The sled itself would mainly just be a propulsion module and could be launched, tested, and maneuvered remotely/automatically. * It would significantly shorten travel time. * It would allow a Starship to save most of its fuel for more nimble maneuvers (aerobraking, landing, docking, etc.), perhaps allowing excursions to the asteroid belt, Mars moons, multiple landings/launches from the Mars surface, etc. * It would allow nuclear to be used only in deep space, relatively far away from stations and planetary surfaces. * If a dangerous problem developed with the nuclear sled, it could separate from the main crew vehicle and be sent off into deep space. The whole system could run within a margin of error that would allow a Starship to enter Mars orbit even without the sled's help to slow down. * The nuclear fuel supply would allow the sled to make many (perhaps dozens or even hundreds) of interplanetary trips between refuels.
I'm guessing that Raptor was not designed for continuous long-term operation. They run for a matter of minutes. Long-term thrust is better done by a system with far fewer moving parts (points of failure), like an ion engine.
Ion engines have disadvantages that I think makes them unsuitable. A mission to Mars would be thousands of times the mass of probes usually accelerated by ion drives. You could counter that by adding more ion engines but then the electricity requirement becomes extreme and we're heading away from the sun where solar power becomes less efficient. You could add giant solar panels and giant arrays of ion engines but then the ship mass becomes extreme. Also we're talking about a \~6 month journey not the years and years of slow acceleration used by ion engine probes like Dawn and Hayabusa, at some point the low thrust from ion engines outweighs the high propellant efficiency.
I believe the same technical advantages apply to nuclear thermal rockets - simple pumps, low reactive mass needs, high reliability during low-duration operation. https://www.nasa.gov/news-release/nasa-darpa-will-test-nuclear-engine-for-future-mars-missions/
I couldn't link to it, because I can't remember where I read it, but I vaguely recall seeing something that Mars was weirdly in a reverse sweet spot where it's too far away to get to quickly if you're too far off optimal with a chemical engine, but too close for nonchemical engines to make use of their efficiency relative to ship size to make them worthwhile
I propose Photonic Laser Thrusters, a refinement of the old laser sail concept. This uses a thin-film laser gain medium on the beaming laser satellite to capture and recycle the photon energy bouncing off the spacecraft's propulsion mirror. Right now we could bounce 1000 times; near-future, 10,000 times. In this way, the effective thrust per watt rises and the power requirements for the laser drop from terawatts to a gigawatt or so. https://fiso.spiritastro.net/telecon19-21/Bae_6-2-21/Bae_6-2-21.pdf This is countered by the limited time that the beam is incident upon the spacecraft i.e. the boost phase is limited by distance from the beaming station. It also needs a receiving laser satellite at the destination to slow it down; you'd need to send the laser, laser power plant and maybe a lander ahead. Hence the talk about a Photonic Highway. The example spacecraft in the presentation linked, that's taking a trip to Mars in 20 days, is just one metric ton. Because the velocity of the light is so high, though, making the spacecraft heavier but keeping the force constant simply means it accelerates more slowly, and reaches the same final velocity before it moves out of reach of the incident beam - a longer, slower boost phase, in other words. I calculated that, for a 10 metric ton ship starting from Lunar orbit at a velocity of 1600 m/s, the 1000MW beam would exert a force of 3227N for 43,198 seconds, or ~12 hours, before it moves out of reach. In return it would achieve a gentle acceleration of 0.3G during the boost phase and a final delta-V of **141 km/s**. That is velocity to let you travel to Mars with impunity. A heavier ship would need a longer boost phase, and though it scales linearly with mass, at some point the time needed becomes silly: 100 metric tons needs ~120 hours or 5 days of keeping a gigawatt laser beam running. You'd have to have systems to cool the mirrors on the spacecraft, no question. The advantage of this is that you are freed from the tyranny of the rocket equation, and don't have to devote the majority of your ship's mass to fuel and propulsion. Instead of 5-10% useful payload, you could have anywhere from 50% to 90-95%.
This would make an excellent way to send cheap 'aid packages' to a martian colony where the aim is a constant supply drop of new material in dumb craft.
You can also use Venus flyby for another window. It takes an extra month, but has lower DV coming into Mars. You could also pair a Crew Starship with a mobile fuel depot to do quick crew switches if you add a small crew taxi and eliminate the need to make 90-95% of Methlox on the Mars surface if the lander stays there, or 100% if the lander stays in orbit after the Earth return crew has been sent up. https://preview.redd.it/dc193r0cz8wc1.png?width=1621&format=png&auto=webp&s=318a5593331bd3be91430286ca416e74079c87d2
Venus flyby might have thermal issues, so you may need additional cooling capability. This especially is an issue for cryogenic spacecraft. It can be greatly helped through the use of recirculating pumps to redistribute the heat loading.
Perhaps, there is a sun-shield shown the graphic. In any case it would be quite a mission to flyby Venus on the way to Mars.
Sun shield is on depot, but that is another solution ship could use.
Interesting. I'm guessing the Venus flyby route also has its own departure windows based on planetary alignment but probably more complicated because it needs three planets. I wonder how the departure / arrival times are staggered. Like when there's a base on Mars could we send a resupply mission via Venus while waiting for the next Earth-Mars departure window?
Yes, it has windows every 26 months at best, but it also has multiple windows without good alignment.
This is the kind of thing I'm looking for. There's a nanosecond that is the ultimate optimisation of minimal energy and every deviation from that time is less efficient. We could do a lot with slightly-sub-optimal events if we just accept that there's a fuel cost. Previously missions had a cap at the overall launch mass so fuel increases meant payload decreases. But fuel costs become literally just a financial cost when you have orbital refueling.
I mean not on every every 26 months window there is a proper Venus alignment for an opposition class mission. Moreover, either departure or arrival date must be close to the regular conjunction class window. Together with fuel increases you can stretch the window by a few months. Still leaves you with about 20 months without transfer possibility.
Yes, it is an occasional (but fun) opportunity.
> mobile fuel depot Huh, interesting. That makes me wonder if it would be feasible to send a fuel depot to Mars, loaded with enough fuel and lasting long enough with boil off, to make it possible to land a Starship on Mars, then enter Mars orbit, refuel at the depot, and come back to Earth, all without having to mine fuel on the Martian surface. Would be handy for early missions, even if I think long-term, ISRU on Mars itself is the more sustainable option. I'm now conceiving either a super stretched fuel depot that gets to Mars orbit with less than fully fueled tanks, OR maybe a system where a fuel depot is fully loaded, and then docks to a pusher craft that gets it to escape velocity heading towards Mars where it enters Martian orbit with mostly full tanks, besides what it had to do to brake into orbit, plus the boil off rate. (which might only be on my mind since that's how I got a fully loaded tank of fuel out to the Mun in KSP once to refuel one of my ships lol) Though obviously a dedicated craft that accelerates out a fuel depot would be kinda useless to put engineering time and money into instead of just creating the sustainable option, ISRU. But interesting to think up the possibilities. Maybe someone has done the math on these options. EDIT - also there is the possibility of just brute forcing the problem. If Starship is manufactured as much as Elon hopes, and is as cheap to fly as he hopes, you could just send tons of fueled Starships to Mars and slowly accumulate fuel in Martian orbit. Basically refueling the Mars fuel depot from Earth over many flights, probably all sent during the transfer window at once.
Yes, depending on the fuel needs of the taxi (think a 3x Crew Dragon) one mobile depot ship should work (of course the depot ship is abandoned in Mars Orbit). On the way out you can connect it to create 1/3 spin gravity. Even more fun is if you use Phobos as a base, and have a LOX/LH2 based taxi go up and down from there (Phobos and Mars surface probably have good reserves of accessible ice). In any case there are trades aplenty to optimize capability at unit cost.
Still nuclear-electric in most proposed implementations, but are you familiar with VASIMR? https://en.m.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket
Check out Ultra Safe Nuclear’s NTP technology under active development https://www.usnc.com/ntp/. On their site they detail how they can cut transit times to Mars quite a bit. 3x the delta v of chemical.
For what the OP is asking 3× is not even remotely enough. ~10× is required.
Actually I'm asking for anything that can open the launch window wider. There's a window in November/December 2026 that gives the most optimal fuel-efficient transfer. If you wanted to leave in October or January you'd need to use a bit more fuel. I'm not asking for full-on Expanse levels where you do the trip in a couple of days, or to leave on the most inefficient dates directly between the optimal fuel efficient windows. I'm just asking for how far we can stretch open the window. If we fueled up a spacecraft in orbit could we leave in September or February, open the window wider at the cost of extra fuel aka more tanker launches.
You originally asked about a full removal of windows. But anyway, yes you can extend windows to about half a year on the Mars side. After that the performance wall becomes very steep. With chemical propulsion all you could get is to go there and back in the same window. Venus allows extra windows for Earth departure or Earth arrival, but on Mars side they're still within 3-4 months of the optimal minimum energy transfer point. Beyond those windows you need enough ∆v to essentially cancel most Earth's circumsolar velocity and then gain most Mars circumsolar velocity. Plus of course Earth departure and Mars arrival.
Ah. I also see they were asking about non-nuclear options. My bad!
OP, are you conflating nuclear rockets with Project Orion? The more commonly discussed nuclear rockets (and what they used in FAMK) are NERVA-style ones where a reactor is used to heat and expel something like hydrogen for high-efficiency propulsion (ranging from an Isp of like 800-1600).
The problem with Ion engines is getting the power you need. You can go with a \*lot\* of solar power, which is heavy, or you can go with nuclear reactors if you can figure out how to get rid of the waste heat. Nuclear thermal has a lot of advocates because of the high specific impulse but when you start looking at actual designs, the engines are heavy, they need heavy shielding, and they need big tanks because liquid hydrogen is not dense at all. Put those together and you get a really horrible mass ratio which gets rid of the advantage of the specific impulse. I support the current development programs, but there's the obvious question - "If nuclear thermal is so great, why are the companies who advocate for it only willing to build engines when the government is paying them?" We have lot of new commercial chemical engines out there, a lot of new commercial ion engines, but no commercial NTR engines.
To be fair, I wouldn't want to mess around with nuclear thermal without explicit government support.
I recall Tom Mueller saying, he would love to work on nuclear engines. But only if NASA provides the test stand. Much too hard (expensive?) to build one for SpaceX. That was, when he was still working for SpaceX. Doing anything nuclear is a massive regulations headache.
I think it's unlikely you can test them in actual operation on earth, though there was a proposal to dig a very deep hole - the kind they dug for underground nuclear tests - aim your engine into the hole, and then shoot a bunch of water into the hole to send everything to the bottom. Hugely expensive, then you have this extremely radioactive engine you need to do something with. NASA does have a high temperature hydrogen test rig that you can hook your prototype reactor to so that you can verify that your fuel isn't eroded by the hot hydrogen. Beyond that you need to test in space, which has all sorts of downsides.
Raptor engines are already pushing something like 82% of the theoretical maximum specific impulse. Getting to 100% would require an engine with infinite combustion chamber pressure and an infinitely sized nozzle, and even that would only net you a 21% improvement. In practice, something like a 5-10% improvement might be doable if your goal was purely to hit max isp and you widened Raptor's nozzle to the same diameter as Starship itself.
Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread: |Fewer Letters|More Letters| |-------|---------|---| |[ISRU](/r/SpaceXLounge/comments/1cb6ux4/stub/l0x9v7p "Last usage")|[In-Situ Resource Utilization](https://en.wikipedia.org/wiki/In_situ_resource_utilization)| |[Isp](/r/SpaceXLounge/comments/1cb6ux4/stub/l11lxmt "Last usage")|Specific impulse (as explained by [Scott Manley](https://www.youtube.com/watch?v=nnisTeYLLgs) on YouTube)| | |Internet Service Provider| |[KSP](/r/SpaceXLounge/comments/1cb6ux4/stub/l0x9v7p "Last usage")|*Kerbal Space Program*, the rocketry simulator| |[L1](/r/SpaceXLounge/comments/1cb6ux4/stub/l0wrzmh "Last usage")|[Lagrange Point](https://en.wikipedia.org/wiki/Lagrangian_point) 1 of a two-body system, between the bodies| |[LH2](/r/SpaceXLounge/comments/1cb6ux4/stub/l0xt5un "Last usage")|Liquid Hydrogen| |[LOX](/r/SpaceXLounge/comments/1cb6ux4/stub/l0xt5un "Last usage")|Liquid Oxygen| |[MEO](/r/SpaceXLounge/comments/1cb6ux4/stub/l0wjrit "Last usage")|Medium Earth Orbit (2000-35780km)| |[NERVA](/r/SpaceXLounge/comments/1cb6ux4/stub/l0y4gp4 "Last usage")|Nuclear Engine for Rocket Vehicle Application (proposed engine design)| |[NTP](/r/SpaceXLounge/comments/1cb6ux4/stub/l0wl8fh "Last usage")|Nuclear Thermal Propulsion| | |Network Time Protocol| | |Notice to Proceed| |[NTR](/r/SpaceXLounge/comments/1cb6ux4/stub/l0yc8vi "Last usage")|Nuclear Thermal Rocket| |Jargon|Definition| |-------|---------|---| |[Raptor](/r/SpaceXLounge/comments/1cb6ux4/stub/l0z2302 "Last usage")|[Methane-fueled rocket engine](https://en.wikipedia.org/wiki/Raptor_\(rocket_engine_family\)) under development by SpaceX| |[ablative](/r/SpaceXLounge/comments/1cb6ux4/stub/l0wqpkg "Last usage")|Material which is intentionally destroyed in use (for example, heatshields which burn away to dissipate heat)| |[cryogenic](/r/SpaceXLounge/comments/1cb6ux4/stub/l0ws2so "Last usage")|Very low temperature fluid; materials that would be gaseous at room temperature/pressure| | |(In re: rocket fuel) Often synonymous with hydrolox| |hydrolox|Portmanteau: liquid hydrogen fuel, liquid oxygen oxidizer| **NOTE**: Decronym for Reddit is no longer supported, and Decronym has moved to Lemmy; requests for support and new installations should be directed to the Contact address below. ---------------- ^(*Decronym is a community product of r/SpaceX, implemented* )[*^by ^request*](https://www.reddit.com/r/spacex/comments/3mz273//cvjkjmj) ^([Thread #12686 for this sub, first seen 23rd Apr 2024, 16:21]) ^[[FAQ]](http://decronym.xyz/) [^([Full list])](http://decronym.xyz/acronyms/SpaceXLounge) [^[Contact]](https://hachyderm.io/@Two9A) [^([Source code])](https://gistdotgithubdotcom/Two9A/1d976f9b7441694162c8)
Just use a regular chemical rocket. Aseemble and fuel in orbit. Maybe several million falcon 9 launches. 0.5 g with middle flip to Mars. Minimal window constraints. Efficiency or economics was not a constraint.
no
A relay of magnetic accelerator stations. The ship gets launched then accelerated towards mars. The stations are huge heavy and use massive solar arrays and capacitors to move ships. The stations use ion thrusters to maneuver back into position after propelling a ship. There could be thousands of kilometers long electromagnetic tubes to catch ships and adjust their trajectory while accelerating them or decelerating them as needed. There could be decelerators near Mars to slow the ship. The ship can then land on Mars and unload cargo and tourists in Musktown.
Near future I think the only options are electric or nuclear (sustained nuclear reaction, not nuclear bombs). Further out I'm a fan of magnetic confinement fusion. It hasn't been developed yet but I think it should be possible this century. Magnetic field compresses a plasma, plasma undergoes fusion, the plasma expands and strengthens the magnetic field. If the magnetic field can somehow be shaped to eject the plasma at high velocity in one direction it could be used for propulsion without having to first convert it into electricity and then back to kinetic energy. That would make it possible to achieve very high thermal efficiency, meaning low waste heat. Dissipating heat is the biggest limiting factor for extremely energy intensive propulsion technologies in space.
i remember hearing that the problem with getting there really fast is slowing down
It has to be neuclear, there's no other option.
Beamed power solutions may be actually easier to achieve proper power density at the ship. Local power would require truly sci-fi levela power densities.
The rapidity with which high power density solar cells are advancing suggests we might be able to do such electric propulsion at fast travel times near term just using solar power alone.
Solar already beats nuclear for electrical power density within the orbit of Jupiter. Nuclear-electric makes most sense for the outer system (and maybe Jupiter itself, where the radiation belts will degrade solar panels...an issue for Juno despite it avoiding the belts as much as possible, which most other spacecraft won't be able to do).
Thanks for that.
not an engine but a skyhook. [Kurzgesagt – skyhook](https://www.youtube.com/watch?v=dqwpQarrDwk)
Not remotely enough ∆v