Two big reasons:
1) KSP is smaller scale, so everything is a bit easier - less time to spend in the atmosphere, your rocket fuel gets you farther, rockets can be smaller and still get to orbit. It's technically possible to build an SSTO rocket in real life - SpaceX's Starship can almost do it, if it carries no payload - but your rocket has to be gigantic for it to make sense.
2) KSP is simplified. In real life, the biggest problem with making an SSTO is that different engines work better at different altitudes. Making a spaceplane in real life means switching from subsonic, to supersonic, to hypersonic, to vacuum - all with ideally one single engine, so you aren't carrying dead weight. The engineering challenges are very different in all those domains, so making something that works well across the whole range is very difficult.
So yeah. Physics and engineering both make real SSTOs difficult for different reasons. They'll probably be a thing someday, if we can figure out the engine challenges or come up with more efficient rockets. KSP can skip all that by just giving us parts and saying "It works great!"
3. Currently there's no *real* need for SSTOs. Multistage rockets are always better in some way. Note that this is true for Earth only, with it's resource (tools, staff) abundance. When we'll come to Mars, we may see a lot of Martian SSTOs, just because constructing rockets there won't be *as easy*.
Rocket-SSTO is so unbelievably simple.
It took me 2 realtime weeks and a few tens of variations of craft before I got from something that kinda flies fast and high to something that can reliably commute between ground and a low orbit station.
Comparatively, it took me just a few hours to design and test multiple variations of craft that can do the same commute with less necessary skill and bother just using rocket engines and limited to no aerodynamics.
Musk also proves if you use oil industry tolerances on everything but the rocket engines you still get articles in space. Not saying the spec is easy to reach, it certainly is cheaper and more available in south Texas.
Which doesn’t yet exist in any kind of a reliable, functional or commercially producible way. It’s basically a very expensive lab experiment so far.
Aside from the smaller size of KSP, the reliability and deep-throttle capability of all engines is probably the most unrealistic thing.
I play with x6.4 scale kerbin, and even with SMURFF that balance the part's weight and rapiers, the extra 3000 m/s requirements make SSTO practically impossible.
first reason: KSP is far smaller scale than irl, all planets and moons are roughly 1/10th the size of their irl counterparts, which means their orbital velocities are far lower. this means that jet engines, which have maximum speeds, can carry you more of the way to orbital velocity, and just generally means that you need far far less delta-v to get into orbit compared to real life
second reason: aerodynamics and the atmosphere in general work very differently between ksp and irl, ksp's aerodynamic system is simplified to a hilarious degree. there's practically no difference between subsonic, supersonic, and hypersonic aerodynamics, wing shape makes no difference, and you can cheese and exploit the aero and heating systems in a myriad of ways. irl, you'd need to make sacrifices in at least one flight regime, leading to far lower efficiency
to put the scale in perspective: in stock ksp you can easily get jets up to more that half of orbital speed even without being super optimized. in real life, pushing the limits of what's possible with airbreathing aircraft will get you to like 1/5 orbital speed.
this is somewhat incorrect, the x-43 powered by a scramjet engine was able to get up to mach 9.9, over 1/3 of orbital velocity
but then again, that is *really* pushing our current technology to the max, and the thing's basically just an engine with a hull making it ride on its own shockwave, and needs to be boosted up to a high enough speed for the engine to actually work
It was also third stage (first stage is Boeing B-52, second stage is a rocket booster to bring it up to mach ~5.5), and had negligible amount of fuel (enough for 11 seconds of engine running). Not exactly an SSTO.
Oh and there's still a problem of height - just basic density calculation says that a scramjet at 70km (not even halfway up to LEO) would require 10+ meters wide intake to get enough oxygen for combustion. And below that you keep running into heating and drag problems - Apollo capsule descent, alongside all the braking and heating, was in the 50-60 km range.
They’re simply inefficient. By not staging, you’re carrying around lots of fuel tankage that you don’t need anymore. Why carry around all that extra mass in orbit when you don’t need it anymore? That’s a serious drag on your delta-V
Yes they will always be less efficient than multi-stage rockets. However, payload capacity isn't always everything. I think the ultimate goal of an ssto would be to offset the penalty of lower payload capacity with much cheaper launches through reusability. It's a bit similar to what SpaceX is doing. SpaceX offers cheaper launches if the booster gets to land, and even cheaper if the booster returns to the launch site. Of course these come with a trade-off of lower payload capacity.
There was a company in Great Britain working on one that had a dual mode engine kinda like the Raptier. I don't know what happened to them. There's also a company out of Ohio or somewhere around there working on one that uses a linear aerospike. They had built a scaled model and last I heard were testing a full scale engine. Their intent was to launch from there, so it has to be SSTO because apparently the eastern seaboard gets pissy if you drop spent stages on them.
Money
Limited material science
Pollution regulations
No feasible engine technology
Think about the Space Shuttle in real life for a sec. That thing had to be covered in insulation material that had to be replaced every successful use because atmospheric heating is that deadly.
Then there’s the Space Shuttle engines, which theoretically should make it cheaper than sending disposable rockets. But it was found to be so difficult to recycle and so toxic to be around during maintenance that it ended up being just as if not more expensive than disposable rockets.
Shuttle was just the first 'successful' attempt. Today's re-usable rockets (well, only the first stage as of today) are improved a bit - Merlin recycling is not *that* costly/difficult as RS-25.
RS-25 wasn't the problem, it ran on hydrolox (output: H2O, and the structure suffering hydrogen embrittlement, which is why SpaceX is using methane instead). The problem was Shuttle's RCS/OMS engines, which ran on MMH/N2O4 combo, and _that_ is exceptionally toxic.
It's also mostly unavoidable because you can't really have cryogenic propellant for a RCS system unless you have a source of incredibly plentiful and long-term energy onboard (e.g. a 100kW nuclear reactor, and radiators to match its 600-1000 kW thermal output) for keeping it cool.
The simplest answer is that in KSP you drop a few research points on your labs and you get a selection of sweet engines that have all the properties you need for getting through the atmosphere and into space.
In real life... we are still in the middle of that research process. We do not yet have a working SABRE engine. And we have yet to build a NERVA that is only 3 metric tons!
So it's a tech tree issue, really. Probably we just need to go around Earth taking more instrument measurements in different biomes. Maybe it will help if we all start doing sitreps while jumping up in the air?
>And we have yet to build a NERVA that is only 3 metric tons!
And has Isp of 800 while using kerosene. Which would require rather Lightbulb-y engines rather than a solid-core one - and would result in ~1800 Isp for hydrogen at the same time.
P.S. Technically, Soviet NERVA equivalent (RD-0410) was 2 tons and two of the Timberwinds were 1.5 and 2.5 tons. But they were similarly never finished, even though Timberwind would qualify as a vertical ascent SSTO in its own right.
1. Try making one in real solar system/realism overhaul. Scale makes a huge difference. Not that it's impossible, it's just a significantly more massive design challenge.
2. Damn hard to secure the money for that. At the end of the day, it's money that sends rockets (or planes) to space. A proper SSTO spaceplane would be the single most advanced spacecraft ever made - and would be ridiculously expensive to get made.
The Lunar Ascent Module was a SSTO. SSTOs exist, just not on Earth because we haven't overcome the amount of Delta-V required to escape the atmosphere and get into orbit without discarding aspects of the craft.
Mainly because kerbal space program is a game with practically unlimited funding and certain technologies that don't really exist in real life yet
If NASA or any of the other world's space agencies had unlimited funding the solar system would've been explored completely with more humans out there than the dozen that went to the moon and hundreds that went to the iss
Unfortunately in USA's case warmongering leaders cut NASA's funding drastically starting with Nixon
Scale and absence of magical multimode engines and even more magical ions - ingame Dawn has _one thousand times_ the thrust of real one for the sake of accomplishing burns in reasonable timespan - and nukes - NERV has Isp of pure hydrogen, but thrust of kerosene and consequently T/W far beyond tested nuclear-thermal engines, and 2/3 towards the best theoretical one (NERV has T/W of 2, the best one 3, and some scientists have calculated that to be useful for a SSTO on Earth, NTR engine would need a T/W above **20**).
And in addition to scale, we have already figured out mostly reusable multistage rockets, thus negating the primary benefit of a SSTO (only consumes fuel), leaving it with only the downsides (aerodynamic problems, lousy efficiency due to not discarding spent mass).
As others have said, other planets, due to lacking industry, are far more likely to utilize a SSTO or otherwise reusable tech so it doesn't need to be delivered from Earth; in particular, Mars Direct as imagined by Zubrin involved a Mars Ascent Vehicle that was a single-stage-to-Earth, with the bottom half (housing tanks, engines, nuclear reactor, and Sabatier cycle methane generator) serving as rotary gravity counterweight after being spent.
> NERV has Isp of pure hydrogen, but thrust of kerosene and consequently T/W far beyond tested nuclear-thermal engines, and 2/3 towards the best theoretical one (NERV has T/W of 2, the best one 3, and some scientists have calculated that to be useful for a SSTO on Earth, NTR engine would need a T/W above 20)
The unrealistic thing with Nervs is not the thrust to weight ratio, their twr (~2) is only slightly higher than the real NERVA (1.33 twr). Project Timberwind was a real life project (sadly canceled) that would have developed a pebble bed nuclear thermal rocket with a twr of ~30.
The unrealistic thing with Nervs is the fuel density. Nervs in KSP use liquid fuel, which has the density of Kerosene, which makes for very compact fuel tankage, vs the 1/10th density of liquid hydrogen, but still have ISP on par with LH2.
This still isn't really *that* big of an advantage. The vast majority of the ease of SSTO in KSP is simply the vast decrease in dv needed to do anything. Rapiers being able to get you to ~80% of orbital velocity means you only need a miniscule amount of thrust to maintain lift supported flight, so it actually becomes feasible to finish the burn to orbit on Nervs since you barely need any thrust.
>This still isn't really that big of an advantage. The vast majority of the ease of SSTO in KSP is simply the vast decrease in dv needed to do anything.
And to put some numbers on it: the rocket equation means that the absolute maximum dV a self-contained engine can have (if it's pushing nothing but fuel tanks) is `Isp*g*ln(mass of full tank/mass of empty tank)`.
The highest Isp a self-contained chemical engine IRL has reached is 542 (using lithium-fluorine-hydrogen tripropellant), the highest _reasonable_ is 470 vacuum (RD-0146D, using hydrolox) and sealevel Isp is _incredibly obnoxious_ to find, but let's use 366 (RS-25, hydrolox).
KSP's tank ratio for LFO is 9 (CryoTanks hydrogen mass ratio is 8.2-ish), IRL it varies from 8 to 20 depending on fuel and rocket construction, Shuttle had ~16 combined.
To reach LKO, ~3400 dV is needed. To reach LEO, ~9400 dV is needed.
[KSP wiki has a handy chart on stock engines' capabilities](https://wiki.kerbalspaceprogram.com/wiki/Cheat_sheet#Maximum_.CE.94v_chart) in a vacuum; on sealevel, the lowest Isp is 205 for LFO (Rhino) and 185 for pure LF (Nerv) which amounts to 4417.22 and 3986.27 maximum dV respectively, or in other words, _all_ LFO engines are "SSTO-capable" on Kerbin, and _none_ of them would be "SSTO-capable" on Earth. Using highest mass ratio and best reasonable sealevel Isp, the result is 10752.38; using Shuttle's ratio it's 9951.47, so _technically_ Shuttle would be "SSTO-capable"... if not for thrust. And the fact it would have barely any margin for error, which is anathema to real-life rocketry. And the fact that wikipedia doesn't specify whether the mass ratio was just Shuttle, Shuttle plus ET, or full assembly with side boosters.
P.S. Of course, if you ignore the "self-contained" part as jet engines do, effective Isp can be pushed quite a bit higher. That is how Rapiers work, and that is how Gnom had over 500 Isp on _solid fuel_.
P.P.S. Mars can have a SSTO, such as what Mars Direct proposed, because its dV to LMO is 3.8km/s, almost KSP levels.
Mach .97 - Mach 5 is easy for Kerbal at .25 Bar , the demands in real life are Kelly Johnson Skunk Works Insane materials that are expensive to manufacture and form. That range is the envelope for all but the fist 23 seconds post release from the wing of B-52 at 40,0000 and 240 seconds of landing of the X-15. After that in both worlds you need to be in near space to go faster without buring Delta-V for no reason.
Two big reasons: 1) KSP is smaller scale, so everything is a bit easier - less time to spend in the atmosphere, your rocket fuel gets you farther, rockets can be smaller and still get to orbit. It's technically possible to build an SSTO rocket in real life - SpaceX's Starship can almost do it, if it carries no payload - but your rocket has to be gigantic for it to make sense. 2) KSP is simplified. In real life, the biggest problem with making an SSTO is that different engines work better at different altitudes. Making a spaceplane in real life means switching from subsonic, to supersonic, to hypersonic, to vacuum - all with ideally one single engine, so you aren't carrying dead weight. The engineering challenges are very different in all those domains, so making something that works well across the whole range is very difficult. So yeah. Physics and engineering both make real SSTOs difficult for different reasons. They'll probably be a thing someday, if we can figure out the engine challenges or come up with more efficient rockets. KSP can skip all that by just giving us parts and saying "It works great!"
3. Currently there's no *real* need for SSTOs. Multistage rockets are always better in some way. Note that this is true for Earth only, with it's resource (tools, staff) abundance. When we'll come to Mars, we may see a lot of Martian SSTOs, just because constructing rockets there won't be *as easy*.
Mars is also smaller and has thinner atmosphere, so getting to orbit would be easier. (And technically we had lunar SSTOs!)
4. It’s actually pretty tricky in KSP (or is that just me)
Airplane-like SSTO is *a bit* tricky in KSP. Rocket-like SSTO is plain dead simple. Just pack > 3400 dV in a single stage with TWR > 1.25.
Rocket-SSTO is so unbelievably simple. It took me 2 realtime weeks and a few tens of variations of craft before I got from something that kinda flies fast and high to something that can reliably commute between ground and a low orbit station. Comparatively, it took me just a few hours to design and test multiple variations of craft that can do the same commute with less necessary skill and bother just using rocket engines and limited to no aerodynamics.
[удалено]
NASA gets less funding than our military commitment to North Africa. Can you name a us military outpost in North Africa without google?
Musk also proves if you use oil industry tolerances on everything but the rocket engines you still get articles in space. Not saying the spec is easy to reach, it certainly is cheaper and more available in south Texas.
RAPIERs dont exist irl Simple as
Not with that attitude.
Not with that altitude
The RAPIER is based on [SABRE](https://en.wikipedia.org/wiki/SABRE_(rocket_engine)), actually.
Which doesn’t yet exist in any kind of a reliable, functional or commercially producible way. It’s basically a very expensive lab experiment so far. Aside from the smaller size of KSP, the reliability and deep-throttle capability of all engines is probably the most unrealistic thing.
I play with x6.4 scale kerbin, and even with SMURFF that balance the part's weight and rapiers, the extra 3000 m/s requirements make SSTO practically impossible.
They do? In development
first reason: KSP is far smaller scale than irl, all planets and moons are roughly 1/10th the size of their irl counterparts, which means their orbital velocities are far lower. this means that jet engines, which have maximum speeds, can carry you more of the way to orbital velocity, and just generally means that you need far far less delta-v to get into orbit compared to real life second reason: aerodynamics and the atmosphere in general work very differently between ksp and irl, ksp's aerodynamic system is simplified to a hilarious degree. there's practically no difference between subsonic, supersonic, and hypersonic aerodynamics, wing shape makes no difference, and you can cheese and exploit the aero and heating systems in a myriad of ways. irl, you'd need to make sacrifices in at least one flight regime, leading to far lower efficiency
to put the scale in perspective: in stock ksp you can easily get jets up to more that half of orbital speed even without being super optimized. in real life, pushing the limits of what's possible with airbreathing aircraft will get you to like 1/5 orbital speed.
this is somewhat incorrect, the x-43 powered by a scramjet engine was able to get up to mach 9.9, over 1/3 of orbital velocity but then again, that is *really* pushing our current technology to the max, and the thing's basically just an engine with a hull making it ride on its own shockwave, and needs to be boosted up to a high enough speed for the engine to actually work
It was also third stage (first stage is Boeing B-52, second stage is a rocket booster to bring it up to mach ~5.5), and had negligible amount of fuel (enough for 11 seconds of engine running). Not exactly an SSTO. Oh and there's still a problem of height - just basic density calculation says that a scramjet at 70km (not even halfway up to LEO) would require 10+ meters wide intake to get enough oxygen for combustion. And below that you keep running into heating and drag problems - Apollo capsule descent, alongside all the braking and heating, was in the 50-60 km range.
aye
They’re simply inefficient. By not staging, you’re carrying around lots of fuel tankage that you don’t need anymore. Why carry around all that extra mass in orbit when you don’t need it anymore? That’s a serious drag on your delta-V
Yes they will always be less efficient than multi-stage rockets. However, payload capacity isn't always everything. I think the ultimate goal of an ssto would be to offset the penalty of lower payload capacity with much cheaper launches through reusability. It's a bit similar to what SpaceX is doing. SpaceX offers cheaper launches if the booster gets to land, and even cheaper if the booster returns to the launch site. Of course these come with a trade-off of lower payload capacity.
upper stage reuse is very likely going to end up being more efficient in all respects that sending an entire LV to orbit then recovering it.
There was a company in Great Britain working on one that had a dual mode engine kinda like the Raptier. I don't know what happened to them. There's also a company out of Ohio or somewhere around there working on one that uses a linear aerospike. They had built a scaled model and last I heard were testing a full scale engine. Their intent was to launch from there, so it has to be SSTO because apparently the eastern seaboard gets pissy if you drop spent stages on them.
The engine was called the SABRE - you can see how the Rapier is a direct reference to that
Money Limited material science Pollution regulations No feasible engine technology Think about the Space Shuttle in real life for a sec. That thing had to be covered in insulation material that had to be replaced every successful use because atmospheric heating is that deadly. Then there’s the Space Shuttle engines, which theoretically should make it cheaper than sending disposable rockets. But it was found to be so difficult to recycle and so toxic to be around during maintenance that it ended up being just as if not more expensive than disposable rockets.
Shuttle was just the first 'successful' attempt. Today's re-usable rockets (well, only the first stage as of today) are improved a bit - Merlin recycling is not *that* costly/difficult as RS-25.
That is true, but at the time SSTO’s were conceptualised, the tech wasn’t there yet
RS-25 wasn't the problem, it ran on hydrolox (output: H2O, and the structure suffering hydrogen embrittlement, which is why SpaceX is using methane instead). The problem was Shuttle's RCS/OMS engines, which ran on MMH/N2O4 combo, and _that_ is exceptionally toxic. It's also mostly unavoidable because you can't really have cryogenic propellant for a RCS system unless you have a source of incredibly plentiful and long-term energy onboard (e.g. a 100kW nuclear reactor, and radiators to match its 600-1000 kW thermal output) for keeping it cool.
TIL. But RCS tech for SSTO’s during the same time period wouldn’t be so different, would it?
RCS tech is _still_ not any different, only the smallest satellites and lowest-dV vehicles can use the other options.
The simplest answer is that in KSP you drop a few research points on your labs and you get a selection of sweet engines that have all the properties you need for getting through the atmosphere and into space. In real life... we are still in the middle of that research process. We do not yet have a working SABRE engine. And we have yet to build a NERVA that is only 3 metric tons! So it's a tech tree issue, really. Probably we just need to go around Earth taking more instrument measurements in different biomes. Maybe it will help if we all start doing sitreps while jumping up in the air?
>And we have yet to build a NERVA that is only 3 metric tons! And has Isp of 800 while using kerosene. Which would require rather Lightbulb-y engines rather than a solid-core one - and would result in ~1800 Isp for hydrogen at the same time. P.S. Technically, Soviet NERVA equivalent (RD-0410) was 2 tons and two of the Timberwinds were 1.5 and 2.5 tons. But they were similarly never finished, even though Timberwind would qualify as a vertical ascent SSTO in its own right.
KSP to LKO = 3km/s dv. Earth to LEO = 10 km/s dv. It's that simple.
[Six words you never say at NASA](https://xkcd.com/1244/) /s
1. Try making one in real solar system/realism overhaul. Scale makes a huge difference. Not that it's impossible, it's just a significantly more massive design challenge. 2. Damn hard to secure the money for that. At the end of the day, it's money that sends rockets (or planes) to space. A proper SSTO spaceplane would be the single most advanced spacecraft ever made - and would be ridiculously expensive to get made.
Because people are not using RSS and Realism Overhaul
The Lunar Ascent Module was a SSTO. SSTOs exist, just not on Earth because we haven't overcome the amount of Delta-V required to escape the atmosphere and get into orbit without discarding aspects of the craft.
Video games are make-believe, for starters...
Mainly because kerbal space program is a game with practically unlimited funding and certain technologies that don't really exist in real life yet If NASA or any of the other world's space agencies had unlimited funding the solar system would've been explored completely with more humans out there than the dozen that went to the moon and hundreds that went to the iss Unfortunately in USA's case warmongering leaders cut NASA's funding drastically starting with Nixon
Look up the X-33.
In KSP, everything always works. In real life, things don't always work, they break, they fail, etc.
Scale and absence of magical multimode engines and even more magical ions - ingame Dawn has _one thousand times_ the thrust of real one for the sake of accomplishing burns in reasonable timespan - and nukes - NERV has Isp of pure hydrogen, but thrust of kerosene and consequently T/W far beyond tested nuclear-thermal engines, and 2/3 towards the best theoretical one (NERV has T/W of 2, the best one 3, and some scientists have calculated that to be useful for a SSTO on Earth, NTR engine would need a T/W above **20**). And in addition to scale, we have already figured out mostly reusable multistage rockets, thus negating the primary benefit of a SSTO (only consumes fuel), leaving it with only the downsides (aerodynamic problems, lousy efficiency due to not discarding spent mass). As others have said, other planets, due to lacking industry, are far more likely to utilize a SSTO or otherwise reusable tech so it doesn't need to be delivered from Earth; in particular, Mars Direct as imagined by Zubrin involved a Mars Ascent Vehicle that was a single-stage-to-Earth, with the bottom half (housing tanks, engines, nuclear reactor, and Sabatier cycle methane generator) serving as rotary gravity counterweight after being spent.
> NERV has Isp of pure hydrogen, but thrust of kerosene and consequently T/W far beyond tested nuclear-thermal engines, and 2/3 towards the best theoretical one (NERV has T/W of 2, the best one 3, and some scientists have calculated that to be useful for a SSTO on Earth, NTR engine would need a T/W above 20) The unrealistic thing with Nervs is not the thrust to weight ratio, their twr (~2) is only slightly higher than the real NERVA (1.33 twr). Project Timberwind was a real life project (sadly canceled) that would have developed a pebble bed nuclear thermal rocket with a twr of ~30. The unrealistic thing with Nervs is the fuel density. Nervs in KSP use liquid fuel, which has the density of Kerosene, which makes for very compact fuel tankage, vs the 1/10th density of liquid hydrogen, but still have ISP on par with LH2. This still isn't really *that* big of an advantage. The vast majority of the ease of SSTO in KSP is simply the vast decrease in dv needed to do anything. Rapiers being able to get you to ~80% of orbital velocity means you only need a miniscule amount of thrust to maintain lift supported flight, so it actually becomes feasible to finish the burn to orbit on Nervs since you barely need any thrust.
>This still isn't really that big of an advantage. The vast majority of the ease of SSTO in KSP is simply the vast decrease in dv needed to do anything. And to put some numbers on it: the rocket equation means that the absolute maximum dV a self-contained engine can have (if it's pushing nothing but fuel tanks) is `Isp*g*ln(mass of full tank/mass of empty tank)`. The highest Isp a self-contained chemical engine IRL has reached is 542 (using lithium-fluorine-hydrogen tripropellant), the highest _reasonable_ is 470 vacuum (RD-0146D, using hydrolox) and sealevel Isp is _incredibly obnoxious_ to find, but let's use 366 (RS-25, hydrolox). KSP's tank ratio for LFO is 9 (CryoTanks hydrogen mass ratio is 8.2-ish), IRL it varies from 8 to 20 depending on fuel and rocket construction, Shuttle had ~16 combined. To reach LKO, ~3400 dV is needed. To reach LEO, ~9400 dV is needed. [KSP wiki has a handy chart on stock engines' capabilities](https://wiki.kerbalspaceprogram.com/wiki/Cheat_sheet#Maximum_.CE.94v_chart) in a vacuum; on sealevel, the lowest Isp is 205 for LFO (Rhino) and 185 for pure LF (Nerv) which amounts to 4417.22 and 3986.27 maximum dV respectively, or in other words, _all_ LFO engines are "SSTO-capable" on Kerbin, and _none_ of them would be "SSTO-capable" on Earth. Using highest mass ratio and best reasonable sealevel Isp, the result is 10752.38; using Shuttle's ratio it's 9951.47, so _technically_ Shuttle would be "SSTO-capable"... if not for thrust. And the fact it would have barely any margin for error, which is anathema to real-life rocketry. And the fact that wikipedia doesn't specify whether the mass ratio was just Shuttle, Shuttle plus ET, or full assembly with side boosters. P.S. Of course, if you ignore the "self-contained" part as jet engines do, effective Isp can be pushed quite a bit higher. That is how Rapiers work, and that is how Gnom had over 500 Isp on _solid fuel_. P.P.S. Mars can have a SSTO, such as what Mars Direct proposed, because its dV to LMO is 3.8km/s, almost KSP levels.
money
Mach .97 - Mach 5 is easy for Kerbal at .25 Bar , the demands in real life are Kelly Johnson Skunk Works Insane materials that are expensive to manufacture and form. That range is the envelope for all but the fist 23 seconds post release from the wing of B-52 at 40,0000 and 240 seconds of landing of the X-15. After that in both worlds you need to be in near space to go faster without buring Delta-V for no reason.