PURE rocket SSTO

[N_las]

Rockets are disposable, sstos are not.

Being able to reach orbit with a single stage or not has nothing to do with being disposable or not. You can have disposable sstos or reusable mulit-stage rockets just fine.

[Streetwind]

As mentioned above, you have your definition off "SSTO" wrong. The term simply means you achieve low earth orbit without decoupling any part of your craft. If you want a returning, reusable vehicle, then you need to look at that separately. It does not in any way, shape or form depend on your number of stages, nor does it directly result from eliminating staging.

But, I also think you're not quite realizing the reality of what "payload fraction" actually means, and what you must go through to achieve it.

Look up the rocket equation. At a high level, it is a very simple equation; if it wasn't for the logarithm, an elementary school kid could solve it, and then there's the fact that everyone uses a calculator for the logarithm anyways. You simply have a division, and the number resulting from that division gets (after having something funny done to it) multiplied with another number, and the result of that is once again multiplied with another number. That's all there is to it. One of the two multipliers is a physical constant, the other one is Isp... which, if you are intending to use a specific engine technology, is also more or less a constant. So the only thing that really changes the result of the rocket equation is that division in the back. And that division is describing something called "fuel mass fraction": the amount of weight of your rocket that is fuel compared to the amount of weight of your rocket that isn't fuel.

The problem with the fuel mass fraction is that it approaches a limit that it cannot exceed: namely the values of the fuel tank you use. In stock KSP for example, your tanks weigh 1 ton for every 8 tons of fuel they contain. You want the result of the division to be as large as possible, so that the end result after multiplying with the two "constants" is also as large as possible. But no matter what you do, in KSP it will never be larger than 9/1, even if you add the entire observable universe's mass in fuel. In real life the numbers are different, but it does not change the fact that there is still a number you cannot exceed no matter what. And therefore there is a dV number you cannot exceed no matter what. A single stage is hard limited in the amount of dV it can have. Assuming you are using the best possible fuel mass fraction, the only way to increase your dV is either by staging, or by using the one other part in the rocket equation that can be variable: Isp.

As a result, you can express the maximum dV a single stage vehicle can achieve as a multiple of Isp. For stock KSP specifically, if you take the natural logarithm of 9 and then multiply it with KSP's gravity acceleration constant 9.82, you get ~21.577. This shortens the rocket equation to dV = Isp * 21.577 - or, in other words: you are never going to get more dV out of a single stage than 21.577 times your engine's Isp. Ever. Period. If you are using a LV-T30, and your average Isp over the entire burn time is ~365 because as a SSTO you are spending most of your time in very thin atmosphere, then you're not going to get more than 7875.6 dV.

And now comes the part that is sometimes called "the tyranny of the rocket equation". First, you obviously understand that you cannot build a rocket with the ideal fuel/mass fraction of the tanks, because with only tanks you can't fly. You have no engine. You have no control elements. You have no aerodynamic structure. You have no flight computer. You have no communication devices. You have no safety and redundancy elements. Want your rocket to be reusable? Well, you also do not have any heat shielding, nor parachutes, nor wings, nor landing gear, nor reserve fuel for deorbit burns. All these things add weight, and that weight is not launch fuel. Therefore it ruins your fuel mass fraction, making it a lot worse than the ideal values of your fuel tanks. And if you attempt to remedy this by adding more fuel tanks, you are running headfirst into rapidly diminishing returns. After all, the fuel tank has mass that is not fuel as well. The more fuel you add, the less is makes a difference, because you can only slowly approach your ideal value, never reach or exceed it. You could flat-out double the size of your rocket and only gain 2%-3% improvement in dV in some cases.

In practical application (and in the KSP example), this means that you're not only never going to get more than 21.577 times Isp in dV... no, you're not even getting that. You're not getting 21 times dV either, and you're not getting 20 times Isp either unless you deliberately build something utterly impractical just to prove me wrong. By the time you're at 15 times Isp, you're getting massively diminished returns already which may start to render the exercise somewhat pointless for the sheer effort involved in building and later flying that monstrosity - and keep in mind, this is without having an actual budget to adhere to, and a supervisor yelling at you if you use more than three fuel tanks!

And after all of that, you know what's still missing? Payload is still missing. Fuel mass fraction is what determines your dV, and it is the ratio between fuel and not fuel; and payload falls firmly in the category of not fuel. More than that, not only is it not fuel, but it's also not something else the rocket needs to fly. It doesn't help with steering, or stability, or landing... it comes in addition to all of that, and it doesn't do anything at all beyond further worsening your fuel mass fraction. That is why payload is so expensive... and that is also why SSTO designs are so incredibly hard to pull off IRL.

Because you need such a large amount of dV to get into LEO, any single stage you use must either have an incredibly high Isp, or an incredibly high fuel mass fraction. Alas, since the space shuttle main engines hit 366/452 Isp in the seventies, we haven't managed to improve the Isp of our launch capable engines at all, and that's assuming you're using LH2/LOX. If you want kerosene, well... our best kerosene engine is the one ULA is no longer allowed to buy from Russia, the RD-180 at 311/338 Isp. Not exactly very reassuring numbers there. So a high fuel mass fraction it is. And you know what you absolutely cannot have when you want a high fuel mass fraction? Payload. You absolutely cannot have any significant amounts of payload. That is the reason nobody has built a SSTO rocket yet, and especially not one running on kerosene: it's incredibly difficult to make something cost effective with a payload fraction that tiny.

The Skylon spaceplane actually aims not to simply "save dV" by flying on airbreathing engines for the first half of the trip. Much rather, it aims to decrease the required fuel mass fraction. That works because airbreathing engines have a much higher Isp than rocket engines (projected 3500 Isp). If you then take the average Isp over the entire flight from liftoff to orbit, it manages an Isp number that no chemical rocket engine could achive. It is basically trying to use the other method besides having a large fuel mass fraction to man up the excessive dV required for a single stage to orbit. And in doing so, it hopes to free up more dry mass allowance than is consumed by the extra equipment necessary for the dual mode engines, which ultimately makes room for more payload.

Skylon hopes to be the first SSTO to ever fly, because it aims to be cost-effective by offering payload fractions competitive with (or better than) staged rockets. A Falcon 9 sends 13 tons payload to the ISS with 506 tons of launch mass; Skylon aims to carry 11 tons of payload to the same destination with only 345 tons launch mass.

Edited June 4, 2014 by Streetwind

[TeeGee]

As mentioned above, you have your definition off "SSTO" wrong. The term simply means you achieve low earth orbit without decoupling any part of your craft. If you want a returning, reusable vehicle, then you need to look at that separately. It does not in any way, shape or form depend on your number of stages, nor does it directly result from eliminating staging.

But, I also think you're not quite realizing the reality of what "payload fraction" actually means, and what you must go through to achieve it.

Look up the rocket equation. At a high level, it is a very simple equation; if it wasn't for the logarithm, an elementary school kid could solve it, and then there's the fact that everyone uses a calculator for the logarithm anyways. You simply have a division, and the number resulting from that division gets (after having something funny done to it) multiplied with another number, and the result of that is once again multiplied with another number. That's all there is to it. One of the two multipliers is a physical constant, the other one is Isp... which, if you are intending to use a specific engine technology, is also more or less a constant. So the only thing that really changes the result of the rocket equation is that division in the back. And that division is describing something called "fuel mass fraction": the amount of weight of your rocket that is fuel compared to the amount of weight of your rocket that isn't fuel.

The problem with the fuel mass fraction is that it approaches a limit that it cannot exceed: namely the values of the fuel tank you use. In stock KSP for example, your tanks weigh 1 ton for every 8 tons of fuel they contain. You want the result of the division to be as large as possible, so that the end result after multiplying with the two "constants" is also as large as possible. But no matter what you do, in KSP it will never be larger than 9/1, even if you add the entire observable universe's mass in fuel. In real life the numbers are different, but it does not change the fact that there is still a number you cannot exceed no matter what. And therefore there is a dV number you cannot exceed no matter what. A single stage is hard limited in the amount of dV it can have. Assuming you are using the best possible fuel mass fraction, the only way to increase your dV is either by staging, or by using the one other part in the rocket equation that can be variable: Isp.

As a result, you can express the maximum dV a single stage vehicle can achieve as a multiple of Isp. For stock KSP specifically, if you take the natural logarithm of 9 and then multiply it with KSP's gravity acceleration constant 9.82, you get ~21.577. This shortens the rocket equation to dV = Isp * 21.577 - or, in other words: you are never going to get more dV out of a single stage than 21.577 times your engine's Isp. Ever. Period. If you are using a LV-T30, and your average Isp over the entire burn time is ~365 because as a SSTO you are spending most of your time in very thin atmosphere, then you're not going to get more than 7875.6 dV.

And now comes the part that is sometimes called "the tyranny of the rocket equation". First, you obviously understand that you cannot build a rocket with the ideal fuel/mass fraction of the tanks, because with only tanks you can't fly. You have no engine. You have no control elements. You have no aerodynamic structure. You have no flight computer. You have no communication devices. You have no safety and redundancy elements. Want your rocket to be reusable? Well, you also do not have any heat shielding, nor parachutes, nor wings, nor landing gear, nor reserve fuel for deorbit burns. All these things add weight, and that weight is not launch fuel. Therefore it ruins your fuel mass fraction, making it a lot worse than the ideal values of your fuel tanks. And if you attempt to remedy this by adding more fuel tanks, you are running headfirst into rapidly diminishing returns. After all, the fuel tank has mass that is not fuel as well. The more fuel you add, the less is makes a difference, because you can only slowly approach your ideal value, never reach or exceed it. You could flat-out double the size of your rocket and only gain 2%-3% improvement in dV in some cases.

In practical application (and in the KSP example), this means that you're not only never going to get more than 21.577 times Isp in dV... no, you're not even getting that. You're not getting 21 times dV either, and you're not getting 20 times Isp either unless you deliberately build something utterly impractical just to prove me wrong. By the time you're at 15 times Isp, you're getting massively diminished returns already which may start to render the exercise somewhat pointless for the sheer effort involved in building and later flying that monstrosity - and keep in mind, this is without having an actual budget to adhere to, and a supervisor yelling at you if you use more than three fuel tanks!

And after all of that, you know what's still missing? Payload is still missing. Fuel mass fraction is what determines your dV, and it is the ratio between fuel and not fuel; and payload falls firmly in the category of not fuel. More than that, not only is it not fuel, but it's also not something else the rocket needs to fly. It doesn't help with steering, or stability, or landing... it comes in addition to all of that, and it doesn't do anything at all beyond further worsening your fuel mass fraction. That is why payload is so expensive... and that is also why SSTO designs are so incredibly hard to pull off IRL.

Because you need such a large amount of dV to get into LEO, any single stage you use must either have an incredibly high Isp, or an incredibly high fuel mass fraction. Alas, since the space shuttle main engines hit 366/452 Isp in the seventies, we haven't managed to improve the Isp of our launch capable engines at all, and that's assuming you're using LH2/LOX. If you want kerosene, well... our best kerosene engine is the one ULA is no longer allowed to buy from Russia, the RD-180 at 311/338 Isp. Not exactly very reassuring numbers there. So a high fuel mass fraction it is. And you know what you absolutely cannot have when you want a high fuel mass fraction? Payload. You absolutely cannot have any significant amounts of payload. That is the reason nobody has built a SSTO rocket yet, and especially not one running on kerosene: it's incredibly difficult to make something cost effective with a payload fraction that tiny.

The Skylon spaceplane actually aims not to simply "save dV" by flying on airbreathing engines for the first half of the trip. Much rather, it aims to decrease the required fuel mass fraction. That works because airbreathing engines have a much higher Isp than rocket engines (projected 3500 Isp). If you then take the average Isp over the entire flight from liftoff to orbit, it manages an Isp number that no chemical rocket engine could achive. It is basically trying to use the other method besides having a large fuel mass fraction to man up the excessive dV required for a single stage to orbit. And in doing so, it hopes to free up more dry mass allowance than is consumed by the extra equipment necessary for the dual mode engines, which ultimately makes room for more payload.

Skylon hopes to be the first SSTO to ever fly, because it aims to be cost-effective by offering payload fractions competitive with (or better than) staged rockets. A Falcon 9 sends 13 tons payload to the ISS with 506 tons of launch mass; Skylon aims to carry 11 tons of payload to the same destination with only 345 tons launch mass.

Jeez guys, SSTO = Single stage = NO STAGING. When I say ssto that cuts back on cost, I mean a reusable craft that returns after achieving orbit in 1 stage.

What is the point of an ssto that makes it into orbit with a small payload and can't return? That's just silly.

Payload fraction is payload fraction.

[NathanKell]

What is the point of an ssto that makes it into orbit with a small payload and can't return? That's just silly.

Cost. Is a thing. That exists in the world outside KSP.

[shynung]

What Streetwind meant by that wall of text, is that a practical SSTO using current technologies would result in very low payload mass fraction.

It's not the SSTO concept that's flawed, but rather the technical limits that's being pushed too far.

[Streetwind]

Indeed.

By the way, shynung - that Aquarius study you linked. Did anything ever come out of the research around that vortex combustion chamber? Looked interesting even if the program in general felt of questionable value.

[shynung]

I didn't follow the study closely; I just stumbled into it when I was researching about cheap orbital rockets about 2 years ago. The study itself was from 2005, so if it does produce an actual engine, it should be well known by now.

Though, I have a feeling that the rocket designs there make great SRB alternatives, if it wasn't for their low reliability.

Edited June 4, 2014 by shynung

[Nibb31]

Jeez guys, SSTO = Single stage = NO STAGING. When I say ssto that cuts back on cost, I mean a reusable craft that returns after achieving orbit in 1 stage.

Yes, but that's not what "SSTO" means. If you mean "reusable", then say the damn word "reusable", not "SSTO", which means something else than "reusable".

And reusable only cuts back on cost if there is demand for a high launch rate that makes reusing worthwhile. Current flight rates do not make reusable spacecraft worthwhile.

When you only host a single party for 50 people every year, it makes sense to buy disposable plates and cups instead of investing in a full porcelain dinner set for 50 people and washing the dishes afterwards. If you will be hosting 50 people every night, then it's more economical to buy and maintain the dinner set.

[TeeGee]

Yes, but that's not what "SSTO" means. If you mean "reusable", then say the damn word "reusable", not "SSTO", which means something else than "reusable".

And reusable only cuts back on cost if there is demand for a high launch rate that makes reusing worthwhile. Current flight rates do not make reusable spacecraft worthwhile.

When you only host a single party for 50 people every year, it makes sense to buy disposable plates and cups instead of investing in a full porcelain dinner set for 50 people and washing the dishes afterwards. If you will be hosting 50 people every night, then it's more economical to buy and maintain the dinner set.

I love this analogy!

[Albert VDS]

When you only host a single party for 50 people every year, it makes sense to buy disposable plates and cups instead of investing in a full porcelain dinner set for 50 people and washing the dishes afterwards. If you will be hosting 50 people every night, then it's more economical to buy and maintain the dinner set.

Instead these plates are made of gold compared to what's on them.

[TeeGee]

While kerosene tanks can be 1% of the weight of their contents, hydrogen tanks often must weigh 10% of their contents. This is because of both the low density and the additional insulation required to minimize boiloff (a problem which does not occur with kerosene and many other fuels). The low density of hydrogen further affects the design of the rest of the vehicle  pumps and pipework need to be much larger in order to pump the fuel to the engine. The end result is the thrust/weight ratio of hydrogen-fueled engines is 30–50% lower than comparable engines using denser fuels.

ME: I can't believe I figured this out in a VIDEOGAME!

- - - Updated - - -

While single-stage rockets were once thought to be beyond reach, advances in materials technology and construction techniques have shown them to be possible. For example, calculations show that the Titan II first stage, launched on its own, would have a 25-to-1 ratio of fuel to vehicle hardware.[8] It has a sufficiently efficient engine to achieve orbit, but without carrying much payload.[9]

http://en.wikipedia.org/wiki/Single-stage-to-orbit#Examples

[sojourner]

Just remember that KSP is a game. Even with RSS added in. Don't think that something you build in a the game will work in the real world. And seriously, if it were so easy, don't you think it would have been done by now?

[NathanKell]

Yeah, um, while I sure try to make RF (and everything else) as realistic as possible, they're not perfect even for what they model, and there are bajillions of constraints that can't be modeled.

Also, imma need a CITE on that difference in mass ratio. For example, Shuttle ET is about 3.6% dry fraction...

[TeeGee]

Same citation as the lower quote Nathan.

Yeah I know this is a game but I'm learning the basics of rocket science through a form of entertainment. Kerbal truly is a very special videogame.

[TeeGee]

I'm beginning to think that ssto's, while technically possible, are not worth the engineering effort. They can't lift very much payload, are extremely complex, and relatively inefficient lifters.

I also worry about WHEN we develop SSTO's like the skylon, we'd make space more accessible to private companies who otherwise wouldn't be able to pay to put things into orbit like a satellite. The easier the access to space, the cheaper launches will be and the more junk will be floating around in orbit up there.

I think SSTO's should only be used to ferry people into space like commercial jet liners. Tickets to get into space would be cheaper and we wouldn't be putting something up in orbit on a permanent basis to gunk up our orbital lanes.

[shynung]

I'm beginning to think that ssto's, while technically possible, are not worth the engineering effort. They can't lift very much payload, are extremely complex, and relatively inefficient lifters.

That's why the people at REL (the company that designed the Skylon) choose to use a hybrid spaceplane design, rather than a traditional rocket design. They are positing that by using a hybrid engine design, enough mass could be lost from the lowered fuel requirements to yield a usable cargo capacity.

[SciMan]

I also worry about WHEN we develop SSTO's like the skylon, we'd make space more accessible to private companies who otherwise wouldn't be able to pay to put things into orbit like a satellite. The easier the access to space, the cheaper launches will be and the more junk will be floating around in orbit up there.

I think SSTO's should only be used to ferry people into space like commercial jet liners. Tickets to get into space would be cheaper and we wouldn't be putting something up in orbit on a permanent basis to gunk up our orbital lanes.

That's simple enough to fix, but at the same time bewilderingly complex.

Just get a law on the books that makes some kind of "One up, One down" practice mandatory (one satellite launch also pays for one satellite disposal). Simple to understand, but then there's the actual "Just get a law on the books" part... not so easy.

Lots of satellites boost themselves to a "graveyard" orbit a couple thousand kilometers above geosynch.

There's plenty of useful components up there, not just raw materials either. Stuff like solar arrays, thrusters, transmitters, antennas, perhaps even the odd not-quite-empty propellant tank. Unsurprisingly, someone has had the bright idea of launching "salvage and repair drone" satellites that would be able to simply go out and grab something like a solar array off of one of those dead satellites and attach it to a working satellite that has a damaged solar array, to keep it operational for longer. At this point, that's all ideas, and I can't remember the name for it, but it's on Wikipedia somewhere.

To be honest, I think the reason we haven't seen SSTO's yet is that currently there just aren't enough satellites looking for rockets to take them to space for re-usable SSTOs of any type (rocket or spaceplane) to make economical sense.

The SSTO might save money in the long term, but right now it just doesn't reach monetary break even fast enough because the launch market isn't there.

Hopefully the reusable Falcon 9 program is a sign that SSTO's might become viable sometime in the next 50 years. Or 5 years. Or whatever. It all hinges on who wants to put up the next Iridium-scale satellite network. Communications sats, nav sats, it doesn't really matter what for, so long as it's commercial. Governments like to do their spy sat launches in-house, for a few obvious reasons (and probably a few not-so-obvious reasons).

Edited June 5, 2014 by SciMan

[sgt_flyer]

One of the problems, is the payload costs a lot - the launcher's price is only a small part of the price tag for a full launch, without even accounting for all that happens around : calculating ascent path / final orbit, if there's going to need to move other satellites, and the launch installations and crews running costs.

Add to that the pricey insurances for the payload

In the end, a reusable system might lower a bit the full price - after a while. At first, launch safety / insurances will cost a lot more for an unproven system, and they're going to have an hard time finding customers who might be willing to pay for a more expensive launcher with reliability records. If they don't have enough flights per year, a reusable systems running / maintenance costs might offset the gains you have from reusability.

[Nuke]

i think that once we get the cost of launch down to sufficiently cheap levels, there will be money in operating orbital junk yards. especially out at geo and other high traffic areas. on site repair of satellites might be a thing.

Edited June 5, 2014 by Nuke

[TeeGee]

Well I think that if we open up space monetarily, our orbits will be gunked up with a LOT of space crap. Orbits will be rented to companies.

I forsee that something like Planetes (anime) might become a reality.

[Nibb31]

Well I think that if we open up space monetarily, our orbits will be gunked up with a LOT of space crap. Orbits will be rented to companies.

Why this focus on orbital debris? There really isn't much of it. Just about everything launched into orbit for the last 20 years must be disposed of. At any rate, it has nothing to do with SSTO or MSTO, because rocket upper stages are either deorbited or sent to a graveyard orbit. Very little debris ends up in LEO, and when it does, it decays naturally.

[Iskierka]

The space junk problem is currently self-feeding. Because it's so expensive to go into orbit, no-one's going to do it for anything they don't profit from reasonably quickly - that includes cleaning up orbit. As soon as we can get to orbit cheaply, the sunk cost is relatively low, so investors are less concerned about getting paid back, and can be happy with knowing it will allow more profit in future. In short, cheap space access will increase Kessler syndrome, but it will remove the elements of this system that cannot actively fight the increase. Active satellites can avoid each other, the concerns are with the large regions of junk around 1000-2000 km that are out of control and just building up more debris.

Similarly, the large costs of satellites currently are because of large launch costs. When a launch costs $100m, you don't want to spend that and then not have your satellite either turn on in the first place, or last for its intended lifetime. As such, the company will probably spend $100m on the satellite for a $100m launch, to make sure things don't go wrong on their end. Make the launch cost $5m, and the satellites will start costing $5m, because it's considered to not be so great a loss, and a failed satellite can be replaced at that cost.

While kerosene tanks can be 1% of the weight of their contents, hydrogen tanks often must weigh 10% of their contents. This is because of both the low density and the additional insulation required to minimize boiloff (a problem which does not occur with kerosene and many other fuels). The low density of hydrogen further affects the design of the rest of the vehicle  pumps and pipework need to be much larger in order to pump the fuel to the engine. The end result is the thrust/weight ratio of hydrogen-fueled engines is 30–50% lower than comparable engines using denser fuels.

This, however, is ignoring that the hydrogen mass becomes a tiny fraction of the overall fuel mass, so the 10% mass figure is negligible overall, and likely slightly exaggerated, particularly for modern tanks. The oxygen tank, which is a more similar percentage to kerosene, makes up the majority of the overall tank mass, leading to, ex, the 3.6% figure for the shuttle's external tank. Which isn't even purely tank, it includes piping and such that is not counted as the tank's mass when designing the container.

Plus the shuttle's engines run very hydrogen-rich, meaning that 10% tank mass figure becomes much more significant - for a reusable SSTO, more development would be put on the engine performance, helping it run at higher temperature and thus leaner than the shuttle, reducing average tank mass further. Aside: Skylon wouldn't benefit from this despite running much purer, but this is because of the small amount of additional fuel to make first ascent. Tank mass would likely still be 3-4%

30% is a very high estimate of the difference, 50% would only be seen on very poor LH2 designs versus very good RP-1 designs. More likely is between 20-30% of the TWR, and this results in the overall dv still being much higher. That same link you quoted specifically points out the overall mass of LH2 SSTOs is significantly lower than for RP-1 for the same payload, despite any drawbacks to LH2.

[Streetwind]

Plus the shuttle's engines run very hydrogen-rich, meaning that 10% tank mass figure becomes much more significant - for a reusable SSTO, more development would be put on the engine performance, helping it run at higher temperature and thus leaner than the shuttle, reducing average tank mass further.

Nitpick: the RS-25 doesn't run fuel-rich to facilitate combustion due to low chamber temperatures. In fact, the RS-25 has extremely high chamber pressures and temperatures when compared to most engines. No, it - like most other American designs - runs fuel-rich for safety and reliability reasons. At the temperatures and pressues in question, oxygen is insanely corrosive. Running fuel-rich therefore reduces the wear on the engine, allowing you to use it for longer before it breaks. With the RS-25 being designed to be reusable, that was a major factor.

The Russians have shown that you can run oxidizer-rich mixtures at high pressures and temperatures. But then, those engines are not reusable (nor were they ever meant to be).

[Iskierka]

Oxidiser-rich is not pure. And further, quickly doing research on Encyclopaedia Astronautica, I found only one Russian engine that exceeded the shuttle's LOx ratio, achieving 6.2. The majority matched, at 6, and maybe a third were lower, most around 5.8, some as low as 4.2. Running pure for hydrogen would require a ratio of 8, which would have to be exceeded to run oxygen-rich, meaning no Russian engines did run lean as you suggest.

Additionally, I never said it had to create low chamber temperatures - it just has to lower them from the pure temperatures, which can relatively easily reach 4500 K and beyond, over 1000 beyond the temperatures actually used. And, in theory, this is still a low temperature - if you could get 100% combustion of hydrogen, the peak temperature would theoretically be around 6700 K, but in practice this has never yet been observed, as at these temperatures hydrogen and oxygen begin refusing to bond to each other. As a result, what should be H2O + lots of energy, becomes some mixture of H2O + HO- + H+ + O(2-) + a lot of the energy, but considerably less.

For more fun, if anyone can design a rocket that can somehow overcome this and create a rocket with 100% hydrogen usage, then even a pure rocket SSTO would require a mass ratio of just 5.5, as the exhaust velocity would reach around 5600 m/s. Hydrogen -should- be the best fuel by far, instead, it's just the best fuel by an appreciable margin.

[Hodo]

Hi everyone.

Question:

Why don't engineers design a purely rocket ssto?

Step 1) Use an engine with excellent thrust (http://en.wikipedia.org/wiki/RD-171#RD-171)

Step 2) Attach kerolox fuel (highest thrust to volume fuel)

Step 3) Add wings and a cargo bay

Step 4) Launch like a rocket

Step 5) Land like a glider

I've done it with KSP using RF, Stockalike, FAR, engine ignitor (with 1 ignition on my main engine), B9 parts, DR and stock engines.

That way we can get heavy cargo into space without sacrificing thrust.

I know I am not an aerospace engineer but I was wondering what the technical limitations of doing what I am proposing has.

Anybody can comment, I welcome any and all opinions.

I have been toying with this concept recently in RSS+RO, and the best I can achieve in 8 hours of tinkering is 450m/s short of full orbit at 190km. I am currently using 3 Aerospike type engines from the RO RftS pack, and it is close but not quite.

The aircraft weighs in at over 300 tons sitting on the runway which is MASSIVE and it doesn't even have a cargo bay.