Why don't we have SSTO's already?

[magnemoe]

Of course, but the crux is that the bulk of the launch cost isn't in fuel. If you make a larger, heavier ship, that needs way more fuel, but is 100% reusable, it's still cheaper to launch things. That's why SSTO concept exists, and why it received a lot of attention in the past couple of decades as it became clear that we can shift cost-efficiency balance that way.

On the other hand, SpaceX et alii push for re-using lower stages instead. That's also a viable option, since lower stage, certainly, is both the heaviest and most expensive part.

It has been many plans for two stage reusable launchers. Most based on an fast plane with an shuttle on top.

Benefit of an reusable two stage system over an SSTO, is that you only have to carry the upper stage to orbit, not the entire ship, you also only need heat shielding on an much smaller and lighter part.

[K^2]

Eh. Plane launches aren't terribly useful. Not enough starting velocity to really bother with. But if you replace plane with a good rocket stage, then we're cooking with gas.

But yeah, either way, that's the idea. But description I was replying to makes it sound like they want to use something like Dragon as final stage, with first and second stages being reusable. And that sounds silly to me.

[magnemoe]

Eh. Plane launches aren't terribly useful. Not enough starting velocity to really bother with. But if you replace plane with a good rocket stage, then we're cooking with gas.

But yeah, either way, that's the idea. But description I was replying to makes it sound like they want to use something like Dragon as final stage, with first and second stages being reusable. And that sounds silly to me.

I talk about ideas for reusable two stage launchers. Most of the old concepts used an hypersonic plane as first stage, yes the cost and engineering issues with building an huge hypersonic plane caused none of them to be build.

[Kulebron]

I suggest to anyone asking why not SSTO, install real Earth mod and try making an SSTO rocket with a payload of 5-10 tonnes. Not even plane, which is harder, but just a single-stage rocket, and get it to orbit.

[K^2]

I suggest to anyone asking why not SSTO, install real Earth mod and try making an SSTO rocket with a payload of 5-10 tonnes. Not even plane, which is harder, but just a single-stage rocket, and get it to orbit.

KSP rocket engines have TWR way bellow that of real world engines. Tank useful weight fraction is also different. So is aerodynamics. You'd need to install a lot of mods before comparison is half reasonable. Simply replacing Kerbin with Earth and running everything else vanilla would make for a far less efficient SSTO than would be in real life.

[trekkie_]

All this talk of 2 stages.....and people seem to forget the space shuttle was a 2 stage craft. The first stage wasn't exactly reusable, but they did recollect the materials and remanufactured them, which was a decent savings since the metals were pretty much still purified and of generally the same mass. Much better than having to remanufacture them from new raw materials.

[Nibb31]

Much better than having to remanufacture them from new raw materials.

Not really. The cost of sending out ships to recover the spent SRBs, repairing, refurbishing, and checking them, was higher than making new steel casings from scratch. They were just steel tubes. The expensive parts of the SRBs (skirts, parachutes, cones) weren't reused.

[Ziff]

We don't have single-stage-to-orbit launchers because they suck at launching stuff. Dropping stages along the way makes your vehicle lighter and therefore much more efficient.

SSTO is easy as long as you don't care about the payload. The ancient Mercury-Atlas was nearly an SSTO (it only dropped its engines on the way up),

The ancient mercury-atlas was an ICBM capable of delivering a payload of about 3,000lbs to orbit. The planned Skylon SSTO is capable of delivering 33,000lbs to orbit, and is reusable. 10x the payload and reusable hardly counts as "sucking at launching stuff".

The thing is, designing an SSTO launcher with a sucky payload is pointless when you can launch a multi-stage rocket with a significant payload.

Again, the proposed Skylon's payload would be more than even the Falcon-9.

PS. And yes, I know you weren't really asking about "SSTO" but about "reusable spaceplanes". So please use the term "reusable spaceplane" when talking about reusable spaceplanes and not "SSTO", which is a flight profile, not a type of vehicle. It's quite possible to envision SSTO rockets, reusable rockets, multi-stage spaceplanes, and all sorts of other combinations...

The correct term is HTOL SSTO (Horizontal Take Off-Landing, Single Stage to Orbit). There are also reusable spaceplanes that would not be SSTO, such as the Space Shuttle Orbiter. It was certainly a reusable spaceplane but by no means was it an SSTO.

- The reason we don't have SSTOs is because MSTOs are much more efficient.

When we are talking about efficiency in space programs what we are really talking about money. MSTOs aren't efficient when directly compared to theoretical SSTOs. MSTO stages have to be recovered and refurbished before they can be reused. For example, the Space Shuttle solid rocket boosters were often recovered and reused. However, over 5000 (5000!!!!) parts were refurbished in EACH booster. That inherent cost is not efficient.

- The reason we don't have spaceplanes is because they are much heavier than rockets because you need to carry all that plane stuff in addition to all your rocket stuff. Therefore you can't carry any payload unless you invent some new breakthrough material and breakthrough engines.

The Falcon-9 weighs 557 tons. The proposed Skylon has a weight of 303 tons, and has a slightly higher payload than the Falcon-9.

- The reason we don't have reusable launchers is because the demand for orbital launches doesn't allow launch rates frequent enough to make them economically viable.

This is the real reason. The cost associated with developing the technology for these HTOL SSTOs is seen as wasteful, unnecessary, and there is little to no interest in space anymore.

The US Defense budget is over 700 BILLION dollars, yearly. The entire 50+ year running budget for NASA totals about 800 Billion.

Think about that for a second. The US spends almost as much money, EVERY YEAR, on it's military, as it has on NASA's entire space program since it was founded 56 years ago.

If we reversed those numbers we would have had HTOL SSTOs about 25 years ago.

[Elthy]

The Problem with Skylon is that there is no constructed hardware. The X-33 was way further in developement (there were eninge test firings allready) when they noticed things dont work in reality like they did on paper...

Also Skylon would need refurbishment, too, its not just refill and fly again. The loads are a little more than for a normal aircraft.

[Red Iron Crown]

Atlas wasn't exactly SSTO either, it dropped engines on the way up. You know, the most expensive part of a rocket?

Skylon's cost estimates are wildly optimistic. They think developing several new technologies will cost less than developing a conventional airliner. I wish them luck, but I think they'll miss their cost estimates by an order of magnitude at least, if the program ever comes to fruition.

[Nibb31]

The ancient mercury-atlas was an ICBM capable of delivering a payload of about 3,000lbs to orbit. The planned Skylon SSTO is capable of delivering 33,000lbs to orbit, and is reusable. 10x the payload and reusable hardly counts as "sucking at launching stuff".

Skylon is a hypothetical design. It might really well on paper, with highly optimistic margins and fantasy funding, but it is unlikely to ever fly, for reasons that have been explained in a dozen threads in this forum already.

Again, the proposed Skylon's payload might be more than even the Falcon-9.

FTFY.

The correct term is HTOL SSTO (Horizontal Take Off-Landing, Single Stage to Orbit). There are also reusable spaceplanes that would not be SSTO, such as the Space Shuttle Orbiter. It was certainly a reusable spaceplane but by no means was it an SSTO.

Partially reusable. It was an awe-inspiring machine, but it was also a fundamentally flawed design that didn't make any sense economically.

When we are talking about efficiency in space programs what we are really talking about money. MSTOs aren't efficient when directly compared to theoretical SSTOs. MSTO stages have to be recovered and refurbished before they can be reused. For example, the Space Shuttle solid rocket boosters were often recovered and reused. However, over 5000 (5000!!!!) parts were refurbished in EACH booster. That inherent cost is not efficient.

Theoretical rockets are always better than operational rockets on paper. Reuse isn't always economical. There are many sectors where disposable is more economical than reusable. It all depends on the volume.

A reusable launcher isn't necessarily more economical than an expendable one.

The Falcon-9 weighs 557 tons. The proposed Skylon might have a weight of 303 tons if the proposed materials actually work exactly as planned and nothing goes overweight, and might have a slightly higher payload than the Falcon-9 if the engines work exactly as on paper with no underperformance.

Experience proves that aerospace projects ALWAYS end up at least slightly overweight, slight underpowered, and hugely overbudget. The truth is that nobody has the faintest idea how Skylon will perform exactly, what its margins will be, and what its operating costs are going to be.

This is the real reason. The cost associated with developing the technology for these HTOL SSTOs is seen as wasteful, unnecessary, and there is little to no interest in space anymore.

It's an investment. And an investment calls for a return on investment. A fully-reusable launcher is only viable if you fly it frequently. There simply isn't a market to support frequent orbital flights. It's not a matter of interest, it's a matter of return on investment. At the current flight rates, vertical launch rockets simply offer a better return on investment.

The US Defense budget is over 700 BILLION dollars, yearly. The entire 50+ year running budget for NASA totals about 800 Billion.

Think about that for a second. The US spends almost as much money, EVERY YEAR, on it's military, as it has on NASA's entire space program since it was founded 56 years ago.

If we reversed those numbers we would have had HTOL SSTOs about 25 years ago.

Quite possibly. But is it NASA's role to spend billions of taxpayer's money on developing new transportation technology when there is no market to back it up?

[Duxwing]

Quite possibly. But is it NASA's role to spend billions of taxpayer's money on developing new transportation technology when there is no market to back it up?

I think he meant:

Were we interested in space, we would have built SSTOs.

We were not interested in space.

Therefore, we have not built SSTOs.

-Duxwing

[Albert VDS]

^That's assuming that SSTOs are the best option.

[Duxwing]

Quite possibly. But is it NASA's role to spend billions of taxpayer's money on developing new transportation technology when there is no market to back it up?

I think he meant:

Were we interested in space, we would have built SSTOs.

We were not interested in space.

Therefore, we have not built SSTOs.

Experience proves that aerospace projects ALWAYS end up at least slightly overweight, slight underpowered, and hugely overbudget. The truth is that nobody has the faintest idea how Skylon will perform exactly, what its margins will be, and what its operating costs are going to be.

Were Skylon merely half the craft it's claimed to be, it could still lift 16,500 pounds to orbit for almost nothing. The potential markets would be vast:

-Launching disposable CubeSats

-Retrieving reusable Cubesats

-Orbital adventuring

-ISS crew transfer

-Satellite maintenance and disposal

-Manned orbital experiments

-Duxwing

[Kryten]

You don't need a very big loss of I_sp from the theoretical model to reduce the payload by a lot more than half. The rocket equation doesn't treat single-stage rockets terribly kindly.

[Albert VDS]

Were Skylon merely half the craft it's claimed to be, it could still lift 16,500 pounds to orbit for almost nothing. The potential markets would be vast:

-Launching disposable CubeSats

-Retrieving reusable Cubesats

-Orbital adventuring

-ISS crew transfer

-Satellite maintenance and disposal

-Manned orbital experiments

-Duxwing

But Skylon is not a manned craft.

[Red Iron Crown]

A passenger module is planned for Skylon.

[Frozen_Heart]

Skylons main problem is that the company developing it isn't capable of anything more than powerpoint presentations. If it was developed by an effective company it may have stood a chance.

The X-33 could have potentially worked as well. However it was shut down due to the fuel tanks that they were trying to use being many times weaker and heavier than a normal fuel tank.

Edited December 15, 2014 by Frozen_Heart

[Iskierka]

May -have- stood a chance? It's still in development, and the company proposing it does not intend to be the manufacturer, they are in discussions with other companies that have the resources. REL simply solved the key limiting problem in the way of the hybrid rocket engine it uses.

[Albert VDS]

A passenger module is planned for Skylon.

Just Googled it... Are they serious? So they this is a safe idea to have a module hidden underneath cargo bay doors?

To get out safely in an emergency the craft would need to have zero rotation, the cargo bay door need to work and get out of the way and the explosion and debris needs to stay clear of the ejected passenger module.

So what can go wrong?

[Red Iron Crown]

No escape system will be necessary because Skylon will be perfectly reliable, just like it will be perfectly reusable.

[Frozen_Heart]

Just Googled it... Are they serious? So they this is a safe idea to have a module hidden underneath cargo bay doors?

To get out safely in an emergency the craft would need to have zero rotation, the cargo bay door need to work and get out of the way and the explosion and debris needs to stay clear of the ejected passenger module.

So what can go wrong?

I'm guessing that the payload bay doors are forcefully opened while abort SRBs shoot the passenger module free. Like a giant ejector seat. It would then be logical to have parachutes on it...

[Iskierka]

The multi-passenger module would most likely not be ejectable, just as airliners are not ejectable, as there is no need for the functionality above a certain level of reliability. The only point the vehicle is likely to fail such that you would like to be away from it is re-entry, in which case you do not want to eject - lower down, anything bar total control failure (which is liable to prevent escape systems from working anyway) is fail-safe as the vehicle can re-land easily due to high glide ratio. For initial human rating a smaller 5-man crew module would be used, which would not feature full module ejection, but individual crew ejection with blasting holes through the cargo bay doors.

Skylon is a hypothetical design. It might really well on paper, with highly optimistic margins and fantasy funding, but it is unlikely to ever fly, for reasons that have been explained in a dozen threads in this forum already.

The Falcon-9 weighs 557 tons. The proposed Skylon might have a weight of 303 tons if the proposed materials actually work exactly as planned and nothing goes overweight, and might have a slightly higher payload than the Falcon-9 if the engines work exactly as on paper with no underperformance.

Experience proves that aerospace projects ALWAYS end up at least slightly overweight, slight underpowered, and hugely overbudget. The truth is that nobody has the faintest idea how Skylon will perform exactly, what its margins will be, and what its operating costs are going to be.

Except for that the paper estimates do everything reasonable to under-estimate, using heavier fuel tank alloys despite better being known and proven, engine performance as absolutely certain and ignoring numerous improvements being investigated, and with comfortable over-estimations on the heat shielding. The supposed guarantees of it never flying are based on other projects that have had to push as hard as possible to sell themselves and say "I'm going to be the best, I'll take over!", Skylon can make performance with less, and so one of the very things it is selling itself on is the fact that they are very, very, VERY sure of the performance. Quoted numbers should be considered probable minimums, not hopes. Almost every single aspect of Skylon would have to go wrong for it to not be at least competitive.

Quite possibly. But is it NASA's role to spend billions of taxpayer's money on developing new transportation technology when there is no market to back it up?

And what market exactly does the military serve? The one it makes itself that would not exist without its own funding? Hmm, perhaps if NASA had a chance ...

[Albert VDS]

I'm guessing that the payload bay doors are forcefully opened while abort SRBs shoot the passenger module free. Like a giant ejector seat. It would then be logical to have parachutes on it...

SRBs in the vicinity of a parachutes doesn't seem like a good idea.

And what market exactly does the military serve? The one it makes itself that would not exist without its own funding? Hmm, perhaps if NASA had a chance ...

The purpose of the military is to protect it's country, that and soothing the paranoid people who see danger around every street corner.

Edited December 15, 2014 by Albert VDS

[Moar Boosters]

SRBs in the vicinity of a parachutes doesn't seem like a good idea.

That's how most ejector seats work.