Hypothetical SpaceX SSTO

[hkultala]

SSTO doesn't work for Earth.

.. because you say so, without any calculations to prove your point?

SSTLEO does work fine and could be done on todays or even tens of years old technology, but it just not has been economical to make, because
1) without air-breathing engines the payload fraction is so bad that the rocket is much bigger and more expensive for same payload, and suitable air-breathing engines have not yet been developed(though SABRE development is underway)
2) Not very many commercial launches are to LEO


I think you are messing up SSTO and reusable SSTO. Reusable SSTO is much harder than SSTO, but even that can be done, but with a very high development cost.

[Impaler]

I think that is exactly what Cat meant, it won't work for SpaceX Earth launches because they care about little things like having their launch vehicle be cost effective.

BTW some discussion earlier about heat-shielding on entry, my understanding is that you always save mass by making the entry object more compact and the surface area of shielding smaller even if it must be thicker.  The reason is that your dissipated heat dose not scale linearly with vehicle mass divided by deceleration energy, a more compact area experiences a shorter higher temperature pulse but less total heat actually directed into the vehicle due to the detached shock-layer effects which put more of the energy into the atmosphere as radiation.

The g-forces that humans can survive is the limiting factor in a manned capsule, if we could arbitrarily compress the crew and make them invulnerable to g-forces then we would design the vehicle like a sample return capsule with very small area of thick shielding.

So when returning a rocket stage you would always orient it vertically to present the smallest area, also it's the only direction that the tank structure has any strength and can survive in.  In addition the engine will have to point forward because it dominates the remaining mass and trying to go nose first would be unstable with the vehicle wanting to flip over.

The heat-shield for a 2nd stage is either going to be some kind of clam-shell inside the inter-stage which opens after separation but before ignition and then closes again for entry (maybe opening again to act as landing legs), or just a quantity of propellant for SRP to create shock layer standoff from the engine.

[rsdavis9]

So when returning a rocket stage you would always orient it vertically to present the smallest area, also it's the only direction that the tank structure has any strength and can survive in.  In addition the engine will have to point forward because it dominates the remaining mass and trying to go nose first would be unstable with the vehicle wanting to flip over.

so what was the CG of the space shuttle? It had 3 engines in the back. Have all reentry vehicles had the CG near the front? It sounds like a hard balancing act to have the CG in the back but if it could be done it would simplify the heat shield placement.

[Impaler]

Center of Gravity on the Shuttle during re-entry was DANGEROUSLY far to the rear and presented a constant danger that the vehicle would tumble during re-entry because the shape was unstable, which is what ultimately happened to Columbia.  This is one of the reasons Buran was considered superior to the Shuttle, not having the main engine in the orbiter made the entry hugely safer because center of mass was basically in the center of the vehicle.

The X-33 project was abandoned in large part because the center of gravity of the empty re-entry vehicle could not be moved far enough forward due to the mass of the rear-engine, the vehicle would have again been dangerously unstable on re-entry.

Even the Skylon concept only works because the engines are placed at the center of the vehicle on stub-wings, the earlier HTOL concept that it evolved from had the same fatal rear-engine flaw. 

Basically every attempt to re-enter a tubular object with the center of mass in the rear has been an deemed an engineering failure, if even regular air-space engineers have learned their lesson then Musk will likely avoid that mistake when he tries to re-enter a 2nd stage.

[Elmar Moelzer]

Center of Gravity on the Shuttle during re-entry was DANGEROUSLY far to the rear and presented a constant danger that the vehicle would tumble during re-entry because the shape was unstable, which is what ultimately happened to Columbia.  This is one of the reasons Buran was considered superior to the Shuttle, not having the main engine in the orbiter made the entry hugely safer because center of mass was basically in the center of the vehicle.

The X-33 project was abandoned in large part because the center of gravity of the empty re-entry vehicle could not be moved far enough forward due to the mass of the rear-engine, the vehicle would have again been dangerously unstable on re-entry.

Even the Skylon concept only works because the engines are placed at the center of the vehicle on stub-wings, the earlier HTOL concept that it evolved from had the same fatal rear-engine flaw. 

Basically every attempt to re-enter a tubular object with the center of mass in the rear has been an deemed an engineering failure, if even regular air-space engineers have learned their lesson then Musk will likely avoid that mistake when he tries to re-enter a 2nd stage.

Not if you reenter bottom first.

[ChrisWilson68]

Center of Gravity on the Shuttle during re-entry was DANGEROUSLY far to the rear and presented a constant danger that the vehicle would tumble during re-entry because the shape was unstable, which is what ultimately happened to Columbia.  This is one of the reasons Buran was considered superior to the Shuttle, not having the main engine in the orbiter made the entry hugely safer because center of mass was basically in the center of the vehicle.

The shuttle had various issues, and having the CoG toward the rear was not the greatest of them.  It was an entirely manageable issue.

Engineering is about trade-offs.  Every choice has positives and negatives.  Stating some negatives and then just entirely dismissing a choice is not justified.

The X-33 project was abandoned in large part because the center of gravity of the empty re-entry vehicle could not be moved far enough forward due to the mass of the rear-engine, the vehicle would have again been dangerously unstable on re-entry.

Most people think there were several reasons X-33/VentureStar was abandoned, and CoG was again not the biggest of them.  The hydrogen tanks with complex shapes and linear aerospike engine were looking like they could not be produced with low enough mass to come even close to the necessary mass fraction.

[The Amazing Catstronaut]

.. because you say so, without any calculations to prove your point?

The calculations are old, readily available, and the same calculations done by one more amateur isn't going to shift anyone's reasoning.

SSTLEO does work fine and could be done on todays or even tens of years old technology, but it just not has been economical to make, because
1) without air-breathing engines the payload fraction is so bad that the rocket is much bigger and more expensive for same payload, and suitable air-breathing engines have not yet been developed(though SABRE development is underway)
2) Not very many commercial launches are to LEO

SSTO is fine if you want to lift the mass of the SSTO into LEO. If you want to carry payload to anywhere other than a few rigid trajectories, you need a massive SSTO. An expendable TSTO is objectively more sound on a bang-for-buck basis than an equivalently voluminous expendable SSTO using the same engines and fuel types. Fiddling with the structural mass doesn't bias performance towards the SSTO either, as both architectures benefit from structural improvements.

Air breathing engines make your mass problem worse because you end up carting them with you. Combined cycle engines of the SABRE ilk have terrible thrust-to-weight margins when compared with equivalent conventional rockets. There's a vast margin of the ascent phase where your wings are more of a hinderance than a help, and once you're up in orbit all that mass dedicated to atmospheric flight is going to be carted around with you to wherever in the solar system you happen to be going to.

I can see a winged SSTO being used for orbital space tourism, delivery of passengers to an LEO space station, max, and there might be an economic case for one then. Hypothetical SSTOs work for LEO servicing, would doubtless be aesthetically sexy, but beyond that, what market is there? If we're trying to force SSTOs to fit mission parameters that vehicles with staging events have been able to conduct for decades - that's not progress. That's retrogression.

SSTOs seem like a great way to spend a lot of dollars for a vehicle tailored to very specific mission parameters.

So yes, my statement that SSTO doesn't work on Earth, was an economic statement. I think it's a sexy sci-fi idea we keep toying with because it appeals to that part of us that believes that LVs need to be aircraft-esque to become aircraft-practical, but it's not a logical connection. SpaceX is not going to build one unless the tech paradigm massively evolves. 

[QuantumG]

SSTO is fine if you want to lift the mass of the SSTO into LEO. If you want to carry payload to anywhere other than a few rigid trajectories, you need a massive SSTO. An expendable TSTO is objectively more sound on a bang-for-buck basis than an equivalently voluminous expendable SSTO using the same engines and fuel types. Fiddling with the structural mass doesn't bias performance towards the SSTO either, as both architectures benefit from structural improvements.

I don't know where you get this from. An SSTO carries a payload of about 2% of GLOW to orbit. An expendable TSTO carries a payload of about 4% of GLOW to orbit. So far we "know" that a reusable first stage eats about 2% of the TSTO performance. Minimum gauge analysis tells us that the lower GLOW for a fully reusable system would be with the SSTO. Reusability economics suggests that the fully reusable SSTO would beat the fully reusable TSTO. There's still a long way to go before we know enough about reusable systems to make any more definitive statements.

[Impaler]

Center of Gravity on the Shuttle during re-entry was DANGEROUSLY far to the rear and presented a constant danger that the vehicle would tumble during re-entry because the shape was unstable, which is what ultimately happened to Columbia.  This is one of the reasons Buran was considered superior to the Shuttle, not having the main engine in the orbiter made the entry hugely safer because center of mass was basically in the center of the vehicle.

The X-33 project was abandoned in large part because the center of gravity of the empty re-entry vehicle could not be moved far enough forward due to the mass of the rear-engine, the vehicle would have again been dangerously unstable on re-entry.

Even the Skylon concept only works because the engines are placed at the center of the vehicle on stub-wings, the earlier HTOL concept that it evolved from had the same fatal rear-engine flaw. 

Basically every attempt to re-enter a tubular object with the center of mass in the rear has been an deemed an engineering failure, if even regular air-space engineers have learned their lesson then Musk will likely avoid that mistake when he tries to re-enter a 2nd stage.

Not if you reenter bottom first.

I think your confusing my response with someone else's, I'm saying that bottom first aka the heavy engine and Center of Mass being first is the only stable and viable configuration, so your agreeing with me but your wording clearly implies disagreement, can you clarify.

[Elmar Moelzer]

I think your confusing my response with someone else's, I'm saying that bottom first aka the heavy engine and Center of Mass being first is the only stable and viable configuration, so your agreeing with me but your wording clearly implies disagreement, can you clarify.

I apologize. Was a misunderstanding on my side.

[ChrisWilson68]

SSTO is fine if you want to lift the mass of the SSTO into LEO. If you want to carry payload to anywhere other than a few rigid trajectories, you need a massive SSTO. An expendable TSTO is objectively more sound on a bang-for-buck basis than an equivalently voluminous expendable SSTO using the same engines and fuel types. Fiddling with the structural mass doesn't bias performance towards the SSTO either, as both architectures benefit from structural improvements.

I don't know where you get this from. An SSTO carries a payload of about 2% of GLOW to orbit. An expendable TSTO carries a payload of about 4% of GLOW to orbit. So far we "know" that a reusable first stage eats about 2% of the TSTO performance. Minimum gauge analysis tells us that the lower GLOW for a fully reusable system would be with the SSTO. Reusability economics suggests that the fully reusable SSTO would beat the fully reusable TSTO. There's still a long way to go before we know enough about reusable systems to make any more definitive statements.

What SSTO gets a payload of 2% of GLOW to orbit?  Is that an expendable SSTO or reusable SSTO?  And is it projections for a proposed SSTO or an actual proven number?

[Impaler]

And most importantly at what ISP do you expect that kind of payload.  SpaceX doesn't use Hydro-Lox which has long been considered the only possible propellant that could give SSTO.  For the expected 380s ISP of Raptor the propellant mass alone would need to be 93% for a 9800 m/s launch.  That leaves 7% for all vehicle mass, payload and landing propellant.  A fantastically light rocket would be 3.3% dry-mass, landing propellant is likely to be more then the remaining ~4% because the estimated retained propellant for F9 to do a down-range landing are around 5%, so their is really nothing left for payload.

[QuantumG]

SpaceX doesn't use Hydro-Lox which has long been considered the only possible propellant that could give SSTO.

Whitehead. PDF

[Elmar Moelzer]

And most importantly at what ISP do you expect that kind of payload.  SpaceX doesn't use Hydro-Lox which has long been considered the only possible propellant that could give SSTO.  For the expected 380s ISP of Raptor the propellant mass alone would need to be 93% for a 9800 m/s launch.  That leaves 7% for all vehicle mass, payload and landing propellant.  A fantastically light rocket would be 3.3% dry-mass, landing propellant is likely to be more then the remaining ~4% because the estimated retained propellant for F9 to do a down-range landing are around 5%, so their is really nothing left for payload.

That is obviously not correct. The Falcon9 rocket has an amazing mass fraction to orbit and it only uses RP1.
Isp is not everything.

[sevenperforce]

I'm inclined to agree. For SSTO, impulse density and TWR vastly outweigh specific impulse. If your impulse density is high enough, it really doesn't matter how low your ISP is as long as your TWR can keep pace.

[Impaler]

And most importantly at what ISP do you expect that kind of payload.  SpaceX doesn't use Hydro-Lox which has long been considered the only possible propellant that could give SSTO.  For the expected 380s ISP of Raptor the propellant mass alone would need to be 93% for a 9800 m/s launch.  That leaves 7% for all vehicle mass, payload and landing propellant.  A fantastically light rocket would be 3.3% dry-mass, landing propellant is likely to be more then the remaining ~4% because the estimated retained propellant for F9 to do a down-range landing are around 5%, so their is really nothing left for payload.

That is obviously not correct. The Falcon9 rocket has an amazing mass fraction to orbit and it only uses RP1.
Isp is not everything.

I applied the F9 Heavy side boosters anticipated 30:1 wet:dry ratio, as I was giving SpaceX the most generous numbers possible least someone accuse me of sandbagging them (alas to no avail), but it still doesn't work.  What is obviously not correct is to try to present personal incredulity as an argument in an engineering discussion.

Try http://www.silverbirdastronautics.com/LVperform.html and see if you can make a SSTO rocket with a positive payload.

SpaceX doesn't use Hydro-Lox which has long been considered the only possible propellant that could give SSTO.

Whitehead. PDF

I wasn't trying to imply that I agreed with the consensus around Hydrogen fuel superiority, simply that it was the dominant view and you have to factor in the increased difficulty that results from lower ISP.  The paper in question was clearly against the mainstream thought of it's day.

The study is comparing the portion of final orbit reaching mass that is composed of known propellant density driven components.  While the fraction of orbital mass that was the dry hardware of the propulsion system is comparable, the low ISP based vehicle needs to have a higher gross take off weight to do the same job.

The biggest factor being ignored in a SpaceX SSTO vehicle is that SpaceX will certainly be using a retro-propulsive landing method which means the vehicles DeltaV needs are well in excess of just going to orbit.  It's more like going to GTO.