Author Topic: Feasibility of a Raptor SSTO and Comparisons to A Fully Reusable Launch System  (Read 20857 times)

Offline dante2308

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I suppose this topic deserved its own thread in the sense that it was a bit more than a conversation about 'possible configurations for Raptor-based rockets.'

Again, I'd like to do the math when I have time, but I am aware that SpaceX has already met the dry mass fractions required to put a non-zero payload in orbit with an SSTO with the intended specs of the Falcon Heavy boosters.

The reason one stages a rocket is to increase (or make non-negative) the payload fraction. However, when we add reusability to the equation, the first stage RTLS actual invites a disadvantage in delta-v which may or may not exceed the disadvantage of SSTO. Even if the two are not on the same order, the feasibility for rapid reusability and manufacturability of an SSTO may exceed the TSTO model and thus generate a potential cost savings.

Does anyone have any thoughts on the matter? Some numbers might greatly aid the discussion as well.
« Last Edit: 11/16/2013 07:46 PM by dante2308 »

Offline Elmar Moelzer

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I hope that Gary will find the time to chime in on this. He is my "hero" when it comes to SSTO RLVs. I think it is very doable with a non zero payload on a reasonably sized LV. You are right that the return to launch site cuts significantly into the payload. I am not sure that it is more than a TSTO with first stage RTLS, but the difference might be small enough that the improved handling of an SSTO outweights the payload penalty.
If you do something like the Phoenix, you can save on a lot of aspects of LV operations compared to a TSTO.

Offline simonbp

It's not that hard to make an SSTO, but it is very hard to make an SSTO that will reenter and land in one piece. A Delta IV first stage launched alone could make it to orbit, but that would no longer be the case if you added enough recovery gear to it that it would make it down to the ground intact.

The two-stage approach that SpaceX have taken is attractive because most of the vehicle can be recovered without having to worry about covering the first stage in heavy reentry-grade TPS. It may take a significant advance in TPS to really make an SSTO worth it.

Offline Rabidpanda

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A vehicle that is mostly empty tanks won't need as much TPS as a capsule like Dragon.

One challenge would be using the engines at multiple altitudes. I see several options:
1. Use engines that are optimized for somewhere between sea level and vaccuum and take the performance hit.
2. Use two different types of engines on one vehicle, i.e. 8 Raptors with 1 Raptor Vacuum.
3. Use a design that compensates for the altitude, such as an expansion deflection nozzle.

Offline PlanetStorm

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It's not that hard to make an SSTO, but it is very hard to make an SSTO that will reenter and land in one piece. A Delta IV first stage launched alone could make it to orbit, but that would no longer be the case if you added enough recovery gear to it that it would make it down to the ground intact.

The two-stage approach that SpaceX have taken is attractive because most of the vehicle can be recovered without having to worry about covering the first stage in heavy reentry-grade TPS. It may take a significant advance in TPS to really make an SSTO worth it.

I wonder if you are missing the point of the OP.  My reading of it is that the OP is weighing a TSTO with reusable first stage against a non-reusable SSTO. Both approaches have the disadvantage of cutting deeply into payload compared to a non-reusable TSTO, but both have advantages too. The non-reusable SSTO has the advantage of simplicity, whereas the two stage has the advantage of reusability of (at least) the first stage. So the question of the OP is, which approach wins in the economic stakes?

Of course, I might have misunderstood the OP too ;)

 

Offline TaylorR137

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Boosted SSTO makes the most sense. The boosters are reusable and flyback, the core refuel-able in LEO.

Offline Rabidpanda

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Boosted SSTO makes the most sense. The boosters are reusable and flyback, the core refuel-able in LEO.

This may be just semantic, but if you have boosters then it's not an SSTO. Otherwise the shuttle would have been considered an SSTO.

Offline guckyfan

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Boosted SSTO makes the most sense. The boosters are reusable and flyback, the core refuel-able in LEO.

This may be just semantic, but if you have boosters then it's not an SSTO. Otherwise the shuttle would have been considered an SSTO.

It's more than semantics. If you have boosters you have staging events. You have to integrate components on the ground. So you lose every advantage a SSTO might have operationally.

Offline ChrisWilson68

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Boosted SSTO makes the most sense. The boosters are reusable and flyback, the core refuel-able in LEO.

Boosted Single-Stage-To-Orbit is the definition of Two-Stage-To-Orbit.

Offline ChrisWilson68

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I suppose this topic deserved its own thread in the sense that it was a bit more than a conversation about 'possible configurations for Raptor-based rockets.'

Again, I'd like to do the math when I have time, but I am aware that SpaceX has already met the dry mass fractions required to put a non-zero payload in orbit with an SSTO with the intended specs of the Falcon Heavy boosters.

The reason one stages a rocket is to increase (or make non-negative) the payload fraction. However, when we add reusability to the equation, the first stage RTLS actual invites a disadvantage in delta-v which may or may not exceed the disadvantage of SSTO. Even if the two are not on the same order, the feasibility for rapid reusability and manufacturability of an SSTO may exceed the TSTO model and thus generate a potential cost savings.

Does anyone have any thoughts on the matter? Some numbers might greatly aid the discussion as well.

I believe SpaceX has said the first stage is 70%-80% of the cost of a Falcon 9.  It's hard to see how a SSTO could be much simpler and cheaper than a F9 first stage.  So SSTO reduces costs by (at best) 20%-30% versus TSTO while first-stage-reuse reduces them by 70%-80%.  And the payload cut for F9 first stage reuse is reported 15%-30%.  So, even if the SSTO F9 first stage lofted the full F9 payload, it would still be more expensive as an expendable than the two-stage F9 with first stage re-use.  And, of course, it would be wildly optimistic to say the F9 first stage could be made SSTO with full F9 payload.

My point is that it's easy to get an upper bound that suggests that with anything like today's chemical rocket technology, first-stage reuse in a two-stage system is far cheaper than an expendable SSTO.

Offline 93143

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first-stage-reuse reduces them by 70%-80%.

That only works if the cost to manufacture the stages is the only cost associated with the launch.  Using your model, a Falcon 9 launch with both stages reused would be completely free of charge.

Offline mlindner

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I'm not sure on the argument of a SSTO with reuse versus a TSTO with full reuse. You save greatly on TPS mass by only putting it on sections that actually have to deal with hypersonic re-entry. Also, the complication of a single staging mechanism isn't that bad, especially if it doesn't use any explosives to do so.

Granted you save some tank-end mass (four tank-ends removed) and engine mass, but you lose a lot in extra TPS and strengthening, not to mention tank dead weight being brought to orbit. TSTO also allows incremental re-use rather than either full re-use or complete non-re-use.
« Last Edit: 11/17/2013 10:31 AM by mlindner »
Internal combustion engine in space. It's just a Bad Idea.TM - Robotbeat

Offline dante2308

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I was comparing a reusable SSTO to a fully reusable (first and second stage) TSTO. You can't avoid a TPS with either configuration. Any other combination is still on topic. SSTO needs some attention and exploration anyway.

Offline dante2308

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I'm not sure on the argument of a SSTO with reuse versus a TSTO with full reuse. You save greatly on TPS mass by only putting it on sections that actually have to deal with hypersonic re-entry. Also, the complication of a single staging mechanism isn't that bad, especially if it doesn't use any explosives to do so.

Granted you save some tank-end mass (four tank-ends removed) and engine mass, but you lose a lot in extra TPS and strengthening, not to mention tank dead weight being brought to orbit. TSTO also allows incremental re-use rather than either full re-use or complete non-re-use.

You also have to carry extra fuel and oxidizer for the first stage to return to the launch site plus the second stage engine and interstage. You can get pretty tricky with the heat shield diameter but PICA-X doesn't weigh that much. By not weigh much, I mean you can cover a meter square with about 5 kg. In fact it's so light, it's almost not even worth debating. It is less than the mass of the RTLS fuel in any case.

My calculations are progressing. Adding drag now....
« Last Edit: 11/17/2013 01:59 PM by dante2308 »

Offline neviden

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Elon has talked in one of his interviews about reusability and has pointed out that it has everything to do with how big of a fraction of your rocket can you deliver to orbit. Reusabiliy part will take about 2%. If you can make your rocket deliver 4%, then you can deliver 2% and still get your rocket back. It's much easier to get robust and reliable 4% TSTO than SSTO.
`
So, if can we make a robust SSTO that goes well over 2%, then it can work. Can we and at what price?

Offline cleonard

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Elon has talked in one of his interviews about reusability and has pointed out that it has everything to do with how big of a fraction of your rocket can you deliver to orbit. Reusabiliy part will take about 2%. If you can make your rocket deliver 4%, then you can deliver 2% and still get your rocket back. It's much easier to get robust and reliable 4% TSTO than SSTO.
`
So, if can we make a robust SSTO that goes well over 2%, then it can work. Can we and at what price?

I have a feeling that once you total up all needed extra's for reusability the delivered mass fraction will be quite small or even negative.   
 

Offline Jcc

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The only SSTO design that has a decent chance of working is as far as I can see is Skylon, but that is still several years before it can be tested. They still need to prove the engines can work. The idea that a big low density airframe can reenter at hypersonic speeds, be strong enough to withstand dynamic pressure and heat without the need for an ablative TPS needs to be proven.

Elon is skeptical about air breathing launch vehicles in general, but I think he wouldn't consider the technology too closely since it can't scale up to MCT class or go BEO, so why bother?

Offline dante2308

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To service the 100% of the launch market that isn't colonizing Mars?

Offline beancounter

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The only SSTO design that has a decent chance of working is as far as I can see is Skylon, but that is still several years before it can be tested. They still need to prove the engines can work. The idea that a big low density airframe can reenter at hypersonic speeds, be strong enough to withstand dynamic pressure and heat without the need for an ablative TPS needs to be proven.

Elon is skeptical about air breathing launch vehicles in general, but I think he wouldn't consider the technology too closely since it can't scale up to MCT class or go BEO, so why bother?

Several years?  That's a bit of an understatement if ever there was one.  They're still developing a test version of the engine let alone the vehicle to go with it.  I'd reckon they'll be lucky to get there under 10 years.  How long have they been going so far?  In fact, I'll go so far as to predict that Elon'll be on Mars before Skylon takes to the sky with an operational vehicle  ;)
Beancounter from DownUnder

Offline IRobot

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Given Skylon's projected cost, IMO it will never be built, even if the engine works.

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