Author Topic: MCT Speculation and Discussion Thread 4  (Read 503183 times)

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2460 on: 07/21/2016 02:24 AM »
I estimate a direct return requires 3x the propellant at Mars surface, which is nothing to sneeze at when we know so little about how hard that propellant will be to acquire.  Note that envy887 just gave us the TEI numbers not the TOTAL the vehicle would need because he didn't include the DeltaV for propulsive landing on Earth, that's going to add around another 500 m/s, that along with a safety margin reserves means your looking at 8 km/s to do the direct return which starts to look prohibitive.

Also no one seems to be taking into account how tripling the propellant capacity of the BFS affects it's design and adds to it's dry mass which has multiplicative penalties not just at return but through the entire mission.  You can not simply take the marginal propellant mass multiply by a tankage fraction of 4% and call that the added dry mass, the larger vehicle needs more thermal protection, stronger landing gear to support it's mass at launch, more thrust to liftoff and land, more systems to chill and hold the propellants.

The DeltaV number I'm targeting is slightly above that required just for launch to mars orbit because I see potential for a surface-orbit-surface cycle and returning a completely empty vehicle to Earth via a slow transfer.  I estimate a vehicle mass of 75 mt and thus a propellant capacity of 300 mt gets both of these capabilities, having 6 km/s when empty and 5.1 km/s when carrying the 25 mt return cargo spec which allows 4.1 km/s to orbit and and 800 m/s on landing with cargo (taking on cargo in orbit means it dose not sum to 5.1 km/s).

A direct return vehicle would have a higher mass, 25 tons more for a total of 100 mt and would require a 900 mt propellant capacity and that is JUST the 4% tank fraction of the marginal propellant as 600 * .04 = 24 so the vehicle could be considerably heavier.  Take off dry mass at Mars would then be 125 (100 dry + 25 cargo) rather then 100.  Now consider total initial mass in LEO that this system would need to do the TMI with it's outbound 100 mt cargo would be 1125 mt.

A smaller BFS using a 2nd stage massing 50 mt for assist in TMI is almost identical at 1151 mt IMLEO for the same trajectory which is split between the 2nd stage and the BFS, with a reserve for the 2nd stage to do a full propulsive reversal and return to LEO.  So

Now how much mass is needed for the refueling tanker.  The smaller BFS has the DeltaV to go to very near the 5 km/s Mars Escape from the surface so the refueling tanker can be placed in a high Martian orbit. The BFS would then need 3 km/s after the refueling to archive a comparable fast transit to Earth as the 8 km/s direct launch.  To do that it needs to take on 125 mt of Propellant.  If the tanker masses 50 mt and will need to return to Earth on a hohmann after the fuel transfer it needs to retain about 13 mt for that, so mass in high Mars orbit is 187 mt and that means an initial mass in LEO of 936 mt, an 80% increase in total IMLEO.

While 936 might seem prohibitive, it is substituting for 600 mt of propellant MADE on the surface of Mars, so the ratio of substitution is ~3:2 when propellant is delivered to a high Mars orbit.  This is quite different then the traditional view that Mars ISPP will have a IMLEO reduction factor of 10x - 20x but thouse estimates are based on taking all propellant down to the Martian surface which is an additional 10 km/s DeltaV, this scenario puts the refueling at the optimum point for efficient delivery.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2461 on: 07/21/2016 02:33 AM »
We don't know isn't a valid reason. Same line of reasoning would give us Apollo-like hypergols.

SpaceX is going to find out. So make a first-try estimate. You'll see that a ton of water per day isn't unreasonable to harvest since you need to harvest at least hundreds of kilograms per day anyway. Better than tripling the required number of launches and the complexity of the architecture.
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Offline mfck

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Re: MCT Speculation and Discussion Thread 4
« Reply #2462 on: 07/21/2016 06:48 AM »
mfck you had it right, I am saying their is an irrational fear of rendezvouses in mission planning. 
....

NASA assesses the chance of a crewed Orion capsule with a contingency EVA option failing to dock with a lunar assent vehicle at just 1 in 546.  https://www.nasa.gov/pdf/140639main_ESAS_08.pdf (Page 13)
...

So I would estimate a failure chance of the BFS to achieve refueling in Mars orbit at around 1:1000 and not at all something worth of concern. 
....

I am not an engineer, but those risk figures seem odd. 1:546 might be considered OK for Orion, a craft that will hardly (imo) fly 100 missions combined, but a 1:1000, for a mass transportation architecture that MCT is, seems unacceptable. What am I missing? What are the comparable risks in, say, aviation?
« Last Edit: 07/21/2016 06:49 AM by mfck »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2463 on: 07/21/2016 09:05 AM »
mfck you had it right, I am saying their is an irrational fear of rendezvouses in mission planning. 
....

NASA assesses the chance of a crewed Orion capsule with a contingency EVA option failing to dock with a lunar assent vehicle at just 1 in 546.  https://www.nasa.gov/pdf/140639main_ESAS_08.pdf (Page 13)
...

So I would estimate a failure chance of the BFS to achieve refueling in Mars orbit at around 1:1000 and not at all something worth of concern. 
....

I am not an engineer, but those risk figures seem odd. 1:546 might be considered OK for Orion, a craft that will hardly (imo) fly 100 missions combined, but a 1:1000, for a mass transportation architecture that MCT is, seems unacceptable. What am I missing? What are the comparable risks in, say, aviation?

Maybe you misunderstood, 1 in 546 is the chance of failure PER attempt, the number of missions an individual vehicle makes isn't relevant to that figure. 

A refueling failing on the first few missions when their is only ONE BFS at Mars and no assistance available could result in a loss of crew.  But when whole fleets of vehicles are operating continually then their is no more danger from a refueling failure then when an airplane flight gets canceled, people are just transferred to another vehicle for Earth return.

No one has any concern about loss of life from a failure to fill up propellant depots at Earth because we have all our resources available here to render aid to any stricken vessel, the lack of that aid chance is WHY distant exploration is dangerous.  If Mars gets a functioning base and a propellant depot in orbit then the rescue contingencies become more robust and mechanical failures that are not 'kinetic' aka don't cause immediate destruction or lose of life become lower impact and thus lower risk because risk = probability * impact.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2464 on: 07/21/2016 09:24 AM »
We don't know isn't a valid reason. Same line of reasoning would give us Apollo-like hypergols.

SpaceX is going to find out. So make a first-try estimate. You'll see that a ton of water per day isn't unreasonable to harvest since you need to harvest at least hundreds of kilograms per day anyway. Better than tripling the required number of launches and the complexity of the architecture.

At least quote my numbers correctly when you give a rebuttal, it was an 80% increase in IMLEO in exchange for a 66% reduction in propellant produced at Mars.

And while we don't know the efficiency of water collection yet this shows what the bar is in performance needed to make it attractive.  Basically propellant production at Mars has a marginal utility that drops as you produce more of it and burn it farther away from Mars due to both the rocket equation and the closer supply from Earth.  The most efficient use is assent to LMO, then rising to high Mars orbit, then TEI, then Earth landing.

Lastly it's quite clear that the vehicles I'm proposing are no more complex then your larger vehicle alternative, they are very likely much simpler to produce due to lower mass.  Likewise the mission profile though it is more costly due to IMLEO and launch increases isn't doing anything that isn't already expected, you have BFS and Tankers and propellant transfer, something we always knew would exist, it's just repeated more times.  And in exchange the longest pole of the whole system the ISPP is cut by 2/3rds

Offline MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #2465 on: 07/21/2016 09:40 AM »
While 936 might seem prohibitive, it is substituting for 600 mt of propellant MADE on the surface of Mars, so the ratio of substitution is ~3:2 when propellant is delivered to a high Mars orbit.  This is quite different then the traditional view that Mars ISPP will have a IMLEO reduction factor of 10x - 20x but thouse estimates are based on taking all propellant down to the Martian surface which is an additional 10 km/s DeltaV, this scenario puts the refueling at the optimum point for efficient delivery.

The 2nd stage is not just substituting for propellant made on Mars. It is a whole new reusable long duration spacecraft which needs to be designed and manufactured. It will have heatshield, landing legs (and engines?), deep space avionics, solar panels (+ radiators?), propellant cooling loop, docking (berthing?) adapter, etc. While not quite as complex as a do-it-all BFS, it is still a major project, costing lots of up-front development effort and testing.

The BFS might be a bit simpler using a second stage, but not by much, all major subsystems would still be present, the only advantage is that it would need a lower propellant mass fraction. An do-it-all BFS has as far as we can tell a reasonably high, but not exceptional propellant mass fraction.

Using a hohmann transfer for the 2nd stage reduces its reuse to every 2 synods (it might be every 3 synods because I think that refuelling the BFS in Mars orbit occurs after the hohmann transfer window closes - I haven't checked this).

Adding in a 2nd stage and refuelling in Mars orbit adds in complexity and a lot of extra failure modes, this is not just docking, but failures in aerocapture, propellant transfer, BFS engine restarts, etc. These failure modes then drive extra abort scenarios.

In my opinion these extra up-front costs and complexity mean that a design like this with a 2nd stage is not optimum in the short term. In the long term the higher IMLEO and longer reuse cycle (2-3 synods) would mean it is likely more costly as well. It really only makes sense if the do-it-all BFS becomes difficult or impossible because of its extra propellant mass requirements.

Edit:

The 2nd stage (burn out) + BFS + cargo would mass more in LEO than a do-it-all BFS (burn out) + cargo, I estimate about 20% more. Which would mean that the BFR would need to be 20% larger, and cost more. This extra cost is mostly offset by the 2nd stage in tanker mode carrying more propellant.

Turning the rocket into 3 stages (BFR + 2nd stage + BFS)  with the 2nd stage not reaching orbit and landing downrange, could lead to a smaller BFS. Even with the expense of downrange recovery this might end up cheaper.
« Last Edit: 07/21/2016 09:56 AM by MikeAtkinson »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2466 on: 07/21/2016 10:21 PM »
A stage with all of the capabilities I've described for the 2nd stage is a necessity under any architecture, others have simply been calling it a 'BFS-Tanker' as it is clear that the cargo carrying BFS vehicle would not be an efficient tanker to LEO and something specialized for that purpose is necessary.

The 2nd stage would need longer duration thus solar panels and radiators but these can be put directly on the surface of the cylindrical body as in the Dragon2 trunk.  It needs to be able to re-enter and land but only at Earth onto a concrete pad and without any cargo meaning it would land nearly empty of propellant, it would have a dry mass about twice that of the F9 first stage and can use a similar set of fold down legs which are nothing compared to the legs systems that the BFS needs to be able to land on rough ground.  Sending the tanker off to Mars is just 2 engine restarts, the rest of the duration it's just free floating in space in a thermal environment much cooler then remaining near the Earth.

As for synod cycle times we already have a terrible rate on the BFS, the one synod cycle Musk has speculated about require insane DeltaV levels and In my opinion they will never happen, not least of which is the fact that the BFS will have nothing to do in Earth space until the next Mars transit window opens.

Your comment on long and short term costs seems to be based on a false premise, that the refueling plan I described is intended for long term usage.  If you had read my comments fully it would be clear that I intended this as just a bootstrapping technique for early missions.

This is one of the more annoying aspects of most discussion on this thread, most commentators are seemingly incapable of conceiving of a MCT SYSTEM which evolves or grows over time in any capacity other then sending larger and larger numbers of the same vessels.  Every proposal is evaluated as if that system MUST satisfy all transport needs from the first human landing to the 10 millionth colonist.  Nothing could be farther from reality, Musk looks for the minimum viable product and then improves and adds to it, a Mars transport system will also expand with additional components that act as force multipliers on the whole system.

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Re: MCT Speculation and Discussion Thread 4
« Reply #2467 on: 07/27/2016 07:03 PM »
A stage with all of the capabilities I've described for the 2nd stage is a necessity under any architecture, others have simply been calling it a 'BFS-Tanker' as it is clear that the cargo carrying BFS vehicle would not be an efficient tanker to LEO and something specialized for that purpose is necessary.
...

Musk said that (paraphrasing) "he's tempted to pursue stage 2 reuse on FH, but it's probably better to focus on Mars architecture".

Since a reusable upper stage on FH is a natural iterative development towards any potential reusable S2 on BFR, I think it's highly probable based on this statement that the Mars architecture won't include a S2 at all. If BFR did have a S2 on it, any focus on developing a smaller version would help (not detract from) focus on the larger one.

So I think the Falcon upper stage is an architectural dead end, and BFR/BFS will derive from the vehicles SpaceX is currently reusing: Falcon Stage 1, and Dragon. That wouldn't mean that specialized (e.g. Tanker) version wouldn't exist, just that they would share heritage with Dragon and not with the F9 S2.

Most of the specializations (lightweight legs, insulation, cryocoolers, extra PV arrays, bigger tanks) will fit just as well in a similar outer mold line to BFS, even though internal and structural components would differ. It would be more like the relation between FH center and side boosters than between F9 S2 and Dragon: looks very much the same, but a bit different inside.
« Last Edit: 07/27/2016 07:47 PM by envy887 »

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2468 on: 07/27/2016 10:04 PM »
BFS will not be anything like Dragon except in the broadest terms. The delta-v will be nearly an order of magnitude different. About as similar as a jellyfish and a rat. "Derive" is not the right word.
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Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2469 on: 07/27/2016 11:58 PM »
To be clear, I'm referring to Crew Dragon, most specifically Red Dragon. And I think there will certainly be more than superficial parallels: both will be interplanetary spacecraft capable of using hypersonic retropropulsion for high-precision EDL optimized for Mars and Earth - but potentially on a variety of Solar System bodies. If SpaceX thinks they have found good Earth and Mars EDL profiles after landing Crew and Red Dragon, they will probably try to keep similar EDL profiles for BFS to minimize risk from new variables.

Of course that doesn't mean the spacecraft will be at all internally similar or have at all similar performance in all respects. They will have different dry mass fractions, payload capabilities, prop mass fractions, very different structures, fuels, pressure vessels, engines, Isp, materials, manufacturing methods, etc. Delta v is just a function of Isp and PMF, and clearly those will be very different.

But that doesn't mean the EDL profiles cannot be similar. The outer mold lines and engine position/orientation can be roughly similar (yes, I know, cosine losses and vac bells, etc... but those are definitely not complete showstoppers). As long as the AOA vs. L/D curves, L/D ratios, COG and COP locations, drag coefficients, mass/area ratios, ballistic coefficients, etc. are similar, they can retire a lot of risk by flying Red/Crew Dragon as a subscale model and applying scale factors.

If you're looking for a animal analogue, try honeybee vs hummingbird: very different scales, structures and physiology, but the aerodynamics scale to allow similar flight profiles.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2470 on: 07/28/2016 01:44 AM »
Outer mold line similarities are n8ot en8ough t8o c8onsider 8one vehic2le derived
A stage with all of the capabilities I've described for the 2nd stage is a necessity under any architecture, others have simply been calling it a 'BFS-Tanker' as it is clear that the cargo carrying BFS vehicle would not be an efficient tanker to LEO and something specialized for that purpose is necessary.
...

Musk said that (paraphrasing) "he's tempted to pursue stage 2 reuse on FH, but it's probably better to focus on Mars architecture".

Since a reusable upper stage on FH is a natural iterative development towards any potential reusable S2 on BFR, I think it's highly probable based on this statement that the Mars architecture won't include a S2 at all. If BFR did have a S2 on it, any focus on developing a smaller version would help (not detract from) focus on the larger one.

So I think the Falcon upper stage is an architectural dead end, and BFR/BFS will derive from the vehicles SpaceX is currently reusing: Falcon Stage 1, and Dragon. That wouldn't mean that specialized (e.g. Tanker) version wouldn't exist, just that they would share heritage with Dragon and not with the F9 S2.

Most of the specializations (lightweight legs, insulation, cryocoolers, extra PV arrays, bigger tanks) will fit just as well in a similar outer mold line to BFS, even though internal and structural components would differ. It would be more like the relation between FH center and side boosters than between F9 S2 and Dragon: looks very much the same, but a bit different inside.

Not a sound basis for speculation.  No reusable 2nd stage for F9 has always been stated to be an economic choice due to lack of margin and most customers wanting GTO launches. 

But we now know that a Raptor upper stage is being made, which IS exactly what would make sense as a for runner to a reusable BFS upper stage, the new F9 upper stage can do controlled re-entry burn tests  while still being disposed of, just as the F9 first stage was splashed into the ocean several times before even trying to put legs on it.  The new upper stage lets SpaceX do it's tests at customer expense, which is how they like to do all their testing.

Offline GORDAP

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Re: MCT Speculation and Discussion Thread 4
« Reply #2471 on: 07/28/2016 08:35 AM »
...
But we now know that a Raptor upper stage is being made, which IS exactly what would make sense as a for runner to a reusable BFS upper stage, the new F9 upper stage can do controlled re-entry burn tests  while still being disposed of, just as the F9 first stage was splashed into the ocean several times before even trying to put legs on it.  The new upper stage lets SpaceX do it's tests at customer expense, which is how they like to do all their testing.

Err, how exactly do we now know that a Raptor upper stage is being made?  I thought the only things we know are (1) The Air Force has thrown some money SpaceX's way to develop a Raptor vac engine, and (2) Musk has recently said that doing a new, reusable upper stage for Falcon is tempting, but SpaceX will be concentrating on developing the 'Mars rocket' instead.  This would imply to me that (sadly) there is no effort underway to do a new upper stage for Falcon, and likely won't be in the future.

Offline Semmel

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Re: MCT Speculation and Discussion Thread 4
« Reply #2472 on: 07/28/2016 12:03 PM »
Err, how exactly do we now know that a Raptor upper stage is being made?  I thought the only things we know are (1) The Air Force has thrown some money SpaceX's way to develop a Raptor vac engine, and (2) Musk has recently said that doing a new, reusable upper stage for Falcon is tempting, but SpaceX will be concentrating on developing the 'Mars rocket' instead.  This would imply to me that (sadly) there is no effort underway to do a new upper stage for Falcon, and likely won't be in the future.

The Air Force said specifically that the engine is for F9.

Quote
This other transaction agreement requires shared cost investment with SpaceX for the development of a prototype of the Raptor engine for the upper stage of the Falcon 9 and Falcon Heavy launch vehicles.

http://www.defense.gov/News/Contracts/Contract-View/Article/642983

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2473 on: 07/28/2016 01:35 PM »
There is a whole thread for the Raptor on F9/FH discussion: http://forum.nasaspaceflight.com/index.php?topic=39314.msg1563987#msg1563987

It's important to distinguish a couple things here:
1) the AF is paying for ENGINE technology development; I doubt they care much what vehicle it flies on;
2) there is a difference between a "new" Raptor stage and a "new, reusable" Raptor stage. The minimum viable product for recovering a Raptor upper stage is appreciably more complex than the minimum viable product for launching it.
3) Raptor expendable on F9 might be commercially viable if it allows them to compete with Ariane on 5300+ kg to GTO with booster reuse.

Quote from: Impaler
Outer mold line similarities are n8ot en8ough t8o c8onsider 8one vehic2le derived
I listed several other similarities; whether "derived" is a good descriptor of their relation is semantic.

Quote from: Impaler
...the new F9 upper stage can do controlled re-entry burn tests while still being disposed of...
I don't think it can at all. It will break up from heat load on reentry unless heavily shielded - at which point it's basically a reusable upper stage, which Musk specifically said they aren't working on. The velocity change for a orbital reentry is about 4 to 5x greater, and the max heating rate 20 to 25x greater than what the F9 S1 sees on entry. Even if it's possible to orbit enough fuel retroburn through the peak heating phase (and I STRONGLY doubt that it is), why would they? The stage has a low ballistic coefficient, lots of area to dissipate heat, and slow terminal velocity: shield the front and part of one side, enter nose-first, and keep all the sensitive, expensive parts out of the hypersonic flow and well away from the bow shock. This is exactly how SpaceX envisioned S2 reuse... and they aren't pursuing it.

The same goes for BFR. I'm extremely doubtful that orbital reentry using primarily retropropulsion for shock standoff and cooling is possible and more optimal than nose-first entry for a S2 type vehicle. If there's any evidence to the contrary please point it out.

Offline JamesH65

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Re: MCT Speculation and Discussion Thread 4
« Reply #2474 on: 07/28/2016 01:44 PM »
Err, how exactly do we now know that a Raptor upper stage is being made?  I thought the only things we know are (1) The Air Force has thrown some money SpaceX's way to develop a Raptor vac engine, and (2) Musk has recently said that doing a new, reusable upper stage for Falcon is tempting, but SpaceX will be concentrating on developing the 'Mars rocket' instead.  This would imply to me that (sadly) there is no effort underway to do a new upper stage for Falcon, and likely won't be in the future.

The Air Force said specifically that the engine is for F9.

Quote
This other transaction agreement requires shared cost investment with SpaceX for the development of a prototype of the Raptor engine for the upper stage of the Falcon 9 and Falcon Heavy launch vehicles.

http://www.defense.gov/News/Contracts/Contract-View/Article/642983

Picky, but that statement does not say anything about actually making a Rapter equipped  U/S, just a Rapter proto ENGINE for a U/S. One could argue why do one without the other, but it may just be a proof of principle. Find out whether a Rapter VAC could be use on an F9 U/S

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Re: MCT Speculation and Discussion Thread 4
« Reply #2475 on: 07/28/2016 01:53 PM »
... Find out whether a Rapter VAC could be use on an F9 U/S

Almost certainly yes. Read the last few pages of the Raptor on F9/FH discussion: http://forum.nasaspaceflight.com/index.php?topic=39314

Offline JamesH65

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Re: MCT Speculation and Discussion Thread 4
« Reply #2476 on: 07/28/2016 03:44 PM »
... Find out whether a Rapter VAC could be use on an F9 U/S

Almost certainly yes. Read the last few pages of the Raptor on F9/FH discussion: http://forum.nasaspaceflight.com/index.php?topic=39314

"Almost certainly" is "almost certainly" not good enough for the US DoD!

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2477 on: 07/28/2016 04:26 PM »
It could be, but it's not going to. At least not before MCT flies.

I started at least one thread speculating about a Raptor-based reusable upper stage for Falcon 9/FH, given that hint from the Air Force (which is not new, we've known about that for a quite long while). But now we know from Musk that they're not going to pursue that right now.
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Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2478 on: 07/28/2016 04:41 PM »
"Almost certainly" is "almost certainly" not good enough for the US DoD!

Again, the AF is paying for ENGINE technology development and demonstration; I doubt that what vehicle the engine will fly on (if any at all) is contractually obligated in any way.

If SpaceX prefers to fly Raptor on a 250t to LEO vehicle instead of a 25t to LEO vehicle, all the better for the AF.

Offline AncientU

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Re: MCT Speculation and Discussion Thread 4
« Reply #2479 on: 07/28/2016 07:27 PM »
It could be, but it's not going to. At least not before MCT flies.

I started at least one thread speculating about a Raptor-based reusable upper stage for Falcon 9/FH, given that hint from the Air Force (which is not new, we've known about that for a quite long while). But now we know from Musk that they're not going to pursue that right now.

That leaves the questions of if and when wide open...
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