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If we're playing 'guess-the-acronym', I'm going to depart from something Mars-based and suggest that the 'C' is to do with 'Cryogenic', and that now that they a

If we're playing 'guess-the-acronym', I'm going to depart from something Mars-based and suggest that the 'C' is to do with 'Cryogenic', and that now that they are more confident with engines they might be switching to a harder, but much higher performance, fuel combination like liquid oxygen with liquid hydrogen. Whereas LOX+Kerosene has a Specific Impulse (crudely the amount of 'go' you get from a pound of propellent) of about 340s, LOX+Hydrogen engines have an Isp of about 450. That's really a huge increase in terms of what you can launch with a given size of rocket. Tom Mueller, the head of propulsion at SpaceX, previously worked for TRW (the company that did the US's ICBMs in the 50s) and while there he worked on a LOX+Hydrogen engine, the TR106 ( http://en.wikipedia.org/wiki/TR-106 http://en.wikipedia.org/wiki/TR-106 ), which was a beast. It's harder to work with - both propellants are very cryogenic - but any rocket engine designer will be very aware that LOX+Hydrogen is hovering at the top of the evolutionary ladder looking mighty attractive for those that have the resources to take it on - a position Space X find itself in as they become more mature and heavyweight. They're already looking at it for their replacement Upper stage vacuum engine for Falcon 9 - the Raptor ( http://en.wikipedia.org/wiki/Raptor_(rocket_stage) http://en.wikipedia.org/wiki/Raptor_(rocket_stage) ). Also, almost any Mars mission would involve a lot of heavy lifting into low earth orbit to construct a vehicle that would transport people there and back. A space vehicle that can support several people for say 2 years, and move itself to and from Mars, is going to by way bigger than anything that can be launched by a single rocket all at once. A big heavy-lifter whose engines have a much improved Isp (which rather relaxes the expensive mass saving measures you need in the rest of the rocket to launch a given payload, or alternatively lets you launch a much heavier payload for the same vehicle) is consistant with a mars road map. I doubt they would undertake something like this just for Mars - developing rocket engines is hecka expensive. SpaceX got started by inheriting most of the cancelled FastTrac project from Nasa ( http://en.wikipedia.org/wiki/Fastrac_(engine) http://en.wikipedia.org/wiki/Fastrac_(engine) ) and indeed still outsource their turbopumps (the hardest bit of an engine) to Barber Nichols, who did the design for Fastrac. That was a great move on their part - starting with a blank sheet would have been madly expensive and very slow - a working baseline engine design is a very high-dimensional optimisation problem, before you even get to cutting metal. My point is, if they're now starting clean-sheet engine design, I'm sure they must have in mind some way for it to pay for itself because it's a hugely costly business. We live in interesting times!

nkoren 14y ago
Hydrogen brings absolute oodles of troubles with it. For upper stages, where it is much more critical to minimise the wet weight, that trouble is generally worthwhile. For lower stages, not so much -- it's generally FAR more cost-effective to just build a heavier stages which will do the same job, but with lower ISP. In the past, SpaceX has always made the (correct, IMHO) decision to optimise the cost-efficiency of their rockets, rather than the propulsion efficiency, and hopefully they won't stray from that path. (They've said in the past that they're working on a LH2 upper stage already, so if you see them building up a cryogenic capacity, that would explain why). Also, I kinda miss the days when they could get away with calling their next generation rocket the "BFR", and no true geek needed to even ask what that stood for.
- ballooney 14y ago
  Indeed, as I say it's very much harder to work with, but it's worth a revisit with 2012 technology, I think. I also am involved with oxygen+hydrogen rocket propulsion research but can't really discuss it much here. But things have come on a bit in the last 30 years :) Re: 'BFR', well I have a few hypotheses about 'MCT' in a similar vein but I reasoned they might be a little bit blue for HN! Edit: A better reason for first stage hydrogen occured to me over a cup of tea: > "SpaceX has always made the (correct, IMHO) decision to optimise the cost-efficiency of their rockets, rather than the propulsion efficiency, and hopefully they won't stray from that path." I think the economics change when you try and go reusable in the way that they are. Lox Hydrogen engines and tankage might be more expensive, which is obviously a factor in an expendable vehicle, but if you reuse the vehicle, and have to keep enough fuel back to land, as SpaceX plan, I think actually the equation goes far more back towards optimising for propulsion efficiency in order to get the cost efficiency. The launcher cost metric is basically dollars per kg into orbit, and for a given reusable rocket, lox+hydrogen will get many more kgs into orbit than lox+kerosene - you need to keep much less fuel back in reserve to land the various rocket stages, and the extra fuel you can use on the ascent is also a lot more thrusty per unit mass. I wouldn't be surprised, assuming you sell at a dollar/kg price, if lox+hydrogen ended up being much more profitable in reusables than lox+kerosene, everything else being equal. I have not done the sums though.

nkoren 14y ago

Hydrogen brings absolute oodles of troubles with it. For upper stages, where it is much more critical to minimise the wet weight, that trouble is generally worthwhile. For lower stages, not so much -- it's generally FAR more cost-effective to just build a heavier stages which will do the same job, but with lower ISP. In the past, SpaceX has always made the (correct, IMHO) decision to optimise the cost-efficiency of their rockets, rather than the propulsion efficiency, and hopefully they won't stray from that path. (They've said in the past that they're working on a LH2 upper stage already, so if you see them building up a cryogenic capacity, that would explain why). Also, I kinda miss the days when they could get away with calling their next generation rocket the "BFR", and no true geek needed to even ask what that stood for.

ballooney 14y ago
Indeed, as I say it's very much harder to work with, but it's worth a revisit with 2012 technology, I think. I also am involved with oxygen+hydrogen rocket propulsion research but can't really discuss it much here. But things have come on a bit in the last 30 years :) Re: 'BFR', well I have a few hypotheses about 'MCT' in a similar vein but I reasoned they might be a little bit blue for HN! Edit: A better reason for first stage hydrogen occured to me over a cup of tea: > "SpaceX has always made the (correct, IMHO) decision to optimise the cost-efficiency of their rockets, rather than the propulsion efficiency, and hopefully they won't stray from that path." I think the economics change when you try and go reusable in the way that they are. Lox Hydrogen engines and tankage might be more expensive, which is obviously a factor in an expendable vehicle, but if you reuse the vehicle, and have to keep enough fuel back to land, as SpaceX plan, I think actually the equation goes far more back towards optimising for propulsion efficiency in order to get the cost efficiency. The launcher cost metric is basically dollars per kg into orbit, and for a given reusable rocket, lox+hydrogen will get many more kgs into orbit than lox+kerosene - you need to keep much less fuel back in reserve to land the various rocket stages, and the extra fuel you can use on the ascent is also a lot more thrusty per unit mass. I wouldn't be surprised, assuming you sell at a dollar/kg price, if lox+hydrogen ended up being much more profitable in reusables than lox+kerosene, everything else being equal. I have not done the sums though.