I think SpaceX is more likely to build something like Briz-M than Centaur.
Propulsive landing is not necessary for re-usability. Honestly, I think grasshopper is a pipe dream. Doing everything with engine thrust simply uses too much propellant and too risky in the sense that an engine failure will result in a 30~80 ton object partially fueled with RP-1/LOX crashing down on the launch site.
In the end, I believe that if they achieve 1st stage re-usability, they use engine thrust to reduce the re-entry velocity to about Mach 4~5 from about Mach 9~10 only. As long as the stage survives re-entry and slows down to sub-sonic speeds via atmospheric drag, they'll use parachute(s) to slow it down sufficiently for a water landing -- like they originally intended to do. In this model, the engine does not need to be on the centerline. It just has to be gimballed so the thrust goes through the CG (very much like SSMEs). Besides, there is no reason to fire just one engine they can fire two or all four.
Quote from: dwightlooi on 07/11/2012 10:08 pmPropulsive landing is not necessary for re-usability. Honestly, I think grasshopper is a pipe dream. Doing everything with engine thrust simply uses too much propellant and too risky in the sense that an engine failure will result in a 30~80 ton object partially fueled with RP-1/LOX crashing down on the launch site.How does this differ to landing aeroplanes?That they still maintain some manouverability even without engines?And it's not doing _everything_ with engine thrust. Atmospheric drag helps a lot also.
Elon wants "rapid reusability". That means not needing to spend lots of time and money to recover and fix/refurbish boosters that have been on salty sea water.
Quote from: hkultala on 07/11/2012 07:25 amI consider this a disadvantage. 4 engines would mean* Reusable version could not land with one engine, as the T/W would be too big, and the engine alignment would also be problematic for this* It would have worse engine-out capability than falcon 9.(3) Having 4 engine also gives nearly as good a level of practical engine out capability as having nine.Any basis for this claim?Propulsive landing is not necessary for re-usability. Honestly, I think grasshopper is a pipe dream. Doing everything with engine thrust simply uses too much propellant and too risky in the sense that an engine failure will result in a 30~80 ton object partially fueled with RP-1/LOX crashing down on the launch site. In the end, I believe that if they achieve 1st stage re-usability, they use engine thrust to reduce the re-entry velocity to about Mach 4~5 from about Mach 9~10 only. As long as the stage survives re-entry and slows down to sub-sonic speeds via atmospheric drag, they'll use parachute(s) to slow it down sufficiently for a water landing -- like they originally intended to do. In this model, the engine does not need to be on the centerline. It just has to be gimballed so the thrust goes through the CG (very much like SSMEs). Besides, there is no reason to fire just one engine they can fire two or all four.The thing about engine out capability is that with 4 engines you can lose an engine through about 50~70% of the 1st stage flight envelope depending on how close to the limit the payload happens to be. With 9 engines you can lose an engine through about 60~80% of the flight envelope. It is worse with 4 but not horribly worse. Besides, most engine failures happen either during the initial start and throttling to full thrust, or late in the burn. The former is mitigated by the 2 second hold down, the latter is fine with either 4 or 9 engines.
I consider this a disadvantage. 4 engines would mean* Reusable version could not land with one engine, as the T/W would be too big, and the engine alignment would also be problematic for this* It would have worse engine-out capability than falcon 9.(3) Having 4 engine also gives nearly as good a level of practical engine out capability as having nine.Any basis for this claim?