I posted this idea over on the SpaceX forum but no one commented on it, so I'll expand on it a little and try it here. It has to do with the effort to land the first stage of the Falcon 9 v1.1, or F9R if preferred. As we know, a single Falcon 9 engine, a Merlin D, has far to much thrust to let the returning booster stage hover. I guess, with landing legs and all, empty mass of S1 is about 30 tonnes while lift from a single Merlin D throttled down to 70% is in the vicinity of 100 tonnes so it can not hover or even land slowly. Instead it will come in to the landing pad at terminal velocity and decelerate at over 30 m/s^2 during the final few seconds of flight while aligning vertically with no spin and no lateral motion. The grasshopper's performance isn't even close so don't be fooled by grasshopper. The above scenario presents a very difficult flight controls problem because of the time needed to correct unacceptable motions and attitudes. Computerized flight controls can give the correcting commands instantly but 30 tonnes of stage 1 metal won't respond that quickly. The control system needs more time, or more control authority, hence this idea.Could a rocket engine thrust quencher be made that would act symmetrically on the rocket engine plume? I envision something made like a jet aircraft engine thrust reverser but not very efficient as a thrust reverser. That's because it wouldn't reverse thrust but rather send the engine plume off to the side, or about 60% of the plume anyway, killing that percentage of engine thrust. It would be attached to the rocket, not the engine, so all 9 engines remain identical.It would work like this: S1 returns to the landing pad at terminal velocity but stops a few meters above the pad. It deploys the thrust quencher then throttles to hover. Flight controls then right the vehicle eliminating any undesired motions and offsets then throttles down to land gently. Yes, it would take more fuel, but not much and it would give the thrusters enough time to rotate the 30 tonnes of S1 metal properly for a safe touchdown.Now, an added complexity. If more control power is needed, the thrust quencher could be designed to act as vanes in the engine plume to provide rotational torque or to torque the stage to upright position if needed.I'm not saying that a rocket engine thrust quencher is the be-all, end-all control for landing over powered rocket stages, but it is something I hope we can discuss seriously.
It would work like this: S1 returns to the landing pad at terminal velocity but stops a few meters above the pad. It deploys the thrust quencher then throttles to hover. Flight controls then right the vehicle eliminating any undesired motions and offsets then throttles down to land gently. Yes, it would take more fuel, but not much and it would give the thrusters enough time to rotate the 30 tonnes of S1 metal properly for a safe touchdown.
Some early rocket engines steered using various objects inserted into the exhaust. IIRC surviving in that environment was challenging but they did work. I bet if you tried hard enough you could make a thrust reverser for Merlid 1D. The thrust reverser would be heavy (it has to deal with ~100 klbf afterall) and the propellant to hover would be heavy.
I posted this idea over on the SpaceX forum but no one commented on it, so I'll expand on it a little and try it here. It has to do with the effort to land the first stage of the Falcon 9 v1.1, or F9R if preferred. As we know, a single Falcon 9 engine, a Merlin D, has far to much thrust to let the returning booster stage hover. I guess, with landing legs and all, empty mass of S1 is about 30 tonnes while lift from a single Merlin D throttled down to 70% is in the vicinity of 100 tonnes so it can not hover or even land slowly. Instead it will come in to the landing pad at terminal velocity and decelerate at over 30 m/s^2 during the final few seconds of flight while aligning vertically with no spin and no lateral motion. The grasshopper's performance isn't even close so don't be fooled by grasshopper. The above scenario presents a very difficult flight controls problem because of the time needed to correct unacceptable motions and attitudes. Computerized flight controls can give the correcting commands instantly but 30 tonnes of stage 1 metal won't respond that quickly. The control system needs more time, or more control authority, hence this idea.
I hope you're right ChrisWilson68. Prior posts on this thread lead me to think that if you are wrong, then deep throttling would be a nicer solution than thrust reversers.
As we know, a single Falcon 9 engine, a Merlin D, has far to much thrust to let the returning booster stage hover. I guess, with landing legs and all, empty mass of S1 is about 30 tonnes while lift from a single Merlin D throttled down to 70% is in the vicinity of 100 tonnes so it can not hover or even land slowly. Instead it will come in to the landing pad at terminal velocity and decelerate at over 30 m/s^2 during the final few seconds of flight while aligning vertically with no spin and no lateral motion. The grasshopper's performance isn't even close so don't be fooled by grasshopper.
Quote from: aero on 10/03/2013 06:42 amAs we know, a single Falcon 9 engine, a Merlin D, has far to much thrust to let the returning booster stage hover. I guess, with landing legs and all, empty mass of S1 is about 30 tonnes while lift from a single Merlin D throttled down to 70% is in the vicinity of 100 tonnes so it can not hover or even land slowly. Instead it will come in to the landing pad at terminal velocity and decelerate at over 30 m/s^2 during the final few seconds of flight while aligning vertically with no spin and no lateral motion. The grasshopper's performance isn't even close so don't be fooled by grasshopper. Your numbers are way off. The thrust of a Merlin 1D is about 66 (metric) tons at sea level. Throttled down to 70% it makes about 46 (metric) tons. If the first stage empty weight is 40 (metric) tons, it means 15 m/s deceleration, not 30 m/s deceleration.And if they leave some extra 10 tons of "reverse fuel+oxidizer" to the tanks for most flight, then it's only some 12 m/s.