Author Topic: Rocket Engine Thrust Quencher ?  (Read 4347 times)

Online aero

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Rocket Engine Thrust Quencher ?
« on: 10/03/2013 06:42 AM »
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.
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Offline strangequark

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Re: Rocket Engine Thrust Quencher ?
« Reply #1 on: 10/03/2013 06:56 AM »
Having it survive would be non-trivial. Jet engine exhaust is pretty cool. Any thrust diverter that works in the way you described would be subjected to the full stagnation temperature of the flow, say around 6000-7000F. There have been rockets that have used vanes in the plume for thrust vectoring. These were typically made of very heavy tungsten alloys to resist the environment, and still oxidized substantially. If 70% throttle is insufficient, I think you're going to be better off trying to increase the throttle band on the engine. Pintle injectors are uniquely suited to deep throttling because you can potentially vary the injector flow area with a translating sleeve. I don't think Merlin does this, but it has been done in other engines, most notably the LMDE.

Offline john smith 19

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Re: Rocket Engine Thrust Quencher ?
« Reply #2 on: 10/06/2013 10:41 PM »
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.
I guess the obvious question is can the engine throttle down to 30%?  Note the pintle injector on the Apollo LM was capable of 10:1 throttling.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.
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Offline deltaV

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Re: Rocket Engine Thrust Quencher ?
« Reply #3 on: 10/09/2013 10:03 PM »
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.

Basically your idea would probably work but would be a lot worse than the alternatives. (IANARS however.)

Offline john smith 19

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Re: Rocket Engine Thrust Quencher ?
« Reply #4 on: 10/10/2013 02:06 PM »
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.
Not just early ones. IIRC some of the Polaris missile stages used it for steering, as did various Armadillo Aerospace designs. It was relatively cheap and the performance hit was fairly minor (it varied with how far the obstruction was in the flow).

I think the OP is talking about something that (roughly) diverts thrust sideways and has at least 2 engines running in opposite directions to cancel lateral movement. A 1/4 cup that can be rotated into the flow.

As you note this would have to be pretty robust (Merlin 1d is more like 150Klb+).

Musk has said the F9 1st stage achieves a mass fraction of 30. Assuming a T/O thrust gives 1.25g (Giving an overall acceleration of 0.25g) that gives an engine thrust 1.25*30= 37.5x stage mass.

This is where a multi engine design really scores  :). with 8 engines off the engine thrust is about 4.17x stage mass. At 0.24 of full thrust they balance.  So achieving a throttle down to 24% of full SL thrust lets you land if  you can keep the stage balanced.

This is not "deep throttling," which only starts about 10% of main thrust, despite most engines not throttling at all. It is "deeper" than Spacex has gone so far, but I don't believe it's outside M1d's potential throttling range, as the Apollo LM Ascent stage (IIRC) was designed with a pintle injector good over a 10:1 range.

BTW even the SSME (notoriously cranky operating cycle) has been tested down to 17%. Testers reckoned it could go lower but the size of the valves meant they were operating in the valves "dribble" range IE barely open. The RL10 has been operated down to 1% with (IIRC) GHe injection to keep combustion stable.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.
So you're going to Mars to seek a better life. What does that mean to you? Always spot a fanbois by how they react to their idols failures.

Offline ChrisWilson68

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Re: Rocket Engine Thrust Quencher ?
« Reply #5 on: 10/13/2013 05:23 AM »
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'm afraid your premise is entirely incorrect.  The flight control system has plenty of time to react.

You're dismissing Grasshopper as being irrelevant because the real F9R will be traveling much faster and have a much higher thrust-to-weight ratio.  First of all, even if Grasshopper doesn't prove F9R's control system can't work with such a high T/W ratio, at best that would say we don't know whether F9R's control system can work with such a high T/W ratio.  You provided no evidence at all to believe it will not.  But, as it turns out, you're wrong that Grasshopper testing proves nothing about the F9R landing.  In fact, Grasshopper shows an ability to very precisely control the attitude of the stage.  That ability to control the attitude very precisely will be the same regardless of the T/W ratio or speed of the stage.  The only thing Grasshopper doesn't show is the ability to control the throttle of the engine with the precision necessary to land at high T/W.  But there's no reason to believe SpaceX can't do that.  Attitude control is a harder problem.

So, you're trying to solve a problem that doesn't exist.  The first stage of F9R can land with no problem at a high thrust-to-weight ratio.

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Re: Rocket Engine Thrust Quencher ?
« Reply #6 on: 10/13/2013 06:20 AM »
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.
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Offline john smith 19

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Re: Rocket Engine Thrust Quencher ?
« Reply #7 on: 10/13/2013 11:47 AM »
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.
You seem to think it's a toss up between attitude and throttling. Attitude control is critical whatever approach is used to kill most of the downward velocity.

AIUI Spacex is substituting an "impulse" approach, and trusting the design can handle the -ve g's.

Think of it like hitting an incoming baseball just hard enough to exactly cancel its momentum, so the ball drops at your feet.

Stupidly difficult (for a human) to do but simplifying the engine control problem to a duration burn, rather than a throttling ramp. Presumably Spacex feel this will lower the amount of fuel used and/or eliminate any problems due to the ramp rate needed Vs correcting any attitude instabilities.

That would make this a kind of "bang bang" control rather than a linear control system. IIRC Spacex announced they upgraded their on board computers for V1.1. This might explain why.

"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.
So you're going to Mars to seek a better life. What does that mean to you? Always spot a fanbois by how they react to their idols failures.

Online hkultala

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Re: Rocket Engine Thrust Quencher ?
« Reply #8 on: 10/13/2013 12:37 PM »
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.

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.

Offline cambrianera

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Re: Rocket Engine Thrust Quencher ?
« Reply #9 on: 10/13/2013 01:07 PM »
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.

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.
@hkultala:
you are correct about Merlin D, but F9 v1.1 first stage should be less than 30000 kg (most probably around 20000 kg).
Also propellants for landing should be no more than 3000 kg.
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Tags: SpaceX F9R