Author Topic: Landing an MVac  (Read 10815 times)

Offline IainMcClatchie

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Landing an MVac
« on: 07/29/2016 08:33 AM »
The exit pressure of an MVac is really low.  If you tried to land an F9 US, the engine nozzle would suffer flow separation.  Among other things, the thrust vector could shift around in an unpredictable manner.

The MVac injects the turbopump exhaust into the engine bell.  Suppose, during the landing burn, you ran the MVac turbopump with a richer fuel:oxidizer ratio.  For the same amount of pump power, you'd emit more gas.

Would it be possible to run the turbopump rich enough to inject enough gas into the engine bell to basically force the flow separation to happen at the turbopump exhaust injection site?  If the exhaust vector was stable, and the engine bell wasn't shaking apart, it seems this change might make it possible to land an F9 US.

And, on a Vacuum Raptor, you might inject copious amounts of straight fuel into the nozzle to accomplish the same thing.

Offline mikelepage

Re: Landing an MVac
« Reply #1 on: 07/29/2016 09:11 AM »
Other people on here know much more than me, but my impression was that it was more of an engineer-time/cost trade off than doubt over whether it could theoretically be done.  IIRC, work on recovering the F9 US was discontinued in order to focus on MCT, (which will presumably be full recoverable). 
« Last Edit: 07/29/2016 09:12 AM by mikelepage »

Offline Kaputnik

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Re: Landing an MVac
« Reply #2 on: 07/29/2016 01:17 PM »
It was never the plan to land using MVac thrust- but of course SpaceX have succesfully demonstrated landings with much higher thrust than originally envisaged, with the three engine burn.
I don't know enough about combustion stability and flow separation to comment on the OP's ideas. However one thing to consider is that a reusable US might have a deployable/retractable nozzle extension for easier re-entry (this was the baseline according the 2011 animation) so the problem might go away anyway.
Waiting for joy and raptor

Offline Jim

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Re: Landing an MVac
« Reply #3 on: 07/29/2016 01:38 PM »

Would it be possible to run the turbopump rich enough to inject enough gas into the engine bell to basically force the flow separation to happen at the turbopump exhaust injection site?  If the exhaust vector was stable, and the engine bell wasn't shaking apart, it seems this change might make it possible to land an F9 US.


No,.  The turbo pump operation is not independent of the rest of the engine.  Increase gas generator output, increases the turbo pump output which increases engine thrust.

Offline sevenperforce

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Re: Landing an MVac
« Reply #4 on: 07/29/2016 01:49 PM »
Moreover, the paper-thin MVac expansion bell would be shredded by the aerodynamic stresses of re-entry.

Offline Space Ghost 1962

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Re: Landing an MVac
« Reply #5 on: 07/29/2016 08:12 PM »
Bottom line: Recovering a F9US is a much harder task than a booster, with the tiniest of margin.

The exit pressure of an MVac is really low.
The result of a larger expansion. However, keep in mind that the stagnation pressure and drag/plume/retropropulsion/acceleration is far more massive. You can also handle EI higher. Keep in mind that this would work better for hypothetically landing a F9US on Mars!

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If you tried to land an F9 US, the engine nozzle would suffer flow separation.
Not necessarily. Depends on handling EI. Also, keep in mind how "fluffy" F9US is.

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Among other things, the thrust vector could shift around in an unpredictable manner.
True lower in the atmosphere, like with booster recovery profile. What if we use a Mars recovery profile on Earth instead? ;)

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The MVac injects the turbopump exhaust into the engine bell.  Suppose, during the landing burn, you ran the MVac turbopump with a richer fuel:oxidizer ratio.  For the same amount of pump power, you'd emit more gas.
MVac's advantage in doing that is increased iSP. Suggest you are attempting to use a booster recovery profile for a US recovery, which is built around overwhelming thrust. A better strategy would be you use the US's iSP instead.

So what if we "land" (go transonic) higher in the atmosphere? And then say fall with a chute that's snagged, much like with ULA's "SMART"?

So the profile would be likely to orient EI nozzle first, possibly film cooling the nozzle with fuel, until an acceptable stagnation pressure for atmospheric ignition (e.g. like US's first ignition "box"), then handle a ramp in throttle that kept the plume at a constant angle/envelope covering the shorter stage, as the atmospheric pressure built, using a steeper reentry angle to match the 7-20G acceleration of the empty, payloadless stage, generating extreme drag.

In this case, because you only have one centered engine (and your CG is near its powerhead!), you're intentionally destabilizing/detaching the vortex wake above the top of the stage, so as to dampen/drag like occurs with edge engines the US doesn't have. You could only do this with a high velocity stream from a engine optimized for iSP not thrust. And a short stage.

Use the virtues of the F9US, don't try to turn it into a booster. This would also lower your parasitic mass for recovery down to acceptable levels, for the price of needing a precision, teleoperated aircraft (helicopter?) recovery system in place of legs etc for RTLS.

Offline cuddihy

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Re: Landing an MVac
« Reply #6 on: 07/29/2016 08:57 PM »
Don't you think all those forces on the nozzle extension would rip it apart?

Offline drzerg

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Re: Landing an MVac
« Reply #7 on: 07/29/2016 09:47 PM »
nozzle extension could be ejected from engine. they obviously do test fire on Mvac so it's technically doable. nozzle extension will be expendable.

Offline Space Ghost 1962

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Re: Landing an MVac
« Reply #8 on: 07/30/2016 02:42 AM »
Don't you think all those forces on the nozzle extension would rip it apart?
No. As long as the stagnation pressure holds it under tension (at low densities before start). The concern is oxidation/heating nearing EI, thus the film cooling.

The point in starting the engine as low as possible is to waste as little props as possible, because you need the drag as much/more than the delta-v, and that goes up with density.

The weakness in this approach is the fact you need the engine's plume on the edges and not the center, so the angle of the shock wave flattens/broadens, thus the mention above of intentional turbulent flow generation above the top of the vehicle, acting as a drag chute. That's where things will get rough. What happens as the vehicle slows down, is that will approach the top of the vehicle - vehicle must go transonic before that occurs.

Although the lack of the stiffener ring concerns after engine start for increasing densities out of concern that the TO of the engine and the retro plume chaotically leverage to destroy the nozzle extension. Probably for that reason you'll have to give up some iSP, but suggest one might be able to acoustically modulate combustion to bound the effect.

Online envy887

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Re: Landing an MVac
« Reply #9 on: 07/30/2016 03:24 PM »
Don't you think all those forces on the nozzle extension would rip it apart?
No. As long as the stagnation pressure holds it under tension (at low densities before start). The concern is oxidation/heating nearing EI, thus the film cooling.

The point in starting the engine as low as possible is to waste as little props as possible, because you need the drag as much/more than the delta-v, and that goes up with density.

The weakness in this approach is the fact you need the engine's plume on the edges and not the center, so the angle of the shock wave flattens/broadens, thus the mention above of intentional turbulent flow generation above the top of the vehicle, acting as a drag chute. That's where things will get rough. What happens as the vehicle slows down, is that will approach the top of the vehicle - vehicle must go transonic before that occurs.

Although the lack of the stiffener ring concerns after engine start for increasing densities out of concern that the TO of the engine and the retro plume chaotically leverage to destroy the nozzle extension. Probably for that reason you'll have to give up some iSP, but suggest one might be able to acoustically modulate combustion to bound the effect.
[/quote

Couple things:
The extension is radiatively cooled; radiating heat into the exhaust plume and surrounding opaque plasma during high velocity entry probably won't work well at all.

For any sort of tail-first lifting entry (like the F9 S1) there will be asymmetrical aerodynamic loads on the nozzle extension, which WILL cause bending in the extension with resulting tension and compression loads. A tail-first ballistic entry might be able to keep the loads on the extension in tension only, but at the cost of much higher peak heating rates and drag loads.

Based on these, I think retracting or discarding the extension are the only viable options for S2 retropropulsion in any atmosphere.

Offline Space Ghost 1962

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Re: Landing an MVac
« Reply #10 on: 07/30/2016 07:56 PM »

Couple things:
The extension is radiatively cooled; radiating heat into the exhaust plume and surrounding opaque plasma during high velocity entry probably won't work well at all.
Consider flow rate and the fact that the entry heating is far hotter than exhaust. Also, with radiative heating, the IR passes through the less dense atmosphere and isn't backscattered - that is the reason why you can use such an extension in the first place on a US engine. Duh.

Perhaps you are confusing radiative and convective effects?

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For any sort of tail-first lifting entry (like the F9 S1) there will be asymmetrical aerodynamic loads on the nozzle extension, which WILL cause bending in the extension with resulting tension and compression loads.
Never said lifting. Not, repeat, not like the F9 S1. This is a mostly drag, partially propulsive entry. It is entirely in the least dense atmosphere. Irrelevant to a booster profile. Also, it is a much higher deceleration rate.

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A tail-first ballistic entry might be able to keep the loads on the extension in tension only, but at the cost of much higher peak heating rates and drag loads.
Yes.

add:

Have not given much thought to what happens to the extension post transonic, because the point was to "land the F9US". Yes, atmospheric effects in the lower atmosphere could tear it to shreds tail first, which either that or a tumble will happen given CG.

Didn't care because the point was to get the US to survive en masse to where it could be snagged by another mechanism.

You might be able to stabilize the stage post engine fire nozzle rearward and avoid damage. Passively/actively. Above dense atmosphere. If you care. That wasn't the point here. Like Mars. Mars doesn't have high density atmosphere.

The point is using high altitude, high iSP to recover a US. And I can't "understand it for you" if you choose to use the wrong profile to evaluate the approach. Again, it is not a booster profile. Duh, its a US.
« Last Edit: 07/30/2016 08:14 PM by Space Ghost 1962 »

Online envy887

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Re: Landing an MVac
« Reply #11 on: 07/31/2016 02:07 AM »

Couple things:
The extension is radiatively cooled; radiating heat into the exhaust plume and surrounding opaque plasma during high velocity entry probably won't work well at all.
Consider flow rate and the fact that the entry heating is far hotter than exhaust. Also, with radiative heating, the IR passes through the less dense atmosphere and isn't backscattered - that is the reason why you can use such an extension in the first place on a US engine. Duh.

That's why you can't radiate heat into the plasma wake on entry. It works on the way up because it can radiate to the near absolute zero background of space.

Regen cooling will work, but that means the engine has to be running the whole time to keep pumping cold fuel though the nozzle.

Online envy887

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Re: Landing an MVac
« Reply #12 on: 07/31/2016 02:17 AM »
..
You might be able to stabilize the stage post engine fire nozzle rearward and avoid damage. Passively/actively. Above dense atmosphere. If you care. That wasn't the point here. Like Mars. Mars doesn't have high density atmosphere.

The point is using high altitude, high iSP to recover a US. And I can't "understand it for you" if you choose to use the wrong profile to evaluate the approach. Again, it is not a booster profile. Duh, its a US.

Lifting is still important to reducing terminal velocity, although that may not be necessary for Earth entry.

Mars also doesn't have enough atmosphere to avoid going splat during a ballistic entry, so depending how much Delta v you want to expend, lift might be necessary. Flow separation probably isn't an issue in the thin atmosphere though.

Offline IainMcClatchie

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Re: Landing an MVac
« Reply #13 on: 07/31/2016 03:22 AM »
Terminal velocities vs altitude for a 4500 kg, 3.66 m diameter Cd=1 vehicle.  The stage doesn't have Cd=1 but it will be close enough for this to be a guide.

5 km    736 g/m^3  101 m/s
10 km  413 g/m^3  135 m/s
15 km  194 g/m^3  196 m/s
20 km    88 g/m^3  293 m/s
25 km    40 g/m^3  432 m/s

Helicopter recovery is pretty close to sea level:
CH-53E payload: 14,500 kg
CH-53E service ceiling: 5640 m, 686 g/m^3

Airplane recovery is just a bit better:
C-17 payload: 77,520 kg
C-17 service ceiling: 13716 m, 237 g/m^3

An unmodified MVac will not be firing during capture conditions.  But capture of the upper stage while under a parachute seems plausible.  So that leaves a question of whether the engine can get the stage down to velocities compatible with a parachute.

The erratic gusts of exhaust from the first stage during entry retroburn do not suggest static forces on the engine bell.  This seems like a killer.

Jim, sigh, your answer doesn't seem to address my proposal.  Running the turbopump richer will produce more turbopump exhaust for a given amount of thrust.  I was wondering if there is any guidance available on how much turbopump exhaust would be needed to make the MVac's thrust vector stable in a higher-pressure environment.

Offline Jim

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Re: Landing an MVac
« Reply #14 on: 07/31/2016 04:08 PM »
Jim, sigh, your answer doesn't seem to address my proposal.  Running the turbopump richer will produce more turbopump exhaust for a given amount of thrust.

Sign, still not true.  Turbopump output and thrust are interlinked.  Changing any parameter for a given engine is going to affect thrust.  Adding more fuel (mass) to the gas generator (they are burned in the GG and not the turbopump) is going to affect turbopump output, which directly affects thrust.  Can't change one without affecting the other.   
« Last Edit: 07/31/2016 04:17 PM by Jim »

Offline Jim

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Re: Landing an MVac
« Reply #15 on: 07/31/2016 04:15 PM »
Look which fluid is used to "throttle" the gas generator and hence the whole engine
« Last Edit: 07/31/2016 04:16 PM by Jim »

Offline Jcc

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Re: Landing an MVac
« Reply #16 on: 07/31/2016 05:27 PM »
Terminal velocities vs altitude for a 4500 kg, 3.66 m diameter Cd=1 vehicle.  The stage doesn't have Cd=1 but it will be close enough for this to be a guide.

5 km    736 g/m^3  101 m/s
10 km  413 g/m^3  135 m/s
15 km  194 g/m^3  196 m/s
20 km    88 g/m^3  293 m/s
25 km    40 g/m^3  432 m/s

Helicopter recovery is pretty close to sea level:
CH-53E payload: 14,500 kg
CH-53E service ceiling: 5640 m, 686 g/m^3

Airplane recovery is just a bit better:
C-17 payload: 77,520 kg
C-17 service ceiling: 13716 m, 237 g/m^3

An unmodified MVac will not be firing during capture conditions.  But capture of the upper stage while under a parachute seems plausible.  So that leaves a question of whether the engine can get the stage down to velocities compatible with a parachute.

The erratic gusts of exhaust from the first stage during entry retroburn do not suggest static forces on the engine bell.  This seems like a killer.

Jim, sigh, your answer doesn't seem to address my proposal.  Running the turbopump richer will produce more turbopump exhaust for a given amount of thrust.  I was wondering if there is any guidance available on how much turbopump exhaust would be needed to make the MVac's thrust vector stable in a higher-pressure environment.

How about parachute helicopter recovery? Coming in from orbital velocity, it is going to need TPS, a reentry burn would take too much fuel. So, according to your table above, it will slow to a terminal velocity in the atmosphere that is well within parachute range, then slows to within capture range, without presenting a danger to the helicopter crew. This could happen near the California coast, to avoid overflying land, and the helicopter either brings it in to land or lowers it onto a barge in case the helicopter needs to conserve fuel.

Offline IainMcClatchie

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Re: Landing an MVac
« Reply #17 on: 08/01/2016 11:45 AM »
Well now THAT's interesting Jim.  Thank you.

They throttle the gas generator with fuel.  The GG is run rich, so that means the heat output in the GG's burner is basically constant, and they throttle it by changing between low temp, high volume and high temp, low volume.  I suppose it's a lot easier to throttle the non-cyrogenic fluid.  And a given change to turbine output comes from a much larger change in fuel flow than oxygen flow, so that probably helps get finer control.

Okay, I can see now that you can't use the existing GG throttle to get more GG exhaust volume with lower turbine output.  Thanks.

What about the GG exhaust injection enabling a clean, predictable flow separation from the main engine nozzle walls?  Does that work?  In fact, does it work with the existing GG flow?  The GG exhaust in the nozzle must be subsonic, all the way from injection to exit, right?  So then doesn't that suggest that at higher ambient pressure, the GG exhaust plume will exit at lower velocity and larger cross section, which is pretty much exactly what you'd want in order to squeeze the main flow into a smaller exit cross section?

The F1 engine was overexpanded, but at takeoff it's exhaust plume got bigger as it exited the engine, for about 20 feet before it bent around and got smaller again.  Clearly no separation from the GG exhaust there.  In fact, it looks like around 20 feet after the end of the engine bell, the GG exhaust mixes pretty well with the core gas because the plume gets a lot brighter.

The even more overexpanded SSME exhaust plume at takeoff was decreasing in size right out of the engine bell.  At the 0:16 second mark in this video you can see the flow separation edge getting pushed out of the engine during throttle-up, and you can see how the instability of that separation causes the bell to bend and wiggle.



I wasn't able to find any videos of overexpanded engines with GG exhaust injection at partial throttle.  It would be interesting to see if they shook or were stable.

Offline Jim

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Re: Landing an MVac
« Reply #18 on: 08/01/2016 02:53 PM »
Well now THAT's interesting Jim.  Thank you.

They throttle the gas generator with fuel.  The GG is run rich, so that means the heat output in the GG's burner is basically constant, and they throttle it by changing between low temp, high volume and high temp, low volume.  I suppose it's a lot easier to throttle the non-cyrogenic fluid.  And a given change to turbine output comes from a much larger change in fuel flow than oxygen flow, so that probably helps get finer control.


No,  they throttle by going from low temp, low pressure and low volume to high temp, high pressure and high volume. GG temp, pressure and mass flow increase with increased fuel. 

It has nothing to with the cryogenic properties. It is because it is the fuel. 


What about the GG exhaust injection enabling a clean, predictable flow separation from the main engine nozzle walls?


No.  The CG exhaust has little to do with flow separation from the main engine nozzle walls.  The main flow from the combustion chamber determines that.  GG exhaust injection is for nozzle extension cooling and nothing more.  It just provides a layer of cooler gas between extension and main chamber flow.
« Last Edit: 08/01/2016 03:02 PM by Jim »

Offline IainMcClatchie

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Re: Landing an MVac
« Reply #19 on: 08/01/2016 06:14 PM »
Something subtle is going on.

The oxygen to the GG doesn't have a throttle valve on it.  It'll get a constant fraction of the total oxidizer flow.

The fuel to the GG has a throttle on it.  At part-throttle setting, it'll get a smaller fraction of the total fuel flow than at full throttle setting.  Right?

So, opening the throttle causes the GG mixture to become more rich.  It's always on the rich side of stoichiometric, so the richer it gets, the lower temperature the GG turbine inlet is.  The more open the throttle is, the cooler the GG turbine inlet is.

Here's where I'm confused.  If you open the fuel valve and dilute the GG gas stream with more fuel, you drop the turbine inlet temperature.  I think that the absolute turbine outlet temperature is some constant fraction of the absolute turbine inlet temperature.  So that implies that the increased mass flow through the turbine essentially cancels the decreased temperature drop through the turbine, and you get no change in power.  Or at least, the power change is a second-order effect that stems from a difference in Cp/Cv at different temperatures.  The gain on that will be very low.

This just doesn't seem right at all, so maybe an earlier assumption is wrong.  Does the GG run on the lean side of stoich on the Merlin 2?  I'm quite confident Merlin 1A-D are fuel-rich, I know good reasons for that.  I can't think of a reason to run lean of stoich.

There is a positive feedback loop around the GG throttle, because when you open the throttle a bit, the GG produces more gas volume, which increases the turbine power output, which increases the pump pressure, which increases the fuel and oxidizer flow to both the main chamber and the GG, and that increases the GG gas volume which is positive feedback.

So maybe the instantaneous gain being low is okay because the feedback loop is positive, converging, and really fast?  Once again, this seems crazy for an engine which must be finely and quickly adjusted during landing.

And there is one other thing.  CRS-6 came down hard because of a "stiction in the biprop throttle valve, resulting in control system phase lag" (Elon Musk).  This suggests they are throttling both LOX and fuel to something, I suppose the gas generator.  So that makes it seem like the diagram you have, which is for the Merlin 2, is just a different control system than for the Merlin 1.


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