Author Topic: Landing an MVac  (Read 12173 times)

Offline IainMcClatchie

  • Full Member
  • **
  • Posts: 261
  • San Francisco Bay Area
  • Liked: 142
  • Likes Given: 241
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.

Online 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

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2798
  • Liked: 448
  • Likes Given: 383
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

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31362
  • Cape Canaveral Spaceport
  • Liked: 9635
  • Likes Given: 299
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

  • Full Member
  • ****
  • Posts: 740
  • Liked: 175
  • Likes Given: 221
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.

Online Space Ghost 1962

  • Senior Member
  • *****
  • Posts: 2485
  • Whatcha gonna do when the Ghost zaps you?
  • Liked: 2384
  • Likes Given: 1746
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!

Quote
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.

Quote
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? ;)

Quote
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

  • Full Member
  • ****
  • Posts: 800
  • Liked: 143
  • Likes Given: 148
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

  • Member
  • Posts: 71
  • Kyiv
  • Liked: 14
  • Likes Given: 6
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.

Online Space Ghost 1962

  • Senior Member
  • *****
  • Posts: 2485
  • Whatcha gonna do when the Ghost zaps you?
  • Liked: 2384
  • Likes Given: 1746
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

  • Senior Member
  • *****
  • Posts: 2774
  • Liked: 1280
  • Likes Given: 806
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.

Online Space Ghost 1962

  • Senior Member
  • *****
  • Posts: 2485
  • Whatcha gonna do when the Ghost zaps you?
  • Liked: 2384
  • Likes Given: 1746
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?

Quote
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.

Quote
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

  • Senior Member
  • *****
  • Posts: 2774
  • Liked: 1280
  • Likes Given: 806
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

  • Senior Member
  • *****
  • Posts: 2774
  • Liked: 1280
  • Likes Given: 806
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

  • Full Member
  • **
  • Posts: 261
  • San Francisco Bay Area
  • Liked: 142
  • Likes Given: 241
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

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31362
  • Cape Canaveral Spaceport
  • Liked: 9635
  • Likes Given: 299
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

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31362
  • Cape Canaveral Spaceport
  • Liked: 9635
  • Likes Given: 299
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 »

Online Jcc

  • Full Member
  • ****
  • Posts: 818
  • Liked: 179
  • Likes Given: 77
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

  • Full Member
  • **
  • Posts: 261
  • San Francisco Bay Area
  • Liked: 142
  • Likes Given: 241
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

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31362
  • Cape Canaveral Spaceport
  • Liked: 9635
  • Likes Given: 299
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

  • Full Member
  • **
  • Posts: 261
  • San Francisco Bay Area
  • Liked: 142
  • Likes Given: 241
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.


Offline Jim

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31362
  • Cape Canaveral Spaceport
  • Liked: 9635
  • Likes Given: 299
Re: Landing an MVac
« Reply #20 on: 08/01/2016 06:40 PM »
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.


No, they are the same engine as far as that.  The differences are in the skirt extension and restart hardware.

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.

....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.


No, the CG valves (notice the "S" and the "bi", fuel is the control but ox is adjusted to keep about same mixture ratio) would modulate that and prevent the feedback.
« Last Edit: 08/01/2016 06:58 PM by Jim »

Offline IainMcClatchie

  • Full Member
  • **
  • Posts: 261
  • San Francisco Bay Area
  • Liked: 142
  • Likes Given: 241
Re: Landing an MVac
« Reply #21 on: 08/01/2016 11:03 PM »
Okay, so both the RP-1 and LOX are modulated by throttle valves on the way into the gas generator.  That means that during the landing burn it would be possible to run the gas generator richer than during the ascent burn, so as to increase the amount of turbopump exhaust volume injected into the engine bell for a given amount of turbopump power.  Well, unless something is going to break from running at lower temperature (like maybe gas generator ignition?)

But I haven't found any support for the notion that injected turbopump exhaust might encourage main combustion chamber gas to separate cleanly from the side of an overexpanded nozzle.

Offline Jim

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31362
  • Cape Canaveral Spaceport
  • Liked: 9635
  • Likes Given: 299
Re: Landing an MVac
« Reply #22 on: 08/02/2016 12:53 AM »
Okay, so both the RP-1 and LOX are modulated by throttle valves on the way into the gas generator.  That means that during the landing burn it would be possible to run the gas generator richer than during the ascent burn, so as to increase the amount of turbopump exhaust volume injected into the engine bell for a given amount of turbopump power.  Well, unless something is going to break from running at lower temperature (like maybe gas generator ignition?)

No, changing the mass flow in the gas generator is going change turbo pump output.  There is no way of changing GG output without affecting the turbo pump.

And the mass flow in the nozzle has little effect on flow attachment. 


Offline IainMcClatchie

  • Full Member
  • **
  • Posts: 261
  • San Francisco Bay Area
  • Liked: 142
  • Likes Given: 241
Re: Landing an MVac
« Reply #23 on: 08/02/2016 09:53 PM »
Uh oh, maybe there is something super basic that I'm missing.

Two identical turbines.  Both have the same mass flow entering them.  One gas stream is hotter than the other.  Output ambient pressure is equal.

The hotter turbine will produce more output power on the shaft, right?

Offline CameronD

  • Full Member
  • ****
  • Posts: 1247
  • Melbourne, Australia
    • Norton Consultants
  • Liked: 399
  • Likes Given: 301
Re: Landing an MVac
« Reply #24 on: 08/03/2016 04:03 AM »
Two identical turbines.  Both have the same mass flow entering them.  One gas stream is hotter than the other.  Output ambient pressure is equal.

Since no-one else has chipped in, I'll have a go.. :)

IIRC (and someone please correct me if I'm wrong), mass flow of a gas is related to volume flow rate, density and temperature so the only way to get a condition where the mass flow is the same but one stream is hotter is to reduce the density and/or the volume flow rate of the gas stream to match.  Eg. In a combustion situation, hotter generally means more fuel input and higher density, but to move away from stoich means less power.. so to answer your question:

The hotter turbine will produce more output power on the shaft, right?

AIUI, no, because for the same mass flow rate input the gas output would have to be slower.
   
« Last Edit: 08/03/2016 11:48 PM by CameronD »
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline IainMcClatchie

  • Full Member
  • **
  • Posts: 261
  • San Francisco Bay Area
  • Liked: 142
  • Likes Given: 241
Re: Landing an MVac
« Reply #25 on: 08/03/2016 04:12 AM »
Two identical turbines.  Both have the same mass flow entering them.  Exit pressure is the same.  Let's say the torque load on the shaft is the same.  Turbine A has hotter gas.  That's enough to determine the rest of the conditions.

As you say, PV=nRT.  Turbine A's incoming stream has larger volume.  Output stream will have larger volume.  Shaft output power will be larger, because the shaft will be turning faster.

Right?

Offline Dante80

  • Full Member
  • ****
  • Posts: 879
  • Athens : Greece
  • Liked: 791
  • Likes Given: 499
Re: Landing an MVac
« Reply #26 on: 08/03/2016 05:58 AM »
A strictly hypothetical question. It has been said above that the nozzle extension for MVac is very fragile and would shatter at re-entry. Also, the very large expansion ratio doesn't help with downthrottling stability.

What would be the hypothetical result if the radiative nozzle extension was dropped before re-entry happened?

Offline IntoTheVoid

  • Full Member
  • **
  • Posts: 201
  • USA
  • Liked: 137
  • Likes Given: 53
Re: Landing an MVac
« Reply #27 on: 08/03/2016 06:07 AM »
(not a rocket scientist)
Two identical turbines.  Both have the same mass flow entering them.  One gas stream is hotter than the other.  Output ambient pressure is equal.

The hotter turbine will produce more output power on the shaft, right?

Why would you presume they have the same mass flow, rather than volume?
Why would you presume the output pressure is equal?
How would the pump be 'generating' power, rather than the gas generator?
What I see is the gas generator driving a common shaft to a fuel pump and oxidizer pump, which may not be sized identically. The fuel line then goes through a trim valve and then a MFV which I presume to be a Main Fuel Valve. The oxidizer line then goes to a MOV, which I presume to be a Main Oxidizer Valve.
The pressure in the lines would be based on all of these things, but not necessarily equal in mass, volume or pressure. The only thing I see required to be equal between fuel and oxidizer is the pump shaft rotational rate.

As you say, PV=nRT.  Turbine A's incoming stream has larger volume.  Output stream will have larger volume.  Shaft output power will be larger, because the shaft will be turning faster.

How would the shaft be turning faster than itself?
I could be way off base, but I don't see where you're getting any of your assumptions from the diagram.

Offline Jim

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31362
  • Cape Canaveral Spaceport
  • Liked: 9635
  • Likes Given: 299
Re: Landing an MVac
« Reply #28 on: 08/03/2016 03:57 PM »
Two identical turbines.  Both have the same mass flow entering them.  Exit pressure is the same.  Let's say the torque load on the shaft is the same.  Turbine A has hotter gas.  That's enough to determine the rest of the conditions.

As you say, PV=nRT.  Turbine A's incoming stream has larger volume.  Output stream will have larger volume.  Shaft output power will be larger, because the shaft will be turning faster.

Right?


Can't say exit pressure is the same.  Work was done. 

Still don't see what you are getting at?  There isn't a way to separate GG output from thrust level.  If you want to prevent flow separation from a large nozzle at lower thrust level, a secondary source, separate from the GG is going to have to supply the fluid.  And that still is not likely going to solve the issue, since the secondary fluid injection is going to generate thrust, further exasperating the issue.
« Last Edit: 08/03/2016 04:04 PM by Jim »

Offline IainMcClatchie

  • Full Member
  • **
  • Posts: 261
  • San Francisco Bay Area
  • Liked: 142
  • Likes Given: 241
Re: Landing an MVac
« Reply #29 on: 08/03/2016 08:08 PM »
I'm trying to strip out as many things as possible so we can get to something that we agree on.  IntoTheVoid, that means I'm comparing two different operating points for the turbine that powers the pumps.  I'm not comparing the two pumps.

If both turbines are exhausting to the same ambient, then exit pressures are the same.  Right?

Now I have two turbines producing different shaft power with the same mass flow, because one is running hotter than the other.  Right?

And that means I could reduce the mass flow for the hotter one to make the shaft power the same.  And then I'd have two turbines with the same shaft power emitting different mass flows of gas.  One emits less mass at higher temperature and lower volume.  This demonstrates that, by actuating the turbopump's LOX and RP-1 valves separately, we can vary the GG output mass rate for a fixed turbopump shaft power.  We don't have full freedom, because the GG can't get too hot, but we can go some amount in the other direction and run the GG with more mass flow and lower temperature.

That's not even my main point, that's just a lemma that I figured everyone would agree on because it was obvious.  I've been surprised by how hard it is to communicate.  Sorry about not being more clear earlier.

So then once I can vary the mass flow from the GG into the main engine bell, my question was around how much control I can actually get from this.  The GG exhaust in the bell is subsonic, so it will decelerate and increase in pressure as it expands towards the exit of the bell.  The F-1 engine pictures show that it's possible to get the GG exhaust well past the exit of the bell before it mixes with the main flow.  It seems like this might be a way to make a variable expansion nozzle for a GG type engine.

Offline Jim

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31362
  • Cape Canaveral Spaceport
  • Liked: 9635
  • Likes Given: 299
Re: Landing an MVac
« Reply #30 on: 08/03/2016 08:19 PM »

1.  And that means I could reduce the mass flow for the hotter one to make the shaft power the same.  And then I'd have two turbines with the same shaft power emitting different mass flows of gas.  One emits less mass at higher temperature and lower volume. 

2.  This demonstrates that, by actuating the turbopump's LOX and RP-1 valves separately, we can vary the GG output mass rate for a fixed turbopump shaft power.  We don't have full freedom, because the GG can't get too hot, but we can go some amount in the other direction and run the GG with more mass flow and lower temperature.


1.  Not necessarily.  It is not automatic.  Turbine design has a play in it.

2.  And no, there are other issues with using more fuel such as coking and sustain combustion. 
« Last Edit: 08/03/2016 08:30 PM by Jim »

Offline Jim

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31362
  • Cape Canaveral Spaceport
  • Liked: 9635
  • Likes Given: 299
Re: Landing an MVac
« Reply #31 on: 08/03/2016 08:20 PM »
  The F-1 engine pictures show that it's possible to get the GG exhaust well past the exit of the bell before it mixes with the main flow. .

No, it isn't mixing with the main flow, it is mixing with ambient air.  That is why it ignites.

  The GG exhaust in the bell is subsonic, so it will decelerate and increase in pressure as it expands towards the exit of the bell.

It is decreasing in pressure, that is what the bell does.


So then once I can vary the mass flow from the GG into the main engine bell, my question was around how much control I can actually get from this.

Little to nothing.  It is a small fraction of the engine output and adjusting it as you propose will have little effect on the overall engine thrust
« Last Edit: 08/03/2016 08:28 PM by Jim »

Offline IainMcClatchie

  • Full Member
  • **
  • Posts: 261
  • San Francisco Bay Area
  • Liked: 142
  • Likes Given: 241
Re: Landing an MVac
« Reply #32 on: 08/04/2016 12:03 AM »
Uh... are you just pulling my leg?

The shear between the subsonic GG exhaust (<500 m/s) and the main flow in the F-1 (>2600 m/s) is vastly greater than the shear between that GG exhaust and the ambient air.  Mixing will be greater on the side with more shear.  This ain't subtle.

And, supersonic gases accelerate when the flow cross section expands.  Subsonic gases decelerate.  That's why rocket nozzles coverge and then diverge.  Now you could tell me that the GG exhaust is injected into the bell supersonically, and I'd be surprised, but I suppose it's possible.  Short of that, though... I'm starting to think we're not actually having the conversation I was hoping to have.

That's too bad.  Oh well.

Offline CameronD

  • Full Member
  • ****
  • Posts: 1247
  • Melbourne, Australia
    • Norton Consultants
  • Liked: 399
  • Likes Given: 301
Re: Landing an MVac
« Reply #33 on: 08/04/2016 02:10 AM »
.........
And, supersonic gases accelerate when the flow cross section expands.  Subsonic gases decelerate.  That's why rocket nozzles coverge and then diverge.  Now you could tell me that the GG exhaust is injected into the bell supersonically, and I'd be surprised, but I suppose it's possible.  Short of that, though... I'm starting to think we're not actually having the conversation I was hoping to have.

That's too bad.  Oh well.

Well, it's an interesting conversation nonetheless.. so keep it up. :)

If Jim is saying what I think he is, ISTM, in summary, that (a) you can't achieve what you're hoping to achieve without a complete re-design of the engine plumbing and (b) the GG exhaust isn't going to cut it (pardon the pun).

How about other proposals - like installing a cluster of smaller nozzles (say 4?) around the outside of the main nozzle and using those for landing instead?
« Last Edit: 08/04/2016 02:11 AM by CameronD »
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline Jim

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31362
  • Cape Canaveral Spaceport
  • Liked: 9635
  • Likes Given: 299
Re: Landing an MVac
« Reply #34 on: 08/04/2016 02:38 AM »

The shear between the subsonic GG exhaust (<500 m/s) and the main flow in the F-1 (>2600 m/s)

Where do you get these numbers?
Mixing will be greater on the side with more shear.

Based on what principle?

Uh... are you just pulling my leg?

No, pressure decreases from the throat to the nozzle exit.

  Now you could tell me that the GG exhaust is injected into the bell supersonically,

It is injected at a point that it is at a higher pressure than in the nozzle at that point.   And the flow stays along the nozzle extension to cool it.  Look at the Merlin diagram.  Turbopump exhaust is less than 100 psi and the thrust chamber is at over 1400 psi.
« Last Edit: 08/04/2016 02:46 AM by Jim »

Offline Jim

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31362
  • Cape Canaveral Spaceport
  • Liked: 9635
  • Likes Given: 299
Re: Landing an MVac
« Reply #35 on: 08/04/2016 02:49 AM »
I'm starting to think we're not actually having the conversation I was hoping to have.


Which is the GG output can't be controlled independently of thrust.  Nor can turbopump exhaust be used to prevent flow separation.

Look at the diagram again, the combustion chamber mass flow rate is over 3200 lb/sec and the GG is around 215, not even 7%.
« Last Edit: 08/04/2016 02:52 AM by Jim »

Offline IainMcClatchie

  • Full Member
  • **
  • Posts: 261
  • San Francisco Bay Area
  • Liked: 142
  • Likes Given: 241
Re: Landing an MVac
« Reply #36 on: 08/04/2016 08:43 PM »
The proposal isn't to prevent flow separation, but rather to cause flow separation in an overexpanded nozzle to be stable, primarily to keep the unstable flow from smashing the nozzle.  There might be Isp advantages but I'm doing so badly already I don't want to get into that.

The main flow in the F-1 nozzle at the exit plane has to be a bit above 2600 m/s, because the sea-level Isp is 263 seconds.  The F-1 is overexpanded at sea level, so there is some pressure loss.  There's also a some lower-speed GG flow at the periphery of the nozzle.  Both these effects imply that the flow of gas from the main combustion chamber mas to be over 2600 m/s at the bell exit face.  So that's where that number came from.

I guessed 500 m/s for the GG exhaust.  The maximum theoretical velocity for that stream would be something like 1100 m/s.  But it doesn't go through a simple converging-diverging nozzle.  The more complex duct is going to lose energy, and as I said I was assuming it was injected subsonically.

Mixing increases with increasing shear, but it's a scale-dependent thing.  Hayward-Gordon has a somewhat useful brief introduction.

Pressure decreases from the sonically-choked throat to the exit because the flow is supersonic.  If the flow was subsonic and diverging, pressure would increase.  Obviously the boundary layer flow along the bell has to be subsonic, but I think the boundary layer has huge viscous losses that eliminate any pressure increase.

Online Space Ghost 1962

  • Senior Member
  • *****
  • Posts: 2485
  • Whatcha gonna do when the Ghost zaps you?
  • Liked: 2384
  • Likes Given: 1746
Re: Landing an MVac
« Reply #37 on: 08/04/2016 09:09 PM »
First - don't try to land (lower atmosphere) a US on a MVac. Not at all what its designed for.

Stay only in the "nonsensible" atmosphere. Because that's as low as the US can go, as a propulsion system.

Second - the majority of counter force / "thrust" and drag comes from stagnation pressure and engine operation.

This increases the density and size of the wake surrounding the stage, inflating it like a balloon. Which is torn apart in vortices, which carry away momentum, hopefully above the stage.

Extreme deceleration is the only point of this. You cannot get anything more then what's described here, and you're minimum cost is the props to decelerate with drag and thrust down to transonic. Then, it becomes a different problem.

In this environment, there is only having the drag of a huge wake, and the counter force of the momentum transferred to the combustion products which form the enormous, flattened, wake, which dissipates the total flux.

The transient of EI is where the mechanical effect, first only thermally, comes into play. One leverages the density increase against the flattened plume to transfer momentum through high velocity combustion products for maximum efficiency.

This isn't improved with a mechanism in any way. Only way it can get better is ... iSP. And more props for recovery. 

So stop messing with the engine.

Offline drzerg

  • Member
  • Posts: 71
  • Kyiv
  • Liked: 14
  • Likes Given: 6
Re: Landing an MVac
« Reply #38 on: 03/17/2017 04:37 AM »
i just do not want to make another topic so this seems close enough.

what if reuse just most valuable part of the 2 stage - engine? it has compact size so no need to big heat shield. it has integrated heat tolerant tail stabilizer so no need to worry about center of mass problems. it is light weighted (400 kg?) so no need for big shute an the end.  and even small helicopter could catch it in the air.   

Offline RotoSequence

  • Full Member
  • ****
  • Posts: 779
  • Liked: 572
  • Likes Given: 781
Re: Landing an MVac
« Reply #39 on: 03/17/2017 05:02 AM »
i just do not want to make another topic so this seems close enough.

what if reuse just most valuable part of the 2 stage - engine? it has compact size so no need to big heat shield. it has integrated heat tolerant tail stabilizer so no need to worry about center of mass problems. it is light weighted (400 kg?) so no need for big shute an the end.  and even small helicopter could catch it in the air.

My gut feeling is that it won't pass cost-benefit analysis. You're up against the cost of relatively inexpensive engines Vs flight rate Vs the cost of trained recovery crews, recovery equipment, recovery operations, inspection, re-certification, and re-integration of the engines. It would probably make more sense to re-design Stage 2 around the ITS paradigm than perform airborne helicopter recovery of second stage hardware.
« Last Edit: 03/17/2017 05:04 AM by RotoSequence »

Offline Kaputnik

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2798
  • Liked: 448
  • Likes Given: 383
Re: Landing an MVac
« Reply #40 on: 03/17/2017 07:36 AM »
i just do not want to make another topic so this seems close enough.

what if reuse just most valuable part of the 2 stage - engine? it has compact size so no need to big heat shield. it has integrated heat tolerant tail stabilizer so no need to worry about center of mass problems. it is light weighted (400 kg?) so no need for big shute an the end.  and even small helicopter could catch it in the air.

My gut feeling is that it won't pass cost-benefit analysis. You're up against the cost of relatively inexpensive engines Vs flight rate Vs the cost of trained recovery crews, recovery equipment, recovery operations, inspection, re-certification, and re-integration of the engines. It would probably make more sense to re-design Stage 2 around the ITS paradigm than perform airborne helicopter recovery of second stage hardware.

Another factor is where do you carry this out? Normally second stages are disposed of in remote locations, but in this case you'd presumably need to make a deorbit burn that aims the stage for your engine recovery location. You don't ideally want it to be raining COPVs on a populated area, but doing helicopter recovery somewhere remote is going to add to the operation costs.
Waiting for joy and raptor

Offline drzerg

  • Member
  • Posts: 71
  • Kyiv
  • Liked: 14
  • Likes Given: 6
Re: Landing an MVac
« Reply #41 on: 03/19/2017 09:10 AM »
possible solution is that after deorbit burn engine could detach from stage and make its own additional deorbit "burn" with cold gas from main nozzle. so they could be separated at the point of impact.

Offline Jim

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31362
  • Cape Canaveral Spaceport
  • Liked: 9635
  • Likes Given: 299
Re: Landing an MVac
« Reply #42 on: 03/19/2017 12:35 PM »
Not worth the effort

Offline Kaputnik

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2798
  • Liked: 448
  • Likes Given: 383
Re: Landing an MVac
« Reply #43 on: 03/21/2017 06:27 AM »
possible solution is that after deorbit burn engine could detach from stage and make its own additional deorbit "burn" with cold gas from main nozzle. so they could be separated at the point of impact.


So you're now aiming for a steeper and harsher entry with the engine. And you've just added three axis control to your engine too (or else how do you make sure the extra deorbit burn is in the right direction?).

If you wanted to make this work, it might be better to do the deorbit burn, jettison the engine, but have the stage remain active to perform additional manoeuvres- remember it already has the avionics, thrusters, and propellant. Have it enter short of the engine recovery zone, as its higher ballistic coefficient will tend to do that anyway.

I still don't know if you can get enough separation to ensure safety, though.
Waiting for joy and raptor

Tags: