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#300
by
mlindner
on 30 Sep, 2013 08:39
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Which would cause a sudden pitch, which would be detected and immediately countered by the control system. Something that long, with that much aerodynamic pressure can't rotate that fast.
The center of pressure for a vehicle is typically above the center of gravity, making vehicles inherently aerodynamically unstable. This sudden pitch and angle-of-attack increase can spell disaster if it happens at max-Q. Having each individual engine thrust if not through the CoG, at least *closer* to the CoG does improve controlability. If all vehicles were capable of correcting attitude disturbances that promptly, I assume they also wouldn't be worrying about things like wind shear at altitude, etc.
There is a good reason they did that on v1.0. Heck, even a Zenit vectors out the individual nozzles of the RD-171 immediately after liftoff, despite having only one engine.
Out of curiosity where did you hear they did it on v1.0. Before you mentioned it here, that was the first I'd heard of it.
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#301
by
R7
on 30 Sep, 2013 08:47
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And why would Zenit do it with only one engine?? It doesn't have "chamber-out" capability.
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#302
by
ugordan
on 30 Sep, 2013 08:59
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Out of curiosity where did you hear they did it on v1.0. Before you mentioned it here, that was the first I'd heard of it.
It's hard to see without closeup shots of the engine, see for example the CRS-2 webcast at the 38:40 mark: www
.youtube.com/watch?v=ik0ElKl5kW4#t=2320
And why would Zenit do it with only one engine?? It doesn't have "chamber-out" capability.
I have *no* idea. Makes little sense to me as well.
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#303
by
ugordan
on 30 Sep, 2013 09:02
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Net vector would change even if you were pointing all engine through CG so you have to do TVC to point it towards intended trajectory.
Yes, in one case you have a sideways movement component on the vehicle, in the other you have sideways and a torque imparted until you correct for it. I'm not going to argue any more that these two are the same thing.
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#304
by
mlindner
on 30 Sep, 2013 09:17
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Out of curiosity where did you hear they did it on v1.0. Before you mentioned it here, that was the first I'd heard of it.
It's hard to see without closeup shots of the engine, see for example the CRS-2 webcast at the 38:40 mark: www.youtube.com/watch?v=ik0ElKl5kW4#t=2320
And why would Zenit do it with only one engine?? It doesn't have "chamber-out" capability.
I have *no* idea. Makes little sense to me as well.
Yes I knew about the angling, but is it really for engine out purposes? I think they're mounted that way, not gimbaled to that. Shuttle has each engine angled as well.
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#305
by
R7
on 30 Sep, 2013 09:21
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It's hard to see without closeup shots of the engine, see for example the CRS-2 webcast at the 38:40 mark: www.youtube.com/watch?v=ik0ElKl5kW4#t=2320
That's T-1 minute, is it before or after TVC check? Need to see situation at T-0.
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#306
by
ugordan
on 30 Sep, 2013 09:25
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#307
by
Jim
on 30 Sep, 2013 11:12
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There's no need to point all (or any for that matter) engines through CG in any cluster, only resultant net force vector.
That is a standard practice for many, if not all launch vehicles with multiple engines.
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#308
by
LouScheffer
on 30 Sep, 2013 11:31
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And why would Zenit do it with only one engine?? It doesn't have "chamber-out" capability.
On vehicles with RCS, aligning the thrust vector with the CG makes the RCS use less fuel. I have no idea if this is why Zenit does it.
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#309
by
kevin-rf
on 30 Sep, 2013 11:45
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You don't have to have an engine failure to benefit from pointing the engines through the CG. If one of the engines is under/over performing or running rough, the RCS will not need to counter it.
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#310
by
cambrianera
on 30 Sep, 2013 12:08
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I have a possible explanation for the roll experienced by the first stage the way down. When the vehicle is subsonic, the air impinging on the bottom of the stage must escape sideway to return in the airstream.
The passage between nozzles is partially obstructed by the fuel lines, carrying fuel to the nozzle; this way air is forced to have a swirling motion, causing roll.
In pic 1 is clearly visible the fuel pipe, pic 2 is a reminder of the relative positions of the engines, finally the sketch explain the movement of air.
When the stage is still supersonic, the shock wave from the nozzles should shield the bottom minimizing this effect.
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#311
by
corrodedNut
on 30 Sep, 2013 13:15
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Have we seen this video yet?
Thanks for posting! Mighty interesting stuff going on after stage separation....
Great video, here's my take:
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#312
by
R7
on 30 Sep, 2013 13:18
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There's no need to point all (or any for that matter) engines through CG in any cluster, only resultant net force vector.
That is a standard practice for many, if not all launch vehicles with multiple engines.
Proton can't even gimbal first stage engines along axis required to track CG.
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#313
by
ugordan
on 30 Sep, 2013 13:20
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I wonder about those puffs. If they were cold gas (He or N2) thrusters, I'm not sure we'd be seeing them. Could the ACS have been rigged to use the GOX from the tank boiloff instead?
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#314
by
corrodedNut
on 30 Sep, 2013 13:26
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I wonder about those puffs. If they were cold gas (He or N2) thrusters, I'm not sure we'd be seeing them. Could the ACS have been rigged to use the GOX from the tank boiloff instead?
I think we got lucky with the lighting. I'm guessing these things are the source:
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#315
by
cuddihy
on 30 Sep, 2013 13:43
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I have a possible explanation for the roll experienced by the first stage the way down. When the vehicle is subsonic, the air impinging on the bottom of the stage must escape sideway to return in the airstream.
The passage between nozzles is partially obstructed by the fuel lines, carrying fuel to the nozzle; this way air is forced to have a swirling motion, causing roll.
In pic 1 is clearly visible the fuel pipe, pic 2 is a reminder of the relative positions of the engines, finally the sketch explain the movement of air.
When the stage is still supersonic, the shock wave from the nozzles should shield the bottom minimizing this effect.
In that case I would think just cheap aero "baffles" oriented to redirect half of the air flow the other direction could entirely eliminate the torque... if that's the cause after all.
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#316
by
cambrianera
on 30 Sep, 2013 13:48
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I have a possible explanation for the roll experienced by the first stage the way down. When the vehicle is subsonic, the air impinging on the bottom of the stage must escape sideway to return in the airstream.
The passage between nozzles is partially obstructed by the fuel lines, carrying fuel to the nozzle; this way air is forced to have a swirling motion, causing roll.
In pic 1 is clearly visible the fuel pipe, pic 2 is a reminder of the relative positions of the engines, finally the sketch explain the movement of air.
When the stage is still supersonic, the shock wave from the nozzles should shield the bottom minimizing this effect.
In that case I would think just cheap aero "baffles" oriented to redirect half of the air flow the other direction could entirely eliminate the torque... if that's the cause after all.
Agreed, some aero baffles integrated into the bottom panels.
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#317
by
Ben the Space Brit
on 30 Sep, 2013 14:04
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@ CorrodedNut,
My inner Firefly geek is coming out. Instead of 'full bloom', I briefly thought you'd written 'full burn'.
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#318
by
guckyfan
on 30 Sep, 2013 14:39
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Agreed, some aero baffles integrated into the bottom panels.
Sounds good and involves no additional drag on the way up.
This theory requires that the additional spin came up only when the stage was falling already at a relatively low speed. So SpaceX should know.
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#319
by
simonbp
on 30 Sep, 2013 14:52
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It depends on how cold the gas is and how moist the air is. If they just using LN2 for the propellant, the exhaust could be plenty cold enough to precipitate ice crystals an produce a little cloud.