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#220
by
Antares
on 29 Sep, 2013 21:22
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I'm contending it wasn't aero forces that caused the spin. Aero roll moment of a symmetric vehicle is tiny.
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#221
by
mlindner
on 29 Sep, 2013 21:22
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I'm contending it wasn't aero forces that caused the spin. Aero roll moment of a symmetric vehicle is tiny.
Bull. You're disagreeing with Elon directly. You're wrong. He has data, you don't. Pick your battles better.
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#222
by
clongton
on 29 Sep, 2013 21:23
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well, the Kerbel solution is fins. Aerodynamic control surfaces.
Too inefficient. Grid fins would be better. Better control, less mass.
Would also contribute to deceleration.
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#223
by
sublimemarsupial
on 29 Sep, 2013 21:25
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I'm contending it wasn't aero forces that caused the spin. Aero roll moment of a symmetric vehicle is tiny.
The vehicle isn't axisymmetric, and it is far from aerodynamically clean with the engine bells first.
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#224
by
Antares
on 29 Sep, 2013 21:25
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I'm contending it wasn't aero forces that caused the spin. Aero roll moment of a symmetric vehicle is tiny.
Bull. You're disagreeing with Elon directly. You're wrong. He has data, you don't. Pick your battles better.
I quit. Read my location. I don't trust twitter for my data. Nothing technical can be explained in a press conference sound bite.
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#225
by
sanman
on 29 Sep, 2013 21:26
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I'm contending it wasn't aero forces that caused the spin. Aero roll moment of a symmetric vehicle is tiny.
What about non-roll aero forces? Can axial aero forces amplify the miniscule aero roll forces to create sloshing that converts into much bigger roll and centrifugation for the fuel?
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#226
by
Kabloona
on 29 Sep, 2013 21:28
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Anyway, does it really matter what the source of the roll torque was? Either way, the ACS has a torque to counteract, and apparently it ran out of gas.
Soution: carry more ACS gas, no?
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#227
by
ensrettet
on 29 Sep, 2013 21:30
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I'm contending it wasn't aero forces that caused the spin. Aero roll moment of a symmetric vehicle is tiny.
Any non-roll moments will couple into the roll axis if the center of mass is not on the axial centerline of the rocket
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#228
by
R7
on 29 Sep, 2013 21:30
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1) Propellants well settled. Single engine starts. Heavy rotor spinning several thousand RPM in one direction.
2) All ACS gas gone to overcome rolling torque due to viscous and rolling friction forces inside the single-shaft turbopump transmitting roll torque to the housing and therefore the entire vehicle.
3) Roll moment imparted to the vehicle draws propellant out of the sumps, up the walls. Engine starved.
I'm not getting paid enough to explain this.
Then how come second stage managed to fly without ending up rolling wildly?
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#229
by
Ben the Space Brit
on 29 Sep, 2013 21:32
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1) Propellants well settled. Single engine starts. Heavy rotor spinning several thousand RPM in one direction.
2) All ACS gas gone to overcome rolling torque due to viscous and rolling friction forces inside the single-shaft turbopump transmitting roll torque to the housing and therefore the entire vehicle.
3) Roll moment imparted to the vehicle draws propellant out of the sumps, up the walls. Engine starved.
I'm not getting paid enough to explain this.
Then come come second stage managed to fly without ending up rolling wildly?
It's active roll control works so long as the main engine works.
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#230
by
cambrianera
on 29 Sep, 2013 21:32
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Recognizing cooked eggs from raw eggs is easy, you spin them on a table; if the egg is cooked the rotating movement easily transmits to the inside, giving some momentum to the rotation, if it's raw doesn't trasmit, and the rotation is short (only the eggshell rotates).
What has this to do with F9?
To get centrifuging of propellant it's not enough to spin the tanks, it's necessary to transmit the movement to the propellant inside.
This can't happen in a few second burn, had to begin before, probably from aero forces.
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#231
by
R7
on 29 Sep, 2013 21:34
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Then come come second stage managed to fly without ending up rolling wildly?
It's active roll control works so long as the main engine works.
Certainly, but how come it didn't get saturated during minutes of firing when first stage failed quickly.
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#232
by
ugordan
on 29 Sep, 2013 21:36
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1) Propellants well settled. Single engine starts. Heavy rotor spinning several thousand RPM in one direction.
2) All ACS gas gone to overcome rolling torque due to viscous and rolling friction forces inside the single-shaft turbopump transmitting roll torque to the housing and therefore the entire vehicle.
3) Roll moment imparted to the vehicle draws propellant out of the sumps, up the walls. Engine starved.
I'm not getting paid enough to explain this.
Then come come second stage managed to fly without ending up rolling wildly?
It's active roll control works so long as the main engine works.
Its only got ACS gas just as the 1st stage had. No more vectored roll nozzle on MVac.
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#233
by
Kabloona
on 29 Sep, 2013 21:37
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Then come come second stage managed to fly without ending up rolling wildly?
It's active roll control works so long as the main engine works.
Certainly, but how come it didn't get saturated during minutes of firing when first stage failed quickly.
Both stages have roll control thrusters. Stage 1 may have run out of ACS propellant on the way down.
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#234
by
aero
on 29 Sep, 2013 21:38
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After we get more information, if we do, we can solve this problem.
How much free space is inside the inner stage? W number of small, pop-out fins would work to control high speed aerodynamic induced spin. More RCS fuel would work, too. So would other types of fins. The trade is in mass, drag and complexity. IMO
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#235
by
rst
on 29 Sep, 2013 21:39
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well, the Kerbel solution is fins. Aerodynamic control surfaces.
Too inefficient. Grid fins would be better. Better control, less mass.
Would also contribute to deceleration.
They've already got landing legs to deploy (and a few suggestions that they might be part of a roll control strategy); would it make sense to try to attach control surfaces of some kind to these?
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#236
by
douglas100
on 29 Sep, 2013 21:44
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Remember when the roll control on the M-vac failed on the first F9 flight? The stage started to roll, but it took minutes to slowly build up. It was quite fast at shut down. I remember worrying about propellant sloshing or centrifuging at the time, but it made it to orbit. Something to consider in this discussion.
I don't think we know enough at this early stage to be certain what the cause or cure might be.
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#237
by
sanman
on 29 Sep, 2013 21:45
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They've already got landing legs to deploy (and a few suggestions that they might be part of a roll control strategy); would it make sense to try to attach control surfaces of some kind to these?
Wouldn't that mean deploying the legs earlier at higher velocity, and then also having to contend with any destabilizing effects they have on the stage?
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#238
by
sanman
on 29 Sep, 2013 21:47
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Both stages have roll control thrusters. Stage 1 may have run out of ACS propellant on the way down.
So if there had been more ACS propellant, then is it conceivable that this landing recovery flight might have succeeded?
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#239
by
R7
on 29 Sep, 2013 21:47
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Recognizing cooked eggs from raw eggs is easy, you spin them on a table; if the egg is cooked the rotating movement easily transmits to the inside, giving some momentum to the rotation, if it's raw doesn't trasmit, and the rotation is short (only the eggshell rotates).
What has this to do with F9?
To get centrifuging of propellant it's not enough to spin the tanks, it's necessary to transmit the movement to the propellant inside.
This can't happen in a few second burn, had to begin before, probably from aero forces.
An egg doesn't have slosh baffles, F9 has.