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In the video below we can briefly see the valiant attempt of a lone Nitrogen RCS thrust to keep the Falcon 9 upright during the failed landing attempt of CRS-6.

However, as the stage continues to tip more thrusters start firing [about the 10s mark in the video], but at 90° to the direct the stage is falling. They're obviously not helping keep the stage up.

Why did those thrusters fire?

Did the Falcon conclude that its cause was hopeless and tried to move itself away from the ASDS to minimize damage?

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    $\begingroup$ One possibility is that the thrusters have independent propellant tanks rather than working from a shared tank, and the one thruster simply ran out. Under most circumstances, the thrusters would need to fire only brief pulses rather than sustaining fire for several seconds. I would not expect that the thruster program would include intentional control of the fall direction. In a situation where the stage couldn't be kept upright, I'd think there would be too many unknowns. I think the fall wasn't orthogonal to the one thruster's orientation, so the other thrusters were in use as well. $\endgroup$ – Russell Borogove Apr 18 '15 at 20:47
  • $\begingroup$ Maybe they thrusted upwards because that was the best vector available to their "gimbaling" (if that really is a word in English, rolling my eyes). Pushing the rocket stage downwards is better than doing nothing, and your AI works for your best day and night until death do you apart! $\endgroup$ – LocalFluff Apr 20 '15 at 17:42
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Looking at that video I'm not sure you're describing it accurately. I don't think it's actually 90 degrees from the fall direction. It's pretty much sideways but I'm not sure it's entirely sideways. If there was even a small component in the needed direction I can see the computer firing it because it wasn't programmed to recognize impossible--it saw it was tipped and tried to straighten the rocket using everything it had.

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It is hard to judge what the rocket trying to do (to impute behavior to machine is may be to early). Anyway, the general impression of majority of people commenting these videos is that flight computer is confused.

The cold gas thrusters are used only for roll control at atmospheric decent phase so they should works in pairs. Cold gas thrusters (RCS in your question)are too week even to try to correct excess of angular velocity that killed the booster.

When gambled engine solely is used for attitude control the control forces are applied in single point. This way, the pitch/yawing control moments are coupled with uncompensated lateral force. Consequently, any attempt to compensate significant moments introduced by cross winds or wind shear inserts lateral force and deviation from target path. Imposed sideway velocities induce back aerodynamic pitch/yow moments. On the other hand, any divert move performed by single-ended attitude control is possible only by tricky tilt and set upright maneuver.

Evidently, on certain point, that highly nonlinear feedback becomes pathological and triggered a Stability Loss and hardover beyond recoverability.

Such deadly vacillation is caused of close frequency response of the lateral feedback interfere with the pitch/yaw channel. Figuratively speaking, the guidance computer is in vacillation between contradictive priorities: attitude stability and horizontal deviation.

cold gas nozzles

The thrust of cold gas thrusters may be estimated by their openings but even without calculations one may decide is it possible such two inches nitrogen nozzles may turn back 20 ton, 42 meter long vehicle in seconds.

The thrusters was activated most likely by axial twist after violent contact of legs with platform.

roll thrusters

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    $\begingroup$ "The cold gas thrusters are used only for roll control so they should works in pairs." Do you have any source for this claim? It seems very clear in the video that they're firing to try and stabilize the rocket in the vertical, not to roll it. $\endgroup$ – Russell Borogove May 25 '15 at 20:09
  • $\begingroup$ on 'cold gas thrusters': the source are (1) Elon Musk Q&A; (2) observation that the opposite directions are activated in same time what make sense only when roll moment is inserted; (3) estimation of cold gas thrusters maximum moment. $\endgroup$ – Val May 26 '15 at 7:14
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    $\begingroup$ How do you square that with the situation in OP's video at 0:11, where two thrusters at positions 180º apart on the fuselage appear to be firing in the same direction? $\endgroup$ – Russell Borogove May 26 '15 at 15:09
  • $\begingroup$ please ref the picture added $\endgroup$ – Val May 27 '15 at 3:52
  • $\begingroup$ Please ref the video. $\endgroup$ – Russell Borogove May 27 '15 at 4:00

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