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I understand that the Mars Scientific Laboratory used its Reaction Control System during its entry into the planet's atmosphere. At what altitude was the RCS last used?

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"Seven Minutes of Terror" JPL video

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TL;DR: It is used unto the very end, so somewhat near $0\ \mathrm{km}$.


The RCS is used in re-entry right from the Entry Interface ($125\ \mathrm{km}$ from the Martian surface, $5,900\ \mathrm{ms^{-1}}$), at which it is pressurized and subsequently used for controlling roll, pitch, and yaw as directed by the entry guidance unit. It shares the same propellant (monomethylhydrazine) used by the landing engines.

Shortly (to be precise: 15 seconds) before parachute deployment, it goes through a procedure called SUFR (Straighten Up and Fly Right) after the center of mass offset jettison. During this phase too, the RCS is active, tasked with making the MSL execute a $180^\circ$ azimuth turn and also to dampen any transients. It also takes the angle of attack to zero.

The parachute is deployed at EI+04:05 minutes, at $450\ \mathrm{ms^{-1}}$ and $11\ \mathrm{km}$ altitude. During PD (parachute deployment), RCS is temporarily disabled but is activated once again to dampen wrist mode oscillations that might have arisen during PD.

But the RCS isn't going to retire yet.

Several things happen:

  • Heat shield separation (about 29 seconds post PD)
  • Backshell separation
  • etc..

One may expect no more RCS activity after sky crane deployment but as a matter of fact, the RCS is used unto the very end, even during the powered descent.

The Powered Descent consumes the largest amount of Propellants with an (sic) partly active RCS and eight burning Mars Landing Engines.

The RCS is apparently partially engaged during the final descent stage, perhaps as a dampener or even engaged symmetrically to provide vertical thrust during the final speed-up or slow-down.


References:

  1. MSL Landing special - Spaceflight 101

  2. Mars Science Laboratory - Wikipedia

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