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I have read that as a fallback option, the service module's reaction control system (RCS) thrusters could be used to deorbit the craft in case the normally used "orbital maneuvering and attitude control thrusters" (OMAC) fail.

The Starliner Notebook from Boeing lists 28 RCS thruster at 85 lbf [bloody imperial units!] each, an overall force of 2380 lbf. The 20 OMAC thrusters, by contrast, have 1500 lbf each, an overall force of 30,000 lbf.

I suppose that the force translates fairly directly into burn times needed to achieve a given Δv. This would imply that there is orders of magnitude more fuel for the RCS on board, as a backup, than needed for nominal operation.

I suppose that an emergency deorbit would also follow a different trajectory that can be achieved with a smaller Δv.

How would such an emergency deorbit be performed?

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    $\begingroup$ I couldn't find any definitive information but all the thrusters on the service module use the same fuel so I'd guess they also have a shared fuel supply $\endgroup$
    – Alan Birtles
    Commented Sep 5 at 5:45
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    $\begingroup$ Wikipedia says that the OMAC and RCS thrusters are all using the same bipropellant, which was also true of both Apollo and the Space Shuttle. However, no clear sourcing. $\endgroup$
    – ikrase
    Commented Sep 5 at 5:58
  • $\begingroup$ @ikrase Ah, right, it's the capsule that uses different types of Hydrazine (one as a mono-propellant)! That's a good explanation, why not write a short answer to that effect. Other people can comment on possible different re-entry profiles, if any. $\endgroup$
    – Peter - Reinstate Monica
    Commented Sep 5 at 6:20

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