Spacecraft control ignoring dM/dt

Kirk's _Optimal Control Theory_ ( includes some pr=
actice examples involving spacecraft control, "neglecting the change in mas=
s resulting from fuel consumption" for simplicity. (To be fair, in a later=
section -- after introducing inequality constraints on state & control -- =
he does treat the full lunar soft-landing problem, including the changing s=
ystem mass.)
My question: Is the simplification *ever* reasonable? Are there classes o=
f rocket engines where the consumed fuel comprises so small a fraction of v=
ehicle mass that it can be (to low order) ignored? Nuclear-thermal rockets=
? Ion drives?
Reply to
Joel C. Salomon
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treat the full
of vehicle
Ion drives?
Steering thrusters, perhaps.
If the mission is long enough that the changing mass happens slowly with respect to the required vehicle dynamics (i.e., a trip to Mars on chemical rockets), then the control problem is eased to one of gain scheduling, rather than having to treat the thing as a full-blown time-varying problem.
Reply to
Tim Wescott

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