Spacecraft control ignoring dM/dt

Kirk's _Optimal Control Theory_ (amazon.com/dp/0136380980) includes some practice examples involving spacecraft control, "neglecting the change in mass 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 system mass.)
My question: Is the simplification *ever* reasonable? Are there classes of rocket engines where the consumed fuel comprises so small a fraction of vehicle mass that it can be (to low order) ignored? Nuclear-thermal rockets? Ion drives?
--Joel
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On 05/03/2011 11:19 AM, Joel C. Salomon wrote:

> examples involving spacecraft control, "neglecting the change in mass resulting

> introducing inequality constraints on state& control -- he does treat the full > lunar soft-landing problem, including the changing system mass.)

> rocket engines where the consumed fuel comprises so small a fraction of vehicle > mass that it can be (to low order) ignored? Nuclear-thermal rockets? 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.
--

Tim Wescott
Wescott Design Services
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