Kirk's _Optimal Control Theory_ (amazon.com/dp/0136380980) 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?
--Joel