___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