This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
(very shortened version of a hotly debated topic)
From Alan Jones:
To a very good approximation, optimal thrust for a model rocket (or
HPR rocket) is T = 2W. That is, you thrust at maximum (ideally just an
impulse) until you reach the velocity where drag = weight, thrust at
2W until fuel is exhausted, and coast to appogee. The interesting part
is that the optimal sustaining thrust level is not very dependant on