[Mvandenbergh]

Most rocket engines don't throttle down well. The main reason isn't the turbopumps (although that can also be an issue) but combustion instabilities in the chamber and flow separation in the nozzle.

Nasa overview of liquid engine throttling:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/2010003...

[grinich]

Exactly the answer I was looking for-- thanks!

[Serow225]

That's a very readable reference, thanks.