"Christopher M. Jones" wrote in message ...
"Henry Spencer" wrote:
Greg wrote:
I know that this is the case for regeneratively cooled engines, the heat
taken out is still in the loop. So its no loss.
Am i correct? Can you get away with a very inefficient turbo-pump?

Yes and no. You're correct in thinking that the heat may not get lost,
although it may show up in unwanted places. More significantly, though,
an inefficient pump is likely to be a heavy pump, and pump mass can be
quite significant.

In other words, heavy pumps play on the other side of the
rocket equation, not in Isp but in achievable mass ratios
and in the dry mass fraction of stages.

A couple of quick calculations using data from
astronautix.com, I get an average of about 20% of the
dry mass and a negligable fraction of the gross mass
(~0.2%) as engine mass for the first stages of the
Atlas V, Arianne 5, and Delta IV med. I.e. the
difference in mass ratios of an engineless stage and
an engined stage is about 25% (0.998/0.8), which
means that engine mass decreases the first stage
delta V by *very roughly* 0.22 * Isp * g, or around
1/2 to 1 km/s for most liquid chemical propellants
(again, very roughly).

The optimum thrust to weight at takeoff for a rocket is around 1.5,
and if the engines have a thrust to weight ratio of 100 then the
engine mass is 1.5 percent of the total mass at takeoff. This is more
than 20% of the dry mass.

Zoltan