inefficient turbo-pumps and Isp

Ian Woollard wrote:
Zoltan Szakaly wrote:
The optimum thrust to weight at takeoff for a rocket is around 1.5,

Actually no, it depends on the rocket, mainly on the staging scheme;
some rockets (especially two or more stage rockets) can optimise at
around 3g takeoff.

Gentlemen;
Allow me to introduce you to a concept called 'Max Q',
or maximum aerodynamic dynamic pressure on a rocket
(or generically, air vehicle) as it flies its ascent
trajectory.

If you accellerate too much, your nice thin payload
shroud starts to have to be beefed up structurally
and thermally to resist the ascent loading, as do
the upper stage structures. Eventually, you need
the same sorts of shielding that you need for re-entry,
if you keep adding more and more low altitude velocity.

There is a reason that most launch vehicles have T/W
ratios around 1.25-1.35 at liftoff.


-george william herbert