Rocket thrust

[Bob Norton]

Nope, that's not it, either. The thrust is caused by the pressure on the
forward end of the combustion chamber. Sideways thrust vectors all cancel
each other out but the forward vector has no cancelling force in the other
direction as that force has gone out the nozzle. Traditional nozzle designs
are a matching device between the constant pressure of the thrust chamber
and the changing pressure of the ambient external atmosphere. They are most
efficient at a single external pressure and less efficient above and below
that pressure. The new aerospike engines, like those used on the X-33 test
vehicle, are an attempt to get around this matching problem and provide max
thrust flow under all conditions.

Date: Sat, 19 Feb 2000 10:37:02 -0600
From: Mark Wright AIA <mwaia at yahoo.com>
Subject: Re: GRGR (20) Part 3, Episode 13:  Notes, Part 1 of 2

Howdy
...
The rocket within the atmosphere is not "pushing" against air molecules
for "some of its lift".  Thrust is generated by the accccelllllerated
masss of the exhaust gases as they rrush through the "venturi" of the
nozzle. 
...