On Mon, Nov 07, 2016 at 08:37:25AM +1100, Troy Prideaux wrote:
(1) you already accept that the conservation of momentum isn't violated
by the commonly adopted De Laval thrust equation for
*exceeding* Ve x q
I must be being too subtle if that's the impression I'm giving, since I
certainly
don't accept that. Not in vacuum, anyway; there, the commonly used equation
*does* exceed Ve x q, so it can't be right.
Oh? Can you provide an example of where you think it might be violated?
Your answer to that second question was an answer I'd already rejected on the
grounds that the air on the outside of the nozzle is not moving much.
Indeed,
you could take the entire volume between the base of the rocket and the
nozzle
and enclose it, stilling the air almost completely, without significantly
affecting
the thrust. You could even pump it down to vacuum without significantly
affecting the thrust. Well, that would increase the thrust *as measured at
the
engine mount*, but for the rocket as a whole that increase would be exactly
canceled out by the increased suction on the base of the rocket, so it would
be
an entirely pointless exercise.
Well, your logic works if the molecules in the air were standing
still, but they aren't standing still. They're impacting the external
surfaces of the nozzle/rocket/whatever at velocities of >M1. They're
colliding with the external surfaces of the nozzle wall and bouncing
off at different angles to which they were travelling thereby
imparting *momentum* and force on the surface in which they
collide. It's pretty much exactly the same mechanism that happens on
the other side of the nozzle wall.
