[AR] Re: {Spam?} Re: Pressure Thrust and Momentum Thrust
- From: "Troy Prideaux" <troy@xxxxxxxxxxxxxxxxxxxxx>
- To: <arocket@xxxxxxxxxxxxx>
- Date: Thu, 17 Nov 2016 13:19:54 +1100
The question at hand got resolved while you were away: the difference
between the conservation-of-momentum result and the textbook equation is
that in vacuum, the exhaust continues accelerating after it leaves the nozzle,
since there is still a downstream pressure gradient. So when you calculate Ve
(aka v2) at the exit plane of the nozzle, that's not the final velocity.
I would argue the exhaust doesn't continue to accelerate after it leaves the
nozzle. There is a pressure gradient, but for that pressure to be converted to
an increase in directional flow requires some kind of reactionary surface to
act against (whether material or electromagnetic or whatever) that is
*independent* of the flow material. The conservation of momentum says that
quite unambiguously.
Of course there will be some molecules that are accelerated post exit plane,
but their increase in aftward velocity would be as a consequence of collisions
with other molecules *of the flow* that would experience an equal but opposite
shift in momentum thereby cancelling out any gains in the actual *flow*
velocity.
Post exit plane, any additional static pressure of the flow above the ambient
is purely that - static pressure and will essentially behave like static
pressure - molecules will be accelerated in an outward direction, but that
direction will be completely random.
Troy.
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