fisher@dvinci.DEC (Burns Fisher, MRO3-1/E13, 231-4108) (06/18/84)
> Why is a rocket more efficient in space than in the atmosphere
Let's try this two different ways, first a conventional conservation of momentum
approach, and then using your intuitive argument of pressure in the combustion
chamber.
One explanation of rocket propulsion is the law conservation of momentum
(which is actually derived from Newton's third, I think, but whatever...)
which states that in a closed system, the total momentum never changes. This
means that if some part of the system changes momentum (defined as mass times
velocity), then something else also has to change to compensate. Thus, if
a molecule of fuel suddenly finds itself bound to an oxidizer and shooting out
the tail of a space shuttle, something else has to have moved forward (either
less mass faster, or more mass slower). Notice that pressure anywhere has
nothing to do with this. If the air interferes with the molecule of fuel
coming out so it doesn't have as much velocity (and therefore momentum),
then the shuttle must get less push forward in compensation.
Now let's try it the other way. One intuitive notion is that the rocket
gets pushed forward because of combustion chamber pressure which is greater
in the front than in the back because of the exhaust hole. If air is
"blocking up" the exhaust hole, then as you say, chamber pressure rises. But
the premise of this whole explanation is that it is the differential in pressure
from front to back that causes the thrust. If the exhaust hole is restricted,
the differential is smaller, thus less thrust.
The gun analogy: (1) The atmosphere is not a tube which forces the gases to
apply their energy to anything, (2) there is nothing like a bullet to which the
gases must transfer their energy, and (3) what happens if you make the gun
barrel too long? The bullet speeding through it will build up the air pressure
ahead of it (since the air can't escape the barrel fast enough) and will be
slowed down. Thus there is an optimal barrel length. Similarly there is an
optimal length for the rocket gas expansion. Hopefully this optimal length
is similar to the length of the engine nozzle!
Burns
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