GRGR (20) Part 3, Episode 13: Notes, Part 1 of 2

[Mark Wright AIA]

Howdy

--- Michael Perez <studiovheissu at yahoo.com> wrote:
> 454.27  “The Rocket creating it own great wind . . . no wind
> without both, Rocket and atmosphere”  This is of some
> conceptual importance.  Growing up during the space age as I
> did, some fundamentals of propulsion go unquestioned.  While
> the rocket within the atmosphere is “pushing” against air
> molecules for some of its lift, but fighting against full gravity
> and the weight of the atmosphere that is also being pulled by
> gravity.  Fahringer may also be trying to say that outside the
> atmosphere, there are no air molecules against which to push,
> but also less effects of gravity and no air to swim through.  The
> rocket must rely on its own “wind” - the gases produced by the
> combustion within it - to propel itself.  The speed of the gases
> expelled by this reaction - the rocket’s wind - remains constant,
> but the relative airspeed changes as it is frees itself from the
> effects of gravity.  Note also the capitalization of Rocket in the
> quoted passages.

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.  The design of the bell-shaped nozzle optimizes the flow of the
gases by keeping turbulence to a minimum.  In a vacuum, the exhaust
gases expand at their most rapid rate, as there is no counter-pressure
trying to stuff the gases back "up the venturi".  In the atmospere, the
exhaust gases can't expand as rapidly because they have to expand
against the pressure of the surrounding air -- the exhaust cannot exit
the venturi as fast, raising pressure within the combustion chamber
above the venturi (a bit), reducing (a bit) the velocity of gas in the
venturi, and constricting the expanding cone of expelled gas beneath
the rocket.  TV has shown us this effect.  This is why the incandescent
exhaust appears to shoot pretty much straight down out of a rocket
early its flight, but then when shown away up in the sky throught a
telescopic lens the gases flare out widely, almost like a peacock's
tail.

Will a real rocket scientist step in and correct me where I've gone
wrong?  This really isn't my field.

I think that the Rockets Wind, in this passage, refers to the air
rushing past the rocket in its early flight.  Hell's Angels generate
their own great wind in much the same way.  I've experienced the
mystical wind myself while riding in Porter's open top Triumph.  No
wind without both: moving object and atmosphere....

On the other hand, on occasion I've experienced a wind of a different
sort, when combustible gases expand rapidly through a venturi.  Hell's
Angels can do this too, according to my biker uncle Earl.

Mark
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