Autzoo.1616
net.space
utzoo!kcarroll
Mon May  3 13:09:12 1982
re:Saturn V Plume

   It has been pointed out that during the launch of Saturn V rockets,
a plume seemed to form about half-way down the body of the vehicle,
well above the location of the main engines. When I first noticed this,
I theorized that it was caused by the firing of attitude-control
rockets along the side of the booster. After taking a few fluid-
mechanics courses, however, I came up with a much more satisfactory
explanation. 
   As the rocket passes through the air, it pushes the air aside.
In order to move out of the way, the air must gain velocity
(in a sideways direction, mostly). As the velocity of the air increases,
its relative pressure decreases (due to a well-known fluid-mechanics
result known as Bernoulli's Law), much in the same way as the 
relative air pressure above an aircraft's wing is lower than
the pressure below the wing. As the pressure decreases, any water
vapour that was dissolved in the air comes out of solution,
forming "clouds" that exist only in that particular low-pressure
zone; downstream of the zone, when the pressure increases again, the
water vapour goes back into solution, and the cloud vanishes.
Now, while a Saturn V is pretty well streamlined (ie has a large
fineness ratio; is long and thin) the intersection between the
first and second stages is rather abrupt; the stages are joined
by a short cone, with a side-slope of about 45 degrees.
This is the sort of condition that makes for the above-mentioned
low-pressure zones. As a result, a small cloud tends to form
starting just upstream of the 1st-stage/2nd-stage adaptor,
and extends partway down the length of the 1st stage. Neat, yes?
   I observed this phenomenon first-hand while landing at London's
Heathrow airport on a very humid and cold day, just after a fog had
dissipated. I had a window seat just level with the trailing edge
of the wing; when the flaps were extended and we were just about
to touch down, little lenticular clouds began flashing into
existence just over the bulge of the wing, then disappearing
again. The clouds always appeared in the same spots, relative to the wing;
however, internal structure in the clouds (striations and regions
of varying density) could be seen to move downstream at the same rate the
ground was. The clouds also formed in the slots between the flaps.
Just having learned enough fluid mechanics to come up with the above
explanation, I was enthralled by the entire process.
   Of course, maybe the plumes were caused by side-thrusters, after all....

						Kieran A. Carroll

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