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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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