willner@cfa250.harvard.edu (Steve Willner P-316 x57123) (01/30/89)
The following article was taken from the 1988 December CANOPUS. See copyright information at end. BRIGHT GAS FLOW ON STS-26 IS OLD PHENOMENON - can881228.txt - 12/8/88 Flashes of flame seen between the solid rocket boosters during the STS-26 launch were caused by a well-known "plume recirculation" effect that has been known since the launches of V-2 rockets in the 1940's. The flashes of light were reminiscent of the flashes that preceded the breakup of Space Shuttle Challenger in 1986 and caused some apprehension among observers until booster separation. "The effect is visible -- if weather conditions and camera angles permit -- from around 92 seconds into flight until just before jettison of the boosters," said Aerospace Engineer Lee Foster of NASA's Marshall Space Flight Center. "Even if it's not visible, we've used instrumentation to determine that the effect does indeed occur on every flight." Plume recirculation, Foster explained, is due to the relative location of the booster and main engine nozzles, the expansion of the rocket jets at high altitudes, and the airflow around the Shuttle vehicle and its plumes. "As the Shuttle ascends, atmospheric pressure becomes lower," Foster said. "As a result, the exhaust plumes grow wider as the Shuttle gains altitude. Then, as they get wider, they begin to intersect with each other and this intersection region gets closer and closer to the base region of the vehicle. "Usually around 92 seconds," Foster said, "the pressure in this intersection becomes higher than the pressure in the base region and some of the exhaust plume gases from this intersection are reversed, or recirculated, into the base region. The high-temperature gases heat the aft dome of the external tank and the aft sections of the boosters." By itself, that effect wouldn't result in anything visible. However, another ingredient--oxygen--is added to the picture at this point. "Around the outer boundary of the recirculation region, oxygen from the airflow around the Shuttle mixes with some of the recirculated exhaust gases, allowing residual fuel in the plume to burn. That's what results in the flames seen in the flight videotapes," said Foster. Foster said the plume effect is not a safety issue: it was predicted before the first Shuttle mission and the vehicle thermal protection system designed to allow for it. "We've been building our data base on this subject ever since the mid-1950s," he said. "In fact, plume recirculation occurred from the very beginning, even with the German V-2 rockets. "During the first six flights of the Shuttle, we put a variety of gauges on the external tank and boosters to measure the heating from plume recirculation. These measurements showed that the heating was basically the same on each flight and that the vehicle is well protected from the heating." Copyright information: ------------------------------------------------------------------------ CANOPUS is published by the American Institute of Aeronautics and Astronautics. Send correspondence about its contents to the executive editor, William W. L. Taylor (taylor%trwatd.span@star.stanford.edu; e-mail to canopus@cfa.uucp will probably be forwarded). Send correspondence about business matters to Mr. John Newbauer, AIAA, 1633 Broadway, NY, NY 10019. Although AIAA has copyrighted CANOPUS and registered its name, you are encouraged to distribute CANOPUS widely, either electronically or as printout copies. If you do, however, please send a brief message to Taylor estimating how many others receive copies. CANOPUS is partially supported by the National Space Science Data Center. ------------------------------------------------------------------------ -- Steve Willner Phone 617-495-7123 Bitnet: willner@cfa 60 Garden St. FTS: 830-7123 UUCP: willner@cfa Cambridge, MA 02138 USA Internet: willner@cfa.harvard.edu