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Aviation History
1955
1955 - 0040.PDF
40 FLIGHT Rocket Combustion Instability Investigated ONE of the major problems in the design oflarge-scale, long-range rockets, the pheno-menon known as combustion instability has recently been the subject of experiments at the Stanford Research Institute in the United States. Occurring for no apparent reason, and causing vibrations which can throw a rocket off- course and may be violent enough to destroy it, the effect is variously referred to as rough burning, chugging, humming, groaning, screeching or motor-boating. Investigating the problem for the Soundrive EngineCompany of Los Angeles, who are pioneers in the rocket field, the Stanford research team (which in-cluded an acoustics physicist, a combustion physicist, a mechanical engineer and an electrical engineer)worked on the theory that pressure changes within the rocket motor could be controlled by changing thesound-absorbing characteristics of the motor walls, theoretical and experimental studies having shown thatenergy in acoustic waves could be absorbed by porous materials lining a cylinder. Elaborate instrumentation was devised for studyingthe time required for ordinary burning of gases within a steel tube to build up to detonation point; andexperiments were made with various liners in order to find whether their acoustic qualities would inhibit theformation of detonation waves and so eliminate the combustion instability caused by local pressure build-ups moving about within the motor. A steel tube was fitted at intervals with ionization gaugesmade from ordinary sparking plugs. Using a hydrogen- oxygen mixture of gases and an electric spark for ignition, theflame-front was observed and the detonation induction distance measured. The length of the pre-detonation burning is governedby considerations of fluid mechanics and kinetics, while the forma- tion of the detonation wave depends on the time required to buildup a pressure wave of sufficient strength to create detonation; the velocity of such a typical detonation wave is approximately8,000ft /sec. As the flame-front passed down the experimental tube smallelectric impulses were generated on each gauge. These were amplified and shown on an oscilloscope. Since the detonation timeproved faster than could be indicated on an available oscillo- scope a system of multiple sweeps on a single oscilloscope wasutilized. Ten ionization gauges were used and were placed between zeroand 40in from the spark. At the critical part of the tube—in the region in which detonation would occur—the gauges were set at2.5in intervals to give greater accuracy, whereas wider spacing was allowed in the rest of the tube. By measuring the distancebetween the blips generated by the ionization gauges the detona- A physicist uses the oscilloscope during experimental firing of the tube. On the left is the firing control panel. tion point was determined. The detonation induction distancewas found by computing the velocity of the reaction zone between successive gauge stations, assuming die transition from combus-tion within the 2.5in interval preceding that interval for which detonation velocity was observed.Linings with different acoustic characteristics were used in the tube, die most pronounced effect being noted in the experimentwith a one-inch coarse bronze liner, which had a median value for the detonation induction distance of 22.5 to 25in. A 0.5incoarse bronze liner showed the next most marked effect, with the distance equal to 20 to 22.5in. A one-inch fine sintered bronzeliner gave an induction median of 17.5 to 20in and a 0.5in fine liner a median equalling 15 to 17.5in. All these results comparedwith a 5 to 12.5in spread for induction distance in unlined tubes and showed a marked change in detonation characteristics whenbronze liners were used. The knowledge gained from these studies is regarded as animportant step forward in the understanding of the formation of detonation waves and in the correction, in the design stage, offactors leading to rocket combustion instability without costly trial-and-error methods. J.G. Adjustments being made to the connections of the ionization gauges before one of the tests. The gauges are adapted from ordinary sparking plugs and screwed into the tube. BIG JET—BIG THRUST P run in May 1952, the S.N.E.C.M.A. Vulcain is one ofx the largest turbojets in the world. Its development has so far been comparatively uneventful, and steady increases in thrust havebeen reported culminating in the news that, a few weeks ago, a five- minute run took place at a rating of 6,000 kg (13,228 lb). Thisdirust was recorded dry, without any form of power boosting. The Vulcain is a single-shaft engine, rather similar to thefamiliar Atar; it is scheduled to power several advanced French types. BRAZIL'S AERONAUTICAL COLLEGE CINCE its foundation in 1949, die Aeronautical College of the,c J^Zllia?,Air Force at Guaratingeta, Sao Paulo, has trained 15,000 specialists, and about 1,900 more have been enrolled forthis year's courses. These courses, 20 in number, include wireless telegraphy, photography, all types of repair, maintenance andinstallation work, flying control, and meteorology. Preparations are now being made to give training in gas-turbine maintenance.There are 40 officers and 160 sergeants on the staff. Courses last between 17 and 23 mondis and pupils are paidabout £11 per month during special training. They pass out as 3rd specialized sergeants" and can subsequendy gain commis-sions. They may also leave the Air Force in order to take appropriate employment with the airlines.
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