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Aviation History
1955
1955 - 1076.PDF
188 FLIGHT, 5 August 195b The fiame or the Super .Sprue of full thrust. SUPER SPRITE ... jettisoning follows normal supply-dropping techniques and, withappropriate training, a crew should be able to put down their Super Sprite nacelles within a circle of 200 yd diameter.At the appropriate moment, the nacelles are jettisoned by operating a double-throw rotary switch (with a centre-off posi-tion), which is independent of the master switch. This switch is moved to the left to release the port nacelle and to the right torelease the starboard, the slight delay-spacing, even in a rapid "click-click" motion, being sufficient to separate the nacellesby at least 200ft and so ensure a free fall for each pack. Standard bomb-slips are employed to carry the weight of thenacelles, and these are triggered by the jettison switch. Once released, a nacelle is free to drop vertically until the thrustspigot has completely emerged, when the nacelle can move in any direction. As the drawings show, each nacelle is equippedwith a 4ft drogue, four stabilizing "wings" and a restrainer strop at the forward end. Initially, therefore, the nacelle is stabilizedby the pull of the drogue (which is released by a static line) and by a corresponding pull of 600 lb applied by the restrainer strop.The latter is arranged as follows. At a hard point on the aircraft, close to the nose of the SuperSprite nacelle, is an attachment for a small box which lies just proud of the skin. Inside the box is an assembly of iin rollersaround which is threaded 32ft of flexible steel tape. One end of this tape is coiled up in the top of the box; the other end isled out through an opening and is attached to a bridle on the nose of the Super Sprite nacelle. When the latter is jettisoned,the tape is pulled from the roller box, the resulting pull on the front of the nacelle having a very considerable stabilizing effect.The tension in the tape can be varied between 200 and 800 lb by re-arranging the run of the tape between the rollers, thevalue chosen being matched to the indicated air-speed during jettisoning. The tape remains attached to the Super Spritenacelle, but the roller box is released from the aircraft as soon as the pull of the tape is removed. As soon as the nacelle is free, it tends to assume a nose-upattitude, but the stabilizing surfaces keep the nose down and give a horizontal deployment, as shown in the sequence ofsketches. When the restrainer strop has run out, a time-delay releases the drogue from its rear anchor and transfers its pull tothe main canopy pack. This can be achieved by such methods as: by a static lifting apron breaking a tie and falling free; by rippinga tear-off strip and spring extractor; or by attaching the drogue to the apex of the main canopy to maintain the pull of the formerduring deployment. A small spring-loaded ejector chute then jumps from the top of the main canopy bay and, within halfa second, the main 36ft canopy is withdrawn, together with the pair of 4ft stabilizing parachutes. The drogue is then released, todrop independently, while the main canopy gradually deploys until the entire nacelle is fully established in a stable descent.A Bowden cable from the me;n rigging trips a time mechanism which, as soon as steady descent has been established, operatesa Walter Kidde dinghy bottle containing nitrogen at 1,000 lb/sq in (CO, was originally used, but was abandoned owing to icingtroubles caused by the violent expansion of the gas). The nitrogen pressure is passed through a T-piece and thence to a pair ofDunlop rams which push open two locks restraining the bungee- loaded doors over the bottom of the nacelle. The bungees thentake charge, opening the two doors through 270 deg (in the manner of bomb-doors) and allowing the large landing bag to unfold at thebottom of the nacelle, under the influence of ram pressure from intakes situated underneath. This inflatable bag is attached tothe nacelle around its upper periphery and has sides provided with louvres which are initially tucked in as pockets but whichopen under ram-pressure. The open louvres then spill off the excess air and improve the stability of the nacelle.With a 36ft canopy the fired Super Sprite nacelle descends at about 25ft/sec. At this velocity the landing bag touches theground, the lower surface of the bag being protected by a sheet of plywood. As soon as the weight of the nacelle is taken off themain rigging lines the no-load release is actuated, so releasing the canopy and preventing the nacelle from being dragged along theground by the wind. The precise recovery procedure for A.T.O. rockets of thisnature has not yet been worked out in detail, but it will probably be rather more complicated than, for example, that followedduring the war in recovering tow-ropes jettisoned from glider tugs. Once recovered, the Super Sprite pack would be sent to aninspection bay, where such items as the nacelle weak points, and the rubber seals and valves and catalyst pack would come in forparticularly close scrutiny. Items such as the nylon cord, the explosive bolt and the anti-tilting tape roller box would be re-placed. The complete unit would be passed through a service bay and would then be held in store until needed. It is at presentimpossible to assess the overall performance of an A.T.O. unit of this type, but it is hoped that die life will be at least 50 firings.Serious damage as a result of jettisoning should be very rare, and it may be found that this type of rocket motor will eventuallyequal other aircraft power units in number of operations if not in hours of firing.The nacelle is an orthodox light-alloy stressed-skin structure, and it carries all the weight and thrust of the motor. The SuperSprite is hung within the nacelle by two main central trunnions, which are picked up by Metalastik bushes on the main nacellelongerons. These two trunnions transmit all the thrust. The rear of the motor is suspended by a pair of swinging-link trun-nions, longitudinal movement being provided to allow for mis- alignment and thermal expansion. There is also one nose mountingon the front of the peroxide tank, which centralizes the complete motor in the nacelle.The thrust of the motor is transmitted through the monocoque nacelle structure up to the steel spigot mounted centrally ontop. Side loads are absorbed through a square-section spigot which projects above the forward part of the nacelle, and theinstalled nacelle is braced to the wing by four crutches. The two front crutches are fixed platforms of welded sheet, with rubbertops which can press against the wing. The rear crutches are hinged to the nacelle and can be extended at right angles to thefree position to engage in circular depressions in the wing when in the firing position. The crutches are bungee-sprung to returnto the free position in small compartments in the nacelle as soon as they are released, and are adjustable in length to apply theappropriate crutching load to the nacelle. Originally, a 2ft 7in drogue was employed, but the productionunit has a diameter of 4ft. It is pre-packed on an aluminium ring provided with a fabric cover, which is placed over an oval holein the upper rear skin of the nacelle and secured by Dzus fasteners. The four stabilizing surfaces are each roughly one footsquare and are simple light-alloy assemblies with three ribs carried on a steel tubular spar. They are mounted at a fixed incidence,a very considerable negative angle being selected appropriate to the angle at which the store is mounted on the aircraft.In order to provide access to the motor, the upper part of the nacelle is arranged to lift off. The bottom part of the nacelle isa lightly loaded box containing the landing bag, the longitudinal joints between the various nacelle sections being made by Pipquick-release pins. The anti-tilt restrainer tape is made by Trianco in 20-gaugesoft steel, 32ft in length and fin wide. Both the landing bag and the main caiiopy are products of the G.Q. Company, who werealso responsible for considerable assistance in developing the basic system. The main canopy is of the open-gore pattern andhas a diameter of 36ft. Originally, three separate landing bags were used, made of dinghy material, but the present patternemploys a singie bag made of fabric. An accompanying diagram (p. 183) shows the development, incumulative firings and in significant events—the Super Sprite was the first British aircraft rocket to receive a M.o.S. type-testcertificate—of the de Havilland Sprite and Super Sprite. Test- bed running has established that the bulk of the unit should havean indefinite life, with the exception of the catalyst pack and various seals and gaskets which would be replaced as required.The type-test consisted of 100 firings (plus initial calibration and check firing) without modification or adjustment. A completenacelle has also been subjected to ten drops from a crane to the full "operational" alighting speed, with intermediate firings be-tween each drop. The basic Super Sprite is now cleared for 50 firings and, assuming that no damage resulted from the jettisonand recovery cycle, it is expected that many such 50-firing lives could be achieved.Super Sprite nacelles are now in quantity production by the de Havilland Engine Company and it is believed that the unit isthe first jettisonable liquid-propellant A.T.O. motor to go into service anywhere. Various applications for the motor are currentlyunder consideration. Development of this unit has been of in- estimable value to the rocket technicians of the company who,for several years past, have been working on an increasingly broad front in the development of bigger and better rocket motorsfor the future.
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