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Aviation History
1952
1952 - 2020.PDF
92 FLIGHT, 25 July 1952 The burning gases leave the Snarler at something like three times sonic speed, and shock-wave formations can be seen as coloured flame cones Armstrong 'Siddeleys Oxygen jWater-methanol Rocket Motor : Handling Liquid Oxygen THE existence of the Armstrong Siddeley Snarler liquid-fuel "hot" rocket-motor of 2,000 lb thrust has been public knowledge for only about a year, but work was started on this and other rocket projects six years ago. The three principal dates in the Snarler's development were : Work started, autumn, 1947; special-category flight-test completed, May 1950; unit tested in flight in Hawker P. 1072, November 20th, 1950. When discussions first took place, they centred, as might be expected, on the choice of fuel, and more particularly of oxidant j should nitric acid, hydrogen peroxide or liquid oxygen be selected ? The company chose liquid oxygen with water-methanol, and as the months have passed it has never regretted its choice. For various reasons, particularly those concerned with logistics, the use of methanol is expected to be abandoned in favour of kerosine, or, for some rocket motors, perhaps ordinary M.T. petrol. The use of aircraft gas turbines will require kerosine to be available on all airfields in large quantities, so provision of this same fuel for rocket motors would not complicate matters. A similar argument applies to "pool" petrol. A rocket motor such as the Snarler is relatively simple, consist ing basically of the combustion chamber; the propellant pumps; a number of small control and safety devices, the principal pur pose of which is to operate valves; and, of course, tanks for the two propellants. Power for the pumps may be provided by drives from a conventional engine in the aircraft, or the rocket motor can be made to produce its own power and so be self-contained. From the start the Snarler has been developed as an aero engine —good handling, reliability and safety being always in mind. As a result, this comparatively small unit, which weighs only a few hundred pounds, is now controlled by two electric switches, one to light up and bring in one-third thrust, the other to select full thrust. Normally a pilot would not start with full thrust selected, but, should he do so by mistake, no harm would be done, for the unit lights up at one-third thrust in any case, and then almost instantaneously increases to the full 2,000 lb. The lighting-up process takes only a second or two and the change-over to full thrust no more than half a second. The motor may be switched on or off as often as the pilot wishes, and will run so long as fuel is supplied. Duration is, in fact, purely a matter of fuel capacity in any given aircraft. When the starting switch is moved to "on" the sequence of operations which follows is purely automatic. The fuel and liquid oxygen pumps begin to operate. Both pumps prime, and the igniter valves open automatically. A high-intensity spark plug ignites the fuel, and rate of fuel delivery is related to chamber pressure—hence the automatic start on one-third thrust before full thrust can be obtained. One action triggers the next. Great emphasis is placed upon safety, and devices are included in the Snarler to shut it down in the event of malfunctioning. It is particularly important to avoid an accumulation of fuels, which might produce an explosive mixture and overload the combustion chamber. For this reason equipment is provided to make it impossible for propellants to enter the combustion chamber before the igniter is fired. Should any interruption occur in the supply of propellants to the combustion chamber, or if any component ceases to function correctly, the rocket is automatically shut down. Unlike piston engines, but in a somewhat similar manner to gas turbines, rocket motors are likely to be easier to design in the bigger sizes, and as much as four times the thrust delivered by the Snarler could be obtained from a unit of similar overall dimensions and less than twice the weight. Thrust developed is largely a matter of fuel-pump capacity. Specific consumption for hot rockets is of the order of 200 (i.e. 200 lb thrust per 1 lb of fuel per second). The Snarler has flown in the tail of the specially adapted Hawker P. 1072 fighter with Rolls-Royce Nene turbojet. The pilots were the late T.S. ("Wimpy") Wade and Neville Duke. The tests must have been of a preliminary nature because the P. 1072 was not designed for flight at the speeds and altitudes foreseen for rocket-powered types and required to give com prehensive test results. The noise of the Snarler running at even one-third thrust is shattering; one cannot stand nearby without ear protectors, and even then it is painful to the ear drums. Flame is visible for from 6 to 10ft behind the nozzle. It should be noted that a liquid-fuel rocket of Snarler type maintains a steady maximum thrust output, whereas turbojets do not maintain their static sea-level thrust as they get higher—far from it. Thus, 2,000 lb thrust can be expected to give a very con siderable increase in climb performance. As an example we may recall the Me 263 which, with a take-off weight of 11,650 lb and rocket thrust of 4,400 lb from a bi-fuel H.W.K. 109.509, (but having no turbojet in addition) had an initial climb of 11,8ooft/min and a climb at 32,800ft of 33,50oft/min. If kerosine is later adopted as the fuel for some rockets a small increase in specific impulse will result; on the other hand, cooling problems for the combustion chamber may arise. Whereas water- methanol is a very good coolant for circulation around the com bustion chamber (prior to entry into the chamber) kerosine is one of the worst. Additionally, the flame temperature with kerosine as fuel is appreciably higher. During operation the combustion-chamber pressure must be kept above a certain minimum or specific consumption falls off, but above this minimum figure the rocket motors are not very sensitive to chamber pressure. At high altitudes the reduced atmospheric pressure is advantageous. Minimum chamber pressure is thus determined to some extent by the requirements for good specific consumption at the lower thrust settings. In order to satisfy our curiosity, Armstrong Siddeley's test section at Ansty, Coventry, recently afforded us an opportunity to play with a bucket of liquid oxygen. As used for rocket motors it is an attractive-looking blue liquid and, having a normal
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