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Aviation History
1951
1951 - 1328.PDF
13 July 1951 47 As a precaution against failure of the decoder, a wire record- ing is also made of the incoming signal, so that it can be played back after the flight. The telemetering system origin- ally gave readings from 28 instruments in the rocket, but in more recent tests 60-channel transmissions have been possible. Certain types of data (e.g. solar spectra) are not readily adaptable to transmission by the telemetering service. It is, therefore, desirable to make records within the instrument- head during flight and to recover these records after the rocket has returned to earth. Two possibilities for increasing the chances for physical recovery after impact seemed worthy of consideration. The first was to render the rocket unstable when it re-entered the atmosphere and thus reduce its ter- minal velocity; the second was to eject the equipment near the end of the flight and reduce its velocity by a drogue device. The technique as finally evolved was for the instrument- head to be separated from the rocket by means of explosive placed on the struts beneath the base of the compartment. This was found to be a dependable method of lowering the terminal velocity to a few hundred feet per second, sufficient to achieve reasonably good physical recovery of recorded data such as exposed spectrographic and motion film protected by thick-walled steel cassettes. The U.S. Air Force have since improved the technique further by providing delayed- opening parachutes in the instrument-head. When the nose compartment breaks away from the rocket hull, a 30-ft pilot parachute is ejected. Increasing atmospheric density as the compartment falls causes this initial chute to open fully, which then drags out the main 100ft canopy to complete the descent. Individual experiments, however, demand different tech- niques, and to obtain direct measurements of temperature in the upper atmosphere instruments have been ejected from the rocket-head independently. A device for this purpose, developed by Dr. Severin Raynor of the Armour Research Foundation, Illinois Institute of Technology, consists of a metal sphere, stabilized by three gyroscopes, and with a number of temperature sensors, or thermo-couples, and a recording camera. The gyros are designed to keep the sphere steady both while it is inside the rocket and after it has been ejected. To achieve this, the sphere is free to turn on supporting rings which, consisting of halves held together by explosive bolts, are discarded after ejection from the' rocket. The sphere then falls freely whilst measurements are made. The measuring device itself comprises 14 buttons —each containing a temperature sensor—located on the sur- face of the sphere, and a special slqw-speed camera which records readings on 16mm film. As the device falls back into denser atmosphere, the camera, which is attached to a 400- megacycle transmitter for purpose of tracking, is itself ejected, whereupon a ribbon-parachute opens to check its fall. The Aerobee rocket (Fig. 22) was developed under a Bureau of Ordnance contract awarded in May 1946 by the Aerojet Engineering Corporation and the Douglas Air- craft Corporation, with technical supervision from the Applied Physics Laboratory of Johns Hopkins University. The proposal was to pattern the new rocket generally on the lines of the successful WAC Corporal, but with instru- ment volume and altitude specifications more suited to high altitude research requirements. The principal objectives of the programme were : (a) to provide a relatively inexpensive vehicle for research into the physics of the upper atmosphere; (b) to advance engineering experience and practice in the design, testing and launching of liquid-propellent rockets, while at the same time making available a proven missile of potential military value as a basis for an anti-aircraft weapon; and (c) to provide experience within the U.S. Armed Services (and their contractors) in the practical handling, servicing, fuelling, launching and tracking of rockets of potential mili- tary type, including shipboard launching. The Aerobee has an overall length of 18.8ft and a diameter of 1.25ft. A solid-fuel booster is fitted behind the main bi- fuel propulsion stage, but this is separated from the former by thin thrust webs so that both units can operate simul- taneously, the exhaust from the Aerobee motor being de- flected outwards by a cone on the head of the booster. The Fig. 23. Aerobee rocket, for cosmic ray investigation, in the launching tower aboard the U.S.S. "Norton Sound," floating test station. latter assists at take-off to bring the velocity rapidly up to about i,oooft/sec, then jettisons; propulsion of the main missile continues for 45 sec. At the time of thrust cut-off, the rocket's velocity is approximately 4,iooft/sec (2,790 m.p.h.), and the altitude about 95,000ft. Like the WAC Corporal, the vehicle is unguided but pos- sesses arrow stability by virtue of its three fins and proper location of its centre of mass. Its trajectory is controlled simply by tilting the 140ft launching tower in accordance with wind data obtained from meteorological balloons. Flight data were originally retrieved from the rocket by two methods : (a) radio transmission from a compact 85-mc telemetering set developed by the A.P.L., and (b) physical recovery from the impact wreckage. Explosive separation of the tail-end structure from the remainder of the hull was arranged to occur at a suitable altitude on the descending path of the trajectory. The tail-less portion was thus made aero- dynamically unstable, with the result that it tumbled to earth with a velocity not exceeding I5oft/sec, and certain specially, armoured equipment was satisfactorily retrieved in this way. More recently, it has been possible to recover the complete instrument compartment by ribbon-type parachutes. Following the firing of three dummy missiles containing no sustaining motor, the first live firing of the Aerobee occurred on November 24th, 1947, but, unfortunately, the flight was terminated deliberately by radio command after 35 sec because of excessive yaw at take-off. A second flight on March 5th, 1948 was, however, completely successful and resulted in a summit altitude of 372,000ft (70 miles). The Aerobee has not only featured in ground-to-air tests but several firings have been made at sea from the Navy's floating test base, the U.S.S. Norton Sound, a converted sea- plane tender. A cruise to the Gulf of Alaska in the early summer of 1950 enabled the rocket to be used in an attempt to determine the relation between the earth's magnetic field and cosmic rays. Later, the new Glenn-Martin Viking rocket
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