Introduction: from the Wright brothers to the X‑1
The first successful powered flight took place in the United States. On 17 December 1903, Wilbur and Orville Wright made the first sustained, controlled flight in a powered aircraft, but by 1915 the US government realised that the United States had fallen behind Europe in terms of military aircraft development and set up the National Advisory Council on Aeronautics (NACA). From 1917 NACA produced technical reports on aircraft and engine development and by 1939 it was investigating rotary wing aircraft. In 1941 the Chairman of NACA appointed a Special Committee on Jet Propulsion. Germany had flown turbojets, and her researchers were working intensively on the development of an operational jet‑propelled interceptor. In Britain the propulsion scientist Frank Whittle had designed and built a gas‑turbine engine and had flown a turbojet‑powered aircraft.
By the end of the Second World War the United States had a considerable advantage in terms of long‑range strategic bombers. The superiority of the B‑29 Superfortress was not challenged in combat until the Korean War (1950–3) but by 1945 Germany had developed jet fighters and rocket‑powered interceptors that could fly at 590 miles per hour and climb to 40,000 feet in two and a half minutes. The German jets and rocket planes came into the Second World War too late to have any effect on its outcome, although the new aircraft caused consternation among American aeronautical scientists and military planners. As the rivalry between the former Allies increased, the United States naturally concentrated on developing jet and rocket engines.
Neville Duke was a British test pilot who in 1953 set the record for highest speed in level flight of 727.6 m.p.h. In 1954 he described rocket propulsion:
The other branch of jet propulsion is the rocket. Rockets can be of the solid‑fuel variety used mainly for assisting take‑off; in which the propellant is in the form of a highly compressed powder. This is ignited and burns rapidly producing very hot gases which are discharged under great pressure at very high velocity. Once the charge is ignited, however, there is no control over the rate of combustion or the amount of thrust, and as a means of flight the bi‑liquid fuel rocket is to be preferred. In this case, combustion takes place through the chemical reaction as the liquid propellant and an oxidizer mix in the combustion chamber.
As the rocket carries its own oxygen with it, it is independent of the outside atmosphere and theoretically is therefore not limited in speed or altitude. Its main drawback is its present highly extravagant consumption of fuel, which is up to six times the rate of that of a ram‑jet and from ten to twenty times as much as the turbo‑jet fuel consumption. For instance, the German A.4, or V.2 as it was known in this country, consumed 9 tons of alcohol and liquid oxygen in 7.1 seconds. During that time, however, the V.2 had accelerated to a speed of over 3,500 m.p.h., and a height of 22 miles from which it continued to climb by its own momentum to an altitude of 68 miles before dropping to earth.
The Germans also worked on a number of rocket‑propelled fighters of which the Me163B, powered by an H.W.K. 509 Unit, was the first to see operational service in 1944. The Me 163B had sharply swept wooden wings and a high fin but no horizontal tail. The rocket unit burnt a mixture of concentrated hydrogen peroxide, and hydrazene hydrate mixed with alcohol, which were carried in separate tanks and pumped by a turbine to the combustion chamber. It developed a maximum thrust of 3,300 lb at the cost of a fuel consumption of 1,000 lb per minute, which gave a climb to 40,000 ft within 4 minutes, and a range after reaching that height of 22 miles which could be extended by gliding. Poor aerodynamic qualities restricted the top speed to 550 m.p.h. at 40,000 ft, or a Mach number of approximately 0.84.
To get better endurance, the H.W.K. 509C was developed with a separate auxiliary combustion chamber. For cruising, the pilot switched over from the main combustion chamber, which gave a thrust of 3,740 lb, to the auxiliary which provided 660 lb thrust and therefore had a much lower fuel consumption.
Attempts were also made to combine rockets with turbojets. The B.M.W. 003R, which was fitted into an Me 262, consisted of the B.M.W. 003 turbo‑jet with a 180 lb rocket unti fared into the rear of the engine casing. Using nitric acid as its propellant, the rocket gave a thrust of 275 lb for 3 minutes. Another project was to fit the Me 262 with an auxiliary H.W.K. rocket unit in the tail.
In the United States, initial research into the question of rocket propulsion was carried out by the Aerojet Engineering Corporation, founded in 1942. Within two years they had developed a solid‑fuel Jato (jet assisted take‑off) rocket. This consisted of a single cylindrical chamber inside which the solid propellant and oxidizer were moulded into a cartridge. The cartridge was fired electrically, producing a thrust of 1,000 lb for 14 seconds. Used on the Lockheed F.80, two Jato rocket units reduced take‑off from 3,000 ft to 1,200 ft.
The first American rocket engine designed for straightforward aircraft propulsion was that developed by Reaction Motors. This was the unit used in the world’s first supersonic aircraft, the Bell X‑1. It consisted of four cylindrical combustion chambers, each with a separate igniter so that they could be used individually or together. The chambers, expansion nozzles, and fuel system were supported within a frame of chrome‑molybdenum steel, the whole unit weighing 210 lb. The fuel, a mixture of ethyl‑alcohol and water, was circulated through cooling ducts in the exhaust nozzles and round the combustion chambers. Both the fuel and liquid oxygen were injected separately under pressure into the front of the combustion chamber, where the chemical reaction produced a jet velocity of 6,182 ft per second and a thrust of 1,500 lb from each chamber, or a total maximum thrust of 6,000 lb.
America’s first aircraft designed on the rocket‑cum‑turbo‑jet principle was the Republic XF.9 in which provision was made for the installation of four rocket units in farings above and below the exhaust, to give extra power at take‑off and for climbing. The XF.91 was powered by a General Electric J.47 turbo‑jet engine equipped with reheat.
The Douglas D‑588‑II Skyrocket which reached Mach 1.03 in straight and level flight at 26,000 ft in July 1949, and attained Mach 2.0 at 72,000 ft (about 1,324 m.p.h.) on 11 June 1951, was originally designed to use both rocket and turbo‑jet. Built to fly at 1,820 m.p.h. at 75,000 ft, it was at first equipped with a Westinghouse J.34 turbo‑jet engine supplied with 250 gallons of ordinary aviation petrol giving a 30minute endurance, and the Reaction Motors rocket unit.
This was the same as that used in the Bell X‑1 but it had only one‑third the amount of propellant (3,000 lb) so that the total rocket endurance by using the chambers individually was about 3 minutes. At maximum power, the endurance was less than one minute so to save fuel, Jato rocket units were also used for take‑off.
Later, the turbo‑jet engine was abandoned because it failed to give the performance anticipated, and the space saved was devoted to increasing the supply of propellant for a new Reaction Motors L.R.8‑R.M.6 rocket engine which incorporated certain small modifications on the 6,000 lb C.4 which was used in the Bell X‑I. To enable sufficient altitude to be reached for the high‑speed run, a B.29 Superfortress was used as a mother aircraft to carry the Skyrocket, fitted to the bomb‑bays, to 35,000 ft.
A considerable quantity of fuel was lost by evaporation before the Skyrocket was launched, and in future the mother aircraft will no doubt carry rocket fuel so that it can top up the tank. As it was, by the time the pilot, William Bridgeman, had reached his altitude only 5% of the fuel supply was left. This gave him an endurance of about 3 minutes powered flight for his record breaking run during which he maintained a speed of over 1,000 m.p.h. for about 10 seconds.
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