JET-PROPULSION POSSIBILITIES

By E. COLSTON SHEPHERD

AS a counterblast to secret weapon propaganda by the enemy the release of extremely limited information on British pro- gress with jet-propulsion must be counted a great success. The new facts now made public can be stated in a few sentences. A jet- propulsion unit designed by Group Captain Frank Whittle has been fitted in a new fighter. In that aeroplane, and possibly others like it, some hundreds of flights have been made. The aeroplane takes off normally after a short run and has a good rate of climb. The fighter was designed by Gloster Aircraft, Limited. The first test pilot was the late P. E. G. Sayer, who lost his life in an ordinary aeroplane and not in one jet-driven. Flying tests have proceeded without mishap since May, 1941.

We are accustomed, after four years of war, to having things left unsaid ; and plenty is left to the imagination when the bare bones of this announcement are examined. This knowledge of nearly three years of test flying is more important for what it promises than for any assurance it conveys of technical superiority in the air during the period of the war. Jet-propulsion is not a monopoly of Great Britain. The Italians had a jet-driven aeroplane flying before the British; and, so far as public announcement goes, before anybody else. The Germans (Junkers and Heinkel for certain) were working on the idea before the war. French, Swiss, Swedish and British were engaged on the new principle of generating thrust. Patent specifications in half-a-dozen countries testify to the faith and in- genuity brought to the subject in the past decade.

Most of us wou'd be surprised to find that the British and Ameri- cans.are the only people who have been flying on jet-propulsion during the last three years. What alone could have given some measure of assurance would be performance figures, and, until the full advantage of surprise has been reaped in operation against the enemy, there can be no question of making such things public. The degree of efficiency in the British system of jet-propulsion has there- fore- to be taken on trust. As Governments rarely make rods for their own backs by " boosting " failures, trust can probably be given with some confidence. If that view is accepted, this news means that the principle of the jet, as a method of providing thrust for certain types of aircraft, is established. It does not necessarily mein that Great Britain is ahead of her competitors, except in seizing the publicity.

Jet-propulsion is consequently best regarded as a scientific and engineering development rather than as an infallible element in ensuring qualitative air superiority to the United Nations. The most satisfactory aspect of the news is its evidence that the British have worked steadily and energetically on the new idea. Reports from abroad were not particularly encouraging up to the end of 1941. All that was established was that a jet, powerful enough to propel an aeroplane of fighter size and weight, could be generated, but none of the promises of high speed, high ceiling and high rate of climb had then been fulfilled.

Inventors were promising speeds of more than soo m.p.h., a ceil- ing somewhere in the stratosphere, and a rate of climb far beyond the possibilities of the conventional aeroplane. We still have no proof that any of these prospects have been realised. Contenting u:Felves for the present with assumptions, we have to conclude that five years of sponsored development and three years of test fly- ing, followed by a burst of publicity, must mean that the promises

appear capable of fulfilment. Because that conclusion supports-cer- tain theoretical conclusions, we can accept the jet and many of the prophecies made for it as in process of realisation.

In principle, jet-propulsion is simp'e. It involves using the force of expanding gases to exert pressure on the air. The mechanical processes it demands, therefore, are compression and heating and such control of the subsequent expansion as will cause its force to be exerted at the right place and in the right direction. Compression alone is not enough. Compression plus the heat derived from the exhaust gases of an auxiliary engine driving the compressor is prob- ably not enough. Direct and lavish heating of the compressed air appears to be needed, and most of the specifications so far published provide for a separate combustion-chamber in which the compressed air is raised to a high temperature by burning paraffin or heavy oil, as used in Diesel engines.

Fuel for jet-propulsion is cheap, but the price of the fuel matters less in an aeroplane than the amount which has to be carried for a given journey or duty. Range, particularly in fighters, is one of the most-needed characteristics. And range is a product of three factors —speed, tankage and fuel-consumption. Theoretically, jet-propulsion can improve speed ; it can have no effect on capacity. The equation between speed and consumption is the vital one ; and as a limit to the increase in speed is set, for the present, by the ihock waves which occur at the speed of sound (roughly, 66o m.p.h. at sea-level), any large rise in consumption wou'd mean a sacrifice in range.

In all earlier calculations, there has been some anxiety on this point. The Caproni-Campini jet-propulsion aeroplane, which was

reported to have a loaded weight of ri,000 lb.. (in the big fighter

class of that time) had to make the Soo-mile journey from Milan to Rome in two stages. Its speed was only 130 m.p.h., and so the special benefit promised by the jet was not enjoyed. Nevertheless, the general conclusion was that " it was heavy on fuel." That was in November, 1941, and no one has told the public anything on this subject since. Before jUdgement can be passed on the jet we shall need to know something about speed and consumption.

Supposing that in the last three years a really powerful jet-propul- sion unit has been evolved, the fighter fitted with it should have high speed. The reasons are plain. There is no slip-stream to upset the smooth air-flow over the lifting surfaces. There is no airscrew to add to the drag. There need be only a small under-carriage, because ground clearance has no longer to be allowed for the air- screw. There could be a positive advantage if one effect of the jet were to suck away the boundary layer of " sticky " air close to the surfaces of the wings and so achieve a true laminar flow over them. Those influences would mean much higher speed for the expendi- ture of a given effort in horse-power. And together with the direct thrust of the jet, they would mean a phenomenal rate of climb.

A fighter with those characteristics would undoubtedly outclimb and outpace any orthodox fighter now flying or in prospect. The arguments for jet-efficiency at great heights also have some force, but the tendency in the past two years has been for air battles to be fought at lower levels, although the ceilings of fighters in certain classes have been raised. Speed, climb and power of manoeuvre are of first importance, and height has served lately more as an element in speed by allowing an opportunity to dive on an enemy. The jet- fighter should not need this aid. It should be able, up to the limit of its duration, to prevent any ordinary aeroplane from escaping. And by virtue of the fierce, hot " squirt " at the rear, it should be immune from attack from directly astern.

Nothing in the nature of jet-propulsion prevents it from being applicable to bigger classes of aircraft—to bombers and transport aeroplanes—but practical considerations must determine its uses, and at present the information is incomplete. The jet, as it is now conceived, could have no place on the roads, for its efflux is not far removed from blast. For the same reason, it is not likely to be suit- able for railways, and there seems no immediate chance of applying it to ships. It is essentially an adjunct of flying in its present form ; and yet it may be opening the way to new developments.

This is primarily an application of thermodynamics. It directs attention anew to the possibilities of gas-expansion without requiring the explosive effects of the internal-combustion engine. Steam tur- bines represent an advance in the science of harnessing the expanding qualities of vapour. Gas turbines are beginning to be considered as a means of exploiting similar qualities in gases. They may employ the piinciple of the jet for a different cycle of operations. The jet- driven aeroplane is evidence of wise development by joint private and Government enterprise. It has implications in other spheres which deserve equal attention.