You may soon fly a plane snatched from the pages of a Buck Rogers' fantasy a propellerless aircraft which promises to revolutionize military and civilian aviation.
This new jet-propelled plane opens breathtaking vistas of flight at speeds limited only by the acceleration which the human body can stand. Dr John C Stewart, professor of Astronomical Physics at Princeton, predicts that by 1950 rocket propulsion will produce speeds of 1,000 mph exceeding the velocity of sound by 30%. Dr Stewart also foresees a rocket to the moon at 25,000 mph sometime after 2050 .
Although the American public has not yet seen this propellerless craft, the English people, who have a nodding acquaintance with it, call it the Squirt. When it made its first appearance over Britain, people hurried to air raid shelters under the impression that the screaming sound of its jet was produced by some new type of giant bomb.
Apparently, this plane has been perfected to the point of production line output. With both the Buffalo and Niagara Bell plants producing the ship, Major Alexander P de Seversky indicates it might be used in the coming European invasion.
In England, E Colston Shepherd, secretary general of the Air League, also implied it will soon be in action against the enemy.
Most current conjecture concerns the external appearance of the Bell Squirt. While some writers have described it as a hybrid of a P-38 and a P-39, it actually bears a closer resemblance to the Douglas P-70. As indicated in the drawing, it has twin jet propulsion units probably mounted close to the fuselage to permit single-engine stability. The jet-propulsion motor, developed by AAF and General Electric engineers from Captain Whittle's single-engine design, is actually a very simple machine a gas turbine much like the turbosupercharger. Turbosuperchargers use the waste exhaust gases from an internal-combustion engine to drive a turbine which, in turn, drives the supercharger impeller. In the turbojet propulsion unit, however, the gases are not waste products but are deliberately created for the purpose of driving the turbocompressor which then discharges the gases through a tailpipe nozzle to give the engine its thrust.
Even though the turbine operates at a terrific heat, probably somewhat greater than the turbosupercharger, the gases do not flame as they leave the nozzle. And both the turbine and compressor are on a common shaft, rotating at an extremely high speed somewhat in excess of 10,000 rpm, depending upon the size of turbine adopted.
Because it spins in a column of moving air, there is little drag from the mechanical parts of the turbine. And since there is no propeller in the nose or on the leading edge, no slipstream exists to batter the leading edge or to turbulate other controlled surfaces. Drag differences in and out of the slipstream are therefore eliminated while many obstinate gyroscopic conditions should also disappear.
The engine is conveniently shaped for installation with its simple construction reducing production manhours far below those needed to build a conventional aircraft engine. The two units are started by means of an external source of power which turns a turbo-compressor for a few seconds. Air is admitted at the intake and drawn into rotary compression chambers. The compressor discharges air from its diffuser section into the combustion chambers. Filtered kerosene is injected into the combustion chambers and ignited to create a tremendous internal gas pressure. The heated gases in the chambers disposed circumferentially between the compressor and the turbine expand and flow through the turbine, causing it to revolve and develop power to drive the compressor. The gases, hot, and still above atmospheric pressure, flow from the turbine into a tailpipe. A final pressure drop takes place when the gases rush through a restriction of the tail pipe which forms a nozzle, which action greatly increases the velocity of the gases ejected through the discharge nozzle at the rear of the nacelle and a powerful propulsive thrust results. The amount of air allowed to enter the system is probably controlled by vanes or other adjustable obstructions in the intake duct.
Broadly speaking, the principle of the propeller and that of the jet engine are the same. Each develops thrust to move the plane forward by expelling air rearward at an accelerated velocity, thus changing the momentum of the air. The average reciprocating motor develops its thrust by means of a propeller creating a slipstream aft. This propeller, functioning as a rotary airfoil, draws air from ahead of the aircraft and moves it swiftly backwards outside of the aircraft, in a stream equivalent to a jet stemming out from the arc of the propeller. A motorboat also works on the jet propulsion principle, the "jet" being represented by the current of water pushed backward by the propeller.
In the case of a jet propulsion motor, thrust is developed by sucking air into an orifice, heating and expanding it, and then ejecting the gases at a high velocity through a nozzle to create a slipstream of much higher speed relative to the airplane than that made by the propeller. Also, when the speed of the propeller blade exceeds 450 mph, compressibility shock waves are set up which decrease the thrust-producing ability of the blade. This compressibility effect manifests itself by a sudden decrease in the air flow speed and a sudden rise in pressure and temperature.
Engineers are fast realizing they are reaching the limit of speed that propeller combinations can produce. Even now, the engines are rapidly becoming a mechanic's nightmare with eighteen or more cylinders, automatic accessories and, to top it all, a turbosupercharger.
Consequently, jet propulsion is the next logical step, since the possibilities of propellers have just about reached their limits. Speeds beyond 500 mph are prevented by the piling up of air molecules.
This impedance of air molecules is diminished in the thin air of the stratosphere. The higher you fly, the faster you can go. A propeller, however, cannot function efficiently in a rarefied atmosphere because it literally screws its way through the air. The greater the density of the air, the greater the traction and the greater the efficiency of the propeller. 45,000 feet is the maximum service ceiling the best propeller-equipped plane has reached. For as we rise, air becomes thinner. At six miles, the density is one-third that at sea level; at a height of 10½ miles, it is one-tenth; and at seventy miles, it decreases to one-millionth the density at sea level.
The thermal turbo-jet propelled plane, however, can function in more rarefied atmospheres since it need not cope with the problem of traction. And unlike the gasoline combustion engine, it needs no supercharging. It gets better and better as speed and height increase.
Actually, jet propulsion is not new. Behind the jet propulsion principle lie many centuries of investigation. Essentially, it is based upon Newton's third law of motion: "To every action there is an equal and opposite reaction." It boils down to this: You can make an object move in one direction by sending out a force in an opposite direction.
A rudimentary example of a jet engine is the familiar rotating water sprinkler. Because water under pressure escapes from the nozzle, or jet, it propels the wheel in the direction opposite to the flow of the water. In the jet engine, instead of water, gas is ejected from the nozzle under high pressure. Contrary to popular belief, jet and rocket engines do not work in spurts or a series of explosions but provide a continuous smooth flow of power.
Basically jet propulsion and rocket motors operate on the same principle. They both obtain their forward thrust by expelling gases rearward through jets under great pressure, but there are extensive differences.
The rocket engine carries with it, in addition to fuel, the oxygen necessary to support combustion, while the jet engine carries only the fuel. Since it is not dependent on the outside air, a rocket not only flies at all altitudes, but functions at a maximum in the pure ether of outer space. In fact, it operates most efficiently in a vacuum. A force which would provide only a relatively low rate of speed at the earth's surface would be sufficient to produce and maintain enormously greater speeds at an altitude of 200 miles.
It is, therefore, incorrect to refer to a jet-propulsion plane as a rocket. The jet engine operates well in the stratosphere, but cannot function in a vacuum because it does not carry its own oxygen; it draws its supply from the atmosphere through which it travels. It will function only at a height where the supply of oxygen still sustains combustion.
Theoretically, rocket propulsion is the next logical step in overcoming the limitations of jet propulsion. It is the kind of motor power that can take us beyond the gravitation of the earth into interplanetary space. However, it remains for thermochemistry to give us new fuels for such sustained flight. After all, no one wants to go beyond the gravitational pull of the earth and not have enough fuel to get back.
The problem with a rocket plane will be how to handle heat and how to control the craft in flight. So far, the development of new alloys enables the Whittle-GE jet engine to withstand temperatures of 1500°, produced by the gases in combustion.
Contrary to popular belief, the jet engine compares favorably with the internal combustion engine in regard to fuel consumption. Since the compression ratios are lower than in reciprocating engines, special high-octane fuels are not needed because no detonation problems exist. And because cheap kerosene is used in the J-P engine, a miles per gallon comparison is unfair from cost angle. According to Frank Caldwell, research director of United Aircraft Corporation, at an altitude of 40,000 feet the jet propulsion system is only half as efficient as the engine-propeller combination at 150 mph. But at a speed of about 300 mph, the two are equal in efficiency, particularly in regard to fuel consumption. At a speed of 550 mph, the jet propulsion system becomes about twice as efficient as the engine-propeller method.
Although efforts to harness the forces unleashed by jet reaction date back to the steam-driven aeolipile of Hero of Greece, its first practical use was proposed in 1680 by Sir Isaac Newton, who mounted a spherical steam boiler having a rearward directed escape nozzle on a four-wheeled cart.
But it was the Italians who made the first successful flight in a jet-powered plane at Milan on August 27, 1940. It was designed by an engineer named Secondo Campini, and took off by means of a propeller, then switched to jet propulsion in mid-air. On November 30, 1941, a somewhat improved model was flown 168 miles from Milan to Rome, at a speed of about 130 mph by Col Mario de Bernardi, the Schneider Trophy ace. It dispensed with the propeller, retained regular engine.
As early as January, 1943, the Luftwaffe employed a jet-propelled interceptor plane with telling effectiveness in repulsing RAF bombers at Brunswick. Its rate of climb was 4½ miles a minute 400% better than our fighter planes. At least two J-P planes, one manufactured by Arado, and the other by Heinkel, have been used in actual combat. The Nazis may even utilize ultra-short waves to remotely control and operate elevating and directional fins on their jet machines.
The principle of the Caproni-Campini design was simple consisting of a nozzle duct which ran along the entire fuselage of this machine. Air was sucked in through the open nose by a blower driven by a radial engine. In the forward portion of the duct, a compressor raised the pressure of the air entering at the nose; and the expansion toward the exit was increased by the addition of liquid fuel injected and ignited near the discharge nozzle. The jet consisted of the combustion products plus the exhaust from the radial engine.
During all this excitement, a quiet little Englishman, Frank Whittle, was working away at jet propulsion almost unnoticed. In 1930, he applied for a patent on a thermal jet propulsion system, and in 1936 he outlined the "dual thermal cycle" scheme. This employed a diesel engine and compressor to supply air and combustion products to a turbine, which drove the main compressor. The effluent from the turbine was utilized for an auxiliary propulsion jet. By April, 1937, Whittle had built his first successful jet propulsion engine, and the RAF promoted him to Group Captain, assigned to jet research.
In 1939, the British Air Ministry placed their first order for a jet propulsion plane with the Gloucester Aircraft Co, Ltd. and assigned Whittle to assist Power Jets, Ltd to build the engines. This was The Squirt and it made its first successful flight in May, 1941, piloted by the late Flight Lt P G Sayers, Chief Test Pilot of he Gloucester Aircraft Company.
Three months later the RAF gave full information regarding the engine to General H H Arnold. At his request, the Whittle jet propulsion engine was sent to the General Electric Co in September, 1941. GE was chosen to build the units because of their knowledge of extreme heat on metal working parts gained while constructing a turbosupercharger for the Army. Bell Aircraft Corp was assigned to build an aircraft suitable to take two of these J-P engines.
It was one year later, October 1, 1942, that the first test flight in America of a propellerless plane was made by Robert M Stanley, chief test pilot for Bell. The following day Brigadier General Lawrence C Craigie flew the ship, thus becoming the first Army officer to fly a jet-propelled plane. Since then several hundred successful flights have been made, both here and in England, many of them at extreme speeds and high altitudes, all without a single mishap. This J-P engine presents no new pilot problems beyond an unfamiliar simplicity. According to one airman who has made several flights, experienced pilots transferring to the jet ship must forget more knowledge of plane operation than they must acquire.
Smoothness, simplicity, and evenness of power are the Squirt's principal characteristics. Elimination of the propellers and the fact that all noise is well aft makes for quietness in the cockpit. Thus, pilot fatigue and deafness attributed to the thunder of propeller air blast and the roar of high powered aviation engines is eliminated.
Freedom from vibration should produce handling ease in tight maneuvers, reduction in stress and strain on structure, machinery, and instruments. It has also encouraged pilots to depend more upon their instruments in flight. The plane has so little vibration that a vibrator was placed in the instrument panel so the pilots could be sure the instruments were not stuck and were working properly. Because there is very little engine noise in the cockpit, the pilot must use super-accurate instruments to follow the engines' operation.
There is a definite absence of torque as smooth acceleration is obtained from standstill up to flying speed. The number of gadgets and dials in the cockpit is materially reduced while one throttle does all the work; forward to go, further forward for greater speed, back to slow down or stop.
Free of the ground clearance limitation imposed by large diameter propellers, the Squirt virtually can land and take off on its belly, with a consequent advantage of lowered center of gravity. Built close to the ground, it is much easier for maintenance men to work on. Much heavy and expensive retracting mechanism is eliminated. It will be easier to load and unload cargo and passengers on future jet propelled transport ships. A more nearly horizontal ground position as well as a clean wing allows superb vision for takeoff and landing. The cushioning effect of a wing close to the ground provides improved landing characteristics.
Because of the successful record and obvious advantages of this new type of ship, the Commanding General of the USAAF, the British Air Ministry, and the Ministry of Aircraft Production have directed that a sufficient quantity of these planes be produced for training purposes both in the United States and Great Britain. The AAF is allotting a number of these to the US Navy for additional tests and experimentation.
In flight, the plane makes a weird noise. At some distance away, it sounds like the faraway rumbling of a train. Its approach is not as noisy as propeller-driven planes. You seldom near a jet propelled plane until it is almost past you; as it rushes away you hear the roar of the jet. On its silent approach qualities alone, it is ideal for low-level attack work. With great possibilities as a interceptor, and for high altitude reconnaissance work, it is at least 100 to 300 mph faster than any Allied aircraft now flying.
And this is only the beginning; the future holds immense possibilities. Mankind is equipped for the ultrahhigh speeds of rockets because the human sensor system cannot convey the sensation of constant speed. Each and every one of us, at every moment of our lives is traveling at a constant speed of 65,000 mph in our perpetual journey around the sun without registering any physical sense of motion. It is only rapid acceleration and deceleration which cause us illness. If the acceleration is gradual, a person would experience no discomfort in flying at the rate of even 50,000 mph; but if no precautions were taken at the start and the finish to control the rates of acceleration and deceleration to those within the limits of human endurance, the occupants of the plane would be crushed to death by their own mass.
Whether the Whittle-GE-Bell plane will provide the weapon to break the Luftwaffe remains for the future to decide. But it appears to have provided the Allies with the fastest fighter planes in the world with which to hammer a German air force concentrating on fighters.
This article was originally published in the May, 1944, issue of Air Tech magazine, vol 4, no 5, pp 44-47, 50.
The original article includes a thumbnail photo of Captain Whittle and 10 drawings (including a small 3-view silhouette of the P-59.)
Illustration credits to Aerosphere, Inc, Acme Newspictures, Air Force, British Combine.