Jet Propulsion
From Fancy to Fact
Part I

by Willy Ley,
Formerly Vice-President, German Rocket Society

Those now working to achieve practical reaction-principle flight owe much to the painstaking trail blazing of yesterday's rocket experimenters …. First installment of a revealing two-part series by a participant in the pioneering.

A little over 100 years ago, a British engineering journal published an article violently attacking certain new plans concerning railways and locomotives. It concluded with the sentence: "It would be difficult to conceive of anything more silly and ridiculous than the idea of building locomotives of twice the average speed of a mail coach. One may just as well expect the inhabitants of Woolwich to give their consent to a ride on a Congreve war rocket than to entrust their lives to such an engine."

The immediate result of this attack was that Stephenson named his next locomotive the Rocket. But the joke on the article writer had only just begun, since soon afterward inventors seriously began to consider Congreve war rockets — minus the incendiary or explosive head — as a possible means of transportation.

A considerable number of projects concerning rocket airplanes and rocket airships were advanced during the latter half of the Nineteenth Century. But while these projects are both interesting and amusing, they can all be disregarded in a modern discussion of the application of the rocket principle for aviation since the originators did not really have a free choice.

They did not think of rockets as a means of propulsion for their various "flying machines" because of any special merit of this method. Their choice was simply dictated by necessity. for they did not have any other prime mover at their disposal save clockwork, which was immediately discarded as too weak, and the steam engine, which was too heavy.

Several of these men were, however, intrigued by steam, although not in the form of an engine. A British patent for an airplane propelled by steam reaction was granted to one Charles Golightly in 1849. Unfortunately, we do not know who Charles Golightly was, nor do we know anything about his plans, except for a contemporary cartoon which has become quite famous (Fig 1).

It seems that Golightly wanted to build something similar in outline to a modern monoplane, with a boiler in the fuselage, producing steam which was supposed to move the whole by direct reaction of a rearward pointing jet. As regards the question of probable speed, Golightly seems to have made rather big claims, for the original caption of the cartoon speaks about a "Steam Horse on Which One May Ride from Paris to St Petersburg in One Hour." Golightly's claims must have been high, but they were "theoretically" justified — the standard of theoretical physics in 1849 permitting the conclusion that the speed of such an apparatus would have to be high.

Golightly found a somewhat belated successor in the Russian engineer Fyodor Geschwend of Kiev who, in the '80s, published two small pamphlets advocating propulsion by steam reaction. The first of these two pamphlets dealt with a steam reaction railway; the second, which is of interest here, was printed in 1887 and entitled General Design of a Steam Airship. This was to be a four-wheeled car suspended from two ellipsoid wings, mounted one above the other in the manner of biplanes. Its steam jet unit was to be mounted between the wings and equipped with venturi nozzles. Weight of the proposed machine was estimated at some 2,900 lb, the speed range from 80 to 150 mph.

Another Russian engineer, Alexander Gorokhov, may be credited with the first — and necessarily crude — modern design of a rocket airplane. It was published just before World War I in the Russian aviation journal Vosdushni Put (Airway) and is one of the first projects where the choice of rocket propulsion is really a. choice, since the airplane had by then been invented.

Gorokhov's project was a torpedo-shaped vehicle with very small wings — large stabilizing fins rather than wings. Consequently, Gorokhov advocated a high speed, mentioning the then striking figure of 220 mph. As far as general design goes, Gorokhov's project was worked out in considerable detail. The interior of the torpedo-shaped fuselage was to contain seats for pilot and passengers, as well as the fuel tanks. while the six exhaust nozzles, three on each side, were to be outside the cabin.

Gasoline and alcohol were mentioned as probable fuels and much of the propulsive mechanism was frankly based on suggestions made a few years earlier by the Frenchman René Lorin. Gorokhov knew well that the main difficulties would be the takeoff and landing of his craft, and it seems that he spent many weeks thinking desperately about these points. At any rate, the solutions which he finally advocated do look desperate. The takeoff of the machine was to be accomplished much in the manner of the takeoff of a ski jumper, with a detachable wheeled carriage substituting for the skis. The landing was to take place in a large heap of the finest beach sand available. One wonders why Gorokhov, having thought of beach sand, did not then think of the sea as an even softer medium on which to land his machine.

Those suggestions by René Lorin upon which, to a large extent, Gorokhov's propulsive mechanism was based had been published in the French aviation magazine l'Aérophile in a series of loosely connected articles, beginning in 1908. As regards the aerodynamic features, Lorin was only groping, as might be expected at that early time.

But he did have a definite idea of what he would use as a rocket motor. It was a modified six-cylinder in-line engine to be inserted in the wing. The modifications consisted mainly of the substitution of direct exhaust nozzles for the pistons, connecting rods, and driveshafts. Lorin wanted to place equal numbers of such engines in each wing in such way that the craft would be propelled by a continuous series of exhaust blasts. The Lorin scheme must have looked rather advantageous at that time.

There were no drive shafts and no need for gears or propellers. Many other things looked simplified and, all told, a great saving in weight was indicated. But these minor advantages to the contrary, it was unfortunate (for Lorin) that the single major advantage involved was represented in the already well-established engine-propeller combination.

The fact that spoke against this reaction motor and this form of application of the rocket principle to airplanes was that the engine-propeller combination also utilizes the rocket principle — and in a manner which, for this particular purpose, is far superior to the "straight" rocket.

The stream of moving air produced by the propeller can be compared directly to the exhaust of a rocket, even though it is produced in a different manner. The important differences are of a purely quantitative nature which, however, are decisive as far as efficiency is concerned. A jet from a rocket motor has a very high velocity — around 2,000 m (7,900 ft) per second — but its mass is small, on the order of a few pounds per second. The "jet" produced by a propeller has a comparatively low velocity, roughly some 20% of that of the rocket jet or less. But its mass is large, up to 1,000 lb per second, in some cases even more.

While improvement of the one or the other sort of motor changes the actual figures involved, the tendency remains always the same. The better rocket motor will produce an even higher exhaust velocity without a noticeable increase in mass, while the better engine-propeller combination will produce an even more massive propeller "jet" with a comparatively small increase in speed.

But the efficiency of a jet of given mass depends almost exclusively on the speed of the craft propelled by it, so it becomes evident why the engine-propeller combination is far superior to the rocket jet for use in airplanes — unless the latter attain flying speeds far higher than those which are now customary.

Main advantage of the engine-propeller combination is that the mass composing the jet is simply taken from the surrounding atmosphere so that it does not have to be carried along. Another important advantage is the possibility of adjusting velocity and mass of the "jet" within rather wide limits.

In a rocket motor, the mass which composes the jet has to be carried along, since it consists simply of the combustion products of a fuel like gasoline or alcohol with an oxygen carrier, usually pure oxygen in the liquefied form. And while the mass of a rocket jet can be readily varied by feeding more or less fuel and oxygen into the rocket motor, the exhaust velocity cannot be varied as easily. Reducing the amount of fuel fed into the combustion chamber of a rocket motor has surprisingly little influence on the exhaust velocity, which has a tenacious inclination to remain the same for a given motor and a given fuel.

On the other hand, the rocket motor has the fascinating feature of utmost simplicity. It literally does not have any moving parts at all, consisting simply of a fuel and an oxygen line leading from the pressure tanks into a combustion chamber fitted with an exhaust nozzle. Since the combustion, while rapid and almost explosive in nature, is continuous, there is not even a need for a spark plug. The mixture is ignited only once (through the exhaust nozzle) for each run, and this ignition may be accomplished in quite a number of simple ways.

It is not surprising, therefore, that the utilization of so simple a device as a rocket motor has intrigued engineers as well as laymen and has elicited quite a number of projects. Of these projects, only that of Dr Eugen Sänger deserves serious consideration and discussion, since it is the only one which is based on a thorough investigation.

But before discussing Dr Sänger's work, a number of experiments made in Germany during the years 1928 and 1929 should be mentioned. In these tests, large powder rockets were used, manufactured by the pyrotechnic firm of Friedrich Wilhelm Sander at Wesermunde, originally for Fritz von Opel's rocket automobiles and rocket railroad cars which, of course, have to be regarded merely as a means of attracting publicity. The experiments were carried out by the Rhon-Rossitten Gesellschaft, a then flourishing German glider society which received its name from the geographical locations of its two main camps.

The society furnished several glider models and a full sized glider of the then novel "duck" type with which the first recorded rocket flight was made on June 11, 1928. The rockets at the disposal of the experimenters were of the following types:

  1. 3-sec burning time with 360 kg or 790 lb thrust
  2. 3-sec burning time with 175 kg or 385 lb thrust
  3. 3-sec burning time with 75 kg or 165 lb thrust
  4. 30-sec burning time with 20 kg or 44 lb thrust
  5. 30-sec burning time with 5 kg or 11 lb thrust

Types 1, 2 and 3 had the customary center bore which does not reach all the way through and which has the purpose of increasing the burning surface, while types 4 and 5 were solid rockets without a bore, known in the trade as "branders".

The tests with models were made first, and almost all of them turned out badly. The rockets proved to be far too powerful for the models. They took off "like projectiles", as the reports put it. The rockets tried to tear off the wings. and the models crashed because they [were] either were in unstable positions when the rockets gave out, or had been damaged by the acceleration, or both.

As a result of these experiences, the most powerful rockets were discarded for the duck glider. Only two solid branders, of 26- and 33-lb thrust, respectively, were installed in the craft, which was to be piloted by Friedrich Stamer. The two branders were wired for ignition in such a manner that only one of them could be ignited at a time, no matter what the pilot did.

In spite of all these precautions, Stamer had to report* that the first two attempts failed. In the initial trial, something went wrong with the launching rope and the one brander which had been ignited by mistake before the glider was in the air, merely burned out. In the second attempt, Stamer did not succeed in balancing the glider correctly. He landed after traveling about 700 ft and without igniting the second brander.

The third attempt was made after thorough inspection of the glider. It was then fitted with two 44-lb (20 kg.) thrust solid branders. Quoting Stamer's report:

"The plane took to the air by means of the rubber rope, aided by one rocket (brander). After flying for about 200 m in a straight line (I noticed a slight climb) I made a curve of about 45 deg to the right and flew for another 300 m. Then I turned again to the right, again about 45 deg. Immediately after the (second) turn, the first rocket ceased burning and I ignited the second …. This time I flew about 500 m in a straight line, made a turn to the right of about 30 deg and landed the machine after another 200 m, just a few seconds before the second rocket was exhausted."

The distance covered was about 1 mi in a little more than 1 min of flight. Asked about his impressions, Stamer declared that it had been "extremely pleasant", and he added:

"I had the impression of merely soaring, only the loud hissing sound reminded me of the rockets."

To continue the report:

"For the next flight it was planned to climb over a small mountain. The launching went all right, and while the plane took to the air, I ignited the first rocket. After one or two seconds it exploded with a loud noise. The 9 lb of powder were thrown out, and they ignited the plane instantly. I let it drop for some 20 m to tear off the flames. After succeeding in that, I landed the plane without mishap."

Just after landing, the second brander caught fire, but fortunately it just burned out without exploding. The fire, however, had damaged the glider to such an extent that extensive repairs were necessary. As a consequence, the experiments were then broken off and not resumed. Nevertheless, a few similar experiments followed in the wake of them.

The Raab-Katzenstein airplane works at Kassel built a full-sized plane of the same type as that depicted in Fig. 5. For unknown reasons, the first attempts failed, and Raab-Katzenstein gave up. But at about the same time, Fritz von Opel had a rocket glider built, and he tested it on Sept 30, 1929, near Frankfort-on-Main.

Two tests in the morning failed because the rocket batteries did not develop enough thrust (Opel took off without rubber rope or other aid). The third attempt in the afternoon of the same day led to a flight lasting about 10 min in the course of which 100 mph were attained. But the plane was not rigid enough. Also, the wings caught fire in the air. Opel succeeded in landing unharmed, but the plane was ruined.

Opel's experiment was repeated some three years later in Italy. Then in 1931, the Italian engineer Ettore Cattaneo tested a 620-lb glider-type rocket airplane at Milan. During one of these tests, the craft remained in the air for 34 sec, traveling a distance of 1 km, or roughly two thirds of a mile.

The experiments of the Rhon-Rositten Gesellschaft had a definite purpose. They were to test the feasibility of an oft-made suggestion to substitute rockets for rubber rope or car tow for the takeoff of gliders. Rockets for takeoff seemed promising not only because they would permit a glider pilot to take off without help, but because it also seemed likely that he could carry along two or three rockets for other takeoffs after intermediate landings. The idea was abandoned reluctantly because powder rockets were not considered safe enough, since the tendency for them to explode could not be eliminated.

The idea of the takeoff, or launching, rocket was also advocated for regular planes in German aviation magazines at that time, on the theory that any plane needs more power for the takeoff than it needs in flight. The idea was suggested to save the extra engine power and gasoline consumption by equipping planes with only as much engine power as was needed for cruising and providing the extra power needed for the takeoff by means of rockets.

Prof Hugo Junkers, then still alive, tested at least part of that idea by taking a single-engined Junkers plane with floats — the same type which made one of the first transatlantic flights — and fitting it with a battery of powder rockets. He overloaded it to such an extent that it could not conventionally take off anymore. Just how much it was overloaded was not stated, but the normal takeoff weight of these planes was around 3 tons. At any event, the takeoff of the overloaded plane with the aid of the battery of rockets was successful.

This experiment had been made in great secrecy in Aug 1929, and nothing was heard of it again until it found practical application during the Battle of Britain. The Luftwaffe then launched overloaded bombers from small improvised airdromes in Holland and Belgium. The bombers had a kind of metal grating fitting the bottom of the fuselage and carrying the rockets, which were ignited electrically by the pilot when the engine was running with full power. When the rockets were exhausted, the grating dropped off the plane, which was by then in the air.

* In Zeitschrift fur Motorluftschiffahrt und Flugtechnik, (ZfM) , Vol 19, No 12, June 28, 1928

Part II

Reaction-principle contributions of the post-1918 pioneers — down to and including the successful Whittle-Bell-GE development — are considered by the author in this final installment of his timely series.

The efficiency of the rockets in the various experiments described in Part I of this article was less than 1%, although the rockets themselves were of good make and quality.

This does not constitute an indictment of rocket propulsion in general but only of plans for operations based on assumptions of energy losses of 99% and more. The reason for the magnitude of these losses becomes clear if it is realized that the problem of the efficiency of a rocket motor must be treated in another manner than that of a steam or internal combustion engine. In a rocket motor one always deals with two different%ages — the "inner" or "thermal" and the "outer" or "ballistic" efficiency.

The inner or thermal efficiency may be compared directly with that of an internal combustion engine. While it is very low — around 2% — in small fireworks rockets it is respectable, in the case of larger ones (about 20%) and unusually high (about 60%) in the case of liquid-fuel rocket motors. This high efficiency can be explained by these facts: The combustion temperature is high, there are no moving parts, and the friction of the gas stream is low.

The outer or ballistic efficiency, however, depends solely on the speed with which the rocket is moving. If the rocket is "nailed down" it is simply zero, if the velocity of the rocket equals that of the exhaust it is 100%. (Theoretically, if the rocket moves faster than its exhaust, the ballistic efficiency may seem to surpass 100%, which is, of course, impossible. Actually the rocket, before it does attain such velocity, has to move slower than its own exhaust velocity for quite some time so that the whole performance remains far below 100%.)

Even assuming that the outer efficiency soon approaches 100%, the overall efficiency would equal only the figure for the inner efficiency. Since the exhaust velocities vary from around 700 per second for fireworks rockets to over 2,000 m per second for liquid-fuel rockets, it becomes obvious that the ballistic efficiency, and with it the overall efficiency, has to be very poor indeed unless the rocket is permitted to move with a velocity above 800 mph at the very least.

This was realized by some inventors more than 20 yr ago, and the obvious conclusion was that either the speed of the airplanes had to be increased (in a then incredible manner) or else the velocity of the exhaust had to be reduced very considerably. The latter seemed easier to achieve since it could be done simply by adding mass, ie, air, to the exhaust so that the mass of the exhaust would be increased with a proportional reduction of velocity.

In 1917, O Morize of Chateaudun, France, proposed a power plant which was to achieve this goal. Morize's idea was production of a rocket blast by means of an engine-driven compressor, fuel injection pumps, and combustion chamber with exhaust nozzle, also to surround the exhaust nozzle with a series of venturi nozzles to add large amounts of outer air. This device has later received the name of thrust augmentor and has been the subject of quite a number of experiments and discussions, among them several NACA reports.

But Morize did not build it then. This was left to his compatriot, Henri F Mélot, who used an invention of his own to produce the rocket blast. It consisted of two cylinders, placed end to end, with fuel inlet ports and spark plugs at either end. An unconnected piston was placed inside for compression, and the exhaust gases were led to a common chamber to which the exhaust nozzle was attached. The device was hard to start, but once started (by means of compressed air) it ran nicely, producing an intermittent blast. Mélot first experimented during the period from 1918 to 1920, then again resumed experimentation some years later. But it seems that he never progressed far beyond his original accomplishment.

While not actually successful, the Morize-Mélot scheme of adapting the otherwise inefficient rocket blast to aviation gave rise to a number of other projects.

One of them is the combination turbine and direct-reaction plane patented by Dr Robert H Goddard in 1929. (US Patent No 1,809,271 of June 9, 1931, filed June 28, 1929). The leading idea of this project was to insert two turbine wheels, with propellers mounted inside, into the rocket blast for the takeoff and for flight in the dense atmosphere of low altitudes. Once high altitudes and high speeds were attained the rocket was to act directly.

The mechanical embodiment of this idea, as it was then published, is shown in Fig 1. The rocket motor is located in the tail end of the device, with the propellers mounted inside the turbine wheels. The shafts of the latter are hinged so that the turbine wheels can be removed from the blast by a slight lateral turn of the hinged shafts. It is obvious that this design is most unlikely to perform in reality. In the first place most of the heavy parts are massed in the extreme tail end, and the fuel tanks could hardly be used for balancing the heavy tail, especially since the weight of the fuel tanks would diminish very rapidly.

Other engineering problems — for example a suitable material for the turbine rings which have to withstand the heat of the rocket blast — present additional obstacles, even if the device could carry enough fuel for successful operation under these conditions, which seems exceedingly doubtful.

Another project, in which the leading idea is again the addition of atmospheric air to the rocket blast, rests on two German patents, No 544,834, valid beginning Oct 21st, 1930, and No 596,856, valid beginning Feb 25, 1932, both granted to Wilhelm Goldau of Duisburg-Meiderich.

Goldau's method, which he demonstrated with a number of small functioning models, differs from Mélot's insofar as the atmospheric air is not added to the rocket blast after the generation of the latter, but rather before combustion. Goldau planned very large reaction chambers, working in pairs. At least three such pairs would be required to propel a plane. They could be arranged around the fuselage in various patterns or might also be placed in the wings.

Goldau's long torpedo-shaped reaction chambers are fitted with a valve at either end. At the beginning of each cycle both valves are open so that the air can flow through the entire length. Then a. small amount of benzene, which ignites very rapidly, is sprayed into the chamber. The exhaust valve is then closed, simultaneously with the fuel injection valve. Then the mixture of fuel vapor and air is ignited, with simultaneous opening of the exhaust valve. While the latter is still open the intake valve is opened again and the next cycle begins.

It is important that the pressure developed does not exceed 50 psi, hence the chamber can be light. Air cooling is sufficient, not only because of the large surface of the chamber but also because the air intermittently flows through the chamber, displacing residual combustion gases. The cycles of the two chambers that form a pair augment each other in such a way that the suction produced by the exhaust of the one draws air into the other. The exhaust speed can easily be regulated by metering the fuel and can be reduced to a. few hundred meters per second so that the plane can soon attain a velocity near that of its exhaust.

If political conditions in Germany had not prevented Goldau from continuing his work he might have succeeded in building such a craft in 1936 or 1937. Actually, the first jet-propelled plane, as is well known, was not of the Goldau type but of the type invented by an Italian, Secondo Campini, whose craft was finally built by the firm of Caproni. The first of these jet-propelled planes, the CC-1, was test-flown by Col de Bernardi in Aug 1940 near Milan. It took off by means of a. propeller, then switched to jet propulsion in mid-air. This model weighed around 8800 lb. The CC-2 built later, weighed about 11,000 lb, had a retractable undercarriage, dispensed with the propeller, and was a two-seater. It is to be noted that the CC-2 differed considerably from an early patent obtained by Caproni Aircraft in France in 1932.

The CC-2 was a low-wing instead of a high-wing monoplane, with the cabin perched on top instead of being placed in the nose. The CC-2 flew 168 mi (from Milan to Rome) attaining a speed of 130 mph. The flight took 2 hr 20 min counting in an intermediate landing near Pisa, probably for refueling.

Compared with Goldau's design, the interior of the CC-2 was enormously complicated. It consisted of a blower driven by a radial engine which sucked air through the open nose of the fuselage. Then the air was condensed, fuel injected into it, and the fuel ignited. The exhaust consisted of the combustion products plus the exhaust from the radial engine.

In spite of the voluminous justification of his system published by Campini (translated as Tech Memo No 1010 by the NACA) one may wonder whether that radial engine, if used to drive a propeller, would not have given a higher speed and a longer range to the plane. Goldau's design is justified by its simplicity, whereas the Caproni-Campini design appears to be unnecessarily complex and especially vulnerable to criticism based on the rocket theory in general.

A jet of which atmospheric air is an important component contradicts its own purpose to a large extent. If the jet is to be superior to the very efficient engine-propeller combination it can be only for flight in altitudes where propellers do not work well or for flights of a velocity that propeller driven airplanes cannot attain because of reduced propeller efficiency at very high rotative speeds.

Naturally these two things go together to a large extent, since such high speeds cannot possibly be achieved in layers of high atmospheric density. The jet-propelled plane, therefore, cannot expect to compete with the propeller driven airplane for speeds of less than 400 mph. Jet craft should be faster, which implies higher altitudes with thinner air.

For competition at lower altitudes the Caproni-Campini is unsuited because its complicated mechanism occupies the whole fuselage so that designers have some trouble finding room for the pilot's cabin.

But there exists another possibility for indirect jet propulsion at low altitudes — the jet helicopter, a helicopter with a rotor which is propelled by the reaction of a jet of some kind. Those who consider this a new idea will be surprised to learn that the first helicopter model that actually flew was of the jet-propelled type. It was, in fact, nothing but the old aeolipile of classic antiquity, whirling around and around by steam reaction in the manner of a rotating lawn sprinkler fitted with helicopter blades. This steam-reaction model helicopter was successfully demonstrated at Paris by one "Engineer Phillips" in 1842. It rose to a considerable altitude and covered a distance of almost half a mile. And it resulted in the then apparently inevitable cartoon mocking its possibilities.

The idea was again brought up in 1911 by a German inventor, Wilhelm Gaedicke, who under the nom-de-plume of "Eng Crassus" published a small book, Der gefahr-lose Menschenflug (Human Flight Without Danger). His project consisted of a multi-bladed helicopter with a cabin hanging from it. The rotor was to be turned by the reaction of jets of burning illuminating gas while the horizontal movement was to be accomplished by a single small exhaust nozzle in the rear of the cabin. His contention was that such a helicopter would attain a speed of 20 mph. A second nozzle might be used to increase the speed, he added after clearly stating that he personally failed to see a reason for higher speeds.

"Crassus" Gaedicke was only one of many inventors who advocated reaction-driven propellers or helicopter rotors. The idea of a reaction-helicopter is justified theoretically because of the fact that the blade-tip speed of the rotor is rather high, even if the helicopter itself is hovering. The blade tip speed may easily surpass 1,000 ft per second, which is high enough to make a jet efficient.

The difficulties in this case would be difficulties of design. If the jet generator (whatever that might be) is located in the fuselage it would require a hollow rotor shaft and hollow blades, and there would be the trouble of keeping all connections leak-proof. if the jet generator should be small and light enough to be rotated along with the rotor, most design difficulties. would disappear. It is difficult to judge this problem on a general basis without specific suggestions. But even so it can be said that the difficulties appear to be mainly of the nature of problems of design, instead of fundamentals.

We have seen that the direct reaction plane suffers mainly from the fact that the difference between flying speed and exhaust velocity is too large for efficient propulsion. The modified reaction or jet propulsion plane does not fare much better, for other reasons: The equipment required to work the modifications usually eliminates the theoretical advantages of rocket propulsion, and this tends to lead to schemes which of necessity defy themselves.

Let us now condense a number of theoretical surveys of the problem of the direct reaction airplane made by Dr Eugen Sänger in his book Raketen-flugtechnik and, somewhat later, in a paper which occupied an entire special issue of the Viennese aviation magazine Flug (Dec. 1934, translated as NACA Tech Memo No 1012, Apr 1942).

Dr Sänger began his work with a long series of actual experiments conducted with the facilities of the University of Vienna in 1931 and 1932. His rocket motors were spherical, made of steel, and fueled with a light fuel oil and liquid or gaseous oxygen under very high pressures. Dr Sänger achieved a constant recoil of around 55 lb for periods from 15 to 25 min.

Experimental evidence, as well as theoretical considerations, made it clear that sustained propulsion in the manner of the engine-propeller combination cannot be expected of rocket motors. The whole performance would hinge on the problem of attaining a very high speed — and with it a high altitude — as quickly as possible for the sake of efficiency, and also the problem of converting the resulting high speed into range.

Keeping these conditions in mind, Dr. Sänger arrived at the following conclusions: The plane should take off under an angle of about 30 deg and climb under that angle until the desired high altitude has been attained. Then the craft should level off. After two or three minutes of level flight, the fuel supply would be exhausted, but the plane would go on, slowly losing speed as well as altitude, until time to land. The figures for a typical flight would be: Burning time of the rocket motor 20 min; total duration of the flight, slightly more than 1 hr, and average speed, 1,600 mph.

In general, Dr Sänger came to the decision that for a given payload the fuel expenditure for a certain range would be about the same for such rocket airplanes and for conventional airplanes, the only gain being time.

Somewhat later (Jan 1936) Dr Sänger published a paper on auxiliary rocket motors for propeller airplanes in the Schweizcrische Bau-Zeitung (Vol 107, No 2). Here he assumed that a large rocket motor (Fig 9) would deliver 1 ton of thrust for a period of 90 sec, and that it could be installed in a pursuit plane weighing normally 3,740 lb and capable of attaining a maximum speed of 320 mph. Such a plane conventionally would need 8 min to climb to an altitude of 20,000 ft, but with a rocket motor of 1 ton of thrust, the same plane could climb to 20,000 ft in 86 sec.

The climb, assumed to be made at the advantageous angle of 30 deg, would be accomplished by the rocket motor alone, while the propeller would only furnish the power required to overcome air resistance. This may also be expressed by saying that the engine-propeller would work as if the plane were in a horizontal flight at top speed, while the rocket lifted it.

The whole rocket apparatus would not be too large to fit into such a plane. It would consist of three spherical tanks — one for liquid oxygen, one for fuel oil, and the third for compressed nitrogen, the latter used to force the two liquids into the combustion chamber. The outside diameter of the oxygen tank would be about 30", the ODs of the other two tanks about 2' each. Further, valve of assembly would be rather small, with the rocket motor itself of about 7" dia and roughly twice as long. This motor would be in the extreme tail end. The weights were calculated as shown in accompanying Table I.

Being equipped with such a device, the plane would weigh 4,700 lb (instead of 3,740) with a resulting increase in takeoff speed of about 12%. The gain would be 6 or 6½ min of time for a climb to 20,000 ft, the disadvantage a dead weight about 200 lb.

That a 1-ton-thrust motor could be built after some experimentation is hardly subject to doubt, but it would be difficult to decide whether the 6 or 6½ min gain for the climb would count more than the final dead weight of 200 lb, or vice versa. It seems, however, that the experts of the Luftwaffe decided the handicap would be greater than the gain, because they have obviously failed to make use of Dr Sänger's theoretical groundwork, although they had it available ever since its publication.

It seems, in retrospect, as if the Germans instead of doing experimental work along these lines, waited to see what would happen with the Caproni-Campini jet-propelled type. This was a mistake, for not only did the performance of the Campini plane fail to amount to anything, but the Axis, as became known early last month, was neatly scooped by the British.

At the time the CC-2 limped from Milan to Rome in Dec 1941, the British jet-propelled plane, originated by Capt Frank Whittle, had already made its first successful flight (May 1941), while the first successful run of the engine had been made in complete secrecy as early as 1937. It has been reported that Capt Whittle conceived the idea of his jet-propelled plane in 1933, that the secret was made known to Gen Arnold five months prior to Pearl Harbor, and that the engine with which the first flight had been accomplished was shipped to the United States in Sept 1941.

The first American jet craft by Bell Aircraft Co is reported to have had two engines, presumably looking like a twin-engined plane, and the British model is said to resemble the P-38 Lockheed Lightning in external appearance. It has not been disclosed whether the British craft has one or two jet engines (though similarity to the P-38 implies two) but it has been stated that the type of engine it employs consists of a blower coupled with an internal combustion turbine which burns kerosene.

The plane emits a loud whistling sound when in flight. In fact the reason for releasing the secret was that many people had seen the plane during test flights and guessed from the sound that it must be a new type. The Squirt, as British spectators were quick to call the craft, is said to be especially suitable for flight at high altitudes, and observers have stated that it is so fast "that it makes Spitfires seem slow".

The well known British air expert, Maj Oliver Stewart, admitted during a radio interview that the Whittle plane has the disadvantage of having an extremely high fuel consumption.

There can be no doubt that the new jet types are far superior to the Campini plane. Still, peacetime applications await further experiments.

Table I
Calculated weights for Dr. Sänger's Proposed Auxiliary Rocket Device for Takeoff Climb

(Assumed performance: 1 ton of thrust for 90 sec)
Fuel tank with valves, empty……22 lb
Oxygen tank with valves, empty……45 lb
Nitrogen tank with valves, empty……115 lb
Rocket motor and fuel lines……23 lb
    Total rocket assembly……205 lb
Fuel (oil)……165 lb
Liquid oxygen……510 lb
Compressed nitrogen……70 lb
    Total fuel……745 lb
  Grand Total……950 lb

This two-part article was originally published in the January and February, 1944, issues of Aviation magazine:
    Part I: vol 43, no 1, pp 147-149, 309-311, 313.
    Part II: vol 43, no 2, pp 121-125.
The original article includes 3 photos and 13 drawings and diagrams. Inconveniently, for purposes of conversion to HTML format, the figures are numbered starting at Fig 1 in each Part.
Illustrations in the Figures are not credited.

Notes: Mr Ley's sources of information might be considered inadequate or inaccurate — neither is remarkable in wartime.
The American jet by Bell was the P-59 Airacomet. The two engines were buried in the fuselage and the plane was transported with a fake propeller, passing as and upgraded Bell 'Cobra-series (single-engined) fighter.
The first British jet was the Gloster Meteor. It looked much more like a Mosquito than a Lightning, with two jet engines in mid-wing nacelles.
The de Havilland Vampire was Britain's second jet fighter. Its tailplane was supported on twin booms, with a central fuselage for the pilot, single jet engine, and avionics equipment. With the twin-boom construction, it could be considered to resemble the P-38 Lightning.

The rocket ship in the movie When Worlds Collide, while somewhat more streamlined, looks to have been inspired by Dr Sänger's design (not shown here.)