Aircraft propelled by rockets may one day solve the problem of stratospheric flight
BUILT TO SEND A ROCKET TO THE MOON. This photograph was taken looking up the framework intended to guide a rocket on a projected flight to the moon. Two guiding rails, one on either side, can be seen. The scheme was worked out in America.
A glance at the development of modern heavier-than-air machines shows a constant progress towards ever greater speeds and higher altitudes. It is not surprising that the upper atmosphere should be an attractive goal for aeroplane designers. Travel in these highly rarefied regions would necessitate the expenditure of but a fraction of the power required to overcome the resistance offered by the denser air at the lower levels.
This region of the stratosphere and beyond is also one of constant climatic conditions: no fogs, storms, ice, nor lightning would detract from the safety of travel at these heights.
The graph below shows the altitude records for landplanes between the years 1909 and 1934. The heights are shown on the left, and the corresponding years at the foot. A fair curve drawn as nearly as possible through these points shows clearly the continuous upward trend. It shows equally clearly, however, that the slope of this curve is steadily decreasing as the years progress and indicates that any advance is being made with growing difficulty. Moreover, there are today far more men engaged in research than in the early days.
ALTITUDE RECORDS for landplanes between 1909 and 1934. The difficulty of increasing the height record with orthodox aeroplanes is illustrated by the flattening of the curve.
Despite the progress which has been made during the last thirty years, and despite the undoubtedly great future which awaits the screw-propelled aircraft, there is a limit to the altitude attainable by this type of machine. This is due to the nature of the conditions in which screw-propelled and wing-supported aircraft can function efficiently. The air must at least be dense enough to allow a tractive effort of the propeller, and it must be able to support the machine in forward flight.
Although this limiting altitude will probably be many times that of our present records, screw-driven craft can never penetrate those realms of partial vacuum where resistance losses will be virtually zero. The velocities will be limited by the power available. For medium speeds in air at any given altitude the power required is proportional, not to the velocity, but to its square; for ultra high speeds the power required becomes proportional to the cube and then to higher powers of the velocity.
This is clearly seen from the graph shown below which gives the results of the Schneider Trophy contests from 1913 to 1931. Here again the years are shown at the foot of the graph. The points on the line A give the average speeds for the winning aircraft for the various years; the points on the line B give the corresponding engine brake horse-power. Fair curves are drawn as nearly as possible through these two sets of points. It is easily seen that, with approximately equal increments of record speed the amount of power required is continually increasing at a greater rate.
SEAPLANE SPEEDS in the Schneider Trophy contests from 1913 to 1931. The curve A shows the increase in speed, the dotted curve B the increase in horse-power required to produce the extra speed. More and more power is required to produce smaller and smaller increases in speed.
To the designer seeking high-speed transport, the highly rarefied upper atmosphere and the vacuum of space provide the answer to yet another problem. At our present speeds the heat developed due to the friction of the air on exposed surfaces, and due to the adiabatic compression at the nose, causes no inconvenience and can be neglected. As speeds increase, however, this would no longer be so.
At a speed of 1¼ miles a second the temperature in front of the craft would be 150° C. Some form of heat insulation — probably of the vacuum flask type — would be necessary to prevent the interior conditions of a craft from coming into equilibrium with this temperature. At 4½ miles a second the temperature would be well over 500° C and at 6 miles a second 700° C would be exceeded.
These are the factors, among others, which have turned the attention of a steadily increasing number of scientists and engineers throughout the world to some form of propulsion other than the airscrew: some form which would be entirely independent of any surrounding medium. An answer to this question has long been known, but it is only during the past twenty-five years that it has been treated from a truly scientific viewpoint.
This answer is supplied by the theory of reaction propulsion, which has embodied in it certain peculiarities giving it distinct advantages. The propulsive action is, in this theory, independent of a surrounding medium or any outside agent.
The simplicity of the principles involved makes the theory easily understood. The basis is Newton’s Second Law of Motion, which states, “Rate of change of momentum is proportional to the acting force, and takes place in the direction in which the force acts”, combined with Newton’s Third Law of Motion, “To every action there is an equal and opposite reaction”. It is essential that the fundamental ideas should be grasped at the outset, as they are the principles upon which the whole science of “Astronautics” (a word often used to describe space-flight) is based.
Analogy of Rifle “Kick”
The projectile — freely supported in space — shown in the diagram below is considered, before the explosion between D and E has occurred, to be at rest in relation to a given frame of reference. The explosive charge between D and E is then fired; a large amount of gas is produced which exerts a force in all directions. The force acting on E will be the same as the force acting on D and therefore on the body of the projectile plus.the remaining charges.
THE PRINCIPLE OF THE ROCKET is illustrated by this diagram. When the explosion occurs, it produces an equal pressure in all directions, thus forcing the rocket forward. A rocket will function in a vacuum in the same way as it does in air.
At some instant during the explosion let this force be E, then if m and a¹ refer to the mass and acceleration of E, and M and a to the mass and acceleration of the body A and remaining charges,
Hence a=m/M a¹.
So it follows that, when the force of the explosion is spent, the final velocities will be inversely proportional to the masses involved. That is, if the mass ratio is Q, that is, M=Q m,
Thus, with regard to the original frame of reference, it is seen that the body has had a velocity V imparted to it without the necessity of any outside medium or external agent. This will hold as well for the vacuum of space as it will anywhere else. For a vacuum the efficiency would be maximum because of the absence of a resisting medium.
From this point it is obvious that, by exploding the second charge, that is, the one between C and D, the velocity will again be increased. This time the increment of velocity will be greater than in the previous instance, as the remaining mass has been decreased by one section plus powder charge. This is repeated for the next charge and so on, the final velocity being the vector sum of all the imparted velocities.
A direct example of this is found in the “kick” of a rifle. It is often supposed that this recoil is due to the thrust of the shot and discharged products of the explosion against the outside air. This supposition is erroneous; the true facts show that as the shot moves in one direction, by the equivalence of momentum, the gun tries to move in the opposite direction.
A more striking example is that of the ice-skater who, while standing still, tries to hurl a large stone in a forward direction and finds himself moving with an appreciable velocity in the opposite direction. The principle underlying this phenomenon is exactly the same.
THE MOON ROCKET shown in sectionalized form, which was designed for use in the frame shown at the top of this page. The flash powder was to have exploded when the rocket hit the moon and thus indicated to those watching through telescopes that the rocket had reached its objective.
The above example of the projectile is an instance of the ejection of large masses one at a time. The ejection of small particles continuously at very high speeds would have exactly the same final effect, with the added advantage that the velocity-time graph would show a smooth curve, contrasted with the discontinuous curve of the original example. Such an arrangement producing a continuous tractive effort is seen in the ordinary sky-rocket, in which the gases and products of combustion are themselves being ejected at high speeds and thus cause the resultant reaction. It is the consideration of the rocket which has been the starting point of the real scientific investigation of reaction propulsion.
Although imagined to be a new arrival on the field of science, reaction propulsion has a history dating as far back as that of aircraft. The Chinese developed an efficient form of incendiary rocket which they used with considerable effect against the Tatars as long ago as the thirteenth century.
Cyrano de Bergerac enters the realm of fancy in his book, Voyages to the Moon and the Sun. In this story his explorer fails on his first attempt to leave the Earth, but the second attempt proves successful and he goes in a box-like contrivance propelled by rockets, the fuel for which eventually runs out. That story was written in 1640.
In 1866 Jules Verne published From the Earth to the Moon. Although purely fantastic, the story of this imaginary adventure was obviously guided by a realization of scientific facts, a point neglected in earlier works. Here Jules Verne imagines a vast cannon with a bore 900 feet long to shoot his craft to the moon. How the occupants of this vessel endured the almost instantaneous acceleration to the required velocity of liberation of about seven miles a second is not discussed, but the interesting point is that the author suggested the use of rockets attached to the nose of the craft to check the fall on the moon.
These suggestions were interesting in their originality, and they prepared the way for the serious work which was to follow. Unhappily it must be admitted that the rocket — the most useful representative of reaction propulsion — first showed itself to advantage and won distinction as an engine of war. The pioneer work was done by the military engineer, General Desaguliers; the work was carried on with even greater vigour by William Congreve.
At Woolwich Laboratory, London, an incendiary rocket was produced having a range of two miles. Experiments were continued with success, and so effective were the results that Sir Sidney Smith equipped a number of boats with rockets in the 1805 campaign against the French. They were not used that year, but in 1806 an expedition against the French port of Boulogne used rockets to advantage. In 1812 a Field Rocket Brigade was formed which distinguished itself at the battle of Leipzig in 1813 and at Waterloo in 1815. Congreve even forecast that rockets would eventually supersede artillery and at one time it looked as though this would be so; they certainly played a prominent part in nineteenth century warfare.
Development of a more peaceful nature has been carried steadily forward in the form of signal and life-saving rockets. The invention of the rocket-borne lifeline is attributed to a Frenchman, Ruggieri, who lived during the latter part of the eighteenth century. Henry Trengrouse of Helston, Cornwall, was the first Englishman to conceive the idea of taking a line from the shore to a wrecked vessel. That was about 1806. By 1826 four life-saving stations had been formed in the Isle of Wight under the supervision of Dennet and were equipped with rocket apparatus.
LIQUID FUEL was used in place of powder in rockets with which Professor Goddard made his later experiments. This photograph shows a liquid oxygen-petrol rocket in the frame from which it was fired on March 16, 1926, at Auburn, Massachusetts. One of the advantages of a liquid fuel is that by special feeding methods a constant lifting force may be obtained.
The greatest advance was made in 1855, when Colonel Boxer, working at Woolwich Laboratory, produced a rocket of far greater range by joining two rockets together, the second coming into operation as soon as the first had been burnt out. By 1870 rocket equipment was being used in many stations all round the coast. Since 1870 the rocket has been instrumental in saving over 13,000 lives. At the present day a 6-lb. rocket will carry a 1-inch hemp line some 400 yards.
The serious investigation of the possibilities of reaction propulsion began almost simultaneously in Europe and America. In France it was the scientist and inventor, Robert Esnault-Pelterie, who first considered the problem from a technical point of view. His first calculations were begun in 1907 and were eventually amplified, but it was not until 1912 that the results of his investigations were presented in a paper to the Societe Francaise de Physique.
During this time Professor Robert H. Goddard of America was similarly engaged. Professor Goddard began his investigations in 1909. In the years 1912 and 1913 at Princeton University he considered the possibility of improving the efficiency of the rocket, but it was not until 1915-16 that he carried out his experiments on steel chambers and nozzles at Clark University, Worcester, Massachusetts.
Professor Goddard made an extremely interesting series of calculations and experiments, first with ship rockets and later with the specially designed steel nozzles. The main idea behind the work was to find a means of increasing the performance of rockets and to make it possible by their use to attain very great altitudes for the investigation of the composition of the upper atmosphere and for meteorological purposes.
Professor Goddard used a ballistic pendulum to which the rocket was attached, and from the displacement of this during the firing the velocity of the ejected gases could be calculated. The
efficiency was given by the kinetic energy in the discharged products of combustion and the energy contained in the powder from its calorific value. This efficiency proved to be low, being only about 2 per cent for the common rocket and about 2.3 per cent for ship rockets.
Experiments were continued with the construction of a steel chamber having a divergent nozzle similar to the nozzle used in steam turbine practice, thus increasing the velocity of discharge by expansion in the jet. The charge of smokeless powder was fired electrically and the displacement of a ballistic pendulum was automatically recorded on a smoked glass plate.
This design gave a far higher value for the discharge velocity than in the previous instance, being a little under 8,000 feet per second compared with the 1,000 feet per second for the ship rocket. With this high velocity the efficiency was increased to about 60 per cent. At the time this was the highest speed of discharge ever attained — just over one and a half miles a second.
So far all the experiments had been carried out in air, but Professor Goddard recognized the desirability of conducting similar investigations in a vacuum. With this in view he devised a means of supporting his heavily weighted gun vertically by a spring. The discharge would then cause an upward displacement which could be automatically recorded. The whole apparatus was placed in a large cylindrical tank which was then exhausted of air to a pressure of between 7 and 0.5 mm. of mercury. The results obtained from this method were almost the same as for the previous experiments.
Later, Professor Goddard replaced the cylindrical tank with a long tube made almost in the form of the figure “6”, the apparatus being placed at the top, the pipe exhausted and the “gun” fired electrically in the usual way. This avoided the disturbing influences of gaseous rebound. Results were again virtually the same.
These experiments showed at least that reaction propulsion does function efficiently in a vacuum — a point at one time often strenuously denied.
Professor Goddard’s calculations, experiments and conclusions appeared in the Proceedings of the Smithsonian Institution Miscellaneous Collections, Vol. 71, No. 2, for 1919, under the title “A Method of Reaching Extreme Altitudes”. His work proved to be of such interest and pointed to such possibilities that he received a grant to continue his researches.
In Germany, too, work was progressing under the enthusiastic leadership of Professor Hermann Oberth, a mathematician, physicist and astronomer of the first order.
In America Professor Goddard was able to continue his researches, but he had turned his attention from powdered fuel to liquid fuel. Between 1920 and 1922 he investigated the combustion of liquid oxygen with various hydro-carbons and he realized that by a suitable arrangement such a mixture could be fed into the combustion chamber which would thus give a constant lifting force.
On November 1, 1923, a rocket motor, that is, combustion chamber and nozzle, was worked in the testing frame by this method, the fuel being liquid oxygen and petrol. From that time onwards liquid fuel became more and more widely used. On March 16, 1926, the first ascent of a liquid fuel rocket was made in Auburn, Massachusetts. Further flights were made, but it became increasingly obvious that some form of stabilizing device was necessary.
It was proof enough that the idea of rocket travel was no longer being looked upon as a harebrained scheme of a few visionaries when in 1929 the late Daniel Guggenheim offered a grant of £20,000 to enable the experiments to be continued and the Carnegie Institution of Washington made an additional grant.
In France Robert Esnault-Pelterie had completed for the Astronomical Society of France his paper, which he delivered in an address on June 8, 1927. His book L’Astronautique was published in the following year. In this he covers every conceivable aspect of astronautics or space flight, dealing in detail with celestial mechanics, the theory of reaction propulsion, and combustion and discharges through nozzles. He even looks ahead and considers the possibilities and difficulties of such an undertaking as an interplanetary journey.
In Germany, Oberth, Max Valier and Otto Willi Gail had taken the lead and Fritz von Opel had become interested. In the summer of 1928 Professor Max Valier and Fritz von Opel built a rocket motor car and tested it on the Opel Motor Works track at Russelsheim, near Frankfurt. There were at the rear of the car twelve nozzles arranged in a rectangle. A speed of over 130 miles an hour was reached before the rocket fuel was consumed. Opel built a second, third and fourth car; in each instance the fuel was of the powdered form. His fourth car met with disaster and was wrecked by an explosion in August 1928. Official disapproval deterred him from a fifth attempt.
In the same year Valier made several tests on light wooden cars; his third attempt, however, ended in failure when the car overturned. After this he applied the rocket principle to drive a sledge on the ice of Lake Starenberg and attained a speed of 235 miles an hour.
The first successful flight of an aeroplane propelled only by rockets was conducted on September 30, 1929. Fritz von Opel was pilot and he flew about one and a quarter miles, rising 50 feet in the air and reaching a maximum speed of 85 miles an hour.
Early in 1930 Max Valier drove a car, the rocket motor of which, though weighing only 7 lb., developed over 40 horse-power. Only a month later he was killed by a sudden explosion while standing near his rocket car. He was the first victim of the new science.
In 1930 the Raketenflugplatz (“Rocket Aerodrome”) was established at Reinickendorf, Berlin, and here under the guidance of Professor Hermann Oberth experiments have continued ever since. A certain amount of research was carried out on small rockets to find the effect of the form of the combustion chamber on the performance. It was eventually found that an egg-shaped chamber was the most efficient. In one example the entire rocket with fuel weighed only 9 lb. but developed the surprising upward force of 20 lb.
In 1930 André Hirsch and Robert Esnault-Pelterie offered a prize of 10,000 francs (about £56 at current rate) to be won annually for the best work furthering the development of Astronautics. By 1934 Robert Esnault-Pelterie had found it necessary to write a supplement to his book L’Astronautique; the supplement was published in 1935. Work in Russia, also, under the able leadership of Professor Nicholas Rynin of Leningrad, was progressing quickly.
In America Professor Goddard transferred his activities to Roswell, New Mexico, and during 1930-32 he conducted a series of static tests. In one instance a chamber weighing only 5 lb. produced a constant lifting force of 289 lb. The gases were probably ejected at about an average velocity of 5,000 feet per second, thus giving the mechanical horse-power of the jet as 1,030, which means the truly amazing figure of 206 horse-power per pound of motor.
In March 1935 Goddard produced a gyroscopically stabilized rocket which reached a height of nearly 5,000 feet with a maximum speed of 550 miles an hour. By May 1935 a rocket had been produced which rose to a height of 7,500 feet and attained a speed of 700 miles an hour.
The report of these later flights and the experimental work carried out in New Mexico was published in 1936 in the Smithsonian Institution Miscellaneous Collections, Vol. 95, No. 3.
In Europe and America the work has been steadily progressing. Nothing spectacular is performed and no extravagant claims are made by the serious scientific investigators. Yet reaction propulsion has been continually advancing as a science and may well hold the key to the future of high-speed travel at high altitudes.
With the high speeds which would be possible in the extreme limits of the upper atmosphere peoples of widely separated places could be brought into contact by only a few hours’ travel where today as many days are necessary. To quote Dr. H. H. Sheldon, Chairman of the Department of Physics, Washington Square College, New York University: “There is but one goal left for man to attack — the upper atmosphere and beyond”.
A “ROCKET AERODROME” was established near Berlin in 1930 under the direction of Professor Hermann Oberth. This photograph shows a discharging stand for small rockets. By means of cables, the rockets are fired electrically from the safety of an observation room.