to the new edition of “A Method of
Reaching Extreme Altitudes” and
“Liquid-Propellant Rocket Development”
The writer’s two papers on rockets, published by the Smithsonian Institution in 1919 and 1936, are so different in content that it seems best to write a separate foreword for each.
It is interesting to compare the “state of the art” at the time the above paper was written with the viewpoints as well as the accomplishments of the present moment. Smokeless powder rockets, during World War II, have grown in size and controllability, but have not given much higher jet velocities than were earlier obtained. Multiple charge, or reloading, powder rockets appear still to be in the status mentioned on page 68.
Liquid fuel rockets, which were presented through a patent reference with little emphasis, have progressed very considerably, and have caused the assumed weight ratio of 1/15 for empty to loaded rocket to appear much more attainable. The jet velocities for liquids, however, notwithstanding the higher energies, are still in the smokeless powder range.
So far as general principles and conclusions go, both appear to be as sound as ever. Thus high jet velocity is still of as much importance for long ranges as light rocket construction. Moreover, the jet velocity should be high for the entire propulsion period, even though a low jet velocity in the early part of the ascent would give a greater energy efficiency. This low jet velocity would obviously be disadvantageous in requiring a larger initial fuel load.
The development of the theory on the basis of the conservation of momentum remains to the writer preferable to using energy. Especially worth while is the procedure of varying the thrust in such a way as to maintain an optimum velocity at each point of the flight, in order to minimize the fuel load. The multiple, or step, rocket principle would be as good as ever in maintaining a low ratio of rocket to fuel, if the application of this method had been found necessary.
As was suggested, a start from a great elevation would reduce considerably the initial mass of propellant required. A further reduction, not mentioned in the early paper, could be made by using some of the tremendous energy of the blast to give the rocket a substantial initial velocity.
Finally, the subject of projection from the earth, and especially a mention of the moon, must still be avoided in dignified scientific and engineering circles, even though projection over long distances on the earth’s surface no longer calls for quite so high an elevation of eyebrows. In this connection, it would appear best merely to repeat the concluding paragraph on page 57, which holds good as much now as then, and to remember that, from some points of view, twenty-five years is not so very long, after all.
The 1919 paper indicated the theoretical possibility of obtaining great ranges by means of well designed rockets; showed also that fairly high jet velocities were obtainable, and mentioned progress in the construction of a solid cartridge, magazine-type rocket, in which the weight of explosive charge to rocket could be large.
The next step was to develop such a rocket that would operate for a considerable time, and have satisfactory flight characteristics. While this work was in progress, some simple combustion tests were carried out with liquid oxygen and a number of liquid fuels. These tests indicated that liquid oxygen could be handled without much difficulty, and that satisfactory combustion could probably be obtained in a chamber, or motor, of comparatively simple design.
Proving stand tests were accordingly carried out with liquid oxygen and gasoline from 1920 to 1926, when a liquid fuel rocket flight was obtained. Those who took part experienced a lift in spirit for the reason that the rocket functioned, but the distance attained did not seem great enough to warrant calling attention to the event.
Further short flights were made up to 1929, when Colonel Charles A. Lindbergh came to Clark University to learn of the status of the work. He concluded that the liquid fuel rocket flights, even though short, had significance, and encouraged Mr. Daniel Guggenheim to make a grant for a further development. The work begun in this way was continued later under the Daniel and Florence Guggenheim Foundation, the details being as set forth in the 1936 paper.
Flights of a thousand feet or so, with a relatively heavily loaded rocket, showed at once that automatic stability in flight would be needed before making any attempt to increase the range. Gyroscopically controlled vanes in the rocket blast gave, first, an indication of flight correction and finally continued correcting from side to side, at approximately a 10° displacement angle, as long as the chamber was in operation. Beyond this point the trajectory was curved, as it had previously been during propulsion without gyro control. The flame was clear in some tests, and accompanied by smoke in others, the latter showing the correcting best in flight photographs.
It was believed by 1935 that the results indicated the possibility of making a rocket ascent which would be high compared with the records for both meteorology and aeronautics, when a comprehensive report would, of course, be in order. Nevertheless, it seemed worth while to publish the 1936 paper as a progress report, in order to show what had already been accomplished.
It was thereafter found necessary to carry further the work on flight characteristics in order to obtain correction during the coasting period, as otherwise the speed when the parachute was released was likely to be excessive. This work led to flight correction to within 3°, during both the powered and coasting periods, together with reliable parachute release at the peak of the flight.
There remained the production of a very light weight rocket, namely one having a pump-turbine drive and light tanks, using the guiding system that had already been worked out with pressure tank rockets. Preliminary flights with such a rocket were under way in 1941, at the time the development was dropped because of other, more urgent war problems.
ROBERT H. GODDARD
May 1, 1945