CLARK COLLEGE
worcester, mass.
To The President of the Smithsonian Institution,
Washington, D. C.
My dear Sir;
This communication I had intended sending a little later, I feel that it would not be desirable to delay any longer. Incidentally, I think it would be best not to make it public.
For a number of years I have been at work upon a method of raising recording apparatus to altitudes exceeding the limit for sounding balloons; and during the last two years I have tried-out the essential features of the method at the Laboratory of Clark University with very gratifying results. These experiments are now completed, and I feel that I have settled every point upon which there could be reasonable doubt. Incidentally, I have reached the limit of the work I can do single-handed; both because of expense, and also because further work will require more than one man's time.
My reason for writing just now is the following: My device will be capable of propelling masses, such as expolsives, for very great distances, and hence would very likely be useful in warfare; although for very great ranges the accuracy would not be extreme. Dr. A. G. Webster of Clark University, who, as you doubtless know, is a member of the Naval Consulting Board, has recently broached the subject of my device to that Board, and has asked me when I can give him a statement concerning it. I hesitate, however, permitting Dr. Webster to place the matter before this Board for two reasons: First, I can trust no one to explain the device and its capabilities but myself. If I am not permitted to do this, in a manner satisfactory to myself, the Board may be poorly impressed; and public opinion may thus be influenced against the method, even before it has been given a fair presentation. My other reason is that the device will, I am certain, be of very great importance to pure science, especially to meterology; whereas, although it may indeed have applications, I feel that its possibilities in warfare may be somewhat limited. In short, the exclusive use of the device for warfare would, I am certain, be a loss to science; and I therefore feel that an investigation of my method should be conducted by such a body as the Smithsonian Institution (working independently, or in collaboration with the Naval Board if thought desirable); with the understanding that if, in subsequent work, results should arise which appeared to be of importance for National Defence, the government should be given exclusive data concerning these results.
The following is a very brief description of the method, and of the experimental results:
Four years ago, while at Princeton University, I developed a theory of rocket action in general; taking into account air resistance and gravity. The problem was to determine the initial mass of an ideal rocket necessary, in order that on continuous loss of mass, a final mass of one pound would remain, at any required altitude. On solving the problem (an approximate method was necessary in order to avoid a new problem in the Calculus of Variations) I was surprised at the comparatively small initial masses necessary, provided the gases were ejected from the rocket at a high velocity, and also provided that most of the rocket consisted of propellant material. The reason for this is, very briefly, that the velocity enters exponentially in the expression for the initial mass. Thus if the velocity of the ejected gases be increased, say five fold, the initial mass necessary to reach a given height will be reduced to the fifth root of that required for the lesser velocity.
An apparatus was designed which possessed a number of important improvements over the ordinary rocket. In connection with these improvements, it should be explained that I have protected myself (and I hope meterological science, as well) by certain United States Letters Patent; four of which have already issued:1,102,653
1,103,503
1,191,299
1,194,496.
Inasmuch as designs and theories alone can hardly carry conviction, certain experiments were desirable in order that any doubtful points might be settled. I first found that the efficiency of an ordinary rocket is remarkably small; namely, less than two per cent. Even the one-pound Coston ship rocket, which I found to have a range of a quarter of a mile, has an efficiency of but little over two per cent; the average velocity of ejection of the gases being about 1000 ft/sec.
My next experiments were performed with steel chambers, of the form shown in Fig. 1, with the object of increasing the efficiency of the device, as a heat engine; and of therefore obtaining as high a velocity of ejection as possible. Some of these chambers were made of soft tool-steel, and others of tough nickel steel. Fig. 3 shows a chamber taken apart. A charge of dense smokeless powder, P, Fig. 1, was fired electrically. The resulting gases, after blowing out the small pasteboard wadding, B, entered a tapered nozzle, expanded, and left the nozzle with high velocity. It will thus be seen that I obtained the work of expansion of the gases, much as is done in the De Laval steam turbine.
The efficiencies and velocities which I have obtained in this way are truly remarkable. My highest efficiency, or rather "duty", is over 63 per cent. This is the highest ever obtained from a heat engine; the best reciprocating steam engine giving 21 per cent, and the Diesel (internal combustion) engine, about 40 per cent. My average velocities of ejection are, in most cases, over 7,000 ft/sec, and the highest is slightly under 8,000 ft/sec. This is far in excess of any velocity hitherto obtained except by minute quantities of matter in electrical discharge tubes. The highest velocities and efficiencies were obtained with a large chamber.
After most of the above experiments had been performed, a question arose as to the possible effect of the air in the nozzle and immediately beyond. It seemed likely that some fraction of the above velocity (measured, of course, ballistically, by the recoil of the chamber) might have been due to reaction against the air. This would evidently mean that, as as high altitudes were reached, and the density of the air became small, the effectiveness of the device would decrease.
A number of rather difficult experiments were performed last year to test this point. The chambers, C, Fig. 3, used in air, were suspended by a spring from a 3-inch pipe-cap, A. The mass of the movable system was increased by lead collars, K; and a strip of smoked glass, G, was provided to record the displacements produced by firing. This is, of course, a very condensed description of the apparatus.
In order to approximate as closely as possible to the condition of firing into a tank of infinite extent, a special pipe system was constructed (by autogeneous welding), Fig. 4. The system shown in Fig. 3 was introduced at the top of the upright pipe, P, of Fig. 4. It will be seen that, on firing, the expelled gases passed around the circular part of the 3-inch pipe system until their velocity had been reduced to a small value by friction. It was found that the rebound of the gases was rarely sufficient to tear very thin tissue paper at T, Fig. 3. By means of an entirely different form of tank, which permitted actual measurements of the rebound to be made, it was demonstrated that such rebound was negligable. All of the experiments in vacuo showed that the recoil was practically the same as that at atmospheric pressure, down to a pressure of 0.5 mm. The recoil was therefore the result of an actual jet of gas, and was not due to reaction against the air.
It will be evident that a heavy steel chamber, such as described, could not compete against the ordinary rocket, even with the high velocities which I attained. If, however, successive charges were fired from the same chamber, much as in a rapid-fire gun, most of the mass of the rocket could consist of propellant, and the superiority over the ordinary rocket could thereby be increased enormously. Such reloading mechanisms are the subject of all the above patents except the first, which is chiefly concerned with the nozzle, and what I have termed a "primary and secondary" rocket principle. I have not made a working model of a reloading device, as it is the one feature of the method that is self-evidently operative.
Regarding the heights that could be reached by the above method; I have applied the theory already mentioned to cases which my experiments show must be realizable in practice, and I find that a mass of one pound could be elevated to altltudes of 35, 72, and 232 miles; by employing initial masses of from 3.6 to 13.6, from 5.1 to 24.3, and from 9.8 to 89.6 lbs., respectively. If a device of the Coston ship-rocket type were used instead, the initial masses would be of the order of magnitude of those above, raised to the 27-th power. I hesitate to give my conclusions regarding the possibility of sending small masses (under what I feel sure are realizable conditions) to very much greater heights than those I have just mentioned.
Regarding the possibility of recovering apparatus upon its return; calculations show that the times of ascent and descent will be short, and that a small parachute should be sufficient to ensure safe landing.
I have in manuscript form an extended description of what I have briefly outlined, which is nearly completed; and have also a number of photographs and lantern slides, which illustrate all the steps in the experiments. I doubt if this manuscript could be entirely finished for several weeks, owing to work connected with beginning the college year. A talk could, however, be presented on much shorter notice.
In the meantime, you would do me a great favor if you would give me some information upon the following points:
1st. Would the Smithsonian Institution be willing to bring together a committee to investigate the method of reaching high altitudes which I have herein briefly summarized?
2nd. Upon favorable report of such a committee, would the Smithsonian Institution be willing to take upon itself the recommending of a fund, sufficiently large to continue the work, either from a society, as the National Geographic Society, or from private individuals?
I realize that in sending this communication I have taken a certain liberty; but I feel that it is to the Smithsonian Institution alone that I must look, now that I cannot continue the work unassisted. I am
Yours sincerely,
Robert H. Goddard
This work is in the public domain in the United States because it was published before January 1, 1929.
The longest-living author of this work died in 1945, so this work is in the public domain in countries and areas where the copyright term is the author's life plus 78 years or less. This work may be in the public domain in countries and areas with longer native copyright terms that apply the rule of the shorter term to foreign works.
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