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March 1920 - "Report Concerning Further Developments" in Space Travel

Robert Goddard Report - March 1920 - Title Page


March 1920.

R. H. Goddard. 


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There are several reasons for bringing the attention of the [Smithsonian] Institution to the matters contained in the present Report, although it was not intended that this should be done until later.
First, in the effort to avoid sensationalism, the public has not been made to realize the importance of the work that has been proposed. Interest has, to be sure, been shown in the scientific applications of the new rocket principle, but not nearly to such a degree as has interest been shown in possible applications to planetary investigations. As the matter stands now, however, the public does not appreciate the importance of the researches that have already been suggested in making such planetary investigation possible.
Secondly, there are lines of attack, not hitherto suggested, which are certain to lead to important results, and upon which work can profitably be begun as early as may be possible. Many people do not, of course, realize that, in solving new problems, results come only through the application of research methods, applied to suggestions that have a sound physical basis. The suggestions in the present report, experiments concerning which have in certain cases already been made, have such a basis, and it is certain that a research conducted upon them will lead to a diminution of the initial masses, and also of the times of transit, between planets, from what would appear to be necessary for the rocket as described in the recent publication. Of course experimental difficulties are certain to arise, but ex- 

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capsule, or container, composed of a substance that is solid at ordinary temperatures; for example, some form of smokeless powder. In practice the cartridge will be inside the chamber but a very short time before firing takes place; being fired as soon as it reaches the top of the chamber. Hence there will be very little time for heat conduction.

Helium can, of course, fill the space in the magazine, not occupied by the cartridges, as hydrogen would be undesirable.

In case work is done upon any of the further developments of the method, the application of hydrogen and oxygen should by all means be investigated, as the propellant is ideally suited for the purpose, besides being inexpensive. 

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II. Investigations Conducted with an Operator.

The presence of an operator is desirable under many circumstances, for example in place of an automatic camera, if a planet is encircled. An operator is essential if investigations are made that would necessitate landing on, and departing from, planets.

1. Necessity of Using Hydrogen and Oxygen.

In the case of encircling a planet, and especially in the case of landing on, and departing from, a planet, the use of hydrogen and oxygen as propellant is essential. This is at once evident from the fact that otherwise the initial masses will be almost prohibitively large, as the following illustration will show.

The initial mass required for a start from the earth; landing on the moon without jar by reducing the velocity so that its becomes zero with the surface is reached; departing from the moon; and finally landing on the earth, the final mass being one pound, may easily be calculated as the product of the mass necessary to raise 1 lb. From the earth to "infinity", for any particular acceleration of the apparatus, multiplied by the square of the mass necessary to raise 1 lb. from the moon to "infinity", and multiplying this by the mass necessary to give 1 lb. a velocity of 4.5 miles per second, let us say, which represents the retardation that must be had to slow the 1 lb. mass sufficiently for it to alight on the surface of the earth without jar. This is probably in excess of what would actually 

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be required, for the reason that if the returning mass entered the earth's atmosphere tangentially (the use of a parachute being possible when the velocity had become sufficiently reduced) the atmosphere would act as a brake in reducing the velocity as already explained.

The following table is made, for hydrogen and oxygen used under the same conditions as stated in the recent publication; namely, velocity of ejection 11,900 ft./sec.; proportion of propellant to total weight 14/15; the estimate being made for three accelerations of the apparatus: 35, 100, and 150 ft./sec2.

It is seen that the total initial mass is larger, if the final mass is larger; although not wholly beyond reason.

If the calculation were to be made for landing on and departing from a planet, the initial masses would be even larger. 

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2. Importance of Production of Hydrogen and Oxygen on the Moon and Planets.

From the above table it is evident that, if hydrogen and oxygen were to be produced and liquefied on a planet, a great saving of initial mass would be possible, as the magazine and mechanism would be a small part of the total weight; the magazine to be used on the return being carried empty. The saving would be even greater, of course, if some or all of the metals needed, such as iron, aluminum, and magnesium, for the mechanism, were present in an available form of the planet.

Since the initial mass required for leaving the moon is much less then that required for leaving the earth, the passage to and from a planet could be made much easier if the start were made from the moon. This implies, of course, the presence of water of crystallization on the moon, or at least of oxygen in an available form, which latter constitutes 8/9 the weight of the propellant.

In either case, solar energy would be available for the production of hydrogen and oxygen (except possible on Venus). The best location on the moon would be at the north or south pole, with the liquefier in a crater, from which the water of crystallization may not have evaporated, and with the power plant on a summit constantly exposed to the sun. Adequate protection should, of course, be made against meteors, by covering the essential parts of the apparatus with rock. 

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3. Necessity for Shortening the Time of Transit.

The time required for passage from the earth to a planet, if the path is traversed with a moderate constant velocity, can best be shown by an example. Thus if a velocity of 3000 ft/sec. Were to be maintained from the earth to the planet Mars, the time of transit would be 102.2 weeks or nearly two years. For the sending of inanimate devices, this might not be unreasonably long, but for an operator it would, of course, be desirable to shorten the time.

If a device that could exert a constant propelling force were to be employed, and the speed were increased during the first half of the journey and decreased during the other half, the time could be considerable reduced. Thus, if the average velocity were 7 miles/sec., the time would be reduced to 7.13 weeks. The limit to the reduction of time is imposed by the nature of the device used to secure this reduction.

4. Necessity of Using Solar Energy.

The only energy present in space, in appreciable amounts, is solar radiation. Also, as has already been mentioned, the substances producing most energy, on chemical combination, are hydrogen and oxygen. It is evident, then, that solar energy must be used to give an increased speed to the jet of water vapor resulting from the combination of hydrogen and oxygen. Inasmuch as this is difficult or impossible if the jet is merely heated, the increase of velocity must be produced by electrostatic repulsion. 

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Assuming that the kinetic energy produced by the chemical action is half that available heated energy of the chemical combination, and that the efficiency of the power plant is 90 per cent., a simple calculation will show that a comparatively small average velocity must be used, as otherwise the mirror surface necessary, per unit of mass carried, will be inconveniently large. Thus, if the average mass were to be 500 lbs., and the average velocity 15 miles/sec. (giving a time of transit of 3.32 weeks) the average mirror surface would be 2,740 sq.ft. or over 500 feet square; where as if the average velocity were 7 miles/sec. And 3 miles/sec., respectively, the mirror surface would be approximately 600sq.ft. and 170 sq.ft. In the latter case, the time of transit would be 14.26 weeks; about three months and a half. The mirror surface of 500 lbs. Average mass is, in this case, reasonable.

5. Discussion of General Method.

(a) Outline of Method.

During the initial rise of the rocket in the atmosphere the mass is large, and the acceleration is considerable. It is thus impossible to begin employing solar energy until the parabolic velocity is approached, owing to the large size of the power plant, particularly the mirrors, that would be necessary.

For the initial mass to be a minimum, the mirrors must be of as small weight as possible in proportion to the power plant, which, itself, must be very light. Further the ratio of the mass of hydrogen and oxygen used, en route, to the energy supplied must be such as will give minimum weight consistent with the possible extent of mirror surface. If, for example, hydrogen 

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and oxygen were to be used in too small amounts, the velocity in question would be high, and hence the solar energy needed would be large. The proportion of solar to chemical energy will therefore be determined chiefly by the proportionate weight of mirror to weight of rocket.

On approaching a planet, the velocity can be gradually reduced (not requiring an excessive amount of energy) over a considerable distance, but not over so great a distance as to seriously increase the time of transit.

(b) Mirrors.

On leaving a planet, then, when the parabolic velocity is approached (far above the atmosphere, of course), mirrors of as tine metal foil as possible, on collapsible frames, should be unfolded; the solar energy being reflected into one or more steam boilers, through one or more pyrex glass windows.

In order to retain the energy and prevent radiation, the boilers must contain either porous black material or black particles held in suspension by the water.

A device of this type is, of course, practicable for the reason that a similar solar engine, without the improvements here suggested, is now in use for pumping water in California.

(c) Power Plant.

The boiler should, of course, be as small and compact as possible, and the outer surface should be highly polished, to minimize radiation. Incidentally, the low temperature of space will permit the use of a very small condenser, and will 

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at the same time make possible high thermodynamic efficiency.

The turbo-generator should be made so that a heavy shaft is not necessary. A multiple-stage turbine is therefore suggested, consisting of a series of disks provided with rows of blades, and rows of magnets, in close proximity; the steam being admitted to the rows of blades from a hollow end disk provided with rows of nozzles. In this way, the forces will be between closely contiguous parts, rather then transferred from the turbine to the generator by a strong, heavy shaft, and the lightest possible form of turbo-generator will be the result.

The electrical energy must be supplied as small current at high potential, in order that the gases from a jet may be given a high velocity. This will be discussed in detail below.

(d) Theory.

The theory is simplified if it is assumed that both the energy supplied as radiant energy, as well as the energy supplied as chemical energy, are proportional to the mass of the rocket that remains at any time t; i.e., to (M-m). This is, of course, a reasonable assumption, for the larger the rocket apparatus, the larger will be the mirrors and power plant that it can support. The mirror surface for the minimum thickness of foil possible would, however, increase more than in proportion to the total mass.

We have, first, the same analytical expression for Newton's third law as in the case of rocket action discussed in 

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the recent publication, namely,

Cdm = (M-m)adt, (1)
in which m is the mass that has been ejected up to time t.

The expression of the Law of the Conservation of Energy is different, however, in that external heat energy must be considered, as well as the chemical energy carried. Thus, let

E = the heat energy of unit mass of the propellant (hydrogen and oxygen) as before, and

E' = the heat energy per second received from the sun, by the mirror surface that accompanies unit mass of propellant.

We have, the, at anytime t, provided E and E' are entirely used in supplying energy to the mass dm that is expelled between the time t and t + dt,

dmC2 = E dm + E' (M-m) dt. (2)
The equations (1) and (2) must, however, hold simultaneously, this condition of simultaneity being obtained by substituting dm/dt from (1) in (2), as

the positive sign only being taken, since the negative sign would imply that C was either negative or zero. Incidentally, this equation shows that C, for the above conditions, is a constant. 

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This new value of C may be used in place of the C in the equations for the simple rocket theory, already presented, inasmuch as the equation used in that theory is (1) of the present paper; and calculations for a minimum mass may be made in a similar manner.

It should be mentioned that an exact estimate of the time of transit will take cognizance of the fact that the tangential speeds of the planets are different, and also that solar gravitation opposes motion outward from the earth, and assists during the return.

6. Problem of the Production of an Ionized Jet of Gas.

(a) General Considerations.

1st. Production of ions in a space adjacent to the jet.

It is evident, at the outset, that if the jet is to contain ions which are repelled, these must be introduced into the jet from a space outside the jet, to prevent injury by the rapidly moving gases; at the same time being as near the jet as possible. It is also evident that the production of the ions, and their introduction into the jet, should require a little energy as possible. An advantage, in this connection, is that the device will operate at such a high altitude as to be in practically a perfect vacuum, so that there will be a saving of the energy that would otherwise be required to force the ions through a gas, on passing to the jet.

2nd. Simultaneous ejection of positive and negative ions.

It is evident that, if ions of but one sign were introduced into the jet, the apparatus would become charge to such a high potential of the opposite sign that, after at time, 

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ions of the first sign could no longer escape. Hence ions of both signs must be lost in equal amounts.

It is likely that the jet can be charged easier negatively than positively, although a method of using porous metals as electrodes, as suggested below, may operate equally well with both kinds of ions. Even if it should be difficult to produce the action with positive ions, however, this will present no obstacle, as positive ions can be expelled from an independent electrode, without the electrification of a jet, and without any considerable loss of energy.

3rd. Possible means of producing the ions.

Perhaps the ions most easily produced in large amounts, requiring little energy, are the electrons produced from hot bodies, especially if in a vacuum. All that is needed is a filament or foil of as high metal point as possible. Positive ions can also be produced in this way, but not in such large amounts.

Other means applicable to the production of ions of both signs are the use of sprays of conducting liquids, and of fumes which produce finely divided solid particles, as of ammonium chloride. The ions produced in either of these two latter ways would, of course, be comparatively large.

Another method of producing ions of both kinds, although not yet tested, may have interesting possibilities. This proposed method would consist in passing gas through an incandescent metal, charged either positively or negatively, the object being to charge the molecules of the gases as they leave the pores of the metal. 

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A still further method, although not so simple as the others here suggested, would be to produce ions of both kinds within the jet, before entering, or while passing through, the nozzle, and removing most of those of one sign before the gas passed out of the nozzle.

4th. Use of continuous and of impulsive jets.

If the jet is produced by continuous combustion, the ions should be produced continuously, and the high potential for repulsion of the gas applied to the nozzle continuously. Continuous combustion may, however, make difficult the proper cooling of the chamber and nozzle.

If the jet is produced discontinuously, by explosive burning, the high potential should be applied discontinuously, although this is not necessary, and the charged ions should be produced in large amounts discontinuously. This can very easily be done in the case of electrons produced by a filament heated by sudden rushes of current.

(b) Experiments Already Performed with Electrified Jets.

Experiments were performed at Clark College in 1916-1917 for the purpose of introducing negatively charged particles (electrons, in this particular case) into a rapidly moving jet of air, at atmospheric pressure.

The devices used may be divided into two classes. In the first, ions were caused to pass, by electrostatic means, into a jet 10, which passed through a nozzle 11; the electrodes 12, 13 either being on opposite sides of the jet, Fig. 1, or located, one inside the jet and the other outside the jet, Fig. 2. In these cases, 12 is the anode and 13 is the cathode. The 

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cathode consisted of a platinum wire coil or a platinum foil, covered with sealing wax residue.

In the second class of devises, the ions were produced in an auxiliary tube 14, Fig. 3, through which passed a current of air of moderate velocity; this current not cooling or otherwise interfering with the filament 13, but at the same time carrying into the main stream all the electrons produced by the filament. The anode was a brass tube, 12, concentric with the cathode filament 13.

This device, as a preliminary experiment, proved very satisfactory at atmospheric pressure; it being possible, using 20 lbs. pressure for the main blast, to obtain a potential of 5,000 to 10,000 volts, from only 110 volts potential difference between the brass tube and the filament; the high potential being produced on a receiver consisting of a large tin can into which the ejection gases passed substantially tangentially, so that they traveled spirally over a considerable distance through silvered glass wool, in the can, before emerging; thus giving up most of the charges to the glass wool.

It should be noticed that in all these forms, the ions were produced within a space outside, and protected from, the main body of the jet.

It should also be remembered that if the space in which the ions are produced were to be substantially a vacuum, as would be the case with a high-velocity jet, the number of ions that could be removed from the filament and introduced into the jet would be very much increased. 

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(c) Best Form of Electrified Jet.

Since the high velocity of the jet of gas is produced by the repelled ions pushing the remainder of the mass of gas, it is desirable that the ions be as well mixed with main body of gas as possible.

The two forms of apparatus that would accomplish this result best are those in which the ions would be introduced into the jet as near the throat of the nozzle as possible. If an auxiliary current of gas were not used, the best form is that shown in Fig. 2; but is an auxiliary current of gas were to be used, the form shown in Fig. 4 would be most desirable. In this case, the tube containing the cathode 13 would be within another tube 15, containing the anode 12, in the form of a ring, and carrying a current of gas, to force the ions into the main jet. 

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A method of carrying out planetary investigations, and of communicating with planets by actual contact, is outlined in the present report, together with the previous Smithsonian publication, which method does not presuppose any new discoveries but depends upon well established physical principles.

Progress has already been made upon some of the basic problems that are involved in this method.

It is believed that the method is superior to any other so far proposed for establishing interplanetary communication, in avoiding enormous expense and at the same time not depending upon a high degree of intelligence and special material equipment of the possible inhabitants of any neighboring planets.

The method is moreover, certain to yield results of great scientific value, even aside from the question of interplanetary communication.

In view of these facts, together with the tremendous interest and importance of the subject, it is believed that an appeal for public support is justifiable.

Respectfully submitted,
[signed] Robert H. Goddard 

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perimental difficulties are not great obstacles when the proper method of attack is found, as has already been shown during the present research.

In view of these facts, and of the intense interest that is being taken in the subject at the present time, it seems worth while to present for consideration the researches herein explained, which if given sufficient support are certain to result in important developments along the lines of planetary investigations. 

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I. Investigations Conducted without an Operator.
1. Taking of Photographs near the Surface of the Moon and the Planets.

(a) General Method.

The taking of photographs of the surface of the moon and planets from points near these surfaces, by an apparatus passing around them, would have the evident advantage of employing as high a resolving power as a telescope many feet, or even several miles, in diameter, at the surface of the earth. Thus a telescope of one foot aperture passing within 1000 miles of the surface of Mars would have the same resolving power as a telescope of 6.6 miles aperature at the surface of the earth.

(b) Apparatus.

The telescope, for lightness and compactness, must be reflecting, and be made compact by several successive reflections, on the binocular principle; all reflecting surfaces being as thin as possible, consistent with rigidity.

Inasmuch as the apparatus can be made much lighter than as if an operator where present, smokeless powder can be used as propellant without making the weight unduly large.

(c) Guiding of Apparatus.

The variables which are present are the direction and magnitude of the resultant force of gravitation, and the direction and magnitude of various sources of radiation, as 

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the sun, moon and stars. All of these variables will be functions of the time of transit. If the initial velocity and direction of the apparatus be known, the value and direction of the force of gravitation at any subsequent time can be predicted, and it should be possible to employ a correcting device which depends upon the difference between the predicted and the actual value of the above variables, at particular times; a reference axis being maintained by gyroscopes.

Side jets on the multiple charge principle, in line with the center of gravity of the apparatus, could be operated automatically, depending upon the discrepancy between the actual and the predicted value of the variables chosen. The jet need not be powerful, for the reason that a small sidewise velocity will produce a large sidewise deviation, if given at the start of a long path.

(d) Precautions on Landing.

Although a sufficient excess of propellant might be taken to check the velocity, on landing, it is very likely that this would not be necessary, providing the apparatus were made to traverse the atmosphere tangentially.

In the case of meteors, which enter the atmosphere with speeds as high as 30 miles per second, the interior of the meteors remains cold, and the erosion is due, to a large extent, to chipping or cracking of the suddenly heated surface. For this reason, if the outer surface of the apparatus were to consist of layers of a very infusible hard substance with layers of a poor heat conductor between, the surface would not be eroded to any considerable extent, especially as the velocity of the 

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apparatus would not be nearly so great as that of the average meteor.

Of course the photographic plate or film should be properly protected; but in this connection, it should be remembered that low velocity meteors, i.e., those traveling in the same direction as, and overtaking, the earth, have been known to be so reduced in speed on falling through the atmosphere as to strike on ice without breaking through.

In order to identify the apparatus, the surface could give a succession of colors, on falling through the air; and electric waves of a certain pitch could be generated, permitting the position to be ascertained in the usual way.
2. Communicating with Other Planets, by Contact.

(a) General Method.

The method would consist in the sending of devices to the planets that would produce a succession of colors on falling through the atmosphere: the devices containing metal sheets stamped with geometrical figures; with the constellations, emphasizing the earth and the moon; together with as much of a description as possible of the rocket itself.

Although this method presupposes the existence of intelligent beings on the other planets, it does not presuppose the existence of elaborate wireless receiving stations, and is not dependent upon such matters as the conducting layer of the upper atmosphere, which may make wireless signaling between planets difficult or impossible. Neither would it involve the expense of the wireless trials that have so far been proposed. 

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It is important in this connection to remember that the initial mass necessary to reach a planet is, except for such mass as may be required for directing, en route, not much more that required to reach the moon, as already shown in the recent publication.

3. Importance of the Use of Hydrogen and Oxygen as Propellant.

(a) Reduction of Initial Mass over that Required for Smokeless Powder.

Although the researches just explained could be accomplished with smokeless powder, of the type that has been used, as propellant, it would be highly desirable, nevertheless, to employ hydrogen and oxygen, in the liquid and solid states, respectively.

The reason is, of course, that the heat energy of hydrogen and oxygen, even under this condition, is about three fold greater than that for the smokeless powder that has been used. Thus, from the data already published, the initial mass required to send 1 lb. To "infinity", from a start at 15,000 feet, if for smokeless powder, 438 lbs., and for hydrogen and oxygen only 43.5 lbs; both for the same efficiency.

It should be realized, also, that even with the smokeless powder that has been used, there has no attempt been made to obtain the best shape of nozzle -- which as been found to have an important effect upon the velocity -- nor the best condition of ignition. If these were obtained, the initial masses would certainly be less than those given above, for the same proportionate weight of propellant as that chosen in the calculations. 

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(b) Reduction of Cost over that for Smokeless Powder by Production of Gases under Pressure.

The manufacture of smokeless powder is a best a process involving considerable expense. The material retails for $1.00 per pound, and could hardly be made, probably, for much less than 40 or 50 cents per pound. Where large masses are employed, the question of cost becomes important.

Besides the reduction of initial mass possible with hydrogen and oxygen, these elements are of advantage in being producible at small cost.

The chief expense in the production of liquid hydrogen and oxygen is the compressing of these gases, before liquefying. If, however, the hydrogen and oxygen are produced electrolytically under high pressure, in a closed container, the water vapor being removed by freezing prior to liquefying, the gases would be ready for the liquefier as soon as produced, and the expense would be reduced practically to that required to produce the gases, which would be moderate. Such a device is described in United States Letters Patent No. 1,154,009. Incidentally, the electromotive force to produce electrolysis under pressure does not differ much from that at atmospheric pressure, as shown, up to 1000 atmospheres pressure, by Wulf, Zeitschr. für Phys. Chem., V. 48, p. 89.

It should be possible to make the process practically continuous, the gases being produced simultaneously, with the oxygen liquefier outside the hydrogen liquefier. 

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(c) Convenience of Using Hydrogen and Oxygen.

There are several peculiar advantages that would accompany the use of hydrogen and oxygen. Certain of these will be mentioned below, but one of the most immediate is concerned with the production of hydrogen and oxygen on a high mountain.

It is, of course, advantageous to have the start from as high as elevation as possible, which implies the top of a high mountain. In the case of hydrogen and oxygen, the propellant material is already present in the form of snow; and the low temperature, to assist in the storage, is also present. Further, water power can be had a short distance away, from the mountain streams at the base. Energy of the sun might also be used for this purpose, although not so conveniently.

(d) Successful Application Certain.

It is certain that hydrogen and oxygen, in the form suggested, can be used as an explosive. An experiment is on record in which a spark was applied to a piece of waste, first dipped in liquid air and then in liquid hydrogen. The material exploded "like dynamite".

There is an advantage in the use of liquid hydrogen and solid oxygen in that the ratio of burning can be regulated by the extent to which the oxygen is finely divided. In case considerable heat should be required to fire the fulminate detonator, finely powdered thermite may be placed on the detonator, under the percussion cap.

The hydrogen and oxygen must be enclosed in a solid