Aaf Scientific Advisory Group Explosives & Terminal Ballistics_vkarman_v10

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AND TERMINAL BALLISTICS A REPORT PREPARED FOR THE AAF SCIEN'rIFIC ADVISORY GROUP By D. P. MAC DOUGALL Naval Ordnance Laboratory, Washington, D. C. N. M. NEWMARK Department oj Civil Engineering, University oj Illinois • PMblished May, 1946 by HEADQUARTERS AIR MATERIEL COMMAND PUBLICATIONS BRANCH, '1001 WRIGHT FIELD, DAYTON, OHIO V-46579 The AAF Scientific Advisory Group was activated late in 1944 by General of the Army H. H. Arnold. He se- cured the services of Dr. Theodore von Karman, re- nowned scientist and consultant in aeronautics, who agreed to organize and direct the group. Dr. von Karman gathered about him a group of Ameri- can scientists from every field of research having a bearing on air power. These men then analyzed im- portant developments in the basic sciences, both here and abroad, and attempted to evaluate the effects of their application to air power. This volume is one of a group of reports made to the Army Air Forces by the Scientific Advisory Group. Thil document contolnl Information affecting the notional defenle of the United Statel within the meaning of the Espionage Ad, SO U. S. C., 31 and 32, 01 amended. Its tronsmiulon or the revelation of Its contents In any manner to on unauthorized person II prohibited by low. AAF SCIENTIFIC ADVISORY GROUP Dr. Th. von Karman Director Colonel F. E. Glantzberg Deputy Director, Military Dr. H. L. Dryden Deputy Director, Scientific Lt Col G. T. McHugh, Executive Capt C. H. Jackson, Jr., Secretary CONSULTANTS Dr. C. W. Bray Dr. L. A. DuBridge Dr. Pol Duwez Dr. G. Gamow Dr. 1. A. Getting Dr. L. P. Hammett Dr. W. S. Hunter Dr. 1. P. Krick Dr. D. P. MacDougall Dr. G. A. Morton Dr. N. M. Newmark Dr. W. H. Pickering Dr. E. M. Purcell Dr. G. B. Schubauer Dr. W. R. Sears Dr. A. J. Stosick Dr. W. J. Sweeney Dr. H. S. Tsien Dr. G. E. Valley Dr. F. L. Wattendorf Dr. F. Zwicky Dr. V. K. Zworykin Colonel D. N. Yates Colonel W. R. Lovelace II Lt Col A. P. Gagge Lt Col F. W. Williams Major T. F. Walkowicz Capt C. N. Hasert Mr. M. Alperin Mr. I. L. Ashkenas Mr. G. S. Schairer LAYOUT & ILLUSTRATION Capt M. Miller Capt T. E. Daley ii TABLE OF CONTENTS Part I - Pages 3 to 37 inclusive, has been deleted. Page Part II - Properties of High Explosives Introduction and Summary .........•.........••. ' ......•••••........•.... 41 General Discussion •••.•••.•.•......•.....••..•...........••••...•....•• 43 Solid Explosives. . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . • . • • . . • . .. 47 Solid Explosive Mixtures .............................•......••...... 49 Aluminized Explosives ...............................•........•... " 5 1 Liquid Explosives .•.•.....•...................•..........••••.•••••.•••. 53 Quantitative Comparison ...•.•.•.................•....••.•....••........ 55 Table I ....•••...•.••.•.......••............•..••••••••••.....•......... 57 Table II ............•.•••..•••........... ' ..........••.•..•.••.•••.•••••. 58 Part III - Terminal Ballistics and Destructive Effects Summary •••........•....•...............................•••••••••....... 61 Means of Producing Damage. • . . . . . . . . . . . . . . . . . . . . . • • • . • • . • . • • • • • . • . . . .. 61 Basic Physical Principles ...•..•...............•....•..•...•..•••.••... " 62 Impact and Penetration ..•.........................•....•...•.•••.... 62 Fragmentation. • . • . • . . . • . . • . . • • . . . . • . . . • . . . . . • • • . . . . . . . . . . . . • • • • • • .. 63 Blast in Air. . • . . . . . . . . • . . . • • . . . . . . . . . . . . . . . . . . • • . • . . . . . . . • • • • . • . • . •• 66 Blast in Earth or Water .......•.......................•.•......••.••. 69 Debris ..•.•...•..•.........•••...•................•.••••...•..•••••. 71 Fire, Chemical Actions, etc.. . • . . . . . . . . . . . . • . . . . . . . . . . . . . . • • • . • . . . • . .• 71 Classification of Targets •...•••......•.••..•...........•••••....••••••••• 73 Types of Existing Weapons. • • . . . . • . . . . • . . . . . . . • . . . . . . . • • . • • . . • . • . . . . • . .. 74 Factors Influencing Selection of Weapons for Attack .•.....••••.•••..•.•.. 75 Getting Weapons to the Target ...................................... 75 Countermeasures ....••...•.•.•...............•• ; ..........••.•.....• 75 Fusing ..•............•..••.•............................•••••.••.•.• 76 Major Uncertainties. . • . . . • . • . . . . • . . . . . . . . . . . . • . . . . . . . . . • • . • • . • • • . • .. 76 Present Status of Weapons Selection .•...••...••••..........•...••.•. 76 Future Needs ...•••••..••.••.••..••••...••.......••.....••••...••••••••• 77 iii PART II PROPERTIES OF HIGH EXPLOSIVES By D. P. MacDOUGALL sF $ PART II PROPERTI ES OF HIG H EXPLOSIVES 3 APRIL 1945 INTRODUCTION AND SUMMARY The purpose of this report is to give a brief summary of the present status of our knowledge concerning the properties and utilization of military high ex- plosives. Attempts are also made to give estimates concerning the behavior of several mixtures and compounds which are not now in use, but might be considered for use in the future. This report does not contain a thorough discussion of the terminal. ballistics of explosive-loaded munitions. However, this subject is discussed briefly in order to provide a basis for comparison of different explosives. Particular attention is paid to the performance of high explosives in blast bombs, partly because this phase is probably of most interest to the Air Forces, and partly because the action of ex- plosives in producing blast has been more thoroughly investigated than other types of action. Of the explosives in military use today, HBX or desensitized Torpex appears to be the best all round filler for bombs. A bomb loaded with HBX will produce an area of destruction about 45 % greater than that from the same bomb loaded with TNT. On the other hand, HBX is sufficiently stable and insensitive to shock to be used in G.P. Bombs. If, in the future, conditions arise which permit the use of explosives of much greater shock sensitivity than TNT (for filling some sort of robot bomb, perhaps), the best bet, if the comparison is made on a volume basis, would appear to be an explosive of the Torpex type, but with a larger percentage of aluminum. One might hope to achieve, with such an explosive. a damage area twice that of the same volume of TNT. If the weight, rather than the volume, of explosive is chosen as the basis for comparison, the most effective explosive would be a stoichrometric mixture of liquid hydrogen and liquid oxygen. This mixture might give a damage area four or five times as great as that produced by the same weight of 'fNT. Because of the low density however, this mixture would not be very effective on a volume comparison. There is some discussion in the report of SBX, by which is meant an explosive consisting entirely of a fuel (plus a small amount of conventional H.E. to give dis- persion) and which utilizes atmospheric oxygen in the detonation reaction. Since 41 V.46579 only the fuel must be carried, the amount of enes-gy liberated per unit weight is, in general, large compared with the value for a conventional high explosive, where, in a sense, both the fuel and the oxygen are carried together. The analysis given in the body of the report suggests that gasoline, used as SBX, might put somewhat over twice as much energy into the blast wave as the same volume of TNT. On a weight basis, the comparison is even more favorable to the SBX. However, experi- ments with SBX have shown that while it is effective in confined spaces, it is very in- effective in the open. This is probably due to a slow reaction between the fuel and the oxygen of the air, which may be overcome in the future. This is a subject which might generously repay future investigation. 42 GENERAL DISCUSSION There are many jobs which a high explosive may be called upon to do, and there is not now, and probably never will be, a single explosive which is best for all uses. The properties desirable in the explosive used as the filler of an armor-piercing shell are different from those required for the filling of a depth charge, and a de- molition explosive must meet still other In the present discussion, most attention will be paid to explosives which can be used as fillings for aerial bombs, both because such explosives are of greatest interest to the Air Forces, and because, on a tonnage basis at least, they are the most important. Even in a single type of muni- tion, such as a general purpose bomb, the explosive produces damage by at least three different methods, air blast, fragments, and earth shock. It is not true that a given '!xplosive is equally effective in producing all three types of damage. In selecting an explosive for a particular munition, therefore, it is important to have available information concerning its effectiveness in the specific type of action in- volved. A high explosive is a material which can be induced, at a predetermined time and place, to explode or detonate. In the process of detonation the explosive is con- verted rapidly, in a period of a few microseconds, into a large amount of gas at a high temperature and pressure, with the release of a large amount of chemical energy. An explosive must therefore be a material of controlled stability. If it is too stable, it will be impossible to make it explode, while if it is not stable enough, it may ex- plode before one is ready for it to do so. Due to its latent instability, every explosive can be made to decompose, with varying degrees of violence if heated sufficiently. However, if the material is to be of practical utility, it must not undergo appreciable decomposition during extended periods of storage at temperatures which might be encountered in magazines, storage depots, holds of ships, etc. An explosive which does undergo such decomposition during storage is undesirable because this de- composition may lead either to premature explosion or to loss of performance. One important requirement for a military explosive, then, is that it must possess adequate chemical stability. There is no hard and fast rule for deciding whether or not stability is adequate. The ideal is that it withstand the very highest temperatures to which an explosive might be subjected, for .periods of time upward of twenty years without appreciable change. However, many materials are accepted for use which are much less stable than this, particularly if they possess other advantages. The only com- pletely reliable way to find out whether an explosive will stand a temperature of 65°C, say, for twenty years, is to keep the explosive at this temperature for twenty years. To save time, tests are carried out in the laboratory at considerably higher temperatures for shorter periods, and by this means, a fairly reliable estimate of the stability of a new explosive can be obtained in the course of a few months. All explosives can be made to detonate by subjecting them to a severe enough mechanical shock, but some are set off by this means much more readily than others. 43 For an explosive to be suitable for a given purpose it must withstand, without de- tonating, all of the mechanical shocks to which it may be subjected before the time for its planned detonation. The qlechanical shocks may be accidental, and all ex- plosives must be capable of standing a certain amount of rough handling during shipment, loadisg, etc. In addition, in many munitions, the explosive receives a variety of shocks and stresses in the course of normal operations, such as the forces of setback which occur when a shell is fired from a gun. The intensity of these planned shocks varies widely from munition to munition. In an armor-piercing shell, for example, the explosive must not only withstand the setback forces, but it must not de- tonate or burn when the shell passes through a piece of armor plate. In a high capaci- ty aerial bomb, on the other hand, an instantaneous fuse is normally used, so that the filling is expected to detonate immediately on hitting the ground, and hence the requirements with regard to insensitivity to shock are very much less. Once again, the only completely satisfactory way to determine whether a particular explosive has a degree of insensitivity to mechanical shock adequate for the contemplated use is to carry out actual performance tests of the explosive in the full-scale munition. Again, however, at least a partial answer can be obtained from small-scale labora- tory tests. These tests involve delivering to small samples of the explosive mechanical shocks of carefully controlled type and intensity. The most widely used test con- sists of determining the height from which a standard weight must be dropped on a standard sample of explosive to produce explosion. The absolute value of this height has no great significance, but it does place the explosive on a scale deter- mined by heights similarly determined for other explosives. If, for example, explo- sive A has been widely used as the filling for a particular munition, and has been found to have adequate insensitivity, and a new explosive B is found in the drop- weight test to be more difficult to explode than explosive A, then there is a good probability that explo'sive B will also be adequately insensitive for the munition. If its other properties, or its availability, make its use attractive, then it will be worth- while going to the trouble of carrying out tests in the actual munition. A new explosive which is being considered for use as a filling for aerial bombs will normally be tested in the laboratory with regard to sensitivity to friction and impact, and to bullet impaCt. If it appears to have satisfactory insensitivity for the purpose in mind, the full scale sensitivity tests include firing various types of small arms ammunition at the loaded bomb, and dropping the bomb from various heights on a hard surface. . In addition to adequate stability and insensitivity to shock, the third important attribute of an explosive is its performance. Since, as pointed out previously, explo- sives may do useful work in a number of different ways, no one type of performance test will suffice to give a general evaluation of the effectiveness of an explosive. The three principal factors which determine the potency of an explosive are: the amount of energy liberated during detonation, the volume of gas produced, and the rate at which the conversion from undetonated explosive to final products takes place. For one application, one of these {actors may be of greatest importance, while in other applications, another one may be more important. This is basically the reason why it is not possible to assign a single figure of merit to a given explosive. In a cavity charge, such as the head of the bazooka, for example, the quantity which appears to deter- 44 mine the effectiveness of the explosive filling is the pressure produced in the head of the detonation wave. To obtain the highest possible pressure, the amount of energy and gas liberated should be high, but the conversion should also be as rapid as possible. For this type of action, ..the aluminized explosives are less effective than certain others which liberate less energy, due to the fact that the reaction involv- ing aluminum takes an appreciable amount of time, and so that the peak pressure in the detonation wave is less than it would be if the reactiQn were instantaneous. Now, coming specifically to the performance of aerial bombs, it may be noted that these bombs can produce three destructive agents, namely air blast, fragments traveling at high velocity, and, if they explode under ground, earth shock. If the bomb explodes under water near the hull of a s h i p ~ the agent of damage is a shock wave in the water. Of these agents, perhaps the most important is air blast. When a bomb explodes, the rapid expansion of its contents produces a compression wave in the air, or a shock wave. (A shock wave is similar to a sound wave, but of much greater intensity.) In this shock wave, the pressure rises essentially instantaneously from normal atmospheric pressure to a maximum value, at the front of the wave, and then falls off in a roughly exponential fashion, reaching atmospheric pressure at some distance behind the front, and then continuing to fall to a minimum value which is appreciably below atmospheric pressure, finally reaching normal pressures again at a greater distance behind the front. This shock wave can be characterized by specifying its peak pressure, the momentum or impulse contained in it per unit area, and the energy content per unit area of the front. The positive impulse or momentum is simply the integral of the excess (above atmospheric) pressure against the time for that part of the wave in which the pressure is above atmospheric. The energy content is . found by a similar integration of the square of the excess pressure times the time. The magnitude of the peak pressure and the momentum depend on the weight and type of explosive and on the distance from the point of explosion. For most types of structures, the quantity which determines whether or not they will be damaged by a shock wave is the positive impulse or momentum. However, if the shock wave is of very long duration, which may be due to its being produced by the detonation of a very large quantity of explosive or by the explosion of a very slow acting explosive (especially SBX, mention of which will be made later), then the damage tends to depend on peak pressure, as in the static case. However, for conventional explosives in bombs of normal size, the quantity which determines damage is approximately equal to the positive impulse. The impulse, I, changes with weight of explosive and distance according to the following equa- tion: (1) where W is the weight of the explosive charge and r is the distance from the point of detonation. The constant k depends on the nature of the explosive and on the thick. ness of the case. A heavy-cased bomb gives a weaker shock wave than the same weight of explosive in the form of a bare charge, because a considerable fraction of the explosive energy is given to the fragments of the case, if it is present. The effective- ness of a given explosive in producing damage by air blast can be expressed by giv- ing the value of the impulse produced at a standard distance by a standard weight - I 45 of charge. It is more common, however, to quote the impulse relative to that produced by the same weight or volume of some standard explosive at the same distance. The statement above, that damage depends on impulse, means that for a given type of construction, a certain class of damage (Class B damage, for example) will occur at all points where the impulse is equal to or greater than a certain quantity. Since area depends on the square of distance from the bomb, the relative areas of a certain class of damage for two different explosives will be proportional to the square of the impulse ratio for the two explosives. For example, a suitable way of expressing the effective- ness of Torpex as an explosive for causing.damage by air blast, is to say that the area of damage is approximately 60% greater than that produced by the same volume of TNT. For producing air blast, explosives containing aluminum are in a class by them- selves. The very high energy of reaction of aluminum with the oxygen contained in the explosive more than compensates for the reduction in volume of gas produced. The overall reaction involving aluminum is not as fast as that for a pure explosive compound, but for bombs of any reasonable size, the duration of the shock wave is sufficiently great so that, while a slow reaction may result in a lower peak pressure, the energy is all liberated before the production of the shock wave is completed and so it is all effective. The effectiveness of a bomb in producing damage by fragments depends on the number and average weight of the fragments produced and on the velocity of the fragments. The situation is very complicated because the size of the most effective fragments depends on the type of target being attacked. For personnel and light materiel, a large number of small fragments will have the greatest effect. For heavier targets, the fragments should be larger, and will consequently be fewer. With re- gard to velocity of fragments, there is a fair correlation between fragment velocity and air blast intensity for different explosives. It has been found that when the ex- plosive in a shell or bomb detonates, the case is not immediately ruptured, but swells a considerable amount first. For this reason, there is an appreciable time period during which the highly compressed gas in the bomb can impart velocity to the case. However, the time available is not so long as for producing a shock wave, and in some cases, it is found that an explosive which is very effective in producing air blast is less effective in producing high-velocity fragments. Minol, which is aluminized Amatol, is probably a case in point. In general, it can be stated that the effectiveness of a bomb in producing fragment damage depends at least as much on the ratio of case weight to charge weight as it does on the type of explosive, and it is not possible to give a specific figure to represent the fragment damaging power of an explosive. The ability of an explosive to produce earth shock and cratering action has not been very thoroughly investigated. In general, it appears that the order of effective- ness of different explosives is about the same as is found when air blast is considered, except that explosives containing ammonium nitrate are more effective as cratering agents than they are in producing air blast. The mechanism by which an explosive produces an underwater shock wave is not very different from that by which it produces an air shock wave. However, there are indications that during a bomb explosion in air, there is some energy liberated by 46 reaction with the oxygen of the surrounding air, whereas, this cannot happen under water. Probably for this reason, the relative effectiveness of different explosives in doing underwater damage is not quantitatively the same as that for air blast dam- age, but the two ratios are in general not very different. SOLID EXPLOSIVES The detonation of an explosive compound is essentially an internal combustion; that is, the fuel and the oxygen for its combustion are contained in the same mole- cule. Since the same atoms are present both before and after reaction, detonation must be a rearrangement of the atoms so as to form more stable, stronger chemical bonds. This is accomplished in practice by having the oxygen in the explosive con- nected to the rest of the molecule through nitrogen atoms. After reaction, the oxygen is found to be directly attached to carbon and hydrogen atoms. For this reason, all of the conventional explosive compounds contain nitrogen, either in the form of nitro groups or nitrate groups. The compounds are made by allowing nitric acid to react with the appropriate substance, usually a compound of carbon and hydrogen, with or without some oxygen. Explosives as used may be pure explosive compounds, or they may be mixtures of two or more explosive compounds, or they may be mixtures .of one or more explo- sives with a nonexplosive. In general, a high explosive must contain a certain propor- tion of an explosive compound. A substance such as black powder, which is a mixture of fuel and oxidizing agent, can react vigorously if ignited, but is believed to be in- capable of a true detonation. In the paragraphs which follow, brief discussions will be given, first of the important explosive compounds used in military explosives, and then of the important mixtures. PURE COMPOUNDS TNT or Trinitrotoluene. This explosive is undoubtedly the most important single explosive in use today. It is made by the nitration of toluene. Toluene was formerly obtained only from coal tar, but is now made from petroleum and is available in large quantities. It is a compound melting at about 80°C, and thus can be melt-loaded by the use of steam, which is one of its attractive features. It is a very stable material, as explosives go. lf properly purified, it can be stored for many years without deteriorating. As made dUring World War I, it usually contained appreciable amounts of low-melting im- purities, which resulted in the exudation of an inflammable liquid during storage. How- ever, the highly purified material being made today does not show this effect. TNT is a highly insensitive explosive and on this score is suitable for most applications. However, it does not have the very high degree of insensitivity which is required for filling A. P. shells and bombs. It is the standard fi.lling, or one of the standard fillings for the following munitions: all aerial bombs, except armor piercing; all calibers of high explosive shell; demolition blocks; depth charges and depth bombs; mines. 47 With regard to performance in various weapons, TNT is a moderately powerful explosive but weaker than many of the newer It is difficult to describe the performance of an explosive by giving absolute numerical figures. It is simpler and just as satisfactory, as far as comparison among different explosives is concerned, to pick one explosive as a standard and measure the performance of other explo- sives under various conditions in terms of the performance of the standard explo- sive. Normally. TNT is the explosive which is taken as the standard material. The ratios to other explosives are of two general types. One may use the ratio of the dam- age (of some specified sort) produced by a given weight of the explosive under con- sideration, to the damage produced by the same weight of TNT. On the other hand, it is sometimes more convenient to use the ratio of weights of the two explosives necessary to produce equal damage. Since the numerical values of the two kinds of ratio are in general different, one should be careful to note which one is being used. Tetryl or Trinitropltenylmethylnitramine. This compound can be made by several methods. The chief starting material is benzene. This explosive is definitely less stable than TNT, but its stability appears to be adequate. It is appreciably more sensitive to shock than TNT. Its melting point is about 130°C, at which temperature it undergoes rather rapid decomposition, so that it cannot be melt-loaded. It is generally loaded by pressing into the container. It is used as the filling for certain small caliber shells, such as 20 mm, but is in general too sensitive for use as the main filling of larger munitions. It finds its widest applica- tion as a booster explosive. That is, a pressed pellet of tetryl picks up detonation from the detonator, and in turn sets off the main charge. The very insensitive explosives cannot be set off directly by a detonator. It is appreciably more powerful than TNT, but where used as a booster, it represents such a small fraction of the total charge that its power does not make much difference. Picric Acid or Trinitrophenol. This explosive is made by the nitration of phenol, which in turn is made from benzene. This explosive itself is not used by this country, except as an intermediate in the manufacture of ammonium picrate, but it is in use by certain other countries. Picric acid has a rather high melting point for melt-loading, 122°C. However, by using rather high pressure steam, it can be so loaded. Picric acid is slightly more powerful than TNT, and is somewhat more sensitive, although less sensitive than tetryl. Since it is a rather strong acid, it can react with metals to form salts, which are quite shock-sensitive. The tendency to form sensitive salts and its high melting point are two of the undesirable features of picric acid. However, out of contact with metals, it is a very stable material. Ammonium Picrate or Explosive D. This explosive is the ammonium salt of picric acid, from which it is made. It has a very high melting point, and is therefore always pres-loaded. Its most im- portant characteristic is its great insensitivity to mechanical shock. It is also a very stable material, and unlike picric acid, it does not tend to react with metals. It is somewhat less powerful than TNT. Its chief use is as the filling for armor piercing. 48 projectiles and bombs. In fact, for large A. P. shells, it is the only explosive now in use by our armed forces which is sufficiently insensitive. RDX, Cyelonite, or Cyelofrimefltylene, Trinitramine. This compound has been known for many years, but has come into manufacture and use as a military explosive only during the present war. It is made by the reaction of nitric acid with hexamethylene tetramine or hexamine. The latter is formed by the reaction of ammonia and formaldehyde. The process developed by the British involves the straight nitration of hexamine; this process, with minor modifications, is used in this country at the Wabash River Ordnance Plant. Another process, the combination, or anhydride process, was developed in this coun!ry, and is in use at the Holston Ordnance Works. In this second process, the yield of RDX per pound of hexamine is considerably greater, and the consumption of nitric acid is much smaller. RDX is a high-melting compound, melting around 200°C, and cannot be melt-loaded. As a matter of fact, it is rather sensitive to shock, being somewhat more sensitive than tetryl, and consequently is always used in mixtures either with other explosives of lower sensitivity or with nonexplosive desensitizers. Despite the fact that RDX is a very energy-rich explosive, it is very stable, approaching TNT in this respect. It is a very powerful explosive, approximately equivalent to nitroglycerine. How .. ever, as an explosive to produce air blast, it is exceeded in effectiveness by some of the aluminized explosives. PETN or Penfaeryfltritol Tefranitra#e. This compound is made by the nitration of pentaerythritol, a polyhydric alcohol produced synthetically. This compound is a nitrate ester, whereas all of the explosives mentioned above are nitro compounds. Like other nitrate esters, it is not very stable, but sufficiently so for most purposes. It is somewhat more stable than nitrocellulose, which is the chief constituent of smokeless powder. PETN is a very shock-sensitive explosive, being more sensitive than RDX. As a result, it is used in the pure form only in specialized applications, such as the base charge for some detonators and as the core of detonating cord or primacord. It is a powerful explosive, being in this respect only slightly inferior to RDX. However, its disadvantages with respect to RDX are that it is more sensitive and less stable. It finds application principally as Pentolite, which is a mixture of PETN and TNT. SOLID EXPLOSIVE MIXTURES. Amato/. The best known explosive of this class is undoubtedly Amatol, which is a mix- ture of TNT and ammonium nitrate in varying proportions. The 50/50 and 60/40 mixture with TNT can be melt.loaded as a slurry, in which the solid ammonium nr- trate is carried by the molten TNT. An 80/20 mixture is sometimes used, but this cannot be poured. For most applications, and in particular for producing air blast and fragmentation, Amatol is somewhat less effective than TNT. For air· blast damage, the area of damage for Amato! is about 80% of that from an equal weight of TNT. However, for cratering action and perhaps for producing earth shock, when a bomb explodes underground, Amatol is somewhat more effective than TNT. The presence of ammonium nitrate in Amatol makes it a very hydroscopic explosive, and when 49 moist it is quite corrosive in contact with most metals. This necessitates very care- ful sealing of an Amatol charge against moisture. Because of this hydroscopicity and rather low power, Amatol is considered as a substitute for straight TNT when the latter is scarce. If TNT is in good supply, Amatol is not used for most applications. Composition 8. This explosive consists of a mixture of RDX and TNT, normally in the pro- portion of 60% of RDX and 40% of TNT. One part of wax per hundred parts of explosive is normally added to give some desensitizing action. The mixture with wax is called Composition B, while if the wax is omitted, the explosive is called Com- position B-2. There is sufficient TNT in this mixture so that at temperatures above the melting point of TNT, it can be poured as a slurry. The explosive is fairly insensitive to shock, but of course more sensitive than TNT. It is generally considered to be equivalent to picric acid in this respect. The stability of the mixture of RDX and TNT is not as great as that of either of the pure components (a common situation in explosives), but the stability is more than adequate. Samples of Composition B have been stored for at least three years at a temperature of 65°C without noticeable decomposition. The pressure in the detonation wave is higher for Composition B than for any other explosive in actual military use, but pure RDX gives a higher detonation pressure. Since this is the most important factor in cavity charge performance, this explosive is excellent for such munitions. It also gives high fragment velocity when used in bombs and shells, about 10% faster fragments than TNT. However, as an explosive for producing blast damage, it is of intermediate effectiveness, being su- perior to TNT, but inferior to the aluminized explosives. In aerial bombs, Com- position B gives about 25% greater area of damage than does TNT in the same bombs. If the choice lay solely between TNT and Composition B, the latter would be the choice, since it has better performance and adequate insensitivity, although it is more serisitive than TNT. However, there are other fillings that are still better. Pentolite. This explosive is a 50/50 mixture of PETN and TNT. It is normally loaded as a slurry, although for special purposes it can be pressed. It is more sensitive to shock than either TNT or Composition B. In fact, its sensitivity restricts its use to small munitions which will not be subjected to violent shocks. However, it is safe enough for handling, loading and shipping. The stability of Pentolite is inferior to that of straight PETN, and the explosive has a limited life when stored in very hot climates. However, during wartime, when there is a rapid turnover, its stability is probably adequate but near the lower limit. Pentolite has a high detonation pres- sure, but a little lower than that of Composition B. At present, Pentolite is chiefly used for filling various types of cavity charge munitions, such as the head of the ba- zooka. However, Composition B may replace it for this application. Pentolite is con- sidered to be too sensitive to be used in aerial bombs. Tetryfo/. This is a mixture of tetryl and TNT, usually in the proportion of 75% of tetryl and 25% of TNT. It can be poured and loaded as a slurry. This explosive has a sta- bility and sensitivity intermediate between those of Composition Band Pentolite. 50 Its chief attractive feature is that it is an explosive which has a higher detonation pressure than TNT, but which does not use RDX. During most of this war, tetryl has been in good supply. Tetrytol is used chiefly as a demolition explosive, for which use it is appreciably more effective than TNT. Its disadvantages are appreciable sensitivity, mediocre stability, and tendency toward exudation in hot storage. The exudation results from the fact that the melting point of TNT is greatly reduced by the large solubility of tetryl in TNT. It has no features which make it attractive as a filling for bombs. Compositions A and C. These are mixtures based on RDX. Composition A contains 91 % RDX and 9% wax. It is loaded by a pressing operation. Where press-loading facilities are available, it is an excellent filling for H. E. shell. It can also be used in small caliber A. P. shells. It is quite insensitive to shock, but not sufficiently so to be suitable for large caliber A. P. shells. Shells loaded with Composition A are considerably more effective than similar shells loaded with TNT or D. Composition C, or rather the present version, Composition C.Z, is a plastic ex- plosive based on RDX. It contains somewhat under 80% of RDX, the remainder be- ing a mixture of TNT, DNT oil and MNT. It is a very powerful explosive and for this reason and because it can be molded by hand, it is in great deman d as a demolition explosive. It is also being tested in thin-walled rocket heads and bombs for use in attacking concrete pillboxes. The plastic nature of the explosive enables it to flatten out and make excellent contact with the target, so that the resulting damage is much greater than with an equal weight of a solid explosive. ALUMINIZED EXPLOSIVES At the present time, there are four aluminized explosives in military use: Tritonal, which is aluminized TNT; Minol, which is aluminized Amatol; Torpex, which is aluminized Composition B, with the addition of a little extra TNT; and Torpex D-1 or HBX, which is Torpex containing 5% desensitizer. 1. Torpex. This explosive contains RDX, TNT, powdered aluminum and a trace of wax. Several compositions have been used at various times, but the material in general use at the present time contains 4Z% RDX, 40% TNT, and 18% of Aluminum. As an explosive for producing high velocity fragments, air blast and underwater shock waves, this is the most powerful explosive "in use today. As a bomb filling, it pro- duces an area of blast damage somewhat more than 60% greater than the same volume of TNT, and about 30% greater than the same volume of Composition B. The chemical stability of Torpex is excellent if water is excluded. In the presence of water, gas is given off. However, if the ingredients are thoroughly dried, there is no difficulty, since Torpex is not hydroscopic. The disadvantage of Torpex is that it is a some- what shock- and bullet-sensitive material. It is used as a filler for depth bombs and torpedo warheads, since under water it is equivalent in damaging power to a 50% greater weight of TNT. The sensitivity of Torpex is probably too great to make its use feasible in aerial bombs, at least of the G. P. type, where the explosive may have 51 to withstand a drop on a hard surface without detonating. (See following section on HBX.) 2. Minol. The composition in present use by the British contains 40% TNT, 40% am- monium nitrate and 20% aluminum. This is a powerful explosive for air blast and under water applications, although inferior to Torpex. It gives an area of blast dam- age about 40% greater than the same volume of TNT. Care must be exercised in handling this explosive, since it is hydro scopic, and in the presence of water, re- action with the aluminum takes place and gas is given off. Minol has been used by the British as a filling for high-capacity bombs, but is probably too sensitive for use in G. P. bombs. 3. Tritonal. The present composition is 80% TNT and 20% aluminum, but there is evi- dence that a 70/30 composition is more powerful. This explosive is somewhat in- ferior to Minol, both for air blast and underwater damage, but is still quite power- ful. It gives an area of blast damage about 35% greater than the same volume of TNT. It had been loaded in this country for use by the British for some months, and has recently been adopted for loading into G. P. bombs by our own Ordnance Department. It might be pointed out that Tritonal gives about 10% greater area of blast damage than Composition B. While somewhat more sensitive than TNT, tests indicate that it is sufficiently insensitive for use in aerial bombs (except A. P. bombs). Planes will be 35% more effective in carrying Tritonal-Ioaded bombs than in carry- ing TNT-loaded bombs. The information on the fragmentation effectiveness of Tritonal is meager, but in this respect it is probably at least equivalent to TNT and perhaps better . . 4. Torpex D-J or HBX. This explosive is Torpex to which has been added 5% of a desensitizer which consists mostly of wax. The addition of. inert material decreases its performance slightly below that of Torpex, but it is nevertheless more effective than any other available filling for blast and underwater damage. Tests conducted to date indi- cate that while it is somewhat more sensitive than TNT, it is sufficiently insensi- tive for use in depth bombs, aerial mines, G. P. bombs, etc. It is understood that the U. S. Navy is in the process of converting most of its loading from Torpex and TNT to HBX. It might be mentioned that the 12,000·lb so-called earthquake bombs are being loaded with this explosive and have been performing very well. 52 LIQUID EXPLOSIYIS Up to the present time, liquid explosives have been very little used, because for most applications they have no particular advantages over solid explosives and many disadvantages. Because of the fact that they can leak out of containers so readily they have not normally been considered for use as bomb or shell fillings. As a result, the information available on liquid explosives is less extensive than that on solid explosives. Nitroglycerine, which is a liquid, is of course manufactured on a very large scale because of the low price at which it can be .sold. However, it is almost never used in the liquid state. It is the most important constituent of various types of dynamite, and is also combined with nitrocellulose to form double-base smokeless powder. In general, one can say that the only liquid explosive about which we know very much is nitroglycerine, and it is much too touchy a material to be very attractive for use in the pure state. Nitroglycerine can be desensitized by the addition of various materials, and some work has been done along this line. However, while such products are much safer to handle than pure nitroglycerine, it seems to be true that liquid explosives made by desensitizing nitroglycerine are more hazardous to handle than solid explosives of comparable power. . Most of the other liquid explosives which have been investigated consist of mix- tures of a fuel with an oxidizing agent, neither one being an explosive alone. One such mixture is Dithekite, which has been studied by the British. It consists of a mixture of nitric acid and nitrobenzene, with about 10-13% of water. This material is fairly insensitive toward shock, but is very corrosive due to its nitric acid content. Its power is about the same as that of TNT. A somewhat similar mixture, Anilite, has been studied by the French. This contains benzene or nitrobenzene and nitrogen tetroxide, and arrangements are usually made to mix the two constituents at the last minute. It is obviously not a very pleasant material. If, at the present time, one desires to use a liquid explosive in large quantities, one is more or less restricted .to one based on nitroglycerine, or on a mixture of a fuel and an oxidizing agent, such as nitric acid, because of considerations of sup- ply. However, there are a number of other possibilities which may be made available in the future -if they prove to be useful. One such compound is nitro methane. This compound is now made commercially, but not in sufficient quantities for large scale uses. However, there is no reason why facilities could not be developed for its pro- duction, since it is made from hydrocarbons and nitric acid. While nitromethane has not been extensively studied, it appears to be a stable compound, not unduly sensitive to mechanical shock, and considerably more powerful than TNT on a weight basis. On a volume basis the comparison is less favorable, since a good TNT casting will have a density of around 1. 5 5 gm/ cc, whereas the density of nitromethane is only 1.13 gm/cc. Another liquid oxidizing agent, which may be available in the future, is hydro- gen peroxide. A mixture of this compound with a fuel, ethyl alcohol for example, 53 should be a rather powerful explosive, based on energy content. However, little in- formation is available in this country concerning its behavior and properties. In general, it does not appear probable that any liquid explosives will be devel- oped which will be appreciably more powerful than present solid explosives. How- ever, for applications in which a liquid is definitely desired because of its physical state, liquid explosives may have considerable use in the future. As yet, the field has not been well studied. 54 QUANTITATIYE COMPARISONS While thermal data are available for the various materials in use or contemplated use as high explosives, there is no simple method for making reliable computation of the power of an explosive from these data. Indeed, since, as was pointed out previous- ly, the relative effectiveness of an explosive depends on the use to which it is put, it is obvious that no single quantity, either computed or experimental, can give a unviersal measure of the performance of an explosive. Even if the consideration is restricted to a single type of action, predictions on the basis of thermal data are only approximate. It has long been customary to use both the heat of detonation and the characteristic product (heat of detonation times volume of gas produced) as measures of some sort of effectiveness. In general, it is found that there is at least a qualitative correlation between either of these quantities and, for example, the blast impulse for unit weight of explosive. When an explosive detonates and creates a blast wave, the hot, compressed gases expand, doing work on the atmosphere until the pressure of the explosion gases has fallen to a value of the order of one atmosphere. The amount of work done during such an expansion can be calculated, if the heat quantities for the explosive are known, and some assumptions are made concerning the equation of the state of the product gases during the high pressure stages of the expansion. When such calculations are made for the common high explosives, it is found that after expanding to one atmos- phere, the temperature of the product gases is not far from room temperature. From the laws of conservation of energy, it follows that the work of expansion is equal (approximately) to the energy released on detonation, if the latter is defined as the difference between the energy of the product gases and energy of original explosive, both at normal temperature and pressure. On this basis, one would expect to find a correlation b e t w e ~ n blast effectiveness of an explosive and its heat of detonation. Empirically, it appears to be approximately true that the blast impulse is proportional to the square root of the heat of detonation. For explosives which have a very high detonation temperature, the explosion products will usually be at a temperature appre- ciably above room temperature after expansion, and here the conversion of heat into useful work is not complete. Explosives containing appreciable amounts of aluminum behave in this way, and for these explosives, the value of the heat of detonation some- what over-estimates the magnitude of the blast impulse. The SBX explosives form a class by themselves. The real explosive here is a mix- ture of a fuel, which is actually carried to the target, with air. Obviously, the volume occupied by unit weight of such an air-fuel mixture is very large compared to that occupied by the same weight of conventional high explosive. As a result, the maxi- mum pressure produced when detonation occurs is very much lower for SBX than for a conventional high explosive. Actually, the maximum detonation pressure in the former case' will be of the order of 10 or 20 atm, whereas for a high explosive it is of the order of 200,000 atm. If we now compute the adiabatic for the explosion 55 products of gasoline and air, for example, we find that about half the total energy of the seaction is retained by the explosion products, and consequently, the useful work is only half the heat of detonation. This is due, of course, to the small expan- sion ratio involved. There is some uncertainty in the calculated value of the heat of detonation for many high explosives. This is due to uncertainty as to exactly what the composition of the products is. Many explosives contain insufficient oxygen for complete con- version of the carbon and hydrogen to carbon dioxide and water, and. in fact, there is often not enough oxygen to convert the carbon and hydrogen to carbon monoxide and water. In this latter case, there is doubt as to the way in which the oxygen is divided between the hydrogen and the carbon. While the equilibria involved have been studied at ordinary pressures, they are not known for pressures of the order of 100,000 atm. Indeed, the composition of the products undoubtedly depends on the condi- tions under which the explosive is used. This uncertainty is especially pronounced in the case of TNT, since in this compound there are only six atoms of oxygen for reaction with five atoms of hydrogen and seven atoms of carbon. In Table I, the heats of detonation have been calculated, in general. on the as- sumption that all of the hydrogen is converted to water, and what oxygen remains reacts with the carbon. This procedure tends to overestimate the heat of detonation, but underestimates the volume of gas produced in the detonation. The value of the characteristics product (heat by volume of gas) is not very sensitive to changes in assumptions concerning the composition of the products, since there is this com- pensation. In Table I, many of the quantities have been both for unit weight and for unit volume of the explosive. Depending on the application, one or the other quantity may form the best basis for comparing different explosives. In Table II, some of the important quantities are tabula.ted for various SBX explosives. The heats of detonation have been calculated for the reaction of the fuel with an amount of air sufficient to give complete conversion of the carbon and hydro- gen to carbon dioxide and water. For SBX, it seems evident that the characteristic product has little meaning and the available heat, or available work, forms the best basis for estimating the potential performance of this type of explosive, relative to conventional H.E. As was pointed out in a previous section, SBX explosives have been found effective in confined spaces but ineffective when exploded in the open. 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