or maintained locally would be available to NATO and that in their opinion there was no prospect of defending their lands with these forces, the political leaders would have quickly appreciated the fact that there was little purpose in making a partial effort. They would have selected a policy of neutrality or compromise with the Soviet Union in the face of no prospect for a successful military defense.

The contribution of the Free World's strategic retaliatory force to the security of Europe must be looked upon as an increment which reinforces the maximum forces that can be maintained locally to the level of military effort required to provide both the military and the governments of Europe with confidence that a defense effort is worthwhile.

Acting in the capacity to fill the gap between capabilities for local defense and force requirements, the Free World's strategic air contribution spans a wide range of differences. At the same time it in no way prevents a continuing effort to maximize the local capability wherever possible into a completely self-sufficient defense.

The very fact that NATO force goals constantly exceed the ability of the nations concerned to build the forces is evidence of the effort being made towards self-sufficient defense in Europe.

If, by reducing the strategic retaliatory forces to a point where they could only be used as a last resort, we admit we have no "will" to use the forces in defense of Europe, then we must reassess the security of our NATO partners on the basis of their local capabilities to deter or defend against Soviet attack. If the local capabilities still prove inadequate to deter or fight aggression without strategic air support, the leaders of the countries concerned will quickly recognize this. They then might seek to reorient their foreign policy to seek protection through agreement with the Communist bloc, being unable to find security in any strategy that NATO can afford.

What if it were found possible ultimately to generate a truly effective local defense and deterrent capability around the entire periphery of the U.S.S.R.? Even so, the Soviet Union would certainly take advantage of the gap between the time we could build such a capability and our earlier declarations that the strategic air forces can no longer be counted upon to protect our friends.

In capitalizing on the budgetary attraction of the offensive element in defense, proponents of a reduced and diversified SAC go so far as to rationalize that the classical prewar concept of the shield and the sword is now reversed. The new twist sees SAC as the "shield" which prevents either side from using strategic forces. Behind this "shield" the front-line, limited-war conventional armed masses become the "sword" and can return to the classiIcal land warfare of attack and counterattack.

The minimum-deterrent strategy sought by critics of the existing counter-force deterrent capacity is one which would lead to unlimited requirements for limited war. Such a strategy would eventually become a far greater drain on the taxpayer than the present one-if Europe survived long enough to implement it in the first instance.

Headquarters United States Air Force

...Air Force Review

IS A SONIC BOOM AN EXPLOSION?

DR. JAMES A. FRASER

WE SHALL this problem by tree,

E SHALL approach this problem by considering three important phases

an explosion. First, we shall consider the origin. Second, we shall consider the mode of transmission of the energy from the place of origin to the place where the effects occur. Third, we shall consider the effects.

Since most definitions of explosion emphasize the origin, the origin is not only the first consideration but also the most important. Webster's New International Dictionary, Second Edition, 1958 printing, defines explosion as: "Act of exploding; detonation; a violent bursting or expansion, with noise, following the sudden production of great pressure, as in the case of explosives, or a sudden release of pressure, as in the disruption of a steam boiler; also the noise made by such bursting." Notice that this definition is exclusively concerned with the origin of the energy involved. Funk and Wagnalls' dictionary, 1952, defines explosion as: "The act of exploding; rapid combustion, decomposition, or other similar process resulting in a great and sudden development of gases, and consequent violent increase of pressure, usually accompanied by a loud report." The Van Nostrand Chemist's Dictionary, 1953, defines explosion as: "A chemical change that produces large quantities of energy or an increase in the volume of the system, or both at a rate sufficiently rapid to have considerable effects, often destructive, upon the surroundings." This definition discusses both origin of energy and effects, but it emphasizes origin. The United States Air Force Dictionary, 1956, defines explosion as: "A sudden outburst of particles or gases from a substance that has undergone detonation. Detonation precedes explosion, although the two events are so closely related as to be identified as the same event in loose or elliptical usage."

While all these definitions are satisfactory, I prefer the one offered by Webster because it is more general. In it a pressure difference is assumed as the necessary condition for the initiation of an explosion. This pressure difference may come about by the sudden production of gases under pressure through chemical reaction of an explosive, or it may come about by gradual buildup of pressure in a confined place, such as a steam boiler. The important point is that before an explosion can take place there must be produced by some means a pressure difference. The second necessary condition for an explosion is that this pressure difference be neutralized or eliminated suddenly. In fact this action must be almost instantaneous-time being measured

in milliseconds. Within this definition an automobile tire blowout is an explosion, but a slow leak is not. In both cases the pressure difference was relieved, but in the slow leak the action was not sudden.

This definition should also include the production of gases under pressure by nuclear and thermonuclear reactions. In these cases tremendous pressures of hot gases are produced with very great speed and released to the surrounding atmosphere with a suddenness measured in microseconds.

It should be noted that within this explanation of an explosion, and within all the definitions of an explosion, the explosion is over when the act of exploding destroys the pressure difference which is a prerequisite for explosion. The train of events initiated by the explosion may continue for some time after the explosion, but these definitions imply that the explosion is the initiating act rather than the succeeding consequences of that act.

In spite of this, to get a means of comparison with the sonic boom which will be described in the next section, we shall consider the first consequence of the explosion. The suddenly expanding gases bursting forth from their place of origin press against and compress the surrounding air. This then. forms a sphere of compressed air, if we assume that the burst has taken place in free air. The spherical shape of the pressure wave may be modified by obstructing substances if they are in the vicinity. This compressed air is the start of a shock wave, which in the case of an explosion is often called a blast wave. Three more definitions are now in order:

shock-a finite pressure disturbance having a discontinuous front fol-
lowed by a variation in pressure, the whole of which propagates as a

wave.

shock front-the discontinuity at the head of a shock wave.

blast wave-the pressure wave resulting from an explosion, usually con-
sisting of a shock front and a positive phase followed by a negative
phase.

The sonic boom has received much public notice in recent years. From both a legal and a public relations standpoint the Air Force has felt it important to inform the public on the nature of this phenomenon of supersonic flight. Court cases involving the sonic boom have begun to crop up. The Air Force has at least the indirect interest of clarifying the facts. Insurance companies, for example, insure buildings against damage from "explosions." If damage is claimed as the result of a sonic boom, the question of whether a sonic boom is an explosion becomes central. One such case was recently concluded in the Circuit Court, Montgomery, Alabama. The judgment was that "the phenomenon known as the sonic boom is not an explosion within the wording of the policy issued by the defendant to the plaintiff.” While this one case is perhaps not definitive, particularly since it is being appealed, it does suggest a need for information on the origin, transfer of energy, and effects of a sonic boom compared to conventional and nuclear explosions. Dr. James A. Fraser, Physical Science Analyst, Warfare Systems School, defines the physical distinctions between a sonic boom and an explosion.

The origin of the sonic boom is best explained by considering the difference between subsonic and supersonic flow. To do this we will first examine a pressure wave formed by a disturbing center that is at rest with respect to the atmosphere, then a pressure wave formed during subsonic flow, and finally a pressure wave formed during supersonic flow. Figure 1 is a cross section of a spherical pressure wave emanating from a disturbance at a point O in the center. The pressure wave is transmitted at the speed of sound in the air. Let this speed be represented by the symbol a. Notice that in time t, the pressure wave will travel a distance r = at. This situation would pertain if the medium in which the pressure wave is propagated (air) is at rest relative to the source of the disturbance.

Now consider a body traveling with a subsonic speed in air, as illustrated in Figure 2. At the start of the discussion, let the body be at the position A, moving in the direction of the arrow, with a velocity V which is less than the speed of sound a. It will create a disturbance in the air which will start a pressure wave. After a time, t, the pressure wave will have moved in a sphere to the line represented at 1 (a cross section of the sphere). The distance from A will be at. But in this same time, t, the disturbing body will have moved only to A'-a distance which is less than at because it is moving slower than the speed of sound. The same sort of argument could be repeated for a number of time intervals. Because V is less than a, the pressure wave or pressure front always moves away from the disturbing object, and the disturbing object always remains inside the spherical waves it creates. Thus when a body

moves with a subsonic speed the disturbances created by it clear away from the body.

Figure 3 illustrates the conditions where a body travels with supersonic speed. In time t the pressure wave caused by the disturbing body will move on a spherical wave front a distance of at. But in this same time the body itself has

r = at

body motion

V > a

Figure 3

moved farther than this distance to a point marked A'. In other words, the body will have overtaken and traversed the disturbance it created; thus the disturbance wave front is always downstream from the body. As the body moves it will be creating disturbances in the air continuously. In Figure 4 five successive points of disturbance are shown with their associated expanding, spherical pressure wave: A, B, C, D, and E. Notice that all the disturbance

E

B

body motion

Figure 4

wave fronts merge along the surface of a cone, the cone starting at the disturbing body and trailing behind it. This is a cone bounded by a pressure greater than the ambient pressure, and this pressure has been built up by the addition of many small increments of pressure from each of the merging pres