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Aviation History
1951
1951 - 0168.PDF
108 FLIGHT, 25 January 1951 FIGHTER ARMAME By A. R. WEYL, A.F.R.Ae.S. Le Prieur anti-Zeppelin incendiary rockets on a Horace Farman biplane in 1916. They were stick-stabilized and used a black-powder propellant. PART VII : Simple Rocket versus Guided Missile—and Some General Conclusions THE majority of artillery rockets now used in surfacewarfare, or from the air, are subsonic. Most of thepresent supersonic rocket missiles are large, and intended as guided missiles of the " aerial-torpedo " variety. Small supersonic rockets, used operationally in numbers (i.e., as salvos, bursts, or salvo-bursts) are still rare and, apart from the German R.4/M, repeatedly mentioned in this series, little developed. The unrotated and the spinning types of rocket missile differ basically, the former having fins for stability in flight. The latter usually has helically disposed discharge nozzles ; during the burning period, the ratio of angular velocity to translational speed remains practically constant. Generally, it seems established that, for high-velocity, air- launched missiles, the unrotated finned rocket gives the smallest dispersion. The R.4/M is not only of this variety, but has folding fins, a feature now often adopted (e.g., in the larger, homing edition of the R.4/M about to be introduced by the Americans for jet fighters). Before launching, the fins are folded back against the concave portion of the nozzle, like blades in a pocket knife; when the missile leaves the cardboard launching- tube in which it is packed and conveyed to the launcher, the fins spread. This arrangement greatly facilitates storage, handling and feeding. For small, plain rocket missiles, solid propellants, based on smokeless powders, are preferable. Admittedly, such a means of propulsion offers many and intricate basic-ballistic problems (on which, incidentally, the Americans have done much spade- work). Yet it is the simplest possible: the propellant (in the shape of "grains," i.e., extruded lengths of highly compressed colloidal double-base powder, usually tubular or cruciform in section) is located in the combustion space. On ignition it decom- poses rapidly, producing hot gases at high pressure; these leave through the discharge nozzle or nozzles. There are no moving parts, but, at the same time, there is no regulation of the com- bustion or of the thrust during the burning period. With air-combat missiles, unrestricted burning is the rule, and the burning period is short (in the R.4/M it is 0.8 sec.); in few cases does it exceed two seconds. Hence, though combustion temperatures are of the order of 3,000 deg C, cooling is usually unnecessary in view of the short burning-time and the expend- ability of the missile. The propellant and the nozzle, which together form the "motor tube," take up considerable space; the missiles are, therefore, rather long and slim. This shape suits finned unrotated projectiles perfectly. The propulsive jet not only provides thrust but also exerts influence upon stability, for a steady efflux may well damp out initial launching oscillations. In practice, the jet is usually far from steady, mostly on account of uneven burning and of the chemical and dynamic phenomena occuring whilst the hot gases are passing through the nozzle. Thus, small initial oscil- lations may even be aggravated, and this is the m&jor cause of the flight-path deviations which are still charaaeristic of rocket missiles. According to painstaking American research conducted on the obsolete M.8 unrotated supersonic rocket, the dispersion is about five times that of a good gun. The major dispersion arises during the burning ; when combustion ceases, the missile behaves, In the first of this series (August 24th) Mr. Weyl reviewed the history of aircraft armament; in the second (September 21st) he discussed German equipment; in Part III (October 5th) he analysed the respective claims of guns versus rockets; on November 23rd he dealt with the effectiveness of various types of shells and fuses; in Part V (December 7th) he analysed Luftwaffe combat experiences, particularly with the use of the R.4/M air-to-air rocket missile; and on January 4th he dealt with some further aspects of gun design, discussed sights, and showed how installation problems indicate the use of rockets rather than guns in the newest intercepters. In this final instal- ment he suggests that the guided missile is not yet the answer to the interception problem and concludes by summarizing his views on our past and present aircraft-armament policy. ballistically, like a shell; and there is no basic difference in effect between spin-stabilization and stabilization by guide-fins. The reasons for the high dispersion during burning are not yet clearly understood. The theory of propellant-combustion supposes that the combustion proceeds at a constant rate over the flame-exposed surface, and that equal layers of the fuel are transformed into gas at the same instant. As can be verified from any half-burned piece of celluloid (a near relative of the basic constituent) this is not completely true; moreover, the rate of burning varies during flight. The material is both heat- and pressure-sensitive, and the burning surface varies greatly during consumption. Consequently, gas is not produced at uniform rate. The theory, too, treats of orderly flow through nozzles; yet the behaviour of any water tap will suggest that this is a rather vague approximation. Therefore, between launching and exhaustion of the propellant, the mean thrust experiences an initial rise to a maximum and subsequent slow decrease, both accompanied by minor fluctuations; finally, it drops non-linearly to zero. There is much scope for research into these problems, the solution of which would make for substantial improvement of the ballistic properties of supersonic rocket missiles. For air interception it is important that the emerging jet should be smokeless, and that, immediately after launching, the flame becomes as short as possible. Flash-suppressors can be incorporated in the propellant. A possible way of decreasing dispersion might be to guide the missile mechanically (e.g., along launching rails) during combus- tion of the propeuant. This, however, is not practicable; the R.4/M for instance, burns for a distance of 600ft. Should much higher initial accelerations be employed, however, mechanical guidance might be effective during the first instant of launching. This would then approximate the conditions obtained in a recoil- less gun. An important item of rocket design is the igniter, which has to heat the propellant to ignition temperature besides raising the initial pressure. Air combat demands instantaneous action; any lag renders the weapon useless. The "ignition delay" is, therefore, a vital factor; it ought to be about one five-hundredth of a second. That is quite practicable with electrically actuated squib igniters ; a heated wire ignites a heat-sensitive explosive material—usually either some sort of granulated black powder, or a metal-powder mixture (magnesium or aluminium) with
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