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Aviation History
1945
1945 - 1784.PDF
SEPTEMBER 6TH, 1945 FLIGHT Bombers or Rockets V Some Further Thoughts Prompted J?$ Ihe^tomctf Bomb : Enormous Possibilities of ^Atomic "Drive^for Rockets By A. V. CLEAVJ IN his August oth article in Flight (f Is the BomberObsolescent ? "), Major Robertson discussed the moraleeffect of intensive bombardment which WHS alluded to in my original July 19th article. Suggesting tha lacking to prove that even the prospect of utter destru' of cities would cause a belligerent nation to surrender, he concluded with the unconsciously significant words : " We have yet to see if the fate of Japan will afford any evidence." Now indeed we have seen ; within 10 days of the dropping of two atomic bombs on Hiroshima and Nagasaki, Japan surrendered. These two atomic bombs, as one American commentator pointed^ _______________ f out, were really bigger news tbeEn^ World War II itself; the potentialities of atomic energy in war and peace raise tremendous issues for mankind, ^probably greater than any which have arisen at any one time before. It may be some years — even many years — before the industrial use of atomic energy revolutionises our way of life, but -"-"———-———————— as the national press and radio have exhaustively pointed out, the military problems raised by atomic bombs are already with us to-day, and with a ven- geance. All the sociological and moral implications of war which were briefly referred to in my original article now stare us/in the face in all their stark and hideous reality— no ledger as future issues to be theorised about, but as facts already experienced by the people of Japan. It is the purpose of this present note "to enlarge on one aspect of the original " Bombers or Rockets ? " article, which contained one sentence as unconsciously prophetic in the imminence of its realisation as the one of Major Robertson's already referred to : " If the possibility of increasing the energy of explosives by the application of atomic physics exists as a practical possibility, then this could be used, both to increase the destructive power of a given warhead weight, and also, very probably, to increase range by improving the propellant fuel of the rocket." Regarding th*e first part of this statement, few will argue. The first atomic bombs were dropped from Superfortress heavy bombers (presumably a light bomber snch as a Mosquito could equally have accomplished the mission), in view of the area of destruction possible and the con- uently reduced requirement for aqcuracy, a rocket projectile of sufficient range would have been an alternative, to be preferred on many obvious grounds. Consider the effect on London of a few V.2 rockets with atomic warheads ! No Range Limitations Now, if atomic energy can be utilised for the propulsion of rocket projectiles, there can no longer be any question of their having a range limitation. We are left with the prospect of a future war with the belligerents throwing directed missiles at one another, with atomic warheads and atomic drive. These might possibly be countered by defensive directed missies of a similar nature, probably equipped with Radar devices enabling them tf" home " on the offensive projectiles, and destroy some of them by ramming high in the upper atmosphere or beyond. Aircraft would still find a relatively limited use as troop carriers for occupying forces, and for reconnaissance, and for those missions in which it was desired to inflict damage rather than complete destruction on the enemy. In this speculative article, which follows on his original contribution in our July 19th issue, the author looks ahead and tries to evaluate the sort of powers and velocities that might be expected if we knew how to harness atomic energy for propulsive purposes. The author is chief project engineer (airscrews) of the de Havilland Aircraft Co., Ltd. leral Arnold a/d others, in recent weeks, have fore- Sh the possibilit^of such a war ; it remains only to observe lat it wouldoj^clude the survival of much of the civilisa- tion whj_tk<(flowed it to begin. Atomic Drive for Rockets The performance of a rocket projectile is fundamentally related to tin exhaust velocity of its jet. Where this = v, and L is the energy content (calorific value) of the propellant fuel used, then : fa Vi~- Published statements on the atomic bomb have quoted its weight as about 4001b. and its effectiveness as equivalent to 20,000 tons of bombs with normal explosive warheads; this would give an 20,000 x 2,240energy content ratio of 400 implying an exhaust velocity ratio equal to the square root of this number, i.e., 335. Actually, the increase in possible ———————— exhaust velocity would be larger than this, because the atomic bomb con- tained only 8 1b. of Uranium 235 "explosive," whereas 20,000 tons of normal bombs would contain about 16,000 tons of explosive. These latter figures would suggest a possible exhaust velocity ratio of : 16,000 x 2,240 = 2,120 Assuming even the lower figure (since all these figures are so hypothetical that their relative accuracy is unimportant— it is desired only to'indicate an overwhelming general trend), some startling conclusions can be drawn. If the starting mass of a rocket is denoted by Mo, and it consumes a mass of fuel = Mo — M in accelerating to a velocity = vo at the end of its powered flight, then :— M -r-;0 = e V"lv where v is the exhaust velocity as M before. This simple expression neglects performance losses due to air drag and gravitational attraction during the climb, but will serve to illustrate the point at present in question. The rocket, after its motors cease thrusting, will behave just like, for example, an artillery projectUe, i.e., its total range is a function of the velocity vo at the end of its powered flight. If normal bomb warhead explosives were used as pro- pellant fuels in a chemical rocket motor, they would give an exhaust velocity of about 7,000 ft./sec. Of course, they are not so used, partly because of the difficulty of controlling their action (rate of burning), but also because of their lower energy content as compared with fuels such as alcohol or petrol burned with liquid oxygen; an alcohol- oxygen mixture such as was used in V.2 should give an actual exhaust velocity of about 8,000 ft./sec. (with 100 per cent, motor thermal efficiency, the value would be 13,700 ft./sec). In V.2, with its range of about 200 miles, the alcohol plus oxygen weight consumed was approximately 8 tons, and the starting weight 12 tons. M 12 Thus, the mass ratio = —.- = — o M 12 — 8 t-o (ft./sec.) = 3.0 = e 8-00° .; Vf> = 6.000 rn.p.h. (This is the velocity at end of powered vertical flight which
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