RIDING ON A GAS JET' JOHN K. ROULEAU Boston College, Chestnut Hill,Massachusetts A r m o u o ~military rockets assumed a prominent position in the recent war when the Germans bombarded England with the mighty V-2, rockets are in fact very old military weapons. It is probable that the Chinese employed rockets against the Mongols a t the battle of Kai-fung-fu in A.D. 1232. During the 13th and 14th centuries military. rockets were in general use. Detailed descriptions of these rockets are recorded in Eichstadt's l'Bellifortis" (1405) and deFontana's "Bellicorum Instrumentorum Liber"'(1420). From about A.D. 1500 until A.D. 1800 the development of firearms and cannons, which had the advantaees of accuracy and range, caused a displacement of rockits as military weapons. In the A.D. 1800 period, the successful employment of military rockets by native Indian troops against the British prompted a young British Artillery officer, William Congreve, to investigate the adaptability of pyrotechnic rockets to military use. As a result of his experiments the Congreve Rocket was developed. This rocket, essentially an augmented "skyrocket," weighed about 30 pounds and employed a "stkck" to stabilize i t in flight, the "stick" acting in a similar manner to the feathers on an arrow. The rocket proved so effective that its use a t the Battle of Bladensburg caused the American Troops to break, thereby permitting the invasion and burnine of Washineton. The "rockets' red glare," immortaliz~dby Francis Scott Key in the "star Spangled Banner," was produced by the flame from the Congreve Rockets. Despite the early success of the Congreve Rocket, for which incidentally W'iliam Congreve was knighted, subsequent artillery development caused the heavy gun to supplant the rocket as a military weapon. From approximately 1850 up to World War I1 rockets were used in war solely as signaling devices. During the recent war, however, all belligerents made use of rockets.' Solid-fuel rackets, both fin and "spin" stabilized, ranging in weight from a few pounds to over a thousand pounds, were used in enormous quantities
on all fronts. The maximum range obtained with rockets of this type is about 10,000 yards, while velocities obtained varied from 65 to 1500 feet per second. The outstanding liquid-fuel rocket, the huge V-2, had a launching yeight of over 14 tons, a length of almost 50 feet, a range of 200 miles, and a peak velocity of 3500 feet per second. Rockets may be described as missiles recoilpg from the rearward expulsion of gases internally generated by the combustion of an internally carried fuel and oxygen supply. Rockets and "power jets" are both propelled by the rearward expulsion of combustion gases. They differin that "~oweriets" obtain the oxveen necessarv
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Presented at the 238th Meeting of the New England Associstion of Chemistry Teachers s t Boston College on December 7, 1946.
Couilesy of A r m y Ordnance ~ e g a r m e
~ u sh t fon, ~ i k ~ - ~ f t
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Pholo h~ U. S. Army SIQZMI Corps, C o u r i e ~ yof A r m y Ordnanrr Maaaitne
"v-z in the birW
for t,he combustion of the internally carried fuel from the air that is forced into the front of the jet as it travels through t,he air. Accordingly, "jets" require either auxiliary compressors or starting rockets to initiate flight. Rockets, on the other hand, have a silf-contained oxygen supply. This oxygen supply may .come from a separate container, as in the case of liquid-fuel rockets, or may be present in the fuel itself, as is the case with solid-fuel rockets. The forward propulsion of a rocket is due to the rearward escape of combustion gases in accordance with Newton's Third Law, "To every action there is an equal and opposite reaction." An estimate of the magnitude of the forward thrust may he calculated by multiplying the value for the number of pounds of gas escaping per second by the value of the exit-gas velocity. Since the mass of the issuing gas is relatively wall, a high velocity of the escaping gas is required in order to obtain a high thrust. This high gas velocity is obtained by, allowing the combustion gases to expand through a convergent-divergent nozzle. In general a rocket motor consists of a combustion chamber, closed a t the head end and fitted with an opening at the discharge end which has the form of a convergent-divergent nozzle. When the fuel is ignited, gas pressure builds up in the combustion chamber and the gas produced streams through the nozzle into the atmosphere where the pressure is lower. When the gas, under pressure in the combustion chamber, expands t,hrough the convergent. section of the nozzle, the gas pressure decreases and simultaneously the velocity of t,he gas molecules increases. However, there is a limiting pressure to which the gas will expand in the convergent section and hence there is a limit to which the gas molecules will accelerate. This limiting pressure, called the critical pressure, is a definite fraction of the pressure existing in the combustion chamber. (A
steady-state condition is assumed, once burning is under \vay in the combustion chamber.) If this critical pressure is equal to or greater than the pressure in the region of discharge (i. e., the atmosphere), then the gas will undergo a further expansion in the divergent section, thereby producing a further acceleration of the gas molecules. When these conditions prevail, the rate of gas discharge will he governed by the following factors: (1) Area of the nozzle throat-that portion of the nozzle where the convergent and divergent sections meet (2) The composition of the combustion gases (3) The temperature and pressure conditioni existing in the combustion chamber The velocity of the exit gases will likewise be governed by these same three factors and hence the thrust, for a given propellant and rocket, is determined by the chamber conditions. For steady-state chamber conditions the number of pounds of fuel transformed into gas per second must equal the number of pounds of gas leaving the rocket per second. This may be expressed in the form of an equation lbs. fuel burnt second
=
lbs. gas discharged second
Furthermore, Ibs. gas discharged second = VdS
Here V is the exit velocity, d is the density, and S is the nozzle area. Hence for a .gi& exit gas velocity the number of pobnds of fuel that must be burnt per second. can be determined. In the case of liquid-fuel rockets, such as the V-2 which burns almost 18,000 pounds of alcohol and liquid oxygen in a minute, the fuel and oxygen areinjected a t the propq rate into the comhustion chamber by means of fuel pumps. In the case of solid-fuel propellants, however, the production of gas in the combustion chamber is determined by the rate of burning of the rocket grain and the geometry of the grain. When a solid-propellant grain burns in a closed combustion space, burning is believed to proceed a t the surface in a manner similar to the gradual erosion of a piece of metal suspended in an acid. The burning occurs at all free surfaces in uniform layers perpendicular to the burning surface. Thus if the propellant grain is originally in the shape of a sphere, that form will he retained throughout burning, although of course the sphere will decrease in size until it is entirely consumed. This burning in parallel layers is known as the burning rate and may he expressed in inches per second. Using this concept, an equation can he established, correlating fuel consumption with themburningrate and the geometry of the grain. . Ibs. fuel burnt
=
seeand
d,S,r
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web, definitely limits the size of a grain that can be produced. However, by employing a technique developed by the British, nitrocellulose-nitroglycerin rocket powder may now be produced without the use of solvent so that wall thickness is no longer a controlling factor in the production of big grains. Single grains of powder several inches in diameter, as long as .five feet and weighing about 40 pounds;have been produced. Certain high-velocity rockets employ several of the grains in a single round. Although nitroglycerin-nitrocellulose grains offer a reliable fuel for sockets, other organic and inorganic materials in solid and liquid form are used, particularly vhen thrusts of relatively long duration are required. Many peacetime applications of the rocket principle are now under investigation, and some items as the J.4TO (jet assist take-off rocket) have already been accept.ed in commercial flying. The famous flight of the Navy plane."The Truculent Turtle" began when takeoff rockets helped the heavily loaded plane to become airborne. Rocket "brakes" for high-speed trains, rockets to lift st,alled vehicles out of a mire, rockets to dig postholes, rockets to soar high into the heavens to obtain data on weather and cosmic rays have already Coniiesv o! !he Hercules Powder C o r n p n y shown t.he versat,ility of rocket propulsion. -Gl.*n." of Powder for R 0 c k . t ~ In an article in "Army Ordnance" .Dr. Frank J. Malina discussed the launching of a V-2 rocket in New Here dl is the density of the grain, S, is the area of Mexico that reached an altitude of 300,000 feet and had burning surface, and r is the rate of burning. For best a maximum speed 'of 3500 feet per second. He points i.ha.mber conditions the three factors affecting fuel out that if a second rocket were launched from a V-2 consumption should.he constant. While the density as it sped up~vard,the second rocket would probably and geometrical shape of a propellant grain are fixed reach an altitude of 2,000,000 feet and have a peak during the process of manufacture, the burning rate is . .. controlled by the pressure and temperature conditions existing in the combustion chamber. Since the burning rate increases with high temperature and high pressure, solid fuels, which give such conditions on hurning, produce high exit gas velocities and concurrent high thrust values. Since powder grains ret.ain their original form on hrning (spherical, cubical, etc.), the %urning surface hecomes progressively smaller. By referring to the equation correlating fuel consumption and the area of the burning surface, it ~ lbe lnoted that such geometric shapes cause a gradual decrease in fuel consumption as Imning progresses. This undesirable feature may be circumvented either by blocking off part of the original burning surface with a noncombustible material, which is done with the American cruciform grain, or by forming the grain in the shape of a hollow tube. .A tubular grain, burning simultaneously from the inside and outside surface, maintains a practically constant total area during burning since the inside area increases as the outside area decreases. It is for this reason that tuhuIar grains are widely used in rockets. Although tubular powder grains possess the advantage of a constant burning area, the usual method of manufacture from nitrocellulose and nitroglycerin Cauricsy ol the H e r c r l e s P w d r ; Comadnr scquires the use of organic solvents. Since thissblvent mu5t later be removed by drying, the wall thickness, or Extrudinv Solid Rockat Fud
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velocity of 9500 feet per second. For those whose curiosity is stirred by the "Jules Verne" aspect of soaring almost 400 miles into space, an article by E. M. Rogers entitled "Man-made satellites," in which plans for huge sodium mirrors mounted on "free floating" platforms 5000 miles up, should prove entertaining reading, particularly as these plans are attributed to the same rocket scientists that produced the V-2. REFERENCES (1) KIEFER, P. J.,
AND M. C. STEWART, "Principles of Engineering Thermodynamics," John Wiley & Sons, Inc., New Vork 1030. -----, (2) "U. S. Rocket Ordnance," A r e l w e by the Joint Bomd o n Scientific Information Policy. (3) MALINA, F. J., Army Ordnance, 31, No. 157 (1946). (4) ROOERS,E. M., ibid., 31, NO. 159 (1946).
NINTH SUMMER CONFERENCE Wellesley College, Amgust 18-23, 1947 The Conference this summer revolves about the secluded Wellesley campus, a little outside the huh of Boston. The grounds have a pleasing rural setting with broad easy slopes about the campus lake. In addition to well-appointed dormitories with dining halls and science buildings there are provisions on campus for golf, tennis, canoeing, and swimming. A life guard is in attendance. Wellesley is on the Boston and Albany throughroute to Worcester and is reaohed by bus from several other ditections. The attempt will be made to have a responsible yo& woman each day to look after very little guests who are not accustomed to attending these learned sessions. The evening social hour is now tradition; it will be on this summer's program. Two field trips to nearby industries and a picnic will occupy the broad central partion of the week. A sequence on "Selected Topics from Introductory Chemistry" and a symposium on "The Develop ment of Atomic Structure" fill important places in the program. Individual speakem and their tentative topic titles include:
"Recent Developments in Powder Metallurgyv-Alden Burgbardt, Watertown Arsenal "Bikini Operationn-Royal M. Frey, Boston University "Fluorine and Tooth Decay"-a member of the Faculty of Tufts College Dental School 'The Commercial Production of Fluorine and Its New Uses"John T. Pinkston, Hamhaw Chemicd Company "Antithyroid Agentsn-R. 0. Roblin, Jr., American Cyanamid Comoanv 'Tnduskai Processes"-Emil R. Riegel, University of Buffalo ''Minerals, Fun, and Profit"4ohn B. Lucke, University of Connecticut "Scientific Liaison in the European Theater of Operations"Charles E. Waring, University of Connecticut "The College Entrance Bomd Exrtminatian-W. W. Turnbull, PrincetonUniversity . Attention is called to the fact that the date of the Summer Conferenoe was given incorrectly in the March Report. WellesLey College asked to have the date changed after the copy for the March issue had been prepared and your Editor neglected to make the change in proof. The correct date is August 18-23.
Official Business
0
President Lynn has appointed the following persons to r e p resent the N.E.A.C.T. on the Executive Committee of the National Association of Science Teachers: Millard W. Bosworth, Elbert C. Weaver, John R. Suydam, and Helen W. Crawley.
e
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Officers for 1946-41
Eldin V. Lynn, P~esidmt,Massachusetts College of Pharmacy Boston 15; Dorothy W. Gifiord, Seeretarp, Lincoln School, h v i dence 6, Rhode Island; Lawrence H. Amundsen, Editor of Repwt, University of Connecticut, Stom; Millard W. Bosworth, Immediate Past P ~ e s i d a t ; John R. Suydam, Vice-president; Carroll B. Gustafson, Treasurer; Ledlyn B. Clapp, Southern Division Chairman; Helen Crawley, Central Division C h a i m n ; Jean V . Johnston, Western DivisionChaimn; Lester F. Weeks. NorthDivision Chairman; . . %Iph E. Keirstead, Curator; Elbert C. Weher, Chai~man Endowmat Fund; S. Walter Hoyt, Aditor.
