space resources for teachers
RICHARD M. LAWRENCE WILLIAM H. BOWMANL Ball Stdo Univenily Munrie, Indiana 47306
High-Temperature Reactions Associated with Space Vehicles
Two high-temperature environments are common to many missions in space. One is the inferno in the chemical rocket engine that provides the thrust to launch and guide a space vehicle. The second is that existing in the hot gas cap surrounding a space vehicle passing through a planetary atmosphere. Rocket scientists are attempting to better understand and control these environments through kinetic studies.
During the remainder of the induction period H, 0, and OH free radicals are formed in increasing amounts by branched-chain reactions, eqns. (2)-(4)
Kinetics of Combustion Reactions
The concentration of atoms and free radicals increases exponentially with time. The ignition phase of the reaction is nearly thermoneutral as can be seen by the approximate values of AH for these steps. The ignition phase of the hydrogen-oxygen reaction a t 1400°C is depicted in Figure 1. The concentration
Rocket scientists select propellants and engine geometry to optimize the performance and reliability of rocket engines. To make these selections, they need information concerning the rates a t which propellants react. One approach is to study the bulk rate of combustion for a given set of conditions including the nature of the propellants, the fuel injection and ignition systems, the size and shape of the comhustion chamber, the temperature, and the pressure. Such tests, however, are very costly in terms of personnel, materials, and time; and prediction of the rate of combustion for even slightly different conditions is difficult. As an alternate approach, comhustion scientists are compiling kinetic and thermodynamic data for individual steps in complex combustion reactions. This approach can be illustrated by considering the reaction of hydrogen with oxygen. The hydrogen-oxygen reaction releases large amounts of energy and has been studied in great detail. The technology for manipulating these substances also has been developed. For these and other reasons this pair of substances is an important fuel-oxidant combination for rocket and ramjet engines. I t is believed ( 1 , 2 ) that the reaction of hydrogen and oxygen gases begins with the formation of small amounts of OH free radicals produced in bimolecular collisions between molecular hydrogen and oxygen, eqn. (1)
This article is one of the series of articles based on resource 11. M., A N D BOWMAN, W. H.,"Spce Ite-, units in LAWRISNCK, sources for Teachers: Chemistrv." NASA EP-87. 1871, available t through the Superintendent of l h m e n t s , ~ o v e ; n m e n Printing Office, Washington, 1). C. 20402. Present address: Laboratory of Biochemical Pharmacology, National Institute of Arthritis and Metabolic Diseases, Bethesda, Maryland 20014.
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OH
+ Hz 2 H 2 0 + H
O + H ~ ~ O H + H
" 0 .o 2
. 0
4
AH,,, = -15 kcal
(2)
AH,lir = 2 kcal
(4)
I
8
k 12
REACTION TIME
16
20
(p seconds)
Figure 1. Characteristics of hydrogen-oxygen ignition for hydrogen b u m ine in air at one otmorphere pressure and 1400' C 121. The hydrogen concentration remains effectively constmt for obovt eight microseconds ofter the start of the reaction. The consentrotion of atomic oxygen increaser rapidly and ignition is completed.
of Hz remains effectively unchanged for about eight microseconds after the Hr02 mixture reaches ignition temperature. After the formation of some OH free radicals requiring only a fraction of a microsecond, the concentration of the atomic oxygen increases exponentially with time. This is followed by a rapid change in the concentrations of molecular hydrogen and oxygen, and the ignition phase of the reaction is completed. Following the ignition phase of the reaction, highly exothermic recombination processes among the H, 0, and OH free radicals prevail. These recombinations are of the termolecular type, as illustrated in eqn. (5)
and therefore are much slower than the bimolecular processes occurring during ignition. I n reaction ( 5 ) , M refers to any species which removes sufficient energy in the three-body collision to stabilize the product molecule. These recombination reactions occur mainly in the combustion chamber, the length of which is predetermined on the basis of the speed of the combustion process. A significant number of recombinations also may occur as the gases pass through the exhaust nozzle of the engine (5). The consequent additional release of energy contributes to the total thrust produced by the engine. The rates of these and related steps in the hydrogenoxygen reaction can be used in calculating the performance of arocket engine. Kinetic studies are in progress in several NASA laboratories (4-7), and some representative results for a Centaur rocket engine are given in Figure 2. In the figure, the specific impulse of the 460
EQUILIBRIUM
WEIGHT RATIO OF OXYGEN TO HYDROGEN IN THE PROPELLANT Figure 2. Centaur rocket engine performance (21. The actual psrformance is shown by the open circles and the predicted performance is shown by the solid liner.
Hz-O2upper stage of the Centaur rocket is plotted as a function of the oxidant-fuel ratio. Specific impulse is the amount of thrust produced by a rocket engine in burning a unit mass of propellant per unit time.2 The values of specific impulse given in Figure 2 were calculated on the basis of expansion of the exhaust gases into a vacuum. If the exhaust gases are assumed to be in chemical equilibrium as they expand from the nozzle, the upper curve results. These values of specific impulse are based on the assumption that the steps in the H r 0 2 reaction require much less time than required for the gases to pass from the nozzle. Similar treatment yields values of specific impulse corresponding to the lowest curve in Figure 2, but in this case the rate of the reaction is taken to be so slow that the gases flow from the nozzle without further reaction, so-called frozen equilibrium. Experimentally determined values of specific impulse for the Centaur rocket engine lie
* In the literature the mass of rocket propellents commonly is given in pounds. It follows that the units of specific impulse are conventional pounds force divided by the m a s flow rate (Ib/see).
between these two curves. By incorporating s a c i e n t kinetic data, one should be able to calculate a curve which nearly passes through the observed values. Such a calculation yielded the smooth curve labeled Kinetic in Figure 2. The Kinetic curve is seen to be a much better prediction of the actual performance of the Centaur engine than can be calculated from thermodynamic information alone. As more kinetic data are accumulated, better approximations of the performance of a rocket engine can be made with a wider choice of reactants. Aerodynamic and Radiative ~ e a l i n of~Space Vehicles
Another phenomenon which has prompted space laboratories to study gas phase kinetics is the interaction between space vehicles and planetary atmospheres. When a space vehicle enters a stationary orbit about a planet or makes a landing on a planet, the vehicle's speed must be reduced. If rockets are used to slow the vehicle, these braking rockets must have approximately the same capacity as those which could launch the vehicle from a parking orbit or from the surface of the planet. Such sizable rockets would provide a considerable weight penalty for any mission. Instead, the atmosphere of a planet, if any, can be used to brake the fall of space vehicles. Atmospheric braking is made possible by designing the shape of the vehicle so that the deceleration rate and aerodynamic heating are not excessive. If the vehicle is decelerated too rapidly, the stress on materials and living occupants in the vehicle could become too large; therefore, the vehicle is shaped so that some lift results as it passes through an atmosphere. The speed of the vehicle and the nature of the atmosphere determine the detailed shape of the vehicle for any specific mission. The shape of the vehicle is also designed to reduce aerodynamic heating. If the kinetic energy of the fall to the surface of a planet were absorbed as heat, the vehicle would be destroyed. It has been found that a large portion of the kinetic energy of a pointed object passing through an atmosphere is absorbed by the object through a convective process of heat flow (skin friction). If the object is blunt, as is the Apollo command module, a larger portion of its kinetic energy is dissipated through heating the surrounding air and less is absorbed by the body itself. For pos&Apollo missions, however, the velocities of the vehicles entering planetary atmospheres will be so great, 12-20 km/sec, that radiative heating will also become important. That is, the air around the vehicle will become so hot that the gases will emit continuous radiation, part of which will be absorbed by the vehicle. Therefore, to minimize the heat transferred to the space vehicle, a balance in the sharpness or bluntness of the vehicle must be achieved. When a blunt object such as a meteor or space vehicle enters the Earth's atmosphere, a shock wave (8) is formed in front of the object at a distance of about one tenth the radius of curvature of the object. The shock wave is only three to four mean-free-path-lengths thick. Passage of the shock wave through the quiescent gas heats the gas immediately behind the wave to temperatures in the vicinity of 100,OOO°Kbased on the translational and rotational energies of the molecules. In the region between the shock wave and the vehicle, the Volume 48, Number 7, July 1971
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gas molecules continue to collide and become excited vibrationally a t the expense of their translational and rotational energies. The resulting temperatures are typically so high that the molecules are excited to the point of dissociation into atoms. Furthermore, the atoms become excited electronically and some are ionized. The sinking of energy into vibrational modes, dissociatiotl of molecules, and electrohic excitation of the particles yields temperatures in the range of 1012,000°1< between the shock wave and the vehicle. At these temperatures an object entering the Earth's atmosphere would be surrounded by a high temperature gas composed mainly of N, 0 , N,+ 0,+and electrons. Continuous radiation is emitted as recombinations occur in this mixture (9). To predict the nature of the radiation emitted by the incandescent gars envelope surrounding space vehicles entering atmospheres, as well as to elucidate the processes occurring in rocket engines, the rates of gas phase reactions at high temperatures are being studied a t several NASA 1abor.atories., These studies include gases such as N2, Ar, Cop, CO and others which are prominent in the atmospheres of planets where we plan to orbit satellites or to land space vehicles. Many of these kinetic studies are performed by observing the effectsof shock waves on gases (IO-19). For example, at NASA Ames Researah Center a shock tube of impressive size and versatility was constructed of two 16-in. naval guns welded end to end. A nylon plug can be blown into a constriction a t one end of the shock tube yielding an air stream moving at speeds up to 5 krn/sec. From the other end of the tube
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models can be fired by gas guns into the approaching shock wave, thereby achieving relative velocities of almost 15 km/sec, nearly the speed of an object executing free fall to Earth on 9. return trip from Mars. The experiment in the shock tube lasts only several thousandths of a second, and during this time shadowgraph pictures and spectroscopic observations are made a t up to 16 points along the tube. From studies conducted with this shock tube one can calculate the rate constants for gas phase reactions and find the aerodynamic forces and the effects of radiative and convective heating on model reentry vehicles. Air temperatures up to 9000°K and Ar and Ne temperatures up to 20,00OoI
