Nuclear Propulsion for Space (Rev.)

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The Understanding the Atom Series Nuclear energy is playing a vital role in thfe life of every man, woman, and child in the United States today In the years ahead it will affect increasingly all the peoples of the earth. It is essential that all Americans gam an understanding of this vital force if they are to discharge thoughtfully their responsibilities as citizens and if they are to realize fully the myriad benefits that nuclear energy offers them. The United States Atomic Energy Commission provides this booklet to help you achieve such understanding. Edward J Brunenkant, Director Division of Technical Information UNITED STATES ATOMIC ENERGY COMMISSION THE COVER The cover shows a reusable nuclear stage, powered by the NERVA engine, leaving an earth orbit on a round-trip mission to the moon. The NERVA engine " f i r e s " at least four times on this mission To leave earth orbit, to go into orbit around the moon, to leave the moon, and to return to orbit around earth. This stage was delivered to orbit, where it was assembled, and refueled by multiple flights of the two stage space shuttle. To provide sufficient payload to support a lunar station, the nuclear stage carries 150 tons of liquid hydrogen in eight tanks After the stage returns to earth orbit, it will be r e - fueled, repaired, if needed, and checkedout for additional missions to the moon, to synchronous orbit, or to interplan-

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by William R. Corliss and Francis C. Schwenk ENERGY FOR SPACE TRAVEL HOW A NUCLEAR ROCKET WORKS FROM THE FIRST SPECULATIONS TO NERVA BUILDING AN ENGINE FOR SPACE The Biggest, Hottest Reactors Ever Built The Artificial Maelstrom Where the Work is Done Holding the Pieces Together Bootstrapping and the Full Flow Turbine Drive NINETY MILES OUT IN THE DESERT ENGINES FORTHE MOON AND BEYOND GLOSSARY OF ACRONYMS AND CODE NAMES United States Atomic Energy Commission Division of Technical Information Library of Congress Catalog Card Number 79 171030

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Interest mounts high as technicians and engineers monitor a space propulsion test m the Reactor Control building at the Nuclear Rocket Development Station, Nevada Instruments record per-

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By William R. Corliss and Francis C. Schwenk ENERGY FOR SPACE TRAVEL The secret of space travel is energy—immense amounts of energy. The first stage of the Satiirn-V moon rocket generates as much energy each second as a million automobile engines. The energy leverage of nuclear fission eventually will give lis spaceships that can sweep across the solar system and carry men to the farthest planets. But the uranium nucleus is not broken to man's bidding easily. Our quest is for a means of uniting the almost limitless energy of the atomic nucleus and the rocket's unique ability to thrust through the vacuum of space. This difficult wedding of nuclear fission (age, 30 years) to the rocket (age, 1000 years) is the subject of this booklet. In the words of Dr. Glenn T. Seaborg, Chairman of the ing to make is a flyable compact reactor, not much bigger than an office desk, that will produce the power of Hoover Dam from a cold start in a matter of minutes."

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HOW A NUCLEAR ROCKET WORKS The rocket concept was grasped by the first caveman when he pushed off from a lakeshore on a raft; every action has an equal and opposite reaction according to Newton's Third Law of Motion.* The caveman's action was pushing the shore away with his foot; the reaction was the surge of the raft onto the lake. Actually, it is not necessary to have something solid to push against. The caveman could have propelled himself out on the lake by hurling rocks shoreward; as each rock left his hand, he and the raft would have moved a bit farther. It is the same way in airless space; propulsion in a given direction means throwing something away in the opposite direction. Instead of rocks, the ordinary chemical rocket expels a roaring jet of hot combustion gases. But the effect is the same. To make a nuclear rocket, the uranium nucleus must be fissioned! in such a way that something is expelled from the spaceship. We'll explain how later in Chemical rocket engines, jet engines, automobile eng i n e s — in fact, most of mankind's engines — extract heat from a fuel and turn it into motion through the expansion of hot gases. The nuclear rocket sprouts from the same family tree ; it also creates hot high - pressure gas and turns it into thrust. The nuclear rocket is a direct descendent of built a century or two before Christ by the famous Alexandrian engineer, Hero. It is relatively easy to see how a hot gas expands against a piston in an internal combustion engine to do useful work. The piston presents the gas with something solid to push against. And the rocket engine operates in much the same way, except that the piston is replaced by the rocket nozzle. The hot gases created by chemical combustion or nuclear heaters issue from the throat of the rocket nozzle *Sir Isaac Newton, the great British intellect who lived from 1642 to 1727, formalized the basic laws of motion as we know t F o r an explanation of nuclear fission, see Our Atomic World, another booklet in this series.

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and expand against its flared sides,pushing the nozzle (and the whole rocket)upward.The pressure against the nozzle walls is the reaction to the expulsion or pushing away of the hot exhaust gases. The first reaction engine, bi Hero's aeolipile, steam squirting from the two pipes caused rotation. Looking at it another way, each molecule in the exhaust is like a small bullet shot from a big gun (the rocket en - gine).Indeed, if we think of the rocket engine as a con - tin u ou sly firing shotgun with molecular ammunition we are pretty close to the truth. If the masses of these molecular " bullets " and their " muzzle velocities " are known, the rocket thrust can be computed from the simple equation : m = the mass flow rate of the bullets or propellant, (in kilograms per second), and V = the average muzzle velocity or exhaust velocity along the rocket axis (in meters per second). *Named for Sir Isaac and equal to the force necessary to a c - celerate 1 kilogram of mass 1 meter per second per second; a force of one newton = .225 lbs. Newton also introduced the idea of placing a dot over a symbol to indicate rate of change. For ex-

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Dividing F by m reveals that the thrust produced per unit m a s s flow of propellant is equal to v. In other words, the higher the bullet or propellant velocity, the more thrust Hot gases expand against a piston, left, pushing it upward. Center, hot gases expand against a rocket nozzle, pushing it upward. The reaction thrust, F, can be computed either from the total net pressure against the nozzle and combustion chamber area, or from mv we get from each kilogram of gas that r o a r s out the nozzle each second. We want to have a high exhaust velocity for good rocket performance because we can thereby a c - complish a space mission with less propellant. We will show later that the nuclear rocket produces about twice the exhaust velocity of the best chemical rocket. High exhaust velocity is the greatest advantage of the nuclear rocket. The question then is: How does a nuclear rocket gene r a t e high exhaust velocities? The key to the nuclear rocket's success lies in a simple equation from thermo -

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which says that the exhaust velocity, v, of any rocket is where T = the temperature of the hot gases just before they enter the nozzle throat, and M = the average molecular weight of the exhaust It is obvious that for a high v value we wish to maximize Knowing thatT and M control v, let us first try to mani pu - lateT.Since Tis under the square root sign, it will have to be quadrupled to double v, the exhaust velocity.Chemical Nuclear rockets have twice the exhaust velocity of chemical rockets. Thus, for the same level of thrust, propellant mass flow is halved and power

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rockets already operate at temperatures close to 3000°K;* if the nuclear fuel i s to stay in solid form, it i s apparent it cannot go to 12,000 °K, which i s hotter than the s u n ' s surface. In fact, today's nuclear rocket reactor fuels barely survive at 3000 °K, the same general temperature level achieved in chemical rockets. To avoid this impasse,something must be done to r e - duce M, the molecular weight, rather than to increase T. In a chemical (combustion) rocket,Mdoes not drop much below 18, because the most convenient oxidizers have quite heavy atoms. For example, the advanced Centaur engine burns hydrogen with oxygen to f o r m water (H2O) for which M = 18. Chemical rocket exhaust velocities are thus limited mainly by the high molecular weights of the combustion products.As long as chemical fuels must be burned with oxygen or fluorine, chemical rocket exhaust velocities can - not be greatly improved. In a nuclear rocket, however,combustion is not required: The nucleus fission s without chemical stimulation, and the propellant i s not an engine fuel but a separate substance that IS heated by the fissioning nuclei in a nuclear reactor. A nuclear rocket designer can heat up any propellant he wishes as long a s it does not chemically attack his reactor fuel elements.There in lies the secret of the nuclear rocket ' s high exhaust velocity — it can make use of a propellant with a low molecular weight. In designing a nuclear rocket then, the first inclination •would be to choose hydrogen gasas the propellant because hydrogen i s the lightest of all molecules, with M = 2. Other prove ment s over early kerosene - burning chemical rockets, but show no advantage over the newer chemical engines that burn hydrogen with oxygen. In the e a r l y days of nuclear rocketry, though, ammonia and methane were competitors of hydrogen because hydrogen was feared as a fickle, explosion - prone material that had to be stored as a liquid *Or 2727° centigrade; degrees Kelvin a r e measured above absolute zero and a r e equal in size to centigrade degrees. 0°C =

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was known that ammonia and methane dissociate * to a large extent at temperatures around 3000 °K, as their molecules collide violently with each other.Complete dissociation of methane into its five constituent atoms would bring M down from 16 to3.2,and this would make methane much more ating at 500 pounds per square inch pressure Plots oj exhaust velocity Methane and ammonia nuclear rockets do not appear to be much better than a hydrogen-oxygen chemical engine A hydrogen nuclear rocket IS superior to all attractive as a propellant.Nevertheless,hydrogen finally did win out overall competition,mainly because the dangerous handling problems were licked and because the nuclear rocket needed the even higher exhaust velocity promised by pure hydrogen if it was to achieve its full *Or decompose into the constituent atoms making up their t Hydrogen gas, H2, is a molecule containing 2 hydrogen atoms.

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is negligible at today's nuclear rocket temperatures. If atomic hydrogen could be used as a propellant, M would equal 1, and the exhaust velocity factor would multiply that of molecular hydrogen by V2. This is a development that will have to wait for future exploitation, however. The first operational nuclear rockets will spew hot molecular hydrogen out of their nozzles. With M = 2 instead of 18, as in hydrogen-oxygen chemical engines, the nuclear rocket exhaust velocity still will be more than double that of the best chemical rocket for the same temperature. Doubling the nuclear rocket ' s exhaust velocity has one further consequence: It requires the nuclear heat source to generate more power. A simple equation for the power, P, in the exhaust gases comes from the kinetic energy (KE) but, since power is the rate of energy production, m is r e - placed by m and KE by P. Thus, Using this equation, and F = mv, we can see that if thrust is held fixed and exhaust velocity is doubled, propellant flow will be halved (as desired), but the power required to do this will be doubled. The price of increasing the exhaust velocity is increased power. From this relationship arises another important advantage of the nuclear rocket: The great reservoir of energy contained in its nuclear fuel can be turned into high exhaust velocity. If we keep in mind t h r e e basic facts about nuclear 1. They convert fission-generated heat into the kinetic energy of rocket propellant, 2. Chemical combustion is not needed, and they can use low molecular weight propellants to attain high exhaust 3. Their reactor fuel has a great deal of energy packed we can understand better the thoughts of the pioneers in

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nuclear rocketry as they tried to channel the power of the atom into high - temperature, lightweight reaction engines that would carry men toward the planets. U S Senator Clinton P Anderson of Neu Mexico, Chait man oj the Senate Aei onautical and Space Sciences Committee, and a member of the Joint (Congressional) Committee on Atomic Energy, examines a Kiwi reactor during a visit to Los Alam.os Scientific Labo-

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Chronology of Rocket and Reactor Technology Key Robert Goddard paper A Method of Reaching Wernher von Braun and Alamagordo A - bomb test (7-45) - NERVA XE (experimental engine) tested at full power (6-69) - Ernest Rutherford makes first controlled nuclear - James Chadwick discovers • NAA, Rand reports (7-46) - LASL Nuclear Propulsion Div formed (6-55) -Suscessful test of redesigned Kiwi (5-64)

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FROM THE FIRST SPECULATIONS TO NERVA "The Italian navigator has landed in the new world." This was the text of the cryptic message sent by Arthur Compton to other American scientists working on nuclear research clear chain reaction The precise moment had come at 3:20 p.m. on December 2, 1942, in a squash court at the University of Chicago, as neutron-absorbing cadmium rods were pulled slowly from Fermi ' s "pile" of graphite and uranium. In the context of this booklet, Compton's telephone call was even more prophetic than he intended, because F e r m i had not only discovered the new world of the atom but had also given man an energy source with which he will be able to explore worlds far beyond the earth Fermi ' s success set scientists and engineers to wondering about what constructive things could be done with the results of nuclear research. In 1944, Stanislaus Ulam and Frederick de Hoffman, at a New Mexico center that later became the Los Alamos Scientific …