AlAA 2000-3856 Nuclear Pulse Propulsion - Orion and Beyond G.R. Schmidt, J.A. Bunornetti and P.J. Morton NASA Marshall Space Flight Center Huntsville, Alabama

36th AIAMASMEEAWASEE Joint Propulsion Conference & Exhibit 16-19 July 2000 Huntsville, AIabam a AIAA 2000-3865

NLCLEAR PULSE PROPULSION - ORION AND BE\I*OND

G.R. Schmidt,* J.A. Bonometti** and P.J. Morton+++ IWSA Marshall Space Flight Center, Huntsville, Alnbnnin 35812

A bgtract As alL\a>s. cost is a principal factor driving the need for systems with much greater performance. The race to the Moon dominated manned space However. when considering transportation of human flight during the 1960's. and culminated in Project crews over distances of billions of kilometers, safety Apollo. which placed 12 humans on the Moon. becomes an equal if not more important concern. The Unbeknownst to the public at that time, several U.S. biggest safety issues stem from the severe radiation government agencies sponsored a project that could environment of space and limitations imposed by have conceivably placed 150 people on the Moon, and human physiology and psychology. Although eventually sent crewed expeditions to Mars and the countermeasures. such as artificial graviw. could outer planets. These feats could have possibly been greatly mitigate these hazards. one of the most accomplished during the same period of time as straightforward remedies is to significantly reduce trip Apollo. and for approximately the same cost. The time by travelling at very high-energy. hyperbolic project. code-named Orion. featured an extraordinary trajectories. This will demand propulsion systems that propulsion method known as Niiclear Pulse can deliver far greater exhaust momentum per unit Pni,n:rlsioti. The concept is probably as radical todaj mass (i.e.. specific impulse or Isp) than modern-day as It was at the dawn of the space age. However. its chemical rockets. and thct can operate at significantlj de\ elopment appeared to be so promising that it was larger pouer densities than current high-performance o~l)hi. political and non-tzchnical considerations that electric propulsion s>stems. it ii as not used to extend humanity's reach throughout Many advanced propulsion concepts have been the solar system and quite possibly to the stars. This identified that could. at least theoretically. meet these paper discusses the rationale for nuclear pulse needs. The only problem is that virtually all of these propulsion and presents a general historq of the technologies. such as fusion. antimatter and beamed- concept. focusing particularly on Project Orion. It enerzy sails. have fundamental scientific issues and describes some of the reexaminations being done in practical weaknesses that must be resolved before they this area and discusses some of the new ideas that can be seriousl) considered for actual applications. could mitigate many of the political and environmental For instance. fusion is limited by the fact that we issues associated with the concept. are still far away from demonstrating a device having energy gains sufficient for commercial power. let alone Introduction space applications. Antimatter, while appealing due to its high specific energy, is severely hampered by The 20th century saw tremendous progress in the extremely low propulsion efficiencies and the high science and engineering of chemical rockets. These costs of current antimatter production methods. advances ushered in the deployment of extensive Beamed energy offers great potential too, but requires satellite systems in earth orbit, conveyance of materials far beyond current state-of-the-art and sophisticated scientific probes into the farthest reaches tremendous investment in power beaming of the solar system, and transport of humans to and infrastructure. from the Moon. Although these feats have been We are confident that many of these issues will impressive. chemical rocketry has more or less reached be overcome. but there is no guarantee that systems the limits of its performance. Accomplishing the based on these technologies could be fielded any time future goals of establishing human settlements on soon. This state-of-affairs points to the disappointing Mars. conducting rapid -'omniplanetary" transportation fact that none of the familiar advanced. high-power throughout the solar system. and eventually travelling density propulsion concepts could. with a any degree to the stars will require revolutionary advancements in of certainty. meet the goals of ambitious space flight propulsion capability . within the next 30 or even SO years. This is especially

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Cop! right C 2000 by the American Institute of Aeronautics and Astronautics. Inc. No cop! right is asserted in the United States under Title 17. U.S. Code. The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed here for Governmental purposes. All other rights are reserved by the copyright owner. true in light ot'the consenative tiscal environment of These 5:udit.s identitit'd the two main issues in the post-Cold War era. \.\. hich could limit the jizable attaining a high Isp Lrirh this t>pe ofsjstem. First is inb estment needed to resol\ e the fundamental ij:UtS the snerg) per unit ma.. or \/?C'~.//i~,.i.it'l'lofthe associated with these conc2pts. Moreo\er. deb sloping detonations. The eftec:i~sexhaust Lelocit) and Isp are actual Lehicles based on these technologies Jnd their proponional to the square root of the energ) distributed required infrastructure could realisticall, cost on the over the entire mass of the explosive charge. and point order of hundreds of billions of dollars. to the need to achieve as high of specific yield as The rather bleak prospects for near-term high- possible. The second consideration is designing the Isp high-power densit! propulsion improve hou e\ cr Lehicle to cope \\ ith the mechanical and thermal effects \\hen we reconsider an ?xtraordinan concept :hat gren of the blast. I\ hich placss a maximum limit on the out of nuclear weapons research during World War 11. uti I i zable en erg! ,. This concept, Nuclear Pulse Propulsion (NPP). The next significant step was the idea of using an represents a radical departure from conventional explosive charge with much higher specific energy than approaches to propulsion in that it utilizes the highly dynamite. namely the atom bomb. In contrast with energetic and efficient energy release from nuclear chemical explosives. the specific energies of nuclear explosions directly to produce thrust. reactions are so high that vehicle design constraints At first it would seem ridiculous to think that will limit the performance before the energy limit is anything could survive the hundreds of thousand- reached. Uranium fission has an energy density of degree temperatures at the periphery of a nuclear -7.8 s 10- MI hg. corresponding tu a masiinum explosion. much less than the multi-million degree theoretical Isp of - i .3 Y IO6 sec. Surprisingly, this temperatures at the core. However as nuclear research value is only half the maximum Isp attainable from advanced in the 1950's and 1960's. it became apparent fusion of Deuterium and Helium-3. which yields a that some materials could survive a nuclear detonation. product kinetic energq equivalent to - 2.2 x 10' secs. and survive it well enough to provide a controllable .A proposal for us? of fission-based e\plosives ccni ersion of blast energ) into vehicle kinetic energ) 5rst made t; Stanislaus Ulani in 1946. followed blosr intriguing ot'all is that this approach could b! some preliminarq calculations by F. Reines and deliLer specific impulses bet\.ceen 10.000 secs up to L lam in 1947. The first full mathematical treatment of IO0.000 secs LV ith average power densities equal to or the concept was published b, Cornelius Everett and greater than chemical rockets. using existing Ulam in 1955. [3] The U.S. Atomic Energy technolog,. Commission was auarded a patent for the concept. The development of nuclear pulse propulsion termed "external nuclear pulse method." following during the 1950'5 and 1960's looked so promising that initial application in 1959. [J] it u as onlq through political and non-technical The earliest ph! sical demonstration and proof of circumstances that it neber became a realitq. The bulk the concept's merit occurred in an experiment of this work occurred under the Orior7 program. a 7- conceibed bj phjsicist Leu Allen. Code-named ,ear project sponsored b! the U.S. government from "Viper." the ekperiment \\as conducted at the Eniwetok 1958 to 1965. Had the program progressed to flight Island nuclear facilit) in the Pacific Ocean, and status. it is conceivable that the U.S. would have been involved detonating a 20-kiloton nuclear device IO able to place large bases on the Moon and send human meters away from two - 1 -meter-diameter. graphite- crews to Mars, Jupiter and Saturn within the same time coated steel spheres. [5] The wires holding the spheres period as Apollo, and possibly for the same cost. were vaporized immediately. but not so for the spheres It is highly unlikely that the Orion envisioned themselves, Some time later and several kilometers back then would be acceptable by today's political and from ground zero. the spheres were recovered, with environmental standards. However, it does provide an only a few thousandths of an inch of graphite ablated excellent starting point for presenting some new ideas from their surfaces. [6] Most importantly. their on nuclear pulse propulsion. which could deliver not interiors were completely unscathed. onl, better performance than the original concept but could mitigate many of the issues associated with Types of Concepts nuclear proliferation, environment contam ination. and costl? deployment in space. Two basic hpes of nuclear pulse concepts have been examined over the years. and these are shown Origin of the Concept schematically in Fig. 1. [7] These concepts share man). common features. and differ primarily in how The idea of usins a series of explosive pulses to momentum from the nuclear blast is concerted to propcl :I rochct vehicle can be traced back to Hennann thrust. In a11 cases. :in indi\idual e\plosive device Ciaiis\\ indt. \\ ho published his ideas in the I 8W's. [ I ] (i.e..pulse unit) is cjccted from the vehicle and and K.B. GostkoLbski. u hu issued the tirst scientific detonated at a prcdeterinined standoff distance from the stud, of a concept using d,namite charges in 1900. [2] rear. The resulting explosion vaporizes the entire pulse

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American Institute of Aeronautics and Astronautics unit and causes this “propellant“ to expand as a high- energy plasma. with some fraction interacting with the External NPP with Pusher PlateiMagnetic Field vehicle and providing thrust. A large number of pulses take place. probably at equal intervals. The limits on Isp due to ablation and spallation can be overcome bq using a magnetic field to shield Pulse unit the surface from the high energ) plasma. Magnetic sbrag 7 field lines are generated parallel to the surface of a conducting pusher plate and as the plasma from the explosion eypands it pushes the field lines against the * conductor. increasing flux density. The increased Expndng pdse unit magnetic pressure slows down the plasma, thus reversing its direction and accelerating it away from the pusher plate. The impulse is transferred to the plate by -\ magnetic interactions which spread out the force and protect the plate’s surface from particle impingement. Therefore, the propellant particle energies can be higher than for an unshielded plate, and the Isp’s attained with !he system can also be greater. Pukeunit -/ Exposim igecacn mrnk Magnetic shielding uas first mentioned b> Internal Pulse Everett and Ulam. [3] and the feature has become standard on the high-power fusion pulse vehicles Figure 1 : NPP Concepts studied following Orion. It is important to note that plasma confinement using magnetic fields is not External NPP perfect. and an! high temperature neutral particles will be unaffected. In general. however. magnetic shielding This concept was historically the first to be offers the on14 method of attaining lsp in t‘scess of IOb conceived. The pulse takes place at some distance secs. while nonmagnetic systems will probably be from a pusher plate. which intercepts and absorbs the limited to -10’ secs. shock of the explosion. The momentum conditioning unit smoothes out the momentum transfer betiveer? Internal NPP pulses to provide a nearly constant acceleration, and returns the plate to its proper location for the next In this concept. the explosion takes place inside a pulse. pressure vessel from which heated propellant is The advantage of this approach is that no attempt expanded through a conventional nozzle. When this is made to confine the explosion. Thus. it circumvents method was conceived. it was supposed that use of an the material temperature limits associated with enclosed “reaction chamber” and nozzle would confined concepts, such as solid and gas core nuclear eliminate the energy losses associated with isotropic thermal rockets. The interaction time of the propellant external expansion and lead to greater performance. with the vehicle is so short that essentially no heat Propellant (liquid hydrogen or water) is fed into transfer occurs. The “temperatures” in the propellant the pressure vessel radially through the wall, and serves cloud may be -lo6 K. but as the interaction time can as a coolant. The explosion occurs at the center of the be as low as -0.1 msec, only a small amount of vessel, propagating a shock wave through the material is ablated and lost. This pulsed nature is propellant until it is reflected from the walls. This essential to the concept’s feasibility, for if such high wave is reflected back and forth in the vessel, temperatures were applied for any extended length of increasing the internal energy of the hydrogen until time. the vehicle would be destroyed. equilibrium is established. This takes a few The Isp attainable with the external concept is milliseconds. after which the vessel is refilled with proportional to the product of the propellant propellant. The expansion process is continued until impingement velocity against the pusher plate and the the previous initial conditions in the vessel are re- fraction of pulse unit mass striking it. The established, and the cycle is repeated. impingement velocitj is limited by pusher plate Studies in the 1960’s concluded that Isp greater ablation. and is probably in the range of 100 to 200 than 1,400 seconds would require very heavy engines. km per second. The pulse unit fraction is determined [8] There are two main limitations to the performance b! design ofthe explosive charge and the stand-off of an internal s>stem. One is radiation heating - most of the radiation emitted in the form of neutrons distance. and is in the range of IO to 50OO. The resulting Isp limits are approximately 3.000 to 10,000 and ;/-rays is deposited into the chamber wall. Thus. seconds. the vehicle requires cooling. and this is the dominant 3

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perfornlance-iimitilig factor in the iriternal design. The launch, probhbly from the L.S. nuclear test site in resulting Isp limit depends on the energ) deribed from kevada. The vehicle. u hich is shown in Fig. 2. the ekplosion. but it i$generallq less than 1.500 secs looked like the tip of a buller. was -80-meters high - ar least an order ofmagnitude Uorse than that ofan and had a pusher plats -40-meters in diameter. estemal sqstem \kith the same pulse unit mass. .Anal)ses shoLred that the bigger the pusher plate, the The other limiting factor is the higher mass of the better the performance. internal vehicles. Studies showed that the minimum mass of an external system will always be less than that for an internal system for the same pay load and mission. Payload Project Orion Secfwn / The most extensive effort on fission-based nuclear I pulse propulsion was performed in Project Orion. The i'i I I results obtained during its seven year lifetime from 1958 to 1965 were so promising that it deserves serious consideration today, especially in light of the . I. ,. I serious technological obstacles posed by some of the other advanced propulsion technologies being considered for ambitious human space flight. An excellent description of the project's history is given by Martin and Bond [7]. The following represents a condensed version of the historical summaries in that paper. Figure -3: Early Orion Concept The Beginning (1957 - 1958) The mass of the kehicle on takeoff would have Orion began in 1958 at the General Atomic been on the order of 10.000 tonnes - most of which Division of General Dynamics in San Diego, would have gone into orbit. At takeoff, the 0.1 California. The originator and driving force behind the kiloton-yield pulse units would be ejected at a project was Theodore Taylor. a former weapon designer frequency of 1 per secohd. As the vehicle accelerated. at Los Alamos who seeked a nuclear propulsion system the rate would slow down and the yield would increase that \rould regain American prestige in space in the until 20-kiloton pulses would have been detonated wake of Sputnik. every ten seconds. The vehicle would fly straight up Taylor had encountered the nuclear pulse until it cleared the atmosphere so as to minimize propulsion concept at Los Alamos. Being an expert at radioactive contamination. making small bombs at a time when the drive was Taylor and Dyson began developing plans for toward high-yield weapons, Taylor conceived a system human exploration through much of the solar system. in which the propellant mass was incorporated along The original Orion design called for 2,000 pulse units, with the nuclear charge in simple "pulse units". rather far more than the number necessary to attain Earth than the cumbersome separate diskkharge arrangement escape velocity. Their bold vision was evident in the in Ulam's original proposal. Taylor adopted Ulam's motto embraced at the time, "Mars by 1965, Saturn by pusher-plate idea, but instead of propellant disks, he 1970." One hundred and fifty people could have lived combined propellant and bomb into a single pulse aboard in relative comfort, and the useful payload unit. would have been measured in thousands of tonnes. Taylor and Francis de Hoffman, the founder of Orion would have been built with the robusmess of a General Atomic. persuaded Freeman Dyson. a sea-going vessel, not requiring the excruciating weight- theoretical physicist at Princeton's Institute for saving measures needed for chemically-propelled Advanced Study to come to San Diego to work on spacecraft. Orion during the 1958-1959 academic year. Taylor and The cost of fielding a flight-operational system Dyson were convinced that the approach to space flight was estimated to be $100 million per year for a 12-year being pursued by NASA was flawed. Chemical development program. However, this figure does not rockets. in their opinion. were very expensive. had vep include development costs for the thousands of smaller limited payloads. and tvere essentially useless for items that such a program would require (e.& flights beyond the Moon. The Orion team aimed for a spacesuits and scientific instruments). The Orion spaceship that was simple. rugged. roomy. and program would have most likely utilized the products affordable. Taj lor originally called for a ground from military weapons programs and existing civilian space projects. Still. even if this estimate was off by a 4 American Institute of Aeronautics and Astronautics .. factor of20, the revised total would have been only $24 billion, roughly the same cost as the Apolto propm. rs The ARPA Years (1958 - 1960) realized,earlythat the AU.S8 ome involved if the project was ssing beyond the research resentation to the

military projects,, Orion remained the only major project under ARPA charge, as neither NASA nor the Air Force regarded it as a valuable as

award of $400.000 was made to the project and the following August another million dollars was placed at Orion's disposal to cover the following year's work. The team grew to about 40 the overall project responsibility falling offman. Taylor was appointed project Nance as assistant director (Nance later took over as director when Taylor left the project in 1963). At this time, the Orion team built a series of flight models, called Putt-Putts, to test whether or not pusher plates made of aluminum could survive the momentary intense temperatures and pressudcreated by chemical explosives. Figure 2 shows a photograph of one of these models on display in the National Air and Space Museum in Washington, D.C. Figure 3: Putt-Putt Flight Test A 100-meter flight in November 1959 (Fig. 3), propelled by six charges, successfully demonstrated that impulsive flight could be stable. These The Air Force Years (1960 - 1963) roved that the plate should be thick apered toward the edges tg th to weight ratio. The durability of the plate was a major issue. The securib grounds. Tat I expanding plasma of each explosion could have a temperature of several tens of thousands of Kelvins the Air Force tinall) onlj on the condition

goals of space eyplorati *ere tied. The plan %A:)> to use Orion as a bkeapon platform he1 ium plasma generator. The experimenters found that ill polar orl311 tl>,it ituld piih\ (ner c1t.n point on the the plate would be exposed to extreme temperatures for I-anh', ,tirt,ic: It ciuld .iIw protect itself easily only about one millisecond during each explosion, and against attach. ~iii,illnumber\ of missiles. that the ablation would occur only within a thin surface However. this idea had the same disadvantages as the 5

American Institute of Aeronautics and Astronautics earl? bomb-cam, ing sattllite proposals. Terminal guidance would have been a problem. since the The core of the vehicle was a - IOO.000 kg technology for accuratel) steering warheads had not )et propulsion module with a IO-meter diameter pusher been developed. Furthermore. both the U.S. and the plate. Lchich \\as set b! the Saturn diameter envelope. Sobier Cnion were deploking missiles that were This rather small diameter restricted lsp to I SO0 to capable of reaching their targets in fifteen minutes with 'j00 secs. While extremely lor$ by nuclear pulse multi-megaton warheads. making orbiting bomb standards. this figure far exceeded those of other platforms irrelevant. nuclear rocket designs. The shock absorber system had Little firm information is available but it does trio sections: a primarq unit made up ot'toroidal seem certain that the vehicles were intended to drive a pneumatic bags located directly behind the pusher 900 tonne payload to low earth orbit or to escape from plate. and a secondary unit of four telescoping shocks a threatening surface launch and return to its operating connecting the pusher plate assembly to the rest of the position. The vehicle was most likely propelled by spacecraft. small yield explosions of about 0.01 kilotons, released Two or possibly three Saturn V's would have from the vehicle at IO second intervals and detonated been required to put this vehicle into orbit, and some between 30 and 300 meters behind the pusher plate. on-orbit assembly would be required. Several mission The gross launch weight of the basic vehicle was protiles were considered -the one developed in quoted as 3.630 to?nes. and the acceleration ranged greatest detail was for a Mars mission. Eight from 20 to 90 m:s-. The Isp of 4,000 to 6,000 sec, astronauts. \\ith around 100 tonnes of equipment dnd along with an average vehicle acceleration of 21.25 g supplies. could have made a round trip to Mars in 175 would enable direct launch from the Earth's surface or days (most current plans call for one-way times of at sub-orbital startup. Such vehicles would have a least nine months). Another impressive figure is that propulsion module inert mass fraction of 0.3 to 0.4 as much as 45OO of the gross vehicle weight in Earth and pulsing intervals of about I sec. orbit could have been payload. Presumably the flight would have been made when Mars was nearest to the The SASA Years (1963 - 1965) Earth; still, so much energy was avaiiable that almost the fastest-possible path betheen the planets could have Robert McNamara. Defense Secretary under the been chosen. Kennedy Administration. felt that Orion was not a An assessment at that time placed the militarq asset. His department consistently rejected development costs at $1.5 billion. which suggests a an> increase in funding for the project, which superior economics for nuclear pulse spaceships. effectively limited it to a feasibility study. Taylor and Dyson also felt that Orion's advantages were greatly D) son knew that another moneq source had to be diluted by using a chemical booster - the Saturn V's found if a flyable vehicle was to be built. and NASA would have represented over 50°,0 of the total cost. was the only remaining option. Accordingly. Taylor Von Braun became an enthusiastic Orion and Nance made at least two trips to Marshall Space supporter, but he was unable to make headway for Flight Center (MSFC) in Huntsville, Alabama. increased support among higher-level NASA officials. At this time, Werner Von Braun and his MSFC In addition to the general injunction against nuclear team were developing the Saturn moon rocket. power, very practical objections were raised, such as Consequently, the Orion team produced a new, "first what would happen if a Saturn carrying a propulsion generation" concept that abandoned ground launch and module with hundreds of bombs aboard should boosted into orbit as a Saturn V upper stage. A explode, and was it possible to guarantee that not a schematic of the vehicle is shown in Fig. 4. single bomb would explode or even rupture? Although NASA feared a public-relations disaster and was reluctant to provide money, its Office of Manned Space Flight was sufficiently interested to fund another .- study.

Orion's Death

A fateful blow was dealt to Orion in August 1963 with signing of the nuclear test-ban treat). . Although the tests required for development of an Orion vehicle . _- ._.- . were now illegal under international law. it was still possible that an evemption could be granted for programs that I\ ere demonstrablq peaceful. However. there is no doubt that the treaty greatly diminished Orion's political support. Another problem was that Figure 4: Orion Spacecraft - NASA Version 6

American Institute of Aeronautics and Astronautics because Orion was a classified project. very few people vehicles was immense and capable of transporting a in the engineering and scientific communities were colony of thousands of people to a nearb! star. It aware of its existence. In an attempt to rectify this. would take -.I .OOO hears for the energ>-limited design Orion's manager. Jim Nance. lobbied the Air Force to to reach Alpha Centauri. uhile the momentum-!imited declassi6 at least a broad outline ofthe work that had case would take a mere centup. been done. Eventually it agreed, and Nance published A ne\\ era in thinking about nuclear pulse a brief description ofthe "first generation" vehicle in propulsion began in the late 1960's and early 1970's. October 1964. Spurred by optimism for controlled fusion for power The Air Force. meanwhiie. had become impatient generation. researchers ignored use of fissicnabie with NASA's noncommittal approach. It was willing material. and began to focus on igniting small .milli- to be a partner only if NASA would contribute kiloton" fusion microexplosions. By IoLvering the significant funds. Hard-pressed by the demands of energy of each fusion explosion, the structural mass of Apollo, NASA made its decision in December 1964 a spacecraft could be reduced. Microexplosions also and announced publicly that it would not continue to promised significantly reduced fuel costs because there fund Orion. The Air Force then announced would be no need for fissionable material or elaborate discontinuation of all funding. thus terminating Orion. pulse unit structures. All told, approximately $1 1 million had been Soon microexplosion designs began to push spent on Orion over nearly seven years. Freeman toward theoretical Isp levels near IO6 secs. implying Dyscn stressed the importance of the Orion story exhaust velocities near 3O.O of light veloci? . The "...because this is the first time in modern history that pusher plate become a powerful magnetic field. which a major expansion of human technology has been would channel charged particles into an exhaust. and suppressed for political reasons." In retrospect, there pulse repetition rates increased to hundreds per second. *ere other issues besides politics, and these included: Converging laser beams. electron beams or other driver ( 1 ) the inherent large size of the vehicle made full scale energy sources would ignite the fusion pellets by tests difficult and costly. (2)the nuclear test ban treaty inertially compressing and confining the fuel. Some of excluded testing in the atmosphere or in space, (3) the the energy of the microexplosions would be tapped NERVA solid core nuclear engine provided strong electromagneticall:, to pro\ ide pwver f(jr the laxrs and competition, and (4) no specific mission existed which the pusher plate magnetic fields. that is a bootstrap demanded such a high performance system. process. These systems clearly have extraordinary design requirements and push technological limits. A Orion's Legacy vehicle propelled by a million-second Isp engine could in theory visit any location within the solar s>stem in Although Orion emplo>ed fission as the mode of a matter of months. energy release, use of fusion was always viewed as the Members of the British Interplanetap SocieQ next logical step in the evolution to ever-higher took up the challenge of fusion microexplosion performance. One advantage of fusion is the higher propulsion and conducted the most elaborate stud\. to specific energy of the reaction, but for charged particle date of a robotic interstellar vehicle. From I973 to products, this is only several times that of fission. 1978, the team of I3 members worked on Project The main advantage of fusion is that there is no Daedalus, a two-stage fusion microexplosion spacecraft minimum mass criticality limit, and the detonation can designed to send a scientific payload of 450 tons at be made very small - yields on the order of 0.001 12% light speed on a one-way, 50-year fly-through kiloton and lower. mission to Barnard's star, 5.9 light years distant. In 1968, Freeman Dyson was the first to propose The IO6 sec Isp engines used deuterium and application of fusion pulse units for the much more helium-3 fusion fuel; the latter component. because of ambitious goal of interstellar flight. His rationale was its terrestrial scarcity, would have to be "mined" from simple - the debris velocity of fusion explosions was Jupiter's atmosphere before the flight. Daedulus would in the range of 3,000 to 30.000 km/sec, and the accelerate for about four years under the incessant din geometry of a hemispherical pusher plate would reduce of 50,000 tons of pellets ignited 250 times per second the effective interception velocity four-fold to 750 - by relativistic electron beams. Total departure mass, 15,000 km:s (Isp between 75,000 and 1.5 x IO6 secs). fully-fueled. 54,000 tons - almost all propellant. This made mission velocities of 10' to IO4 km/sec More recent investigations of fusion possible. microexplosions have considered use of laser inertial Dyson considered two kinds of concepts. The confinement. with Lawrence Livermore's VISTA more conservative design was energy-limited, having a concept. [9] and use of combined microfission fusion large enough pusher plate to safely absorb all the with an antimatter trigger. [IO] Although the driver thermal energy of the impinging explosion. without technology in all these cases is ~epdifferent. the basic melting. The other momentum-limited concept concepts a11 have their roots in the earlier concepts of defined the upper region of performance. Each of these fusion-based nuclear pulse propulsion.

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place an upper limit on the performance in terms of Reconsidering Nuclear Pulse Propulsion Isp. Smaller. high-specitk yield pulses combined with more ablation resistant materials would reduce Interest in nuclear pulse propulsion never reallq minimum standoff distance requirements. thereb! died with Orion. it merel? evolved into concepts based increasing Isp considerabl) . on \c hat manq v iew as the tamer and more politicall? Even with the reduction in 4 ield and acceptable process of nuclear fusion. In retrospect. this improbements in performance. use of self-actuating shift in interest was probably premature and based on nuclear charges would still be a political issue. overly optimistic projections of fusion's viability. We However. it can be argued that in some Lvays the now know that the challenge of fusion is much more environment may be more accommodating today than difficult than originally envisioned. In fact, fusion for it was during the politically-charged days of the Cold spacecraft applications may in some respects be harder War. In many ways. international cooperation is more to achieve than for commercial power, due to the need prevalent today, and could conceivably be extended to for lightweight subsystems and high gain. [I I] the peaceful application of nuclear pulse technology. It Recognizing the formidable challenges of fusion. does provide a productive avenue for disposing ofthe perhaps it would be wise to take a step back and substantial stockpiles of weapons-grade fissionable reconsider the use of fission-driven pulses. There have material that exist throughout the world. and the been many changes to the technological and political environmental contamination would be negligible if landscape over the last 30 to 40 years, and it is used at a sufficient distance outside low earth orbit. possible that fission-based systems could be made safe. There is no doubt that political acceptance of such affordable, and even better performing than the designs an idea would demand convincing technological need considered in the Orion program. and international involvement. As of now. there are The most sensitive issue with Orion was its use several propulsion concepts that could be used for of self-actuating nuclear devices. Ironically. this was human missions to blars. However I\ ith conservative aljc one of its rnain strengths. since it eliminated the projections of technolosical readines. these missions need for massive driver and energy storage hardware would be constrained to 2 to 3 year durations. onboard the spacecraft. Still. almost anyone who has Ifthe need arose to conduct a Mars mission much been exposed to the concept feels uncomfortable about faster (say in a year or less) or if there \cere a need to this aspect. and rightly so. since it raises a myriad of transport human or large payloads as rapidly as issues regarding testing. nuclear proliferation, and possible to destinations in the outer solar system (e&, national security. This is particularly true with the Jupiter and beyond), then the use of nuclear pulse > ield of the devices originally considered in the Orion becomes quite compelling. If such missions involved program. Although small by weapon standards, they extensive international cooperation, then there may be were nonetheless in the 0.1 to IO kiloton range, and more acceptance for this tqpe of technology. drove the need for large, robust spacecraft designs. Perhaps the most promising avenue for use of There has likely been considerable progress in the fission-based nuclear pulse lies in the direction of actuation of explosive fissionable charges over the last microfission processes. In these schemes. subcritical 30 to 40 years. and this technology could be applied to targets of fissile material are compressed via a realize smaller yield detonations than those baselined mechanism onboard the spacecraft in a manner similar for Orion. The main challenge is not achieving low- to that in fusion-based concepts. The big difference is yield devices per se, but being able to do so with high that the energy requirements to drive a fission sample energy per unit mass (i.e., high specific yield). Of to supercriticality and high burn-up fractions is course, such information would undoubtedly be substantially less than that for comparable fusion classified and unavailable for openly reviewed processes. spacecraft evaluations. However, the possibility is The advantages of this type of approach are clear. there and could bring the yields down into more It eliminates the concerns over having vehicles that acceptable ranges. caw fully contained -'bombs." Because these systems Another major difference between now and the rely on a compression and energizing source from the time of Orion is the dramatic improvement in materials spacecraft, they cannot be used as a weapon, at least in technology. Orion's pusher plate and momentum any conventional way. Not only does this take care of transfer assemblies were based on 1950's and 1960's the concern over storing thousands of small bombs in technology. and featured common materials, such as close vicinity. but it also removes manq of the issues steel and aluminum. Research over the last 40 years concerning nuclear proliferation. has opened the prospects of advanced carbon structures Only a few studies ofthis approach have been and lightweight refractorq materials which could conducted. but the results look ver) promising. It ma) greatl! reduce the mass and improve the ablative prove to be a more realistic intermediate step between characteristics of nuclear pulse systems. The latter the propulsion systems of today and the fusion- consideration is especially important since it tends to propelled concepts of tomorrow.

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American Institute of Aeronautics and Astronautics References

1. Ganswindt, H., Das jungste Gericht, Berlin, 1899.

-.3 Gostkowski, R.B., Die Ziet, p. 53, Vienna, 28 July 1900.

3. Everett, C.J.and Ulam, S.M., “On a method of propulsion of projectiles by means of external nuclear explosions,” LAMS-I955 (1955). (Declassified, Aug 25, 1976).

4. “Nuclear propelled vehicle, such as a rocket,” British Patent Specification, No. 877, 392, 13 Sept, 1961.

5. Flora, M.R., “Project Orion: Its Life, Death, and Possible Rebirth,” Submitted for the Robert H. Goddard Historical Essay Contest, Nov 24, 1992.

6. Mallove, E. and Matloff, G., The Starflight Handbook: a pioneer’s guide to interstellar travel,” John Wiley & Sons, Inc., ISBN 047 1619 I24 1989, 1989.

7. Martin, A.R. and Bond, A., “Nuclear Pulse Propulsion: A Historical Review of an Advanced Propulsion Concept,” J. of the British Interplanetary Society, Vol. 32, pp 283-310, 1979.

8. Platt, E.A. and Hanner, D.W., “The effective specific impulse of a pulsed rocket engine,” UCRL- 12296 (1 965). Presented at AIAA Propulsion Joint Specialist Conference, 14-1 8 June 1965.

9. Orth, C. D., Klein, G., Sercel, J., Hoffman, N., Murray, K., and Chang-Diaz, F., “VISTA: A Vehicle for Interplanetary Space transport Applications Powered By Inertial Confinement Fusion,” Report UCRL-LR- 1 10500, University of California, Lawrence Livermore National Laboratory, Livermore, CA 94550 (1998).

IO. Gaidos, G., Lewis, R. A., Smith, G. A., Dundore, B. and Chakrabarti, S., “Antiproton- Catalyzed Microfission/Fusion Propulsion Systems for Exploration of the Outer Solar System and Beyond,” Space Technology and Applications International Forum, El-Genk, M. S. ed., 1998.

1 I. Chakrabarti, S. and Schmidt, G.R., “Impact of Energy Gain and Subsystem Characteristics on Fusion Propulsion Performance,” AlAA 2000- 3613. July, 2000.

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American Institute of Aeronautics and Astronautics