Nuclear Pulse Propulsion - Orion and Beyond G.R

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

36th AI AAIASMElSAElASEE Joint Propulsion Conference & Exhibit 16- 19 J u IY 2000 Huntsville, Alabama

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AIAA 2000-3865

NUCLEAR PtrL SE PROPCLSION - ORIO:\f A.'I'D I3EYOND

C .R. Schmidt,· J.A. Bonometti** and P.J. Morton*"'* NASA ,yfarshall Space Flight Center, Huntsville, Alabama 35812

Abstract As alway . cost is a principal fa ctor driving the need for systems with much greater performance. The race to the Moon dominated manned space However. when considering transportation of human fl ight during the 1960's. and cu lminated in Project crews over di stances of bill io ns of ki lometers, safety Apollo. which placed 12 humans on the Moon. become an qual ifnOl more im portant concern. The Unbeknownst to th e publ ic at that time, several U.S. biggest safety issues stem from th e severe radiation government agencies sponsored a project that could environm ent of space and lim itations imposed by have conceivably placed I SO people on the Moon, and hu man ph ys iology and psycho logy. Although eventually sen t crewed ex peditions to Mars and the countermeasures. such as artific ial gravity. could outer planets. These feats could have possibly been greatly mitiga te these hazard. one of th e most accomplished during the same period of time as straightforward remedies i to sign ifi cantly reduce trip Apollo. and for approximate ly the same cost. The time by travelling at ve ry hi gh -energy. hyperbolic proj ect. code-named Orion. featured an extraordinary trajectories. This will demand propulsion systems that propul sion method known as Nuclear Puls e can deliver far greater exhaust momentum per unit Prup!ti ·ion. The co nc~pt is probably as rad ical today mass (i.e .. specific im pul se or Isp) than modern-day as It lI as at the dawn of th e space ag e. However. its chemical rockets. and thN can operate at signiticantl y de\'elopment appeared to be so promising that it was larger power de nsities than CUITe nt hi gh-performance onl~ b) political and non -technica l considerations that ~Iectric propulsion ys teill s. it lIas not used to extend humanity's reach throughout Many advanc ed propulsion concepts ha ve been the so lar system and quite possibly to the stars. Thi identified that co uld . at least theoreticall y. meet these paper discusses the ratio nale for nuclear pulse needs. The on ly prob lem is that vi rtually all of these propulsion and presents a general history of the technologies. such as fusion . antimatter and beamed­ concept. focusing pal1icularly on Project Orion. It energy sa il s. have fun damental scientific iss ue and describes ome of th e reexamination being done in practical weaknesse that mu t be reso lved before they thi s area an d discus es ome of the new ideas that can be se riou s I: considered fo r actual application s. could mitigate many of the po litical and environmental For instance. fus ion is limited by the fact that we is ues associated with the co ncept. are still fa r away from demonstrating a device having energy ga ins sufficient for commerc ial power, let alone Introduction space app lications. Antimatter, while appealing du e to its hi gh spec ific energy. is severely hampered by The 20th century saw tremendous progress in the extremely low propulsion efficienci es and the hi gh sc ience 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 sc ientific probes into the farthest reaches tremendous in vestmen t in power beaming of the solar system . and transport of hum ans to and infrastructu re. from the Moon . A Itho ugh these fea ts have been We are confident that man y of these issues wi ll impress ive. chemical rocketry has mo re or less reached be overcome. but there is no guarantee that system s the limits of its performance. Accom plish ing th e based on these technologies cou ld be fielded any tim e future goals of establishing human se ttl ements on soon. This state-of-affairs points to the disappo in ting lars. conducting rapid "omniplanetary" transportation fact that none of the fam iI iar advanced. high-power throughout the so lar sys tem. and eventuall y travelling density propul sio n concepts co uld. with a any degree to the stars will require revolutionary advancements in of certainty. meet the goals of ambitious space flight propulsion capabi lit y. withi n the ne xt 30 or even 50 yea rs. This is es pecially

Lkput> 'v li.lllag.er. Propul sion Resea rch Cc: nter. ·r. :Vk mb.:r ,\IAr\. ~u<:kar Propul sio n Engin.:er. Prop ul sio n Re "c:an:h C':lller. :Vkmb.:r . \ 1. \ . \ . • • Flight :> st.:m s Engi nee r. Propul sion Research Ce nter. Member AI,\ , \ .

C()p~ righ t C :1000 b. the Ame rican Institute of Aero nautics and As tronautic s. Inc. No cop~ ri ght is asserted in the United States under Title 17. . Code. The U. S. Gove rnm ent has a royalty -free license to exerc ise all rights under the copyright daimed here fo r Govemmental purposes . All oth er rights are reserved by the copyright owne r. true in light of the conservative fiscal envi ronme nt of These studies identified the two mai n issue in the po t- Co ld War era. "hich co uld limit the sizable anai ning a high Isp 1\ ilh [hi s rype of system. First is investment needed to reso lle the fundamenta l issu e- the energ; per unit mas- or speC/jic rield of the a sociated with these concept Moreoler. de'elo pi ng de to nati ons. The effectile exhau t \elocit: and Isp are actual vehi cles based on these technol ogi s and the ir proporti onal to the qua re root of the energ) di tributed required infrastructure could rea listicall, CO St on the ove r th e entire mas of the ex plos ive charge. and point order of hundred s of billi ons of do llars. to the need to achieve as high of specific yield as The rather bleak pros pects fo r near- te rm high ­ po sible. The second consideration is design ing the Isp high-power density propulsion improve hOII'eler vehicle to cope II ith the mec han ical and thermal effects \\ hen we reconsider an extraord ina ry concept that gre\1 of the blast. \\'hich pl ace a maximum limit on the Out of nuc lear weapons research during Wo rl d War II. ut ilizable energy. This concept, Nuclear Pulse Propulsion (NPP) . The next significant step was th e idea of using an represents a radical departure from conventional expl osive charge \:,:, ith much hi gher specific energy than approaches to propul sion in that it uti lizes the high ly d, namite. name ly the atom bom b. In contrast with energeti c and effic ient energy release fro m nuc lear chemical explosives. the specific energies of nuclear explosions directly to produce thrust. reactions are so high that ve hic le des ign constraints At first it wo uld seem ridiculous to think that will Iim it the performance before the energy Ii m it is an yth ing could survive the hundreds of thousand­ reached. Uranium fis sion has an energy density of degree temperatures at the periphery of a nuclear - 7.8 x 10- MJ kg. corresponding to a maximum explosion . much le ss th an the mu lti -million degree theoretical Isp of - 1.3 x 106 sec. urprisingl y, th is temperatures at the core. However as nuc lear research value is only half the maxim um Isp attainable from advanced in the 1950's and 1960's. it became apparent fu sion of Deuterium and Helium-3. which yields a that some materials could surv ive a nuc lear detonation . product kinetic energy equiva lent to - 2.2 x 106 secs. and survi ve it we ll enough to provide a con tro llabl e A propo al for use of fission -based explosives comersion of blast energy into vehicle kin etic energ; . was first made b) tani slaus Ulam in 1946. followed Most intrigu ing of all is that th is approach cou Id b: some preliminary calculatio ns by F. Reines and deli\er ' pecific impul e between 10.000 secs up to ulam in 19-17 . The [-,rst full math ematical treatm ent of 100.000 secs with average power densitie equa l to or the concept wa publi hed by Cornelius Everett and greater than chem ical rockets. us ing existing Ula m in 1955 . (3] Th e .S. Atomic Energy tec hn ology . Commi sion was awarded a patent for the concept, Th e development of nuclear pul se propulsion termed "external nucl ear pu lse method." following during th e 1950' s and 1960's looked so promising that in itial appl icati on in 1959. [4] it \\ as onl y through political and non -technical The ea rli e t ph~ ical demonstration and proof of circumstances that it never became a reality. The bulk the concept' s merit occurred in an experim ent of thi s work occurred under the Orion program. a - concei ed by ph : ici st Lew Alle n. Code-named year project spon sored b: the U.S. government fro m "Viper." th e experim ent was conducted at the Eniwetok 1958 to 1965. Had the program progressed to fligh t Island nuclear facility in the Pacific Ocean, and status. it is co nceivable that the U.S. wo uld have been involved de tonating a lO -kiloton nuclear device 10 abl e to pl ace large bases on the Moon and send human meters away from two - I-meter-diameter. graphite­ crews to Mars, Jupite r and Saturn within the same time coated steel spheres. [5] The wi res holding th e spheres peri od as Apoll o, and possibly for the same cost. were vapori zed imm edi ately, but not so fo r the sph eres It is hi ghl y unlikely th at th e Orion envis ioned themse lves. Some tim e later and several kilometers bac k then wo uld be acceptabl e by today's political and fro m ground ze ro, th e sph eres were recovered, with enviro nm ental standards. However, it does provi de an only a few th ousandth s of an in ch of graphite ablated exce ll ent startin g poi nt fo r presenting some new ideas fro m thei r surfaces . [6] Most importantl y. their on nu clear pul se propul sio n, whi ch could de li ver not interiors were comp letely un scath ed. on I} be tter perfo rm ance than the ori gin al conce pt but could mitigate many of the iss ues associated with Type of Concepts nuclear prol iferation, envi ronm ent contami nation. and costly deployment in space. Two basic types of nuc lear pulse concepts have been exam ined over the years. and these are shown Origin of the C oncept schematical ly in Fig. I. [7] These concepts share many common features. and differ primarily in how The idea of using a series of explos ive pulses to momentum from the nucl ear blast is converted to propel a rocket ve hi cle can be traced back to Hermann thrust. In all ca es. an individual exp losive device Gansw indt. who publ ished h is idea in the 1890' -. [I ] (i ... pul se unit ) is ejected ti'om the veh icle and and R.B. Gos tk ow ki. \1 ho iss ued the tirst sci entific detonated at a predeterm ined standoff distance from the study of a concept using dy nam ite charge in 1900. [2] rear. The resulting explosion vaporizes the entire pulse

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American Institute of Ae ronautics and Astronautics unit and ca uses th is "propellant"' to ex pand as a hi oh­ energy plas ma. with ome fractio n interact ing with th e External NPP with Pu sher Plate/Magnetic Field vehicle and prov iding thrust. A la rge numbe~ of pu lses take place. probably at t!q ual interva ls. The limit on I p due to ablation and pallation can b~ overcomt! by u ing a magnetic field to hie ld Pusher pate tht! su rface fro m tht! high energy pl as ma. Magneti c fie ld lines are gent!rated parall el to the surface-o fa conducting pu sher plate and as the plasma from th e explosion e'\pand it pus hes th e field lin es aga in st th e conductor. increas in g fl ux de nsity. The in cr;ased Expodlng PJlse unit mag netic press ure slows down th e plasma, thu s reversing its direction and acc eleratin g it away from th e External Pulse pusher plate. The impu Ise is transferred to the plate by Pu lse unll magnetic interactions which spread out the force and protect the plate's surface iTom particle impingement. Therefore, the propellant particle energies can be higher than for an unshielded plate and th e [sp's attained with ~he system can also be greater. Pulse unll EXPOSI01 Inl ecb01 O1amtH Magnetic hie ldin g was first menti oned b\ Internal Pulse Everett and Ulam. [3J and the feature has bec o n~e standard on th e hi gh-power fusion pul se vehicles Fi gure I: PP Concepts studied foll ow in g Ori on. It is important to note that pi a ma confineme nt usi ng magnet ic fi elds is not Externlll NPP perfect. and any high tem peratu re neutral pan ic le wi ll be una ffe cte d. [n general, howeve r. magnt! tic shi eldin g Th is co ncept was hi sto ri ca ll y the first to be offers tht! onl ) method of atta inin g Isp in exce s of 10° conceived. Th e pulse takes pl ace at some di stance ·ecs. wh ile no n- magneti c sy tem will probably be from a pusher pl ate. which intercepts and absorbs the limited to - I oj ec . shock of the ex plosion. The momentum conditionino unit smooth es ou t the mo mentum transfer between '" Internal NPP pul ses to provide a nea rl y co nstant acceleration, and re tu rn the plate to its proper locati on fo r the nex t In thi s concept. the ex pl osion takes pl ace in ide a pulse. pre ure vessel from which heated propell ant is . The adva ntage of thi s approach is that no attempt ex panded through a co nventional nozzle. Wh en th i IS made to confin e th e ex pl osion. Thus. it circumvents method was co nce ive d. it was uppo ed th at li se of an the material temperature limits associated with enclosed "reaction chamber" and no zzle would confined concepts, such as so lid and gas core nucl ear elim inate 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 _ 106 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 ifsuch high wave is reflected back and forth in the vessel increasing the internal energy of the until t~mperatures .were applied for an y extended length of h y droge~ time. the vehicle would be destroyed. equilibrium is es tablished. 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 cont inued unti I impingem ent velocity against the pusher plate and th e the previous initial conditions in the vessel are re­ fraction of pulse unit mass striking it. The established, and the cycle is repeated. imp inge ment veloc ity is limited by pu sher plate Studies in th e 1960's concluded that Isp greater abl atio n. and is probab ly in the ran ge of 100 to 200 th an 1,400 seco nds wo uld require very hea vy engines. km per econd. Tht! pul se un it fracti on is determined [8] Th ere are two main limitatio ns to the performance by dt!sign of the explosive charge and the stand-off oran in ternal ~ tem. One i radiatio n heatin g ­ distance. and i in the range of 10 to -0° o. The mos t of th e radiation em itted in the fo rm of n; utrons res ulting Is p limits are approx im ately 3.000 to 10,000 and ,{-ray is deposited into the chamber wa ll. Thus. seconds. the ve hicle req uires coo ling. and thi s is tht! dom inant

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American Institute of Aeronautics and Astronautics pe rform an ce-li miting facto r in (he inte rn al design. The la un ch, probab ly from the U . . nuclea r test site in resulting Isp limi t depends on th e energy deriv;d from . evada. The vehicle. which is shown in Fi g. 2. the e'plosion. but it i generally les than I.: 00 sees looked like the tip of a bullet. was - 80-mete~s hiQ.h - at least an order of magn itude wor e than that of an and had a pusher plate - 40-mete rs in diameter. - external system with the same pul se un it mass. .-\na lyses showed th at the bigger the pu he r plate. the The othe r li m iting factor is th e hi gher mass of the bener th e performance. internal veh icl es. Studies showed that the minimum mass of an external system wil l always be less than that for an in ternal system for the same payload and mi ssion.

\ Payload Project Orion \ Section I The most extensive effort on fission-based nuclear ­ I pul se propulsion was performed in Project Orion. The results obtained during its seven year lifetime from 1958 to 1965 were so promising that it deserves serious consideration today, especially in Iight of the 2M·Stage se riou tech nolo",ical obstacl es posed by some of the Shock PropulSion oth er advanced propulsion technologies being Absoroer Section co nsi dered for ambitious human space fl ight. An excellent description of the project" s hi story is lSI'Stagej~iiiiShock give n by Martin and Bond [7] . The following Absorber represe nts a condensed version of th e hi storic; l ummaries in tha t paper. Figu re 1: Ea rly Orion Concept

Th e B ~g inning (1957 - 1958) Th e mass of the ve hi cle on takeoff wou ld have Ori on began in 195 8 at the General Atomic been on th e order of 10.000 to nn es - mos t of wh ich Division of Gen era l Dynamics in San Di ego, would have gone into orbit. At takeo ff, th e O. I Californi a. The ori ginator and dri vin g force behind the kil oton-yield pulse units would be ej ected at a project was Theodore Tay lor. a fo rm er weapon designer fre qu ency of I per ecor,d. A the ve hi cle acce lerated. at Los lamos who see ked a nu clear propul sion system th e rate wo uld slow down and the yield would increase that \vo uld rega in Ameri ca n presti ge in space in the until 20-kiloton pul se would have been detonated wake of putnik. every ten seconds. The ve hicle wo uld fly straight up Tay lor had encountered the nuclear pul se unt il it cleared th e atm osphere so as to min imize pro pu ls ion conc ept at Los Alam os. Being an expert at radi oactive contam in ati on. 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 disk/charge 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 robustness of a General Atomic. persuaded Freeman Dyson. a sea-going vessel , not requiring the excruciating weight­ theoretical ph ysicist at Princeton's Institute for saving measures needed for chemically-propelled Ad vanced Study to come to San Diego to work on spacecraft. Orion during the 1958-1959 academic year. Taylor and The cost of fielding a fli ght-operational sy stem was estimated to be $100 mill ion per year for a 12-year D ~so n were convinced that the approach to space flight bein g pursued by NASA was flawed. Chemical de velopment program. However. thi s fi gure does not rockets. in th ei r opi nio n. were very ex pensive. had ve rY include de vel opm ent costs for th e th ousa nds of smaller limited pay load . and were essentialI v useless fo r - items th at such a prog ram would require (e.g .. flights beyond the Moon. The Orio n' team ai med fo r a spacesuits and scie nti tic instrum ents) . Th e Ori on pace hip tha t was sim ple. ru gged. roo my. and program wou Id have mo t Iikely utiI i zed the produ cts affordable. Taylor originally called fo r a ground fro m military weapons programs and existing civi lian space projects. Still. even if thi s estimate was off by a 4

American Institute of Aeronautics and Astronautics facror of 20. [he rev i ed [oral would have been onl y la\ r Ul tile 1' 1.1 " i '1: JlIl'JlIl)11 01 hl'..:l :em pe ratures S1-l- billion, roughly th e ame cost as the Apollo \ \ ;1 ~ '>Il ,h"r! 111.11 \ :'- luk /)':;11 11I)\\--:J Il1 to the plate. program . :1I1U :lL.I\ -'~. I' ,~ ,\'1 11'!l.:e':",1r\ r 11;; ;:,\pe rimenters cOl1cluJeJ til,1[ -'Itiler .tllI l1)lllllJl) (ll' 'k'el \\Ll uld be The ARPA Year ( 1958 - 1960) uurJbk <'111H ~II tll ,1Lt J, pl,ne 11).1leI'1J I.

The Orion team realized ea rl y that the .S. government had to become involved if the project was to ha ve an y chance of progressing beyond rh e research stage. In April 1958 Taylor gave a presentation to the Ad~anced Research Agency (ARPA) of the Department of Defense (000). The following July, after a good deal of negotiation, an award of $1 million was made to cover 10 months of work. It was at this time that the code name of Orion was assigned. Shortly after the start of the project. NASA was formed and took over all the civil space projects funded by ARPA. while the Air Force laid claim to all military projects. Orion remained the only major project unde r ARPA charge, as neither NASA nor the Air Force regarded it as a valuable as et. Taylor's efforts to interest NASA at this stage failed. which is difficult to understand in light of the growi ng interest in goi ng to the loon . Figu:'e 2' PIJ I: -ruI: Fli ght \lod el At the end of 1958 an award of $400.000 was made to the project and the following August another mill io n dollars was pl aced at Orion's di sposal to cover th e following year's work . The team grew to about 40 member. with the overall project responsibility falling to Frederic de Hoffman. Tay lor was appointed project director with Jim ance as ass istant director (Nance later took over as director when Taylor left the project in 1963 ). At thi time. th e Orio n te am built a eri es of fli oht models called Putt-Putts. to test whether or not pu~ her plates 'made of alum inum could survive the momentary intense temperatures and pressu res created 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. Fig ure 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 Forc e Year ( 1960 - 1963) experiments also proved that the plate should be thick in the middle and tapered toward the edges to j un cture cam e in late 19:9. whel1 ARPA maximize its strength to weight ratio. decided it could no lo nge r support Orion on national- The durability of the plate was a major issue. The ecurit'v ground '. Ta\ 101' had no choice but to approach expanding plasma of each ex plosion could ha ve ~ th e Ail: F-o rc e for ti.l~ ds. A Ithough it was a hard sell, temperature of several tens of thousands of KelVInS th e , ir Fo rce fina ll ~ decided to pick up Orion , but even when the explosion occ urred hundreds of feet on" all the ol1dition tha t a mi li tar)- use be fo und for from the plate. Following the lead of the Eniwetok it. Th e ir Force contacts we re Yl11pathetic to the tests. a scheme was devised to sp ray graphite-based gOJI of sp:Ke e\r l()r~lilln, but felt that Ih eir hands grease onto the plate between blasts. Extensive work \~ en: lieu. was done on plate eros ion using an explosive-dri ven ril e pld J1 .\ .1' Il' U,-: ( )ril ' Il ,h ,I \\ -:apo n platform hel iu m plasma generator. The ex perimenters found that In pl)I~lr ,11' " ' . , I~ ",'l 1<1 p.1" 1\ er c\ ':1': poin t on the the plate "ould be expo ed ro extrem e te mperatu res fo r I anll', ,url.k,' ',l'uld .1I'l' I'r(llt;cl It"el f ea il y onl y abo ut one millisecond during each ex plosion. and auain s[ ,1ll<1 ek, ''. ,11l,I1lnlllllh-:r, ll l' Illissi k s. that the ablation would occ ur onl y within a thin urface I-~\\ ever. Ihi ., IJ ~; 1 had the sLime disadvantages as the

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American Institute of Aeronautic and ,\ stl'Ona ut ics -~------... --... -.

earl; borr.b-carrying atelli te proposals. Terminal guidance would have been a problem. since the The core orthe veh icle was a - 100.000 kg technology for accurately teering wa rhead had not yet propu lsion module with a 10-meter diameter pusher been developed. Furthermore. both the U. S. and the plate. which was et b: the aturn diameter enve lope. oviet Uni on we re deploying missiles that we re This rather smull diameter restricted Isp to 1800 to apable of reach in g their targets in fi fteen minutes with 2500 ecs. While ex tremel y low by nuclear pul se multi-megaton warheads. making orbiting bomb ·tandard . this tigure far exceeded those of other pl atforms irrelevant. nuclear rocket design . The shock absorber system had Little firm in formatio n is available but it does two sec ti ons: a primary unit made up of toroidal eem certain that the vehicles were intended to dri ve a pneumatic bags located directly behind the pLl her 900 to nne payload to low earth orbit or to escape from plate. and a econdary un it of fo ur 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 yi eld explosions of about 0.01 kilotons, released Two or possibly three Saturn V's would have from the vehicle at 10 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 profiles were considered - the one developed in quoted as 3.630 tonnes, and the acceleration ranged greatest detail was for a Mars mission . Eight 2 from :W to 90 m/s . The Isp of 4,000 to 6,000 sec, a tronauts. with around 100 tonne of equipment and along with an average vehicle acceleration of ~ 1.25 g su ppl ies. could have made a round trip to Mars in 125 would enable direct launch from the Earth 's surface or days (mo t 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 0 propulsion module inert mass fraction of 0.3 to 0.4 as much as 45 0 of the gross vehicle weight in Earth an d pul ing intervals of about I sec. orbit could ha ve been pay load. Presumably the fl ight would have been made when Mars was nearest to th e The NASA Years ( 1963 - 1965) Earth; sti ll, so much energy was available that almost the fastest-possible path between the planets co ul d have Robert Mc amara. Defense Secretary under the been chosen. Kennedy Administration. felt that Orion was not a An assessme nt at that time placed the mil itary asset. H is department consistently rejected de velopment costs at $ 1.5 billion, which suggests a an) increase in funding for the project, which superior economics for nuclear pul e spaceships. effecti vely limited it to a feasibility study. Taylor and Dyson also felt that Orion's advantages were greatly Dy on knew that anoth er money so urce had to be diluted by using a chemical booster - the Saturn V' found if a flyable vehicle was to be built. and A A wo uld have repre ented over 50% of the total co t. wa the onl remaining option. Accordingly. Taylor Von Braun became an enthu iastic Orion and Na nce made at least two trips to Marshall Space su pporter, but he was un ab le to make headway for Flight Center (MSFC) in Huntsville. Alabama. increased support among hi gher-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 sc hematic of the vehicle is shown in Fig. 4. single bomb would explode or even rupture? Although ASA feared a public-relations disaster and was POYoEREUFlfCHI crw:w 3 fAlk>N(:5HELOED) -_•. reluctant to provide money, its Office of Manned Space CRFW ACCOIrotOOA 1lOH Flight was sufficiently interested to fund another study.

Q..Jrr..S1C S T'RUC~ INCl ~ PU...SE-lH' Orion's Death DEliVERY SYSTE '" A fateful blow was dealt to Orion in August 1963 with signing of the nuclear test-ban treaty. Although the tests required fo r development of an Orion veh icle _ _ _ 1.. were now illegal under international la w. it was sti ll pu.:n '-R PI.Art - -

5 J~rr possib le that an exempti on could be granted for u rcrrt)Pu' SI t N T [J'JI""-CE programs that \\ ere demonstrabl y peacefu I. However. pc)r"" Of-Of:'o ~ nOH - - the re is no doubt that the treaty greatl y di m in ished Orion's pol itical support. Another problem was that Figure 4: Orion Spacecraft - ASA Version 6

American Institute of Aeronautics and Astronautics because Orion was a classified project. very few people vehicles was imme nse and capable of tran sport ing a in the engi neering and scient ific co mm uniti es were colony of th ousand of people [0 a nearby star. It aware of it ex iste nce . In an artempt to rec ti fy thi . would take - 1.000 years fo r the energy-limi te d de ign Ori on's manager. Jim Nance. lobb ied the Ai r Force [0 to re ach Alph a Centaur i. whi le the momen wm-li mi ted dec las ify at least a broad outline of the work that had case would ta ke a mere ce ntu r; . been done. Even tu all y it ag reed, an d Nance published A new era in thinking abo ut nuclear pulse a brief descrip ti on of th e "first gen eration " ve hi cle in propul sion began in the late 1960 's and ea rly 19 70 ·s. October 1964. Spu rred by optim ism fo r contro ll ed fusion fo r power The Air Force. meanwhile. had become im pa ti ent generatio n. researcher ignored use of fi ssiona Ie wit h NASA 's nonco mmittal ap proac h. It was willing materia l. and began to focu s on ign iti ng small " mi ll i­ to be a partner only if AS A would co ntribute kil oton" fusio n mi croexplosion . By 10\\'ering the significant funds . Hard-pressed by the demands of energy of each fu sion 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 tission able materi al or elaborate discontinuation of all funding. thus terminating Orion . pul se unit structures. All told, approximately $11 million had been Soon microexplosion designs began to push spent on Orion over nearl y seven years. Freeman toward theoretica l Isp levels near 106 secs. implying Dyson stressed the importance of th e Orion story exhaust vel ocities nea r 3°'0 of Ii ghr ve locir; . Tne t · . .. because this is the first ti me in modern histo ry th at pu her pl ate become a powerful magnetic fi eld. which a major expansion of human technology has been would channel ch arged parti cles into an exhau st. and suppressed for political reaso ns." In retrospect, there pul se repetiti on rates in creased to hundred s per second . we re other issues bes ides pol it ics, and these included: Con ve rgin g lase r beams. electron beam s or other driver ( I) the inheren t large size of the veh icle made fu ll scale energy ou rces wou ld ignite the fus ion pe ll et by tests difficu lt and cos tl y. (2) the nuclear test ba n treary ine rt ially compressin g and confining the fuel. Some of excluded test ing in the atmosphe re or in space. (3) the the ene rgy of the m icroexplosi on wou Id be ta pped NE RV solid core nu clear engin e pro vided strong electromagneticall ; to provide po\. er for the lasers and competition. and (4) no spec ific mi ss ion existed which the pusher plate magnetic fi elds. that is a bootsrrap demanded such a hi gh perfo rmance system . process. Th ese systems cl early have ex traordinar;' design requ irements and pu sh technological limi ts. A Orion's Legacy ve hicle propelled by a million- second Isp engine could in th eory visit any locati on wi thin th e so lar syste m in Altho ugh Orio n empl oyed fi ss ion as the mode of a matter of month s. energy release, use of fusion was always vi ewed as the Me mbe r of the Br it ish Interplanetar; Society nex t logica l step in the evo lut io n to ever-higher took up the chall enge of fusion microexpl osion pe rformance. One adva ntage of fusion is th e hig her propulsion and conducted the most elaborate -tudy to spec ific energy of the reac ti on, but for charged particle date of a robotic interste ll ar ve hi cle. From 19 3 to products, this is only several times that of fission . 1978, the team of 13 mem bers worked on Project The main ad vantage of fu sion is that there is no Daedalus, a two-stage fusion microexplosion spacecraft minimum mass criticality limit, and the detonation can designed to send a scienti fie payload of 450 tons at be made very small - yields on the order of 0.00 I 12% light speed on a one-way, 50-year fly-through kiloton and lower. miss ion to Barnard's star, 5.9 light years distant. In 1968, Freeman Dyson was the first to propose The 106 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 debri s velocity of fusion explosions was Jupiter's atmosphere before the fli ght. Daedulus would in the range of 3,000 to 30,000 km /sec, and the accelerate fo r about four years under the incessant din geometry of a hem ispherical pusher plate would reduce of 50,000 ton s of pellets ign ited 250 times per second th e 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 106 secs). full y-fueled. 54,000 to ns - alm ost all propellant. Th is made miss ion ve locities of 10 ) to 10 4 km/sec More recent in ves tigations of fusion poss ib le. microexplosions have considered use of laser inertial Dyson conside red two ki nds of concepts. The confinement. with Law rence Li vermore's VISTA more conservative design was energy-Iim ited, hav in g a conce pt. [9] and use of co mbi ned m ic rofi ss ion fusion large en ough pu her plate to safe ly abso rb all the wit h an antim atter tri gger. r I 01 A Ithough the ri ve r therm al energy of th e impin gin g explo ion, without tec hn ology in all th ese ca es i ve ry different. he basic mel ting. The other momentum-limited concept concepts all ha ve their root in the ea rlia co ncepts of deti llt:d the upper region of perform ance. Each of these fusion-based nuclear pulse propul sion.

7

American Institute of Aeronautics and Astronautics place an upper limi t on the performance in te rm s of Reconsidering Nuclear Pu lse Propulsion Isp. Sma ll er. hi gh -specific yield pu lses combin ed with more ablation res istant material wo ul d red uce Interest in nuc lear pulse propu l io n neve r rea lly minim um sta ndoff di stance requirement. the reby died with Orion. it merely evo lved into concepts based inc reasing Isp considerably. on what many v iew as the ta mer and more po li tical ly Even with the reduction in yield and acceptabl e process of nu clear fusion. In re tros pect. th is imp rove ments in perfo rm ance. use of se If-actuating hift in inte rest was pro bably prematu re and based on nuclear charges would still be a poli tical issue. overly optimistic projecti ons of fusion's viability. We However. it can be argued that in so me ways th e now know th at the chall enge of fusion is much more enviro nm ent may be more acco mm odating today than diffic ult than ori gi nall y envisioned. In fac t, fu sion for it was du rin g the pol itica ll y-charged days of the Co ld 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 Iig htweight subsystems and hi gh gain . [I I] the peaceful applicati on of nucl ear pul se techn ology. It Recogn izin g the form idable challenges of fusion , does prov ide a productive avenue for di spos ing of the perhaps it wou Id be wise to take a step back and substantial stockpiles of weapons-grade fi ssi onable recons ider 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 negl igible if landscape over the last 30 to 40 years, and it is used at a sufficie nt di tance oll ts ide low earth orbit. possi bl e th at fissio n-based systems could be made safe. There is no doubt th at po l it ica l acceptance of such affo rdable, and even better perfol111 ing th an the design s an idea would demand convincing techn olog ical need considered in th e Orion program. and international involvemen t. As of now. th ere are Th e most se nsitiv e iss ue with Orion was its use several propul ion concepts th at cou Id be used fo r of self-actuating nucl ea r dev ices. Ironically. this was hum an mi ss ion s to Mars. However wi th conserva tive also one of its main strengths. since it elimi nated the projections of techno log ical readiness. these mission s need fo r mass ive dri ve r and energy storage hardware wo uld be constrain ed to 2 to 3 year du ra tions. onboard the spacecraft. till. al mos t anyone who has If the need arose to co nduc t a Mar mi ss ion much been expo ed to the concept fee ls uncomfortable about faste r (say in a year or less) or if there were a need to this aspect. and rightl y so. since it rai ses a myriad of transport hum an or large pay loads as rapidl y as issues regarding testing, nuclear proliferation, and possi ble to destinati ons in th e outer solar system (e.g., natio nal securi ty. Thi s is particularl y true with the Jupiter and beyo nd). th en the use of nuclear pu Ise ield of the dev ices ori gin all y considered in the Orion becomes qu ite compell ing. If such mi ss ions in vo lved program. Al th ough small by wea pon standards, they ex tensive intern ati onal coo perati on. then the re may be were nonetheless in the 0. 1 to 10 kil oton range, and mo re acceptance fo r th i type of tec hnology. drove the need for large, robust spacecraft des ign s. Pe rh aps th e most prom isin g ave nu e fo r use of There has li kely been co nsiderable progress in th e fissio n- based nucl ear pul se lies in th e direction of ac tuati on of ex pl os ive fi ss ionable charges over the last microfiss ion processes. In th ese sc hemes. subcritical 30 to 40 years. and thi s technology could be applied to targets of fissile material are compressed via a rea li ze smaller yield detonations than those baselined mechanism on board 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 hi gh burn -up fractions is course, such infol111ation would undoubtedly be substantially less than that for comparable fusion class ified and unavailable for openly reviewed processes. spacecraft evaluations. However, the poss ibility is The advantages of thi s ty pe of approach are clear. there and could bring the yields down into more It eliminates the concern s over having vehicles that acceptabl e ranges. carry fu ll y contained ·'bomb s." Because th ese system s An oth er major di fference between now and the rely on a compression and energi zin g source from the time of Orion is the dramatic improvement in materials spacecraft , they cannot be used as a weapon, at least in techn ology. Ori on's pusher plate and mom entum an y conventional way. No t only does th is take care of transfer assemblies we re based on 1950's and 1960's the concern over storing thousands of small bombs in technology, and featured common materials, such as close vic inity. but it also rem oves many of the iss ues stee l and aluminum . Research ove r the last 40 years conceming nucl ear proli fe rati on. has opened the pros pects of adva nced carbon structures Only a few studies of this approach have bee n and ligh tweight refractory materia ls whi ch co uld co nducted. but the results look very promisi ng. It may greatl~ red uce th e ma s and improve the ab lative prove to be a more realistic intermediate step between characterist ics of nuc lear pul se systems. The latter the prop ul sion system of today and the fusion ­ co nsi derati on is es pec ia ll y important since it tends to propelled co nc epts of to morrow.

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American Institute of Aeronautics and Astronautics , . .. .

References

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

2. 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- 1955 (1955). (Declassified, Aug 25, 1976).

4 . "Nuc lear 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 Startlight Handbook: a pioneer' s guide to interstellar travel," John Wiley & Sons, Inc., ISBN 0471619124 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 impu Ise of a pulsed rocket engine," UCRL-12296 ( 1965). Presented at AIAA Propulsion Joint Specialist Conference, 14-18 June 1965.

9 . Orth, C. D., Klein, G., Sercel, 1. , Hoffman, N., Murray, K. , and Chang-Diaz, F., "VISTA: A Vehicle for Interplanetary Space transport Applications Powered By Inertial Confinement Fusion," Report UCRL-LR-II 0500, University of California, Lawrence Livermore National Laboratory, Livermore, CA 94550 (1998).

10. 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 , EI -Genk, M. S. ed., 1998.

II . Chakrabarti, S. and Schmidt, G.R. , " Impact of Energy Gain and Subsystem Characteristics on Fusion Propulsion Performance," AIAA 2000- 36 13. July, 2000.

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American Institute of Aeronautics and Astronautics