I - INTRODUCTION
This study is an examination of the current status and plans for the American nuclear thermal space propulsion program. Major issues addressed include:
+ NASA and DOE policy and goals, including funding and program prospects;
+ Activities of other participants, including other government agencies, the Congress, with particular reference to DOE National Laboratories and private contractors;
+ Reactor concepts and mission architectures currently under consideration.
However, there are very few fields of technological endeavor in which the historian's maxim -- that the past is prologue -- is of greater relevance than the realm of space nuclear propulsion. There are at least three senses in which this is true:
Many of the reactor and system concepts that are the subject of contemporary interest have been the subject of study and analysis for over three decades. While new permutations of these concepts have emerged over the years, there have been few qualitatively novel developments. Reviews of nuclear propulsion systems in the late 1980's centered on technology developed in the 1960s, which was evaluated in comparison with other concepts of more elderly vintage. And much of today's technical debate is a rehearsal of long-established positions.
Many of the leading figures in nuclear propulsion came to the field early in their careers, and have remained active in the field since. Thus there is a striking coincidence of the history of nuclear propulsion and the biography of nuclear propulsion technologists. While it may be less than apparent to the novice or the uninitiated, the nuclear aerospace propulsion field today is enriched by an unusual wealth of accumulated personal experience and insight.
For nearly three decades, aerospace nuclear propulsion has been a capability in search of a mission. Despite the nearly $10 billion (1992 constant dollars) invested over this period, resulting in major capability demonstrations, the promises of aerospace nuclear propulsion remained unrealized. Rather, the past four decades have witnessed repeated episodes in which aerospace nuclear propulsion projects are initiated and development and testing takes place, only to be followed by growing doubts about the merit or utility of the proposed applications, leading ultimately to the project's demise. Undaunted by their disappointment, the aerospace nuclear propulsion community has invariably found a new focus for their energies, only to see the cycle repeated.
Prior to the First World War, the early pioneers of spaceflight, such as Goddard and Tsiolkovsky, were quick to recognize the potential applications of nuclear power to astronautics Frustrated by the meager performance of existing propellants, and intimidated by the practical challenges confronting application of more powerful chemical propellants, the energy locked in the atomic nucleus seemed the key to unlocking the gates barring human access to the heavens. But prior to the Second World War, the energy of the atom remained locked away from human use.
The realization of the potential for fission reactions on the eve of the Second World War soon led to speculation on how this new-found source of energy could be put to use. Although atomic bombs and radiological weapons topped the list, workers in America, Britain, and particularly Germany, quickly realized that fission reactors could be used to generate thermal and electrical power.
It was but a short step to the contemplation of how fission reactors could be used to power submarines. Indeed, in wartime Germany, engaged in an intense submarine campaign in the Atlantic, nuclear-powered submarines seemed perhaps the most attractive application of atomic energy.
And less than a decade after the War's end, the commander of the first atomic submarine, the Nautilus, would dispatch the historic signal "Underway on nuclear power."
Though the path to this moment was a difficult one, there could be little doubt that the journey would be completed. Three factors were critical in the application of nuclear propulsion to submarines:
The submarine was an established component of navies around the world. It had proved its potential for decisive military action during the Second World War, and there was little question of its continuing importance.
Nuclear propulsion revolutionized the combat potential of the submarine. Indeed, it is perhaps appropriate to use the term "submarine" only to refer to nuclear-propelled vessels. Non-nuclear boats are more properly considered "submersibles," condemned to spend most of their existence on the ocean's surface, only briefly submerging below the waves. In contrast, atomic submarines are creatures of the deep, roaming the world ocean at will, freed of any connection to the world above. The underwater speed and endurance of the atomic submarine created entirely novel combat capabilities that have proved irresistible to any navy possessing the treasure required to obtain them.
This revolution in military capabilities was achieved in the absence of a revolution in technological capabilities. It does not detract from the ingenuity and skill of the technologists who perfected the atomic submarine to note that the reactors on these vessels were a relatively straightforward extension of land-locked reactors developed over the previous decade. Technological evolution produced a military revolution.
The civil space community has spent nearly three decades attempting to replicate the commercial success of communications satellites. Heroic efforts to place endeavors such as remote sensing and materials processing on the self-financing basis so rapidly achieved by communications satellites have thus far been in vain. The "voices in the sky" remain the exception that proves the rule that space is a very difficult place to turn a profit.
So too do atomic submarines remain an exception that proves a rule -- nuclear energy has proven remarkably resistant to propulsion applications. Nearly four decades of effort by aerospace technologists have failed to replicate the success of atomic submarines. Nuclear airplanes and nuclear rockets (not to mention atomic-powered helicopters and cruise missiles) remain in the realm of artist's concepts.
That this should be the case is not surprising, when the context of nuclear aerospace propulsion is contrasted with conditions that were so conducive to the emergence of atomic submarines.
While atomic submarines, like communications satellites, filled pre-existing needs whose importance and legitimacy was widely appreciated, nuclear aerospace propulsion applications have frequently been at the margins of national priorities. Temporary enthusiasms for aerospace missions that might utilize nuclear propulsion have generally waned before the required technologies could be brought to maturity.
While both atomic submarines and communications satellites promised revolutionary capabilities, nuclear aerospace propulsion has more often than not found itself in sharp competition with a host of non-nuclear solutions that offered comparable, or at least not drastically inferior, performance. The modest performance gains of nuclear propulsion too often seemed negligible reward for the effort expended.
In contrast to atomic submarines or communications satellites, which resulted from evolutionary extensions of the existing state of the art, aerospace nuclear propulsion has required major technological innovations. Aerospace reactors operate at power levels and temperatures far exceeding those of reactors intended for other applications. Modest evolutions in capabilities are purchased at the price of technological revolutions that have challenged the imaginations of a generation of reactor designers.
In the face of these unpromising conditions, what is surprising is not that aerospace nuclear propulsion remains a promise rather than a reality, but that the promise has remained so attractive for so long. With the exception of a brief detour in the 1970s, aerospace nuclear propulsion has been a field of considerable, and essentially continuous, activity for nearly five decades.
In the years immediately following the Second World War, a number of studies were conducted to evaluate the potential of nuclear propulsion to aircraft, missiles, and spaceflight. Although these studies were not followed with development programs, this was as much a function of the ascendancy of the manned bomber in this period, which frustrated the development of rocketry generally, as it was the result of the shortcomings of nuclear propulsion.
From the early 1950s through the early 1960s, several billion dollars were spent toward the development of nuclear-powered bomber aircraft. Though this produced significant technological developments, chemical-propellant bombers and ballistic missiles ultimately proved more attractive to the military.
With remarkable continuity in funding (Figure I-1), interest in and support for nuclear rocketry waxed as the nuclear airplane program waned. Nuclear rockets were seen as the key to piloted missions to Mars, and an important contributor to extended human exploration of the Moon. Though the nuclear rocket program continued through 1972, testing over 20 nuclear reactors and engines, by the early 1960s it was clear that there was little political support for the ambitious and costly space voyages these rockets would support.
Nuclear airplanes and rockets were not the only aerospace propulsion projects of this period. An active testing effort was undertaken for the development of a nuclear-powered intercontinental cruise missile. Other projects included nuclear powered helicopters and dirigibles. These projects came to naught following the cancellation of the nuclear rocket
program in 1972. For the ensuing decade exotic skills of aerospace propulsion designers were preserved in the Clinch River Liquid Metal Fast Breeder Reactor program.
The demise of the Clinch River project in 1982 coincided with a resurgence of interest in aerospace reactor systems, which was greatly animated by the emergence of the Strategic Defense Initiative in 1983. Nuclear reactors were considered as one option for powering the exotic space-based weapons that figured in initial concepts of the SDI program. The full range of nuclear systems developed in previous decades, including those tested under the nuclear airplane and rocket programs, were considered for this purpose. But non-nuclear chemical systems were the preferred solution to this challenge. And the entire question was soon mooted by the declining ambitions and fortunes of the SDI program, as discussed in Chapter II.
Just as the nuclear rocket program emerged from the ashes of the nuclear airplane project in the early 1960s, much of the SDI nuclear reactor effort quickly found new inspiration in the Space Exploration Initiative, announced by President Bush in 1989. Although earlier concepts of human missions to the Moon and Mars had centered on chemical propulsion systems, the Space Exploration Initiative has increasingly focused on nuclear propulsion options. As with the SDI program, evaluation of nuclear reactor options has been an exercise in technological archeology, with old concepts reviewed for their potential contribution to this latest mission requirement. But as with the nuclear airplane proponents in the 1950s and the nuclear space power proponents of the SDI era, advocates of nuclear propulsion for human space exploration have been hard-pressed to demonstrate clear-cut advantages to their preferred solution, as covered in Chapter III.
While most recent work on space nuclear propulsion systems has been of a theoretical nature, one concept, the particle bed reactor, has proceeded to hardware fabrication. This concept, first identified in the late 1950s, and subsequently elaborated in the early 1980s, received nearly $200 million in funding from the Strategic Defense Initiative under the highly classified Timberwind program. However, in 1991 this program was largely declassified, and transferred to the Air Force as the Space Nuclear Thermal Propulsion program. Various strategic defense applications were considered for the particle bed reactor, including generation of electrical power, propulsion of anti-missile interceptors, and use as a high-performance launch vehicle upper stage. Although the complete nature of the SDI Organization's interest in this technology remains shrouded in classification, analysis based on open sources suggests that unique contributions of the particle bed reactor to strategic defense missions are rather difficult to identify. That this judgement was ultimately shared by the SDI Organization is indicated by its decision to discontinue funding the project, as discussed in Chapter IV.
Despite these setbacks, proponents of aerospace nuclear propulsion remain undaunted. Their persistence in the face of this history is not easy to explain. Equally, if not more difficult to explain is the secrecy surrounding the Timberwind program, and the SDI Organization's interest in this project.
Both of these phenomena are largely rooted in the observations contained in the 1989 report of a panel of the National Research Council noted:(1)
"History has shown that it takes longer to develop a nuclear reactor system... than to develop a space mission. Hence today's civil and military space project managers cannot include any nuclear reactor space power system -- or any other system -- in their mission planning until that system has been developed and tested. This dilemma is sometimes referred to as the "chicken and egg syndrome."
The persistence of the aerospace nuclear propulsion community, and the curious history of the Timberwind project, derive from this "chicken and egg" dilemma. Long and hard experience has revealed the transient and ephemeral character of mission requirements, in contrast to the protracted realities of developing workable aerospace nuclear propulsion systems. This has engendered a belief within the aerospace nuclear engineering community that if only reactors could be developed, users would emerge to claim them.
Thus mission analysis becomes an exercise in articulating not-implausible pretexts for continued reactor development, rather than a process for establishing firm requirements. In this sense, for instance, for the past decade the particle bed reactor has been a solution in search of a problem. The bewildering sequence of missions that have been attached to this reactor concept over the years is perhaps the clearest example of how (potential) capabilities drive requirements, in contrast to the more (naively) logical presumption that requirements should drive capabilities.
While proponents of aerospace nuclear technology may have properly identified the locus of their discomfort, it is less clear that they have properly diagnosed the nature of their malady. Is this a problem which admits the possibility of a solution, or is this a dilemma from which there may be no escape?
The suggested solution is hatching the egg -- creating reactors which will call into existence users. But four decades of effort call this solution into question.
Aircraft nuclear propulsion systems were close to perfection when the program was terminated in 1962, and yet no users have come forward in the following three decades.
Nuclear rocket propulsion systems ready for flight testing when the program was terminated in 1972, and yet no users have come forward in the ensuing two decades.
Over $400 million dollars has been expended by several agencies over the past decade on the SP-100 space nuclear power system, and yet the identity potential users have become increasingly elusive over the years.
It is equally likely that the "chicken and egg" phenomenon is a dilemma for which there is no solution, particularly if the eggs produce turkeys when hatched. The continuing frustration of aerospace nuclear propulsion lies, not in the absence of off-the-shelf reactors, but in the failure to meet the conditions that were the key to the success of atomic submarines.
Aerospace nuclear reactors will remain artist's concepts as long as they continue to fail to respond to widely accepted requirements, fail to provide qualitative advantages compared to other technical approaches, and require major engineering innovations.
The engineering challenges of this technology will be met only when clear user requirements are satisfied in a clearly superior manner.
1. National Research Council, Energy Engineering Board, Committee on Advanced Space Based High Power Technologies, Advanced Power Sources for Space Missions, (Washington, National Academy Press, 1989), page 88.