Renewed interest in nuclear thermal propulsion was sparked by President George Bush. On 29 July 1989, the twentieth anniversary of the Apollo moon landing, President Bush delivered a speech which marked the beginning of the Space Exploration Initiative:
" First, for the coming decade - for the 1990s - Space Station Freedom - the critical next step in all our space endeavors. And next - for the new century - back to the Moon. Back to the future. And this time, back to stay. And then - a journey into tomorrow - a journey to another planet - a manned mission to Mars."
And in a speech on 11 May 1990, President Bush subsequently established the timetable for the Space Exploration Initiative to realize these goals as culminating in the first human on Mars by 20 July 2019.
A - PROGRAM HISTORY
Responding to the President's July 1989 speech, NASA prepared a blueprint for achieveing these goals, known as "the 90 Day Study" (discussed below). On 19 December 1989 Vice President Quayle, who chairs the National Space Council, wrote to NASA Administrator Richard Truly requesting study of "different architectures, new systems concepts, promising new technologies, and innovative uses of existing technologies" to implement the SEI. This was included in a Presidential National Security Directive on 16 February 1990, which established the "Synthesis Group" to evaluate these alternatives.
The Synthesis Group was chartered by the National Space Council in the Summer of 1990 to review NASA plans for the Space Exploration Initiative, as well as to incorporate suggestions from other sources. It will recommend at least two alternative architectures for SEI implementation, one of which has been characterized as "nuclear rich." Although the deliberations of the Synthesis Group will continue through March 1991, this review process has already reached a number of preliminary conclusions:
"1 - Contrary to popular opinion, the first trip to Mars may have to be fast rather than slow, because humans are the weak link in the chain; human psychology is a big unknown.
"2 - One architecture proposed by the Synthesis Group will be "nuclear rich" because nuclear is probably safer and cheaper (and faster).
"3 - There has been a discussion about improving the overall system reliability by using multiple engines, i.e., rather than trying to put all of the reliability in one engine, have "engine-out" capability so that the overall system reliability is high.
"4 - Chemical/Aerobrake will probably cost tens of billions of dollars to develop and prove out and doesn't provide much gain. It was described as "disappointing.""(1)
General Stafford has testified that:
"Today it looks like technology has advanced so that in the year 2010 or 2020 a nuclear thermal rocket would certainly be feasible, assuming that you added all the safety criteria and had political acceptance... We are convinced that nuclear rocket propulsion can make an important contribution to the Space Exploration Initiative if it proves feasible and safe and can gain public acceptance. For example, a nuclear thermal rocket can reduce the travel time to Mars by 60-70%."(2)
Following receipt of the NASA 90-Day study, Vice-President Quayle requested a review of this study by a number of organizations, including the National Research Council (the policy research arm of the National Academy of Sciences). Consequently, the Council empaneled a Committee on Human Exploration of Space, a group over a dozen senior space experts, chaired by H. Guyford Stever. Their report was released on 2 March 1990.(3)
A number of the Stever Committee findings pertain to space nuclear propulsion. The committee's report noted that:
"A major advantage of nuclear propulsion is its ability to enable transfer between Earth and Mars in one-half to one-third the time required with single-stage chemical propulsion systems. This advantage could be critical, pending the outcome of research on human performance in space for long periods. The use of nuclear technology in space faces formidable barriers of public acceptance, however, especially if employed in Earth orbit. Therefore, issues of safety are paramount in research and development.
"If careful systems studies, using thrust-to-weight ratios and specific impulses known to be feasible, show a significant advantage for nuclear rockets in trip time or in weight to orbit, an in-space demonstration of this technology should be done as soon as possible -- taking into account requirements for crew, ground personnel, and public safety covering all phases of launch and flight, including mission abort. It will not be feasible to select the nuclear rocket as a baseline in a system architecture until such a demonstration has been conducted.
"A number of gaseous-core reactor concepts were carefully evaluated in the years between 1959 and 1970, but none was found to be technically feasible. Unless a new idea has appeared, which is always a possibility, the committee believes the gaseous-core nuclear rocket technology is too speculative at this time and should be dismissed as a possibility."
The Office of Exploration is the lead NASA activity for implementation of the President's Space Exploration Initiative. This Office was initially established at the Associate Administrator level in 1987, but was downgraded to an office subordinate to the Associate Administrator for Aeronautics and Technology in early 1990. Pursuant to a recommendation by the Augustine Committee on the future of NASA, this Office is being restored to the Associate Administrator level in early 1991.
The most detailed assessment of SEI technical requirements prepared by this office was released in December 1988.(4) Although this study focused on baseline chemical + aerobrake architecture, it concluded that:
"Old SC/NTR (solid core nuclear thermal rocket) technology can provide "new" high-leverage capability for human expeditions to Phobos and Mars. For the all-propulsive split mission to Phobos, reductions in IMEO on the order of 40 to 50 percent appear possible. For split mission to Mars, the NTR (operating all-propulsively) can still provide a 5 to 15 percent savings in IMEO over that of the aerobraked chemical system. With comparable propellant loadings, the SC/NTR could travel faster, higher delta-V transfer orbits than its chemical counterpart, resulting in further reductions in crew trip time.... By appropriately sizing the engine, a single NTR stage could function as a lunar shuttle; by clustering, several NTR stages could be used to support human expeditions to Phobos and Mars."
Exploration Technology Program Manager John Mankin has noted that:
"we want to be able to provide a nuclear propulsion program for the nation in the next 10 to 15 years that can allow us to reach Mars, but whether it's nuclear thermal or nuclear electric, today we don't have the information to say which is the way to go."(5)
A recent NASA report to the Congress concluded that:(6)
"Nuclear propulsion, therefore, is considered a critical major technology alternative to more conventional space transportation technologies for exploration. The Exploration Technology Program strategy with regard to nuclear propulsion is to conduct parallel development in several major technologies, within the areas of nuclear thermal propulsion and nuclear electric propulsion, with down-selection on promising concepts for further development as research and testing warrant."
And in testimony in early 1990, Associate Administrator Arnold Aldrich noted that:
"We believe more sophisticated, advanced technologies in nuclear propulsion are very, very advantageous to the longer range of this HEI (Human Exploration Initiative) program and should be explored in depth as we go forward; and we think that we can achieve beneficial systems and also systems that are environmentally safe to utilize... The nuclear propulsion is primarily beneficial to the Mars phase, by providing increased efficiency in propulsion for those missions, thereby making those missions much more practical to do, in the later phases of the HEI planning. However, because nuclear propulsion is still not an advanced technology, we feel we need to begin now to find ways to proceed in this area so that such a technology can be available in the timeframe that we would be able to use it."(7)
NASA's 1991 $179.4 million budget request for exploration technology included $11 million for nuclear electric and nuclear thermal propulsion. The reductions in the NASA funding request have not yet been factored into the Code RP budget, but it is anticipated that no more than a few million dollars will be available in 1991. The FY1992 budget request is still being formulated, and will be included with the President's budget submission, currently planned for early February 1992. It is anticipated that this request will follow the profile anticipated in the FY1991 request:
Fiscal Year 1991 1992 1993 1994
Amount (millions) $ 11 $ 35 $ 51 $ 65
The key focus of this Office:
"with whatever resources (people, funding, facilities, computer time), the focus for FY 1991 must be on developing unassailable arguments for nuclear propulsion. This should include development of appropriate figures of merit."
In the absence of significant funding during 1991, major activities will focus on those efforts that can be conducted at low cost, such as in-house concept studies with assistance from other agencies, laboratories, industry and academia. High priority would be given to NEP and Hybrid concepts which have received relatively little system design consideration. Other high priority tasks for 1991 include NASA, DOE and DoD laboratory identification of key emphasis areas and unique capabilities, collection of relevant documents into a centralized nuclear propulsion library, and publication of standard-format summaries of each reactor concept with a complete set of references.
Nuclear thermal propulsion has been identified as offering a number of advantages over non-nuclear systems.(8) Propulsive capture at Mars eliminates the need for aerobraking (which faces technical risks and operational uncertainties), though nuclear propulsion can benefit from incorporation of aerobraking if it is available. In addition, nuclear thermal propulsion can reduce total trip times by 50 to 100 days relative to chemical + aerobrake architectures with the same IMLEO, and as a result is less sensitive to launch window constraints.
The Jet Propulsion Laboratory is the lead NASA center for nuclear electric propulsion.
Johnson Space Center was in charge of preparation of the "90 Day Study" in response to President Bush's speech of July 1989.(9) Although the mission architectures discussed in this study used chemical + aerobrake technology, the report noted that:
"... for transportation from Earth orbit to Mars, nuclear propulsion shows a great deal of promise as an option for significantly enhancing mission performance... Chemical propulsion with aerobraking and solid core nuclear thermal rockets are ... essentially equivalent in their advantage over chemical propulsion without aerobraking... gas core nuclear rockets offer the significant advantage of reducing round trip travel time to Mars to less than 1 year."
Johnson Space Center interest in nuclear propulsion derives in part from the presence of the Astronaut Office, with its responsibilities for space crew selection, training and safety. At the November 1990 Houston Feedback meeting, astronaut Franklin Chang-Diaz noted that since JSC is an operations-oriented center, it was appropriate for JSC to begin considering the operational implications of Moon and Mars missions. He indicated that if NASA is serious about planetary exploration, new propulsion systems would be required, since the Astronaut Office was strongly in favor of Mars mission profiles that emphasized short triptimes. For this reason, Gas Core Reactors were highly favored.
The responsibilities of NASA Lewis in NASA's nuclear propulsion program are two-fold:
1 - coordinating NASA work (including NASA JPL nuclear electric work);
2 - acting as the lead center for nuclear thermal propulsion.
In support of this second responsibility Lewis has given substantial attention to the current baseline system, the NERVA-derived reactor.
a - Interagency Nuclear Propulsion Steering Committee
The Nuclear Propulsion Steering Committee is the joint NASA/DOE/DOD panel that is responsible for coordinating development efforts for Nuclear Thermal and Nuclear Electric Propulsion. The members of the Steering Committee are:
Gary Bennett NASA Headquarters Chair
Earl Wahlquist DOE NE-50 Member
Lt. Col Roger Lennard DOD (SDIO) Member
Thomas Miller NASA Lewis Executive Secretary
The initial work of the Steering Committee included two technical conferences, on NEP at JPL in June 1990, and on NTP at NASA Lewis in July 1990. The NTP conference included the formation of a series of panels that evaluated the presentations made at the conference. These Panels (whose work is reviewed below) include:
- Technology Capabilities;
- Reactor;
- Propulsion;
- Advanced Development Plans.
The work of these evaluation panels will form the basis for Steering Committee development of future plans. The principal findings were presented at a Feedback meeting at NASA JSC on 15 November 1990:(10)
"In general, the trend of the panel reports seemed to be than NTP technology is more advanced than NEP technology, and hence, for any near-term (<2016 launch) application of nuclear propulsion, solid core NTP rockets are the most likely candidate. However, it was agreed that NEP should be continued because of its very high specific impulse, and hence longer term payoff potential. Within NTP, the NERVA derivative was judged to be the quickest and least expensive approach, with PBR's and other solid core concepts being in the far term. The Advanced Development Panel noted that the liquid core concepts were probably as far off as the gas core concepts, but with less performance potential, so it is probably not worth pursing liquid core concepts at all."
Several specific comments and recommendations emerged from the Feedback meeting, including:(11)
"There is a need to work public perception and education... A policy of acceptable radioactive release in space must be established, with some sort of public acceptance, because it could affect which concepts can actually be used...
"The volume or surface area of the system launched may be more important than initial mass in low Earth orbit (IMLEO)."
By early 1991 the Steering Committee will be establishing a series of Technical Panels with government, industry and academic participants. These Technical Panels will include:
- Mission Analysis (NTP & NEP);
- Nuclear Thermal Propulsion (which may include specialized Working Groups on specific concepts);
- Nuclear Electric Propulsion;
- Nuclear Fuels and Materials (NTP & NEP).
These Panels are in addition to two Technical Panels which are already being established by DOE:
- Nuclear Safety Policy (including environment, manrating & reliability);
- Nuclear Propulsion Testing and Facilities (NTP & NEP);
Among the major tasks of these Technical Panels will be the establishment of consistent figures of merit for performance and risk for evaluation of propulsion alternatives, with particular attention to crew guidelines. The Steering Committee will also focus on definition of regular project milestones, as well as definition of key proof of concept experiments. Other work will include evaluation of the applicability of DOE fusion work, as well as the potential of the SP-100 Project to serve as a pathfinder for nuclear system launch, safety and environmental impact processing.
1. Clark, John S., (NASA Lewis), "Dear Colleague," letter to November Nuclear Propulsion Feedback meeting participants, 3 December 1990.
2. United States Senate, Committee on Commerce, Science and Transportation, Subcommittee on Science, Technology and Space, NASA Authorizations, 101th Congress, 2nd Session, 1990, pages 282, 297.
3. National Research Council, Committee on Human Exploration of Space, Human Exploration of Space: A Review of NASA's 90-Day Study and Alternatives, (National Academy Press, Washington, DC, 1990).
4. NASA Office of Exploration, Exploration Studies Technical Report, NASA Technical Memorandum 4075, December 1988.
5. David, Leonard, "Nuclear Power Backers Contend Mars Vessels Possible by 2005," Space News, 27 August 1990.
6. NASA, Space Exploration Initiative, Technology Needs and Plans, A Report to the United States Senate, (NASA, Washington, DC, Summer 1990).
7. House of Representatives, Appropriations Committee, Subcommittee on VA, HUD, and Independent Agencies, Departments of Veterans Affairs and Housing and Urban Development, and Independent Agencies Appropriations for 1991, 101st Congress, 2nd Session, Part 4, page 139.
8. Bennett, Gary, and Miller, Thomas, "Planning for the Space Exploration Initiative: the Nuclear Propulsion Option," Proceedings of the Eighth Symposium on Space Nuclear Power Systems, (Albuquerque, NM, 6-10 January 1991).
9. NASA, Report of the 90-Day Study on Human Exploration of the Moon and Mars, (NASA, Washington, DC, November 1989).
10. Clark, John S., (NASA Lewis), "Dear Colleague," letter to November Nuclear Propulsion Feedback meeting participants, 3 December 1990.
11. Clark, John S., (NASA Lewis), "Dear Colleague," letter to November Nuclear Propulsion Feedback meeting participants, 3 December 1990.