Limitations of nuclear propulsion for earth to orbit

An assessment of nuclear propulsion for Earth to orbit (ETO) missions is presented. Emphasis is placed on nuclear thermal rockets, but other propulsion schemes are briefly addressed. The NERVA, CERMET and particle bed reactor (PER) designs are considered and compared to chemical rocket performance. It is shown that nuclear propulsion schemes have some disadvantages for the ETO mission. This is in contrast to many space missions, which require nuclear propulsion to achieve mission goals. Also, first order system analyses are presented to show the effects of engine performance and propellant storage on required lift-off mass and final mass in orbit. The analyses indicate that specific impulse advantages from a nuclear thermal rocket (NTR) are offset by bulky storage of hydrogen propellant. It is also evident that the NERVA and CERMET type reactors do not merit further consideration for ETO unless breakthroughs in power density and specific power are achieved. The analysis does indicate a superior performance for an unshielded PER design operating at 3200 K, when compared to the Space Shuttle Main Engine (SSME). However, the PER system mass growth is much more sensitive to performance due to propellant density. Background and Historical Perspective Shortly after the achievement of the first fission chain reactor in 1942, members of the Manhattan District Project, including renowned scientists Enrico Fermi and Robert Serber, speculated about the possibility of nuclear fission powered aircraft and rockets.' In May of 1946, the Nuclear Energy for the Propulsion of Aircraft (NEPA) program began under the direction of the United States Air Force (USAF). The NEPA program was directed out of the Oak Ridge National Laboratory. The NEPA program ended in 1951, but much of the work continued in the joint Atomic Energy Commission (AEC) / USAF Aircraft Nuclear Propulsion (ANP) program that followed. ANP had ambitious goals of powering an aircraft using nuclear energy to heat air as the propellant, affording an indefinite range and hover capability, or at least until supplies of coffee and donuts on board were exhausted. The program considered various engine cycles including direct air heating and indirect cycles. ANP was cancelled in March 1961. It is important to note that concurrent to ANP, the Union of Soviet Socialist Republics (USSR) attempted development of a nuclear bomber aircraft, which shared a similar fate. The technical achievements, shortcomings, politics and remaining challenges are well documented in References 1-4. Early in the NEPA program, studies were done to consider nuclear ramjets and rockets in addition to nuclear aircraft. The Lawrence Radiation Laboratory (now Lawrence Livermore National Laboratory) was assigned the responsibility of developing a nuclear powered ramjet under the codename, Pluto project. During the Pluto project, a reactor was developed, built, and demonstrated in a ground test." The investigation of nuclear energy for rockets ended up in the hands of a small group of advocates, primarily at the Los Alamos Scientific Laboratory, who were tasked with evaluating nuclear propulsion for an inter-continental ballistic missile. These investigations led to the USAF sponsored Rover Program, which included nuclear thermal rocket (NTR) development work and the before mentioned Pluto project work. The NTR work for Rover was performed at the Los Alamos Scientific Laboratory and at Livermore. Livermore Laboratory was later directed to focus exclusively on the Pluto project portion of Rover. Senior Researcher, Member AIAA t Senior Researcher, Member AIAA ^Research Scientist, Univ. of Alabama in Huntsville on IPA to NASA MSFC, Member AIAA §Deputy Director, Propulsion Research Center, Senior Member AIAA Copyright © 2001 by the American Institute of Aeronautics and Astronautics, Inc. No copyright 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 herein for Government purposes. All other rights are reserved by the copyright owner. (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. Rover later evolved into the Nuclear Engine for Rocket Vehicle Applications (NERVA) project under the direction of NASA in 1958 as part of the space exploration program." NASA continued the NERVA project until 1972. In that time, NERVA accomplished 28 full-power tests with restarts, development of a high temperature reactor fuel and vacuum Isp values of approximately 825 seconds. The NERVA project undoubtedly represents the largest and most productive effort to date on nuclear propulsion of any type, notwithstanding marine propulsion devices such as used in submarines and ships. The NERVA project is responsible for the highest power nuclear reactor ever built, the Phoebus II-A, which was designed for 5000 MW and operated at a power of 4080 MW. Subsequently, the USAF began the Space Nuclear Thermal Propulsion (SNTP) program or Timberwind (unclassified name), which lasted until 1993. The focus of this program was to develop high power density reactors so that thrust to weight ratios required for orbital transfer and potentially upper stage propulsion might be achieved. The reactor type used in Timberwind is called a Particle Bed Reactor (PER) and will be discussed later in this paper. While the Timberwind program did not accomplish the level of development that the NERVA program did, it does represent a significant effort in reactor development and did achieve nuclear tests of single fuel elements, criticality experiments for a prototype 1000 MW core and thermal hydraulics tests in multiple fuel elements to lend observations regarding potential power densities of a PER. Other notable efforts for nuclear propulsion have taken place, but NEPA/ANP, Rover/NERVA and SNTP (Timberwind) were programs that included reactor development and, therefore, represent the most significant contributions to our knowledge on potential flight systems. Of interest in this paper is the number of conceptual vehicles that proposed the use of NTR propulsion for Earth to orbit (ETO) applications. The most comprehensive and ambitious of these concept vehicles is the Aerospace Plane with Nuclear Engines (ASPEN) proposed by Bussard in 1961 in a then classified report. The ASPEN vehicle was a single stage to orbit (SSTO) concept with up to 17% payload fraction to low Earth orbit (LEO). ASPEN proposed an advanced turbojet boost to a ramjet mode to an NTR to orbit. Due to the early timing of the ASPEN concept, much of the weight and sizing information used for ASPEN was speculative and optimistic when compared to technology of the present day. Since it was proposed, many deficits of the analyses have been identified including a subsequent report (ASPEN II) by Bussard, which implies that a 2-stage ETO system would be more appropriate to avoid radiation hazards that were not mitigated in the first proposal. A review and an indepth analysis of the ASPEN vehicle are presented in Reference 8. Other concept vehicle proposals for NTRs in the ETO mission include the Saturn C-3N, a 1961 proposal for the Saturn vehicles to use a 3 stage NTR based on the NERVA technology.' Also, the RITA C, a 1963 concept vehicle by the Douglas Company that proposed an all NTR powered heavy lift vehicle (454,500 kg to 325km orbit).' In recent years, several papers with NTR propulsion schemes for ETO have been published.' Introduction A compelling reason for investigating the applicability of nuclear power in rockets is the incredible energy density of nuclear fuel when compared to chemical combustion energy release. While combustion of hydrogen and oxygen (H2 and O2), has an energy release of 13 MJ/kg, the fissioning of U yields approximately 8 x 10 MJ/kg and the fusion of deuterium and tritium particles (T+D) has a 3.4 x 10 MJ/kg yield. Use of fission energy represents the nearest term application of nuclear power for propulsion application. Several fission based propulsion schemes have been proposed for ETO, including airbreathing, pulsed nuclear explosions and NTRs. NTRs have the highest applicability and, therefore, the bulk of discussion in this paper is focused on them. A small section at the end of the paper addresses other proposed concepts for ETO using different nuclear energy schemes such as airbreathing nuclear rockets, fusion-electric and nuclear explosions. NTRs pass a cooling fluid (or propellant) through a core of material that has been heated by fission. This makes the NTRs effectively a heated gas rocket. Since the NTR is a heat transfer rocket, the propellant can be selected to maximize performance of the propulsion system. It should be noted that this is the primary distinguishing feature of an NTR in contrast to a chemical propulsion rocket. With the present limitations of materials, NTR gas temperatures cannot exceed chemical propulsion gas temperatures (-3650K for H^O^, but because of the selection of propellant, exhaust velocity of chemical American Institute of Aeronautics and Astronautics (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. rockets can be greatly exceeded. This is true when a low molecular weight propellant is chosen compared to the molecular weight of chemical rocket combustion products. As seen from Equation (1), lower molecular weight (MW) and higher temperature increase the exhaust velocity. Equation (1) presents the relationship for an ideal gas flowing through a nozzle from a stagnation pressure and temperature, p0 and T0, to an ideally expanded exit pressure, pe.