Name: Alisha S.
ProgramYear: 2006
Submit Date: Jun 4, 2006
Review Date: Jun 7, 2006

2006 NASA High School Scholar Program Final Project by Alisha Seam May 28, 2006 NUCLEAR SPACE SYSTEMS FOR PROPULSION AND POWER Spacecraft propulsion is used to change the velocity of spacecrafts and artificial satellites, or in short, to provide delta-v (6). Spacecraft propulsion is an active area of research. Most spacecraft today are propelled by rocket engines - by heating the reaction mass and allowing it to flow out the back of the vehicle. Contemporary chemical rockets use bipropellant or solid-fuel for launch, and most satellites and interplanetary spacecrafts have simple reliable monopropellant rockets or resistojet rockets (6). Newer geo-orbiting spacecrafts are starting to use electric propulsion for north-south station-keeping. A few small vehicles have experimentally used ion thrusters – another form of electric propulsion. Electrically operated momentum wheels are also used to change angular momentum and the repositioning of satellites without using chemical fuel. Despite enormous size and on-board fuel storage, the application of today’s chemical jet and rocket propulsion systems has been limited to relatively short missions because they drastically limit the size of the payload, velocity, distance, and in-flight maneuverability. The huge size of the Saturn V rocket was necessary to carry massive amounts of solid and liquid fuel for Apollo missions, which were merely attempting to send a small ‘crude’ spaceship to Earth’s moon. Limitations imposed by chemical propellants and systems have been even more severe on extended interplanetary missions, which have so far allowed only very small and unmanned robots and spacecrafts to land on Mars or fly-by various planets in our solar system. The fly-by spacecrafts, such as Voyager, could carry only miniscule amounts of chemical fuel, which restricted their maneuverability to, when necessary, just correcting their flight path remotely from earth. Although electrical systems based on solar arrays may be adequate for today’s satellites and modest interplanetary vehicles, they, despite the large surface area of solar panels, would be incapable of generating the powerful and sustained thrust required to propel and maneuver manned interplanetary spacecrafts of any significant size. Another problem is that a huge bulk of solar panels would be necessary to generate all the electrical power needed to sustain life on extra-terrestrial human settlements. Large geo-orbiting objects, such as the International Space Station (ISS), may also function with solar arrays, but they too are confined to earth’s orbit and, therefore, do not regularly require the use of thrusters. Chemical rockets, on the other hand, are severely impaired by their size and huge quantities of expendable fuel even for short durations, and, therefore, are unsuitable for the larger and highly maneuverable spacecrafts that are being envisioned to ferry substantial cargo and crew on extended inter-planetary travel and construction missions. What practical, viable and significant energy source can supplant chemical rockets and complement solar arrays for space propulsion for the next 10 to 50 years? The answer is clearly nuclear thermo-electric propulsion systems. Although many design, engineering and prototype development challenges still remain – especially with regards to compactness and safety of space-based nuclear reactors, nuclear propulsion of reaction mass (spacecraft and payload) is the most viable alternative available, given that most other more futuristic solutions - like the electromagnetic plasma-ion rocket, VASMIR – are still in theoretical to conceptual testing stages. As illustrated in attached chart, chemical power generation recedes as a function of duration of use in space, becoming extremely sparse after only a day or so. The chart also shows how nuclear reactors, when augmented by solar electricity generation, can over-compensate for the loss in chemical power, especially during extended periods of in-space or interplanetary missions. For all practical purposes, nuclear reactors are required when moderate to high levels of continuous power are required for extended periods of time. Given the compelling need for a viable alternative to massive multi-stage rockets based on chemical propulsion technology for extended inter-space travel and extra-terrestrial human settlements, expert strategists and planners are betting on nuclear thermal propulsion and power systems. Nuclear power supplies offer significant reductions in mass, especially when tens of kilowatts of power are needed for more than several days (2). The development of innovative and compact nuclear reactors is crucial for powering much larger and more sophisticated transport shuttles and unmanned exploratory spacecrafts, which will travel much longer and farther, and will be far more maneuverable than what exists today. NASA’s strategic initiatives and priorities, such as the establishment of permanent human habitats on Lunar and Mars surfaces, cannot be realized with contemporary chemical or solar powered systems due to their insurmountable physical and operational constraints. Following considerable civilian and military research and feasibility studies in 1950s and early 1960s, intensive government lead efforts to harness nuclear propulsion for producing thrust and electric power for aircraft, rockets and spacecrafts were abandoned (1). The two major American reactor development efforts in the 1960s were KIWI and NERVA and, in 1971, studies were conducted on a Shuttle-launched Nuclear Shuttle System, which used the PEEWEE reactor (3). Renewed interest in nuclear thermal propulsion was sparked by President George Bush in 1989 with his Space Exploration Initiative. More recently, due to its higher operating temperatures, the nascent Particle Bed Reactor (PBR) has been studied extensively as a superior alternative to the earlier solid core reactor models (4). The long term strategy contemplated by the Technology Capabilities Panel envisioned an evolutionary progression in which technology development in the 1990s would lead to component and flight system testing in the following decade, with the initial system supporting Lunar and Mars flight operations in the 2010-2020 period. System upgrades to the initial system would support more ambitious Mars operations in the subsequent decade, with continued development leading to the introduction of more advanced systems, such as Gas Core or Fusion in the post-2030 period. In a nuclear electric rocket, nuclear thermal energy is changed into electrical energy that is used to power one of the electrical propulsion technologies. So technically the power plant is nuclear, but the rest of the process is standard. A number of heat-to-electricity schemes have been proposed. In a nuclear thermal rocket a working fluid, usually hydrogen, is heated in a high temperature nuclear reactor, and then expands through a rocket nozzle to create thrust. The nuclear reactor's energy replaces the chemical energy of the reactive chemicals in a traditional rocket engine. Due to the high energy of the nuclear reactions compared to chemical ones, about 107 times, the resulting engine is at least twice as efficient chemical engines even considering the weight of the reactor, and even higher for advanced designs (6). In a 1989 conceptual design, as illustrated in attached chart, a space nuclear power system converts the energy from a nuclear heat source into electricity to power a particular load or application (2). Beside reactors, radioisotopes have been used as source for space nuclear power. In reactors, controlled fission of Uranium generates heat energy, which is then converted into either thermoelectricity using coolants (static) or ready electricity using turbine/alternators (dynamic). Wasted heat is rejected through a radiator. The electric power is “conditioned” according to the needs of various application (including thrusters) and payload on board a spacecraft. Radioisotope thermal generators produce heat by the natural decay of a radioisotope, such as Plutonium-238. The pebble bed reactor (PBR) or pebble bed modular reactor (PBMR) is an advanced nuclear reactor design. This technology claims a dramatically higher level of safety and efficiency. Instead of water, it uses pyrolytic graphite as the neutron moderator, and an inert or semi-inert gas such as helium, nitrogen or carbon dioxide as the coolant, at very high temperature, to drive a turbine directly. There are a whole host of other programs currently active to develop various forms and sizes of nuclear reactors for a variety of space applications. Some of the significant challenges in the design of a 10 megawatt reactor are as follows (1): • Technical complexities and uncertainties in R&D, engineering and design of high performance miniaturized of high-performance nuclear reactors and high-heat construction materials, equipment and facilities • Safety: design and thorough testing and risk assessment of all safeguards. Problem with radioactivity in case of mission failure • Logistical and technical difficulties in the construction/assembly of nuclear power station in space and on other planets • Transportation of fuel and disposal/recycling of spent fuel • Problems of carrying personnel and equipment for long flight durations In conclusion, a balanced combination of compact nuclear reactors and solar arrays will replace traditional jets and rockets that use chemical fuel to power future interplanetary spacecrafts and extra-terrestrial colonies. Nuclear space systems – such as plasma-ion thermoelectric thrusters and generators - will not only alleviate tremendous limitations of weight, duration and extensibility imposed by chemical rockets on interplanetary spacecrafts, they will also power permanent Lunar and Mars habitats, which are essential elements of NASA’s priorities and strategic initiatives. Of all the concepts and blueprints for future space propulsion and power systems on the table, nuclear-cum-solar option is the most realistic and viable alternatives for safe, efficient, portable, and, most importantly, limitless energy for space exploration and inhabitation. Decades of research and development, testing, and proven technological know-how are available in both nuclear reactor and solar array arenas. Beyond R&D, an intensive public and private effort is currently underway to design and engineer various prototypes of compact nuclear thermal reactors and propulsion systems. REFERENCES (1) Review of Manned Aircraft Nuclear Propulsion Program, a detailed study by Atomic Energy Commission and Department of Defense (1963) http://www.fas.org/nuke/space/anp-gao1963 (2) Background of Space Nuclear Power - Steven Aftergood, Science and Global Society (1989) http://www.princeton.edu/~globsec/publications/pdf/1_1-2Aftergood.pdf (3) Nuclear Thermal Propulsion, Federation of American Scientists http://www.fas.org/nuke/space/index.html (4) Particle Fuels Technology for Nuclear Thermal Propulsion, Horman, F.J., et al, AIAA/NASA/OAI Conference on Advanced SEI Technologies, Cleveland, Ohio, 4-6 September 1991, Paper AIAA 91-3457 (5) ADVANCED COMPACT REACTOR SYSTEM (1. Bartine, D.E, and Engle, W.W., "Space Reactor Shielding: An Assessment of the Technology," in Advanced Compact Reactor Systems, (National Research Council, Washington, DC, 1983) (6) Wikepedia http://en.wikipedia.org/wiki/Spacecraft_propulsion