Nuclear Thermal Propulsion for Advanced Space Exploration M. G. Houts1, S. K. Borowski2, J. A. George3, T. Kim1, W. J. Emrich1, R. R. Hickman1, J. W. Broadway1, H. P. Gerrish1, R. B. Adams1. 1NASA Marshall Space Flight Center, MSFC, AL 35812, 2NASA Glenn Research Center, Cleveland, OH, 44135, 3NASA Johnson Space Center, Houston, TX, 77058 Overview have the potential for providing even higher specific impulses. The fundamental capability of Nuclear Ther- mal Propulsion (NTP) is game changing for Many factors would affect the development space exploration. A first generation Nuclear of a 21st century nuclear thermal rocket Cryogenic Propulsion Stage (NCPS) based on (NTR). Test facilities built in the US during NTP could provide high thrust at a specific Project Rover are no longer available. How- impulse above 900 s, roughly double that of ever, advances in analytical techniques, the state of the art chemical engines. Characteris- ability to utilize or adapt existing facilities tics of fission and NTP indicate that useful and infrastructure, and the ability to develop a first generation systems will provide a foun- limited number of new test facilities may en- dation for future systems with extremely high able a viable development, qualification, and performance. The role of the NCPS in the acceptance testing strategy for NTP. Alt- development of advanced nuclear propulsion hough fuels developed under Project Rover systems could be analogous to the role of the had good performance, advances in materials DC-3 in the development of advanced avia- and manufacturing techniques may enable tion. Progress made under the NCPS project even higher performance fuels. Potential ex- could help enable both advanced NTP and amples include cermet fuels and advanced advanced Nuclear Electric Propulsion (NEP). carbide fuels. Precision manufacturing will also enable NTP performance enhancements. Background NTP will only be utilized if it is affordable. Development efforts in the United States Testing programs must be optimized to obtain have demonstrated the viability and perfor- all required data while minimizing cost mance potential of NTP systems. For exam- through a combination of non-nuclear and ple, Project Rover (1955 – 1973) completed nuclear testing. Strategies must be developed 22 high power rocket reactor tests. Peak per- for affordably completing required nuclear formances included operating at an average testing. A schematic of an NCPS engine is hydrogen exhaust temperature of 2550 K and shown in Figure 1. a peak fuel power density of 5200 MW/m3 (Pewee test), operating at a thrust of 930 kN (Phoebus-2A test), and operating for an ac- cumulated time of 109 minutes (NF-1 test) [1]. Results from Project Rover indicated that an NTP system with a high thrust-to- weight ratio and a specific impulse greater than 900 s would be feasible. Excellent re- Figure 1. Schematic of an NCPS engine. sults have also been obtained by Russia. Ter- nary carbide fuels developed in Russia may The Nuclear Cryogenic Propulsion Stage potential reusability. A strategic method of Project development must be considered; assessing both commonality and scalability for minia- NASA’s Nuclear Cryogenic Propulsion Stage turization or growth. Other strategic consid- (NCPS) project was initiated in October, erations are the testing approach (a combina- 2011, with the goal of assessing the afforda- tion of terrestrial and space testing to validate bility and viability of an NCPS. Key ele- the engine) and the need for sustained fund- ments of the project include 1) Pre- ing. conceptual design of the NCPS and architec- ture integration; 2) Development of a High Power (~1 MW input) Nuclear Thermal The NCPS must show relevance to the U.S. Rocket Element Environmental Simulator space exploration goals and must provide a (NTREES); 3) NCPS Fuel Design and Test- development path toward a feasible, afforda- ing; 4) NCPS Fuels Testing in NTREES; 5) ble, and sustainable Nuclear Cryogenic Pro- Affordable NCPS Development and Qualifi- pulsion Stage. United States’ National Space cation Strategy; and 6) Second Generation Policy (June 28, 2010, pg. 11) specifies that NCPS Concepts. The NCPS project involves NASA shall: By 2025, begin crewed missions a large (~50 person) NASA/DOE team sup- beyond the Moon, including sending humans plemented with a small amount of procure- to an asteroid. By the mid-2030s, send hu- ment funding for hardware and experiments. mans to orbit Mars and return them safely to In addition to evaluating fundamental tech- Earth. The NCPS design will focus on ensur- nologies, the team will be assessing many ing maximum benefit to human Mars mis- aspects of the integrated NCPS, and its ap- sion, although the NCPS could have numer- plicability to enable NASA architectures of ous other applications as well. interest. NCPS mission analysis and definition will Pre-Conceptual Design of the NCPS and stay synchronized with the NASA Human Architecture Integration Architecture Team (HAT) for application to- ward future human missions and the currently The NCPS is an in-space propulsion sys- developing Space Launch System (SLS). The tem/stage using fission as the energy source NCPS will provide input to SLS future block to heat propellant (hydrogen) and expand it upgrades to enhance efficiency for volume though a nozzle to create thrust. The increase and mass constraints that will reduce the in engine performance available from even a number of launches for a mission utilizing the first generation NCPS would enable more NCPS. ambitious exploration missions, both robotic and human. It is the intent of the NCPS pro- NCPS system trade and analysis will natural- ject to develop a pre-conceptual design of a ly optimize the efficiency of the system to first generation stage with one or more nucle- accomplish the mission. The sensitivity of ar thermal rocket(s) capable of interfacing stage performance to specific impulse, engine with soon to be available launch vehicles and thrust-to-weight ratio, and other parameters possible payloads and missions. The design will be assessed to accomplish the mission must utilize technologies that are readily and eventually define the actual size and available with minimal risk to development. weight of the system. The design of the The design must take into account the devel- NCPS will favor proven and tested technolo- opment viability/feasibility, affordability, and gies and the design will also identify critical technologies that will be required for devel- The NCPS must take maximum advantage of opment. technologies, components, and subsystems that are developed elsewhere in the architec- A historical perspective for a common, scala- ture, as well as provide input and require- ble fuel element will help provide flexibility ments to those technologies to obtain the ca- in design. A developmental approach from a pabilities needed for effective integration of small demonstration to full size engines for NTP. The NCPS must also stay connected to human missions can provide a flexible path the SLS and upper Cryogenic Propulsion toward development and must be analyzed. Stage (CPS) projects to take advantage of During the Rover program, a common fuel common elements and to leverage technolo- element / tie tube design was developed and gies and configurations to reduce cost. used in the 50 klbf Kiwi-B4E (1964), 75 klbf Phoebus-1B (1967), 250 klbf Phoebus-2A The NCPS will also evaluate the potential of (June 1968), then back down to the 25 klbf Bi-modal Nuclear Thermal Electric Propul- Pewee engine (Nov-Dec 1968). sion (BNTEP). Although the design of such a system would likely be more complex than NASA and DOE are investigating a similar the design of either a pure NEP system or a approach: design, build, ground and then pure NTP system, there could be potential flight test a small engine that uses a common performance advantages. For applications fuel element that can then be bundled into a requiring small amounts of electricity, a larger diameter core producing greater ther- somewhat simpler system could be used to mal power output for the full size 25 klbf- provide that power, especially for missions class engines that can be used for human mis- that will have limited access to solar energy. sions. The NCPS must be optimally sized to Both propulsion-only and “bimodal” (propul- ensure that it provides significant benefits to sion and power) systems will be assessed un- a wide variety of potential exploration mis- der the NCPS Project. sions, but can be adjusted in size with mini- mal changes to avoid unnecessary cost in re- To support the NCPS design effort, available development if possible. analytical tools will be enhanced and refined. The Department Of Energy (DOE) has devel- The stage will need to leverage technologies oped sophisticated computer modeling tools from other programs and projects. Some spe- for nuclear system design. Since the initial cific technologies that are important for the fuel elements under consideration are very NCPS are listed below: similar to the past work accomplished under the Rover/NERVA and other programs, the o Cryogenic Storage (long duration NCPS will be able to take advantage of these storage, cryo-coolers, zero boil-off, available models. NASA has many rocket zero leakage) system simulation tools. These computation- o Automated rendezvous & docking al modeling tools from DOE and NASA will o Radiation hardening of electronics be used in conjunction to respond quickly to o Radiation shielding needed trade studies and mission analysis. o Enhanced System Health and Status Initial effort will focus on benchmarking of Sensor/ Post operation/test remote in- the nuclear models with test data and/or be- spection evaluation tween similar models. After confidence in the nuclear models has been established, an iterative design process will begin conver- gence of NASA and DOE models for best Development of a High Power (~1 MW in- design solutions.