A Path Toward Nuclear Thermal Spaceplanes

This study seeks to determine if it is both technically and regulatorily feasible to develop and commercially operate a reusable launch vehicle between ground and low earth orbit using a nuclear thermal propulsion engine exclusively.  In other words, this report attempts to answer the question, “Is a launch vehicle utilizing exclusively nuclear thermal propulsion feasible, from technical and (in the US) regulatory perspectives?” To state the conclusion up front: probably yes, though more research and optimization is needed.

The regulatory analysis is necessary because operating an illegal launch vehicle has about the same chance of entering commercial operation as one failing to produce any thrust or lift.  Additionally, regulations impose requirements the technical analysis must meet.  Most notably, regulations and technical realities limit the practical options for nuclear fuel to High-Assay Low-Enriched Uranium (HALEU).  Since legislation is more malleable than the laws of physics, a regulatory analysis like this is necessarily constrained to the regulations existing in a certain place and time.  This report analyzes the situation in the country in which this work was conducted (the United States of America) at the time this report was completed (May 2025).

The study identifies two regulators with jurisdiction over a commercially operated nuclear thermal launch vehicle:
● The Federal Aviation Administration (FAA), within the Department of Transportation, has jurisdiction over the launch vehicle during commercial launch operations, from the moment operations for a given flight begin to the moment they end.  Although certain details change, most notably the presence of regulatory concerns not significant for launch vehicles using chemical propulsion, the core regulatory structure does not.  The spaceport this vehicle launches from will also need to be licensed by the FAA.
● At all other times, the Nuclear Regulatory Commission (NRC) has jurisdiction over the nuclear thermal engine.  In particular, only NRC jurisdiction applies during ground tests, nuclear fuel handling, and maintenance.  These operations must be accounted for during licensing, but the launch vehicle and the spaceport hangar it rests in are licensed in the same manner as any other non-power-generating reactor.

Our technical analysis demonstrates, despite the constraints of HALEU's reduced enrichment and the added mass of radiation shielding, which constraints do not exist for on-orbit nuclear thermal propulsion, achieving a thrust-to-weight ratio high enough to allow launch remains technically feasible.  A significant difference from historical nuclear thermal propulsion engine designs is the insertion of a protective liner between the nuclear fuel and propellant, avoiding ablation of the nuclear fuel and thereby dramatically reducing radioactive content in the exhaust.  While this approach maintains the core principles of the MITEE concept as published by the Department of Energy, this report identifies performance enhancements through targeted modifications, which may be explored further in subsequent work.  To establish practical benchmarks and create a roadmap toward implementation, this report also includes an analysis of a spaceplane flight profile utilizing this propulsion system, to confirm the performance thresholds required for successful operation.

Among the most promising pathways for future work are:
● Improving the engine’s nuclear fuel geometry and hydrogen flow path to put more heat into the hydrogen per unit mass of engine,
● Redesigning to operate at higher temperatures, perhaps 3,600 Kelvin instead of the 3,000 Kelvin considered in this report, to achieve higher efficiency,
● Use of better materials than this study was allowed to cite, such as radiation shielding that provides lower kilograms per square meter shielded for the same dose reduction, and
● Testing to confirm simulated performance, which will be essential to obtain the licenses required to begin operations.

More development is needed to reduce this concept to practice, but this report disproves several of the common assumptions that would, were they true, make this approach impossible.  The result lies within the bounds of what physics, and the laws of the United States of America as of May 2025, allow.