A National Academy of Sciences Report recommends that NASA get moving to develop nuclear propulsion technologies that will get U.S. astronauts to Mars much faster than conventional chemical rockets. The NAS report says that in order for humans to reach Mars, major advances are needed in space nuclear propulsion technologies and soon. The study — undertaken by the Space Nuclear Propulsion Technologies Committee — was sponsored by NASA.

National Academies of Sciences, Engineering, and Medicine 2021. Space Nuclear
Propulsion for Human Mars Exploration. Washington, DC: The National Academies
Press. https://doi.org/10.17226/25977

NASA Will Need Nuclear Power to Get Astronauts on Mars

The NAS report says using nuclear propulsion technologies to support a human mission to Mars by the late 2030s will require NASA to pursue an aggressive and urgent technology development program.

The NAS committee doubles down on its use of term “urgent.” The report calls on NASA to commit by this time next year to conduct an extensive assessment of the merits and challenges of using different types of space nuclear propulsion systems and to making significant technology investments as a result over the next decade. Such a program must include prototype systems, ground testing, and cargo missions as a means of flight qualification prior to first crewed use

Challenges and Merits of Space Nuclear Propulsion Technologies

The report assesses the primary challenges, merits, and risks for developing a nuclear electric propulsion (NEP) system and a nuclear thermal propulsion (NTP) system for a human mission to Mars.

While NEP converts the thermal energy from a nuclear reactor into electrical energy to power electric thrusters, NTP uses the thermal energy from a nuclear reactor to heat a rocket propellant and create thrust. Each system has its own advantages and limitations for use in a crewed mission to Mars.

“Safely transporting astronauts to and from Mars will require advances in propulsion systems to develop spacecraft that are up to the challenge,” said Roger Myers, owner of R. Myers Consulting and co-chair of the committee that wrote the report.

“Nuclear propulsion systems have the potential to substantially reduce trip time compared to non-nuclear approaches. Synergy with other space mission applications and terrestrial power programs is also significant and will bring about added value.”

Studies comparing NEP and NTP systems are needed to assess the viability of each system for a crewed mission to Mars. Given the need to send multiple cargo missions to Mars prior to the first crewed mission, NASA should use those cargo missions as a means of flight qualification of the selected nuclear propulsion system before it is incorporated into the first crewed mission.

NEP – The fundamental challenge for developing an NEP system is scaling up the operating power for each subsystem, something that requires power levels that are orders of magnitude greater than have ever been achieved to date. Another challenge is developing a compatible chemical propulsion system to provide the primary thrust when departing Earth’s orbit and when entering and departing Mars’ orbit.

NTP – The fundamental challenge facing an NTP system is the ability to heat its propellant to the proper temperature, approximately 2,700 K. Other challenges include the long-term storage of liquid hydrogen in space with minimal loss; the need to rapidly bring an NTP system to full operating temperature, preferably in under one minute; and the need to develop full-scale ground test facilities that can safely capture the NTP exhaust.

The NAS committee recommends that the development of operational NTP and NEP systems include extensive investments in modeling and simulation. Ground and flight qualification testing will also be required.

• For NTP systems, ground testing should include integrated system tests at full scale and thrust.
• For NEP systems, ground testing should include modular subsystem tests at full scale and power.

Fuels – In the near-term, NASA and the Department of Energy (DOE), with inputs from other key stakeholders, including commercial industry and academia, should conduct a comprehensive assessment of the relative merits and challenges of highly enriched uranium (HEU) and high assay, low-enriched uranium (HALEU) fuels for NTP and NEP systems as applied to the baseline mission.

There are significant problems with using low enriched uranium as a lot more of it is needed to produce the same amount of power. This adds weight to the launch vehicle and produces a penalty in terms of payload.

Given the need to send multiple cargo missions to Mars prior to the flight of the first crewed mission, the committee also recommends that NASA use these cargo missions as a means of flight qualification of the space nuclear propulsion system that will be incorporated into the first crewed mission.

Cost Benefits of Nuclear Propulsion Systems

In Ars Technical, Eric Berger writes that NASA’s Space Launch System rocket, a chemical rocket, will have a carrying capacity of 105 tons to low-Earth orbit. NASA expects to launch this rocket once a year, and its cost will likely be around $2 billion for flight. So to get enough fuel into orbit for a Mars mission would require at least 10 launches of the SLS rocket, or about a decade and $20 billion. And that’s just for the fuel.

The bottom line Berger writes, “ if we’re going to Mars, we probably need to think about other ways of doing it.”

Berger also noted that nuclear propulsion requires significantly less fuel than chemical propulsion, often less than 500 metric tons.

What would be helpful for a Mars mission is that it would include several advance missions to pre-stage cargo on the red planet. Nuclear propulsion’s fuel consumption is also more consistent with the launch opportunities afforded by the orbits of Earth and Mars. During some conjunctions, which occur about every 26 months, the propellant required to complete a Mars mission with chemical propellants is so high that it simply is not feasible.

Getting NASA to Get a Move On

The problem with developing these types of nuclear propulsion systems isn’t technical feasibility, it is bureaucratic thinking by NASA. According to Berger, if NASA is to use nuclear propulsion in human missions during the 2030s, it must get started on technology development immediately.

So far, Berger writes, the agency has been “reticent” to move quickly on nuclear propulsion. This may be partly due to the fact that the space agency is heavily invested in the Space Launch System rocket and chemical propulsion needed for the Artemis Moon Program.

In recent years, NASA has not asked for nuclear propulsion funding. Congress has appropriated money for the effort anyway. In the fiscal year 2021 budget bill, NASA received $110 million for nuclear thermal propulsion development. Of this amount, $80 million is for work on a future flight demonstration program.

“Space nuclear propulsion technology shows great potential to facilitate the human exploration of Mars,” said Bobby Braun, director for planetary science at the Jet Propulsion Laboratory and co-chair of the NAS committee that wrote the report.

“However, significant acceleration in the pace of technology maturation is required if NASA and its partners are to complete this mission within the stated timeline.”

Military Interested in Space Nuclear Power

In a separate report on Ars Technical Berger writes that the US Department of Defense is getting interested in space-based propulsion. In May 2020. through a pre-solicitation, the US Defense Advanced Research Projects Agency announced its intent to have a flyable nuclear thermal propulsion system ready for a demonstration in 2025.

Through this Demonstration Rocket for Agile Cislunar Operations, or DRACO program, the defense agency seeks technology that will allow for more responsive control of spacecraft in Earth orbit, lunar orbit, and everywhere in between, giving the military greater operational freedom in these domains.

“Activity in cislunar space is expected to increase considerably in the coming years,” Maj Nathan Greiner, manager of the DRACO Program, told Ars. “An agile nuclear thermal propulsion vehicle enables the DOD to maintain Space Domain Awareness of the burgeoning activity within this vast volume.”

According to the solicitation . .

The space domain is essential to modern commerce, scientific discovery, and national defense. Maintaining space domain awareness in cislunar space – the volume of space between the Earth and the Moon – will require a leap-ahead in propulsion technology.

The goal of the Demonstration Rocket for Agile Cislunar Operations (DRACO) program is to demonstrate a nuclear thermal propulsion (NTP) system on orbit. NTP uses a nuclear reactor to heat propellant to extreme temperatures before expelling the hot propellant through a nozzle to produce thrust.

Compared to conventional space propulsion technologies, NTP offers a high thrust-to-weight ratio around 10,000 times greater than electric propulsion and two-to-five times greater specific impulse (i.e. propellant efficiency) than chemical propulsion.

The DRACO program anticipates two tracks.

• Track A will include the baseline design of a NTP reactor and culminate in a baseline design review.
• Track B will include development of an operational system concept to meet operational mission objectives and a demonstration system design that is traceable to the operational system but focuses on demonstrating the propulsion subsystem. Track B is anticipated to culminate in a technology maturation plan review for the demonstration system.

BWXT’s Nuclear Propulsion Efforts

In a separate interview with Berger, Jonathan Cirtain, president of advanced programs at BWXT Technologies, told Ars Technica’s reporter that DARPA’s decision to push forward with development of nuclear thermal propulsion comes as critical enabling technologies are maturing.

Cirtain’s company, which makes most of the nuclear reactors found on US Navy submarines and aircraft carriers, is working with NASA on the design of a reactor to enable Mars missions. (BWXT Space Nuclear Propulsion Program)

Conceptual Design of BWXT Nuclear Propulsion System; Image: BWXT file

A key technical milestone is the ability to manufacture metals which are extraordinarily resistant to heating. To operate efficiently, Cirtain said, an engine must be able to withstand huge temperature and pressure changes across just two meters in length. Hydrogen fuel is stored at just 19 Kelvin and heated to 2,500 Kelvin or higher.

Cirtaina also said that engineers designing nuclear reactor cores have access to computational power that allows them to iterate new designs—calculating such variables as neutron flux and fluid dynamics—quickly.

“Now, with supercomputers on your desk, you can go from years’ worth of calculation time to days, and iterate to a design solution much faster than you could previously.”

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