
The Rolls-Royce Micro-Reactor could enable many space operations, including propulsion for satellites. Credit Rolls-Royce
View from North America
Dear MMTA Members,
Good afternoon from Cleveland Heights, Ohio this Labor Day.
And it is quite stunning weather here: sun shining, not too hot and no humidity. It is also the Cleveland National Air Show, with the incredibly noisy F/A-18 Super Hornets of the U.S. Navy Blue Angels roaring overhead at just a few hundred feet. Anyway, the noise they make got me thinking of rocket-ry, propulsion systems and, once again, space!
Nuclear Propulsion in Space
Whilst the concept of nuclear propulsion in space has been around for quite some time, back at least ‘til the ‘60s, a couple of years ago it appeared to have taken on a somewhat of a new lease of life. There are, currently, two main types of nuclear propulsion under further development for possible future use in space: nuclear thermal propulsion (NPT) and nuclear electric propulsion (NEP).
Nuclear Thermal Propulsion
If we exclude H.G. Wells1 and his thoughts that one might be able to use radium for propulsion in space, the concept (using nuclear bombs no less) was next championed by the physicist Freeman Dyson in the 1950s. Fortunately, the idea was dropped before the bombs themselves could be. After further thought as a method of propulsion, it was considered just a little too dangerous.
The idea was explored once again in the 60s, and through until 1972, as part of NASA’s Nuclear Engine for Rocket Vehicle Application (NERVA) programme.
During this period, a number of NTP propelled rockets were successfully built and tested by scientists at the Los Alamos National Laboratory (once home address of the Manhattan Project) outside Santa Fe in New Mexico.2
In the 1980s, NTP powered rockets (more powerful than chemical rocket engines) were revisited by the Space Nuclear Thermal Propulsion (SNTP) programme, part of US President Ronald Reagan’s Strategic Defense Initiative. Because of both cost and faulty fuel cells, the programme was subsequently axed in the 90s.3
More recently, NASA and the US Department of Energy “started working with industry to develop updated nuclear thermal propulsion reactor designs”4 and, in July 2021, they chose three reactor design concept proposals for a nuclear thermal propulsion system.5
Interest in the field, however, is not restricted just to the US. In the UK, the UK Space Agency, under the National Space Innovation Programme (NSIP), is currently looking at nuclear micro-reactors for use in space.6 As have both the European Space Agency and the Australian Nuclear Science and Technology Organisation (ANSTO) in Australia.

NTP System Source: NASA
So, what is NTP? Using NASA’s words, NTP “ … provides high thrust and twice the propellant efficiency of chemical rockets. The system works by transferring heat from the reactor to a liquid propellant. That heat converts the liquid into a gas, which expands through a nozzle to provide thrust and propel a spacecraft.”7
Nuclear Electric Propulsion
Nuclear electric propulsion systems, on the other hand, “ … use propellants much more efficiently than chemical rockets but provide a low amount of thrust. They use a reactor to generate electricity that positively charges gas propellants and pushes the ions out through a thruster, which drives the spacecraft forward.”8
Why Nuclear Propulsion in Space … Excluding the Many Military Reasons?
While I know it sounds silly to say so, space remains inconceivably big! And it can take a very very long time to get from one place to another, for example, from Earth to Mars. Using nuclear propulsion (perhaps a mixture of both NTP and NEP), NASA could both “reduce [up to 25%] significantly how long it takes to get there and back and carry greater payloads than today’s top chemical rockets.”9 “Combined with a much higher thrust-to-weight ratio, NTP could get a rocket to Mars in just 500 days, rather than 900.”10
How can they can achieve this? They can do so because, being more “energy dense”, NPT rockets are twice as efficient than their chemical peers. (I believe I may have touched on the subject before, but if I’ve not, the energy density of a fuel is the amount of energy stored per unit volume of it—whether gas, solid or liquid.)
In engineering terms, in a rocket, specific impulse measures its performance, i.e. how much thrust a specific amount of propellant provides. As NASA describes it: “The specific impulse of a chemical rocket that combusts liquid hydrogen and liquid oxy-gen is 450 seconds, exactly half the propellant efficiency of the initial target for nuclear-powered rockets (900 seconds). This is because lighter gases are easier to accelerate.” And: “When chemical rockets are burned, they produce water vapor, a much heavier by-product than the hydrogen that is used in a NTP system. This leads to greater efficiency and allows the rocket to travel farther on less fuel.”11
But what about NEP? Whilst, NTP can provide short high-thrust bursts, NEP can provide low thrust for longer periods. I.e., not only steady, reliable power, ideal for long trips, but also a possible source of such power once a destination is reached … if one is actually sought. That being the case, one likelihood is that, should research in this field prove successful, spacecraft may employ both NEP and NTP, one complementing the other. Or use the different modes of propulsion for different kinds of mission.
Metal Used
Quite apart from the metals used in the construction of the spaceship itself, it’ll probably come as no surprise that these propulsion units themselves use a number of interesting metals.

Energy Density Graph Source: ANSTO
Uranium aside, perhaps most obviously, looking at one of the companies (Materion) to have been co-awarded one of the Nuclear Thermal Propulsion Reactor Concept con-tracts through the DoE’s Idaho National Laboratory (INL), there is beryllium. In a typical NFT propulsion unit, the metal is to be found (as beryllium oxide—BeO) in the unit’s “[m] oderator assembly … to moderate or slow down neutrons to achieve the appropriate neutron energies/velocities for nuclear fission.”12 (As is zirconium in the form of zirconium hydride—ZrH.) Beryllium can also be found at the outer radius of the unit’s reflector: “[to] minimize neutron leakage … to reflect or “scatter” neutrons back into the core that would otherwise escape. The reflected neutrons can then cause more fissions and improve neutron economy of the reactor.”13
In the nozzle units (depending upon the type used), the other metals used include refractory metals like molybdenum and tungsten — the latter because of its high melting point and excellent thermal conductivity. And the former because it, too, has a high melting point and is more easily machinable than tungsten.
There is tantalum as well. In addition also to having a high melting point, it has good resistance to corrosion. Ceramic materials employed include zirconium dioxide for both its high melting point and low thermal conductivity and gadolinium zirconate because of its “exceptional thermal and chemi-cal stability, resistance to thermal shock, oxidation, and corrosion, and low thermal conductivity.”14 And, finally, there are the additives, like chromium and cobalt, to be found in the superalloys used.
Conclusion
Sadly, there’s not enough space to go into very much detail about both NTP and NEP systems, in particular these last. I believe that, as (and if) we try to explore further into deep space, embark upon further expeditions to the moon or, in-deed, attempt a peopled mission to Mars, we shall hear a great deal more about both modes of propulsion.
I shall, inconclusion, leave you with some words from the story about Rolls-Royce in The Engineer: “To realise global ambitions in space, reliable power and propulsion is needed. Limitations of existing power sources, such as solar, creates (sic) operational challenges to which nuclear fission reaction technologies are widely considered the solution, and an essential enabler for lunar surface activity.”15
In July this year, Rolls-Royce received £4.8 mln (c.$6 mln) from the UK Space Agency (UKSA) to test and demonstrate its modular Micro-Reactor for propulsion in space. It follow its work in collaboration with U.S. firm BWXT Advanced Tech-nologies LLC (BWXT) to find optimum technologies, also funded with £1.8mln from UKHSA. Last year Rolls-Royce demonstrated a concept nuclear module for powering a lunar module, also with UKHSA funding via the Lunar Surface Nu-clear Power Contract. so it’s investigations are twofold: into using its modular reactors for propulsion of space vehicles, and also into providing a reliable power supply for a stationary base.
For those of you interested in staying close to the cutting edge of all this, I would also recommend keeping an eye on DARPA’s (the US Defense Advanced Research Project Agency’s) Demonstration Rocket for Agile Cislunar Operations (DRACO) programme, the goal of which is to “demonstrate a nuclear thermal rocket (NTR) in orbit.”16
In the meantime, with the Blue Angels having flown their last set piece, from a gorgeous Cleveland Heights, Ohio, as always
I remain
Yours
Tom
©2024 Tom Butcher
Tom Butcher, formerly Director of ESG there, is now a Mar-keting Advisor at Van Eck Associates Corporation (“VanEck”). The views and opinions expressed herein are the personal views of Tom Butcher are not presented by or associated with VanEck or its affiliated entities. Please note that VanEck may offer investments products that invest in the asset class (es) or securities mentioned herein. This is not an offer to buy or sell, or a recommendation to buy or sell any of the securi-ties/financial instruments mentioned herein.
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1 H.G. Wells: The World Set Free, 1914
2NASA: 6 Things You Should Know About Nuclear Thermal Propulsion, December 10, 2021
3Physics World: Nuclear-powered spacecraft: why dreams of atomic rockets are back on, Richard Corfield, February 1, 2024
4NASA: 6 Things You Should Know About Nuclear Thermal Propulsion, December 10, 2021
5NASA: NASA Announces Nuclear Thermal Propulsion Reactor Concept Awards, July 13, 20-21
6The Engineer: Rolls-Royce awarded funding for nuclear Micro-Reactor, July 22, 2024
7NASA: Space Nuclear Propulsion,
8Ibid.
9NASA: 6 Things You Should Know About Nuclear Thermal Propulsion, December 10, 2021
10Physics World: Nuclear-powered spacecraft: why dreams of atomic rockets are back on, Richard Corfield, February 1, 2024
11NASA: 6 Things You Should Know About Nuclear Thermal Propulsion, December 10, 2021
12NASA: Components of a Nuclear Thermal Propulsion System
13Ibid.
14Refractory Molybdenum: Common Types of Rocket Nozzle Materials
15The Engineer: Rolls-Royce awarded funding for nuclear Micro-Reactor, July 22, 2024
16Defense Advanced Research Projects Agency: Demonstration Rocket for Agile Cislunar Operations (DRACO), Dr. Matthew D. Sambora,