Atomic rockets are back

By Rob Coppinger,
Published by The Royal Aeronautical Society, 15 September 2023

While the current Oppenheimer blockbuster film focused on the destructive power of nuclear weapons, more peaceful uses of atomic propulsion for space exploration are now gaining once again momentum. ROB COPPINGER reports.

Nuclear fission and fusion power propulsion are under investigation in Europe and the US with an in-space engine demonstration planned by 2027 – with the news last month that Lockheed Martin had been selected to develop a nuclear thermal propulsion system for DARPA’s DRACO programme (see below).

Nuclear propulsion is attractive as it is far more efficient and powerful than conventional chemical rocket engines – with nuclear thermal propulsion (NTP) having twice the propellant efficiency of chemical rockets. SpaceX plans to use its Starship Heavy rocket, propelled by liquid oxygen and methane, to take Elon Musk’s colonists to Mars. NASA’s decades of research have also concluded that NTP is the best choice for crewed missions to the red planet with its Human Exploration of Mars Design Reference Mission 5.0, published in 2009, making clear NTP’s advantages. With NTP, a propellant, liquid hydrogen, is propelled by the heat from a nuclear reactor. It offers a high thrust-to-weight ratio around 10,000 times greater than nuclear electric propulsion (NEP) and two-to-five times greater specific impulse than in-space chemical propulsion. An NEP system uses a nuclear reactor to provide electricity for an electric engine that accelerates an ionised propellant, typically xenon in Hall-Effect electric thrusters, to achieve thrust. With NEP, the engine can provide extremely high specific impulse of more than 10,000 seconds but with low thrust and limits on mass to power ratios.

Britain joins the atomic space race

Pulsar Fusion’s concept for a NEP deep space rocket. (Pulsar Fusion)

It is not just NASA. Last December the UK Space Agency (UKSA) awarded funding to the UK rocket company Pulsar Fusion to develop nuclear fission-based power systems for NEP. “These things are match funded,” said Pulsar’s founder and chief executive officer (CEO) Richard Dinan. “We are going to be putting in way more [than the UK government] because it benefits us [and] we don’t want to be stopped by the level of the funding that the [UK] space agency is offering.” He also has concerns about UK regulation.

Dinan points out that UK rules do not include launching fission cores into space, while the US regulatory regime does. “The one thing that needs to change is the regulation. At the moment is doesn’t really consider people launching fission cores. It’s something that in the US you can do. It’s just the UK regs are pretty prohibitive all round,” he explained. For the UKSA funded work, Dinan and his team are working with the Nuclear Advanced Manufacturing Research Centre (AMRC) at the University of Sheffield. “The Nuclear AMRC have been brilliant, really amazing, knowledgeable,” he said.

Pulsar is working towards plasma experiments using a test chamber that it took delivery of this year. “We’ve just taken delivery of a seven metre chamber, which we’re going to put a two tesla electromagnetic field in so we can do real size, real world plasma shots at Pulsar,” Dinan explained. “We’ll start with things like Argon, we won’t be using Tritium or anything, but just so we can model it to scale.” Pulsar sells a 1.5kW Hall-Effect Thruster (HET), but nuclear power can transform the thrust levels that have been seen historically from Hall Effect technology. “We know that [nuclear] propulsion can offer several hundred times the particle exhaust speed compared to the Hall Effect Thruster,” said Dinan.

Pulsar Fusion, as the name indicates, wants to use fusion reactors for propulsion systems for interplanetary travel. Its plan includes developing a nuclear fusion propulsion prototype by 2025 for a static demonstration and then from 2027 to manufacture a rocket for in-orbit testing. “Fusion propulsion can literally be directed and fired. It also doesn’t have to be efficient. We can actually lose money and energy because speed and space is fungible with money. I can charge you more if I can get you there faster.” For Dinan, the enormous thrust levels that can be achieved mean a speed of service that has a good value proposition.

Rolls-Royce aims for Mars

Rolls-Royce’s small modular reactors, along with its nuclear submarine propulsion experience, could be leveraged for long-duration space missions. (Rolls-Royce) 

In April this year, Rolls-Royce participated in the Space Foundation’s National Space Symposium, a civil and military space exhibition and conference held in Colorado every spring. It had already launched its Micro-Reactor concept in 2021, with plans for a first of a kind space model ready by 2029. The UKSA has awarded Rolls-Royce £2.8m for its micro-reactor work. However, Rolls-Royce was not available for comment.

Two months before Rolls-Royce staff went to the NSS, on 9 February a systems engineer with Lockheed Martin UK, Sam Brass, presented to the RAeS Bedford branch the results of a joint Lockheed Martin UK and Rolls-Royce study. Brass described two fission propulsion technologies and compared one, NTP, to conventional chemical engines and their respective performance under a Mars mission scenario. The joint study used a Rolls-Royce NTP engine and assumed the dry mass of the spacecraft was always the same, whether it was nuclear or chemically propelled.

The Rolls-Royce NTP engine had a mass of 7 tonnes, an output power of 200kW and a lifespan of 10 years of continuous use. Brass said that Rolls-Royce had been investigating space faring nuclear reactors since the 1980s and that the recent work had included designing spacecraft structures. The Rolls-Royce NTP spacecraft would protect the crew with high-density shielding and the fissile fuel storage would be located between the reactor and the crew cabins. The reactor would be cooled using radiators, radiating heat into space just like the International Space Station.

The conclusions of the study were that NTP would allow 19% more cargo to be delivered and would reduce the transit time to Mars by 17%. Using chemical propulsion, the journey, the study only examined a one-way trip, took 192 days, while the NTP trip required just 160 days. Brass added that these journey times did not include any optimisation. The departure dates for a cargo mission were 2035 and for the crew mission, 2037. Another conclusion was that future work should study NEP because of the huge specific impulse that that propulsion system provides, up to 10,000 seconds compared to NTP’s 800-900 seconds.

NASA goes back to the future

NASA crewed atomic rocket concept from 1964. Note crew hab module and spaceplane well away from the engines. (NASA Glenn Research Centre)

Meanwhile, in January this year, NASA announced that it was to work with the US government’s Defense Advanced Research Projects Agency (DARPA) on that agency’s Demonstration Rocket for Agile Cislunar Operations (DRACO) programme. NASA nor DARPA were available for comment. NASA is interested in nuclear propulsion because it means a truncated journey to Mars, cutting it down from the nine months with conventional chemical rockets. NASA is joining DRACO for phases two and three of the DARPA NTP project. The phase one for DRACO was 18 months long and saw General Atomics work on a preliminary design of an NTP reactor. But NASA has a 60-year-old history with nuclear propulsion that dates back to 1961, when the space agency embarked on the Nuclear Engine for Rocket Vehicle Application (NERVA) programme.

For NERVA, over several years nuclear reactors and rockets were designed, built and tested. Sixty years on and this year NASA is funding NTP work at the University of Florida under the agency’s NASA Innovative Advanced Concepts programme. The University of Florida research predicts that a “bimodal design” enables a journey time of 45 days to Mars. A bimodal NTP produces electricity for the spacecraft’s onboard systems as well as delivering the thrust. NASA and DARPA are aiming to ground test a nuclear thermal rocket engine in 2025 with an in-space demonstration planned by 2027. On 26 July, Lockheed Martin was named as the prime contractor to develop the DRACO uncrewed spacecraft with BWX Technologies providing the reactor and fuel.

DARPA is aiming to test DRACO in space by 2027. (Lockheed Martin) 

DARPA leads the DRACO programme, managing the integration of the engine with the spacecraft, and is overseeing the entire NTP demonstrator including the fission reactor. Under the DRACO agreement with DARPA, NASA’s Space Technology Mission Directorate will lead nuclear thermal engine technical development. In its January 2023 announcement, DARPA’s DRACO programme manager, Dr Tabitha Dodson, said: “We will conduct several experiments with the reactor at various power levels while in space, sending results back to operators on Earth, before executing the full-power rocket engine test remotely.”

In parallel, under its own independent NTP efforts, NASA had announced in July 2021 that it had selected three NTP reactor design concept proposals along with related contract awards worth $5m each. The winning companies were naval nuclear reactor maker BWX Technologies, General Atomics Electromagnetic Systems and Ultra Safe Nuclear Technologies. “These design contracts are an important step towards tangible reactor hardware that could one day propel new missions,” said NASA’s then space technology mission directorate associate administrator, Jim Reuter, in the agency’s July 2021 announcement.

Fuel for this NASA research was delivered in December 2021 by BWX Technologies. The Lynchburg, Virginia based company manufactures nuclear reactors for the US’ Navy’s submarines and aircraft carriers. General Atomics is the company linking the civil and defence spacecraft nuclear reactor work, it has been contracted by NASA and the DoD separately.

Dinan is aware of the interest from defence departments and ministries. “There’s a defence application and it’s worth [researching] because if you can get to point X quicker than anybody else [it has value],” Dinan explained. “The other nice thing about fusion is it’s not dependent of on one of these very fast propulsion designs [where] you have to sort of slingshot round planets or you have to all these very elegant little tricks.” For Pulsar’s fusion plans, Dinan is working with Princeton Satellite Systems, a small New Jersey based firm. The goal is to create a fusion powered propulsion system whose propellants are protons, the subatomic particles.

“If you can do it, that would be most ultimate source of propulsion,” said Dinan. “What Princeton has studied and what Pulsar are doing is ‘Linear Fusion,’ so straight electromagnetic field lines and you are using deuterium and helium-3 because that reaction is aneutronic,” which means it does not produce many neutrons. Neutrons are produced in very large quantities by other nuclear reactions, and they are high energy particles that will damage materials they encounter. This aneutronic fusion process produces protons that are then fired “out the back of your electromagnetic field”. The limitations of NEP’s gaseous propellant source or NTP’s liquid hydrogen are overcome.

Other Studies

ESA NTP concept. (NASA)

A nuclear powered cis-lunar transportation capability has been proposed by the Mitchell Institute of Aerospace Studies, a think tank that is an affiliate of the US Air Force. In the Institute’s January 2022 policy paper, its senior fellow for space studies, Christopher Stone, sets out an argument about a new space race. He claims Russia and China are already developing nuclear propulsion propelled satellites for manoeuvre warfare in space in the 2030s. While the UKSA funds domestic research and Rolls-Royce is sending people to the US, which has its own substantial projects underway, the European Space Agency (ESA) is funding NEP studies.

Electric propulsion for cislunar and Mars crew and cargo transportation and exploration missions beyond Mars are the focus. Three organisations were awarded €250,000 each for 11 months work and that started in the first and second quarters of this year, explained ESA’s Future Space Transportation Preparation senior adviser, Jerome Breteau. The first reports come at the halfway stage, so ESA will have received those already, with each firm providing a final report next year. The three organisations are German space firm OHB’s Czech subsidiary OHB Czechspace, French engineering company Tractebel Engineering and French Alps’ region laboratory CNRS. The studies will include the safety of NEP in space and launching these nuclear systems from Earth. No follow-on projects are planned for, but the results will be used as the basis for other activities.


NASA NTP concept. (NASA)

Once a Cold War curiosity developed in the belief that the Third World War could be fought in orbit, nuclear propulsion is now returning centre stage for in-space propulsion. Once the arena of only small chemical rocket designs and then electric Hall Effect Thrusters with their gaseous propellant, the nuclear rockets of the 21st Century could send military satellites on their wide-ranging patrols around the Earth, crew and cargo to Mars and robotic probes to the outer planets, and some of it could have a UK badge on it.

See: Original Article