A new project from Bletchley space propulsion firm Pulsar Fusion, revealed in March at the Space-Comm Expo in London, will instead attempt to achieve useful fusion in space. The Sunbird Migratory Transfer Vehicle – developed in secret over the past decade – is designed to dramatically shorten interplanetary travel times by harnessing the power of fusion, working as a tug to propel spacecraft to destinations throughout the solar system.
In-orbit demonstrations of some key components are planned for as soon as 2027. With fusion on Earth said to be ‘always 30 years away’, such a target might seem incredibly ambitious. But CEO Richard Dinan hopes that working in space could actually make some aspects easier.
“Where do you see fusion already? In space – the Sun. That's where fusion naturally occurs. It’s very unnatural to see fusion on Earth,” he says, speaking to Professional Engineering at the Excel Centre in London on 11 March.
In his earlier keynote, which was moved to a larger stage due to popular demand, he explained one of the reasons why the process – combining two light atomic nuclei to form a single heavier one, releasing huge amounts of energy – is so difficult on Earth. “People say ‘OK, that’s really hard, doing it in space must be a crazy proposition.’ But actually, there’s a lower bar in space, because half of the problem is doing it in the atmosphere,” he said.
Pulsar Fusion CEO Richard Dinan
The atmosphere needs to be removed to provide the necessary conditions for controlling fusion reactions, preventing particle collisions with ‘background’ gas molecules and ensuring the purity of the plasma, the ionised cloud of fuel where fusion happens. This means that experiments on Earth need giant vacuum facilities as large as the Excel’s auditorium, Dinan said, requiring huge amounts of energy to maintain the incredibly low pressures inside.
In space, however, there is no atmosphere to keep out. “The point that I make is, if you can do fusion, then doing it in space is actually easier than doing it in the atmosphere.”
He expands on this at the Pulsar Fusion stand, which throngs with visitors after the announcement. “Secondly, there's a scaling problem on Earth. The fact that the bigger it gets, the bigger it needs to be; the bigger it gets, the more expensive it gets… this scaling problem is kind of unavoidable at the moment on Earth.”
That is why terrestrial fusion projects such as ITER in France are so big, he claims, as it is the only way to guarantee the ‘Q’ – the ratio of output power to external heating power required to start and maintain the reaction. Larger facilities are able to target a Q of more than one thanks to stronger magnetic fields, larger reaction volumes and better control of the plasma.
On Earth, many projects use a fuel pair of deuterium-tritium, which offers a large ‘cross section’ – the probability that two nuclei will fuse when they collide – at temperatures of about 100 million ºC. That pairing produces high-energy neutrons, however – which, Dinan points out, are less than desirable on a spacecraft.
“I would not want a neutron source in space. It's going to spray neutrons into my spacecraft that I've spent all that money to put up there. It's going to degrade it,” he says. “You can't direct neutrons out the exhaust… no matter how clever you are, you're not going to be able to tell a neutron which way to go.”
Instead, Pulsar’s ‘aneutronic’ approach will use deuterium and helium-3 fuel to power its Dual Direct Fusion Drive (DDFD) engine, creating protons that would be channeled out of the back of the spacecraft to generate thrust. The company hopes that this approach, which will involve solving a number of major engineering challenges such as directing particles and handling plasma instabilities, could also make it more economical by creating a ‘self-sustaining’ fuel supply, potentially using helium-3 from tritium decay or fusion byproducts.
‘Fusion is game-changing’
If Pulsar manages to overcome the significant challenges of fusion-powered spacecraft propulsion, the opportunities could be vast. Potential applications described on the company website include rapid cargo delivery to Mars, probe deployment to Jupiter, transfer from low Earth orbit (LEO) to the Moon, and transport of tools and materials for asteroid mining.
An animated video released as part of the announcement shows how the system would work: rocket-propelled spacecraft would make their own way to orbit, where they would meet a space station-like dock holding multiple Sunbirds. One of these would then detach and use the company’s Hall effect thrusters to gently manoeuvre to the spacecraft, attaching close to its base. It would then activate the DDFD, racing away from Earth. At its destination, the Sunbird would disconnect again, docking with another nearby station to refuel. It could also provide 2MW to the payload on arrival.
By stationing the Sunbird in orbit, Pulsar aims to lower the ‘delta-V’ – the change in velocity needed to complete a mission – for spacecraft. Just launching to LEO requires about 9.4 km/s, which could be handled by the launch vehicle. By avoiding that delta-V, the Sunbird could then provide the power for the remaining journey.
“If I save you time and space, I save you weight, I save you fuel; I save you fuel, I save you weight; and that delta-V starts to go in your favour,” Dinan says.
An artist's illustration of a look inside Pulsar Fusion's Sunbird
Once in space, many spacecraft use solar panels to power propulsion. These face limitations as they travel further from the Sun, however, which Pulsar hopes to avoid with its compact, linear fusion reactors.
“I'm not naive as to the challenges that things like fusion in space are going to require, because that is a really enormously ambitious task,” Dinan told the keynote audience. “But the reason we at Pulsar remain so focused on fusion is because at some point, as our satellites and payloads get bigger and we start going into deep space missions, we have a problem, and that is the fact that solar intensity starts to fade away. So these enormous solar panels, which can drive your ion thrusters or your conventional propulsion engines, are going to need to be replaced by something.”
He continues at the booth: “If we want to travel meaningful distances with hardware in space, if we really are going to do all that stuff, then fission is good. Fusion is game-changing… if the engines are there waiting, and you can just get yourself to low Earth orbit, attach with a better propulsion system (which is only for a vacuum) then as long as humans can do fusion, then it will be much easier to do – for some of those reasons – in space.”
With high specific impulse of 10,000 to 15,000 seconds and exhaust speeds of up to 147km/s, Pulsar hopes to halve journey times to solar system destinations. This could mean flights to Mars in less than six months.
Turning up the heat
The most immediate question about the project is how it will achieve net gain (meaning a Q of above one), says Professor Gianluca Gregori, professor of physics at the University of Oxford, who is not involved in the work. That elusive goal has only been briefly achieved on Earth.
Pulsar’s aneutronic approach “brings an additional level of complexity… because the cross section requires much higher temperature for the plasma to be sustained,” he adds.
Some aspects of Dinan’s claim that “doing it in space is easier” do have merit, he says. “You could have a situation where some of the processes could be easier to achieve in space because there's no gravity, there’s no material around you.”
But Pulsar’s project will be “even more challenging, because they’re [aiming] to do something in space, where you cannot bring all the equipment you want to bring, because you are limited by the payload requirements… that doesn’t mean it shouldn’t be tried, but it has to be put into perspective.”
Tackling this issue will involve having to miniaturise the entire fusion plant so it can be brought from Earth to space, he says. If Pulsar is successful, he predicts wider application in the energy sector.
Despite the challenges it is “a very cool idea,” he concludes. “If they’re successful it should be pursued. I'm very positive that the research should be done. Whether or not it can be completed in our lifetime, I don't know.”
‘I don't want to be the sucker who's obsessed with fusion’
A prototype Sunbird should cost £70m to build, Dinan claims. The technology could also be applied in smaller craft for defence applications where speed is key, he adds.
The first simulation models have been completed and the company will soon start testing technology in two recently-commissioned propulsion testing chambers. The in-orbit demonstration in 2027 will not involved an entire Sunbird, but is aimed at proving the accuracy of the company’s simulations. “If it is accurate, [we’re] in a very good position to say to customers ‘We can save you all this money, now off that we will raise a prototype,’” Dinan says.
The company will react to clients’ demands, he adds. “If clients say ‘Richard, we don't want fusion’, I want to stop quickly. I don't want to be the sucker who's obsessed with fusion, trying to make fusion happen, even if no-one wants it.”
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