New fusion rocket design could cut Mars trip to under 4 months

It seems too good to be true, but UK-based Pulsar Fusion has revealed its new Sunbird self-contained nuclear rocket tug that uses a fusion propulsion engine that could reduce a trip to Mars to under four months and Pluto to under four years.

Imagine a near-future where a spacecraft the size of a SpaceX Starship is setting out on a mission to Mars. Under current plans, that would mean a rendezvous with multiple rockets ferrying fuel from Earth to tank up the Mars ship for the year-long journey to the Red Planet.

However, things are different now. Instead of tankers, small, black, polygonal modules leave a stick-like space station. They dock with the stern of the Mars ship and, once in position, their exhausts glow blue, then violet as the ship accelerates faster and faster, the self-contained nuclear fusion engines burning longer and with more power than anything seen before as the ship slips into a transfer orbit that will see it reach Mars in record time.

Science fiction? Maybe not.

Founded in 2013, Pulsar Fusion has been on our radar for sometime and it's been the cause of much head scratching at New Atlas editorial meetings. On the one hand, it seemed to be a very serious company producing a solid line of electric space propulsion systems as well as a hybrid liquid/solid rocket engine and space-based nuclear fission reactors, along with getting some serious development money from the British government. On the other hand, it was also making noises about a nuclear engine project that sounded so crazy that it seemed like it had to be vaporware put out for publicity.

Called Sunbird, we now have more details on the nuclear fusion rocket project that is so far along that the company expects to demonstrate it later this year and begin orbital tests in 2027.

What made Sunbird seem impossible when we first heard about it was that it was based on a fusion rocket engine. If anyone has followed the attempts to build practical commercial fusion reactors, they'll be aware that the goal is right where it was back in the 1950s – always 20 years in the future. If that's the case, how could Pulsar Fusion be claiming to be building something that would make SpaceX's Starship look like a horse-drawn Zeppelin?

According to Richard Dinan, CEO of Pulsar Fusion, the answer is that Sunbird uses a completely different kind of fusion to what you find in a tokamak reactor – one with a completely different purpose in a completely different environment where weaknesses become strengths

In a tokamak reactor, fusion is based on Deuterium-Deuterium (D-D) or Deuterium-Tritium (D-T) reactions, where the hydrogen isotopes deuterium and tritium are subjected in confinement to immense pressures and temperatures until a reaction is set off that fuses the hydrogen atoms into helium, releasing energy and neutrons.

Since a tokamak has to run continuously and the energy is collected as heat to generate steam to run a turbine, the reactor has to be built on a gargantuan scale with giant magnetic coils, cryogenic coolant systems, heavily armored containment vessels, and lots of radiation shielding to deal with neutron erosion.

On the other hand, the Sunbird seems to have been designed in an absentminded fashion. Or rather, absentmindedness with a clear purpose in mind. Its fusion reactor works on the Dual Direct Fusion Drive (DDFD) principle, which harkens back to an early fusion reactor design that's been long abandoned except for laboratory research.

Basically, it's a linear reactor where, instead of going around in circles, there's a straight chamber with a pinched bit. It involves pulsed magnetic fields that accelerate plasma into the chamber, where the particles collide, producing fusion. It works, but it was rejected early on because it tends to leak at the ends.

However, what a power engineer would regard as a flaw, a rocket engineer sees as a feature.

In the DDFD, this leakage is actually desired because it's used for thrust. In addition, there's no need for much of the tokamak gear because it isn't being used to generate heat, it doesn't need to sustain the reaction indefinitely, and it doesn't even need nearly as much shielding.

That's because Sunbird uses what is called aneutronic fusion. Instead of fusing deuterium and tritium to make helium and neutrons, Sunbird uses a mix of deuterium and helium-3. For the mathematically minded and the masochistic, that's expressed by the formula D+3He→4He(3.6MeV)+p+(14.7MeV). Translated, it means that the deuterium and helium-3 fuse to produce the stable isotope helium-4, lots of energy, an alpha particle, and a positively charged proton. In practice, there are many fewer neutrons and a plasma that can be used for rocket thrust. The positive charge of the protons means that the plasma can be readily controlled.

That may sound a bit like the ion thrusters and variants used on today's deep space missions and ones that Pulsar Fusion builds, but there's a huge difference in scale, like comparing a Dyson ducted room fan to a Rolls-Royce Ultrafan jet engine.

Sunbird uses a converging pair of plasma streams confined by a proprietary technology that Pulsar Fusion really doesn't want to talk about. This is all a massive oversimplification based on incomplete knowledge, but the upshot is that, if Pulsar Fusion can get this to work, it has very serious implications for the future of space travel.

The biggest thing is that the Sunbird combines high thrust with high specific impulse, which is a measure of a rocket's efficiency. The higher the number, the greater the efficiency. For example, a chemical rocket can generate a million pounds of force, but its specific impulse, measured in how many seconds it takes to consume a unit of fuel, is only 500 seconds.

That means a lot of thrust but for only a short time. An electric propulsion system like an ion or Hall thruster has a specific impulse of up to 5,000 seconds but a thrust of only about the equivalent to the weight of a sheet or paper or a coin. Rockets using nuclear fission do better than chemical rockets with a specific impulse of up to 900 seconds, though the thrust on the experimental variants has only reached a mere 75,000 lb.

On the other hand, Sunbird is a bit startling – at least on paper. Its specific impulse is up to 15,000 seconds, which is three times that of ion drives. Thrust? Not so much on the current test visions at only 2.2 lb, but the theoretical limit is in the one million-pound range.

Long story short: Sunbird combines very high efficiency with very high thrust. Oh, and that plasma blasting out of the engine can be passed through a magnetohydrodynamic coil to generate 2 MW of electricity. That's pretty good, considering that the most powerful system is currently the solar array on the International Space Station (ISS) that manages 120 kW.

The Sunbird Migratory Transfer Vehicle, as the complete package is called, is a modular system that's designed to act as a tug rather than an integrated rocket engine that's part of a spacecraft. It's designed to be stationed in orbit on a refueling mothership. As needed, it can dock in groups with another craft and act like a sort of super high-tech cosmic outboard motor. With its greater thrust and efficiency, it could reduce the travel time to Mars to as little as three months and to Pluto to just four years.

Sunbird could also potentially propel much larger payloads and provide them with undreamed of energy levels for long-duration missions into deep space. Not to mention that each Sunbird is reusable and can be stationed not just in Earth orbit, but around the Moon and other planets as required.

The technology is currently under development by Pulsar Fusion using vacuum test chambers that are the size of a double-decker bus. Earthside demonstrations of Sunbird are expected this year with in-orbit tests of key components in 2027.

"I expect that, as of today, Sunbird is already being closely analyzed by our global competitors," said Dinan. "Pulsar has built a reputation for delivering real technology – not just concepts. We have recently commissioned not one, but two of the largest space propulsion testing chambers in the UK, if not all of Europe. Pulsar is now an international space propulsion testing powerhouse, and we have ambitious plans to expand rapidly. We actively welcome visits from appropriate partners and prospective international clients and look forward to sharing more exciting developments we have in the pipeline."

Source: Pulsar Fusion