Star potential

Fusion propulsion has been the space industry’s Holy Grail ever since it was first proposed in the late-1950s. The idea of interplanetary craft crewed by earthlings whooshing through space is so appealing to an industry that prides itself on breaching frontiers that protagonists for the technology have remained despite a paucity of projects for turning the dream into reality. The indications are, however, that the fusion propulsion bandwagon is now gathering momentum.

NASA says that to fulfil its plans to send a crewed craft to Mars it might consider a fusion-powered spacecraft. Next month ESA will publish its first study into the feasibility of open magnetic fusion propulsion. And experiments are being conducted in Russia, the US and Japan that could pave the way for investigating the technology’s use for space travel.

‘Interest has never been higher in fusion propulsion than it is now,’ said Francesco Romanelli of the nuclear fusion division at the Italian National Agency for New Technologies (ENEA) in Rome. ‘There's a fighting chance that we’ll see some kind of demonstration within maybe 30 years.’

Fusion propulsion depends on replicating and controlling the kinds of reactions that go on in the core of the sun and using the enormous energy to provide thrust instead of heat and light. At its simplest two atoms of hydrogen collide, forming a slightly heavier helium atom and two surplus protons. Energy is also released and, if it can be expelled in the desired direction, will provide thrust.

The good thing about fusion propulsion is that it might just be able to propel a crewed craft to Mars in two or three months. The bad thing is that it involves creating, sustaining and containing very hot, dense plasma, which is extraordinarily difficult on Earth, let alone in a steerable container in space.

A team at the Lewis Research Centre, Ohio was among the first to consider the potential of non-terrestrial applications for nuclear fusion in 1957. At the time Lewis was run by the National Advisory Committee for Aeronautics before it evolved into NASA and the theoreticians faithfully believed that a Moon landing was only a staging post on the way to a manned Mars mission. They knew that chemical propulsion would not be sufficient to get people to and from the red planet so they broadened their outlook.

It seems they were ahead of their time, or, at least, ahead of NASA’s budgets.

Nevertheless, the idea took hold and has been handed down from one researcher to another over the decades. Meanwhile, the Lewis Centre has been renamed in honour of the second human to orbit Earth, John H Glenn, and nobody has stepped on to the Moon after the Apollo 17 crew collected 110kg of rock from its surface in 1972.

NASA, though, is not an organisation that likes to rule anything out. It has plans to get people back to the Moon by 2020 and at some, as yet undefined, time later to send a crewed craft to Mars. Earlier this year Dr George Schmidt, the recently appointed manager of NASA’s Propulsion Research Centre at the Marshall Space Flight Centre in Huntsville, Alabama, said that: ‘We really need new technologies for this. It opens up a whole load of propulsion options… If you are sending a crew [beyond the asteroid belt] you need to think about anti-matter or fusion-powered spacecraft.’

In April, Schmidt took time off in between managing research into solar sails, tethers, advanced electric and other exotic propulsion concepts, to expand on NASA’s present stance on fusion propulsion.

‘The current priority of this research is low, as it does not meet the requirements of any presently contemplated mission,’ he admitted. ‘We are conducting some exploratory research on a pulsed fusion propulsion concept and this effort mainly involves assessing concept feasibility through small theoretical and experimental activities.’ The key word here would seem to be ‘experimental’ because it clearly indicates more than just having a ‘think’.

Schmidt also revealed that work at Marshall includes ‘several other cross-cutting technologies that could be applicable to future [fusion propulsion] systems, such as pulsed power switches and magnetic nozzles. We are interested in pulsed, magneto-inertial approaches to fusion, as they seem to offer the best chance of developing a high specific-power system,’ he said. ‘In other words, a system that delivers a lot of jet power for the smallest mass.’

Among the few dozen fusion propulsion protagonists worldwide who continue to pursue relevant research is Terry Kammash, professor emeritus of nuclear engineering at the University of Michigan. He spent 20 years looking at how nuclear fusion might be used to generate power on Earth before one of his students ended up working for NASA and got him into exploring the idea of fusion propulsion. ‘That was 15 years ago and I've been at it ever since,’ said Kammash. ‘You could say I’m one of the old-timers.’

It also quickly became obvious that he is an energetic advocate with a persuasive line in logic when he explained in detail the pros and cons of different fusion propulsion concepts. ‘There are two main forms: magnetic confinement fusion (MCF) and inertial fusion. I believe the concept of MCF is promising, particularly that of the gas dynamic mirror,’ said Kammash. ‘It could shape a plasma which is so dense and hot that it behaves like a fluid. We like dense, hot plasmas because they have a high specific impulse (Isp) to give good propulsion.’

The higher the isp the more efficient the use of propellants. Chemical propellants such as those used by the Saturn rockets have an Isp of 450 seconds. A nuclear thermal system to heat a propellant such as hydrogen has an isp of 450–900 seconds which is better ‘but not enough for deep space’, according to Kammash. Fusion, however, has an Isp of 100,000 seconds, ‘which is a great, great attribute’.

But Kammash does see problems with MCF. ‘You need very large magnetic fields to confine the plasma and that makes it heavy,’ he said. ‘Three of the components are quite big and one is very big indeed. The reactor will weigh 86 tonnes, the injector that keeps the plasma hot is another 70t and other vital components weigh more than 100t. But these pale in comparison to the radiator. It will have to weigh 660t. ‘It would be pretty expensive to put all that 900-plus tonnes into space at current costs of about $10,000 (£5,000) per kilogram.’

The system would only be about 40m long, compared to the 100m of the familiar Saturn system, and there are suggestions that a radiator at least half the weight could do the job. ‘The conventional heavy radiator is simply a bunch of pipes carrying a coolant but a liquid droplet radiator is also being considered. The droplets of water would lie on a large surface and radiate away into space,’ said Kammash. ‘Super-conducting magnets could also lower the mass of the rocket.’

Having explained his reservations about MCF, Kammash went on to describe the second form of fusion propulsion, inertial fusion, in which a small pellet of tungsten is filled with the appropriate kinds of hydrogen then zapped with a laser beam to compress it to extremely high densities, one or two orders of magnitude higher than solid state densities. ‘You need thousands of joules of laser and it’s vital that the energy is delivered to the core of the pellet at exactly the same moment as the high compression,’ said Kammash. ‘If that can be achieved a hot plasma is created and exhausted through a nozzle to give thrust.’

Kammash favours a slightly different approach, magnetically insulated inertial confinement fusion (MICF), which was suggested two decades ago by Japanese researchers. ‘It means you don’t have to worry about compressing the pellet,’ he said. ‘Instead, the pellet has a hole in one side. The laser enters the hole and ablates the inside surface, creating a very hot plasma which burns to produce fusion energy. It also creates a magnetic field which serves as a thermal insulator to retard the flow of heat so that the burn time becomes longer.’ There’s a bonus, too. At the end of the burn there will be a blob of hot plasma mixed with hot tungsten ions which can also be ejected through a nozzle to give thrust.

The trouble with MICF, however, is that it needs lasers and that’s not very practical in space. So Kammash is keen on another way to make a dense, hot plasma in which fusion can take place. ‘I have been looking at using anti-protons,’ he said. ‘They are extremely efficient at creating plasma because the annihilation energy is enormous when they encounter a proton. A craft could be propelled with a milligram or even a few micrograms of protons and anti-protons.’

Unfortunately only a few nanograms of anti-protons are currently produced each year, by CERN in Switzerland and the Fermi National Accelerator Laboratory in the US. But Kammash is not disheartened. ‘An anti-proton trap has been built, 1m in diameter, 3–4m long and big enough to contain 1,012 anti-protons,’ he said. ‘There are projections that by 2020 worldwide production of anti-protons will be several grams a year and some at Lawrence Livermore National Laboratory in California are rather more optimistic, quoting kilograms.’

The current shortage of anti-matter is inversely proportional to the price of fuel pellets. General Atomics of San Diego, California, currently sells a 20mm-diameter tungsten or gold pellet with a pinhole of up to 15 microns for about $5,000 (£2,600). So filling an MICF-propelled spacecraft would not be cheap.

Kammash’s enthusiasm is not necessarily misplaced, though — there are signs that fusion propulsion is increasingly being given attention. This month ESA will publish its first study into the feasibility of open magnetic fusion propulsion. The organisation has little historical baggage of its own to influence its choices, and little money to waste on pursuing chimeras, so it is reviewing its options carefully.

‘It’s very early days for us,’ said Dr Roger Walker, head of propulsion at the agency's advanced concept team in Noordwijk, Netherlands. ‘From an engineering point of view, open magnetic fusion (OMF) looks the most advantageous and appears to win against inertial confinement and closed systems.’

Walker cited four relevant terrestrial lab experiments: the gas dynamic trap at the Budker Institute in Russia, the gas dynamic mirror at NASA’s Marshall Space Flight Centre; the star thrust experiment at the University of Washington; and the gamma-10 tandem mirror at Tsukuba University, Japan.

The ESA study is being conducted by Francesco Romanelli of the nuclear fusion division at the Italian National Agency for New Technologies, Energy and the Environment (ENEA) in Rome and Claudio Bruno from the University of Rome. ‘ESA is just testing the water and it might continue,’ said Bruno. ‘In Framework 7 there will be a substantial investment from the EU into space propulsion.’ He believes that technology showcasing fusion propulsion in space could be on the cards within 30 years.

George Schmidt at NASA is more cautious and believes there is some way to go before a fusion system that generates more energy than it uses can be demonstrated. ‘At a minimum it is necessary to demonstrate an energy gain greater than one with a relatively low mass device,’ he said.

‘The other hurdles that need to be overcome depend on the approach taken. A steady-state fusion system would be likely to require advances in super-conducting magnet technology. A pulsed system would probably need advances in electrical pulsed-power technology.’

So how much time will it take before fusion propulsion will become a practical solution for deep space crewed missions to other planets within our solar system and possibly beyond? ‘Certainly not within a decade,’ said Schmidt, ‘unless there was a mission that gave it great importance, such as planetary defence. Otherwise a 50–100 year time-frame is more realistic.’