Humanity has always looked up at the stars with nothing but a deep desire to travel to them. For decades we have yearned to sail with a great cosmic ocean unlocking mysteries of other worlds. But to conquer the cosmos we fist need a way to travel across it and with our space technology we have a long way to go, before this becomes a reality. One big obstacle between humans and their space Empire is fueling the rocket to travel such vast distances. With the technology we currently have, an interplanetary trip will require a spacecraft to carry an extremely large fuel supply which may not be a viable option in the long term. So what are our alternatives to do this better?
Well…to be able to do this we clearly need more advanced technology, so that we can for as much as possible reduce the payload, reduce the costs and of course travel faster. Such technology is nowadays a very much a big challenge, but at least theoretically is not impossible. Much powerful and faster rockets could be propelled with fuel generated by Nuclear Fusion, and one idea to use this technology for interplanetary trips, would be to make fusion-powered rockets propelled by Helium-3 (3He).
WILL THE HELIUM-3 MEET THE REQUIREMENTS FOR INTERPLANETARY TRAVEL?
If we manage to harness a mature technology for this purpose then 3He could be a very good candidate, the answer is very likely YES. Unfortunately we are not there yet and we are long way to go until we will be. To understand where we currently are, let’s take a quick look at NASA’s Artemis 1 rocket.
Artemis 1 Mission is the beginning of NASA’s long-term goal to put a sustained human presence on the Moon. The propulsion for Artemis 1 spacecraft involves using over 750.000 gallons of ultra-cold liquid Hydrogen (H2) and Oxygen(O2). Combining the H and O results is a violent combustion reaction with temperatures more of 2700°C. This reaction produces nearly 8 million pounds (or 3,63 million kg) of thrust which is the equivalent weight of 16 Boeing 747 aircraft. That’s impressive. But now comes the question: How would 3He stand up the competition if used as a fuel source?
For some years already, there are a lot of trials ongoing trying to make nuclear fusion a feasible and accessible technology. One of the most studied reaction used in Nuclear Fusion trials is between Deuterium and Tritium. Both of these elements are Hydrogen isotopes.
- Deuterium is a stable hydrogen atom with 2 protons and 2 neutrons in its nucleus (noted as 2H or D), hence not radioactive.
- Tritium is a hydrogen atom with 2 protons and 3 neutrons in its nucleus (noted as 3H or T) is an unstable atom, hence highly radioactive
If 2 atoms like these, fuse together a huge energy output is generated. The basic reaction is defines as follows:
2H + 3H → 4He + n => total energy output: 17.6MeV (2.8 x 10-12 Joules)/reaction or 3.37 x 10¹⁴ Joules per kilogram of fuel.
This value represents the energy released when hydrogen nuclei combine to form heavier elements, such as helium, the same process that occurs in stars like the Sun. . This process converts a small amount of mass into a massive amount of energy, as described by Einstein’s equation, E = mc².
Hence, the D-T fusion reaction is considered the most promising reaction for future terrestrial fusion power plants due to its high energy output and relatively lower temperature requirements compared to other fusion reactions. If one day this technology will become commercial available, then we can very well use it to power spacecraft for interplanetary travel as well. For example a trip to Mars currently takes about 9 Months one-way with conventional rocket fuel. With rockets propelled by fuel made through nuclear fusion a one-way journey from Earth to Mars could be drastically reduced at 3 months or even less.
The nuclear fusion technology is still in its early stage of development, but the trials are promising and one day in the near future, I am optimistic that humanity will finally benefit from it for real. Likewise , in terms of resource supply, we don’t have to worry much because even if Hydrogen gas is rare on Earth, we have plenty of it in water and very likely hydrogen gas deposits are available in the Earth crust too.But Of course, here again we don’t have yet a commercially well enough developed technology to easily extract hydrogen from water or to mine it efficiently from the underground.It is still a lot of work to do for this as well, but this a topic for another article.
But lets come back to Nuclear Fusion, and let’s take a look at Helium-3. As I’ve mentioned earlier, hydrogen is abundant on Earth in water molecule. Yet Helium is scarce and very expensive to get. Helium is an inert element, it is super light, it doesn’t from compounds, and once released it easily escapes the Earth atmosphere with other words is gone forever, hence is a nonrenewable resource. In this case:
Why to bother trying to use Helium in a nuclear fusion application such as powering rockets for interplanetary trips? Why not just let Hydrogen do the job?
There are benefits and risks in nuclear fusion technology both for hydrogen and helium alike. Yet the Helium isotope known as Helium-3 has few net benefits versus any hydrogen isotope. Compared with hydrogen, Helium is not flammable, doesn’t release any radioactive waste in a nuclear reaction and generates a huge amount of energy too. Hence it is definitely interesting to investigate what happens if we replace Tritium with 3He in a usual D-T fusion reaction. In this case we will have a reaction between 2 non-radioactive elements as follows:
2H + 3He → 4He ++p => total energy output: 18.3MeV (2.93 × 10-12 joules)/reaction or 6.3×1014 Joules per kilogram of fuel
So as we can see there is net gain of more energy and the generated products are an alpha particle (as Helium-4) and a proton. In a pure reaction, there is no neutron released like in D-T reaction, hence the reaction is generally considered of aneutronic nature. Becasue in case of D-T reaction there is a charged neutron released, the D-T fusion reaction is considered of neutronic nature. Let’s see the more about the difference between the 2 reactions.
In case of D-T reaction = When energy comes out as an energetic neutron (charged with 14.1MeV in D-T), it needs to hit something (another atom) to heat it, and then you get the energy out with a thermal engine (like a steam engine), which has poor efficiency (you lose about half the energy) and large capital cost for building and maintaining the thermal machine. Definitely not ideal for a spacecraft design.
In case of D-3He reaction = When energy comes out as an energetic ion, that is actually electric current – basically a (very fast) moving charged particle. So you get the energy with direct conversion, potentially with high efficiency, but most importantly with much smaller and cheaper machines – no moving parts, and this is really interesting for a optimized spacecraft design. That said, aneutronic fusion is potentially more (net) energetic that neutronic fusion.
Because of this fuel efficiency a 3He-powered spacecraft could carry significant less mass than its traditional chemical propulsion based counterparts even when accounting for the weight of the He and deuterium as well as the fusion engine. This weight reduction is really handy when we are talking about interplanetary trips. Additionally because most of the energy emitted from D –3He fusion is in the form of charged particles such as 4He, an ion collector or a similar apparatus could be used to convert these particles into electrical energy which could then be used to power certain components of the spacecraft. By using 3He fuel, approximately 60- 70% of the potential electrical energy from this fusion is estimated to be usable, which is more than double amount of energy versus the current status which is around 30-35¨%. So as we can see, the energy potential of 3He is enormous.
However, like Hydrogen, unfortunately 3He too doesn’t come without its drawbacks.
D-3He fusion is possible only under very high temperature conditions (Temperature needed = 100 million Kelvin) and building an engine that can facilitate this fusion will cost a hefty amount. After all we need a design that is an appropriate size and weight and can supply the level of energy needed to facilitate D –3He fusion and withstand the resulting reaction as well.
There’s also the question of abundance. 3He is very hard to come by here on Earth and is primarily created through the radioactive decay of Tritium (3H), which can be found in nuclear warheads. However 3He is likely available in large quantities on the Moon in the lunar regolith, also the gas giants of our solar system contain an abundance of deuterium and 3He. So if we ever need to refuel on interplanetary expeditions, we could potentially count on our cosmic neighbors to pitch in. Of course the technology required to mine 3He from these gas giants is out of reach currently and may not be available until a couple of decades of even centuries from now.
Achieving practical helium-3 fusion is extremely challenging due to the scarcity of the fuel, the exceptionally high temperatures required, and significant plasma physics difficulties. While a pure 3He fusion reaction would be aneutronic, meaning it produces no radioactive neutrons, the inevitable side reactions with a Deuterium-3He fuel mix still generate some neutron radiation. However if an efficient enough method is found to heat and confine fusion fuel, and a good source for the fuel found, then D-3He is more interesting than any other fusion reaction. A general overview as comparison between the 2 nuclear fusion processes discussed above is generally given in Table 1.
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