To start a fusion reaction, you have to create extreme conditions. A combination of stellar temperatures, incredible pressures and lightning-quick energy dumps have all been tried to create these conditions, with varying degrees of success.
In this post, we'll look at a low-cost, low-energy method of achieving nuclear fusion. It's not Cold Fusion, it's Gun Fusion.
Understanding what's difficult
Nuclear fusion, as you may already know, is the addition of two atomic nuclei to create products with a slightly lower mass. The difference in mass is released as energy.
|A view inside a Tokamak. The pink rink is the plasma.|
If you've sat through basic chemistry, you'll know that atoms contains electrons, protons and neutrons. Two of these are charged - they act through electromagnetic forces to repel or attract each other.
You will therefore realize that trying to push two nuclei full of positively-charged protons together so that they can fuse means trying to overcome the electromagnetic forces keeping them apart.
|The nuclear force and the coulomb barrier.|
These forces, on an atomic scale, are measure in electron-volts. The energies discussed here are on the mega-electron-volt scale, or MeV.
Defeating the electromagnetic repulsion in practice means giving each fusing atom large amounts of energy.
For example, the easiest fusion reaction to achieve, between Deuterium and Tritium, requires a temperature of about 40 million K.
Current methods and applications of spacecraft
As mentioned, several methods have been used to ignite fusion already.
The most commonly discussed, and the subject of large-scale, multi-billion dollar experimentation, are magnetic confinement and inertial containment.
The magnetic confinement method relies on very large electromagnets generating incredibly powerful magnetic fields to contain an ionized fusion fuel plasma. They are necessary as like any gas, heating the plasma increases its pressure. Heating it to multi-million Kelvin temperatures generates incredible pressures, which have to be countered with equal force by the magnetic fields. Examples of this approach are the Tokamak and the Wendelstein 7-X Stellarator.
|Inertial Confinement Fusion at the National Ignition Facility|
The inertial containment method attempts to bypass the requirement of containing the plasma by working on extremely short timescales. A zap of energy is delivered very very quickly, in the form of a laser or ion beam, to a frozen ball of fusion fuel. The ball's exterior vaporizes and expands, thereby pushing against the ball's interior. If done properly, the compression and heating of the interior is sufficient to ignite a fusion reaction before the whole thing explodes.
As a result of their popularity, they are featured heavily in both science fiction and in real-world fusion designs.
The focus of this blog is space and spaceships, and naturally, we must consider the myriad proposals for fusion-driven spaceships.
Fusion is an excellent choice for propelling spacecraft. It is much more powerful than chemical fuels, but does not have the rare fuels and radioactive residues of fission. However, the disadvantages of using fusion with the aforementioned methods are equal in measure to its disadvantages.
|A fusion rocket using a gas-dynamic mirror.|
A magnetic containment fusion spacecraft has to carry the mass of huge electromagnets and their cooling equipment. It has to power both with a continuous, powerful energy source, which might mean another nuclear reactor. Furthermore, it has to find a way to extract the energy from the reactor and apply it to the propellant. This can be done by extracting the heat from the reactor walls indirectly, with associated conversion losses, or pumping propellant into the reaction chamber and risking the plasma being destabilized.
|VISTA fusion rocket.|
Inertial confinement is better suited to propelling spacecraft, but has its own complication. It requires petawatt lasers or a particle accelerator, with their associated mass. They have to be powered by capacitors, which are notoriously low on energy density. A fusion pellet can easily be placed inside a cloud of propellant for propulsion, but the challenge becomes transmitting the energy pulse through the propellant and onto the pellet without losses or deviation. The entire ignition process also has to maintain a degree of precision even when under acceleration and or when subject to external bumps.
So, fusion spacecraft have large mass overheads, require significant external energy input, and have various difficulties when it comes to using fusion energies for propulsion.
What is proposed here is a method to ignite fusion using well-understood technologies, requiring only minimal power inputs yet remaining well adapted for use in propulsion.
The design is composed of two railguns, each accelerating a specially configured bullet, to be launched at each other with the intention of igniting fusion within a solid propellant cylinder. It involves a 5 step process:
The bullets contain three elements: a thin (100nm) faceplate of gold, a chamber containing a gas mixture of deuterium, tritium and hydrogen, and a solid 'tail'. They are launched using a railgun to velocities of 20-100km/s depending on configuration.
|The basic bullet configuration|
Upon impact, the gold faceplates vaporize as kinetic energy is converted into thermal energy (10% efficiency). The heat radiated by the impact raises the temperature of the gas mixture (1% of bullet mass) to 3.125 million K. This lowers pressure requirements for ignition by a factor of 100000.
|Diagram of impact: 2) Gold plate. 3) Gas mixture. 4) Optional backing plate (solid fusion fuel). 6) Point of impact 7) Radiated energy. 8) Vaporizing backing plate.|
The momentum of the tails makes them act as pistons. Their movement compresses the gas mixture to over 80 million atmospheres. Temperatures rise to over 50 million K. A fusion reaction is ignited.
|Diagram of compression: 3) Gas mixture being compressed. 4) Optional backing plate (solid fusion fuel). 5) Tail pieces.|
The impact takes place under a half-sphere of polyethylene or other suitable propellant. The energy released by the fusion reaction vaporizes and ionizes the propellant. The momentum of the propellant is captured by electromagnetic or mechanical means.
|Configuration for gun-fusion drive|
This 'gun' fusion has several clear advantages over the aforementioned methods of igniting fusion.
Railguns are simpler than electromagnets cooled to superconducting temperatures or petawatt lasers using heavy, bulky supercapacitors. While the total energy involved is equal to or greater than that of inertial confinement fusion, the power levels are much lower and vastly more manageable. The fusion fuel does not have to be handled at cryogenic temperatures either.
With today's technologies, railguns are lighter than particle accelerators or Tokamaks. They are also more efficient than petawatt lasers, and the fusion mechanic is more robust. In the special designs discussed below, they can be replaced by even lighter methods of accelerating the bullets.
Another important characteristic for fusion propulsion is that the fusion equipment can be placed arbitrarily far away from where the fuel is ignited. This lowers shielding requirements.
We will start with a payload mass, a mission and then go from there to work out a spaceship design.
We'll assume a 100 ton mission to Mars. The mission is an impulse trajectory requiring 100km/s of deltaV and crossing a 54.6 million km distance is about 12 days.
|Deuterium-Tritium fusion products.|
To stick close to the findings of the cited research, we will be using D-T fuel. 20% of the energy is released as soft X-rays, with the remainder in 14MeV neutrons. It is therefore essential to capture the neutron's energy. For this reason, polyethylene is the propellant of choice. It is light, and absorbs over 98% of neutrons within 4cm depth. A 4cm radius hemisphere has a volume of 134cm^3 and a mass of 128 grams.
|Polyethylene has p: 0.96g/cm^3, A: 28g/mol|
Let's say we ignite 0.1g of deuterium/tritium fuel, within two 10g bullets, with a fuel burn-up of 6.6% (we need 3g/cm^2 for 33%, but we're using a sphere of room-temperature deuterium/tritium at 0.2g/cm^3).
We obtain 4.5GJ per impact-fusion.
The fusion products are released in a spherical fireball, so about 50% reach the polyethylene half-sphere. 98% of neutrons are absorbed, 100% of X-rays too. It is safe to say that 50% of the energy released is used to heat the polyethylene.
The polyethylene gains 2.3GJ. It has a heat capacity of 1.25J/g/K, so it rises to a temperature of 14 million K. Using the average molecular kinetic energy equation, we reach a particle velocity of 111.7km/s. We obtain 35kN of thrust, depending on the efficiency of the electromagnetic nozzle, assumed to be 85%.
|The Mini-Mag Orion, which uses a similar propulsion system to our worked example spaceship|
Accelerating 10g bullets to 20km/s (staged compression) requires 2MJ. A 16% efficient railgun would require 10MJ. If we include a heatsink, this figure rises.
If the spacecraft uses an electromagnetic system for capturing momentum from the polyethylene particles, then we might use a 20% efficient MHD generator. It would produce 460MJ per pulse, more than enough to power two railguns. A simpler thermoelectric generator with 5% efficiency still produces a sufficient 115MJ. The latter can be integrated into the radiation shielding on the railguns.
|How a thermoelectric generator works|
400MJ can be contained in 4 tons of supercapacitors, for two cold re-starts. The current-generation 64MJ railgun masses 67 tons (including capacitors). We can extrapolate that two 10MJ railguns might mass 20 tons.
The ICAN-II spacecraft places engine mass at 27 tons to handle an output of 11.7GW. If the gun-fusion is fired once per second, power output is 2.3GW, so an extrapolated figure of 5.3 tons might be appropriate to cover the electromagnetic particle capture system.
The mass of the polyethylene propellant ejection system should be insignificant (less than 1 ton). Structural mass is roughly estimated at 10% of dry mass, so approximately 12.9 tons. This includes suspension for the nozzle, to smooth out accelerations.
Payload: 100 tons
Drive system: 29.3 tons
Structural mass: 12.9 tons
Drive power: 2.3GW (fusion), 1.96GW (nozzle).
Exhaust velocity: 111km/s
Firing rate: 1Hz
Mass flow: 148g/sec (of which 128g/sec is propellant).
Consumables mass: 53.8 tons (711TJ mission energy/1.955GJ per impact)
Total mass: 196 tons
In short, a small, simple-to-construct spaceship can reach Mars in under a month.
The design mentioned in the 'Gun fusion' section requires that the bullets impact at 100km/s. In fact, studies on impact fusion typically recommend velocities approaching 1000km/s, where a microgram pellet of fuel is compressed in one dimension against an immobile target. The solution to vastly reducing velocity and energy requirements is staged compression.
|Staged Compression bullets. (50) are spacers. (51) are cavities filled with gas.|
Each stage compresses and heat the gas in the following stages. With a sufficient number of stages, the fuel bubble can achieve ignition temperatures and pressures at velocities of only 20km/s.
Another 'improvement' is the use of various acceleration mechanisms. A two-staged light gas gun could theoretically achieve the necessary velocities by adding a third stage. A coilgun can be used, with improvements in efficiency. It also be made circular to make a more compact accelerator.
Ablative propulsion has been proposed. A laser or ion beam vaporizes the tail of the bullet, accelerating it. It would be very useful if efficiencies rise. Their greatest advantage is gradual acceleration: the bullet can be accelerated up to the point of impact. However, they would require unobstructed line of sight and any fault in the laser's accuracy or the ablative material's manufacture will cause the bullets to miss.
A final improvement is the magnetic manipulation of the bullets. By making the bullets' tails out of a ferromagnetic material such as iron, their trajectory can be modified and corrected by magnetic forces. This would allow accurate impacts from further away (important for very large spaceships using larger yields) or off-angle acceleration of the bullets.