Fission-Powered Fusion Rocket

The Fission-Powered Fusion Rocket (FPF) is a type of an in-space rocket engine that contains a fission power-plant that makes electricity to power a fusion thruster. In essence, it as a hybrid engine containing both fission and fusion reactions. The FPF could be thought of as a "turbocharged" variant of NEP (Nuclear Electric Propulsion). The primary difference of an FPF from an NEP, is that the nuclear power-plant powers a fusion-engine instead of an ordinary plasma thruster.

Now, your first reaction is surely to disregard this article because you read the word "fusion". I probably wouldn't blame you. However, there is a fundamental difference between an FPF and all other proposed fusion rockets. In an FPF, the fusion portion does not generate any electrical energy. The fusion thruster is NOT a net-positive device (a net-positive device is one that generates more electrical energy than it consumes). In the case of the FPF, all fusion energy is converted directly into thrust and nothing else. Since we are not extracting electrical power from the fusion thruster it becomes a significantly simpler device.

Let’s briefly explore the fundamentals of fission and fusion to determine the optimal way in which the two can be combined.

We have decades of experience with fission reactions. Given the right amount of radioactive material and the proper containment geometry, it is relatively easy to initiate, throttle, and terminate fission reactions. The vast majority of energy from fission reactions gets carried away by neutrons. Since neutrons have no charge, they are very difficult to control and are impossible to focus into a jet using modern technology. As a result, the most practical thing we can do with fission-reactions is to generate heat and electrical energy (via some form of thermal cycle for example). There is of course the Nuclear Thermal Rocket (NTR), which ejects hydrogen coolant directly from the nuclear core through a nozzle. The NTR, however, presents only a small improvement over conventional chemical rockets, with no application to high-speed, deep-space propulsion.

With fusion reactions, the tables are completely flipped. Fusion reactions are difficult to initiate, throttle, and sustain but not impossible. We proved that we have the means to generate these types of reactions (take the National Ignition Facility as an example). The real reason why fusion is always "10 years away" is because nobody really knows how to get more electrical energy out of fusion than what it consumes. It is clear that a practical fusion power-plant is indeed decades away, but if our goal is to simply eject fusion byproducts into space - now we are talking about a significantly more practical machine. In contrast to a fission reaction, a fusion reaction will emit the majority of its energy in the form of charged particles (like protons and helium nuclei). Unlike neutrons, charged particles may be easily controlled and funneled using magnetic fields. Henceforth, a magnetic nozzle can convert fusion energy directly into thrust, something that cannot be accomplished with fission reactions.

Clearly, the optimal rocket configuration uses a fission reactor to generate electricity to power a fusion reactor which ultimately makes the thrust.

The fission portion of the FPF could be developed from the Prometheus concept originally proposed by Ashcroft and Eshelman in 2007. This type of reactor is cooled by a combination of water and liquid salt and uses an array of turbo-alternators to generate the required electrical power. Power output for this type of reactor is only limited by the mass of the radiator that the rocket can carry. To understand this concept, here is a quick Thermodynamics 101.

To generate electricity from heat we require a thermal cycle. A thermal cycle needs the system to contain a heat source and a heat sink. In space, there is neither conduction nor radiation, so the only way for us to release heat is by means of black body radiation. The rate at which heat is radiated is proportional to the temperature of the radiator. Unfortunately, a hot radiator makes for an inefficient thermal cycle, so ultimately we are stuck with a very heavy radiator.

In terms of the fusion thruster, as always, we seek to find the lightest solution. Since the thruster will operate in the vacuum of space, the reactor will be relatively light as no vacuum vessel is required. The fusion driver, on the other hand, is one of the heaviest pieces of equipment. A "driver" is defined as an electrically powered machine responsible for creating the temperatures and pressures needed for fusion reactions to take place. Drivers come in all shapes and sizes. The most popular type of fusion reactor, the Tokamak, contains a combination of drivers such as Neutral Beam Injectors (which inject high energy neutral particles to heat the plasma) and Cyclotron Heaters (electromagnetic radiators). The driver at National Ignition Facility (NIF), for example, consists of photonic amplifiers, mirror, optics and lasers. All these drivers have one thing in common, they convert electricity into a different form of energy which can then be used to initiate fusion. If we find a way to use electrical current to directly drive the fusion reactions, we can get rid of a whole bunch of weight (and complexity).

Enter the Z-Pinch machine. Unlike other drivers, the Z-Pinch machine is able to pass electrical current directly through a mixture of fusion-fuel. The Z-Pinch is actively being researched at Sandia National Labs. The idea behind operation of the Z-Pinch can be observed in the diagram below, summarized in 3-step process:

1- We begin with an ionized mixture of fusion-fuel which is injected into a cylindrical magnetic chamber. The mixture is hot enough to form the fourth state of matter: plasma. Like an electrical wire, the resulting cylindrical plasma is conductive.

2- An array of capacitors releases a sudden burst of current into the system. An anode and a cathode are present on both ends of the chamber. The electrical current flows through the plasma. Due to the Lorentz force, the plasma cylinder begins to radially compress (this is why it is called a "pinch"). Inside the cylinder, the temperatures and pressures are continuously increasing.

3- At some point the pressure inside of the cylinder equals to the pressure trying to collapse it. This is the point of stagnation. For a fraction of a second the plasma cylinder basically freezes. During this time, most of the fusion-fuel inside the plasma will undergo reactions (also known as ignition).

Once the capacitors are depleted and the current stops, the internal pressure pushes the fusion byproducts outwards. A magnetic nozzle redirects all these charged particles to the aft of the engine, thereby generating thrust. As the jet of particles leaves the engine, the capacitors are recharged from the power source (fission-power plant in our case). After this, the whole cycle is repeated over and over. By controlling the frequency of the pulses, we can also control the throttle of the engine.

Obviously, we have quite a long way to go to see an FPF flying around, but there is a clear path forward. The first step is to fully mature NEP (Nuclear Electric Propulsion) technology. In parallel, we should continue developing the Z-Pinch technology with a big focus on electrodes (since the Cathode and Anode get completely eroded after every pulse). Once both technologies are sufficiently mature, we can bring them together to build an FPF. Keep in mind that the development timeframe for the Z-Pinch machine can be greatly reduced if we get rid of the requirement for net-positive energy production (as mentioned earlier).

When considering all the technologies that go into an FPF, it is very likely that one can be built much sooner than an earth-based fusion power-plant. From childhood we were taught that once humans learn to harness fusion-energy we will be able to build clean, zero-pollution power-plants. In reality it is likely that the dawn of the fusion-era will not happen here on Earth but rather in space when we turn-on the very first FPF spacecraft.