Fusion Energy and Space Transportation
Exploring advanced propulsion techniques for interstellar travel.
What energy sources have been tied to space travel more than any other? The list of leading power sources used in science fiction include the following (some of which exist today):
Ion Drive (existing): Uses an electrical field to ionize a non-reactive gas such as xenon, which is then expelled at high speeds. The electricity is typically generated by solar cells. Today, this is the propulsion source of choice for deep space probes.
Solar Sails (existing): If you have read (not watched the movie) “Planet of the Apes” by Pierre Boulle, you know that at the end of the novel two space travelers (Jinn and Phyllis) unfurled the solar sail on their spacecraft. While solar sails actually do work, the further from a star you get, the less photons you have to accelerate the ship. High-power space lasers may be an option, but not yet.
Plasma Propulsion Engine (in development): Similar to an ion drive, but uses even more powerful magnets and radio waves to “supercharge” the thrust. The Variable Specific impulse Magnetoplasma Rocket (VASMR), which is under development, is the most advanced example of this today.
Nuclear Fission Powered Spacecraft (previously in development): Nuclear fission power could be used for a number of things related to space transportation. One of the most developed concepts was called the Nuclear Engine for Rocket Vehicle Application (NERVA), which was being developed by the US in the 1960s-1970s. The reactor in NERVA super-heated fuel to create a plasma, similar to a plasma propulsion engine. Several successful ground-based tests occurred with NERVA before the program was cancelled during the height of the Vietnam War.
Antimatter Engine (postulated): When you read or hear about interstellar travel, antimatter engines usually figure largely in the equation, whether in science fiction (the USS Enterprise from Star Trek) or in proposals for interstellar rockets. The problem is that antimatter (whether positrons or antiprotons) is incredibly difficult to create even small quantities, let alone what would be needed for interstellar travel.
That leaves fusion energy as the most promising technology for interstellar space travel that is a near-term possibility. Fusion has long been a staple of science fiction as a propulsion source for interstellar spaceships. Star Trek’s impulse engines fused deuterium and tritium (D-T fusion) to produce a high-energy plasma which was then accelerated to propel the spacecraft, as did the “Epstein Drive” of the Rocinante from the TV series “The Expanse”. Both are similar to plasma propulsion engines and NERVA, but with significantly higher energy. In all situations, the use of aneutronic fusion fuels (those where the fusion reaction does not produce excess neutrons) is preferable – aneutronic fusion wouldn’t require shielding for high-energy neutrons, and wouldn’t have the energy loss due to the stray high-energy neutrons.
Fusion has the benefits of high energy density, (relatively) available fuel, and no long-term radioactive materials. Moreover, work on both large-scale fusion experiments and commercial fusion machines have provided a strong theoretical and practical understanding of the physics and engineering of controlled fusion. One company working on turning this into a practical rocket is UK-based Pulsar Fusion. Pulsar, which is a developer of ion-based Hall Effect Thrusters (HETs), is developing a Direct Fusion Drive (DFD) that utilizes the fusion of deuterium and helium3 (D-He3) to power and propel the spaceship. Pulsar’s concept is to utilize a rotating magnetic field to turn the plasma created by a field-reversed configuration fusion machine to create thrust.
One of the other “big” concepts for a fusion-powered rocket is (unsurprisingly) inertial confinement fusion (ICF). The approach would be at a base level similar to that used in the National Ignition Facility (NIF) fusion machine. The difference is that one end of the chamber would be open, and the other end would use large electromagnets to create a “pusher plate” for the plasma, creating forward thrust. This was one of the approaches the UK’s Project Daedalus identified for interstellar space travel. One of the other big differences is that NIF utilizes D-T as its fuel. The positive is that D-T is far easier to “ignite” than other fusion fuels; however it also produces energetic neutrons. That is why researchers looking at ICF for space travel are also looking at D-He3 as a fuel. The primary question is whether (or hopefully, what) types of lasers would be required to fuse D-He3.
A number of other approaches to fusion for space travel have been postulated, including magnetized target fusion (MTF), inertial electrostatic confinement (IEC), and the Bussard ramjet (it gathers hydrogen from free space, and then fuses it). How soon we will see one of these systems in production in the near future is an open question. However, it is clear that if we want to reach beyond near-Earth objects such as the Moon or Mars, we will need the a new class of rockets to reach and orbit objects such as Jupiter, Saturn, and their moons in something less than the five or more years that it takes to day with our fastest unmanned spacecraft. Fusion looks like one of the most promising means to move people and cargo quickly to such locations.