What is the most promising fusion technology for rocket propulsion?

Question

Given current fusion technology (barely above q=1 as of the 12/12/22), is a fusion plasma torch rocket engine viable?

I'm asking in terms of basic science, not engineering, as I know this would involve decades of work to get to a working model.

Answer 1

Answer: Magnetized Target Fusion

Most proposed fusion reactors are grossly impractical for space propulsion.

However, General Fusion’s Magnetized Target Fusion (MTF) https://en.wikipedia.org/wiki/General_Fusion overcomes many of the difficulties. General Fusion has raised over $430M in funding and is building a 2/3 commercial scale reactor in England.

The reactor fuses a magnetically confined deuterium-tritium plasma. The fuel is injected into a vortex cavity formed by spinning a sphere of molten lead (orange in diagram). Rams (green) create an acoustic shock wave which compresses and ignites the fuel. For power generation, heat is extracted in a heat exchanger from the molten lead, about 1000*C.

The lead is laced with Lithium, which is converted to Tritium by the fusion reaction. The Tritium is recovered from the molten lead and used as fuel.

The molten lead absorbs the neutron flux from the fusion reactions, shielding the spacecraft and the reactor itself. This extends the lifetime of the reactor since neutron embrittlement puts a limit on the service life of most reactors.

For propulsion, the reactor would be used to heat reaction mass. Although the lead is 1000*C, the fusion products would be much hotter, so the Isp would be limited only by the engine parts exposed to rocket exhaust. The throat and bell could be cooled by the molten lead, so exhaust temperatures of several thousand degrees are attainable. This engine should be capable of similar Isp to gas core reactors (thousands of seconds) since it is, in fact, a gas core reactor.

Sending a motor made of lead into space may sound like a "lead balloon" joke. But in microgravity, the creation of a vortex may require a much smaller mass than in Earth gravity.

Answer 2

Currently fusion is not economically viable as power generation technology, but there's a very promising research into utilizing it as simultaneously an extremely high density power storage and a superb, high-performance propulsion.

That whole q<1 isn't that big of a problem if you're able to spend the excessive "input energy" needed to provide all the necessities for fusion here, on Earth, in ground-bound production facilities and later extract the fusion energy up in space - even if you extract less than you used.

Answer 3

No, the equipment needed for current fusion technology is too heavy by many orders of magnitude and the ratio between the energy generated by the fusion and the energy needed to start a fusion is much too small. The maximum duration of a continuous fusion is very small and the minimum time interval between two fusions is much too long.

Answer 4

I thought about using the General Fusion reactor as a propulsion system but so far it looks that it would only be useful as a power source for an electric propulsion system. The issue would be the fusion products being absorbed entirely by the molten lead-lithium mix. 80% of the reaction energy is lost as neutrons with only 20% of the remaining energy coming out as charged particles (Chapman p.1) as that could be directed by a magnetic nozzle but 20% is the max. Now, there are designs for a fission-fusion hybrid system where the lithium blanket for breeding tritium is replaced with a blanket of U-238 which can be split via fast neutrons (> 1 MeV) (Manheimer p.6). So, tritium can be bred ahead of time or (because of the short 12 year half-life of tritium), bred in a second reactor, extracted, and used to fuse with deuterium to produce the fast neutrons needed to fission U-238. The fission products are positively charge (Brown p.3) (Ceyssens p.4) so magnetic fields can deflect them out a nozzle and provide thrust.

Chapman, John J.,"Advanced Fusion Reactors for Space Propulsion and Power Systems", NASA, Langley Research Center at https://ntrs.nasa.gov/api/citations/20110014263/downloads/20110014263.pdf

Manheimer, Wallace, "Fusion breeding as an approach to sustainable energy" Discover Sustainability November 20202 at https://www.researchgate.net/publication/346534693_Fusion_breeding_as_an_approach_to_sustainable_energy (Downloadable PDF)

Ceyssens, Frederik, Kristof Wouters, and Maarten Driesen "Fission Thrust sail as booster for high Δv fusion based propulsion" at https://arxiv.org/pdf/1408.6225

Brown, L.C "Direct Energy Conversion Fission Reactor Annual Report for the Period of August 15, 2000 Through September 30, 2001", Nuclear Energy Research Initiative (NERI) Program No. DE-FG03-99SF21893 for the U.S. Department of Energy at https://www.osti.gov/servlets/purl/805252