Path to Megawatt Direct Drive Fusion Propulsion Has No Scientific Roadblocks

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A Masters Thesis analyzed a mission to Saturn’s moon Titan using the Princeton Plasma Physics Laboratory Direct Drive Fusion system. The Direct Fusion Drive (DFD) is based on a D-3He fueled, aneutronic, thermonuclear fusion propulsion system.

He considered a 2-MW-class single DFD module, which provides 8 N of constant thrust and a specific impulse of 10,000 seconds. Two different profile missions have been considered: the first one is a thrust-coast-thrust profile with constant thrust and specific impulse and the second is a continuous and constant thrust profile, with a switch in thrust direction operated in the last phases of the mission. Each mission is divided into four different phases, starting from the initial Low Earth Orbit departure, the interplanetary trajectory, Saturn orbit insertion and the Titan orbit insertion. For all mission phases, maneuver time and propellant consumption have been calculated. The results of calculations and mission analysis offer a complete overview of the advantages in terms of payload mass and travel time. The first scenario analyzed is the thrust-coast thrust profile mission which is based on the assumption that the DFD is capable to turn off and on the thrust generation, though without restart the engine.

The paper is Trajectory design for a Titan mission using the Direct Fusion Drive by Marco Gajeri, July 2020.

Princeton Satellite has had various NASA, SBIR and IR&D grants to develop a multi- megawatt-class nuclear fusion propulsion and space power system. This funding has enabled them to precisely simulate their designs and performed experiments. They have stated they would need $100 million and five years to actually make a full megawatt propulsion system. They have not received the level of funding needed to proceed with the main development. Studies of electron heating in PFRC-2 surpassed theoretical predictions and recently reaching 500 eV with pulse lengths of 300 ms, and experiments to measure ion heating with input power up to 200 kW are ongoing. When scaled up to achieve fusion parameters, PFRC would result in a 4-8 m long, 1.5 m diameter reactor producing 1 to 10 MW. DFD uses an innovative radiofrequency (RF) plasma heating system.

Several previous attempts to heat FRC plasmas with RF only reached near-FRC plasma with “open” field lines. The “open” field lines let the plasma to escape.

The PFRC exploits a rotating magnetic field (RMFo) with odd-parity symmetry, produced by the oscillation of the current in four quadrature-phased radio-frequency (RF) antenna. The magnetic field on one side of each figure-8 is in the opposite direction as the other side and closed field lines in the generated FRC. The closed field lines keep the plasma trapped when it is heated.

Current is generated and conditions heat the plasma ions and electrons. These enable compact devices and excellent stability due to the fact that a small, high-temperature FRC plasma, it is said to be kinetic rather than fluid-like and is stable against the tilt mode.

In an FRC reactor, current-carrying electrons will have very high peak energy, about 5 times greater than in D-T tokamak fusion reactors.

The RMFo method improves energy confinement, current drive, plasma heating, and plasma stability.

The simple geometry of the machine, low radiation, and moderate magnetic field strength all contribute to lowering development and maintenance costs. There are no hazardous fuels or materials required. The DFD has been designed to be safe and affordable.

A conservative value for the specific power was chosen for the Masters thesis study. 0.75 kW/kg results in an engine mass of about 2660 kg, which is the estimated mass used for the calculations. A Brayton cycle was chosen for electric power generation.

Current space-qualified radiators will be too heavy but there are upcoming radiator materials that will make the radiators mass a small fraction
of the engine total. NASA is currently supporting research on carbon-carbon radiators. The goal is to reduce the areal mass of radiators from about 10 kg/m2 currently to 2 kg/m2 or less and an average temperature of 625 K. Other essential topics are the superconducting coils and the shielding design. Available data suggests that research in all these areas has made tremendous progress and no roadblocks have been identified.

The analysis shows that the direct drive fusion mission could bring large payloads to Titan in less than two years.

SOURCES – Princeton Satellite, CUNY & Torino Masters Paper – Marco Gajeri
Written By Brian Wang, Nextbigfuture.com

17 thoughts on “Path to Megawatt Direct Drive Fusion Propulsion Has No Scientific Roadblocks”

  1. Except it is not designed for power generation, only for thrust. Containment is the hardest part, all they want is raw thrust out the back.

  2. There are 3 main types of spacecraft that interest me.
    1. Getting into orbit.
    2. Reaching Mars and mining the asteroid belt.
    3. Going to other star systems.

    This thing won't get us to Alpha Centauri in 5 hours, so 3 is out of the question.
    You need brute force to get out of Earth's gravity well, so 1 is out also.

    Saturn is cool to look at, and I'd dearly love to do that in person some day, but it's of no real benefit to Mankind. But if you can use it to mine asteroids and increase the global GDP by 100 times or more, then we're talking.

  3. Apologies, I did not mean my comment to slam anyone. There are a lot of bright people here (engineers and physicists) and I am sure they have a reason to be negative about this proposal. My interest was what can we build today, with no new unobtanium or pie in the sky technologies. We will need powerful drives sooner rather than later, powerful enough to move asteroids or other very heavy objects like true spacecraft (too heavy to launch from earth). As orbital launch costs drop whole new possibilities and investment opportunities are opening.

  4. Thanks for the link. That clarifies things a bit. I was going by the record for withstanding G-forces, which is over 46Gs: https://www.grunge.com/165460/the-highest-g-force-a-human-can-stand/
    but only for a very short time. Fighter pilots fly in planes designed to produce up to 9Gs, because that's all they can withstand for a short period.
    Your link says a continuous 2G trip to Mars would take just 24-34 hours, so I guess a trip to Titan would be a week or so. OK, maybe we have to decelerate at fewer Gs over a longer period; I'm not smart enough to do the calculations of a trip to Titan at 1G with a survivable deceleration at the end – which would also change the total flight time correspondingly. But it certainly seems doable within 1 week for the Fusionfantasy Drive. 🙂 Burn, baby, burn…

  6. It's not negative because people oppose the concept of fusion-powered rockets. The comments are negative because of all of the broken promises that fusion is just twenty years away or Mars by 1986. I used to feel the same as you did. I'd wished the naysayers would just shut up, but some of those naysayers do want humanity to expand out into space and do want nuclear fusion to be a reality but past experience has been rather…discouraging. Let someone launch a fusion-rocket prototype from LEO and you'll see attitudes do a complete 180° and it is highly likely it will happen. It's only a matter if you and I will be alive to see it.

  7. A lot of negative comments on this proposal. If this is not feasible, what do people say IS the most powerful, feasible and cost effective drive we can build today and get working that is not a standard rocket? If it is built in space and comes nowhere near the earth even an Orion drive could be constructed politically, for use interplanetary. Civilian and military usage would likely have different answers same as right now. Anyone?

  8. Any comparison with John Slough's Fusion Driven Rocket? It was funded by NASA as a joint venture between the University of Washington and MSNW LLC, and was reported on NBF. What happened?

  10. Laser fusion system experts claim they have to get 5k
    or higher to achieve fusion not
    500 eV which is what the Princeton device delivers.

  11. Nah, if we have the Fusionfantasy drive, just skip the rotating gravity habitat and accelerate at 1G for 7/8 of the trip to Saturn, then flip the ship 180 degrees and decelerate at 8G for 1/8 of the trip, and you'll get there in days, maybe hours. No more space sickness and no need to store huge amounts of food and games to alleviate boredom!
    The problem has never been speed, it's been the fuel to thrust ratio, or more realistically, just fuel.

  14. If that's for real

    And you've zeroed in on the one flaw in the story.

    Not real. There is no fusion power plant. This is just a science fiction story where they've bothered to do the maths correctly. But the maths is still based on made up numbers.

  15. It looks good for deep Space, where there is no alternative, but boiling water is too expensive a way to make and distribute baseload Earth grid electricity, even if the heat is free, which this reactor probably is not. Power beaming and Space Solar are just too good.

  16. Of course it has no scientific roadblocks. It is so unscientific that is not o even in the scope of science. If you have no working engine all of this is as possible as building antimatter matter engine. Just a few handwavevium engineering requirements.

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