A New Vision for Fusion Energy Research: Fusion Rocket Engines for Planetary Defense

Abstract

We argue that it is essential for the fusion energy program to identify an imagination-capturing critical mission by developing a unique product which could command the marketplace. We lay out the logic that this product is a fusion rocket engine, to enable a rapid response capable of deflecting an incoming comet, to prevent its impact on the planet Earth, in defense of our population, infrastructure, and civilization. As a side benefit, deep space solar system exploration, with greater speed and orders-of-magnitude greater payload mass would also be possible.The US Department of Energy's magnetic fusion research program, based in its Office of Science, focuses on plasma and fusion science [1] to support the long term goal of environmentally friendly, socially acceptable, and economically viable electricity production from fusion reactors [2]. For several decades the US magnetic fusion program has had to deal with a lack of urgency towards and inconsistent funding for this ambitious goal. In many American circles, fusion isn't even at the table [3] when it comes to discussing future energy production. Is there another, more urgent, unique, and even more important application for fusion? Fusion's Unique ApplicationAs an on-board power source and thruster for fast propulsion in space [4], a fusion reactor would provide unparalleled performance (high specific impulse and high specific power) for a spacecraft. To begin this discussion, we need some rocket terminology. Specific impulse is defined as I sp ¼ v e =g, where g is the usual Earth's gravitational acceleration constant and v e is the rocket propellant's exhaust velocity. The rocket equation, M f /M o = exp (-DV/v e ), allows us to relate the final mass M f of the rocket divided by its initial mass M o , to the change in velocity DV that it is capable of achieving. The rocket requires a power source with an output power P = aM s , where we define a to be the specific power (W/kg), and M s as the mass of the power supply (including the power conditioning, structures, and any waste heat radiators).Today's best chemical rockets produce propellant exhaust velocities (v e ) up to 4.5 km/s. Fission (nuclear thermal) rocket engines [5] could roughly double that, to about 8.5 km/s (\ eV/amu temperature equivalent), constrained by material limits [6]. Electrically driven thrusters [7] are already quite efficient and have higher propellant exhaust velocities (corresponding to *5 eV/ amu) but are usually limited in power resulting in low thrust, and are driven by limited electrical power/energy sources (photovoltaic or radioisotope). Development of high power, high thrust plasma thrusters has not been a

Potential Early Markets for Fusion Energy

“…Finally, although this paper focused on commercial energy-related markets, fusion may also have a range of space applications, e.g., see [48,49], with different techno-economic requirements. Assessing space applications as a potential early market for fusion, though warranted, was beyond the scope of this paper.…”

Potential Early Markets for Fusion Energy

“…Finally, although this paper focuses on commercial energy-related markets, fusion may also have a range of space, e.g., see [50,51], and defense applications with different techno-economic requirements. Assessing these applications was beyond the scope of this paper.…”

Constraints on the abundance of $0.01\,c$ stellar engines in the Milky Way

“…where M i and M f are the initial and final masses of the star, whereas v ex is the velocity at which the propellant is expelled. If one considers v ex that is a few times higher than the stellar escape velocity, or equivalently the stellar wind velocity, and choose a mass ratio of ∼ 10, we would end up with v ∼ 0.01 c. It should be noted that this setup calls for an increase in the exhaust velocities by only a factor of order unity compared to current designs (Cassibry et al 2015;Wurden et al 2016). From a conceptual standpoint -although they are not readily implementable with current human technology -relativistic rockets reliant on hydrogen (or its isotopes) as the fuel have the capacity to reach weakly relativistic exhaust velocities (Winterberg 2019;Holmlid & Zeiner-Gundersen 2020); in this context, the realization of v max ∼ 0.1 c does not seem wholly impossible.…”