Case and Development Path for Fusion Propulsion

Abstract

This paper discusses the importance of fusion propulsion for interplanetary space travel, illustrates why the magnetoinertial fusion parameter space may facilitate the most rapid, economic path for development, justifies the choice of pulsed Z pinch, and provides a potential development path leading up to a technical readiness level 9 system. Round trips of less than one year to Mars are only possible using fusion propulsion systems. Such a system will require an onboard nuclear fission reactor for reliable startups, and so fission and fusion developments for space are mutually beneficial. The paper reviews the more than 50 year history of fusion research and summarizes results from a recent study of the fusion parameter space for terrestrial power, which suggests magnetoinertial fusion can provide the smallest, most economical approach for a fusion propulsion system. Emerging experimental data and theory show pulsed Z-pinch fusion solves some of the most deleterious instabilities and scales to fusion breakeven within reach of current pulsed power facilities. The paper illustrates a potential development path to a technical readiness level 9 flight system, starting from an assumed technical readiness level 2 for the current state of fusion propulsion. Nomenclature a = acceleration, m∕s 2 g 0 = gravitational acceleration at Earth's surface, m∕s 2 J = mission difficulty parameter, m 2 ∕s 3 k = ratio of tank to propellant mass m = mass, kg _ m = mass flow rate, kg∕s N = total number of stages, number of fusion reactions n = number density P = power, W R = distance traveled, m T = total trip time and time, s t = time, s V = reacting volume, m 3 v j = jet or exhaust velocity, m∕s Y = fusion energy yield, J α = propulsion system specific mass, kg∕W β = mission type multiplication factor γ = ratio of propulsion system to initial mass of nth stage Δv = velocity increment, m∕s λ = payload mass fraction τ = characteristic time, s Subscripts burn = propulsive burn c, coast = unpowered coasting conf = confinement d = dwell, as in dwell time τ d f = final fus = fusion jet = jet, as in for jet power P jet n = number of the stage opt = optimum p = propulsion time pay = payload pn = propulsion time for nth stage (as in for T pn ) pr = propellant ps = propulsion system t = tank 0 = initial 1, 2 = dummy subscripts indicating different species

The future of energy: Energy prospects for a budding inter-planetary civilization

“…Nuclear fusion is a highly desirable energy source that could greatly improve humankind's prospects for successfully establishing colonies beyond Earth. As an added bene t, fusion reactors could also be the basis for fusion-based propulsion in future spacecraft [48]. Nuclear fusion could therefore contribute to space colonization in two ways: By allowing us to harness great amounts of energy, and by allowing us to travel through space faster and more e ciently.…”

Pulsed fusion space propulsion: Computational Magneto-Hydro Dynamics of a multi-coil parabolic reaction chamber

“…A rapid, affordable and efficient (i.e. capable of a highpayload capacity) access to deep space would indeed require systems with both high I sp (> 10 3 s) and high thrust (> 10 4 N), two characteristics one can hardly find together in any of the space propulsion concepts of today: specific impulse is usually increased at the expense of thrust [4,5,6].…”

“…Nuclear power is a promising alternative, for the high energy density of its fuel is particularly attractive for rocket engines. Besides, taming nuclear fusion may be the only option we have to make a 1-year interplanetary round trip possible [5,7,8]. Interest in applications of fusion power has a long history behind and despite breakeven point may still be far to achieve for mankind, several space propulsion concepts have been investigated in the last half-century [9,10,11,12,13,14,15,16].…”

“…The options discussed are many (i.e. ICF, IECF, MCF, MIF, MTF), however, Magneto-Inertial Fusion has been projected to give the smallest solution and is thus seen as the most favourable alternative for space propulsion applications [5,12]. Discussing the state of the art of the subject is however far beyond the scope of this paper.…”

“…Discussing the state of the art of the subject is however far beyond the scope of this paper. The authors will thus limit to saying that the z-pinch reactor (mentioned in the next), is one MIF device recognised by many as the most suitable for rockets [5,17,13]. What will be hereby discussed, instead, is the way thrust can be generated in a fusion rocket.…”