Historical research guide · 0951 · 2014
Using VASIMR® for the Proposed Europa Mission
Edgar A. Bering III · Matthew Giambusso · Mark Carter · Andrew Ilin · Christopher Olsen · Jared P. Squire · Franklin Chang Díaz · Benjamin W. Longmier
In one minute
Getting a spacecraft to Jupiter normally means years of looping past Venus and Earth to beg gravity for speed. Edgar Bering of the University of Houston, with Franklin Chang Díaz’s Ad Astra Rocket Company and Benjamin Longmier of Michigan, propose doing it with sunlight and a plasma engine instead. Their idea is a reusable catapult. A VASIMR-powered stage first thrusts backwards to fall in close to the Sun, where sunlight is strongest; there it turns around, spends that abundant power on a hard sustained burn, and flings its passenger onto a direct transfer orbit before returning to Earth to do it again. The engine is not hypothetical: the 200 kilowatt prototype it is based on has been run in a vacuum chamber at 72 per cent efficiency. On their trajectory models a four-tonne payload reaches Jupiter in about 35 months — roughly half the time Galileo took — with the ship, not the planets, providing the energy.
Connection
Chapter 8 asks what a craft can do once its performance stops being set by chemistry, and this paper is the honest top of the ladder you can climb while still throwing mass out of the back: a measured engine, a real power budget, and a Jupiter mission that halves the transit time. Chapter 9 gets the machine itself — a plasma made by a helicon antenna, heated at the ion cyclotron frequency and released through a magnetic nozzle, with no electrode anywhere in the beam.
From the source
The concept is a reusable probe catapult. A VASIMR stage sends a 4000 to 5000 kilogram spacecraft to Jupiter on a Hohmann-like transfer orbit arriving in just 36 months elapsed time, by making a slingshot pass close to the Sun and using the high level of solar energy available there to produce a sustained burst of high thrust; the probe is released with enough speed to reach Jupiter in three years and the catapult returns to Earth for another mission.Abstract
The engine has no electrodes in the plasma, and that is the whole design. A helicon discharge makes the plasma, an ion cyclotron heating stage accelerates the ions with a circularly polarised radio wave tuned to their cyclotron frequency, and a two-stage magnetic nozzle turns their gyration into directed momentum as the field weakens — thrust following from conservation of the first adiabatic invariant. Radial magnetic confinement keeps both ions and electrons off the walls, which is what allows the engine to process power densities of order 6 megawatts per square metre and still expect a long life.Section I, Introduction, and Figures 1 and 2
The performance numbers are measured, not projected. The VX-200 couplers were designed for 60 per cent thruster efficiency at 5000 seconds specific impulse on 200 kilowatts of direct-current input, equivalent to 186 kilowatts of coupled radio-frequency power. Running the full 200 kilowatts of radio-frequency power, the measured performance was 72 per cent thruster efficiency at a specific impulse of 4900 plus or minus 300 seconds. Plasma production came in below the 100 electronvolts per ion that efficient operation requires, with a minimum ionisation cost of 87 plus or minus 9 electronvolts per ion, measured with the engine in a 150 cubic metre vacuum chamber held below one hundredth of a pascal.Section III, Efficiency, subsection A, Thruster Performance at 200 kW; Figures 3, 8 and 9
The mission runs in three phases and the geometry is specific. In phase one the spacecraft thrusts antiparallel to its heliocentric velocity, deliberately slowing down so that it falls in toward the Sun. In phase two it accelerates along its velocity vector while the solar flux climbs, passing inside the orbit of Venus to a perihelion of about 0.54 astronomical units. Phase three begins at about 0.73 astronomical units, where the payload already has enough energy to coast to Jupiter. The solar array is taken to be a flat planar array rather than a concentrator, because a concentrator would overheat the cells that close to the Sun.Section IV, subsection A, The VASIMR Ejector Catapult, and Figure 11
The vehicle is massed against a real spacecraft. The catapult is scaled from the MESSENGER design with two changes — thermal control multiplied by about eight, and propellant plus tankage increased by more than an order of magnitude — giving a full system wet mass in low Earth orbit of about 22 to 25 tonnes. A 25 tonne initial mass carrying a 5 tonne payload reaches Jupiter in 36 months, half the time Galileo needed; optimised solutions for a 4 tonne payload on a 22 tonne initial mass deliver it in 34.7 months.Section IV, subsections B, C and D, with Figures 12, 13, 15, 16 and 17 and Table I
What to watch is the return leg, and the authors say so themselves. Bringing the catapult home costs more argon than it delivered payload, and the reuse case assumes ten deep-space missions over twenty years, which they call unlikely in the present funding environment. Their alternative is to keep thrusting instead: carry on accelerating out past the orbit of Mars, jettison the argon tankage, magnets and thermal system in mid-course, insert into Jupiter orbit with a chemical stage, and arrive with 20 kilowatts of solar power still running and the engine’s radio-frequency generators repurposed as the transmitter of an ice-penetrating radar. That alternative has not yet been worked out quantitatively.Section IV, subsection E, An Alternative Idea, and section V, Conclusion
The way in
https://doi.org/10.2514/6.2014-4344How to cite it
Edgar A. Bering III, Matthew Giambusso, Mark Carter, Andrew Ilin, Christopher Olsen, Jared P. Squire, Franklin Chang Díaz, Benjamin W. Longmier (2014) Using VASIMR® for the Proposed Europa Mission doi:10.2514/6.2014-4344