Historical research guide · 0934 · 1998

Interplanetary missions with the GDM propulsion system

William Emrich · Terry Kammash

In one minute

Terry Kammash of the University of Michigan and William Emrich of NASA’s Marshall Space Flight Center ask a plain question about a fusion rocket: how long is the trip? Their machine is the gasdynamic mirror, a long thin tube of magnetic field holding deuterium plasma so dense that the ions collide constantly and the whole plasma behaves like a gas escaping from a vessel with a hole in each end. Out of one end the particles leave as thrust. Out of the other they are caught by a converter that turns their energy back into electricity and feeds the plasma’s own heating. Kammash and Emrich draw that power budget as a flow diagram, solve it, and find the reactor need only return about 1.22 times the power it draws — a modest demand next to a terrestrial power station. Then they fly it: a round trip to Mars comes out near 170 days, Jupiter 494, Pluto 1566, burning a propellant load smaller than the ship’s own dry mass.

Connection

Chapter 8 is about what a craft can do once it stops being a chemical machine, and this is the honest arithmetic of the last stage before you leave reaction mass behind altogether: the fuel is a few per cent of the vehicle, the launch window stops mattering, and the outer solar system comes inside a single mission. Chapter 9 gets the physics underneath it — a long, dense, self-heating column of magnetically confined plasma that leaks from its ends by design rather than by failure.

From the source

In the gasdynamic mirror the plasma is deliberately made so dense that the ion-ion collision mean free path is much shorter than the length of the machine, so the plasma behaves like a fluid and escapes the mirror exactly as a gas escapes into vacuum from a vessel with a hole in it; the particle confinement time is then the mirror ratio times the plasma length divided by the ion thermal velocity, and the mirror ratio the plasma sees is degraded from the vacuum value by the ratio of plasma pressure to magnetic pressure.Introduction and Basic Principles; Equations 1 and 2
Because a mirror machine can hand back the energy of the charged particles leaving one end through a direct converter, the fusion energy multiplication a self-supporting engine needs is small. For a symmetric mirror the critical value is independent of the fuel cycle, and at a direct-converter efficiency of 0.9 and a thermal-converter efficiency of 0.45 it works out at 1.222 — quite modest and easily achievable compared with the large Q-values required by terrestrial power reactors.Power flow diagram, Figure 2; Equation 3 and the paragraph following it
The deuterium-tritium engine is specified in full: plasma density ten to the sixteenth per cubic centimetre, ion temperature 10 kilo-electronvolts, plasma radius 5 centimetres, plasma length 44 metres, central magnetic field 9.21 tesla, fusion power 2730 megawatts against 2233 megawatts of injected power, an engine mass of 101 tonnes inside a 422-tonne vehicle, specific power 13.4 kilowatts per kilogram and a specific impulse of 1.268 times ten to the fifth seconds.Table I, GDM Plasma and Propulsion Parameters, deuterium-tritium column
The clean deuterium-helium-3 cycle is priced honestly in the same table and costs time. It needs 60 kilo-electronvolts rather than 10, a 24.73 tesla field rather than 9.21, a plasma 1297 metres long rather than 44, and a 4434-tonne vehicle rather than 422 — and although it generates larger propulsion parameters than the deuterium-tritium system it results in a 35 per cent increase in travel time, because of the significantly larger vehicle mass.Table I, deuterium-helium-3 column; Mission Analysis, first paragraph
Flown on a continuous-burn trajectory at constant thrust and constant specific impulse, and ignoring the gravity of the sun and planets and the motion of the Earth during the flight, a deuterium-tritium gasdynamic mirror rocket reaches Mars in 59 days for a fly-by, 84 days to rendezvous and 169 days for a round trip; Jupiter in 170, 241 and 494 days; and Pluto in 517, 733 and 1566 days.Equation 4; Table 2, Solar System Missions with GDM, engine-to-payload mass ratio zero
The authors name the small propellant load as the single most striking feature of the design: the propellant for a Mars round trip with about 50 tonnes of payload is about 7 per cent of the total vehicle mass, and for a Jupiter round trip at 25 per cent payload about 20 per cent, so the propellant required for round trips inside the solar system may be significantly smaller than the dry mass of the vehicle. The engine model rests on the companion claim, argued in the authors’ 1998 Physics Basis paper, that a high-aspect-ratio gasdynamic mirror plasma is stable against both magnetohydrodynamic and microinstabilities, which if allowed to arise or persist can destroy its propulsive capability.Summary; Introduction and Basic Principles, final paragraph; reference Kammash and Emrich 1998

The way in

https://doi.org/10.1063/1.54732

Historical source-access record. This guide links to the original; it does not reproduce the paper text.

TEXT. The full six-page text was read for this sheet and every claim below is located against it by section, equation and table number. The paper was fetched from the copy the University of Michigan deposited in its Deep Blue repository, handle 2027.42/87425, file 1145_1.pdf, read through the Internet Archive capture of 2 December 2023 because the repository itself now answers 403 to a script. RIGHTS. It is not promoted past abstract-only. Although the first author, W. Emrich Jr, gives his address as Marshall Space Flight Center and the acknowledgment reads ‘This work was supported by NASA’, the copy that exists is the conference-proceedings version, and its first page carries the line ‘CP420, Space Technology and Applications International Forum-1998, edited by Mohamed S. El-Genk, DOE CONF-9801039, 1998 The American Institute of Physics’. A NASA-funded paper published by a society press is not a NASA document, so only the published abstract is reproduced here, and it is the abstract as the publisher deposited it. A search of the NASA Technical Reports Server returns ten gasdynamic-mirror items by this group — the Marshall experiment papers, the microinstability papers and the system description — but not this one; the Office of Scientific and Technical Information holds it as bibliographic record 21179650, pages 1145 to 1150, with no full text. NAMES. Publisher and repository records give the authors as W. Emrich, Jr. and T. Kammash; OpenAlex expands the first to William Emrich and the second is Terry Kammash of the University of Michigan, who is named in full on the companion Journal of Propulsion and Power papers this one cites. AFFILIATIONS as printed: W. Emrich, Jr., Marshall Space Flight Center, Huntsville, Alabama; T. Kammash, Department of Nuclear Engineering and Radiological Sciences, The University of Michigan, Ann Arbor. On this site, the fusion-propulsion neighbours are the two Defense Intelligence Reference Documents on aneutronic fusion propulsion at [/library/stm-b5e092d030](/library/stm-b5e092d030) and [/library/stm-08b7559cf0](/library/stm-08b7559cf0), the inertial-confinement route at [/library/stm-b87332054a](/library/stm-b87332054a), the mirror-adjacent power cycle at [/library/stm-6625a82f89](/library/stm-6625a82f89), and the wider survey of what a propulsion breakthrough would need at [/library/stm-00a7ce4729](/library/stm-00a7ce4729). Kammash appears again in the Defense Intelligence Reference Document on invention at [/library/stm-19f5f06922](/library/stm-19f5f06922).

How to cite it

William Emrich, Terry Kammash (1998) Interplanetary missions with the GDM propulsion system doi:10.1063/1.54732

In one minute

From the source

The way in

How to cite it

Where it sits in the curriculum