NEEP602 Course Notes (Spring 1996)
Resources from Space

Lecture #31: Opening the Solar-System Frontier

Title: Fusion Propulsion

April 10, 1996

Almost done, but requires some fusion-reactor configuration figures to be scanned. Some of the black and white figures will be replaced with the colored versions used in my lecture.

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Developing a frontier

  • Q: Thomas Jefferson reputedly (by Willy Ley) predicted in 1803 that it would take 800 years to settle the Louisiana Purchase. Nevertheless, most territories in the region achieved statehood before 1900. What happened?

  • A: The introduction of a new technology, the train, shortened the travel time from New York to Chicago from six weeks to two days---enabling the development of the American West. The year 1803, coincidentally, saw the first operation of a train for commercial freight transport. Fusion propulsion, I believe, can be a similarly enabling technology for the space frontier.
Palmerston Train

From the Rheilffordd Ffestiniog Railway Web pages

Selected events in the history of fusion for space propulsion

Two interesting early papers on D-3He space-propulsion reactors are Englert (1962) and Hilton, et al. (1964). These papers followed much of the same logic given in the present discussion to propose using the D-3He fuel cycle in linear magnetic fusion reactors. Although we now know that the simple-mirror concept used in the earliest papers cannot achieve a sufficiently high Q (ratio of fusion power out to required input power), which probably must be on the order of 10 or more, they present many interesting ideas and recognized several important engineering approaches.

Later papers, such as Roth, et al. (1972), examined the idea of adding `bucking' coils to extract a magnetic flux tube from a toroidal magnetic fusion reactor and exhaust the thrust. Although this geometry may work in relatively low magnetic field toroidal reactors, it would require massive coils and be extremely difficult for the present mainline concept, the tokamak (see lecture 26), where the magnetic fields in conceptual designs approach practical limits of about 20 T.

Arthur C. Clarke's perspective

The recognition that fusion might prove attractive for space applications was widespread. Fusion's advocates included Arthur C. Clarke, who wrote in 1961 that

``The short-lived Uranium Age will see the dawn of space flight; the succeeding era of fusion power will witness its fulfillment.''

Magnetic fusion fuels for space applications

Advantages of D-3He magnetic fusion for space applications

D-3He fuel is more attractive for space applications than D-T fuel.

Shown below are the fusion power density in the plasma and the fraction of fusion power produced as neutrons for D-T and D-3He fuel.

Fusion Power DensityNeutron Power Fraction
Fusion power density versus ion temperature Neutron power versus ion temperature

The high fusion power density in the plasma favors D-T fuel, but the reduced neutron power fraction favors D-3He fuel. This trade-off exemplifies the competition between physics and engineering in fusion energy development. In reality, a balance among these and other considerations must be found. For space applications, D-3He fuel has usually been projected to be most attractive. The key reason for this is that the most important factor is not the fusion power density in the plasma (kW-fusion/plasma volume) but is the engineering power density (kW-thrust/mass of reactor and radiators). Several factors contribute to the dominance of D-3He fuel:

Regarding the last point, magnetic-fusion configurations can be classified as in the following table:

B field at limitB field near limitRelatively low B field
Superconducting tokamakCopper tokamakField-reversed configuration
TorsatronTandem mirror
Bumpy torus
Reversed-field pinch

Energy density of space-propulsion fuels

A fundamental limit on the specific power available from a fuel is the energy density of that fuel. A realistic assessment, of course, requires the detailed design of fuel storage and a means of converting fuel energy to thrust. Nevertheless, a high fuel energy density is desirable, because it facilitates carrying excess fuel, which contributes to mission flexibility, and indicates the potential for a high specific power.

Fuel energy density for various fuels The argument is often forwarded that antimatter-matter annihilation is the best space-propulsion fuel. A key difficulty exists, however: antimatter takes much more energy to acquire than it produces when annihilated with matter. Presently the ratio is about 104, and there appears little likelihood that ratios below about 102 are accessible. Antimatter, therefore, will probably be of limited use for routine access to the Solar System, although it will be the fuel of choice for specialized applications, such as interstellar missions. The energy needed to acquire various fuels is compared with the energy released in burning them in the figure at right.

High efficiency is critical in space

The ratio of useful thrust energy, which scales with efficiency, to the waste heat, which scales with (1 - efficiency), is a strong function of the efficiency of converting the fusion power to thrust. Because radiators often contribute a substantial fraction of the total rocket mass, efficiency generally is an important parameter. Ratio of useful energy to waste heat

Fusion Reactor Designs for Space Applications

Conceptual designs of magnetic fusion reactors for space propulsion during the past decade have generally calculated specific powers of
1--10 kWthrust/kgreactor. An argument supporting the general performance level, based on a `generic' analysis, appears in Santarius and Logan, 1993. The projected specific powers for selected designs appear in the table below.
NB: Widely varying assumptions and levels of optimism have gone into these conceptual designs and the resulting specific powers. For perspective, perhaps, note that my main involvement with these studies was with those that had two of the lowest specific powers listed, at about 1 kW/kg.

First AuthorYearConfiguration
Santarius1988Tandem Mirror1.2
Haloulakis1989Colliding Spheromaks--
Bussard1990Riggatron Tokamak3.9
Carpenter1992Tandem Mirror4.3
Kammash1995Gas Dynamic Trap21(D-T)
Kammash1995Gas Dynamic Trap6.4(D-3He)

Various fusion-reactor configurations have been considered for space applications. Generally, the key features contributing to an attractive design are

Some candidate concepts for both magnetic and inertial-electrostatic fusion are illustrated below, with links to selected references at the end of this document.

Field-reversed configuration (FRC)Tandem mirror
Figure to be scannedFigure to be scanned
SpheromakInertial-electrostatic Confinement
Figure to be scannedFigure to be scanned
Some Inertial-confinement fusion (ICF) reactors for space propulsion have also been designed. One example is VISTA, shown at right. Because of fusion burn dynamics, D-3He fuel is much harder to use in ICF reactors, and VISTA used the D-T fuel cycle. The British Interplanetary Society's earlier Daedalus study used D-3He fuel, but had to simply assume that the physics would work. VISTA ICF space-propulsion design
Figure to be scanned

D-3He Fusion Capabilities

Space propulsion option operating regimes The performance that D-3He fusion appears capable of providing for space propulsion is shown in the figure at right. In particular, as discussed in lecture 28 on spacecraft trajectories, fusion is the only option that achieves the most important regime for Solar-System travel: exhaust velocities of 105 to 106 m/s at thrust-to-weight ratios of 10-3.

Earth-Mars and Earth-Jupiter cases Earth-Mars and Earth-Jupiter cases Also recall from lecture 28 that such levels of performance allow both fast human transport and efficient cargo transport, as shown in the figure at right.

Implications for space development

Although the emphasis of this document has been space propulsion, relatively small mass penalties would allow the magnetic fusion systems described here to produce electricity or to be used for materials processing by the hot fusion plasma. These applications, along with fusion's propulsion capabilities, would enable

Consequently, I confidently predict that, as the train functioned in opening the American West, D-3He magnetic fusion will open the Solar-System frontier.

Useful references


Selected references on fusion for space propulsion

NB: Widely varying assumptions have gone into the conceptual designs presented in these papers. Proceed with caution!

Worldwide Web

Selected fusion sites

Space-related, government, and other potentially useful Web sites


  1. Discuss the advantages and disadvantages of the key fusion fuel cycles D-T and D-3He for space applications.
  2. Compare the key characteristics of D-3He fusion and fission for Mars missions.
  3. Compare the key characteristics of D-3He fusion and fission for missions to access asteroid resources.
  4. Explain the main reasons that linear geometry is often preferred over toroidal geometry for magnetic fusion space propulsion.
  5. Discuss ICF fusion for space propulsion.

next previous up
Next: Lecture 32: Orbiting solar satellites
Previous: Lecture 30: Plasma and electric propulsion
Up: Resources from Space syllabus

Dr. John F Santarius
Fusion Technology Institute,
University of Wisconsin-Madison
1500 Engineering Dr.
Madison, WI 53706

415 Engineering Research Building
e-mail:; ph: 608/263-1694; fax: 608/263-4499
Last modified: April 12, 1996
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