NEEP602 Course Notes (Fall 1997)
Resources from Space

Lecture #32: Opening the Solar-System Frontier

Title: Fusion Propulsion

November 14, 1997

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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 presented 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 Density
Neutron Power Fraction 
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 limit B field near limit Relatively low B field 
Superconducting tokamak Copper tokamak Field-reversed configuration 
Stellarator Heliotron Spheromak 
Torsatron Tandem mirror 
Bumpy torus 
Reversed-field pinch 

Energy density of space-propulsion fuels

Fuel energy density for various fuelsA 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.    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. The projected specific powers for selected designs appear in the table below. Note: Widely varying assumptions and levels of optimism have gone into these conceptual designs and the resulting specific powers. An argument supporting the general performance level, based on a `generic' analysis, appears in Santarius and Logan, 1998. 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 Author Year Configuration Specific Power 
Borowski 1987
Tandem Mirror
Colliding Spheromaks
Riggatron Tokamak
Tandem Mirror
Gas Dynamic Trap
Gas Dynamic Trap
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. Clicking on the thumbnail figure will give a larger version. Clicking on the title will put the appropriate reference at the top of the page.
Field-reversed configuration (FRC) 
Plasma Engineering schematic (DT version) 
FRC plasma  FRC engineering 
Tandem mirror engine 
Tandem mirror 
Tandem mirror with life support and other systems 
by UW EMA 569 senior design project class 
Tandem mirror 
Spheromak  Inertial-Electrostatic Confinement (IEC) 
Spheromak  IEC propulsion device 
VISTA ICF space-propulsion design  VISTA 
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. 

D-3He Fusion Capabilities and Space Development

Space propulsionoption operating regimes 

The performance that D-3He fusion appears capable of providing for space propulsion is shown in the figure above. In particular, as discussed in lecture 8 on spacecraft trajectories, fusion is the only option that potentially 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. Also recall from lecture 8 that such levels of performance allow both fast human transport and efficient cargo transport, as shown in the figures below. 

Earth-Mars and Earth-Jupiter cases  Earth-Mars and Earth-Jupiter cases

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
 Fusion applications in space 

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.

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Next: Lecture 33: Economics of Large Space Projects
Previous: Lecture 31: 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: Tue Jul 21 16:57:47 CDT 1998 

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