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.

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

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.

From the Rheilffordd Ffestiniog Railway Web pages

Selected events in the history of fusion for space propulsion

1950's to 1970's: Conceptual designs were formulated for both D-³He and D-T fusion reactors for space propulsion. These included
- Simple mirror reactors and
- Toroidal reactors with magnetic divertors
Both type-II superconductivity and fusion power were new concepts in the early 1960's when the first D-³He design was performed.
Three recent advances enhance the feasibility of D-³He reactors for space applications.
- Lunar ³He resources
- More credible physics concepts
- Advances in technology

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

No radioactive materials are present at launch, and only low-level radioactivity remains after operation.
Conceptual designs project higher specific power values (1--10 kW-thrust per kg) for fusion than for nuclear-electric or solar-electric propulsion.
Fusion gives high, flexible specific impulses (exhaust velocities), enabling efficient long-range transportation.
D-³He produces net energy and is available throughout the Solar System.
D-³He fuel provides an extremely high energy density.

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

High charged-particle fraction allows efficient direct conversion of fusion power to thrust or electricity.
- Increases useful power.
- Reduces heat rejection (radiator) mass.
- Allows flexible thrust and exhaust velocity tailoring.
Low neutron fraction reduces radiation shielding.
D-³He eliminates the need for a complicated tritium-breeding blanket and tritium-processing system.

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

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:

Reduced neutron flux helps greatly
- Reduced shield thickness and mass
- Reduced magnet size and mass
- Increased magnetic field in the plasma
Direct conversion can be used to increase the net electric power if plasma thrusters are needed.
Many configurations can increase magnetic fields (B fields) in the fusion core, gaining power density from a B⁴ scaling.

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

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.

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.

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 kW_thrust/kg_reactor. 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 Author	Year	Configuration
Borowski	1987	Spheromak	10.5
Santarius	1988	Tandem Mirror	1.2
Chapman	1989	FRC	--
Haloulakis	1989	Colliding Spheromaks	--
Bussard	1990	Riggatron Tokamak	3.9
Bussard	1990	Inertial-Electrostatic	>10
Teller	1991	Dipole	1.0
Carpenter	1992	Tandem Mirror	4.3
Nakashima	1994	FRC	1.0
Kammash	1995	Gas Dynamic Trap	21(D-T)
Kammash	1995	Gas Dynamic Trap	6.4(D-³He)

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

D-³He fuel
Solenoidal magnet geometry (linear reactor geometry) for the coils producing the vacuum (without plasma) magnetic field.
Advanced fusion concepts that achieve high values of the parameter beta (ratio of plasma pressure to magnetic-field pressure).

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 scanned	Figure to be scanned
Spheromak	Inertial-electrostatic Confinement
Figure to be scanned	Figure 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.

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

Human settlements on the rocky planets or in space;
Scientific outposts near the gas-giant planets and elsewhere; and
Access to the vast resources of the asteroids.

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

Miscellaneous

Arthur C. Clarke, ``The Planets Are Not Enough,'' in The Challenge of the Spaceship (Ballantine, New York, 1961).
Willy Ley, Satellites, Rockets, and Outer Space (New American Library, New York, 1958), p. 17.
Francis F. Chen, Introduction to Plasma Physics and Controlled Fusion (Plenum, New York, 1983).
N.A. Krall and A.W. Trivelpiece, Principles of Plasma Physics (McGraw-Hill, New York, 1973).
T. Kammash, ed., Fusion Energy in Space Propulsion (AIAA, Washington, DC, 1995).

Selected references on fusion for space propulsion

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

A. Bond, A.R. Martin, R.A. buckland, T.J. Grant, A.T. Lawton, et al., ``Project Daedalus,'' J. British Interplanetary Society 31, (Supplement, 1978).
S.K. Borowski, ``A Comparison of Fusion/Antiproton Propulsion Systems for Interplanetary Travel,'' AIAA/SAE/ASME/ASEE 23rd Joint Propulsion Conference, paper AIAA-87-1814 (San Diego, California, 29 June--2 July 1987).
S.A. Carpenter and M.E. Deveny, ``Mirror Fusion Propulsion System (MFPS): An Option for the Space Exploration Initiative (SEI),'' 43rd Congress of the Int. Astronautical Federation, paper IAF-92-0613 (Washington, DC, 28 August--5 September, 1992).
S. Carpenter, M. Deveny, and N. Schulze, ``Applying Design Principles to Fusion Reactor Configurations for Propulsion in Space,'' 29th AIAA/SAE/ASME/ASEE Joint Propulsion Conference, paper AIAA-93-2027.
R. Chapman, G.H. Miley, and W. Kernbichler, ``Fusion Space Propulsion with a Field Reversed Configuration,'' Fusion Technology 15, 1154 (1989).
G.W. Englert, ``Towards Thermonuclear Rocket Propulsion,'' New Scientist 16, #307, 16 (4 Oct 1962).
J.L. Hilton, J.S. Luce, and A.S. Thompson, ``Hypothetical Fusion Propulsion Vehicle,'' J. Spacecraft 1, 276 (1964).
T. Kammash and M-J. Lee, ``Gasdynamic Fusion Propulsion System for Space Exploration,'' J. Propulsion and Power 11, 544 (1995).
H. Nakashima, G.H. Miley, and Y. Nakao, ``Field Reversed Configuration (FRC) Fusion Rocket,'' Proc. 11th Symp. Space Nuclear Power and Space Propulsion Systems (Albuquerque, NM, 1994).
C.D. Orth, et al.,``The VISTA spacecraft--Advantages of ICF for Interplanetary Fusion Propulsion Applications,'' Proc. IEEE 12th Symp. on Fusion Engineering, Vol. 2, p. 1017 (IEEE, Piscataway, NJ, 1987).
J.R. Roth, ``Space Applications of Fusion Energy,'' Fusion Technology 15, 1375 (1989).
J.R. Roth, W.D. Rayle, and J.J. Reinmann, ``Fusion Power for Space Propulsion,'' New Scientist 54 #792, 125 (20 Apr 1972).
J.F. Santarius, ``Magnetic Fusion for Space Propulsion,'' Fusion Technology 21, 1794 (1992).
J.F. Santarius, ``Magnetic Fusion for Space Propulsion: Capabilities and Issues,'' Proc. Amer. Nuc. Soc. Topical Meeting on Nuclear Technologies for Space Exploration, p. 409 (Jackson, Wyoming, August 16--19, 1992).
J.F. Santarius, ``Magnetic Fusion Propulsion: Opening the Solar-System Frontier,'' Proc. Second Wisconsin Symposium on Helium-3 and Fusion Power, p. 137 (1993).
J.F. Santarius and B.G. Logan, ``Generic Magnetic Fusion Rocket Model,'' 29th AIAA/SAE/ASME/ASEE Joint Propulsion Conference, paper AIAA-93-2029 (Monterey, California, 28--30 June 1993).
N.R. Schulze, ``Figures of Merit and Attributes for Space Fusion Propulsion,'' Fusion Technology 25, 182 (1994).
E. Teller, A.J. Glass, T.K. Fowler, A. Hasegawa, and J.F. Santarius, ``Space Propulsion by Fusion in a Magnetic Dipole,'' Fusion Technology 22, 82 (1992).

Worldwide Web

Selected fusion sites

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

Bookmarks

Questions

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