Fusion Propulsion: Reaching the Stars by Wielding the Power of the Stars

The following blog is a guest post from Kevin Schillo who is currently pursuing a Master of Science in Aerospace Systems Engineering at the University of Alabama in Huntsville. His research is focusing on pulsed fusion propulsion, which could help to open up the solar system for routine manned exploration. Kevin has always had a profound interest in science fiction and the future of space development, a passion which he has expressed in the form of his first publication, the novella “Apotheosis,” which was published in the anthology “Against a Diamond Sky” as part of the Orion’s Arm Universe Project. Kevin further demonstrated his literary prowess at expressing the beauty and majesty of the cosmos and humanity’s role in it in the form of his essay “Allure from Afar,” which garnered him the student essay award at the Next Generation Sub-Orbital Researchers Conference in 2011.

Magneto-inertial fusion (MIF) has been shown to offer a short development path and small reactor size[1] and z-pinch in particular shows very favorable scaling for fusion breakeven.[2]

Fusion propulsion in general enables rapid interplanetary space travel with trip times significantly shorter than that offered by chemical, solar electric, nuclear thermal, and nuclear electric propulsion systems. The reasons for this include the 106 greater energy density in fusion compared to chemical reactions, significantly higher thrusts compared to electric propulsion, higher exhaust velocities compared to nuclear thermal, and direct conversion to thrust, which reduces radiator mass significantly compared to nuclear electric propulsion. Combined, this enables missions that are 1/2 to 1/4 of the typical mission times required by current technologies.

As an example, if fusion can be fully developed, it could enable astronauts to make roundtrip voyages to Mars lasting a total of about six months. Manned voyages to Jupiter and beyond could also be accomplished within a year.[3,4] This is clearly advantageous over chemical propulsion, which necessitates the usage of a prohibitively massive spacecraft and/or long voyage times for any manned mission to Mars or beyond. In addition to this, a fusion spacecraft could be single-stage and reusable, which would enable manned interplanetary missions to become routine.[3,4]

A roundtrip manned mission to Mars exemplifies the great advantage fusion has over other propulsion systems. The propulsion systems examined in this case are chemical, nuclear thermal nuclear electric, and two fusion systems, with specific powers of 1 kW/kg and 10 kW/kg. Assuming the mission objective is to deliver a payload mass of 100 mT to the surface of Mars, the mass of the spacecraft throughout the mission can be plotted in the figure below:

 

Figure 1. Spacecraft mass for roundtrip Mars mission.

 

This graph illustrates the massive vehicle size and/or long voyage times associated with chemical propulsion. The mission becomes feasible with the utilization of nuclear thermal and nuclear electric systems. However, manned voyages to Mars can only be made routine if fusion propulsion is used.

Despite the great potential that fusion offers humanity, much research remains to be done in order to make fusion propulsion a reality.  One of the key technologies that has yet to be developed is the nozzle, which is required in order to redirect the isotropic energy yield into a direct motion for thrust. The extremely high temperatures of fusion plasma exhaust would cause any solid-state nozzle to undergo physical damage and failure.[5] Because of this, a magnetic nozzle is necessary to redirect the plasma.

The type of nozzle depends on the confinement concept used to contain the high temperature plasma. Fusion propulsion systems can be classified as two general concepts: steady-state and pulsed systems. Steady-state systems confine high energy density plasma at high temperatures for relatively long periods of time. Pulsed systems contain the plasma for relatively brief periods of time.

In a steady-state magnetic nozzle, the plasma follows the magnetic field lines in a manner that mimics the geometric shape of a solid-state converging-diverging nozzle. This causes the plasma to choke at the nozzle’s throat and then expand supersonically as it enters the nozzle’s diverging section.

In a pulsed magnetic nozzle, magnetic field lines absorb the kinetic energy of an expanding plasma sphere. The field is compressed by the plasma until the magnetic pressure is equivalent to the dynamic pressure of the plasma. The plasma is then ejected from the nozzle as the magnetic field rebounds to its initial position.[6] This is illustrated in the figure below:

 

Figure 2

Figure 2. Pulsed Magnetic Nozzle Operation

 

The overwhelming majority of research and development in magnetic nozzles has focused on steady-state devices, and very little has been done with pulsed magnetic nozzles.[7,8]

A pulsed z-pinch fusion system has the potential to offer lightweight reactors, which would be crucial for any spacecraft using fusion propulsion.

Magnetic nozzle performance is measured by the ratio of the axial kinetic energy of the exhaust gases to the initial thermal energy of the plasma.  An efficiency derived from this has always been assumed and scant theories and absent experiments mean that there is a rich opportunity for exploring the physics and engineering needed to understand, characterize, and improve magnetic nozzle performance for pulsed systems.

 

Planned Experiments

The University of Alabama in Huntsville is working in partnership with Boeing and Marshall Space Flight Center to construct the Charger-1 facility at the Aerophysics Lab on Redstone Arsenal. The Charger-1 is a ~500 kJ pulsed power facility capable of 2 MA discharges at 3 TW of instantaneous power, and it provides a unique opportunity to explore and develop technologies that can help to make pulsed fusion propulsion a reality. Once operational in the spring of 2013, the Charger-1 will be used to conduct fusion experiments via z-pinch confinement, and will be the largest and most powerful puled power machine used by a university. The team is working with ORNL and Y-12 to create lithium deuteride (6LiD) in order to conduct the subscale thermonuclear fusion tests. 

 

Figure 3

Figure 3. Charger-1 device.

 

To conduct propulsion research, an experimental magnetic nozzle apparatus initially consisting of a single conducting ring will be assembled and coupled to the Charger-1 device in order to examine techniques to capture and redirect the plasma exhaust via magnetic fields. A 300 kW DC power supply will be utilized to drive the current through nozzle field coil. The magnetic nozzle will be generated by this current ring. Multiple nozzle field strengths, field geometries, and z-pinch implosion locations will be experimented with to determine an optimal nozzle design. Laboratory equipment consisting of thrust stands and a data acquisition system will be needed in order to measure the performance of the nozzle.

Due to volatile NASA funding, it is crucial that alternative sources of funds are found to support this research. For this reason, a crowdfunding campaign has been launched on RocketHub. Any amount that can be contributed to this campaign would be greatly appreciated.

Click here to support this research.

 

References

1. Lindemuth, Irvin R, and Richard E Siemon. 2009. “The Fundamental Parameter Space of Controlled Thermonuclear Fusion.” American Journal of Physics 77 (5): 407–416.

2. Slutz, Stephen A., and Roger A. Vesey. 2012. “High-Gain Magnetized Inertial Fusion.” Physical Review Letters 108 (2) (January 12): 025003. doi:10.1103/PhysRevLett.108.025003.

3. W.E. Moeckel, Journal of Spacecraft and Rockets 9, 863 (1972).

4. W.E. Moeckel, “Propulsion Systems for Manned Exploration of the Solar System,” NASATM-X-1864, (1969).

5. Maslen, Results for Icarus optimistic scenario withS.H., Fusion for Space Propulsion. IRE Transactions on Military Electronics, 1959(Mil-3, 52).

6. Orth, C.D., VISTA – A Vehicle for Interplanetary Space Transport Application Powered by Inertial Confinement Fusion, 2003, Lawrence Livermore National Laboratories: Livermore, California.

7. Bond, A., Martin, A.R. et.al., Project Daedalus – The Final Report on the BIS Starship Study. Journal of British Interplanetary Society, 1978.

8. Adams, R.B., et al., Conceptual Design of In-Space Vehicles for Human Exploration of the Outer Planets, 2003.