NASA SBIR Contract NAS8-97136
Advent Project NA0002

SBIR Phase I Final Progress Report PR-NA0002-03

1. Background and Objective:

A magnetically confined plasma which utilizes fusion reactions is to be used as a propulsion system for an upper stage of a launch vehicle that could be employed in interplanetary travel. It makes use of a confinement scheme known as the magnetic mirror in which a hot plasma is radially contained in a solenoidal geometry by a magnetic field with a configuration that allows a certain fraction of the plasma to escape axially producing thrust. Unlike the ordinary mirror machine, the particle density in the proposed device will be sufficiently high to allow the plasma to behave like a continuous medium - a fluid with properties that render it especially suitable for advanced propulsion. Such a system can be constructed with almost present day or near-term technology drawing on relevant research that has accumulated over the past three decades.

The main objective of the effort was to demonstrate thrust enhancement of GDM. This was achieved by showing that when a supplementary propellant like hydrogen is employed, the thrust can increase significantly albeit at the expense of reduction in the specific impulse. This means that GDM can indeed serve as a variable thrust variable impulse propulsion system.

2. Work Performed:

In order to meet the technical objectives, four tasks were defined and carried out:

1. System Analysis of Power Plant
2. Critical Q-value and its Impact on Propulsion
3. GDM Performance with Different Fuel Cycles
4. Thrust Enhancement Approaches

Details of the work performed are described further below:

In order to obtain a critical Q-value, an analysis of the system which consists of the Gasdynamic Mirror (GDM), an injector, a thermal converter, a direct converter, and other components, was performed. One of the other components considered was a "hydrogen preheat" unit whereby preheated hydrogen may be injected into the system to enhance the thrust. It was found that the symmetric mirror, in which half the charged particle power goes to the direct converter to be converted to electric power, does not necessarily provide the most optimum performance. If less power is used by the direct converter (called an asymmetric mirror), then travel time can be reduced

Deuterium - tritium (DT) and deuterium - Helium-3 (D-He³) fuel cycles were examined to study the GDM performance with different fuel cycles. The first is considered the fuel for the first generation fusion reactors since the ignition temperature is the lowest for all the fuels considered for these systems. Most of the energy in the DT case appears in the neutrons and the radiative power (bremsstrahlung and synchrotron) is comparatively modest. Since the ignition temperature for the D-He³ case (» 60 keV) is significantly higher, the radiative power is correspondingly larger, and the neutron power density arising from the satellite reactions is significantly lower.

As noted in earlier reports, the GDM fusion propulsion system is capable of producing very high specific impulses (>10⁵ seconds), but at modest thrusts. To provide enhancement in this area we considered employing a supplementary propellant which may be pre-heated by regenerative cooling of the nozzle or other components of the system if sufficient power is available. If not, a heating unit such as an electro-thermal unit can be incorporated into the power flow system so as to heat the auxiliary propellant before injecting it into the system as illustrated in Figure 1. Once inside the chamber, it will be further heated by the radiation (Bremsstrahlung and synchrotron) emanating from the fusion plasma. If the propellant in question is hydrogen then some seeding will be utilized in order to enhance its absorption of the radiation.

To estimate the heating that results from the absorption of radiation we employ a simplified model which ignores the emissivity of the hydrogen gas and neglects the heat transfer in the direction of motion as is usually done for heat conduction in moving fluids. The appropriate energy balance is then given by

(1)

where p_r is the radiated power per unit volume or the heat source,is the temperature change in the direction of the flow and u the axial flow velocity. Noting that where n_H is the hydrogen particle density and m_H its molecular mass, Eq. 1 can be rewritten as

(2)

which upon integration becomes

(3)

where is the inlet temperature and the hydrogen residence time given by

(4)

This model was applied to a symmetric mirror and the two different fuels mentioned earlier. Several other candidate propellants were examined to establish which provides the best enhancement.

3. Results Obtained

The results of this model are shown in the following table:

Table: Parameters for simple thrust enhancement model

It should be noted that the quantities in the parentheses for the last three parameters in the table reflect the results for the un-enhanced system. It should also be noted that for both fuel cycles the effective thrust has increased significantly albeit at the expense of a sizable reduction in the specific impulse. This is reflected in the trip time which increased significantly in the DT case and less dramatically in the D-He³ case. This is due to the fact that the radiated power in the DT case is a much smaller fraction of the fusion energy produced. The significant change in the propulsive performance in the thrust-enhanced GDM stems from the fact that the propulsion parameters are determined almost totally by the hydrogen propellant rather than by the fusion plasma.

We have examined several candidate propellants to establish which provides the best enhancement. The results are shown in Figure 2 and Figure 3. Since they all have different properties, we decided to evaluate them on the basis of one mass flow rate, namely 3 kg/s. We see from the chart (Fig. 2) that helium and hydrogen provided the largest thrusts followed closely by lithium. The same can be said of the specific impulse but not of the exit temperatures. When the mass flow rate was allowed to vary, it is seen from the graph (Fig. 3) that hydrogen is indeed the desirable propellant followed by lithium. This is especially important for lithium since in a DT fusion propulsion system, lithium can be used as a breeding material of tritium as well as a good heat transfer medium. It is envisaged that lithium could be effectively used in a Brayton thermal conversion cycle that is coupled to a liquid drop radiator for dispensing with waste heat. These considerations may very well result in drastic reductions in total vehicle mass in a GDM propulsion system since the radiator constitutes a major portion of such a mass.

4. Assessment of the Project’s Technical Merit and Feasibility

The study revealed that GDM as a variable thrust, variable impulse system is feasible based on the underlying physics and engineering considerations which are will understood and easy to implement.

5. Assessment of the Potential Applications of the Project Results in Phase III

This study has revealed that an advanced propulsion concept that utilizes fusion reactors to generate radiation that heats a propellant can be utilized for NASA’s missions in space exploration. A commercial application of such a system might manifest itself in a "plasma torch" in which the fusion plasma and the radiation emitted there can be utilized in the treatment and safe disposal of hazardous waste.

6. Assessment of Whether the Results Justify Phase II Continuation

It can be argued that the results of Phase I can justify a Phase II continuation but the cost of an actual experimental device that can operate at the specified plasma parameters is not possible within present Phase II and III budget limitations.

Prepared by:

David A. Horvath, PE

Principal Investigator

List of Attachments

1. Figure of Thrust-enhanced GDM Propulsion Engine

2. Properties of Different Propellants at 3 kg/s Mass Flow Rate

3. Thrust and I_sp versus Mass Flow Rate for Different Propellants

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