Comparison Of Fusion And Antiproton Propulsion For Interplanetary Travel.pdf

1. Abstract
Rocket propulsion driven by either thermonuclear fusion or antiproton annihilation reactions is an attractive concept because of the large amount of energy released from a small amount of fuel. Charged particles produced in both reactions can be manipulated electromagnetically making high thrust/high specific impulse (Isp) operation possible. A comparison of the physics, engineering, and costs issues involved in using these advanced nuclear fuels is presented.

2. Introduction
A high-performance rocket system must operate with both a high-specific impulse Isp and a low-mass powerplant (MW) capable of generating large amounts of jet power (Pjet). Because the thrust-to-engine weight ratio [F(kg)/MW(kg) = 2000 αp(kW/kg)/Isp(s)] of a spacecraft is directly proportional to the engine specific power (αp ≡ Pjet/MW), large values of αp are required to provide the acceleration levels necessary for rapid transportation of cargo and personnel throughout the solar system.

3. Comparison of Fusion/Antiproton Propulsion Systems
Nuclear propulsion is currently receiving greater attention by both NASA and the U.S. Air Force. In Refs. 3 and 4, the Air Force has identified the direct fission thermal rocket and the antiproton annihilation engine (MAR) as systems worthy of development. The interest in antimatter is attributed to the fact that it is a highly concentrated form of energy storage.

4. Nuclear Propulsion
The SCR has a specific impulse potential comparable to the electrothermal (ET) thruster (~103 s) yet delivers thrust levels equivalent to those of chemical engines (~105 lbf). The performance of the SCR is limited, however, by the melting temperature of the fuel, moderator, and core structural materials.

5. Antimatter Propulsion
A milligram of antihydrogen –H [consisting of an antiproton p– and a positron e+ (an antielectron)] reacted totally with the same amount of normal hydrogen possesses an energy content equivalent to ~ 13 t (1 metric ton (t) = 103 kg) of LO2/LH2. Although synthetic –H is definitely a “high-test” propellant, it requires a large energy investment and will be expensive to manufacture (~$1010/g assuming commercial electricity usage), and difficult to store and manipulate.

6. Fusion Propulsion
The energy yield per reaction and its distribution among charged particles, neutrons, and gamma radiation, and the requirements on engine design of utilizing stable vs unstable reaction products for propulsive purposes are examined. The characteristics of the p–p reaction and the various fusion fuel cycles are presented.

7. Antimatter and Fusion Propulsion Concepts
A variety of antimatter and fusion propulsion concepts are described. Comparisons are also made between the different MAR configurations and their fission/fusion analog. Simple weight estimates and engine performance parameters are presented and used to quantify the advantages and disadvantages of each system.

8. Mission Capability, Fuel Costs and Availability, and Technology Requirements
The areas examined will include mission capability, fuel costs and availability, and technology requirements. The purpose of this chapter is to compare various antimatter and fusion rocket designs in an effort to obtain a clearer understanding and potentially quantify the advantages and disadvantages of each system.