Defense Intelligence Reference Document Aneutronic Fusion Propulsion(1)

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Germany called the Silvervogel that was intended for use in suborbital bombing of
distant targets. The shuttle requires expendable rockets to attain orbit. Several
efforts are underway to manufacture aircraft that can attain orbit with less cost and
environmental damage as the current rocket technology. While the Spaceship One
and Spaceship Two designed by Scaled Composites and Virgin Atlantic can carry
passengers into space, they lack the energy to propel a payload into orbit at the
required 17,000 miles per hour. The Boeing X-37 is designed to carry unmanned
payloads into orbit, yet still requires an Atlas V rocket to launch. The DARPA/USAF
Falcon is designed for hypersonic flight at Mach 6, far short of the Mach 23 or so
required for orbit. Through the use of RAM jets and hybrid systems (air
breathing/rocket), the goal of SSTO is achievable over the next 30 years.
LEO TO MARS (34 TO 249 MILLION MILES)
Chemical rockets have been used to launch probes to Mars with great success. The
transit time is typically 9 months, each way. Aerobraking in the Martian atmosphere is
used to slow down the vehicles, resulting in a considerable savings in fuel. For human
flight to Mars, the transit time must be as short as possible to minimize radiation
exposure from cosmic sources, including the proton flux from the Sun. The specific
impulse of chemical rockets is low; Isp = 421 seconds for the last two stages of the
Saturn V, for example. Ion thrusters are also existing technology and can generate
much higher specific impulse (3,000 seconds for xenon electrostatic drives to 30,000
seconds for VASIMR). Ion drives, however, typically generate very low thrust.
A high performance Hall effect ion drive with lsp = 8,000 seconds generates only 2.5
newtons of thrust, for example, which is enough to accelerate one kilogram of mass at
0.25 g, where g = earth's gravitation acceleration. This drive requires 140 kW of
electricity to operate and a supply of xenon gas as a propellant. For a probe having the
mass of the International Space Station (370 metric tons), we can use equation 6.1 to
calculate the time required to transit to Mars with our Hall effect ion drive.
1 ~ ✓ 2 ~ m (6.1)
In this equation, tis the transit time, F the thrust, d the distance, and m the mass of
the object to be accelerated at a constant rate. Results show that it will take at least 4
years to make this transit. The use of multiple drives may decrease this time.
For manned flights to Mars, an estimated transit time of 30 days would be considered
appropriate in order to minimize radiation exposure to the crew. For this scenario,
10,000 Hall effect ion drives would be needed along with 1.36 gigawatts of electrical
power, slightly more than the power generated by a single reactor at the San Onofre
Nuclear Power Plant. An alternative ion drive design is the Variable Specific Impulse
Magnetoplasma Rocket (VASIMR) developed by Franklin Chang-Diaz in 1977. These
systems use hydrogen, argon, or neon gas and generate 1 newton of thrust for 100 kW
of power. The VX-200, a 200-kW VASIMR engine, will be tested on the International
Space Station in 2011 or 2012. 21
Chemical rockets and nuclear electric propulsion can be used to reach Mars; both are
based on existing technology. A third candidate is nuclear thermal propulsion. Rockets
of this design were tested at the Nevada Test Site and were intended to launch
payloads from the Earth's surface. With Isp = 850 seconds, the performance more than
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