AAWSAP DIRD, Aneutronic Fusion Propulsion I, November 2010

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Ion Drives
Ion thruster electrical propulsion provides a convenient and efficient method of
generating thrust. A gas that is easily ionized, such as xenon, is carried onboard as a
propellant. The voltage difference between an electrode and a metal screen accelerates
xenon ions toward the screen and out the back of the spacecraft, generating thrust.
The specific impulse of this kind of drive is about 3,000 seconds. It is relatively
common for satellites to use ion drives, generating minute forces measured in
millinewtons, to maintain orbit. Solar energy and radioactive decay are possible
sources of electric power for satellites in Earth orbit. In deep space, fusion or fission
reactors could provide electrical power. However, once the xenon propellant has been
expended, the ion drive is no longer useful.
Ion drives include the VASIMR (Variable Specific Impulse Magnetoplasma Rocket)
designed by the Ad Astra Rocket Company in Webster, Texas. This system uses two RF
radiowave antennae to couple energy into an ionized gas that is used for propulsion in
space. While ion drives are often used to help maintain orbit for satellites circling the
Earth, the VASIMR technology has been proposed for use in moving payloads
throughout the solar system. The specific impulse for this technology is cited as 5,000
seconds compared to ~3,300 seconds for typical ion drives. 5
Photonic Propulsion
Photons of visible light, infrared radiation, or x-rays can produce thrust through
momentum transfer, where the momentum of each photon is given by p = h/A (h =
Planck's constant, A = radiation wavelength). Photonic propulsion has been explored by
Y. K. Bae Corporation (http://www.ykbcorp.com) who holds a patent on a photonic
laser thruster.
These drives generate no contaminants and require a source of
electricity to produce photons.
Their photonic laser thruster (PLT) uses an active
resonant optical cavity formed between two mirrors on a pair of spacecraft to generate
thrust. Photonic drives would be viable on fusion or fission -powered spacecraft if their
power output were used to generate electrical power that could provide light.
RADIATION SHIELDING
Radiation shielding will be important for astronauts traveling to the Moon, to the other
planets, and to other star systems. Radioactive particles and cosmic rays left over from
the big bang, radiation from supernovae, x-ray emissions from black holes, and a
constant flux of energetic protons from our own sun all contribute to the radiation dose
received by humans in space.
Radiation levels are typically measured in sieverts (Sv), expressing the amount of
energy deposited in human tissue from radiation. One sievert is equivalent to 1 joule of
energy absorbed for every kilogram of tissue. 6 • 7 An older unit, the rem (Roentgen
equivalent, man) is still in common use: 1 rem = 0.01 Sv.
Sieverts are now the
international standard unit.
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