Dave Dietzler
The Basics
SMART-1 demonstrated the feasibility of ion drives for spacecraft propulsion by successfully reaching the Moon. It took about a year to do the job which is acceptable for cargo, but not for manned flight. However, ion drives of the future could be speedier. Ion drives use electrostatic fields to accelerate ions of krypton, xenon, or mercury and some have suggested the use of sodium or lithium. Ion drives have high exhaust velocities but low thrust and use meager amounts of reaction mass.
Higher thrust ion drives could be devised by increasing the exhaust velocity but the energy requirement for that goes up to the square of the velocity. KE=½mv2 with m in kilograms and v in meters per second to get the answer in joules or watt-seconds. The mass flow through ion drives could be increased a bit to improve thrust and this will require more energy to accelerate that mass and possibly a larger drive, and there are limits to the number of ions that can be forced into such a drive because of ion repulsion.
Ion drive units could be clustered together to improve thrust and reduce flight times also. More thrust from clusters of drive units, like more exhaust velocity, will require more energy but this increases directly and not exponentially so it seems like the better way to go. More energy means we need more efficient and larger powerplants. This cuts into payload mass and hurts mass ratio and top speed.
The Vapor Core Reactor
Scientists of the Innovative Nuclear Space Power Institute at the University of Florida are working on a high temperature vapor core nuclear reactor that uses UF4 gas and MHD power generation systems. By adding turbogenerators it may be possible to reach thermal to electric efficiencies of over 70% which is unheard of when most nuclear space power systems are only about 5% efficient.
Even better, they predict specific powers of better than one kilowatt per kilogram, even almost three kilowatts per kilogram for the whole system and that includes pumps and waste heat radiators as well as the reactor itself. Since net power generating fusion reactors don't even exist and it looks like they will be massive machines suitable for stationary power generation but not mobile systems, the VCR-MHD may be the space powerplant for ion drives and VASIMR drive systems of the future.
VASIMR drives are similar to ion drives in that they eject an ionized plasma but they use extreme temperatures rather than electrostatic acceleration of ions. If a VCR energized ion drive system can be developed that has enough thrust to reach escape velocity in days or weeks instead of months, it could speed up freight transportation and even be used for manned missions to Mars or Venus. Manned ships would have to depart from GEO, well above the Van Allen belts to avoid being poisoned by radiation during several days or weeks of acceleration up to escape. Astronauts would take small rapid chemically propelled taxis to GEO and board their ships. Upon escape the ion drives would keep thrusting for weeks until even higher speeds and shorter times of flight to Mars are acquired.
VASIMR Thrust
A plasma drive like VASIMR can develop one newton of thrust per 100 kilowatts. One newton equals about 0.225 pounds. A one thousand ton Mars/Venus clipper with a 100 ton VCR generating 200 MWe will get about 450 pounds of thrust from a VASIMR drive. This will allow an acceleration of 0.0022 meters per second per second. In one hour the ship will speed up by 7.94 meters per second. In one day 190.5 m/s and in one week 1333 m/s or 1.33 kps. In four weeks 5.33 kps and in eight weeks 10.66 kps. Constant low thrust can lead to high speeds. VASIMR can be operated at higher power levels than electrostatic ion drives.
Reaction Mass
Reaction mass is needed for electric drives but elements like xenon, krypton and mercury are rare on other worlds. Hydrogen is typically envisioned as the reaction mass of choice for VASIMR. We will never "fuel" fleets of hundreds of large ships with Xe, K, or Hg, and hydrogen, though abundant in the universe, is not found in large amounts on the Moon, Mercury or Venus. Until we mine Mars or the ice moons of the Outer Planets we won't have much hydrogen.
Other elements can be used in electric drives like ion or VASIMR. Sodium has potential and is found on the Moon, however, magnesium is more abundant and has been overlooked as potential ion drive reaction mass. There is only about 0.29% sodium in the lunar regolith and about 6% magnesium. The presumption made here is that lunar regolith will be the prime source of materials for large scale operations in the future, and that is a reasonable prediction. Metals have to be vaporized and passed through charged screens to ionize them before acceleration by electrostatic plates in ion drives or super heated in VASIMR. Magnesium boils at only 1120 C versus 900 C for sodium. That's not much higher. Magnesium has an atomic mass of 24, similar to sodium at 23, and has a valence of +2 versus +1 for sodium. It will be as easy to accelerate magnesium ions as sodium ions with thermal energy in VASIMR.
Magnesium is plentiful on the Moon and will probably be found on other worlds. Most silicate bodies have iron cores, ferromagnesian suite mineral mantles that often reach the surface as basaltic lavas, and anorthositic crusts. Viking landers detected magnesium at 3.6% in martian soil. We can expect to find magnesium throughout the solar system and not just on the Moon. We will not be forced to center ion drive fueling operations around the Moon beyond Earth orbital and cis-lunar space. Magnesium will be mined on many worlds including Mercury and asteroids that were once part of the mantles of shattered parent bodies.
Other Electric Drives
Magnetoplasmadynamic or MPD and Pulsed Inductive Thrusters or PIT have potential. MPDs generate lots of thrust compared to other electric drives from a small sized thruster but they run hot and have electrodes that can burn up. PITs have no electrodes to burn up and can operate for great lengths of time. Thrust can be scaled up by increasing the pulse rate. Hydrogen, lithium and argon have been tested in MPDs and PITs. Intuition tells me that PITs may function better than VASIMR with magnesium vapor and that PIT is preferable to MPD because it has no electrodes to wear out. As far as I know, no experimtation has been done with magnesium in any of these drives because NASA planners focus on single missions launched up from Earth where they can use anything for reaction mass that they want. Magnesium may not be as good as hydrogen for instance, but it will be cheap and plentiful when the Moon is mined and huge numbers of ship ply the solar system.
Planet Fall
Upon approach to another planet an electric drive propelled ship could start decelerating weeks ahead of time and enter a tightening elliptical orbit around the planet over the course of several weeks. Weary space travelers could become impatient. It will be possible for the ship to rendesvouz with a truly high thrust taxi powered by chemical or nuclear thermal engines. The travelers will board the small taxi and rocket over to a space station in low orbit then transfer to a lander. The flight will only last a matter or hours.
The interplanetary ship could remain on automatic control while it slowly circularizes its orbit and docks with a space station where it is serviced and reloaded with reaction mass. In Mars orbit this reaction mass may come from Deimos or Phobos that are certain to contain magnesium and in Veneran orbit it may come from a solar-mag sail robotic freighter pipeline from the Moon or Mercury where magnesium is mined and launched into space with surface based mass drivers.
Another trick would be to use Al+LUNOX boosters when leaving a Lagrange point in Earth orbit to soar up to escape velocity and use a gravity assist from Earth to get going. The electric drives will operate for weeks to increase speed and shorten time of flight to Mars or Venus where someday floating cities may emerge at an altitude of about 50 km.
