It’s heartening to see that NASA has inked an agreement with a commercial firm to get its VASIMR (Variable Specific Impulse Magnetoplasma Rocket) technology into the private sector. Houston-based Ad Astra Rocket Company is actually located within the Johnson Space Center and, under the direction of president and CEO Franklin Chang-Diaz, focuses on the development of plasma rocket technologies. Its agreement with NASA should further work on a design widely thought to offer powerful advances over conventional chemical rockets.
Chang-Diaz is a veteran of seven Shuttle flights who retired from NASA last July to focus on the Ad Astra/VASIMR connection. The design, which he conceived back in 1979, uses magnetic fields to channel a plasma exhaust that would melt conventional rocket nozzles. An additional beauty of the concept is that both thrust and specific impulse can be varied during the course of a mission. Ad Astra compares this to the transmission in an automobile; the exhaust characteristics are varied while constant power is maintained, resulting in the shortest trip time with the highest payload for a given amount of fuel.
Plasma itself is a high-temperature ‘soup’ of charged particles that can be manipulated by a magnetic field. It’s fascinating to reflect that 99 percent of the visible universe — the Sun and other stars — is in a plasma state of some kind. And ponder this: the particles in plasmas capable of being created today move at velocities of 300,000 m/sec at temperatures comparable to those inside a star.
To create the plasma for VASIMR’s use, a propellant like hydrogen is injected into a system of magnetic cells, where it is first ionized and then heated by radio-frequency excitation before being exhausted out the back of the rocket by a magnetic nozzle. “The promise this system holds could dramatically reduce the travel time for interplanetary missions,” says Chang-Diaz, “cutting trip times to Mars by one half or better.” Another benefit: the engine’s residual magnetic field can theoretically provide shielding against radiation, while the variability of thrust and specific impulse offers a wide range of abort options.
Centauri Dreams‘ take: Using hydrogen as a fuel for VASIMR makes refueling along the route a viable option; hydrogen is going to be available at any destination reachable with this technology. The Mars mission that Chang-Diaz talks about could refuel on the surface of the planet, and hydrogen could also be used as a radiation shield during the long cruise. But for now the real story is the interface between NASA and Ad Astra, because getting advanced concepts into commercially viable frameworks is the key to serious progress.