From his corner office at Ad Astra Rocket headquarters near Houston, Franklin R. Chang Díaz envisions his fission-powered variable specific impulse magnetoplasma rocket propelling a crew to Mars and back:
With a power plant similar to the ones on nuclear submarines, the plasma rocket could carry several people from Earth to Mars in 39 days, as opposed to what would be at least a 180-day journey on a chemical rocket, Chang Díaz says. The savings in food, water, air, tedium, and cosmic-ray exposure would be immense. In 2012, Ad Astra plans to test a prototype — using solar power rather than nuclear — on the International Space Station. An astronaut will spacewalk out to attach the 200-kilowatt engine, and if all goes well, it will bump the ISS into a more attractive orbit with about 5 newtons of thrust. The tests will begin to indicate whether VASIMR can figure in NASA’s grand plan to shuttle people and cargo to the moon and perhaps Mars over the next couple of decades. In particular, engineers will analyze two things: how efficiently the engine uses its electricity to produce plasma and how fast its radiator can siphon away excess heat.
His office may be in Houston, but Ad Astra’s warehouse lab is in Liberia, Costa Rica:
To test their radiator, the engineers prepare to fire the thruster. They settle into chairs at a row of desks facing a vacuum chamber the size of a school bus. Attached to one side is the business end of the apparatus: permanent magnets, a radio-frequency generator, a tank of argon gas, and the tube where they will generate the plasma before venting it into the vacuum chamber. The argon is flowing, and the magnets are powered up.”Cinco, cuatro, tres,” Jorge Oguilve-Araya, a lead engineer, chants into a walkie-talkie. ”Dos, uno. Pulso!” The RF generator switches on and releases a torrent of RF waves into the argon stream. The gas heats up and ionizes, turning into a plasma of about 50 000 kelvin. Magnetic fields generated by the permanent magnets hold and channel the viciously hot material, protecting the thruster walls from melting on contact. A purplish light fills the vacuum chamber before fading to black.
There’s a similar setup in Houston, but with one more stage. Another antenna generates an electric field to heat the plasma to a million kelvin. When the ions’ rotation frequency matches the frequency of the field, the potential energy in the electric field changes into kinetic energy for the ions, accelerating them in a direction perpendicular to the magnetic field lines. This configuration forms a magnetic beach — waves on which the particles then surf their way out of the rocket.
The key to the VASIMR is its variable specific impulse:
Rocket engineers love the idea of variable specific impulse, because it allows a spacecraft to behave more like a race car, adjusting its acceleration at each turn around a track. Chemical rockets are fixed at a relatively low specific impulse of around 450 seconds. They need lots of propellant and can produce lots of thrust. Heading off to Mars, a chemical rocket would thrust for half an hour to escape Earth’s gravitational well and then coast the rest of the way. VASIMR, on the other hand, can run at specific impulses between 5000 and 15 000 seconds using deuterium or as low as 4000 with argon. For wandering the interplanetary voids, high specific impulse — or low thrust — is good: With highly efficient propulsion, the engine can keep firing until it reaches a high velocity, generating minimum thrust near the middle of the trip.
NASA has always wanted a nuclear rocket — and they got much closer than most people realize:
Almost immediately after the agency was formed in 1958, it began working on nuclear reactors for space, under a program known as Rover/NERVA, which stands for ”nuclear engine for rocket vehicle application.” In the spring of 1969, just before Neil Armstrong planted his boot in the Sea of Tranquility, the NERVA team finished ground testing its first complete mock-up of a nuclear reactor, the NRX-XE. The reactor went through 28 start-and-shutdown cycles at the Nevada Test Site, where the United States tested nuclear bombs.During the 13 years of its existence, the program’s engineers built and tested 20 reactors and nearly produced a flight-qualified propulsion system. They measured thrust and vented radioactive exhaust at an isolated spot known as Jackass Flats, bordered by mountains and mesas. They demonstrated systems with half the mass of a chemical rocket and a specific impulse of about 845 seconds. They tested engines that could get a crew to Mars and back in 80 days. But before the reactors could fly, the program ended. It was the 1970s, and political pressures were marginalizing space science.
The Soviet Union kept nuclear reactors in play a bit longer. Between 1965 and 1988, it launched a series of naval satellites with small reactors on board. At least two of them failed, releasing radioactive materials and spooking politicians worldwide.
To use nuclear reactors for a trip to Mars safely, a launch vehicle would deliver a spacecraft with three inactive nuclear engines to low Earth orbit. Around 220 nautical miles up, at roughly the altitude of the International Space Station, the reactors would start up. They’d run for no more than 45 minutes, producing about 330 000 newtons of thrust and kicking the ship beyond gravity’s grip. Like a chemical rocket, the vehicle would coast most of the way to Mars and then fire its engines briefly to decelerate. The vehicle would ease into orbit and be greeted by a lander vehicle. The lander would ferry the astronauts to the surface, where the real mission would begin.
At the peak of nuclear rocket research, engineers were reaching thrust levels of almost a million newtons, well beyond what they’d need. ”We’re the only propulsion technology that I think is scaling down in size,” says Stan Borowski, an engineer pursuing nuclear thermal propulsion at NASA’s Glenn Research Center, in Sandusky, Ohio. In a typical nuclear rocket design, the fuel consists of graphite pellets mixed with particles of uranium-235 and bundled into fuel rods. Channels perforate the bundle, enabling hydrogen or helium coolant, which is also the propellant, to flow through. The nuclear reaction heats the rods and the propellant, which blasts out into space.
Apparently Americans used to have a can-do attitude.