Chair Force Engineer

Thursday, January 26, 2006

Mars or bust

NASASpaceFlight.Com is reporting that NASA is conflicted about propulsion systems for a post-2030 mission to put humans on Mars. Both choices would use a nuclear reactor. The first method would employ a nuclear-thermal rocket, which would force vaporized hydrogen through a nozzle. The second, nuclear-ion rockets, would use the reactor's electricity to propel charged Xenon gas from a set of thrusters.

The tradeoff between the two systems is huge. Nuclear-thermal rockets have moderate efficiency (Isp between 800 and 900 seconds) and moderate thrust (tens of thousands of pounds.) Ion rockets have extremely high efficiency (Isp ~5000 seconds) but very weak thrust (only fractions of a pound.)

There is also a third way which has been woefully underfunded over the years--VASIMR. It would use large nuclear reactors (producing megawatts to electricity) to power microwave generators. The microwaves would excite a hydrogen plasma that would be ejected from the rocket by magnets. VASIMR offers similar Isp to ion rockets with sufficiently more thrust. Still, there is a long way to go until VASIMR is ready, and a lot more money will need to be invested in the technique.

The idea was pioneered by astronaut Franklin Chang-Diaz (one of the most experienced US astronauts, with 7 shuttle missions to his credit.) NASA has now decided to commercialize the concept with Chang-Diaz's company, Ad Astra Rocket Co. Perhaps some forward-thinking company will subsidize Ad Astra's continued research into VASIMR, with hopes it will be ready for primetime by the year 2030.

If we must choose between nuclear-thermal and nuclear-ion, I hope that a combination of both is selected. While a pure-ion rocket can get away with less propellant mass than a nuclear-thermal one, the mission time is made much longer due to the low-thrust spiral out of earth orbit and the low-thrust spiral to brake into Mars orbit. Humans will probably be cooked alive during a slow transit through the Van Allen Radition Belt. I would propose a reusable, nuclear-thermal tug to at least accelerate the crewed spacecraft to an orbit higher than the radiation belt, before the ion rockets are activated.

Besides the propulsion system debate, NASA has interestingly endorsed artificial gravity. This addition presents some challenging tradeoffs. It's known that prolonged exposure to weightlessness wreaks havoc on the human body, but the extent of the damage is not known (because the longest continuous stay in space for a human is 439 days, about half the length of a Mars mission.) Artificial gravity fights the ill-effects, with the drawback of dictating a larger and heavier spacecraft (to create the centripetal "artificial gravity" without the Coriolis force which leaves humans dizzy and disoriented.)

At the same time, an artificial-g spacecraft would require plenty of testing in earth orbit before it was ready for a Mars mission. Perhaps artificial-g will create more problems than it solves. Much like the exploration of Mars, if we never go, we'll never know.