Artist illustration of a nuclear thermal rocket. Image credit: NASA/Pat Rawlings

Jun 11, 2015 Half the time to Mars - the how and the why

Sen—In a speech given at the Aerojet Rocketdyne plant in Canago Park, California on May 28, NASA administrator Charles Bolden strongly suggested that advanced propulsion technologies are needed for effective travel to Mars. Specifically he stated that a 50 per cent reduction in travel time would be a reasonable goal to aim for. Typically this means a reduction from an 8-month one way trip to a 4 month trip.

I say “strongly suggested” because with all the will and technology in the world, without funding, it’s just hot air, and sadly, hot air does not move manned rockets. Well…that’s not technically true. Hot GAS does indeed move rockets, and air IS a gas. So has NASA finally figured out a way to harness hyperbole, or does Bolden have something a little more realistic in mind? And what are the benefits of cutting flight travel time?

To answer the flight time question first, the benefits are many. Firstly, there is the question of radiation exposure. Thanks to the eight month journey of NASA's Curiosity rover to the Red Planet, we know from the onboard Radiation Assessment Detector (RAD) that the average dose for a trip between Earth and Mars comes in at 300 mSv (measured in Sieverts, the Sievert being a unit of measurement showing absorption of radiation by the human body). To put that in perspective, 300 mSv is roughly the same amount of radiation one would experience if they undertook 24 CAT scans.

In short, it is still within the acceptable level for total dosage in an astronaut’s career. Although certain radiological effects can be predicted to a reasonable level of confidence (such as radiation sickness which occurs at a fairly constant level for all humans), so-called “stochastic effects” (random events, such as leukemia) are not so easy to predict. Also, the longer one spends in space, the higher the chance of experiencing some form of high radiation dose from a solar event (such as a solar flare), so all in all, any effort to reduce that transit time is going to be welcomed.

In addition to the radiation exposure, and the heavy shielding required to protect from radiation, there is the question of supplies. Astronauts need to eat and drink. The requirements for daily calorific intake remain constant throughout the trip, so the reduction of transit time by 50 per cent translates directly into a 50 per cent reduction in the mass of food carried.

And of course, all of this mass saving results in a saving of fuel needed for the trip, which in turn equals even less mass, which means it costs less money for the mission, because in space, mass = money.

So overall, it is a very desirable thing to reduce transit time. That is the “why”…so what about the “how”?

There is a lot of talk about exotic propulsion methods in the media at the moment. Many involve Star Trek style warp drives and quantum something-or-others, but Bolden seems to be focused on tried and tested methods rather than flights of fancy which may or may not be real.

Rockets, be they chemical, electric or nuclear, all operate on the same principle, the principle being Newton’s 3rd Law. If you chuck something out the back of something else, it will move forwards. Or to put it more succinctly, for every action, there is an equal and opposite reaction.

Some rockets operate by flinging a large mass out of the back fairly slowly (such as chemical rockets) and some operate by hurtling lots of tiny masses out of the back at ridiculously high velocities (such as ion engines).

Solar Electric Propulsion (SEP) systems, as mentioned by Bolden at Rocketdyne, fall into the Ion Propulsion category. SEP systems typically require ten times less propellant to move through space than traditional chemical rockets. The propellant in a SEP system is ionized and then accelerated out of the back of the spacecraft by an electric field, thus producing a gradual buildup of thrust over time. The actual thrust of the SEP system is dependent on the amount of electrical power available on the spacecraft.

During the talk at Rocketdyne, Bolden was joined by Rocketdyne president Scott Seymore who highlighted this fact. "We're now trying to get to higher power levels- that's the next step” Seymore told the audience. "Fifteen kilowatts would be the next step, and then to cluster them together ... then, in the long term, 50 to 100 kilowatts”.

Aerojet Rocketdyne is currently building a 4.5kW system, which has in fact already been tested in space. Their 4.5kW Hall Effect (HET) ion thruster was first flown in space in 2010, and to date is the most powerful HET ever flown.


Image: Hall Effect Thruster...thrusting.  Credit: JPL / Caltech

And of course, anything entering the megawatt range would require something a little beefier. Something more nuclear flavoured, such as a Nuclear Thermal Engine.

Typical chemical rocket engines get their thrust from the exothermic decomposition of a fuel and another reactive chemical (typically an oxidizer). In layman’s terms, you mix two reactive chemicals together; they explode and create a lot of heat. Simply shoot that explosion out of the back of the spacecraft and thrust is generated.

This means carrying a LOT of propellant. Nuclear Thermal engines do the same job as a chemical engine but instead of getting the energy from chemical reaction, the fuel (normally hydrogen) is heated by a nuclear reactor before being allowed out of the rear of the rocket. You still get the same kick as you do from a chemical rocket, but as you are using half of the mass of a chemical rocket then the propulsive efficiency increases. In short, you can put a lot more payload into orbit than a chemical rocket for the same price (over twice the usable payload).

NASA has in fact researched nuclear rocketry before, in the form of the NERVA project (Nuclear Engine for Rocket Vehicle Application). Apparently early tests of sub-systems showed that the principle was completely viable; however, the loss of political will from Congress and the subsequent lack of funding to the project meant that it was doomed to never see the light of day.

And that brings us full circle back to my opening statement regarding hot air. It would be nice to see some funding dedicated to these realistic projects for a change, rather than relying on pretty graphics depicting theoretical fantasy propulsion systems to spur the public imagination.