Fusion reactions release an enormous amount of energy, which is why researchers are devising ways to harness that energy into a propulsion system. A fusion-powered spacecraft could move up NASA's schedule for a manned Mars mission. This type of spacecraft could cut travel time to Mars by more than 50 percent, thus reducing the harmful exposure to radiation and weightlessness.
The building of a fusion-powered spacecraft would be the equivalent of developing a car on Earth that can travel twice as fast as any car, with a fuel efficiency of 7,000 miles per gallon. In rocket science, fuel efficiency of a rocket engine is measured by its specific impulse. Specific impulse refers to the units of thrust per the units of propellant consumed over time.
A fusion drive could have a specific impulse about 300 times greater than conventional chemical rocket engines. A typical chemical rocket engine has a specific impulse of about 450 seconds, which means that the engine can produce 1 pound of thrust from 1 pound of fuel for 450 seconds. A fusion rocket could have an estimated specific impulse of 130,000 seconds. Additionally, fusion-powered rockets would use hydrogen as a propellant, which means it would be able to replenish itself as it travels through space. Hydrogen is present in the atmosphere of many planets, so all the spacecraft would have to do is dip down into the atmosphere and suck in some hydrogen to refuel itself.
Fusion-powered rockets could also provide longer thrust than chemical rockets, which burn their fuel quickly. It's believed that fusion propulsion will allow rapid travel to anywhere in our solar system, and could allow round trips from Earth to Jupiter in just two years. Let's take a look at two NASA fusion propulsion projects.
Variable Specific Impulse Magnetoplasma Rocket
VASIMR is actually a plasma rocket, which is a precursor to fusion propulsion. But, since a fusion-powered rocket will use plasma, researchers will learn a lot from this type of rocket. The VASIMR engine is quite amazing in that it creates plasma under extremely hot conditions and then expels that plasma to provide thrust. There are three basic cells in the VASIMR engine.
- Forward cell - The propellant gas, typically hydrogen, is injected into this cell and ionized to create plasma.
- Central cell - This cell acts as an amplifier to further heat the plasma with electromagnetic energy. Radio waves are used to add energy to the plasma, similar to how a microwave oven works.
- Aft cell - A magnetic nozzle converts the energy of the plasma into velocity of the jet exhaust. The magnetic field that is used to expel the plasma also protects the spacecraft because it keeps the plasma from touching the shell of the spacecraft. Plasma would likely destroy any material it came in contact with. The temperature of the plasma exiting the nozzle is as hot as 180 million degrees Fahrenheit (100 million degrees Celsius). That's 25,000 times hotter than gases expelled from the space shuttle.
On a mission to Mars, a VASIMR engine would continuously accelerate for the first half of the journey, then reverse its direction and slow down for the second half. A variable exhaust plasma rocket could also be used in positioning satellites in Earth orbit.
Gas Dynamic Mirror Fusion Propulsion
Being developed simultaneously with VASIMR is the Gas Dynamic Mirror (GDM) Fusion Propulsion system. In this engine, a long, slender, current-carrying coil of wire that acts like a magnet surrounds a vacuum chamber that contains plasma. The plasma is trapped within the magnetic fields created in the central section of the system. At each end of the engine are mirror magnets that prevent the plasma from escaping out the ends of the engine too quickly. Of course, you want some of the plasma to leak out to provide thrust.
Typically, plasma is unstable and not easily confined, which made early experiments with mirror fusion machines difficult. The gas dynamic mirror is able to avoid instability problems because it is constructed in a long and thin manner, so the magnetic field lines are straight throughout the system. Instability is also controlled by allowing a certain amount of plasma to leak past the narrow part of the mirror.
In 1998, the GDM Fusion Propulsion Experiment at NASA produced plasma during a test of the plasma injector system, which works similar to the forward cell of the VASIMR. It injects a gas into the GDM and heats it with Electronic Cyclotron Resonance Heating (ECRH) induced by a microwave antenna operating at 2.45 gigahertz. Currently, the experiment is designed to confirm the feasibility of the GDM concept. Researchers are also working on many of the operational characteristics of a full-size engine.
While many of NASA's advanced propulsion concepts are decades from being achieved, the foundation of fusion propulsion is already being built. When other technologies are available to make a Mars mission possible, it could be a fusion-powered spacecraft that ferries us there. By mid-21st century, trips to Mars may become as routine as trips to the International Space Station.
For more information on fusion propulsion and other advanced propulsion concepts, check out the links on the next page.