Future Science
 

Electromagnetic Propulsion:
Jolting Into Space
The U.S. Department of Energy (DOE) is typically not in the business of developing propulsion systems for NASA, but it is continually working on better superconducting magnets and very rapid, high-power solid-state switches. In the mid-1990s, Goodwin chaired a session for NASA's Breakthrough Propulsion Physics Project, which is working to design propulsion systems that have no propellant, use a very high energy system and can eventually overcome inertia.
"It seemed that there should be some way to use this technology that [DOE scientists] were developing to help NASA meet their goals, and it basically sprang from that," Goodwin said. What sprang from the DOE research was Goodwin's idea for a space propulsion system that uses super-cooled, superconducting magnets vibrating 400,000 times per second. If this rapid pulse can be directed in one direction, it could create a very efficient space propulsion system with the ability to achieve speeds on the order of a fraction of 1 percent of the speed of light.
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The heart of the system is the super-cooled, solenoid-style electromagnet and the metal plate that causes an asymmetry in the magnetic field.



During the first 100 nanoseconds (billionths of a second) of an electromagnet ramping up, the electromagnet is in a non-steady state that allows it to pulse very rapidly. After it ramps up, the magnetic field reaches a steady state and no pulsing occurs. Goodwin describes the electromagnet he is using as a solenoid, which is basically a superconducting magnetic wire wrapped around a metal cylinder. The entire structure will have a diameter of 1 foot (30.5 cm), a height of 3 feet (91.4 cm) and a weight of 55.12 pounds (25 kg). The wire used for this propulsion system is a niobium-tin alloy. Several of these wire strands will be wrapped into a cable. This electromagnet is then super-cooled with liquid helium to 4 degrees Kelvin (-452.47 F / -269.15 C).

For the magnet to vibrate, you need to cause an asymmetry in the magnetic field. Goodwin plans to deliberately introduce a metal plate into the magnetic field to enhance the vibrating movement. This plate would be made of either copper, aluminum or iron. The aluminum and copper plates are better conductors and have a greater effect on the magnetic field. The plate would be charged up and isolated from the system to create the asymmetry. Then the plate would be drained of electricity in the few microseconds (millionths of a second) before the magnet were allowed to oscillate in the opposite direction.

"Now, the catch here is, can we use this non-steady state condition in such a way that it only moves in one direction?" Goodwin said. "And that's where it's very uncertain that that can be done. That's why we would like to do an experiment to find out." Together with the cooperation of Boeing, Goodwin is seeking funding from NASA to perform such an experiment.

The key to the system is the solid-state switch that would mediate the electricity being sent from the power supply to the electromagnet. This switch basically turns the electromagnet on and off 400,000 times per second. A solid-state switch looks something like an oversized computer chip -- imagine a microprocessor about the size of a hockey puck. Its job is to take the steady-state power and convert it to a very rapid, high-power pulse 400,000 times per second at 30 amps and 9,000 volts.

In the next section, you'll learn where the system draws its power from and how it may send future spacecraft beyond our solar system.

Beyond Our Solar System
The U.S. Department of Energy also is working on plans for a nuclear space reactor for NASA. Goodwin believes that this reactor could be used to power the electromagnetic-propulsion system. The DOE is working to secure funding from NASA, and a 300-kilowatt reactor could be ready by 2006. The propulsion system would be configured to convert the thermal power generated by the reactor into electric power.
"For deep space, Mars and beyond, you pretty much need to go nuclear if you are going to move any mass," Goodwin said.

The reactor will generate power through the process of induced nuclear fission, which generates energy by splitting atoms (such as uranium-235 atoms). When a single atom splits, it releases large amounts of heat and gamma radiation. One pound (0.45 kg) of highly enriched uranium, like that used to power a nuclear submarine or nuclear aircraft carrier, is equal to about 1 million gallons (3.8 million liters) of gasoline. One pound of uranium is only about the size of a baseball, so it could power a spacecraft for long periods of time without taking up much room on it. This kind of nuclear-powered, electromagnetically propelled spacecraft would be able to traverse incredibly large distances.



Thermal energy from a nuclear reactor could be converted into electricity to power the spacecraft. Here we see a uranium-235 nucleus splitting through induced fission.

"You couldn't make it to the nearest star, but you could look at missions to the heliopause," Goodwin said. "If it worked extremely well, it could hit speeds of a fraction of 1 percent of the speed of light. Even at that, it would take hundreds of years to reach the nearest star, which is still impractical."

The heliopause is the point at which the solar wind from the sun meets the interstellar solar wind created by the other stars. It is located about 200 astronomical units (AU) from the sun (the exact location of the heliopause is unknown). One AU is equal to the average distance from the sun to the Earth, or about 93 million miles (150 million km). For comparison, Pluto is 39.53 AU from the sun.

In order to move people, a much larger device would have to be built, but the 1-foot diameter, 3-foot-tall electromagnetic could push small, unmanned spacecraft like an interstellar probe to very far distances. The system is very efficient, according to Goodwin, and it puts a lot of power through a superconductor. The question is whether scientists can convert that power to propulsion without destroying the magnet. The rapid vibration would likely bring the magnet to the edge of its strength.

Skeptics of such a system say that all Goodwin will accomplish is to vibrate the magnet very rapidly, but it won't go anywhere. Goodwin admits that there's no evidence yet that his propulsion system will work. "It is highly speculative, and on my most wildly optimistic days, I think there's one chance in 10 that it might work," said Goodwin. Of course, 100 years ago, people believed we had even less of a chance of ever getting to space at all.

What is Fusion?
We and our planet are the beneficiaries of millions of nuclear fusion reactions taking place every second inside the sun's core. Without those reactions, we wouldn't have any light or warmth, and probably no life. A fusion reaction occurs when two atoms of hydrogen collide to create a larger helium-4 atom, which releases energy. Here's how the process works:
Two protons combine to form a deuterium atom, a positron and a neutrino.
A proton and a deuterium atom combine to form a helium-3 atom (two protons with one neutron) and a gamma ray.
Two helium-3 atoms combine to form a helium-4 (two protons and two neutrons) and two protons.
Fusion can only occur in super-heated environments measuring in the millions of degrees. Stars, which are made of plasma, are the only natural objects that are hot enough to create fusion reactions. Plasma, often referred to as the fourth state of matter, is ionized gas made of atoms stripped of some electrons. Fusion reactions are responsible for creating 85 percent of the sun's energy.
The high level of heat required to create this type of plasma makes it impossible to contain the components in any known material. However, plasma is a good conductor of electricity, which makes it possible to be held, guided and accelerated using magnetic fields. This is the basis for creating a fusion-powered spacecraft, which NASA believes is achievable within 25 years. In the next section, we will look at specific fusion engine projects in development.

Flying on Fusion Power
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.
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Photo courtesy NASA
Artist's concept of a fusion-powered space vehicle approaching the Saturn moon Titan