Researchers Forge Tools to Conquer The Final Frontier

April 2005
By Henry S. Kenyon

 
The ghostly blue glow of electrically charged xenon gas radiates from a 50-kilowatt Hall ion thruster at NASA’s John H. Glenn Research Center. Scientists at the center are developing a variety of electric propulsion systems to efficiently propel spacecraft into the outer solar system.

Ion engines and inflatable antennas bring manned missions to Mars and beyond closer to reality.

To meet a bold presidential mandate for space exploration, NASA scientists are developing new power and propulsion systems for future generations of manned and unmanned missions. These applications will allow spacecraft to travel efficiently far into the outer solar system, preparing the way for a manned return to the moon and eventually a manned flight to Mars. And new technologies to stay in touch with these distant missions may revolutionize near-earth communications as well.

In January 2004, President George W. Bush launched a new deep-space exploration policy. The goal of this effort is to develop a sustained and affordable human and robotic program that will begin to extend human presence across the solar system over a 25-year period. The plan calls for the development of new manned exploratory vehicles followed by a return to the moon in the 2015 to 2020 time frame. Coupled with an ongoing and extensive robotic exploration program, the effort would set the stage for a manned mission to Mars around 2030. To achieve these goals, the agency is conducting basic research on improving the propulsion and power systems that will carry future explorers across the void.

NASA’s John H. Glenn Research Center, Cleveland, Ohio, has been developing power and propulsion technologies for space exploration since the 1950s. Its special facilities for simulating deep-space environments and in-house expertise provide the center with research capabilities matched by only a few laboratories around the world.

One key research area is electric, or ion, propulsion. This work focuses on high-power systems for large deep-space probes and possible manned spacecraft and lower power thrusters for satellites and small probes, says Robert Jankovsky, chief of Glenn’s electric propulsion branch. Unlike conventional rocket engines that use chemical fuels to produce thrust, electric propulsion uses an electric field to ionize small amounts of gas. Commonly referred to as ion engines, these systems cannot be used in the atmosphere because they produce minute amounts of thrust, but they are very efficient for propelling vessels with constant acceleration across great distances in space.

The new initiative calls for more powerful thrusters to propel vehicles to Mars and beyond. Spacecraft such as those envisioned by NASA’s Project Prometheus would rely on nuclear power and require engines using 30 to 40 kilowatts of power. Also, plans exist to develop thrusters in the multihundred kilowatt range, Jankovsky says.

Although it produces little thrust, electric propulsion is more efficient for long-duration missions than are rocket engines. Jankovsky explains that the gas mileage of a rocket is measured by its specific impulse, the measure of thrust per pound of fuel. Chemical rockets typically have a specific impulse of 300 to 450 seconds.

By comparison, electric thrusters’ specific impulse ranges from 600 to 9,000 seconds, which is 20 times more efficient than chemical propulsion. Jankovsky stresses that no spacecraft has ever achieved such a high specific impulse before. “That’s a big deal. This allows us to go places we couldn’t go before and stay in orbit a lot longer,” he says.

Researchers also are examining the use of low-power electric thrusters. These systems are powered by solar cells and generate between 2 and 6 kilowatts for propulsion. This type of ion engine flew on NASA’s Deep  Space 1 probe, which tested a variety of technologies and was the first spacecraft to rely solely on electric propulsion for thrust and maneuvering power.

Ion engines have been used for decades in satellites in lieu of chemical thrusters for orbital station keeping. The economic advantage of these systems is that they take up less space than rocket engines, allowing for smaller, lighter spacecraft and less expensive launches. Explaining that power levels on commercial spacecraft are approaching the 20-kilowatt range, Jankovsky predicts that, within a decade, electric propulsion will begin to replace rockets for orbit insertion. One type of ion engine called a Hall thruster is very efficient for this type of operation because of its 1,500- to 2,500-second-specific-impulse range. He notes that the Russians have used Hall thrusters as maneuver systems on some of their satellites for the past 15 to 20 years.

Because electric drives produce very low acceleration, Jankovsky believes the military probably will retain some chemical rocket systems to maneuver some spacecraft quickly—for example, shifting a surveillance satellite from orbit over Korea to Iraq. A typical chemical engine for orbit insertion generates about 100 pounds of thrust, whereas an electric thruster creates roughly 80 millinewtons of thrust—equivalent to the force applied by the weight of a grain of rice, he observes.

But a tradeoff exists between thrust and fuel efficiency. Instead of running for minutes, electric thrusters run for hours. The engines for the Deep Space 1 probe ran for more than 30,000 hours in ground tests—approximately 3.5 years. The engines for the Prometheus spacecraft will have to operate for 10 years. Jankovsky notes that one of the challenges is to develop the background data to qualify these initial spacecraft. “If we had the thruster ready today and turned on, we could not demonstrate full life until after launch,” he says.

Ion engines do use fuel, usually an inert gas such as xenon, but they require much less of it. For example, a rocket-propelled space probe may need 100 to 200 kilograms of chemical propellant, while a spacecraft with electric thrusters might need 10 kilograms of fuel for the same or greater performance. “That’s a lot of mass, and in some missions it’s not an insignificant amount. But relative to what you would have to do chemically, it’s an order of magnitude less,” he maintains.

Despite these challenges, ion propulsion is a proven and tested technology. Jankovsky explains that power increases up to 20 kilowatts do not pose major technological challenges. But once power levels reach the megawatt range, ion thrusters become inefficient because they must be concentrated, increasing the weight and size of a spacecraft. So, engineers have the option of using experimental thrust technologies, or they might choose nuclear thermal propulsion, which operates by introducing water or some other propellant into a reactor chamber and using the resulting gas and heat for thrust. But, he observes, it will be another 20 to 30 years before engineers are forced to reconsider propulsion systems because of increasing power requirements.

The power generated by a spacecraft’s reactors has to be converted into energy to drive systems such as ion engines, sensors and scientific experiments. Scientists at the Glenn Research Center are improving heat-engine technology to convert thermal energy into power. Thermal electric generators have flown in more than 22 spacecraft since the 1960s, but they are not very efficient, converting only 4 to 6 percent of heat energy into electrical power, explains Richard Shaltens, chief of Glenn’s thermal energy conversion branch.

NASA scientists are working on systems to convert energy from power sources such as the nuclear reactors that will fly on the Prometheus spacecraft and the surface power systems for future manned and robotic facilities on the moon and Mars. Because advanced heat engines are more efficient, converting from 20 percent to more than 30 percent thermal energy into electricity, they can generate more power from less fuel. For example, where previous heat engines drew 4 watts per kilogram of plutonium, the new systems can create between 6 and 8 watts. Improved thermal conversion also means that spacecraft will require less nuclear fuel, creating a variety of economic benefits such as less expensive launches and lighter spacecraft.

 
Deep-space exploration will require high-data-rate communications with Earth. Researchers at the Glenn center are developing inflatable offset parabolic membrane antennas that easily can be stowed aboard spacecraft. Once inflated, these lightweight aerials will allow space probes to transmit messages at speeds of more than 100 megabits per second.
Thermal engines resemble closed-cycle turbines or pistons that use gases such as helium or helium-xenon to drive their internal systems through heat exchange. As with electric propulsion systems, extending thermal engines’ operational life for deep-space missions remains a challenge. Shaltens explains that terrestrial thermal engines have 2,500-hour operational lives with many on/off cycles. Existing databases for ground systems are only about 10,000 hours long, but space applications will require studies of nearly 100,000 hours to fully eliminate the potential for system failures, he says.

Now if these propulsion efforts are successful, NASA will have to find new ways of keeping in contact with these spacecraft. The Glenn center is conducting research into communications systems to link future deep-space and planetary probes with terrestrial command centers. One project involves inflatable antennas for space and ground applications. According to Robert Romanofsky, senior research engineer at NASA Glenn’s Communications Technology Division, the antenna work originated from a concept for deep-space relay satellites and as high-data-rate links back to Earth from Mars orbit. These spacecraft would be stationed at La Grange points, fixed areas in space where a planet’s gravity interacts with that of its moons to form a region where both of their gravitational fields are nullified. Any object placed in a La Grange point will remain there permanently without the need to fire its thrusters repeatedly to maintain its position.

A major attraction of inflatable antennas is that they occupy very little space and weigh little when stowed aboard a spacecraft. Romanofsky notes that the theoretical ratio for the deployed-to-packed volume of such a device is 75-to-1. Extremely light, an average aerial will weigh about 0.17 kilograms per square meter. He adds that NASA intends to deploy antennas in the 10- to 25-meter size range in the near future. In line with the president’s space exploration initiative, NASA is interested in systems that can return data rates of more than 100 megabits per second from Mars. “To do that, you need either a large aperture or lots of transmit power, or both, as well as large, sensitive receivers here on Earth,” he says.

Another advantage of inflatable antennas is that a very large structure can be deployed that is more sensitive, uses less power and is lighter than aerials made out of conventional materials. For example, antenna gain is measured by the square of the aperture size. A 10-meter antenna has 100 times the gain of a 1-meter antenna. The increase in size also means that designers can reduce transmitter power by the same factor, Romanofsky says.

The inflatable antennas are made of a polymer used in a variety of space applications such as satellite thermal insulation and solar sails. To fully inflate, the antenna opens like a clamshell to form two parabolic surfaces. The polymer is 1-millimeter thick and coated with a 1,000-angstrom layer of silver to form a highly reflective surface. Romanofsky explains that the work behind the antennas originated in another division at NASA Glenn, which was developing an inflatable solar concentrator to focus sunlight on spacecraft solar cells. The antenna uses the same shape as the collector, which is effective for microwave antenna work, he says.

But Romanofsky cautions that there is still much to do before an antenna can be tested in space. Engineers must select a gas to inflate the structure, and once inflated the antenna must be made rigid so that it will not deflate if it is pierced by a micrometeorite. “We would like to have some sort of ultraviolet cure occur that would make the membrane or at least the canopy a solid surface once its inflated and rigid so that if there is a penetration, it wouldn’t matter,” he explains.

Glenn engineers currently are evaluating a 4-meter x 6-meter offset parabolic membrane antenna at the center’s near-field facility. The antenna will be tested from X band through Ka band, roughly 8 gigahertz through 32 gigahertz. In addition, the tests will study how errors and imperfections in the surface coating affect efficiency and performance.

Another aerial type under consideration is a Cassegrain antenna. This device has a transparent central aperture, with the feed and most of the hardware located behind the antenna. A hyperbolic subreflector would be the focus. This system may be easier to inflate and deploy than the clamshell version, Romanofsky explains. The center also is working on other materials such as using shape-memory polymers and composite materials instead of inflatable membranes. Within two years, he believes, one of these approaches will be selected as the most effective technology for deep-space missions.

 

Web Resources
NASA Glenn Research Center: www.nasa.gov/centers/glenn
Ion Propulsion: http://www.nasa.gov/centers/glennlioin_propulsion.html
Project Prometheus: http://exploration.nasa.gov/programs/prometheus.html