Solar Electric Propulsion: NASA’s engine to Mars and Beyond
NASA is hard at work developing what they believe is the best space engine for future missions to Mars and beyond. It’s not warp drive. No, nothing so exotic or dreamy. In fact, it already exists. The challenge is to enhance it for our needs in space in the coming decades. That is the hope, and the goal, of NASA’s continuing development of solar electric propulsion (SEP).
Solar electric propulsion uses electricity generated from solar arrays to ionize atoms of the propellant xenon. These ions are then expelled by a strong electric field out the back of the spacecraft, producing thrust. So, in short, SEP is a propulsion system that is a combination, or coupling, of solar array technology and ion thruster technology.
The NASA Glenn Research Center in Cleveland, Ohio, has been a leader in both ends of this technology for decades. Its work with ion thruster technology began with the Space Electric Rocket Test 1 in 1964. Today, ion thrusters are used to keep over 100 geosynchronous Earth orbit satellites in their locations, a process called station keeping. The Deep Space 1 mission, which made flybys of asteroid Braille and the comet Borelly between 1998 and 2001, used the NASA Solar Technology Application Readiness (NSTAR) ion propulsion system.
“The greatest example of the technology we’ve developed here that is currently in space is the Dawn mission,” said NASA Glenn senior propulsion engineer Dave Manzella told SpaceFlight Insider.
The Dawn spacecraft, launched in 2007 atop a United Launch Alliance (ULA) Delta II booster, used three NSTAR ion thrusters to propel it on its voyage to the asteroid belt where it has orbited the giant asteroid Vesta and, currently, the dwarf planet Ceres. Dawn was the first spacecraft to go into orbit around two extraterrestrial bodies – an impressive feat that, if it had used chemical rockets, would have required a much heavier spacecraft and a tremendous amount of fuel to achieve.
“The characteristic of SEP that makes it so attractive is basically its fuel economy,” Manzella said. “We talk about a parameter called specific impulse. It is a measure of the average exit velocity of the gas that’s coming out.” SEP can provide specific impulses that are an order of magnitude higher than can be achieved with chemical systems. That means SEP can get ten times the fuel economy, which means it can use an order of magnitude less propellant to do the same mission. Why is that important? “Because with all our missions, much of what we launch is just propellant to go other places,” Manzella said. “If you can impact the amount of that mass, you can really impact the economics of doing things in space.”
So why isn’t SEP being used everywhere?
For chemical propulsion, the thrust is generated by a chemical reaction that frees the energy that is in the chemical propellant itself. That energy is what is used to create the propulsive force.
“For electric propulsion, we basically take electrical energy that we generate from a solar array and we add that to the propellant, and that’s how we create the thrust,” Manzella said. “So the amount of thrust that we generate is related to how much electrical energy we can produce.”
Currently, the state of the art for electric propulsion systems are systems that operate at single digits of kilowatts, from 1 kW to 5 kW.
“A kW is about the amount of power you’d have in an electric hair dryer,” Manzella explained. “It’s not a huge amount of power, and the impact of that is that the amount of thrust is not great. So that’s not good for pushing big things far distances.”
That is the reason that up to now SEP has been used almost exclusively for station keeping on geostationary satellites.
“But even at that low kW power level, you can do primary propulsion missions like Dawn,” Manzella said. “However, that mission was launched in 2008 and had very long trip times for using that low power electric propulsion system. Ultimately we would like to have systems that are on the order of hundreds of kW. That would allow us to generate thrusts that are multiple pounds that can move things in months, with relatively large payloads. That’s what we’d like to have in support of human crewed missions to Mars. Even at hundreds of kW, those forces on the order of pounds, we very likely wouldn’t be pushing crew. We would be using that to pre-position assets or logistics. Things like habitats or return vehicles that we can put into orbit around Mars. Then send the crew with direct chemical propulsion trajectory.”
SEP systems operating at hundreds of kilowatts – that is the goal. However, the jump from 5 kW to hundreds of kW requires advancing the technology to an interim stage. NASA Glenn Director James Free recently identified this effort as one of his center’s biggest challenges for 2016.
“The solar electric propulsion technology we’re developing is a system that basically is a factor of 3 higher performance than anything we’ve done thus far,” Free said. His center, along with the Jet Propulsion Laboratory (JPL), is in the process of awarding contracts to engineering firms to fully develop the new systems. “We will be working on handing that over so that the contractor can go make that hardware and deliver it in 2019. Awarding that contract and handing it over are the two biggest technical challenges we have on the space side this year.” NASA recently awarded the first of these contracts, to further develop an ion propulsion unit called the Nested Hall Thruster, to Aerojet Rocketdyne. More contract awards are expected in the spring.
NASA Glenn’s goal for this interim stage is to advance the technology to the 50 kW level. Once the 50 kW hardware is fully tested, it will be made part of a demonstration mission. This mission will prove the technology’s capability to support missions in the area that NASA refers to as the proving grounds – missions in cislunar space, out as far as the Moon. Such missions are hours or days from Earth where technologies can prove themselves before advancing to missions that are more Earth-independent.
“The first mission we are looking at for demonstration of this 50 kW class of SEP technology is the Asteroid Redirect Robotic Mission (ARRM),” Manzella said.
ARRM is a mission that was highlighted as a priority by the Obama administration. ARRM would use the new 50 kW solar electric propulsion robotic spacecraft, fly it to a near-Earth asteroid and take a piece of that asteroid or asteroid material, something 10–16 feet (3–5 meters) in diameter, and return it to orbit around the Moon. Once it is in orbit, astronauts aboard the new Orion spacecraft would go investigate it.
“We could do that mission using only a few thousand kg of xenon,” Manzella said. “Using that, we could bring back something that is 20–30 metric tons. So we would really prove out that higher level SEP capability that could be used for enabling that kind of primary propulsion for a mission.”
The mission is currently planned to lift off sometime in the early 2020s. It is being led by JPL. Development of the capture system method is headed by NASA’s Goddard Space Flight Center.
“Our portion of that mission is the SEP,” Manzella said.
SEP Begins with Solar Arrays
The largest solar arrays currently flying today on high power robotic spacecraft produce 20–25 kW. Those are aboard large power geostationary communication satellites. The goal at NASA Glenn is to go a factor of two greater than that, with a solar array that can stow compactly on the spacecraft within a launch fairing and then autonomously deploy.
“So, engineering the solar arrays is a [structural] problem as much as anything,” Manzella said.
NASA Glenn recently concluded two development projects designed to come up with concepts for those solar arrays. One was the MegaFlex array, built by ATK Space Systems. MegaFlex folds out like a fan during deployment. A smaller version of it called UltraFlex was recently demonstrated on the Cygnus resupply vehicle to the ISS, and it is aboard the planned Insight mission to Mars. The second array design is Deployable Space Systems, Inc.’s Rollout Solar Array, or ROSA, which uses a pair of composite slit booms to roll out the array, like rolling out a carpet. These designs are ultimately intended to produce power at hundreds of kW power levels, but will first be tested at the 50 kW level.
“We look at the efficiency with which they stow, and we look at how many kW we can get in a cubic meter, and their mass, how many kW can we provide per kg,” Manzella said. “These new arrays substantially outperform the existing state of the art, which is what we call rigid panel arrays. These are solar cells basically bonded to [large composite honeycomb] panels. The new arrays are what we call flexible blanket arrays. They would have the same kind of solar cells bonded to a kind of flexible substrate or a mesh type material.”
The solar arrays on the ISS are also flexible blanket arrays that produce hundreds of kW. However, the ISS solar power system was assembled in orbit after multiple launches and construction spacewalks. The challenge for the teams at NASA Glenn has been to develop equivalent scaled solar power systems that can be launched in a single mission, and then autonomously deploy. So far, they have met that challenge successfully, as both designs have been deemed flight-ready.
The Thrust of SEP is Ion Propulsion
As the solar arrays generate the electricity, it is used to power the system’s ion thruster. NASA Glenn is in the process of developing a 15 kW class SEP system. The 15 kW units would be combined into multiple strings to get the system to the 50 kW power level. For the ARRM, they will use four of those individual strings.
“The electric propulsion system is made up of several individual components,” Manzella explained. “One is the thruster. It uses a propellant, in this case, the xenon gas. We introduce the xenon into the thruster and we ionize that gas, and when we accelerate the ions we produce an electric field across a voltage. In the system we’re using, that voltage might be from 600–800 volts to produce the thrust that we need. Then there is also a cathode which is the neutralizer that produces an electron stream that joins with that positive ion stream to neutralize it, so we don’t just send a bunch of charge out the back that would then come back and be attracted to the spacecraft.
“We also developed the power processing unit that takes the power generated by the solar array and converts it to the voltages and currents that we need to operate that thruster.”
All electric propulsion works by using electricity to ionize the propellant and accelerate it to high exit velocities.
There are two main types of thrusters: gridded ion thrusters, and Hall thrusters. The difference in them is how they apply the physics to produce the thrust. With gridded ion thrusters, the ion plasma is created in a can or thrust chamber and then accelerated by a voltage that is supplied between two semi-permeable screens.
“If you think of like two screen doors, and that voltage in between there accelerates those ions out, ” Manzella said.
Previously mentioned NSTAR, the engine for Deep Space 1 and the Dawn spacecraft, is a gridded ion thruster. The NASA Evolutionary Xenon Thruster (NEXT) is a gridded ion thruster that operates at three times the power of NSTAR. NEXT has been tested at NASA Glenn for 51,000 continuous hours, the equivalent of more than six years of operation, to demonstrate that the thruster could successfully operate for long duration missions. NASA Glenn’s patented Annular engine is another gridded ion thruster that has the potential to outperform the capabilities of NEXT and other electric propulsion thrusters. It yields a total ion beam area two times greater than that of NEXT. It ultimately has the potential to achieve very high power and thrust levels.
The Hall Thruster, which will be further developed through the recently awarded contract with Aerojet Rocketdyne, working closely with JPL, the University of Michigan, and Silicon Turnkey Solutions, is a little different.
“The Hall thruster is also an electrostatic device that creates ions and accelerates it,” Manzella explained, “but it doesn’t do it with an actual physical grid. It does it in a region that we establish with a magnetic field by basically trapping the electrons in a way that they go in an azimuthal or circumferential direction relative to the thruster. And that circulating current is called the ‘Hall current’, and that’s why they’re called Hall thrusters.”
SEP and RTGs
Are SEP engines better than Radioisotope Thermoelectric Generators (RTGs)? The answer is both yes and no.
Spacecraft like the New Horizons mission to Pluto used an RTG instead of SEP. An RTG uses the natural radioactive decay of plutonium to produce electricity, just as the solar arrays of a SEP system produce electricity. However, while RTGs provide electric power, they are technically not a part of the spacecraft’s propulsion system, which is instead provided by conventional chemical rockets. But considering the lighter mass and less propellant required with a SEP system, why no SEP on New Horizons and other deep space science missions?
“New Horizons and other outer Solar System probes have used RTGs out of necessity,” Manzella said. “Those spacecraft are going where it is difficult for a solar array to generate sufficient power at those very high heliocentric distances away from the Sun. You need a very large solar array to generate sufficient power at those distances.”
For now, at least, SEP engines will be at their greatest level of utility for spacecraft flying to destinations in the inner Solar System.
Next Step For SEP
“One of the reasons that we want to get to higher powered systems is to try to move heavy things quicker than we can with the existing lower kW systems,” Manzella said. “The NEXT system was in continuous testing operation here for almost seven years. But ideally, we want to have propulsion systems that don’t necessarily need that long duration. With the higher kW systems, we would like to have transfers that are on the order of two to three years. The Asteroid Redirect Robotic Mission (ARRM) is currently planned as a five-year mission; that includes coasts – time at the asteroid and time back.”
But ARRM, a mission which is expected to prove this higher level SEP system, is navigating an uncertain path to the launch pad.
Tune in to SpaceFlight Insider tomorrow for the second half of our review of NASA’s Solar Electric Propulsion technology.
Video courtesy of NASA
Michael Cole is a life-long space flight enthusiast and author of some 36 educational books on space flight and astronomy for Enslow Publishers. He lives in Findlay, Ohio, not far from Neil Armstrong’s birthplace of Wapakoneta. His interest in space, and his background in journalism and public relations suit him for his focus on research and development activities at NASA Glenn Research Center, and its Plum Brook Station testing facility, both in northeastern Ohio. Cole reached out to SpaceFlight Insider and asked to join SFI as the first member of the organization’s “Team Glenn.”