Space Technology and Innovation | NASA Banking on Solar Electric Propulsion’s Slow but Steady Push
WASHINGTON — The list of technologies NASA says it needs for a crewed mission to Mars notionally envisioned for the 2030s includes large-scale electric propulsion systems that dwarf those used today aboard many satellites.
“The type of solar electric propulsion that is flying now, at the 4 kilowatt or 5 kilowatt level, is very useful for doing things like station keeping,” said David Manzella, an engineer with the solar electric propulsion group at NASA’s Glenn Research Center near Cleveland. “But it doesn’t provide enough thrust to move heavy payloads in short times,”
Neither would the first in a series of scaled-up solar electric systems NASA is working on now, which are being paid for by the agency’s Space Technology Mission Directorate.
Just as NASA envisions sending astronauts to a relocated asteroid before leaping on to Mars, it has also planned an intermediate step between the small, electric attitude control systems of today and the massive space tugs of tomorrow: a system that generates between 25 kilowatts and 30 kilowatts of power, and that would be first flown in space between 2016 and 2019, Manzella said.
A rig with that much power would mark a manifold increase compared with the electric propulsion systems flying today, but is still far less powerful than the 300 kilowatt to 500 kilowatt systems NASA believes it needs to tug supplies to Mars in advance of a crewed mission.
A potential beneficiary of the mid-range solar electric system NASA is preparing now is the asteroid retrieval mission announced in April. NASA’s notional plan is to launch an uncrewed solar-electric spacecraft to capture an asteroid around 10 meters in diameter and tractor it back to the vicinity of the Moon, where astronauts could visit it as soon as 2021 aboard the Orion crew capsule now in development.
Work on technology for that potential mission is under way, with early efforts focused on the large solar arrays needed to power an all-electric craft that eschews chemical-fired engines in favor of slower but more efficient electric thrusters.
For the solar panels, NASA in September awardedSpace Components and Deployable Solar Systems, both of Goleta, Calif., $6.9 million and $4.6 million in funding, respectively, to work on collapsible photovoltaic arrays big enough to power a spacecraft fitted with a cluster of electric thrusters about twice as powerful as those flying today.
NASA wants thrusters “in the 10 kilowatt to 15 kilowatt power range as the propulsion to go with those solar array systems,” Manzella said.
To meet that power requirement, ATK is developing a circular photovoltaic array about 10 meters in diameter, a larger version of what it is making for the Lockheed Martin-built Orion spacecraft. The 10-meter array could generate about 35 kilowatts of spacecraft power, said David Messner, vice president and general manager of solar arrays and deployables for ATK Space Components.
“One of the neat things about a configuration that’s round is that from a structural standpoint, it’s more efficient than linear, rectangular kind of arrays,” Messner said. Rectangular arrays, such as those used aboard the international space station “have to carry wires all the way back to the spacecraft, so as you get really, really long, it requires more mass because of all the wire you need.”
ATK is planning to test one of these circular arrays, which it calls MegaFlex, in a 30-meter-diameter thermal vacuum chamber at NASA’s Plum Brook Station near Sandusky, Ohio, in early 2014. Messner expects this round of MegaFlex work, which began in September 2012, to wrap up in March.
Meanwhile, Deployable Solar Systems, the other company working on solar arrays that might find their way onto NASA’s asteroid tug later this decade, is sticking with the tried and true rectangular solar panel shape — with a few modifications.
“We don’t have any motors, we don’t have any complex mechanisms, complex hinges or anything,” Brian Spence, founder of Deployable Space Systems, said. Once in space, “the array unrolls like a carpet” using the pent-up force of a spring-loaded boom to unstow itself.
Deployable Space Systems, a 20-employee company founded by former ATK Space Component employees in 2008, is planning to vacuum test its novel Mega Roll-Out Solar Array in “the first quarter of next year,” Spence said May 14.
A possible spot for the test is Boeing Defense, Space & Security’s thermal vacuum chamber in El Segundo, Calif., Spence said.
Meanwhile, with ATK and Deployable Space Systems working on the arrays that will generate power, others are working on the thrusters that will spend that power.
“We have 12-kilowatt systems in development that could be used in much higher power-level systems,” said Julie Van Kleeck, vice president of space programs at Sacramento, Calif.-based Aerojet. The company tested one such thruster at Glenn last year, Van Kleeck said.
A 12-kilowatt engine more than doubles the power of the 4.5-kilowatt, xenon-fueled BPT-4000 Aerojet provided for the Air Force’s Advanced Extremely High Frequency communications satellites.
Aerojet built the thrusters for the Air Force satellite by licensing technology from a Natick, Mass., company called Busek Space Propulsion and Systems. In April, Busek won $5.1 million from NASA’s Small Business Innovation Research program to work on bigger solar electric thrusters in the 10 kilowatt to 20 kilowatt range.
A Little Thrust Can Go a Long Way
Although NASA insists on pushing the state of the art for the asteroid retrieval mission and later Mars missions, even the comparatively small and weak electric propulsion systems flying today are powerful enough to be useful. The Air Force andproved that in 2010, when the service’s Advanced Extremely High Frequency satellite lost its main onboard engine and had to rely on Aerojet-built electric thrusters to boost from the transfer orbit where its rocket left it up to geostationary orbit. The trip took nine months.
The electric thrusters provided by Aerojet for the Air Force satellite — which are among the most powerful being flown today — produce only about 250 millinewtons of thrust. NASA’s Dawn spacecraft, which launched in 2007 to explore two of the largest asteroids in the solar system, gets a whopping 90 millinewtons or so from its xenon-fueled ion engine.
For comparison, a commercially available 8-gram, solid-fuel motor for a model rocket can produce just over 10 newtons of thrust at its peak, making it about 1,000 times more powerful than Dawn’s engine — for the fraction of a second the toy motor is capable of firing.
Dawn’s engine, like other electric thrusters, can stay lit for much longer than that. And they need to. The tiny amount of thrust such engines produce means spacecraft that use them must perform long burns in order to get anywhere.
Despite that handicap, electric propulsion systems offer a large reduction in mass at the launch pad, which has obvious advantages.
Boeing Space and Intelligence Systems, for example, is now planning to do on purpose what the Air Force and Lockheed only did by accident: transfer communications satellites to geostationary orbit with no chemical propulsion at all. In 2012, the Seal Beach, Calif., company sold four of its all-electric 702SP satellites in a joint order from Asia Broadcast Satellite of Hong Kong and Satmex of Mexico.
These satellites are scheduled to launch in pairs aboard Space Exploration Technologies Corp. Falcon 9 rockets in 2014 and 2015. Like electric thrusters on other Boeing satellite models, those on the 702SP are powered by xenon propellent, of which each satellite requires only about 350 kilograms — substantially less than the 2 metric tons of fuel needed aboard satellites with chemical-fired thrusters, Boeing estimates.