SDL–Built Payload Ready to Help NASA Refueling Experiment on Space Station
December 25, 2018 | HJNews
It’s winter, so chances are you’re not very far from an insulated flask keeping someone’s drink warm. A device similar to a vacuum flask in principle but much, much more advanced could someday help humans travel to and from other planets.
Such a device has been developed by the Logan-based Space Dynamics Lab.
The Methane International Space Station-Dewar, or MISS-D for short, is an insulated tank designed and built by SDL that can keep 50 liters of liquid methane very, very cold for a long time. MISS-D is a key component of the third phase of NASA’s Robotic Refueling Mission, which successfully launched to the International Space Station on Dec. 5. The space agency hopes to demonstrate that cryogenic liquids like methane can be stored and transferred between tanks in the extreme conditions of space.
“It’s been exciting to be a part of this and help NASA do this,” said Glen Hansen, SDL’s group lead for mechanical engineering and program manager for MISS-D.
RRM3 Hardware Manager Mark Neuman at NASA’s Goddard Space Center in Maryland said he’s happy with SDL’s contributions to the mission.
“Overall, I think they were very professional, did a great job,” Neuman said. “The component they supplied us was kind of like the key piece of the mission, the storage tank with the liquid methane. All our cryotransfer couldn’t happen without the main source dewar.”
Cryogenic dewars are specialized vacuum flasks built to store extremely cold materials.
If RRM3 is successful, it may pave the way to robotic refueling of satellites, extending the operational lifetime of devices that can cost hundreds of millions of dollars to build and launch. Communications satellites need to carry a relatively small amount fuel for station-keeping ― staying in precisely the right area in relation to Earth despite factors like solar wind and the gravitational fields of the Earth and moon. When that fuel’s used up in about 15 years, the satellites are often pushed into a “boneyard” orbit where they won’t interfere with other spacecraft, even if the rest of the hardware is still functioning ― because right now, we have no way to refuel them.
“Being able to go out and refuel them allows greater flexibility,” Hansen said. “They can stay on orbit longer.”
Refueling a spacecraft isn’t as simple as pumping gas into your car. Working with cryogenic materials like liquid methane is tricky even on Earth, and the extreme conditions of outer space impose even more significant challenges.
Because methane is a gas under normal Earth conditions, unless liquid methane is kept high pressure and very cold temperature, it starts to boil off.
“As the cryogen begins to boil off, it’s going to create a bubble,” Hansen said. “Here on the ground, with gravity, that bubble will just rise to the top. But when you’re out in orbit, in microgravity, that bubble doesn’t rise to the top of the tank, and if it’s in the wrong place then it could block transfer of the fluid, of the cryogen.”
Demonstrating that the technology in RRM3 can transfer the cryogenic liquid correctly in microgravity is one of the mission’s main goals, according to Neuman.
In orbit, MISS-D can keep methane in its liquid state by a solar-powered cryo cooler, Hansen said. The current experiment aims to demonstrate that MISS-D can store the material with no boil-off for up to six months.
On top of that, NASA needed MISS-D to be able to keep methane with very little boil-off even if it went without external power for up to five days to give the agency flexibility in different launch scenarios, Hansen said.
“We had a cryo cooler on board that would help maintain the cold temperatures of the cryogen,” Hansen said. “But once it was on the launch pad, then the power was pulled and it had to be able to sit there and not lose very much cryogen in the time that it took to get into orbit and before the solar panels could be deployed and they could get power from them to start the cryo cooler again.”
To help keep the methane at cryogenic temperatures, MISS-D essentially built like a vacuum flask.
“Basically you can picture it as a Thermos bottle,” Neuman said. “It has an inner aluminum tank and there’s a vacuum shell around it. Kind of like a two-layer tank.”
But whatever material you use to support the inner tank within the outer shell will transfer heat. MISS-D had to limit that heat transfer as much as possible while staying within NASA’s volume and mass restrictions.
“They had an innovative fiberglass nested rings system, where the outer ring was attached to the outer shell, and then the inner ring was attached to the inner tank,” Neuman said. “And these two rings kind of fit together in a very low thermal mass way so it did not transfer heat through it very well.”
MISS-D’s 50 kilogram allotment made it pretty tricky to design something that was functional, fault-tolerant and safe, Hansen said.
“We had to spend a lot of time in analysis and design coming up with a tank that would fit within the mass budget and still be strong enough to not pose any problems to anybody on the ground or to the space station and the personnel on board,” Hansen said.
The ability to refuel spacecraft could greatly aid space explorers in visiting the moon or even other planets. It takes tremendous energy to propel payloads into space ― so if the fuel was already in orbit, it wouldn’t cost as much to get the other parts of an interplanetary mission off the ground.
“They could actually go into orbit and then they could be refueled up there with what they would need to get to the moon or to other planetary missions that they would want to do,” Hansen said. “So it helps in that aspect, to not have to take everything with you the first time off the ground.”
RRM3 could be a step toward refueling depots in orbit around the moon or Mars, Neuman said.
“We’re trying to push the limits of exploration, and setting up like a gateway space station around the moon is one of the newest things they’re talking about,” Neuman said.
NASA recently confirmed the existence of ice at the moon’s polar regions, which could theoretically be used to produce drinking water and oxygen for life support as well as hydrogen for rocket fuel.
“If we could mine some of the water that’s on the moon that we’re recently finding, we could split it into hydrogen and oxygen and those type of gases, you would store them as supercold liquids,” Neuman said. “Hydrogen could also be a rocket fuel that could be transferred as a cryogenic fluid. So the demonstration we’re doing now could be applied for use with different cryogens. Same with methane. We could possibly produce methane on Mars and have a supply of that and use that for refueling rockets for return flights.”