July 20, 2016 – Crew members aboard the International Space Station (ISS) are receiving a unique hardware delivery today that can help shape NASA’s human journey beyond Earth and into deep space.
The Phase Change Material Heat Exchanger (PCM HX) Demonstration Facility hitched a ride to the space station on SpaceX’s Dragon cargo craft, which launched July 18 on a Falcon 9 rocket from Cape Canaveral Air Force Station in Florida. Dragon arrived at station in the early hours of July 20, and crew will soon begin unloading the spacecraft of the nearly 5,000 pounds of science, research and hardware for the orbiting laboratory.
This hardware is one of NASA’s Game Changing Development program’s efforts that will advance space technologies and may lead to entirely new approaches for the agency’s future missions and solutions to significant national needs. Even more novel is that this high-tech gear is stuffed with wax that has a crayon-like texture.
The use of wax dates as far back as 221-206 B.C., but it may not come to mind as ideal for 21st century space travel. Rubik Sheth, project manager and systems engineer in the Thermal Systems Branch at NASA’s Johnson Space Center in Houston explains how it could be important.
“It gets really hot when the spacecraft is between the sun and the moon,” Sheth said, so sending humans to deep space around the moon is a thermal challenge. “We need these phase change material heat exchangers to absorb the excess waste energy that Orion will take in.”
One future destination for NASA’s Orion spacecraft is supporting a crew in cislunar space and Sheth notes that heat exchangers freeze or thaw a material to sustain critical temperatures inside a spacecraft, thus protecting crew members and equipment.
That material of choice to be showcased on the phase change material heat exchanger aboard the ISS is N-pentadecane. “It’s pretty much like crayon wax in its consistency and the way it feels,” Sheth says.
How It Works
The phase changer material heat exchanger — PCM HX for short — stores energy by thawing a phase change material, in this case wax, using hot coolant. That energy is later rejected by the spacecraft’s radiator, then refreezes the wax and prepares it for the next spike of heat load. This new type of heat exchanger could help offset heat experienced by Orion and better regulate temperatures, said Sheth. “That’s why we’re flying it to the space station to see how it works in microgravity, and then take the next step in implementing the concept.”
Using wax in a PCM HX was tried out in trial and error fashion on NASA’s Skylab experimental space station that housed crews in 1973-1974. Similarly, wax was utilized earlier as a passive means of cooling instrumentation on lunar rovers used in the Apollo moon-landing project. However, results were inconsistent, Sheth said.
“We took a total re-look,” Sheth said. Working with United Technologies Aerospace Systems of Windsor Locks, Connecticut, the wax-based PCM HX was built for flight demonstration. The test facility for ISS uses a thermal control system with built-in heaters and thermoelectric devices that aids in the freezing and thawing cycles of the PCM HX.
A removable kitchen drawer-like section of the PCM HX facility is loaded with about 10 pounds (4.5 kilograms) of wax.
“The wax itself can hold 200 kilojoules of energy per kilogram. So for every kilogram of wax I can stuff 200 kilojoules of energy in there.” Sheth said.
That is the equivalent of about eight hours of energy to light a compact fluorescent light bulb.
A PCM HX using wax, as contrasted to using gallons and gallons of water, equates to a potential mass savings for Orion spacecraft builders.
Back On Earth
Aboard the ISS, the equipment can operate day and night. But it’s a power-hungry unit when working to lower temperatures down to between 10 and 30 degrees Celsius. That means having to share power with other station payloads; choreography is needed to distribute electricity between experiments.
“We want to run through December of this year,” Sheth said.
By year’s end, the wax is to be removed from the facility and then returned to Earth. The actual demonstration facility will remain onboard the ISS, ready for other experiments that require coolant temperatures below -10 degrees Celsius, Sheth points out.
Once back in NASA hands, the wax will be visually inspected for any deformities and then cut it in half. That appraisal could make a future wax-based PCM HX even more efficient.
“We want to see how the wax maintained the internal geometry of the heat exchanger unit itself,” Sheth said.
Sheth said the goal is to give the Orion spacecraft team a report for Orion’s Exploration Mission 2, or EM-2, subsystem critical design review process for the phase change material to be chosen for EM-2, slated to be the first crewed mission on NASA’s Space Launch System rocket.
The ISS PCM HX Demonstration Facility endeavor took some two years of time to develop.
“It has been rewarding in so many ways,” Sheth said. “From engineering the facility, getting it approved for the ISS, and sending the hardware up to station on Dragon, it is incredible what we have accomplished.”