One of NASA’s least known funding mechanisms is the Small Business Innovation Research (SBIR) program. This program, mandated by the Baye-Doyle Act of 1980, allocates part of the budget of every US federal agency (including NASA) to developing small businesses to commercialize new technology. NASA’s SBIR program usually focuses on commercializing technologies that can be used in space, and often funds a university that does some work alongside the small business that received the grant. A company called Air Squared Inc, based in Broomfield, Colorado, recently received one of these SBIR grants and worked with a mechanical engineering laboratory at Purdue University and Whirlpool, one of the world’s largest equipment companies, to make a necessary component for extended periods of time Space mission – a refrigerator.
While it seems like a pretty mundane way to spend research money, getting a refrigerator to operate in space is not an easy task. There are two main problems that must be addressed by a refrigerator in space. First is a lack of gravity which can affect a refrigerator’s compressor. Second is the orientation of the system which can destroy the vapor compression cycle the refrigerator uses to cool down.
Purdue video with details on how to operate the refrigerator.
Photo credit: Purdue Department of Mechanical Engineering
Let’s look at gravity first. Refrigerators have a machine called a compressor. This is the machine that is in operation when a refrigerator is turned on. The job of the compressor is to pressurize the “hot” refrigerant line so that it can participate in the steam cycle described below. The compressor itself contains moving parts that are normally lubricated with oil, much like what would be the case with a gasoline engine.
The problem is that if there is no gravity, the oil is not sure where to go and can spread too thinly to lubricate the compressor, causing it to stick. Without a compressor, the entire cooling system is out of whack.
Learn the technical video that explains the principles of refrigerator operation.
Photo credit: Lesics YouTube Channel
Another way to unbalance this system is to change its direction. A key component of refrigerators is the throttling system, which essentially acts as a bridge between the hot liquid and cold vapor lines. The throttling system forces a significant pressure drop in the hot line liquid, causing some of that liquid to evaporate. Evaporation of this liquid requires energy extracted from the heat of the evaporated liquid, which makes the vapor much colder on the other side of the throttle where it condenses.
The problem is that if the orientation of the refrigerator tips over, the liquid and vapor can switch places, creating bubbles and destroying the compressor that is trying to pressurize the liquid. For this reason, you shouldn’t lay a refrigerator flat when moving it.
CAD model of the new refrigerator design that is part of NASA’s SBIR program.
Photo credit: Air Squared, Inc.
When a spaceship moves to a different gravity well, it is very likely that what was considered “up” during its launch will be considered “down” once it has landed in a new location, which will result in the entire process goes into chaos. Any typical cooling system is therefore useless in any environment in which its orientation changes so.
Air Squared and Purdue believe they have a better solution. First, Air Squared designed a compressor that does not require oil, eliminating the possibility of it self destructing in low gravity environments.
Close up of the prototype refrigerator in a unique orientation.
Photo credit: Purdue University / Jared Pike
The details of how they’re going to deal with the orientation problem aren’t quite as clear, but it wouldn’t come into play until gravity is a factor again. The solution could be as simple as designing a compressor that reverses direction and swaps where the tubes go so that pressure can be applied to both sides of the throttle depending on the orientation of the system.
Fortunately, they have plenty of opportunities to test their solutions to both problems. The program was funded from the Flight Opportunities program, which allows the team to fly its refrigerator design on four “zero gravity” flights. The flights, performed by Zero Gravity Corporation’s weightless research laboratory, simulate microgravity dozens of times over the course of a flight for around 20 seconds.
If it passes its microgravity tests, the next step is to test it in space. It is not the first time that cooling in weightlessness has been attempted. In fact, the ISS currently has a cooling system in which biological samples are stored. However, it is bulky and uses significantly more electricity than a normal refrigerator on earth. Since performance is scarce in space, this is a less than desirable design.
The Air Squared / Purdue prototype is much more user-friendly. It’s about the size of a microwave and is specially designed to keep food cold. Such a cooling system could increase the shelf life of astronaut food from around 3 years to around 5 or 6 years.
That extended shelf life would be invaluable to some of the longer-manned missions that are on the space exploration roadmap. There’s still a lot to research and invent, but hopefully a small startup in Colorado, with the help of some of the best engineering organizations in the world, can introduce consistently reliable cooling for space exploration missions.
Purdue – astronauts need a refrigerator. Engineers build one that works in weightlessness – and backwards.
Interesting tech – engineers develop a zero-G refrigerator for space missions
Eckhard Groll (left), head of the mechanical engineering department at Purdue, and doctoral student Leon Brendel stand next to their reorientable refrigerator.
Credit: Purdue University Photo / Jared Pike