NASA's Heat Pumps in Outer Space

May 28, 2007
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The Orion lunar exploration vehicle uses a heat pump developed for outer space by Mainstream Engineering, Rockledge, Fla. (Photo courtesy of NASA.)

ROCKLEDGE, Fla. - Heat pumps designed for NASA’s International Space Station (ISS) and the new Orion lunar crew exploration vehicle are not pie in the sky. They are real, functioning HVAC units that must actually work in outer space.

Space exploration heat pumps (and their counterparts here on earth) share the same refrigeration cycle principles as well as components, such as evaporators, condensers, compressors, etc. However, designing heat pumps for zero gravity, extreme heat, and other demand elements has taken years of design work from one of NASA’s top design-build contractors, Mainstream Engineering.

Heat pump technology is now widely accepted for HVAC requirements on most space exploration projects, but it was Mainstream Engineering that sold NASA on the concept after nearly 15 years of individual heat pump component prototypes, demonstrations, and paper presentations.

The first hurdle overcome by President Robert Scaringe, Ph.D., P.E., was convincing NASA scientists and engineers that heat pump technology was far superior to the more favored concept of the era - pumping hot liquids in a loop with huge radiators for heat rejection. This was back in 1986, when Scaringe founded Mainstream.

ORION VEHICLE'S DESIGN

The HVAC design of the Orion space exploration vehicle, which is designed for terrain transportation, had to solve the problem of heat rejection in a very hot environment. The moon’s high ambient temperature is much warmer than the rejection heat temperature of a conventional heat pump, which makes convection or conduction through condensers impossible. Therefore, radiators must be used for heat rejection.

The Orion project uses a Mainstream Engineering-designed heat pump that raises the heat rejection temperature to 120°C, which is significantly higher than the moon’s temperature; rejected heat radiates away.

However, radiator technology poses its own set of challenges in space. Every pound of payload on a rocket launch is critical; therefore, a space exploration vehicle with an abundance of large, heavy radiators isn’t practical. Aside from weight, huge folding radiators on a space exploration vehicle present more moving parts that can fail on a mission.

Mainstream’s solution was to provide higher heat rejection temperatures. Besides making heat rejection possible in hot environments, higher heat rejection requires less radiator surface area and thus, a smaller payload with less moving parts.

CHANGING MINDS

Once Scaringe had believers in heat pump technology, he had to reverse the belief among some scientists that higher heat rejection temperatures would require too much of the space vehicle’s energy supply, which is mainly solar energy from photovoltaic cells.

“Although the heat pump is consuming power to reject heat, the overall system is now more efficient and therefore the net power is less,” said Scaringe, who was one member of a three-engineer Mainstream design team that developed the Orion’s heat pump-based refrigeration system.

Pumping liquid in a loop was the original favored method for space vehicles. Therefore, Scaringe had to convince officials that a compressor would be more reliable than a typical circulating pump.

“Equipment that compresses a vapor has more reliability and longevity than a device that pumps a liquid, which is very prone to problems such as cavitations,” explained Scaringe.

“One of the many papers we presented had to relieve the fears NASA scientists had about compressors used in heat pump technology.”

UNPREDICTABLE OIL

Another invention Mainstream developed as a subcontractor for primary contractor-integrator Lockheed-Martin (Bethesda, Md.) is an oilless compressor for the current ISS heat pump system. A typical refrigeration system uses gravity to return the circulating oil to the compressor crankcase for separation from the refrigerant and preparation for the next refrigeration cycle. This type of system would fail in outer space: The absence of gravity might pool the oil in one place, or upset other flow processes.

To solve this challenge, Mainstream invented a gravity-insensitive compressor for the ISS heat pump that needs no lubrication.

Oilless compressors, such as this model developed by Mainstream Engineering for heat pumps used aboard spacecrafts, have found their way into more earthly applications. This sometimes happens with technologies developed initially for government use. (Photo by Mainstream Engineering.)

LAUNCH BREAKS

Ensuring that the refrigeration system withstands a launch into space is equally challenging for Mainstream, which fabricates most equipment at its Rockledge headquarters. Aside from the tremendous vibrations, a launch also subjects onboard components (as well as personnel) to 6-G (six times gravity) gravitational conditions.

“Space is a smooth environment for equipment to operate, but it’s the tumultuous launch to get it up there that worries most contractors that make equipment for NASA,” said Scaringe.

The heat pumps of the Orion and the ISS were first tested on Earth under normal conditions, then tested under a vacuum similar to outer space conditions. Once they passed, they were tested in zero-gravity space conditions on prior launches a few years ago before NASA finally approved them for each application.

SIDEBAR: EARTHLY RESULTS

Mainstream’s design and development does filter down to the HVACR industry for practical applications here on Earth. For example, en route to designing Orion’s heat pump, Mainstream had to solve the reduced efficiency problems produced by long idling periods in outer space. The result (QwikBoost®), invented by Mainstream and marketed by InterDynamics in auto parts stores nationwide, offers a 10 percent performance boost in R-134a systems.

Another offshoot product, now distributed through HVACR wholesalers, has been helping solve acid and moisture problems in refrigeration systems. QwikShot® is an acid flush product Mainstream developed for the ISS. Since system failure on the ISS is not an option, the refrigeration system was injected with the acid flush before launching. It remains in the system and vaporizes acid and moisture if and when it occurs over the next 10 years or longer.

PuraClean®, a filter spray that produces electrostatic effects that increases the dirt-attracting and -holding capabilities of a disposable HVAC system filter, was originally developed to improve the IAQ in space vehicles and stations.

Publication date: 05/28/2007

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