Space Cryogenics

Figure 1: Hydrogen and oxygen tanks developed by Ball Aerospace for the Apollo Program. Image: “An overview of Ball Aerospace cryogen storage and delivery systems,” J. Marquardt et al. Adv. Cryo. Engr. (IOP Conf. Series: Materials Science and Engineering 101, 2015). http://2csa.us/creativecommons

Figure 1: Hydrogen and oxygen tanks developed by Ball Aerospace for the Apollo Program. Image: “An overview of Ball Aerospace cryogen storage and delivery systems,” J. Marquardt et al. Adv. Cryo. Engr. (IOP Conf. Series: Materials Science and Engineering 101, 2015). http://2csa.us/creativecommons

Space cryogenics is, somewhat obviously, the application of cryogenics to space exploration and science. The use of cryogenics in space optimizes the launching of vehicles, provides power and life support to spacecraft and is critical for many scientific observations.Space cryogenics has a number of challenges and has driven both pure and applied research and development in areas including He II; cryocoolers; liquid acquisition, cryogen storage and adiabatic demagnetization refrigerators; phase separators; and zero boiloff systems.

This work has also been important in training specialists in cryogenics who now work throughout the cryogenics industry.

An early use of cryogenics in space systems was in the liquefaction, transport and storage of liquid hydrogen (LH2) (Defining Cryogenics, Cold Facts April 2014) and liquid oxygen (LOX) for use as rocket fuels. The advantage of cryogenics here is the substantial volume reduction found in converting these materials into liquids, permitting a sufficient mass of the fuel to be carried within a rocket’s limited available volume.

Launch systems including the Saturn V, Space Shuttle and Ariane 5 have all used cryogenic fuels, and the application continues to be important. The Space Launch System currently under development by NASA, for example, also uses cryogenic fuels, and the need to maintain cryogenic fuels and LOX supplies for long duration missions is a principal motivation behind the development of zero boiloff systems.

LH2 and LOX are also used in fuel cells to provide spacecraft power, while LOX is additionally used, of course, to provide breathable air for astronauts. Figure 1 shows LH2 and LOX tanks developed for the Apollo Program.
Another important application of cryogenics in space is the cooling of sensors and instruments to the proper operating temperature to allow scientific measurements. These temperatures range from sub-Kelvin temperatures for X-ray detectors to 2 K for infrared astronomy and up to tens of K for Earth observation instruments.

More and more, these cooling applications are being carried out by mechanical cryocoolers, eliminating the need for cryogenic liquids. A disadvantage of cryogenic liquids is that once they have boiled away, the cooling and thus the observations stop.

At the lowest temperatures, engineers typically employ adiabatic demagnetization refrigerators. Cooling by passively radiating heat into space is also an option but has some limits, and the lowest achievable temperature depends on how close the spacecraft is to Earth, moon and sun. Typically, engineers use passive radiators in the 30 – 100 K range. Figure 2 shows the Integrated Science Instrument Module for the James Webb Space Telescope with the Near Infrared Spectrograph prominently displayed. The sensors of the spectrograph will operate at around 40 K.

Space cryogenic applications include not only the equipment launched into space but all the support, test and research facilities on the ground. As an example, very large cryogenic vacuum chambers are used to test a spacecraft’s ability to function in the cold, airless environment of space.

There are also unique challenges in space cryogenics not found in other areas of cryogenics. The allowable weight and size of cryogenic systems used in space is restricted and the sytems must withstand the severe shock and vibration of launch.

Once in space, there is no gravity to separate the liquid from vapor in cryogenic tanks and so engineers are required to use approaches such as wire meshes that use surface tension to direct liquid and porous plug phase separators to vent He II storage tanks.

Given the remoteness of space, reliability is paramount. Significant development work has been done over the years to improve the reliability of terrestrial cryogenic systems, such as cryocoolers, so that they are suitable for space applications. This improved reliability then benefits Earth-bound applications.

Figure 2: A view of the Integrated Science Instrument Module for the James Webb Space Telescope showing the Near Infrared Spectrograph on the right hand side. Image: NASA/Chris Gunn

Figure 2: A view of the Integrated Science Instrument Module for the James Webb Space Telescope showing the Near Infrared Spectrograph on the right hand side. Image: NASA/Chris Gunn

Additional information on space cryogenics is available at the biannual Space Cryogenics Workshop (www.spacecryogenicsworkshop.org), and papers on space cryogenics are also presented at the Cryogenic Engineering Conference and the International Cryogenic Engineering Conference.

A good survey of cryocoolers for space applications is given in “Space Cryocooler Developments,” L. Duband, Proc. ICEC 25 (Physics Procedia 67 – 2015). A broad survey of instrument cooling approaches is provided in “Cryogenics in Space: A Review of the Missions and of the Technologies,” B. Collaudin et al., Cryogenics 40 (2000). A detailed case study of a space cryogenic system is “The Superfluid Helium On-Orbit Transfer (SHOOT) Flight Demonstration,” M. DiPirro in Cryostat Design, J.G. Weisend II (Ed), Springer (2016).

A recent example of a cryogenic propulsion system is “Cryogenic Propulsion for the Titan Orbiter Polar Surveyor (TOPS) Mission,” S. Mustafi et al., Cryogenics 74 (2016). And “Simulated Propellant Loading System: Testbed for Cryogenic Component and Control Systems Research and Development,” J. Toro Edina et al., Adv. Cryo. Engr. (IOP Conf. Series: Materials Science and Engineering 101 – 2015) provides a good example of the scale of ground test equipment required for space cryogenics. ■