Loop heat pipe

Lore & Background

Loop heat pipes were patented in the USSR in 1974 by Yury F. Gerasimov and Yury F. Maydanik, with a US patent filed in 1982. The first space application occurred aboard a Russian spacecraft in 1989. LHPs are now commonly used in space on satellites including the Russian Granat and Obzor spacecraft, Boeing's (Hughes) HS 702 communication satellites, the Chinese FY-1C meteorological satellite, and NASA's ICESat. They were demonstrated on the NASA space shuttle in 1997 with missions STS-83 and STS-94.

LHPs overcome limitations of conventional heat pipes, which are mainly confined to transferring relatively small heat loads over short distances when the evaporator and condenser are at the same horizontal level. These limitations arise from pressure losses from liquid flow through the porous structure and viscous interaction between vapor and liquid phases. Conventional heat pipes are also sensitive to orientation in gravity, with unfavorable slopes adding pressure losses. LHPs combine spatial separation of the transportation line with non-capillary arteries, allowing operation in any directional orientation.

Construction involves controlling volumes of the reservoir, condenser, and vapor and liquid lines so that liquid is always available to the wick. The reservoir volume and fluid charge ensure fluid remains in the reservoir even if other lines are completely filled. Small pore size and large capillary pumping capability are necessary in the wick, balanced with wick permeability. Common coolants include anhydrous ammonia and propylene.

Reader's Guide

Loop heat pipes are important parts of systems for cooling electronic components, particularly in space applications where reliability and orientation independence are critical. Their ability to transport large heat loads over long distances with a small temperature difference makes them suitable for satellites and spacecraft, as demonstrated by their use on multiple Russian, American, and Chinese missions. The loop scheme forms the basis of the physical concept of Two-Phase Loops (TPLs), extending the functional possibilities of two-phase systems to applications involving otherwise inoperable slopes in gravity. By spatially separating vapor and liquid phases and using non-capillary arteries, LHPs maintain normal operation in any directional orientation, overcoming the gravitational sensitivity that limits conventional heat pipes. Their development from the 1974 Soviet patent through to modern satellite applications illustrates a steady progression from theoretical concept to practical thermal management solution.

Did You Know?

• The first space application of a loop heat pipe occurred aboard a Russian spacecraft in 1989.
• Common coolants used in LHPs are anhydrous ammonia and propylene.