Performance comparisons of space borne cryostats

We discuss the performance of liquid helium cryostats that have flown in space or are planned for space flight. Usual figures of merit are total cryostat mass or depletion rate of the cryogen. These often fail to accurately represent other important characteristics which affect cryostat performance such as the method used to survive launch lock-up or the temperature of the cryostat vacuum shell obtained by radiative cooling. To address these issues, we define the ratio, H/R in W day/l as metric to judge cryostat performance. The parameter H=σ_BA_tank(T⁴_shell−T⁴_tank) is the Stefan Boltzmann law for energy transfer to the helium tank, where σ_B is the Stefan Boltzmann constant, A_tank is the surface area of the cryogen tank and T_shell and T_tank are the vacuum shell and cryogen temperatures. The average cryogen depletion rate R=V_f/t is computed using the total cryogen volume, V_f, at the last fill before launch, including the volume of ‘booster tank' cryogen if used and the cryogen lifetime, t, to depletion on-orbit. Cryostats launched on the Space Shuttle have the same H/R≈60 W day/l whether the cryogen was liquid helium or solid neon, and for a broad range of vacuum shell temperatures 113<T_shell<300 K, cryogen volumes 2200>V_f>85 l, and mission times, 9 days to >2 years. Cryostats launched on unmanned rockets have a higher H/R≈300 W day/l. Only one, the X-Ray Spectrometer (XRS), out of the four solid neon and two solid hydrogen cryostats showed a clear advantage of using a cryogen other than liquid helium.