Tests conducted at the NASA Lewis Research Center's Supplemental Multilayer Insulation Research Facility (SMIRF) demonstrated that a hybrid thermal control system could eliminate boiloff of cryogenic propellants. This is significant because of the substantial mass and cost savings that could be achieved for any long-duration space mission that requires cryogenic propellants.
With long-duration cryogenic storage, propellants will boil off because of the environmental heating of the tank. To accommodate these losses, extra propellant is required along with larger propellant tanks. Analysis of Mars mission scenarios using space-transfer cryogenic stages showed that significant savings in propellant mass and tank size could be achieved if it were possible to eliminate or significantly reduce propellant boiloff.
Engineers and technicians at NASA Lewis designed, built, and tested a hybrid thermal control system to eliminate or significantly reduce cryogenic propellant boiloff. The system consists of an active cryocooling system using a cryocooler in addition to the traditional passive thermal insulation, as shown in the photo.
Test article prior to installation into the vacuum chamber. Mated to the cryocooler is a condenser (not shown) which was designed to transmit heat entering the tank by condensing hydrogen vapor. This extended into the ullage of the liquid hydrogen tank.
Testing conducted over the summer of 1998 compared the performance of the hybrid system to that of the traditional passive-only insulation solution. The hardware consisted of a pressurized 50-ft3 tank insulated with 34 layers of multilayer insulation (MLI), a condenser, and a Gifford/McMahon Cryocooler that has a cooling capacity of 15 to 17.5 watt (W). Liquid hydrogen was the test fluid. The test tank was installed into a vacuum chamber, simulating space vacuum.
First, the passive insulation system was tested to determine the boiloff under steady-state conditions. The measured steady-state boiloff rate was approximately 0.12 kg/hr, equating to a heating rate of approximately 14.5 W, slightly higher than the prediction of 13 W and the historic data of 11.5 W (1978 tests). Even so, the heating rate was less than the cryocooler capacity (15 to 17.5 W).
Next, a series of tests was conducted with the cryocooler in operation. Late into these tests, after the chamber vacuum pressure finally inched upward to approximately 8.5x10-5 torr, which was still a full one-half decade worse than that of our previous test, we achieved zero boiloff (0.00 kg/hr). At this time, the test tank pressure and temperatures decreased. This tank pressure decrease was approximately 0.07 psi/hr, and the condenser temperature decrease was approximately 0.12 K/hr. The testing was terminated when the tank pressure dropped near atmospheric pressure. This was to prevent air from being accidentally drawn into the tank.
These tests confirmed that it is possible to achieve zero boiloff with cryogenic propellant storage using a hybrid thermal control system, and they provided the foundation for the more detailed analysis and testing needed to support future long-duration space missions.
Lewis contact: David W. Plachta, (216) 977-7126,
David.W.Plachta@grc.nasa.gov
Authors: David W. Plachta
Headquarters program office: OSF (Advanced Projects), AST at MSFC
Programs/Projects: Propulsion Systems R&T, AST
Last updated June 16, 1999, by Nancy.L.Obryan@nasa.gov
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