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The long-term storage of cryogenic propellants on orbit under thermal and pressure control is a promising enabling technology for future space exploration. Composite insulation composed of polyurethane foam (Foam), variable density multilayer insulation (VD-MLI), and a vapor-cooled shield (VCS) is considered as an effective passive thermal control method for such missions. This paper presents a theoretical model that considers three heat transfer mechanisms simultaneously within the VD-MLI and convective heat transfer inside the VCS, to predict and optimize the thermal performance of the insulation combination. The model is validated by experimental data. The influences of VCS position and warm boundary temperature (heat flux) on the thermal insulation performance are investigated for LH2, LN2, LO2, and LCH4 tanks. The temperature profiles within the insulation material with or without VCS are compared. In addition, the contributions from the VCS to reduce the heat flux into the tank are evaluated. The conclusions are valuable for the optimal design of future composite insulation with Foam/VCS/VD-MLI for cryogenic storage tanks on orbit.

Cryogenic propellant storage on orbit is a crucial part of future space exploration. Efficient and reliable thermal insulation is one of the dominant technologies for the long-duration missions. This paper presents theoretical and experimental investigation on the thermal performance of variable density multilayer insulation (VDMLI) with different configurations and spacers. A practical method for optimizing the configuration of VDMLI was proposed by iteratively predicting the internal temperature profiles and maximizing the thermal resistance based on the basic layer by layer model. A cryogen boil-off calorimeter system was designed and fabricated to measure the temperature profile and effective heat transfer coefficient of the VDMLI samples over a wide range of temperature (77–353 K). The experimental data confirm that the optimized sample as predicted does have the minimum effective heat transfer coefficient in the control group. The results indicated that the insulation performance of MLI could be improved by 45.5% after replacing the regular uniform configuration with the optimized variable density configuration. For the same optimized configuration, the performance was further improved by 54% by changing the spacing material from none-woven fiber cloth to Dacron net. It was also found that the effective heat transfer coefficient will be much less sensitive to the MLI thickness when it exceeds 30 mm for on-orbit thermal environment.

The paper presented experimental investigation on the heat transfer of MLI with different rarefied gases at different pressures. The investigations were carried out using an innovative static liquid nitrogen boil-off rate measurement system in the case of the small temperature perturbations of cold and warm boundaries. The heat fluxes for a number of inert and some polyatomic gases have been analyzed at different heat transfer conditions ranging from molecular to continuum regime, apparent thermal conductivities of the multilayer insulation were measured over a wide range of temperature (77 K–300 K) and pressure (10 −3 –10 5 Pa) using the apparatus. The experimental results indicated that under degraded vacuum condition, the influences of rarefied gas on the MLI thermal performance very depend on the gas rarefaction degree which impacted by the MLI vacuum degree. Under the condition of molecular regime heat transfer, the MLI thermal performance was greatly influenced by gas energy accommodation coefficients (EAC), when under the continuum regime, the performances depend on the thermal conductivity of rarefied gas itself. Compared to the results of N 2 , Ar, CO 2 , Air and He as interstitial gases in the MLI, Ar was the better selection as space gas because of its low EAC and thermal conductivity characteristics on the different vacuum condition ranging from high pressure to vacuum. So different residual gases can be utilized according to the vacuum level and gas energy accommodation coefficient, in order to improve the insulation performance of low vacuum MLI.