Architecture study for in-orbit long term cryogenic storage

Abstract

The space community is currently focusing on defining system architectures able to perform multiple interplanetary missions to support deep space exploration. In particular, placing orbital propellant depots in strategic locations in space would allow to increase the useful payload mass at launch. Despite all the efforts that are being invested in finding effective propellant solutions, cryogenics are still widely used within the space industry. In particular, the hydrolox combination is the most used as it returns the best Δ𝑣 performance thanks to its high specific impulse. However, this class of propellants faces an issue that could jeopardize all the mass savings: boil-off. This consists in propellant evaporation due to heat penetration in the tank structure, and it could become so severe for long storage times (in the order of months) that the additional propellant mass used to make up for the boil-off losses would nullify all the benefits that come from the high impulse of these mixtures.

The approach taken in this work consists in developing a propellant depot sizing model that allows determining the effect of different thermal control design options, thermal environments, and depot configurations for varying mission durations. The model performs a multi-nodal thermal analysis to estimate boil-off rates and allows for taking into account different thermal control methods, which include Multi-Layer Insulation with uniform and variable density distributions and Vapor Cooled Shields.

Using the developed tool, the analysis of different depot architectures is performed. In particular, Low Earth and Geostationary orbits are investigated as locations for a depot, with the possibility to extend the results to the Earth-Moon Lagrange point L1. As for the depot, both single and multi-tank configurations are studied.
Results showed that variable density MLI (VDMLI) improves the effectiveness of the thermal insulation compared to the uniform density case: the most mass efficient designs in terms of insulation plus boil-off mass are achieved with less total number of layers when VDMLI is used. The use of a vapor cooled shield (VCS) additionally reduces the propellant boil-off. However, for the VCS to become advantageous in terms of mass, a minimum storage duration is needed: this corresponds to two months for the LEO depot, and at least six for the GEO depot. This is due to the mass penalty introduced by the addition of the metal shield into the system.
Lastly, it is shown that multi-tank depots have increased boil-off rates than single tank depots because of reflected thermal fluxes among the depot elements. However, some propellant savings come from placing the cryogenic tank in the shadow of the other tanks of the depot station.