Zero-Boil-Off (ZBO) or Reduced Boil-Off (RBO) technologies

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 During long space voyages Heat conducted through support structures or from the radiative space environment can penetrate even the formidable Multi-Layer Insulation (MLI) systems of in-space propellant tanks, leading to boil-off or vaporization of the propellant and causing tank self-pressurization. The current practice is to guard against over-pressurizing the tank and endangering its structural integrity by venting the boil-off vapor into space. Onboard propellants are also used to cool down the hot transfer lines and the walls of an empty spacecraft tank before a fuel transfer and filling operation can take place.  Thus, precious fuel is continuously wasted during both storage and transfer operations, rendering long-duration expeditions—especially a human Mars mission—infeasible using current passive propellant tank pressure control methods.

Zero-Boil-Off (ZBO) or Reduced Boil-Off (RBO) technologies provide an innovative and effective means to replace the current passive tank pressure control design. This method relies on a complex combination of active, gravity-dependent mixing and energy removal processes that allow maintenance of safe tank pressure with zero or significantly reduced fuel loss.

Zero Boil-off Storage and Transfer: A Transformative Space Technology

At the heart of the ZBO pressure control system are two proposed active mixing and cooling mechanisms to counter tank self-pressurization.  The first is based on intermittent, forced, subcooled jet mixing of the propellantand involves complex, dynamic, gravity-dependent interaction between the jet and the ullage (vapor volume) to control the condensation and evaporation phase change at the liquid-vapor interface. The second mechanism uses subcooled droplet injection via a spraybar in the ullage to control tank pressure and temperature. While the latter option is promising and gaining prominence, it is more complex and has never been tested in microgravity where the phase change and transport behavior of droplet populations can be very different and nonintuitive compared to those on Earth.

Although the dynamic ZBO approach is technologically complex, it promises an impressive advantage over the currently used passive methods. An assessment of one nuclear propulsion concept for Mars transport estimated that the passive boil-off losses for a large liquid hydrogen tank carrying 38 tons of fuel for a three-year mission to Mars would be approximately 16 tons/year. The proposed ZBO system would provide a 42% saving of propellant mass per year. These numbers also imply that with a passive system, all the fuel carried for a three-year Mars mission would be lost to boil-off, rendering such a mission infeasible without resorting to the transformative ZBO technology.

The ZBO approach provides a promising method, but before such a complex technological and operational transformation can be fully developed, implemented, and demonstrated in space, important and decisive scientific questions that impact its engineering implementation and microgravity performance must be clarified and resolved.

NASA

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