Demonstration of Optimal Chilldown Methods for Cryogenic Propellant Tanks in Reduced Gravity

Future lower-earth-orbiting (LEO) propellant fuel depots and human-carrying orbital transfer spacecraft flying to the moon and Mars will have to utilize the high thrust and high efficiency of liquid cryogenic chemical propulsion or nuclear thermal propulsion. Efficient in-space tank-to-tank propellant transfer (propellant fuel depot to orbital transfer spacecraft) of cryogenic propellants is an enabling technology for the planned DRM 9 Crewed Mars Surface Mission. The transfer of cryogenic propellants in space, however, has yet to be accomplished, solely due to the unavailability of cryogenic quenching heat transfer data during chilldown (quenching) and filling of the propellant receiver tank in reduced gravity and microgravity as liquid propellant cannot be stored in a liquid state until the tank is quenched down to the liquid temperature. 

In order to maximize the storage tank chilldown efficiency, the technology proposed includes cryogenic spray cooling, Teflon thin-film coating of the tank inner surface, and spray flow pulsing. The completed flight experiments successfully demonstrated that cryogen spray cooling is the most efficient cooling method for the tank wall chilldown in microgravity. Teflon coating together with flow pulsing was found to substantially enhance the chilldown efficiency in microgravity. The feasibilities of charge-hold-vent for tank chilldown and no-vent-fill for tank filling in microgravity were also successfully demonstrated. 

The completed parabolic flight experiments were performed on scaled down models using liquid nitrogen as a simulant for actual propellants. Therefore, we only verified the physical and technical concepts, and proved the feasibility of the proposed technology. As a result, more engineering and development work is needed before it can be utilized by NASA. 

With this data it will now be possible to develop reliable and robust heat transfer correlations for cryogenic spray quenching. The new correlations developed from this data will enable engineers to accurately design in-space cryogenic propellant tank chilldown and fill systems. Therefore, the data from these flights will be a part of an advance for in-space cryogenic propulsion and future human flight into deep space. 

Continuing work and plan:

1) Continue research to perfect the technology by finding the maximum achievable spray cooling efficiency, the optimal coating materials and thickness for tank inner surface coating, and the pulse flow thermal-fluid operating conditions. 

2) Develop mathematical and engineering models verified and validated by flight results for NASA engineers to design future propellant transfer and storage systems. 

3) Investigate the long-term effects of cryogenic flow on the integrity and life-cycle of coated surfaces. 

The current technology and concepts were specially aimed at their applications in the space microgravity environment. We would not have been able to be the first to verify and prove their feasibility and validity in space without the financial support for these parabolic flights from the NASA Flight Opportunity program.