As space travel extends to greater duration and distance, missions may require a propellant refill in space. To achieve this, spacecraft may require larger tanks and efficient refueling along with tanks that have the capability of isolating propellant from ullage fluid (a gas and vapor mixture) during a vent. The goal of this Challenge is to develop a novel solution for the venting of ullage contents from a partially full propellant tank, in microgravity, with minimal loss of propellant. This ullage venting solution would help allow the adjustment of pressure in the receiving tank prior to, during, and/or after the liquid propellant transfer.  

This Challenge is seeking solutions to propellant tank venting in micro-gravity with minimal loss of propellant. Although all concepts will be considered, solutions that are external to the propellant tank are preferred as they could use existing (heritage) propellant tanks and avoid development costs related to designing and qualifying a new (or modified) tank.  

On Earth, liquid propellant tanks typically (and easily with the assistance of gravity) allow receiver tank venting. To enable a high mass fill fraction and a timely liquid fluid transfer in space, there must be a way to vent residual gas from the receiver tank in microgravity with a minimal loss of liquid propellant in the process. Fluid surface tension properties will dominate in large vessels/tanks when a fluid transfer is performed in low or zero gravity, resulting in somewhat unpredictable fluid (particularly liquid) distribution within the tank; therefore, it may be difficult to separate liquid from gas. Additionally, direct venting to space could alter the velocity of the spacecraft or generate undesirable stress on or contamination of components. Controlled propellant tank venting in space is historically rare but may be required for future missions where an in-space propellant transfer is needed. These future examples may include long duration missions to the Moon, Mars, and beyond, or operations to repurpose or extend the mission life of satellites or observatories.  

At present, there is no viable, efficient, multi-use, reasonably sized solution for venting propellant tanks that are non-bladder or diaphragm type. As some of the fluids (such as nitrogen tetroxide) being transferred are extremely corrosive and damaging, there does not exist an elastomeric material approved for long term life cycle use. Moreover, a metal bellows type tank solution is not a viable option because of quantity transfer limits, reusability (material fatigue), and tank sizing design challenges. A recent NASA project opted to perform mission operations for refueling without venting, relying instead on a larger tank volume and a lower propellant fill fraction. A negative consequence of this design is a progressive decline in thruster performance as the propellant tanks are drained and depressurized with propellant consumption through the thrusters. Also, the non-optimal refill Concept of Operations (ConOps) leads to a higher technical risk to the hardware since the tank would be compressed as it is filled, thus potentially heating the ullage pneumatic gas to near the system upper thermal limits. To address this, the project team developed a refueling ConOps solution that required multiple heating and cool down cycles to transfer propellant. This was not an optimal solution, due to its inherent complexity, fueling duration, and, as mentioned above, gradually decreasing efficiency of thruster performance, but it did allow the design of the system to meet the mission schedule and budget constraints. Future missions with larger tanks and/or more rapid transfer timelines will need a better solution. NASA anticipates that the winning Challenge solutions will include a gas venting capability that avoids loss of liquid and does not require any major increase in spacecraft mass, size, or performance. Moreover, they will not introduce greater technical risk of possibly exceeding current propellant tank thermal qualification limits. 

Developing an effective venting solution could result in higher propellant transfer efficiency, improve safety, reduce task duration, increase mission life cycle usage durations, and decrease operational and system complexity. This capability will increase the operational capabilities of  NASA, DoD, and commercial spacecraft allowing mission operators to complete long distance, duration, or unplanned repurposing of missions with increased confidence. Winning solutions may be considered for continued support to advance the concepts using existing NASA programs such as The Space Technology Research Grants (STRG) Program. 

In an ideal space fluid transfer system, the primary goal would be to transfer liquid propellant from one tank to another using a motive force (e.g. pump or other similar process) and achieve the maximum propellant fill level of approximately ​​​​95% by volume with the lowest risk and in the shortest time. The transfer would be performed with the two tanks connected to each other (ullage and propellant) as shown in the image attached (in Files tab). Pressure levels would be regulated to desired flowrate parameters between the tanks via this receiver vent while the liquid propellant flows from one to the other over an approximate period of hours to days depending on mission requirements and volumes being transferred. A vent would also be required in other (non-ideal) scenarios in which the receiving propellant tank is depressurized (vented to space) prior to (or after) the propellant refill. 

Please submit your questions in the challenge Clarification Board or via email at: WhoLetTheGasOut@freelancer.com