No matter what mode of transportation you take for a long trip, at some point, you’ll have to refuel. For cars, this could be a simple trip to a gas station, while planes, trains, and ships have more specialized refueling services at their depots or ports. However, for spacecraft, there is currently no refueling infrastructure whatsoever. And since the fuel spacecraft use must be stored cryogenically, and the tanks the fuel is stored in are constantly subjected to the thermal radiation from the Sun, keeping enough fuel in a tank for a trip to Mars with astronauts is currently infeasible. Luckily, NASA is currently working on it and recently released a detailed look at some of that work on a blog on their website.
The problem definition is very clear – cryogenic hydrogen and oxygen are used as fuel on most spacecraft missions. Once in space, the tanks the fuel is stored in heat up due to the constant solar radiation they’re subjected to. Since there’s no air, there’s no way to radiate out that heat, so eventually, it can get through even the most sophisticated passive thermal insulation system. When it does, the fuel starts to boil, and mission planners typically have chosen to eject the vaporous fuel out into space rather than leaving it as a potential medium to heat the rest of the fuel faster.
This resultant fuel lost to this sublimation can cost as much as half of the cryogenic fuel needed for a 3-year mission to Mars – in just a single year. In short, crewed trips to Mars are impossible using the current fuel storage technology in space. However, there are alternatives, known as Zero Boil-Off (ZBO) or Reduced Boil-Off (RBO) systems. These advanced tanks use a combination of “active” processes to maintain tank pressure and not allow too much loss of fuel during long space flights.
An “active” process must be actively controlled and typically requires some sort of power input. In particular, ZBO systems rely on two technology ideas – a jet mixing of the propellant and a droplet injection technology. Let’s take a look at the mixing technology first.
In this example, part of the fuel would be forcibly mixed back into the vapor space in a particular way that would allow it to control the phase changes of the vapor/fuel interface. In essence, it would stop the fuel from sublimating into a vapor in the first place. Similarly, a droplet injection system would use a novel type of spray bar to inject fuel droplets into the vapor area, causing it to condense and remove some of the pressure from the system.
To add another layer of complexity to these already complicated fluid dynamics systems, this all must be done in microgravity, where things like droplet formation and liquid mixing don’t always happen the same way as they do on Earth. So, NASA decided to do what it does best and run some experiments – in this case on the ISS.
Credit – NASA
Back in 2017, NASA started the ZBOT-1 Experiment on the ISS. It was intended to quantify how the jet mixing would behave in microgravity, and the result of some 30+ tests was that we still understand very little about how these systems work in microgravity. While how they were is different than what most fluid engineers are used to, they are still acting according to physical laws, so more experiments would help narrow down the models that tank designers can use to understand how these ZBO systems might best be used.
Two other experiments are focused on furthering that understanding – one called the ZBOT-NC Experiment, is due to be launched to the ISS in 2025. It will study the effects of microgravity on “non-condensable gases,” which can be used to control the pressure inside the fuel tank. Data from its observations can also be fed into the CFD models, allowing scientists to understand better how the model differs from reality in microgravity.
The final test in the series will focus on droplet phase changes. Known as the ZBOT-DP test, this is the most ambitious of the three, as it tests a technology that has never been used in microgravity at all before. It will focus on understanding how droplets interact with their surroundings, including superheated tank walls, in microgravity environments. They could eventually lead to a fully functional droplet system and an active control system to ensure no tank boil-off happens.
Credit – VideosFromSpace YouTube Channel / NASA Technology
That’s still a long way off those, with no planned date for the ZBOT-DP test. Given the importance of this technology to missions like the crewed Artemis mission planned in the next few years, it seems that the successful completion of these experiments and the design and testing of a fully ZBO fuel tank should be very high on NASA’s priority list. While the agency’s already supporting it, let’s hope that the researchers involved can prove their ideas before they’re needed for a real human mission.
Learn More:
NASA – Zero-Boil-Off Tank Experiments to Enable Long-Duration Space Exploration
UT – Why Build Big Rockets at All? It’s Time for Orbital Refueling
UT – There’s Now a Gas Station… In Space!
UT – Robotics Refueling Research Scores Huge Leap at Space Station
Lead Image:
The Gateway space station—humanity’s first space station around the Moon—will be capable of being refueled in space.
Credits: NASA