China, India and the U.S. have all achieved landing on the Moon within the 2020s.
Once there, their eventual goal is to arrange a base. But a successful base – together with the spacecraft that may carry people to it – have to be habitable for humans. And a giant part of making a habitable base is ensuring the heating and cooling systems work.
That’s very true since the ambient temperature of potential places for a base can vary widely. Lunar equatorial temperatures can range from minus 208 to 250 degrees Farenheit (minus 130 to 120 degrees Celsius) – and similarly, from minus 225 F to 70 F (minus 153 C to twenty C) on Mars.
In 2011, the National Academies of Science published a report outlining research within the physical and life sciences that scientists would want to do for the U.S. space program to succeed. The report emphasized the necessity for research about constructing heating and cooling systems for structures in space.
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I’m an engineering professor, and when that report got here out, I submitted a research proposal to NASA. I wanted to check something called the liquid-vapor phenomenon. Determining the science behind this phenomenon would help with these big questions around keeping structures in space a snug and habitable temperature.
Over a decade after we submitted a proposal, my team’s project is now being tested on the International Space Station.
Going with the ‘flow’
Liquid-vapor systems – or two-phase systems – involve the simultaneous flow of liquid and vapor inside a heating or cooling system. While many business air conditioners and refrigeration systems on Earth use two-phase systems, most systems utilized in spacecraft and on the International Space Station are purely liquid systems – or one-phase systems.
In a single-phase systems, a liquid coolant moves through the system and absorbs excess heat, which raises the liquid’s temperature. This is comparable to the best way cars use radiators to chill. Conversely, heated liquid within the system would eject the warmth out to the ambient area, lowering the liquid’s temperature to its initial level.
But liquid-vapor systems could transfer heat more effectively than these one-phase systems, they usually’re much smaller and lighter than purely liquid systems. When traveling in space, you have got to hold the whole lot on the craft with you, so small and lightweight equipment is important.
There are two key processes that occur in a closed, two-phase liquid-vapor system. In a single, the liquid changes to a vapor during a process called “flow boiling.” Identical to boiling water on the stove, in flow boiling the liquid heats up and evaporates.
In systems utilized in space, the two-phase mixture passes through heat exchange components that transfer the warmth generated from electronics, power devices and more into the mixture. This progressively increases the quantity of vapor produced because the system absorbs heat and converts liquid to vapor.
Then, there’s flow condensation, through which the vapor cools and returns to a liquid. During flow condensation, heat leaves the system by radiating out into space.
Scientists control these two processes in a closed loop so that they can extract and use the warmth that’s released during condensation. In the long run, this technology could possibly be used to regulate temperature in spacecraft going to the Moon, Mars or beyond, and even in settlements or habitats on the lunar and Martian surfaces.
Constructing and testing
With the grant from NASA to do that work, I designed an experimental program called the “Flow Boiling and Condensation Experiment.” My team built a fluid management system for the experiment and two test modules: one which helped us test flow boiling and one which helped us test flow condensation.
Without delay, the equipment used for heating and cooling in space was designed based on experiments in Earth’s gravity. Our flow boiling and condensation experiment seeks to vary that.
First, we tested whether the system and modules we built worked when subjected to Earth’s gravity. Once we learned they did, we sent them up in a parabolic flight aircraft. This craft simulated reduced gravity so we could get an idea of how the system performed in an environment just like that of space.
In August 2021 we accomplished the flow boiling module and launched it to the International Space Station for testing in zero gravity. By July 2022 we’d accomplished the boiling experiments. In August 2023 the flow condensation module followed, and we’ll start working on the ultimate condensation tests soon.
Responding to reduced gravity
Liquid-vapor flow systems are much more sensitive to gravity than the purely liquid systems used now, so it’s harder to design ones that work under reduced gravity.
The mechanism behind these systems has to do with the motion of liquid relative to the vapor, and what that motion looks like depends upon an idea called buoyancy.
Buoyancy is set by gravity in addition to the density difference between liquid and vapor. So any change in gravity affects the system’s buoyancy, and thus the movement of the vapor relative to the liquid.
In space, there are also different strengths of gravity that the systems might have to operate under. Space vehicles experience microgravity – near weightlessness – while a lunar habitat would operate under gravity conditions about one-sixth the strength of Earth’s gravity, and a Martian habitat could be operating under gravity three-eighths the strength of Earth’s gravity.
Our team is working on designing flow boiling and condensation models that may work under all these levels of reduced gravity.
Applications for space habitats
This equipment could someday go right into a human habitat on the Moon or Mars, where it could help maintain comfortable temperatures for people and machinery inside. A heat pump using our flow boiling and flow condensation systems could extract the warmth that astronauts and their machines give off. It will then send this collected heat out of the habitat to maintain the within cool – just like the best way air conditioners on Earth work.
The temperatures in space could be extreme and hostile to people, but with these technologies, my team might someday help create craft and habitats that allow people to explore the Moon and beyond.