The heat of reentry is incredible. And not in the way we tend to use that word these days, but truly difficult to believe. And due to the Leidenfrost effect, we haven’t used liquid-cooled heat shield technology. This effect is the same thing that makes water droplets skitter over a hot pan and keeps the heat energy from transferring entirely to water. So instead of the water mass absorbing the heat, it sits on an insulating film of water vapor.
Most aerospace vehicles rely on air cooling to keep critical surfaces below their transition point. Air Cooling is effective up to a certain point; energy is energy, after all. The more mass flow rate of coolant, the better the cooling effect. When using something with low density, like air, the solution is to send large amounts of it past the hot surfaces to create a fast-moving boundary layer. This boundary layer can separate hot exhaust gasses from critical components. This is how engineers have cooled rockets, aircraft engines, and even electric motors for generations.
Air vs Water – Liquid Cooled Success
City University in Hong Kong is host to a research team led by Professor Wang Zuankai. His team has created a composite material that will absorb and use liquid for cooling past 1000 degrees Centigrade. This breakthrough is possible by taking advantage of the capillary effect to draw the liquid into the superheated material, absorbing the energy and transitioning to a gas. This gas then escapes through a series of microchannels, taking the unwanted energy with it.
The Future of Sustainable Aerospace
While this technology is revolutionary, it begs the question; so what? Is this type of heat shielding really that much more effective?
Currently, the trend in aerospace is a race towards large-scale reusable craft, that can attain orbit and return with large payloads and relatively little maintenance. SpaceX has led the way in the industry, however the are many other challengers.
The main maintenance issue with any sort of vehicle that undergoes reentry is tending to the heat shield. The NASA Shuttle was famous for issues with heat shielding, and every one of the ceramic tiles needed to be checked and potentially replaced after every mission.
With this type of Structured Thermal Armor, the cost of vehicle maintenance drops. Additionally, vehicles can now take advantage of atmospheric braking, reducing the fuel requirements and vehicle weight. Sustainable access and operations in space will rely on just this sort of innovation.
So while it may not sound as revolutionary as a scramjet engine or hypersonic air travel, the applications of this material will reshape the landscape of the aerospace world just the same.