If a base does indeed appear on the Moon over the next few years, the question of regular takeoffs and landings will arise. And for them, specialized buildings and structures are needed, which are already being developed by engineers.
Launch pads on the Moon as a new engineering challenge
But when attempting to build structures on other worlds, such as the moon, engineers don’t yet know much about the local materials. Still, due to the costs of getting large amounts of materials off of Earth, they will need to learn to use those materials even for critical applications like a landing pad to support the landing/ascent of massive rockets used in resupply operations. But when attempting to build structures on other worlds, such as the moon, engineers don’t yet know much about the local materials. Still, due to the costs of getting large amounts of materials off of Earth, they will need to learn to use those materials even for critical applications like a landing pad to support the landing/ascent of massive rockets used in resupply operations.
A new paper published in Acta Astronautica by Shirley Dyke and her team at Purdue University describes how to build a lunar landing pad with just a minimal amount of prior knowledge of the material properties of the regolith used to build it.
Why build a landing pad at all, though? Couldn’t Starship or a similarly heavy capacity rocket simply land wherever its flight algorithm deems is a flat enough patch of ground? In theory, yes; however, the plume from the retrograde rockets will kick up a massive amount of rock and dust, potentially damaging not only nearby structures (like a fledgling lunar base), but also potentially the rocket itself.
To avoid that fate, mission designers generally agree on the need for a more structured “landing pad” similar to what we use on a near-daily basis on Earth. The question arises: is it possible to recreate such runways on the Moon using local lunar materials?
Not in the same way we would build them here. Building a landing pad on the moon would require using local regolith, since the cost of shipping enough concrete to the lunar surface to build a landing pad using Earth-source materials would be prohibitively expensive.
But, according to Dr. Dyke, there’s still so much we don’t understand about the mechanical properties of the moon’s regolith – especially about the strength a member has when it is sintered together, which is the current favorite process for creating a cohesive, hard structure out of in-situ regolith that would serve as a landing pad.
Lunar regolith as a building material for launch pads
Anyone familiar with lunar regolith testing is probably asking – why don’t they just use simulants to perform some preliminary tests? This material is the closest we can get to the real deal on the moon, and it’s been used for everything from beneficiation tests to growing plants. But, “simulants are called simulants for a reason,” according to Dr. Dyke. While some of the material properties might be the same, the only way to truly know how a material will react, especially in as unique an environment as the moon, is to test it in situ.
There are two main considerations when designing a landing pad – its mechanical properties (i.e., stress/strain under force) and its thermal properties (i.e., how much it expands/contracts at different temperatures). While many of the properties of sintered regolith materials are unknown, the authors were able to estimate structural properties based on what little information is available in the literature.
One theory was that the sintered regolith would be brittle and weaker in tension (pulling apart) than in compression (pushing together). It’s also expected to be very thermally insulative, so even a direct blast from the retrorocket of a Starship would only dramatically heat the top 8cm of a slab. However, this results in cracking each time that the ship launches from the pad.
Risks of destruction for the lunar launch pad
But the act of landing/ascending isn’t the only major stress the pad would undergo. It would also be affected by the 28-day lunar day/night cycle, where temperatures would vary wildly. The expansion and contraction that the pad would undergo during that cycle would be resisted by friction with the loose regolith soil underneath it – another mechanical property that we don’t understand.
The authors do understand that, if temperature changes aren’t spread evenly throughout the slab thickness, the expansion of the hot layer could cause the entire slab to curl, creating a warping stress that could lead to fracturing.
Taking these behaviors into account, the team suggests that, for a 50-ton lander, the pad should be about one-third of a meter thick (or 14 inches for the Imperially inclined). When asked why not simply make it denser to provide a sufficient margin of error, Dr. Dyke pointed out in an interview with UT that increasing the depth would make it more likely to fracture under thermal stresses, actually causing the pad to fail more quickly than a smaller version.
Platform integrity
There are some failure modes that are expected to happen, though. Spalling is one. In this process, chips of the pad crack off due to the thermal expansion/contraction. While the pad can be designed to maintain its overall structural integrity, over time, with repeated rocket blasts, this could degrade the structural integrity of the pad, causing it to be unable to support the same size of rockets.
But perhaps the biggest concern is the fracturing of the pad itself. This could be caused by thermal stresses, spalling, degrading its integrity, or even a rocket coming down at a bad angle. Uncertainties crop up in the design process at almost every turn, which is why Dr. Dyke and her co-authors suggest a simple plan for proving the pad will work – in situ testing.
Most likely, the first step of lunar exploration will not be to build a pad for consistent rocket landings/launches. Early missions could collect more data on the material to be used for the pad, and are especially well-placed to do in-situ testing under lunar gravity and atmospheric conditions that are hard to reproduce here on Earth.
Prospects for the construction of a launch pad
Once a landing pad is finally in place, instrumenting it and taking data will help improve the design over time. Dr. Dyke is most interested in how the pad is deforming under load, but also during the extreme thermal day/night cycles. With that knowledge, she could predict how cracks would form and potentially come up with a mitigation strategy in advance.
Mitigating those cracks and building the pad more generally will likely be the purview of robots – either teleoperated or completely autonomous. Trying to build such a pad using human labor, especially when that human is enveloped in a bulky space suit that is the only thing keeping them alive in the vacuum of space, is infeasible. So, according to Dr. Dyke, robots will be an absolutely critical part of the equation for building the landing pad – and maintaining it once it is already built.
That first build is probably still years away at this point, as NASA and other agencies are still actively working on getting astronauts back to the moon. As that process continues, hopefully, engineers back on Earth will get some more data to improve their models about how a landing pad might work on the moon.
But even if they don’t, the iterative testing, learning, and design process suggested in the paper could eventually result in a structurally sound and therefore safe entry point to our nearest interplanetary neighbor.
Provided by: phys.org
