The first advanced lunar transportation concept is the idea of intentional hard lithobraking landings of bulk raw materials. The idea was recently suggested by frequent Selenian Boondocks commenter Paul Dietz in the comments to this previous post. Basically you would launch a rocket with one or more large chunks of raw material on a trajectory that grazes the surface of the Moon, using the lunar regolith to decelerate the raw material to the point where it can be subsequently collected. Even if the payload is coming in on a hyperbolic trajectory from Earth without stopping in low lunar orbit, it’s theoretically possible for a very shallow impact to not vaporize the payload, leaving it largely intact.
Artificial Shallow-Impact Meteorites
In Dennis Wingo’s excellent book MoonRush, he makes the argument that due to the Moon’s shallow gravity well, it’s possible for a subset of lunar nickel/iron meteorite impactors to arrive at low enough relative velocity and a shallow enough angle for the majority of the material to survive intact in a large chunk.
This concept though would be basically trying to create that phenomenon artificially, with an earth-launched chunk of raw material, such as a big ball of some alloy that may be hard to produce on the Moon (copper conductor alloys? high strength aluminum alloys?). Impact conditions could be carefully controlled using the upper stage as a shepherding vehicle, similar to what was done with the LCROSS mission1, but potentially with the upper stage performing a last minute maneuver to avoid intercepting the lunar surface, allowing it to return to earth for recovery/reuse.
Why would you want to do something like this?
Primarily because it would dramatically reduce the cost of delivering some bulk raw material to a lunar settelement site. The delta-V from LEO to a lunar soft-landing is approximately 6km/s. With high performance LOX/LH2 propulsion, that means that only 1/4 of your LEO mass will be dry mass on the lunar surface. For instance, an ACES stage with 70mT of prop loaded in LEO could deliver approximately 24mT to the lunar surface, with the stage and Xeus landing hardware taking up at least 6mT of that (leaving a net ~18mT of payload). On the other hand, the same stage could hard land over 65mT of bulk material while recovering the stage back to LEO. In the case of a big chunk of raw material, unless it is something really exotic, most of the cost is going to be in the launch of the material and the propellant to get it to the moon. If you can increase the payload to the moon for the same propellant by a factor of 3.6x, that’s likely going to decrease the cost on the lunar surface by a somewhat similar margin.
What would you use this for?
As mentioned above, the most likely use of this would be to hard land bulk raw materials. Specifically you’d want something somewhat tough so it could survive the impact intact, in spite of impact forces probably in the 1000s of Gs, most likely a large chunk or ball of metal. Here are some specific applications I can think of:
- Copper conductor materials — copper is relatively rare on the Moon, but is used a lot as a conductor for power transmission, and could be a key element in manufacturing electromagnets or conductors for other propellantless launch/landing systems. Landing equipment that could metal and process high-purity copper or copper alloys into useable conductors may be a lot easier than landing big spools of conductors, if this approach allows you to cut the cost of the copper by almost a factor of 3.6x.
- High-Strength Aluminum Alloys — while aluminum itself is pretty common on the Moon, most high-strength alloys, particularly ones you’d want to use for structural components or pressure vessels, are ones that require alloying elements that may be hard to source locally, and which frankly will likely take a long time to build up the smelting capabilities on the Moon even if the alloying elements are available. These alloys could be alloys that can be cast using locally produced sintered regolith molds. Or they could be alloys designed to be converted into 3D printing powder. Or it could be wrought alloys that are designed to be melted and converted into sheet or bar forms. Once again, processing equipment to convert the bulk raw material into usable forms is likely going to be massive enough that you’ll be somewhat limited, but with proper thought the 3.6x cheaper aluminum alloy raw material may still be useful in some applications.
- Toughness-Enhancing Alloys for Ni/Fe Asteroidal Material — in the ongoing discussion on the previously mentioned ESIL-8 post, it was mentioned that many examples of meteoric steel, while beautiful and stainless steel-like are actually rather brittle. It might be possible to send along the right mixture of alloying materials to mix with magnetically recovered Ni/Fe asteroid fragments on the Moon to turn it into a much better and more useful grade of stainless steel, for 3d printing, and/or production of pressure vessels for habitation or material storage.
Some potential implementation issues that may need to be addressed for this concept include:
- What materials can actually survive an impact at the velocities associated with a hyperbolic trajectory from Earth?
- What is the optimal impact trajectory for controlled landing in a recoverable manner? Does too shallow result in large dispersions? Does too steep result in vaporization or the material being buried below the surface? How much of this can be analyzed up-front versus needing to be experimented with?
- How big of an impact crater are you going to form? How far will regolith fly?
- What is the optimal shape for the material? A solid sphere? A hollow sphere?
- From an earth TLI, where on the Moon can you realistically target an impact point for that has the right impact flight path angle?
- Where would you want material to be targeted relative to a settlement, to minimize risk to the settlement while simultaneously maximizing the odds of being able to recover the material easily?
- Can you design a trajectory that allows the shepherding upper stage to avoid lunar collision and return to Earth for recovery and refueling?
- Do you need to “prepare the ground” any by having a robot remove large boulders from the target area, and/or “rake” the regolith?
There are probably other questions, but the potential of being able to cut the cost of raw materials delivered to a lunar site by 3.6x makes this intriguing, even if a little on the messy side.
Next Up: Propellantless Soft-Landing Options