I suggested in the middle of a couple of recent posts that the hoverslam techniques pioneered by SpaceX with the Falcon9 be used for Lunar landings. It was a kind of throwaway thought along with several other suggestions. As I think about it though, it seems to me that there might be a serious possible schedule and reliability gain from adapting the technique to Lunar development. That’s why I’m putting it up as a separate post.
I didn’t think hoverslam was a viable technique until it had been demonstrated. I was wrong. Now that it has been demonstrated multiple times, it may be time to see if there are more applications in which it might give an advantage. Lunar landings being the application under discussion recently, I want to lay out a few possibilities.
First thing would be a discussion with the team that is already using the technique in operational vehicles. From the outside looking in, it appears that hoverslam is a software solution to landings that was previously considered a hardware development problem. If this thought is accurate, then it may not be necessary to develop engines and control systems that allow an empty tank vehicle to hover in 1/6 gee. It seems that it is a requirement to bring velocity to zero at the instant that altitude is zero with thrust/weight being far less relevant than most of us previously thought possible. It seems that the SpaceX team is landing with thrust/weight levels of well over two on Earth, which would be well over a dozen at the Lunar surface.
If a Lunar lander is at 20 tons at touchdown, then the hovering that most of us consider a requirement would need engines capable of throttling to 3 tons for a gentle descent at very low velocities. The experience of Apollo 11 finding a clear landing area validates this opinion. This is however, not 1969. The Lunar surface is not only far better known now, but any potential landing sites could be imaged to near centimeter precision at relatively low cost. So hovering while making sure of a clear landing zone may not be a requirement. Navigation to the clear areas is also much less of a challenge than a half century ago. So it may be possible to go straight in to a site on near side without even orbiting first. It may be possible to land that 20 ton vehicle with engines that will only throttle down to 50 or 60 tons.
Doug believes that getting funding authorities to sign off on Lunar hoverslam would be a nonstarter. He is right unless the technique is fully validated just as it was on Earth/barge. I suggest the first step would be an RFI to SpaceX to confirm that it would or would not be possible to use the technique in this manner. If the answer is affirmative, then a test mission could be envisioned. For a test mission, perhaps an upper stage of the Falcon9 could be refueled by a Facon9 tanker in Earth orbit to validate tanker technology as well before sending it on to the Lunar surface.
The Falcon9 upper stage with one refueling should be able to place well over 5 tons on the Lunar surface during the test mission if the concept is valid. Depending on the flight backlog and the interest of both NASA and SpaceX, this could fly by Q4 2018. I doubt any other system could land a comparable payload in anything close to that time frame regardless of interest. Cost would be for two Falcon9s plus payload and Lunar operations. 5 tons in useable condition on the Lunar surface would go a long way towards convincing a funding authority to further use the technique for unmanned payloads.
Central to acceptance of the concept would be the failure modes. Obviously a high enough speed impact would destroy the stage and cargo. Hitting a rock with a landing leg and tipping over could be almost eliminated with a good survey and navigation. A sideways vector on landing that tipped the stage over should not be a factor with the current experience level. The most likely failure modes would seem to be engine failure at altitude from fuel depletion, and excess velocity at touchdown from software or navigation error.
Payloads on the first flight(s) should be very robust as well as being useful so that good work can be done even with a less than successful landing. During an excessive velocity landing, the stage propellant tanks provide a crumple zone if done right. An impact at 100 m/s (200 mph) in the vertical orientation could subject the payload to under 10 gees which is survivable to most hardware. It should be expected that the first payload may have to cut its’ way out of the wreckage before deploying solar panels and starting the primary mission. If the stage soft lands but with a side component that tips it over in the 1/6 gee, the payload should also see less than 10 gees.
The spectacular failures we saw from the early Falcon9 barging attempts were almost all from residual propellant exploding. Though technically not detonations, the burns were fast enough that most of us would call it a good boom. The vertical and horizontal vectors on most of the early Falcon9 barging attempts would have been payload survivable without the propellant reactions on impact. In the vacuum at the Lunar surface there would be no reaction from residual propellants in a crash other than fast evaporation and site contamination. All of those spectacular RUDs on the barge would have been stage lost and payload delivered on the Lunar surface.
I suggest that this concept be considered at some low level to see if there is any merit to it. If there is, it could speed up Lunar development by several years and save a few Dirksens.