As I mentioned in the first post in this series, I wanted to paint the overall picture first, and then flesh out the details as time and interest permits. For this post, I want to discuss an interesting lander concept that could work well with the mission model I discussed in Part I. I may discuss some thoughts about how to do lunar lander reuse in a future post in this series.
By way of introducing the concept, I wanted to point out some material that LM/ULA came up with two years back that got me thinking in these directions. While many have read the AIAA paper ULA published in 2006 about various Centaur-derived manned lander schemes as alternatives to the ESAS LSAM, there was also some less-well-known material they had developed for Centaur-derived robotic landers that I found interesting. I just noticed today that a paper containing the information I had previously seen about this concept is up on the ULA site, here, so I figure it’s now ok to talk about this idea.
Basically, the second paper goes into some work LM/ULA had done for the Lunar Precursor Robotics Program back in the 2006 timeframe. They had looked into converting an existing Centaur into a lunar lander for robotics payloads, by adding a “Extended-Duration Mission Kit” and a “Lunar Lander Kit”. These kits, which the Centaur team has already detailed to some extent, would add things like better passive cryo insulation hardware, sunshields, solar panels, upgraded avionics and batteries, landing gear, landing propulsion systems, etc. The concept was based on launching the whole Centaur lander stack into LEO on an HLV.
The paper also went into a 4-person lander using the same Centaur-derived concept but extending it a bit further. A version of this concept was further discussed in the first paper. The manned lander would be two-stage with a hypergolic biprop system for ascent, and the lander would include hardware for supporting at least two-week lunar surface stays.
What I was interested in was what those concepts could do if they were used with in an architecture that included a LLO depot/waystation. In the case of the robotic lander, the lander itself also performs the TLI and LOI burns, which means that most of its propellant is used up before it gets to LLO. For the human lander, while they assumed the use of another stage to do the TLI/LOI burns, the system was constrained to be launchable with an Atlas V HLV, which meant that a full Centaur-load worth of propellant couldn’t be used for it either. Plus, with the use of a hypergolic ascent stage, the ascent fuel weighs a lot more than it would in a reusable scenario. Fortunately, this paper gives a mass budget, so we can do some number crunching.
For the robotic lander, it used a Centaur dry mass of 2500kg, a Extended Duration Mission Kit mass of 800kg, and a Lunar Lander Kit of about 1000kg, with 1500kg of LLK propellant, and 21000kg of Centaur propellant. Now assume a mission concept where you tank the whole Centaur stage up in LLO, the Centaur propulsion provides most of the delta-V except for the final touchdown/hover, the hypergolic landing engines provide landing/hover thrust as well as enough ascent thrust to get the vehicle up a couple hundred meters before relighting the RL-10s for ascent.
Depot-Enabled Centaur-Derived Manned Landing Missions
Factoring in some extra hypergolic propellant for both a long-duration hover (>90s) during landing, and enough propellant to get the vehicle up to a decent altitude before lighting the RL-10s, I estimate a payload in the 7500-8850kg range (you can download a copy of the spreadsheet I used here). The lower number was assuming a 2500m/s ascent delta-V (ie ascent DV plus some plane change propellants), while the higher payload was for a 2200m/s ascent delta-V, which is probably closer to what you would nominally need (when you have backup systems like tugs, depots, and a second lander in orbit, you don’t need as much in the way of contingency margins on any individual flight).
By way of comparison, the mass of the Apollo LEM minus main propellants was 4200kg, and most of that was stuff that would already be provided by the lander. So, it’s pretty safe to say you could haul at least four people up and down in such a system. For another comparison, at the higher end of the hauling capacity, you could haul a full Bigelow Sundancer module to and from the surface. Lastly, comparing it to the two-stage lander that they analyzed, if you ditched the ascent propulsion system and propellants and used the Centaur stage, it looks likely that you could haul 6-8 people and several tons of cargo for a two week stay without much difficulty. I can check on that if I can get better numbers from somewhere of the mass breakdown for their concept–the numbers given in paper #2 aren’t very clear on which weight in the ascent stage is for stage and propulsion mass, and which is for crew accomodations, pressure shells, etc.
Note, that at that point, this is anything but a “light scout lander”. Such a system would likely provide a substantial increase in capability compared to the ESAS LSAM, while only requiring a marginal 25-26 metric tonnes per mission worth of propellants/consumables. I don’t have the latest ESAS numbers, but for a sortie mission from them, you’re talking at least 65 metric tonnes, for only 1/2 to 2/3 as many people.
Also note that all of this is based on the existing Centaur design, not the Wide Body Centaur/ACES stuff that ULA has been investigating over the past several years.
Depot-Enabled Centaur-Derived Cargo-Dropoff Lander Missions
Now, what if instead of hauling a crew module up and down from the lunar surface, you were just hauling one-way cargo down to the lunar surface? For that scenario, I’m getting about 23,900kg of landed mass. Which is probably enough to deliver a full Bigelow Nautilus module to the surface. Or just about any piece of equipment you could imagine. Once again, this is with a stage based on the existing Centaur stage, not anything fancy like the ACES stage.
Admittedly, getting a payload that big to lunar orbit is actually the bigger challenge. You would need to use something bigger than a single Centaur derived transfer stage like I had talked about in Part I. Solar electric tugs, multiple Centaur stages in series, or a WBC/ACES derived transfer stage would be required. Or just finding a way to offload a decent amount of weight from the module itself, and outfitting it in lunar orbit before landing it.
Anyhow, I think this concept shows that using propellant depots in lunar orbit can greatly enhance a lunar exploration/development program, while also making the transportation phase of the program much safer. This performance benefit is not just with tiny sortie missions, but also with missions much more capable than what could be done with the planned ESAS architecture. Depots just make too much sense.