I don’t know for sure what it is, but I just don’t have a lot of mental bandwidth anymore these days. I’ve realized that I don’t have as much time or energy to pour over all the details of an idea before posting. Especially when a lot of these ideas may be a lot farther out from being implementable than I may have thought in the past. So, rather than just let potentially useful ideas go undiscussed, I’m going to try putting some of these ideas out in a more basic form first. If there’s interest in the concept, and based on the discussions, I’ll try to flesh out the concept with more detail.
The first concept I wanted to talk about was one I had several months back (and which I briefly mentioned tangentially on the blog), the concept of going to what I call a depot-enabled multi-sortie mission model. The basic concept is fairly simple. There are lots of varied approaches you could take, but the common elements would be:
- A LEO Propellant Depot
- Small Lunar Orbital Propellant Depot/Way Station
- A Reusable Lunar Lander
In fact, even the LEO propellant depot isn’t 100% necessary for the concept–it just makes everything easier.
A couple of notes on the second and third items:
- The lunar orbital depot/way station could possibly be built around commercial modules, such as the Sundancer and/or Nautlius modules under development by Bigelow Aerospace. I’ve got some ideas that I’ve been working on for how you could convert such modules into the foundation of a propellant depot, but with luck, I will be turning those ideas into an honest to goodness conference paper this year, so I won’t go into details now.
- The depot/waystation would include both modules for propellant storage, as well as habitable space, and life support for two sortie crews (which depending on crew size could be 4-8 people total).
- The depot would have facilities for long-term propellant storage (sunshields, advanced passive cooling systems, and probably some active cooling as well)
- The landers, which could possibly be based on some of the Lunar Lander work done at LM/ULA, only need to be reusable for a limited number of times for this to make sense.
- I think that many of the key concerns with lunar landers (the impact of dust, rocks kicked up by landing engines, etc) can probably be managed for a limited number of flights even without heavy maintenance so long as certain precautions are taken.
- Because this is for short duration sortie missions, and because the propellant depot has long-duration storage capabilities, all stages can benefit from higher performance cryogenic propellants (LOX/LH2 is still my preference, but LOX/CH4 for some parts might still be worthwhile). The only place where hypergols might really be necessary is for something like the lunar ejector seat I described previously.
While there are tons of possible variations on the theme, the basic concept would go something like this:
- Once you have the LEO propellant depot setup, you send the lunar propellant depot over piece-by-piece. With propellant depots, an existing Atlas V Centaur stage, modified with a “Lunar Mission Kit” that has a few components needed for a long duration mission, and then refueled on orbit, should have more than enough margin to place a Sundancer module all the way into lunar orbit. In fact, if you could off-load about 3000lb worth of stuff from the Sundancer modules (might be possible if you use prox-ops tugs at both ends instead of having all the modules also be independent spacecraft with their own RCS/ACS systems, among other things), you could possibly reuse the Centaur LTV using nothing fancier than propulsive braking.
- Send an “unpacking crew” to help setup the lunar depot/way station. They would help unpack the modules, hookup any plumbing and wiring, verify all the systems are ready to go, and generally get things ready for exploration. They could be sent either using a crew vehicle per se, or by a self-ferrying lunar lander as per the next item. This crew might not need to be the full size of a landing crew.
- Launch two lunar landers, tank them up, and then send them both to the lunar orbital station. The delta-V needed for a lunar orbit to surface and back round-trip (with margins and reserves for plane changes and such) is pretty close to the same performance needed for the landers to ferry themselves to lunar orbit. This also functions as a “shakedown cruise” for both vehicles, allowing you to test out all the various subsystems and verify in-space that they are properly functioning before you have to risk your life on them.
- Start launching tankers to the lunar orbital station. These could possibly use Centaur-derived tankage sent using Centaur-based transfer tugs as mentioned earlier for delivering the Sundancer modules. You could probably deliver about 15,000lb of LOX/LH2 per tanker flight (about 1/4 of the propellant pumped in LEO reaching lunar orbit for a reusable system that doesn’t use aerobraking).[Update: I made a math error here, and I didn’t keep the spreadsheet where I made it so I’m not sure what I did wrong. Anyhow the correct tanker delivery mass is about 7500lb for a purely propulsive system that reuses the Centaur, and about 18000lb for a system that uses partial aerobraking to reuse the Centaur]
- Send your sortie crews once you have enough propellant for two missions.
- Tank up both landers, and send first one to the surface for a sortie. Continue propellant deliveries. The second is available on-orbit to mount rescue missions. If no rescue mission is needed, propellant can either be transfered back to the station (if enough propellant hasn’t arrived during the sortie for another sortie), or the next sortie can be launched.
- Keep rotating landings with one crew on-orbit during one landing ready for backup, and then the next landing they’re the crew.
- Rotate in crews on a six-nine month basis. Rotate in new landers on a regular basis, determined by the reliability/reusability of the systems.
Basically, once you have the system setup and working, each crew will probably go on 3-4 (or more) sorties during their stint. So, instead of having to ship out a new lander and a new capsule and a new transfer stage for each mission, you only have to ship out the marginal propellant needed for a single landing. Depending on the details of the setup, this could possibly yield the following benefits:
- Much higher safety. Most of the risk in a lunar mission revolves around landing on, ascending from, and departing from the Moon. With a depot/way station and a backup rescue vehicle, you can greatly increase the odds of getting a crew back in case something goes wrong. In many cases failures go from being life-threatening to just plain boring. Your return vehicle engine fails to light? You just sit it out and wait at the lunar station for the next crew rotation. Your lander fails to ascend? You have a rescue mission. You have to bail out during an aborted landing/ascent using a lunar ejector seat? You could actually have a rescue mission on-hand in hours or minutes instead of days or weeks. While depending on the inclination of the lunar station and the latitude of your sortie site, you might not have anytime aborts from the surface, you can at least greatly reduce the risk of losing a crew due to propulsive events.
- Lower marginal mission costs. Instead of making expensive hardware that mostly only gets used once, you can now eke out at least a few missions each from the transfer stages and landers.
- Much lower IMLEO per sortie. It might be possible to add another sortie for only a single DIV-H launch worth of propellant. Lower IMLEO, and lower hardware costs, can give you much better bang for your bucks.
- Lunar landing crews build a lot more experience quickly. Of the precious few hours the six crews during Apollo spent on the lunar surface, how much time was spent just figuring out how to function in 1/6g? How much time was spent figuring out how to work in that environment? How to work with the tools sent from earth. Sure you can train for some of that stuff, but there’s probably only so much you can figure out without being there.
- More data on the impact of lunar gravity on the human body. As I’ve mentioned before, we only have six data points that aren’t at either full 1g (billions of data points) or microgravity (hundreds of data points). With so little data in the middle, you really can’t make any meaningful claims about how much gravity a human body really needs. The human body could be extremely frail and optimized for 1g, or it could turn out that it works fine over a very broad range of gravity. The fact of the matter is that we don’t know for sure, and anyone who tells you otherwise doesn’t know what they’re talking about. By having multiple data points for the same people over time, it should be a lot easier to get good data on the impacts of lunar vs. micro gravity.
There are probably other benefits, and plenty of various nuances that I didn’t go over. For instance, I think that coupled with a two-person architecture, like I explored in the past, you could visit a lot more sites, and do a lot more real exploration and prospecting than you could do with a much bigger crew. But propellant depots also allow you (depending on their size) to launch larger missions as time goes by. Even if you start with two-person crews with no landed cargo, such a system could easily expand eventually to 6-8 person crews or substantial surface cargo capabilities. I also think that advances like orbital RLVs, WBC/ICES/ACES derived transfer stages, etc will only continue to add to the capability and flexibility of such an approach.
In short, I think this overall approach has a lot of benefits compared to more traditional architectures, and is worth further investigation.
What do you all think?