Lunar Depot-Enabled Multi-Sortie Missions

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:

  1. A LEO Propellant Depot
  2. Small Lunar Orbital Propellant Depot/Way Station
  3. 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:

  1. 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.
  2. 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).
  3. The depot would have facilities for long-term propellant storage (sunshields, advanced passive cooling systems, and probably some active cooling as well)
  4. 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.
  5. 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.
  6. 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:

  1. 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.
  2. 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.
  3. 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.
  4. 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]
  5. Send your sortie crews once you have enough propellant for two missions.
  6. 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.
  7. Keep rotating landings with one crew on-orbit during one landing ready for backup, and then the next landing they’re the crew.
  8. 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:

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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?

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Jonathan Goff

Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
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38 Responses to Lunar Depot-Enabled Multi-Sortie Missions

  1. Paul Talbot says:

    My initial thought was whether crew might be nervous being next to such a large quantity of cryo propellants, but assuming they’re cool with that, it might also answer my next question, which was what to do in the event of a big CME.

  2. Bill White says:

    Where would you locate these lunar orbital station(s)?
    My understanding is that low lunar orbit is not a reliable place to deploy due to the Moon’s “lumpy” gravity arising from irregular mass concentrations and that achieving global lunar access is quite a challenge if you start from low lunar orbit.
    EML-1 and EML-2 appear (at least to me) to be a dependable and useful location allowing 24/7 global lunar access and equal access from every LEO orbital inclination.

  3. kert says:

    One of the immediate problems i see is lunar station rad shielding of crew.

  4. Jonathan Goff Jonathan Goff says:

    Paul,
    You keep the two propellants separated. Really, such a setup should be much safer than traveling on a mars ship or even current lunar vehicles. With those you often don’t even have a 1/4″ of metal between both propellants, nothing to protect you from MMOD, etc.

    Plus, as you point out, you have a large amount of propellant in the station, and the possibility of adding extra water and other materials to form a storm shelter of sorts. If anything, such a setup would vastly safer when it comes to solar radiation events than anything planned.

    ~Jon

  5. Jonathan Goff Jonathan Goff says:

    Bill,
    Lunar orbit just requires station-keeping propellant (as does L1/L2 for that matter). While you could do this out of L1/L2, the case isn’t as obviously useful. Your reusable lunar landers have to be a lot bigger since you need more delta-V and they also need to house the crew for much longer, and any rescue mission is now back to days away instead of hours or minutes. In fact the delta-V penalties are enough that it no longer saves you a lot versus just going directly with each mission. It’s safer, but you lose a lot of the clear-cut economic benefits by doing that. I’d rather bite the bullet, and take a low lunar orbit, and just eat the worse station keeping losses. I think you’ll come out ahead for a first-generation sortie station.

    Later, once you’re setting up more permanent facilities, or doing longer duration sorties, having an L1/L2 base is a good idea. But even then, I still think that keeping a LUNO station or two or three (depending on how light/cheap they are) around for Search and Rescue operations might not be out of order.

    ~Jon

  6. Bill White says:

    Jon,

    EML vs LLO could simply be a numbers crunching issue that requires figuring out where a LLO fuel depot might wander off to in terms of orbital inclination. A sufficiently large plane change could be significant in terms of delta v and delta t for travel to and from a given point on the lunar surface.

    Likewise, LEO to LLO re-supply flights (fuel tankers!) will be launch window constrained and LEO orbital inclination constrained.

    That said a LLO depot to stage multiple sorties would be a HUGE improvement over current plans to sortie a single LSAM leaving no meaningful infrastructure in place, improvement as a function of objectives. If the Moon is merely practice for Mars, infrastructure is a bug rather than a feature.

    = = =

    Using Lissajous orbits around EML-1 and EML-2 appears to diminish station keeping requirements and allows direct line of sight with Earth (relevant for EML-2) and at a minimum, placing com-sats at EML-1 and EML-2 would permit a 24/7 lunar global communications net.

  7. Bill White says:

    A quote and a link for interesting reading on lunar orbits. 86 degrees could be a winner:

    “Be careful of the orbit chosen for a low-orbiting lunar satellite. “What counts is an orbit’s inclination,” that is, the tilt of its plane to the Moon’s equatorial plane. “There are actually a number of ‘frozen orbits’ where a spacecraft can stay in a low lunar orbit indefinitely. They occur at four inclinations: 27º, 50º, 76º, and 86º”—the last one being nearly over the lunar poles. The orbit of the relatively long-lived Apollo 15 subsatellite PFS-1 had an inclination of 28º, which turned out to be close to the inclination of one of the frozen orbits—but poor PFS-2 was cursed with an inclination of only 11º”

    http://science.nasa.gov/headlines/y2006/06nov_loworbit.htm

  8. Will McLean says:

    Sorties are so much less efficient than one or more surface outposts, even with an orbital depot, that it make little sense to let them drive your architecture.

    For the marginal cost of three sorties from orbit, totalling three weeks on the surface, you could probably land the same crew at an outpost with enough consumables for a year. And has been pointed out, it’s a lot easier to give them robust radiation protection on the surface.

  9. Jonathan Goff Jonathan Goff says:

    Will,
    Yes and no. If you assume that long-distance lunar surface transportation is going to be easy, maybe. But the reality is that due to safety considerations, you’re probably not going to have big roving missions without the ability to do a rescue sortie if the thing breaks down outside of the walk-back range. While outposts are good for some things, they limit the amount of sites you can visit. If you think that the poles are the only interesting places on the moon like some do, then sure just plop an outpost down. But the reality is that there are lots of interesting places out there. And if you want to explore them for longer, you cargo-land some light temporary outpost structures. This approach can handle outpost missions just as easily as a direct from earth approach. But the difference is, if you have a transportation failure you’re not 100% screwed. And you have more flexibility.

    Also, it’s not clear that you couldn’t have a small storm shelter on orbit without much trouble. You’ve got a lot of propellant mass, and finding a way to use some of that as shielding shouldn’t be that hard. As it is, most of the recent NASA outpost stuff I’ve seen doesn’t actually have the habitats buried, so in reality they would probably be worse risks for radiation than what I’m talking about here.

    ~Jon

  10. Bill White says:

    Will, wouldn’t sorties help in choosing where to best locate an outpost.

    Also, robotic sorties could be a precursor approach. Start with an operational Google X Prize lander (once one or more of those become operational) and modify (upgrade) to permit autonomous refueling in LLO and repeated flights to and from lunar surface.

    Hop around Luna collecting samples and taking measurements as a preliminary scouting mission to help ascertain where it would be best to send humans.

  11. Eric Collins says:

    Jon,

    Could you please post the delta-v requirements for LLO vs EML1, EML2 rendezvous? It would also be helpful to see what that means in terms of propellant mass. I know the amounts will probably vary widely depending on the locations of the landing sites and inclinations (and phase) of the LLO depot. But it would help me out if I could see rough orders of magnitude.

    Thanks

  12. Formerly known as Skeptic says:

    Here is some relevant info from the CEV studies:
    http://www.astronautix.com/craft/cevthrop.htm

    “In supporting its decision to move from an L1 scenario to an LOR scenario, Northrop examined the necessary additional delta-V required to get ‘anytime’ back from a variety of lunar surface base locations to a CEV waiting in lunar orbit. These were as follows:

    * Scenario: CEV parked at L1 Lagrangian point, lunar base anywhere on lunar surface. Delta-V from lunar orbit to L1: 677 m/s. Maximum ascent plane change delta-V required for anytime return: 0 m/s. Total delta-V: 677 m/s.
    * Scenario: Scenario: CEV in equatorial lunar orbit, lunar base at lunar equator. Delta-V from lunar orbit to earth: 987 m/s. Maximum ascent plane change delta-V required for anytime return: 0 m/s. Total delta-V: 987 m/s.
    * Scenario: Scenario: CEV in polar lunar orbit, lunar base at 70 deg lunar latitude. Delta-V from lunar orbit to earth: 987 m/s. Maximum ascent plane change delta-V required for anytime return: 560 m/s. Total delta-V: 1947m/s.
    * Scenario: Scenario: CEV in polar lunar orbit, lunar base at 56 deg lunar latitude. Delta-V from lunar orbit to earth: 1387 m/s. Maximum ascent plane change delta-V required for anytime return: 942 m/s. Total delta-V: 2329m/s.
    * Scenario: Scenario: CEV in polar lunar orbit, lunar base at 43 deg lunar latitude. Delta-V from lunar orbit to earth: 1387 m/s. Maximum ascent plane change delta-V required for anytime return: 1285 m/s. Total delta-V: 2672 m/s. “

  13. Bill White says:

    Skeptic, you left off the final sentence from your link:

    “Despite the anytime return LOR scenario requiring four times the delta-V of the L1 scenario, NASA favored the approach. ”

    FWIW

  14. Jonathan Goff Jonathan Goff says:

    Bill,
    I guess it boils down to a question of which sort of emergency you want to optimize for. If you want to be able to do a medical evacuation from the lunar surface at any time, with minimal delta-V penalty then L1/L2 have real benefits. But, if you’re more concerned about aborts during ascent/descent, then LUNO makes a lot more sense. Unless you burn a ton of propellant, L1/L2 is typically a day or more from the moon. That drives up the size/weight/cost of your lunar landers, and also make is that much less useful for emergencies.

    It all depends on what you’re *really* trying to achieve. I’ve longtime been a proponent of L1/L2, but I don’t think it makes sense in all cases. Once you’re talking long-duration landers, and big cargo landers and such, L1/L2 might make a lot more sense.

    ~Jon

  15. Bill White says:

    Jon, I do agree with you and as I wrote in comment #6 above:

    Deploying an “LLO depot to stage multiple sorties would be a HUGE improvement over current plans to sortie a single LSAM leaving no meaningful infrastructure in place” especially for more preliminary recon work.

    There is no right or wrong answer other than architectures should best fit the objectives of the check writers, a/k/a “The Golden Rule”

  16. Eric Collins says:

    Thanks for the link and the numbers, former-skeptic.

    I also did a few seconds googling for ‘cislunar delta-v’ (sorry for being lazy before) and came up with a paper by Wendell Mendell(Stategic Considerations for Cislunar Space Infrastructure). The delta-v numbers provided in the paper are mostly for LEO to various other destinations, but they also provide the mass-ratio surcharge. From their data the L1 to lunar surface leg would require about 2.52 km/s and a surcharge of 4.53 kg of propellant per pound of payload delivered.

    I thought that the delta-v to return to L1 would be about the same, but these numbers are quite a bit larger than those quoted in the astronautix article for the first scenario. Am I missing something?

  17. Bill White says:

    Eric,

    For whatever it is worth, Wikipedia has a comprehensive chart of cis-lunar space delta v numbers. Maybe I should post this link at nasaspaceflight and ask if anyone sees errors.

    http://en.wikipedia.org/wiki/Delta-v_budget#Earth-Moon_space_budget

  18. Exploration Fan says:

    Personally my favorite aspect of depot based architectures is the flexibility.

    A simple initial LEO based depot could support robotic missions proving the technical capability of a depot including rendezvous, propellant transfer and cryo storage. The robotic lander could be an early version of the LSAM, but starting from LEO deliver modest payloads to the lunar surface while providing early demonstration of this critical transportation element.

    The LEO depot than evolves to support a larger propellant capacity. Large robotic missions and initial crewed lunar sortie missions expand the exploration program, utilizing a LLO depot, as Jon suggests, or L1 depot to refuel the lander prior to going to the surface. Thus the lander performs both the Earth departure and landing functions, avoiding the cost of dedicated stages for these two functions (sort of reuse for free).

    This architecture can be improved by utilizing advanced in space propulsion (as it becomes available) to deliver propellants to the LLO depot. High ISP options include solar thermal or solar electric hugely reducing the per-mission launch mass required.

    Insitu production of LO2 at the moon can further enhance this mission architecture by supporting the lunar ascent transportation.

    Use of reusable stages as Jon discusses can be introduced as they become available and demonstrate lower costs.

    This architecture provides the perfect foundation now for asteroid, Mars or other missions (crewed or large robotic). Use of a depot at L1 provides a great starting point missions beyond Earth-Lunar space.

  19. kert says:

    One thing i’d like to point out regarding sorties vs. surface travel. A battery + solar array + brushless motor, i.e. a simple electric wheeled vehicle ought to be several orders of magnitute more reliable method of transportation than anything that lights a rocket. This unfortunately is and will be the case for probably at least a few decades still.
    Especially when you factor in in-situ repair/maintenance of both.

  20. Exploration Fan says:

    Let me provide a couple of examples for a multi use lander (capable of starting from either LEO or LLO or L1):

    I’ll assume an efficient (LO2/LH2) lunar lander having the following: 50,000 lb propellant, an ISP of 460 sec and a mass fraction of 0.8.

    LEO = 6,800 lb payload (~20,400 fps to lunar surface).

    L1 = 56,000 lb payload (~8,300 fps to lunar surface).

    LLO = 72,000 lb payload (~7,000 fps to lunar surface).

    This shows that a single lander can support both moderate robotic missions and very robust crewed or lunar base emplacement payloads. Such a lander would have to have very robust thrust needing to throttle thrust down to <3,000 lb for the lighter payloads and around 14,000 lb for the heavier payloads to enable soft landing. But the thrust would have to be much greater during the descent sequence to minimize gravity losses.

  21. Exploration Fan says:

    Here is an example of how Jon’s propellant depot concept would benefit from the use of enhanced orbital propulsion. I’ll assume the goal is to move a 100,000 lb payload from LEO to LLO. This payload consists of a tank carrying cryo propellant to replenish the LLO depot. I’m assuming a velocity change from LEO to LLO of 13,500 fps.

    The following masses are for a propulsions stage (propellant+stage) capable of delivering the 100 klb from LEO to LLO:

    Chemical = 193 klb (ISP=460, MF=0.9)
    Solar thermal = 55 klb (ISP=900 sec, MF=0.85)
    Solar electric = 11 klb (ISP=3,000 sec, MF=0.8)

    I’m probably being overly optimistic with the mass fraction for the Solar electric stage. This example shows how the Earth launch requirements for replenishing the LLO propellant depot can be cut by almost a factor of 3 (from 293 klb to 111 klb) by improving the in space propulsion system. This offers space exploration a huge lever to reduce one of the major costs: Launch.

  22. Jonathan Goff Jonathan Goff says:

    Kert,
    “One thing i’d like to point out regarding sorties vs. surface travel. A battery + solar array + brushless motor, i.e. a simple electric wheeled vehicle ought to be several orders of magnitute more reliable method of transportation than anything that lights a rocket. This unfortunately is and will be the case for probably at least a few decades still.”

    Not so much when you factor in the lunar environment. Sure it might be easier to repair per se, but what happens when you break down 500km from your base?

    ~Jon

  23. Will McLean says:

    If you want to visit many locations on the Lunar surface, then a mobile base is a better solution than a lot of short sorties. The Lunox outpost and Morphlab were both proposals for how this could be done in a way that provided a lot of redundancy and capability.

  24. Jonathan Goff Jonathan Goff says:

    Will,
    It would at least seem to me that “robust radiation shielding’ and “mobile” are somewhat incompatible goals for a base. Sortie missions don’t have to just be the lander itself. It’s possible to do hybrids of the two options, as I discussed previously in this blog post:
    http://selenianboondocks.com/2006/11/lunar-surface-rendezvous-and-light-scout-outpost-missions/

    ~Jon

  25. Karl Hallowell says:

    Not so much when you factor in the lunar environment. Sure it might be easier to repair per se, but what happens when you break down 500km from your base?

    Or your vehicle had a brake failure while going down a long slope?

  26. Habitat Hermit says:

    LEO to EML1 with a delta-v of only 0.1 m/s, a mere 10 cm per second.

    It will take about 17 and a half days using a 3-impulse trajectory according to page 8 of this 2007 presentation from a European workshop on space mission analysis called “Trajectories to/from the Earth-Moon Lagrangian points L1 and L2 for the human exploration of the moon”.

  27. Brad says:

    I am in favor of the concept of propellant depots in general. I even think cryogenic depots in Earth orbit could be useful. But I worry about using a depot to support a reusable LH2/LOX lunar lander.

    I think the most difficult aspect of such a scheme is the lunar ascent phase. Keeping boiloff of the ascent propellant to acceptable levels over a prolonged period might be too challenging on the daylit lunar surface for a lander which by neccessity must use the lightest possible structure.

    Didn’t the Lockheed-Martin dual-axis thrust lander concept use an ascent stage and leave the bulk of the lander’s structure behind on the lunar surface? It may not be practical for a fully reusable LH2/LOX lander to linger on the moon.

    Perhaps active cooling could solve the problem? Or a softer cryogen than hydrogen (such as methane)?

    I’ve got an even wackier idea. What if the reusable lander is multi-engined? Or multi-fueled?

    A multi-engined lander might have a LOX/LH2 descent engine and a storable propellant ascent engine. The lander could be configured like the Lockheed-Martin dual-axis thrust lander. Since the storable propellant engine would handle the final hover and touch-down the LOX/LH2 engine wouldn’t need deep throttling or rapid engine response features and could be optimised for maximum ISP.

    A multi-fuelled lander might use LOX/LH2 for the descent phase of flight and LOX/CH4 for the ascent phase with a single engine. Though this would be a considerably more difficult engine to develop.

  28. Exploration Fan says:

    Brad,
    I share your scheptisism regarding reuse of the lander, at least during the early days of lunar exploration. During this period of lunar infrastructure development I can see the large descent stage having more use on the lunar surface than trying to reuse it for transportation. Heck, one just went to a lot of trouble to land this stage on the moon, why not use it in place. The large tanks, high pressure bottles, valves, computers all would be extremely useful in supporting a lunar base development.

    At some point the lunar base may be “finished” and there are more trips to the base than one needs hardware, spares, etc. At this point it may make sense to start reusing hardware. The dual axis lander concept seems to be particularly suited to reuse. The RL10 is kept well off of the surface, out of danger of being damaged by debris kicked up during landing. At this point it may make the most sense to store the LO2 and LH2 in surface dewars. These dewars could be spent lander tanks with lots of added insulation to support very efficient lunar surface storage. Upon landing, exess propellant from the lander would be transfered into these surface dewars. When ready for ascent the ascent vehicle would be loaded with enough LO2 & LH2 for the ascent trip. Descent propellant would be supplied from the orbital depot.

    As you point out alternative propellants are an option. I still favor LO2 and LH2 because the are very useful for the base as well. You can make potable water as well as store energy for the fuel cells. If one wants to store power during the lunar night you are going to need a lot of LO2 and LH2. Having a common comodity (LO2 & LH2) for all of these uses would simplify the infrastructure of the lunar base.

  29. Will McLean says:

    Jon at #24:

    Providing shielding for a mobile base is more difficult than at a fixed one, but easier than doing it for a “light scout oupost”

  30. Brad,
    It depends a lot on the details, like how long of a stay you’re doing for the stage. I really don’t like the idea of going with a two-stage design, because you lose any benefits this approach might have had. Going LOX/CH4 for the ascent stage might be better if storing LH2 is too hard, but at this point I’m not sure we really know how hard it will be to store LH2. There’s some real advantages to keeping the number of fluids, tanks, and feed systems down to a reasonable number.

    ~Jon

  31. Exploration Fan,
    I think there’s some benefit to not tieing yourself down to a base so early in the exploration phase. Though I do agree that when you start needing to retire the reusable landers, retiring them to the surface to serve as surface infrastructure doesn’t seem like a bad idea. And yes, the LM horizontal lander approach seems like it would lend itself well to a lot of this.

    ~Jon

  32. Will,
    Providing shielding for a mobile base is more difficult than at a fixed one, but easier than doing it for a “light scout outpost”

    Do you have any evidence for this belief? Because I really don’t see much difference. If anything, burying a “mobile base” module that has to be able to move again later seems even harder than burying a light habitat module that you’re planning on leaving at the outpost site.

    I guess I’m just not so much a fan of tying yourself to one location so early on. With the light-outpost approach, you have more flexibility to mix base and sortie expeditions, while with the existing architecture, you can only really do base missions. If your goal is to really explore the moon and really find out about where the valuable resources are, etc. tying yourself to one base so soon seems to be very suboptimal. And such a base-centric architecture does absolutely nothing for reducing most of the key risks of a lunar mission–the descent, ascent, and departure phases of the transportation.

    ~Jon

  33. Exploration Fan says:

    How should we explore the moon? I’m personally very much a fan of a fixed base, right from the beginning. Developing a “home” base to learn how to live on another planet is very important to me. I really believe that people need to start emigrating from our single nest. Early wide spread exploration of the moon can be done very well robotically with the occasional sortie mission. I actually think that a very robust robotic exploration program should proceed human missions to define where the base should be located as well as improving our understanding of how machinery functions on the moon.

  34. Jonathan Goff Jonathan Goff says:

    Exploration Fan,
    I think that a more hybrid approach will likely yield a better result. I agree that for long-duration “figuring out how to live on another planet” stuff, a fixed base makes a lot of sense. However, I think that needs to be balanced with not jumping the gun and tying yourself to one location too soon. I don’t think that a single central base is really going to work very well at least for purposes of exploring and prospecting the moon itself. It’s better than no base at all, but the moon is big (surface area of Africa) and the terrain and environment is rough. Having the ability to have smaller base camps or outposts set up in regions of high interest (say in the Aristarchus region, down near Shackleton crater, up near the north pole, and a few other regions) seems more prudent. And having at least some of the infrastructure in orbit also seems to make a lot of sense both from safetly and logistics standpoints. Moving goods from a given point on the moon to some point hundreds of kilometers away is going to be a major undertaking for the foreseeable future, whereas landing stuff at any given point may not be that big of a deal relatively speaking….

    Anyhow, I think I need to write a few more blog posts, fleshing out this idea a bit further. I don’t think that the lunar depot concept I’m talking about here is mutually exclusive with the concept of building a fixed base first if that turns out to make the most sense. But I do think it gives you more flexibility so you’re not forced into a base-first solution if you’re not sure yet where your base should be.

    ~Jon

  35. Exploration Fan says:

    Jon, I completely agree with you that a depot supports fixed base, sortie or robotic options:) As can a reusable lander as you are describing.

    A discussion on the progression of exploration: robotic, sortie, base, would be interesting.

  36. Will McLean says:

    I think that sorties and light scout outposts are a profoundly inefficient way to explore the moon. Consider your options using the following hypothetical system. You have landers that can deliver either two crew from lunar orbit to the surface and back, or 8 metric tons one way. The system can make up to twelve landings a year and (optimistically, I believe), NASA can afford to make that many missions.

    A light scout outpost masses eight mt and can support two men for 14 days (the early lunar access habitat was estimated at 8.5 plus 1 mt of solar arrays and consumables delivered on an earlier flight. If use of an inflatable structure cut structure mass in half, eight mt would be plausible.

    A four man fixed base as per First Lunar Outpost would be 36 mt not including pressurized rovers, and capable of supporting eight month rotations. A mobile base of the same capacity, as per Lunox of Morphlab, would be 55 mt, and include two general purpose pressurized rovers, 5 mt each, and two power carts, 1.5 mt each. In three months the mobile base can move itself up to 1000 km through a combination of autonomous navigation and teleoperation.

    An 8 mt logistics module can support 16 man/months of operations, i.e two men for eight months or four for four.

    Now suppose that you have decided to devote half of the landings to building up and supporting a four man base, but you also must make a total of 12 manned visits within four years to widely separated locations on the lunar surface. The main base counts as one, so this can be accomplished by setting down 11 light scout outposts and visiting them. This takes 22 landings, leaving you two to land additional logistics and outpost equipment to expand one outpost to an 18 mt two man base and keep the crew there for six months. Total surface man months for this campaign, 17.

    Or you could put down two two-man mobile bases (55 mt) plus two extra pressurized rovers and power carts so each mobile base has redundancy (13 mt) This takes nine landings. You land 11 crews to different locations, moving the bases between visits, and enough logistics to give each a 3 month stay. Total surface stay time, 66 man months, with greatly enhanced mobility at each location.

    Alternatively, you could reflect that the moon isn’t going away any time soon, and that you can afford to be methodical. You set down a four man mobile base and make four four man visits to four locations, staying eight months each time. Total surface stay time, 128 man months.

    The longest possible stays are vastly more cost effective. And if you expect to be preparing for Mars, you should really be working towards two year rotations if possible.

    Note too that if your metric is risk per amount of the moon explored, the long stays get a lot more stay time out of each risky manned rocket voyage.

  37. Martijn Meijering says:

    Over on nasaspaceflight.com, on the DIRECT thread we were talking about L1 gateways and depots a bit. Actually I was advocating them and the DIRECT guys wanted to build a big cryogenic upper stage first.

    Their current plan is for a quick stunt to the moon, Apollo 8 style, once J-120 is operational, around 2013. Assuming they get their way of course. They’d use a Delta IV Heavy Upper Stage as an EDS. It’s a bit like the architecture you discussed a while back, just with a DIVHUS instead of a Centaur, the advantage being that it’s bigger.

    The DIVHUS gets tantalisingly close to placing an Orion into a circular LLO, so close that I wondered if you could close the gap by using direct ascent. Another trick the DIRECT team is studying is putting the Orion into a highly elliptical orbit to same some fuel. Either way it would be very cool.

    I was really excited about this, because it promised good results soon without spending outrageous amounts of money and while using some off the shelf hardware.

    A little calculation showed something I found even more exciting: a J-120 + DIVHUS plus Orion comfortably gets an Orion to L1. It also gets the standalone space station ESA is planning on building to L1. This means that within 4 to 5 years we could have a functioning L1 gateway up there!

    Of course the next thing the DIRECT guys want to do is to build a big upper stage to they could get an Altair up there too. Since, probably like most of you, I’m more interested in economic development of LEO (and then beyond) first, and only then in going to the moon, I wasn’t so enthusiastic about that.

    But the Altair is quite heavy, so a single DIVHUS wasn’t going to be enough. At first this seemed like a serious spanner in the works. But then it hit me: what if you made the Altair use hypergolic fuels, just like the Apollo LEM did? Then you could send cargo, propellant and lander to L1 on separate DIVHUS’s. No need to develop an expensive upper stage! No excuse for postponing depots and tugs!

    Of course a hypergolic lander would be less efficient, but it would allow commercial participation early on. It should be relatively cheap to convert the Orion SM into a tug that can pick up small or large dumb payloads launched on commercial launchers. Building a hypergolic depot should be much easier than a cryo depot. You could get the bugs out of the proximity operations very early on. You could reuse that knowledge (and the commercial launch infrastructure!) once you got cryo depots up and running.

    Then Chuck Longton came up with a brilliant suggestion: with refueling, you can afford to make your lander bigger. And that means you can land your future moon base in bigger chunks without having to build a monster rocket like Ares V. And I understand that even with Ares V there having mass problems. All of this goes away with L1 refueling. And hypergolic fuel transfer on orbit is a proven technology!

    Hypergolics have further advantages as well. Since they are storable, you could use Belbruno trajectories to offset some of the inefficiencies due to lower Isp. Once you develop SEP, either from LEO to L1 or from L1 to LLO, you can reduce inefficiency a bit further. Also, hypergolics are denser than H2 so you save may some tanking mass. They also allow you to build a more squat lander that doesn’t tip over so easily, another problem Constellation is having to deal with.

    So all in all, a lot of advantages. Key point: J-120 + Orion + ESA L1 gateway + hypergolics may be the quickest way to the sort of commercial infrastructure operations we’d all like to see. This stuff could happen really, really soon.

  38. Frigate says:

    @Jonathan Goff.
    Could you please upload article ”Trajectories to/from the Earth-Moon Lagrangian points L1 and L2 for the human exploration of the moon” from ESA Mission Design conference. Unfortunately ESA link is dead.

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