The Slings and Arrows of Outrageous Lunar Transportation Schemes: Part 1–Gear Ratios

[Note: I haven’t quite finished with my Venus ISRU series, but some of the articles I’ve read over the past few days drew me back to this series on propellantless lunar launch approaches that I started writing four years ago but never finished. While Venus ISRU is interesting, I still think it’s pretty likely that the first operational ISRU (ie beyond demo or pilot-plant scale) will be on the Moon.]

One of many important issues that doesn’t get enough airtime when discussing lunar ISRU is how to efficiently get the propellants and other materials off the lunar surface. There seems to be a line of thinking that could be called “all we need is ISRU” that says that lunar ISRU is the most critical technology and everything else is just a distraction.

While it is possible to take propellant produced on the lunar surface up to LLO or to one of the Earth-Moon Lagrange points using similar rockets to what you landed with, and then deliver this to LEO using entirely propulsive tugs with no new technology, this isn’t very efficient. You end up spending a significant fraction of the lunar derived propellant lifting both the delivery propellant and the landing return propellant, as well as the propellant to ship the cis-lunar tanker back to LEO and bring it back for refueling near the Moon.

To give you an idea of how inefficient, I’m attaching a spreadsheet with some back-of-the-envelope level calculations to illustrate this point. In the spreadsheet I model a Lunar Surface to LLO or EML-2 and Back tanker, and then an LLO or EML-2 to LEO and Back tanker. In both cases, I assumed they were about Centaur size (~23tonnes), and used RL-10 based propulsion. For the reusable lunar surface tanker, I gave two propellant mass fractions–90% (aggressive once you factor in landing hardware) and 85% (more conservative). For the cislunar tanker, I assumed a 90% propellant mass fraction, and also analyzed cases where an aerobrake was provided that weighed 5% of the GTOW and 10% of the GTOW.

In the most extreme case of “all you need is ISRU” thinking, where you use entirely existing chemical propulsion systems for getting propellants from the lunar surface to LEO, only 9-11% of the propellant produced on the Moon actually makes it to LEO.  Alternately, this means you have a “gear ratio” (ratio of propellant extracted on the Moon to propellant delivered to LEO) of 9-11. Not only is this very wasteful, but it means that you would need to size your ISRU capacity significantly higher than if you had a more efficient system.

Of the approximately 12km/s of round-trip Delta-V from the lunar surface to LEO and back, there are several options you can use to improve your gearing ratio, each of which attack a different leg of the journey:

  1. Stage and refuel in LLO or EML-1/2 (which was already assumed for this analysis).
  2. Aerocapture/braking to go from your Trans-Earth Injection trajectory into LEO
  3. Propellantless methods for launching from the lunar surface to LLO, EML-1 or 2, or even directly to LEO.
  4. Propellantless methods for landing on the Moon from LLO or EML-1 or 2
  5. Propellantless or high-Isp methods for traveling from LEO to LLO or EML-1 or 2.

This series is focused on options #3 and #4, though #2 is also low-hanging fruit (and provides about a 2-3x gear ratio improvement over the baseline “all we nee is ISRU approach).

Next up: Five Propellantless Lunar Transportation Approaches

This entry was posted in ISRU, Lunar Commerce, Lunar Exploration and Development, Space Settlement, Space Transportation. Bookmark the permalink.

66 Responses to The Slings and Arrows of Outrageous Lunar Transportation Schemes: Part 1–Gear Ratios

  1. Hop David says:

    Should have said 0º/180º *longitude* line. Rats.

  2. George Turner says:

    You could probably come up with a scheme for you polar launch to shoot the payload to the lunar equator, with as little vertical velocity as possible, and then using fixed lasers for mid-course corrections, very accurately touch down on another rail system while retaining the full horizontal velocity. From there you could either slow down to a stop, then make a completely new launch to your intended orbit, or design the receiving rail so that it curves 90 degrees in a path tens of miles long, so you can hit your intended orbit without braking and re-accelerating. It’s double or triple the trouble, but the alternative is to move the ice or water overland by vehicle or pipeline.

  3. Andrew Swallow says:

    @gbaikie The hydrogen peroxide thruster is the NASA Mighty Eagle being developed at the Marshall Space Flight Center in Huntsville, Alabama.

    Morpheus is a rival VTVL system that uses methane/LOX being developed at Johnson Space Center in Texas.

  4. gbaikie says:

    Andrew Swallow says:
    January 2, 2014 at 7:48 pm

    Thanks.
    I couldn’t remember it very well.
    But don’t know if it’s a rival.
    Though I would generally be happier if there was more rivalry. And I think I am more impressed with how Mighty Eagle is doing things vs Morpheus.

    But anyways, so not sure if anything like Mighty Eagle might used on the Moon.
    But perhaps if marry, Mighty Eagle as first stage, and this reclamation tube, it might be useful way to do things on the Moon.
    Though not sure what’s involved with making hydrogen peroxide on the Moon-
    I guess look again at that.

  5. gbaikie says:

    Re: hydrogen peroxide
    It seems quite easy.
    And was reminded of how useful hydrogen peroxide could be, not used as a rocket fuel.
    So it seems if splitting water, getting hydrogen peroxide from the H2 and O2 is apparently simple.
    But what I was also wondering about is does require less energy, as compared to splitting water
    And it seems one make hydrogen peroxide from oxygen and electrolysis , so one use the expected over-abundance oxygen to make hydrogen peroxide, though not sure how electrical power is used.
    Anyways, can be done but still unsure about which path one take for lowest electrical usage. But then again, not sure, getting lower/lowest electrical cost is particularly important. Or making a lot of electrical power and cheaply, should a key focus in regards to the Moon.

  6. Andrew Swallow says:

    It should be possible to use hydrogen peroxide to power heating systems and electrical generators. The nights on the Moon can be very long. It may even be possible to power land vehicles and mining machines.

  7. George Turner says:

    In my thoughts above I’d neglected the drag along the tube walls, which won’t affect the rocket but will effect the tube pretty dramatically unless it’s heavy (perhaps weighted down) and anchored, because the momentum of the supersonic exhaust eventually gets transfered to the tube, and from there to the lunar surface.

  8. George Turner says:

    I’ll toss out another crazy architecture that just occurred to me.

    Supposing you’re wanting to deliver ISRU resources from the moon to low Earth orbit, using some form of propellantless delivery system for the lunar launch and hurling the payload directly on an Earth return trajectory. By the time you get to the Earth the payload is traveling at about 11 km/sec, so you need to bleed off about 3 km/sec to establish a low orbit. You could do that with aerobraking, but then you’ve thrown away your excess energy instead of using it for something else. You could burn propellants in a rocket engine to kill the 3 km/sec, but then you’re wasting mass.

    So instead you launch a pair of vehicles connected by a tether, which is played out to a full length of dozens of miles during the coast to Earth. As the vehicles hit perigee, they take advantage of the Oberth effect and the tether to reel towards each other at high G’s, perhaps using a geared gas turbine such as you’d use to power a turbopump. which is reasonably fuel efficient. The turbopump exhaust is condensed back into water that is retained on board either craft, so no mass is lost.

    So vehicle 1 slows down by 3 km/sec, allowing it to enter Earth orbit, while vehicle 2 is accelerated from 11 km/sec to whatever results from the relative masses of the two craft. If they were equal in mass, the second vehicle accelerates to 14 km/sec and it’s specific kinetic energy would increase from 60.5 MJ/kg to 98 MJ/kg while only expending about 4.5 MJ/kg of energy, due to the Oberth effect. By adjusting the relative masses of the two vehicles, the second can be sent to virtually any destination, and no mass was lost during the entire sequence from lunar launch to LEO capped with an interplanetary trajectory.

  9. Doug Jones says:

    I’m impressed by the work at Tethers Unlimited- see http://www.tethers.com/papers/CislunarFinalReport.pdf for details. A balanced tether system could transport goods between LEO and the lunar surface at near-zero propellant cost, very slick.

  10. Karl Hallowell says:

    For me the fundamental gear ratio for any space activity which starts from Earth, is the ratio of the value of the output given the cost of what was put in from Earth in terms of resources, effort, and so on.

    For example, if I had an inefficient but substantial lunar launch infrastructure that only delivered 9% of its propellant to low Earth orbit, but the infrastructure, aside from an initial investment, only took one Earth-side mouse clicker to run, then that would be a huge marginal return on Earth-side investment. It wouldn’t matter very much (aside from the opportunity cost of not displacing more Earth-side launched propellant) since that propellant has no effective value to us until it gets into LEO and can save us some Earth-launched mass.

    Everything in space is effectively free to use once you first get to the point that you can do so. It just takes some degree of Earth-side resources in order to access, exploit, and transport to a desired location. If those resources cost less than what you gain, then you make a profit of some sort.

    As the Moon becomes more developed, then usual economics kicks in. The lunar resources and other goods and services have some value independent of what Earth-side value they possess.

    So to summarize, economically, you can think of the early cost and value of space resources solely from an Earth-centric viewpoint. That is the “gear ratio” that I think we should be considering at first.

  11. Karl,
    While I agree with you, I think my point somewhat matters even to the gear ratio you’re focusing on. If you take a propellant inefficient route, and it takes 3x as much lunar throughput for a given unit of LEO output, you’re going to have to ship probably 3x as much hardware to the moon to get it going. The question becomes which of the “more efficient” routes actually pay for themselves, and how fast do they pay for themselves.

    ~Jon

  12. Andrew Platzer says:

    What about a literal sling? 2.5 km/s at a 6 km radius generates a bit over 100g. A calculation shows a 2.4 cm diameter kevlar string would support the weight of 1000kg plus the string itself with a 33% margin. The string would weigh in at 3,600 kg.

    Spin it up to 4 RPM using solar power and then release. At first, the payload might need run on a track until the acceleration levels out the payload to almost horizontal.

    For safety, build it inside a crater with a notch cut out in the direction of flight. That way if the string breaks prematurely, the payload will slam into the crater wall.

    Not sure how one would handle the snapback on release and then the deceleration. Perhaps a small rocket at the attachment point to handle the sudden 20% drop in mass. It can control the end as it slows down. There’s a heck of a lot of kinetic energy to dump – 7GJ by my guess if you include a matching a counterweight.

  13. George Turner says:

    At 100 G, I think you could accelerate linearly in half that radius. One thought I had was to just use a cable traveling around two sheaves at the required velocity and have the payload grab the cable via a slip clutch of some sort, kind of like a cable car. But when the cable travels around the sheaves it experiences extremely high centripetal accelerations, which as far as tensile strength calculation can be treated the same way as hoop stress in a pressure vessel. According to my spreadsheet, the resulting tensile stress is independent of sheave diameter but highly dependent on cable velocity. Kevlar could get me to 2,000 m/sec, whereas lower density Spectra was good to 2,700 m/sec.

    In that kind of system you’d probably treat it as a momentum transfer mechanism and store the required kinetic energy in the spinning sheaves, which of course have to keep spinning at high speed to meet the final velocity requirements, but the system doesn’t require ever slowing them down so you might maintain a pretty high launch rate. I’m sure many people have done detailed work on such a system, but I am not one of those people.

    I also wondered if the stress problem could be solved by having the cable lay in a trough built into the sheave so that it’s experiencing compression instead of extra tension, but that just moves the stress problem over the sheave, so I’m not sure if it buys anything.

    It might be that such a system would work quite well at the lower velocity ranges but start having all sorts of problems as it’s pushed to the design limits (and lunar orbital velocity is looking awfully close to such limits). But if you perhaps combine it with something like my above thoughts on an exhaust gas recovery tube or an electromagnetic system (or orbital tether), the cable gets the payload to some fraction of the final velocity and then the second system would accelerate it further. If the cable system is simpler, lighter, and cheaper to install per meter than the second system, then getting as much velocity as feasible from the cable would make a lot of sense.

    You could also pair down the rotating cable in the same way, having its release point fixed mechanically (almost like a pop-up cable cutter set into the floor of the crater) so the payload always enters the second system on a very narrowly controlled trajectory.

  14. Paul D. says:

    I’ve been wondering what can be made with 3D printers on the moon. Don’t many 3D printers require gravity to hold down a powder, so it can be melted? The moon might be the ideal place to use them in the inner solar system.

    It would be really cool to use modern 3D printer technology and go back to old studies and see just how much of a lunar colony could be made in situ (including structural parts for more 3D printers.)

    For the purpose of this thread: I suspect aerobraking heat shields and much of the mass of rocket motors could be made this way.

  15. Paul,
    That’s an interesting point. I was actually discussing that very topic with some of my colleagues on the drive back from SmallSat yesterday. Most of those 3d printing methods still require good, pure feedstocks, which may take some time before they can be supplied by purely ISRU sources, but it may definitely shift what you get from Earth versus local manufacture. In my copious free-time I may have to do a blog post about Peter Kokh’s old “MUScle” concept for ISRU, and where that stands these days with the advent of 3d printing.

    ~Jon

  16. Paul D. says:

    Most of those 3d printing methods still require good, pure feedstocks,

    I wonder if feedstocks, or at least materials that can be turned into feedstocks, could be delivered to the lunar surface by crashing. As long as they don’t become too dispersed/mixed with regolith, material deposited that way could be reprocessed into a powder, probably more easily than could be extracted from the original regolith. And you wouldn’t have to expend any impulse for a soft landing.

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