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: The Beachhead Analogy

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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 the founder and CEO of Altius Space Machines, a space robotics startup that he sold to Voyager Space in 2019. Jonathan is currently the Product Strategy Lead for the space station startup Gravitics. 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.
Jonathan Goff

About Jonathan Goff

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 the founder and CEO of Altius Space Machines, a space robotics startup that he sold to Voyager Space in 2019. Jonathan is currently the Product Strategy Lead for the space station startup Gravitics. 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.
This entry was posted in ISRU, Lunar Commerce, Lunar Exploration and Development, Slings and Arrows of Outrageous Lunar Transportation Schemes, Space Settlement, Space Transportation. Bookmark the permalink.

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

  1. George Turner says:

    The delta V to launch from the moon to LLO or EML2 is just within the range of muzzle velocities of light gas guns, such as NASA uses to simulate high-velocity projectile impacts. But I’ve never run any numbers on how efficient such guns are with their hydrogen relative to the projectile mass. The method would require a very, very large diameter gun or deliver fuel in very small cans, but the equipment is fairly simple.

  2. Jonathan Goff Jonathan Goff says:

    Admittedly while Light Gas Guns are a potential option, I don’t know enough about them at this point to include them in the five others–plus they aren’t propellantless even if they might be lower propelleant than the others. The five I plan on covering are: mass drivers, lunar tether slings, lunar launch loops, lunar rotovators, and lunar elevators/beanstalks.


  3. Robert Clark says:

    I’m looking forward to your discussion. We could have practically “free” propellant at LEO if we could get these propellantless propulsion methods in operation on the Moon. This of course would have a profound effect on a possible manned Mars mission, or to any destination in the inner solar system.
    About your 9-11% estimate though I think that is if you use propulsion to enter Earth orbit. However, it should be possible to use aerobraking for the purpose. It may take multiple passes as we do for circularizing the orbits of satellites at Mars, but it should be doable.

    Bob Clark

  4. Robert,
    I mention towards the end of this post that Option #2 (aerobraking/capture) does improve your gear ratio to 3-4 from 9-11. I showed the math I used in the spreadsheet. I’ve actually got a promising aerobraking/capture related technology I need to write-up, but wanted to go on to the propellantless lunar launch options first.


  5. George Turner says:

    The five I plan on covering are:

    Hey! Spoiler alert! ^_^

    Light gas guns are essentially just glorified pellet guns, where you slam a large diameter piston into some lightweight gas, which then gets funneled into a much smaller diameter barrel to propel the projectile. The advantage over a mass driver is that they can run at very high pressures (limited by barrel materials, like a regular cannon), and so generate extremely high accelerations in a short distance. One of NASA’s light gas guns hits 6 km/sec with a 1 meter barrel length. However, for a given pressure and projectile area density, you have less acceleration, so barrel length increases linearly with projectile length (for a given projectile density). Also, the hotter you run the gas (via compression heating and possible with an electric arc), the lower the ratio of gas mass to projectile mass.

    Light gas gun wiki

    If you put something like a very, very large silencer over the end of the muzzle, and had a plug that stops the very end quickly after firing, you could probably lose very little gas at all. If you accelerated the large piston electromagnetically, instead of with a chemical propellant, you wouldn’t even need to recover and re-use a chemical propellant, just the hydrogen gas used as the velocity multiplier.

    Many years ago Lawrence Livermore had a project to use a light gas gun to launch payloads into Earth orbit (New York Times story), and launching from the moon would be far, far easier than doing that. It’s what Jules Verne would do!

    I could see some serious development and implementation headaches, but also the possibility that the initial working system could be delivered in a fairly small lander, allowing you to bootstrap much earlier than a mass driver project if the system actually works.

  6. born01930 says:

    Although it is not one of your items for discussion, I would think a rail gun would be suitable. Solar powered, projectile shape/size doesn’t matter, make the rails long to allow for ramped up acceleration.

  7. Hop David says:

    The most troublesome leg is LEO to EML1/2. This uphill leg can’t be helped with aerobraking.

    I like to imagine LEO tethers flinging tankers back to EML2.

    We usually think of tethers catching stuff from lower orbits and flinging to higher orbits. These actions subtract orbital momentum. Doing this repeatedly would de-orbit the tether.

    Catching stuff from higher orbits would restore lost momentum. Thus the incoming EML2 tanker’s momentum might become a valuable commodity.

  8. Andrew Swallow says:

    Do not forget mixed systems. Getting the item to say half orbital speed will save a lot of propellant. The sling or rail gun would be considerable lighter, cheaper and quicker to build. The ‘first stage’ can be enhanced later.

  9. Heinrich Monroe says:

    Not to be picky, but it’s Lagrange (as in Joseph-Louis Lagrange), and not LaGrange (as in La Grange, Texas, the former home of the Chicken Ranch brothel). Miss Jessie would be offended, as probably would Joseph-Louis.

    But very interesting point about the inefficiency of lunar ISRU propellants at LEO.

  10. George Turner says:

    At the other end of the technological spectrum is using a simple rocket that burns oxygen and very hot, liquid aluminum, which is heated way above its boiling point prior to launch, along with all the required plumbing. Since the moon offers unlimited quantities of aluminum, oxygen, iron and titanium (which would form the tanks for the molten aluminum), you wouldn’t be using up any rarer carbon and hydrogen just to transfer mass off the lunar surface. However, I don’t know what the ISP of LOX/molten aluminum is.

  11. john hare` says:

    A regular propellant gun is quite enough to get a vehicle from the surface to lunar orbit. See tank gun velocities with sabot. It is also possible to recover a percentage of the gas with check valved storage tanks connected to a barrel extension. Experience and innovation over time could possibly rever >90% of the propellant gas, which does not necessarily involve hydrogen.

  12. Andrew Swallow says:

    This video shows a rocket burning frozen aluminum and water, Isp 210s.


  13. Andrew W says:

    It’s tethers all the way for me. Hypersonic velocity to LEO, LEO to LTO, LTO to lunar surface, just handing off from one to the next.

  14. Robert Clark says:

    Another possibility is to use ion propulsion, i.e., solar electric propulsion(SEP), for TEI or TLI, with the chemical propulsion for the landing or lift off from the Moon.
    Assuming we will be launching the propellant or materials continuosly we wouldn’t care that the SEP leg might take months. I estimate if you only use chemical propulsion to lift off from the Moon with SEP to transport back to LEO, you might be able to double your payload over the fully chemical propulsion approach.

    Bob Clark

  15. Jonathan Goff Jonathan Goff says:

    Andrew W,

    I’ll get into my thoughts on momentum exchange tethers during the next post in the series (they’re one of the five options).


  16. Jonathan Goff Jonathan Goff says:

    Andrew S,

    Regarding your video of LOX/water rockets: Yeah, but the ISRU hardware needed to extract pure enough aluminum and process it into a form that can be used as rocket propellant is non-trivial, and at 210s Isp, you’re going to need a ton of it. I don’t think the “low-tech rockets” approach really solves the gear-ratio issue I described above. If anything, I think it makes it harder.


  17. Jonathan Goff Jonathan Goff says:

    Sorry for the typo! I keep forgetting which is the “right” way to spell that name.

    As for the inefficiency of lunar ISRU propellants at LEO…I was hoping the takeaway was more going to be “the inefficiency of lunar ISRU propellant delivery to LEO without any new technology development.”


  18. Jonathan Goff Jonathan Goff says:

    Agreed it’s an option (and one of the five I intend to write about).


  19. George Turner says:

    I hope you also briefly touch on this story, in which Greek researchers determined that french fries would be soggy if fried in low gravity due to the weaker convective heat transfer. Future lunar settlers will probably have to use a centrifuge for fried foods, which would be a tad inconvenient.

    I’m not sure what other ISRU processes would be impacted by the same problem.

  20. Andrew Swallow says:

    Aluminum is a building material that can be used for walls, tables and electrical wires. Setting up an ISRU just to use aluminum as a fuel would be very expensive. Where as using say 10% of your output as fuel may be viable.

  21. Jonathan Goff Jonathan Goff says:

    Andrew S,
    I’m more worried about the total amount of Aluminum you’d need to extract, purify, and prepare for propellant usage per pound of useful propellant delivered to the earth.

    Running the numbers using that spreadsheet I provided, that propellant combination doesn’t close at all (even when I boosted the achievable pmf by 5% in each case) except for the option that used aerocapture on the way back to earth and use EML2 staging. And at that point the gear ratio is 30:1, ie it takes ~96.7kg of lunar water/aluminum propellant to deliver 3kg of propellant to LEO using your approach.


  22. Andrew Swallow says:

    I personally do not expect lunar propellant to be used in LEO, working on Earth is so much easier.

  23. gbaikie says:

    “I personally do not expect lunar propellant to be used in LEO, working on Earth is so much easier.”
    Plus why not use mass driver from Earth to deliver rocket fuel to LEO?

    Now if there was existing market of rocket fuel in LEO, one then might
    consider getting lunar rocket fuel to LEO. If there was a fairly large market
    in LEO for rocket fuel and the price was high.
    But it seems even without Earth mass drivers, if lunar rocket were to create
    a large market for rocket fuel in LEO, Earth launched rockets might deliver the rocket
    fuel at lower costs.
    Whereas in high earth orbit, any earth launched rocket fuel will cost twice the cost to deliver as compared to LEO.

    So it seems best market location for lunar rocket fuel is lunar surface.
    And to increase the lunar surface market, one delivers to Lunar orbit.
    And if rocket fuel needed to beyond lunar orbit, it seems one will limited
    to high earth orbits and orbits on other planets.

    So lunar water miner should be advocate humans going to Mars- sell them
    rocket fuel and water.
    One should more interested in selling lunar water at high earth orbit to those going to Mars than rocket fuel.

    To sell rocket fuel to LEO, one would need lunar rocket fuel to be around $1000 per lb at lunar orbit and less than $500 per lb on lunar surface. And have lunar water around $100 per lb.

    Suppose instead lunar water was price at 500 per lb and rocket fuel at 1000 per lb of LOX and over 4000 per lb for Liquid Hydrogen.
    So this allows a price of lunar water at low lunar orbit to be about $2000 per lb, LOX
    to be 3000 or less, and LH to be $5000.

    If you consider you constrained by electrical energy available [and you will be], ship lunar water beyond Moon, is a lunar rocket fuel market at lunar surface.
    Or on the moon, comparatively, one have lots of water, fair amount LOX, and at least amount of hydrogen.

    And for L-1 or anywhere in high earth tack on another $1000 per lb for water, LOX, or LH. So water in high Earth for $3000 per lb. And tack on another $1000 for LEO.
    So if LEO one could get water for $4000 or LOX for $5000 and Hydrogen for $9000.

    Would anyone need water at $4000 per lb?
    Or could buy lunar water and split it and sell it for less than LOX for $5000, Hydrogen for $9000?
    Or if you want to ship to LEO, maybe you focus shipping water. Say a discount at $2000 to $3000 per lb. Any buying 1000 tons of gets at $2000 per lb.
    And essential that means you selling 1000 tons of lunar rocket fuel at lunar surface at
    discount of maybe $800 LOX and $3000- Or translates to 1000 tons of LH&LOX lunar rocket fuel at $1114 per lb.

  24. gbaikie says:

    So that above, leads me to conclusion that good way to subsidize or help commercial lunar water mining is to buy 500 tons of water at L-1 for $2000 per lb, 4 million per ton.

    Now how many tons of water does NASA need for a Mars program?

    Maybe instead, instead say, 300 tons of water at $2000 per lb and 200 tons of LOX at $3000.
    So total cost of 600 million of water and 600 million of LOX.
    And water tanks could be arranged to block sunlight [at distance] to make passively cooling LOX easier.
    And obviously it’s cheaper for NASA then shipping these from Earth- so lowers Mars
    costs. OR in terms of SLS- 130 ton, it about 8-10 of these launches.

  25. Andrew Swallow says:

    To a first approximation, to be financial viable an ISRU process has to make at least twice as much water, by mass, as the equipment weighs. The process includes the mining equipment as well as the refining equipment and replacement parts.

  26. Peter says:

    @George Turner

    Regarding aluminum propellant I recall reading a demonstration study Purdue University did with aluminum/ice solid rocket. They achieved a peak ISP of 210s. I don’t know how well that would compare to the LOX/molten aluminum rocket you have described.

  27. Andrew W says:

    A particularly appealing propellant combination is lunar oxygen plus lunar metals, especially lunar oxygen and lunar aluminum. Aluminum and oxygen alone will provide a specific impulse somewhat lower than most hydrocarbons. Brower et al. expect a value of 285 seconds

  28. George Turner says:

    Thanks. I also found a 1992 NASA paper on using liquid aluminum/LOX from the moon, along with aluminum/LOX hybrids, aluminum slurry, and an aluminum LOX monopropellant.


    It gives an ISP of up to 290, but 285 is more likely because some of the aluminum condenses into particles in the exhaust. So it would perform almost as well as RP-1 or hypergolics without using any carbon, nitrogen, or hydrogen.

  29. DougSpace says:

    Discussions about ballistic launches from the Moon with dumb mass puzzle me. How do you trap the mass into a new stable orbit without it either intersecting the Moon again or intersecting the atmosphere again in the case of aerobraking. If using a “catcher” what keeps that catcher from being moved out of place and out of use?

  30. Warren Platts says:

    There’s a long thread at NSF somewhere on “ALLOX” rockets. The Isp is low, but because of the high density of Al, they produce a lot of thrust. It’s good for lifting heavy payloads to LLO, but not much else. Maybe a 2-stage system, with an ALLOX 1st stage and a cryogenic 2nd-stage would work. Might be hard to engineer since the exhaust is basically garnet–the same stuff they make sandpaper out of.

    An LH2/LO2/Al tripropellant might be interesting. Same Isp as LH2/LO2 (~450 s), but with a lot higher density.

    @ Jon: a while back you mentioned somewhere, IRRC, the possibility of using some sort of tether based, electromagnetic braking system, that wouldn’t require atmospheric entry and would mass a lot less than traditional aerobraking systems?

  31. Warren Platts says:

    In defense of the “All you need is ISRU” strategy, efficiency must also take into account dollar costs. I mean, if the propellant is cheap enough, then who cares if the “gear ratio” is horrible? E.g., if gasoline is 20 cents/gal, who cares if you got a gas guzzler that gets 5 mpg–it’s still cheaper to drive than a car that gets 50 mpg when gas is $4/gal. There’s little point to developing fuel efficient cars when gas is dirt cheap.

    It all depends on the scale of production of the lunar mine and its runout costs. If the Moon base has an annual overhead of $1B/year and can produce 10,000 t of LH2/LO2 at lunar surface per year, then the cost is $100/kg at lunar surface: that would translate to a cost of $1000/kg at LEO, which is competitive.

  32. Jonathan Goff Jonathan Goff says:

    The problem is that your fixed costs are likely going to scale with throughput. If your gear ratio is crappy you’ll need a lot more hardware on the moon to get a given pound of propellant back to LEO. And since the hardware has to be delivered from earth, it’s going to be really expensive. To produce 10,000 tonnes per year of prop on Luna you’re likely going to need 100s or 1000s of tonnes of hardware on the lunar surface. You can boostrap eventually (using say an early LOX/LH2 pilot plant to lower the cost of getting your full-scale system down), but doing things the “easy way” is often a way to force costs into other parts of your system.


  33. Jonathan Goff Jonathan Goff says:

    There are some reasonably feasible “catcher” architectures where a truly dumb mass launch system can work, but my intuition tells me the simplest system will more likely need at least some sort of stage for orbit circularization. And you’re probably going to want to do things so you can reuse that circularization bit multiple times. The key is keeping that small compared to your delivered cargo so the cost of freighting it back to the lunar surface isn’t ridiculous.


  34. Andrew W says:

    There’s a long thread at NSF somewhere on “ALLOX” rockets.

  35. George Turner says:

    Regarding the circularization of the payload’s orbit after hurling it off the moon, perhaps that’s the ideal application for a very simple hybrid AL/LOX pressure fed engine, where the payload is spun up prior to launch in the correct orientation for the eventual circularization burn, which is initiated by a simple timer.

    With some cleverness about the orbital placement and the mascons, you could probably set it up so that payloads will be in a small range of paths for a finite time (perhaps a week or two), and then their orbits would decay in a predictable pattern that would have them impact somewhere far away from any lunar base, while any payloads that whose circularization burn fails to ignite will likewise fall back to a known location away from any base.

  36. Warren Platts says:

    The problem is that your fixed costs are likely going to scale with throughput.

    Hi Jon,

    I see what you’re sayin’, but mining throughput scales faster than fixed costs, at least in theory. Crudely, your throughput is proportional to the volume of your digger buckets and dump truck beds, but the mass (and hence cost) of these items is proportional to the area of the buckets, so at the least throughput = ~cost^(3/2)

    But hey, mass drivers and space elevators are fun–looking forward to the future articles!

  37. Paul451 says:

    Speaking of Verne guns, there’s also the nuclear Verne steam cannon. Drill a tunnel at a good launch angle, widen the base into a cavern. Fill the cavern with water. Suspend a low kiloton nuclear warhead at the centre. At the bottom of the main tunnel, put your payload. Set off the nuke, insta-vapourisation of water, the pressure launches the payload at horrific g-loads. (Recovery system at the top of the tunnel to freeze out the steam for re-use.) Estimates I’ve seen are usually 1:1 payload:yield or better. (Ie, thousands of tons of payload to escape velocity from a single kiloton yield nuke.) Very little radiation is released, so the area around the site will be fairly clean. Meaning that future tunnels can be drilled fairly close to previous ones. (And no biosphere or hydrosphere for the radiation to leach through.)

    Works on Earth too. But you’ve got some added complexity due to the atmosphere. (Oh and the “detonating nukes on Earth” thing.)

  38. Paul451 says:

    “(Ie, thousands of tons of payload to escape velocity from a single kiloton yield nuke.)”

    Oh, that’s on Earth. I’m not sure what the multiplier would be on the moon.

  39. Peter says:

    “Maybe a 2-stage system, with an ALLOX 1st stage and a cryogenic 2nd-stage would work. Might be hard to engineer since the exhaust is basically garnet–the same stuff they make sandpaper out of.”

    Does this mean the rocket would effectively be a sandblaster?

    I’m eager to read the next installment in this series. I think a cost analysis of lunar launch platforms is important. I consider it analogous to the activation energy in a chemical reaction: a cost-effective launch mechanism might bootstrap a space economy if done intelligently.


  40. Andrew W says:

    Is Kirk still involved with Selenian Boondocks? I get the impression he’s expert with the orbital dynamics of using tethers between the Earth and Moon, and I understand that Apsidal Regression, is a problem in using such a system frequently due to the Moons inclination (?)

  41. Jonathan Goff Jonathan Goff says:

    Andrew W,
    Kirk’s not super involved anymore (Nor is Ken Murphy–John’s really the only coblogger who’s still very active). But you’re right, he’s done a lot more work on tether-approaches than I have. Unfortunately, he’s pretty tied-up these days with his Liquid Fluoride Thorium Reactor work at Flibe, so I’d be surprised if he had time to write anything detailed on tethers these days. But I’ll ask–you never know..


  42. Andrew W says:

    Thanks, one possible solution might be to have the LEO tether rotating in an Earth equatorial orbit but have its axis of spin offset by more than 10 degrees to that orbit, so that the tether launches payloads in the same plain as the Moons orbit.

    Maybe that makes sense, I wouldn’t know, I’m out of my depth 🙂

  43. George Turner says:

    Well, I’ll suggest a cheesy way to make a conventional launch system that recycles the propellant, though it’s not cheap because it uses an RL-10 or an RS-25 hurtling down a tube that’s probably made of Bigelow fabric held into a cylinder by carbon fiber or locally-made steel or aluminum stiffening rods.

    The RL-10 didn’t give me much payload capability in a reasonable tube length, so I’ll give some numbers for the RS-25 (SSME).

    To hit a low lunar orbit of 2,312 m/sec with an ISP of 452.3, you need a mass ratio of 1.684. I’m using the SSME as a starting point and picking M0 as 77,809 lbs and M1 of 46,200 lbs, with an engine+tank dry weight of 9.880 lbs, a payload mass of 36,319 lbs, and a fuel mass of 31,609 lbs. The initial acceleration is 6.58G’s and the final acceleration is 11.09 G’s, and the horizontal run length is 20.04 miles. You can greatly shorten the run length by reducing the payload so the accelerations are higher, of course.

    That’s just a conventional rocket launch along a horizontal plane. So, the key step is to recycle and re-use the precious rocket fuel, and we do that by having the rocket traveling down a guide rail that’s surrounded by something like a Bigelow fabric. The rocket exhaust is always traveling backwards down the tube because Ve > Vc, so there would be plenty of time to seal the end of the tube before any of the exhaust even escaped due to the internal pressure. So then you have a 20 mile tube filled with
    31,609 lbs of steam, which you then pump out and convert back to hydrogen and oxygen via electrolysis.

    You could also have the payload detach from the engine and tank, and have the rocket slow down via friction so you can keep using it over and over without wasting fuel flying it back down from orbit. Since it won’t be lit during the slow-down phase, that doesn’t need to occur inside the fabric tube.

    It would be like a mass driver for people who don’t like electromagnets very much, and can’t bring themselves to abandon legacy Space Shuttle components.

  44. gbaikie says:

    -DougSpace says:
    January 1, 2014 at 10:04 am

    Discussions about ballistic launches from the Moon with dumb mass puzzle me. How do you trap the mass into a new stable orbit without it either intersecting the Moon again or intersecting the atmosphere again in the case of aerobraking. If using a “catcher” what keeps that catcher from being moved out of place and out of use?-

    With Lunar water and/or lunar dirt, you put it in highly elliptical LEO orbit fairly easily. Say 300 km by 50,000 km.
    So 100 km to GEO is GTO, so from a 100 by 100 Km it’s about 1.5 Km/sec, and 100 km to lunar distance is about 2.5 Km and to get to that from Lunar orbit is about 1 km/sec and you can pick the inclination of Earth orbit.

    So if not concern about going to thru Van Allen belt- any electronics could made to handle it, and doesn’t affect payload. Then from lunar orbit you hit Earth orbit at 50 to 100 km orbital height, and at 7.8 + 2.5 km, one would get significant drag. And so few passes to lose .5 Km/sec, thereby get about 1/2 halfway between Earth and Moon in terms of Apogee. One could continue this and get to say 500 km apogee and with less than .5 Km delta-v, raise perigee. Or less delta-v than Shuttle de-orbit burn [your apogee is higher and your perigee is higher]. Of course this would take a long time- and time is money, etc. And in terms of time is money what gets you out of lunar orbit, if ion tug, you don’t want a ion tug brought down gravity well to LEO [not normally]. One could use ion to get it to the LEO to lunar distance orbit, and then have ion go back to Lunar orbit, or L-1 or somewhere. And carrying lunar water, you use it as very poor propellent for the stationkeeping type delta-v needed for where ever orbit you want. It seem main issue is whatever container mass one is using- or where do get this container which starts this journey from Lunar orbit.
    Or instead going all way down gravity well from the 50 to 100 perigee to half way to Moon apogee, one raise perigee using little delta-v [water as propellent] so it’s out of the way, so say 600 km perigee and halfway to Moon- though you could be crossing GEO traffic. Or pick 300 km and cross more LEO traffic.
    If you bringing lunar dirt, you might want to pick to Earth surface and sell it for about price of gold per gram. Therefore one bring a container/re-entry vehicle from Earth to go to this orbit, dock with it, and take some of lunar dirt payload and return to earth
    with it. That vehicle could also lower it’s apogee, or just do direct re-entry to Earth.
    Or one could sell lunar dirt at highly elliptical orbit to anyone wanting can come and get it, and be able to return anywhere on Earth surface. So if they getting $50 per gram [$50,000 kg, $50 million per ton] how much do you sell for at the highly elliptical orbit? One could get an empty dragon to that orbit for about 100 million
    and it return 3 tons from LEO, so should able to sell dirt for few thousand per kg at this orbital location. But what you actually selling is lunar rocket fuel at lunar surface and the ion engine use to get it out of lunar orbit, and ion engine returns and gets the empty contains which at the highly elliptical orbit, so it can fill with it more dirt from the Moon.
    Or one could sell lunar water at the highly elliptical orbit. From the highly elliptical you get anywhere in LEO- so you get to ISS. So if water was needed at ISS. Or one make rocket fuel from this orbit, so if ISS want water, LOX, or LH, or lunar dirt for few thousand dollars a kg.
    The highly elliptical orbit is gets almost twice solar energy as LEO, so if you split water in orbit, it’s about best place to split water. And if going to Moon, it’s a good place to refuel. Or if want things in GEO [dead satellites] it’s also good palace to refuel,
    though one also bring the rocket fuel down to LEO- but the rocket would cheaper at the highly ellipical orbit, so only reason to bring rocket to fuel to LEO, if destination is beyond LEO, is if want to lift the largest payload one can from Earth with the rocket available.
    So at moment Heavy Falcon, doesn’t appear to lift mass well above LEO, so one lift as massive as 53 ton payload to LEO, refuel, get to elliptical orbit, refuel, then get 53 ton payload to Mars or somewhere.

  45. George Turner says:

    Let me revise my above proposal somewhat, and suggest stiffening the launch tube by inflated fabric rings and longerons, so you have a zero to some partial atmosphere launch tube held open, straight, and stiff by attached inflated structures. The inside of the launch tube is protected from heat by metal foil, and the start section could be solid aluminum or steel, with perhaps a steam catapult to get the velocity up to a hundred or two hundred knots prior to main engine ignition.

    The bed for the launch tube is smoothed and graded, the tube is laid down and inflated in sections, and sewn into the tube are conductive wire loops for a passive maglev system to support the launch vehicle. Where the tube has to cross significant craters, it’s supported on large airbags.

    An alternative to that is to build a physical steel rail out of lunar iron, and anchor it into the soil. I think just blowing up an inflatable structure would be simpler, and a laser profiler sweeping the intended launch path would provide all the data needed to sew the appropriate fabric structures to mate with the surface profile.

    The length of the launch run is inversely proportional to the thrust to weight ratio, and if you let it go to about 44 G’s at the end a 5 mile tube would start operations. Then you keep adding length until your G’s loads are small enough that crews can use it, and then you can launch into lunar orbit (or to Earth orbit, once the tube is lengthened sufficiently) without having to mine any extra materials because you keep reusing the same hydrogen and oxygen for your launch system.

    Another advantage is that if you have an engine failure. you just coast to a stop on the deceleration section that was going to slow down the engine anyway. Until you hit orbital velocity, separate, and fire a vertical thruster, you’re just on a crazy train ride.

    I’m starting to like the idea because it’s low-tech (as these things go), repairable by driving a little cart down the tube, and doesn’t require a whole lot of new innovations, orbital genius, or extremely precise maneuvers.

    The unknowns to me are will the steam exhaust freeze against the walls of the launch tube, how easily can it be pumped out as a gas or liquid, etc.

  46. Hop David says:

    “It would be like a mass driver for people who don’t like electromagnets very much, and can’t bring themselves to abandon legacy Space Shuttle components.”

    Just so. Until I reached that final paragraph I was thinking “George is describing a mass driver.”

    There are rail advantages even without H2O recapture or electromagnets. If you can achieve orbital velocity along a horizontal track, you have virtually no gravity loss. Sans gravity loss, you don’t need to worry about T/W ratio and the rocket engines can be a smaller part of the ship’s dry mass.

    Even with electromagnets, the rail would need a honking big power power source to achieve 1.6 km/s much less 2.3 km/s. An adequate power source would take some time to build. Ships using early versions of the rail might use a combination of electromagnetic energy and old fashioned chemical energy.

    I wonder what the radius of a H2O reclamation tube would be? If it’s too small, the hot rocket exhaust would melt it.

  47. George Turner says:

    Expanding into a 20 foot diameter should get SSME exhaust down to about 600F, I think. Since the walls of the reclamation tube (nice term!) will radiate heat really well, it could probably be a lot smaller than that, depending on the type and thickness of the inner liner.

  48. gbaikie says:

    “Expanding into a 20 foot diameter should get SSME exhaust down to about 600F, I think. Since the walls of the reclamation tube (nice term!) will radiate heat really well, it could probably be a lot smaller than that, depending on the type and thickness of the inner liner.”

    Hmm, it seems reclamation tube is weird idea. So 20 feet in diameter and very thin walls- so 1/16th of inch or less. I would put ice on floor.
    And say 100 feet long. Or that’s too short.
    One could have steam rocket type stage. Not sure what it would look like, but assume high thrust. And if directed downward exhaust would tend to go downward, so this reclamation tube could get the steam some distant above the tube.
    So this is in dark craters, and one has everything pretty cold at start with a coating of cold ice better than cold metal.
    So rather imagine some steam rocket one look at hydrogen peroxide engine which Morpheus is testing with. And this look like one could get high thrust fairly easily.
    And I doubt the steam would be as billowing in a vacuum as indicated in their recent tests.
    So assume 30 m/s/s. And 5 seconds that is 375 meters and mere 150 m/s.
    Or 3 seconds is 135 meters and 90 m/s.
    So go for 100 meter tall and 40 feet diameter. For pipelauncher, I noted:
    “cubic feet of steel 40 dia and 1000 feet long which is 1/4 thick walls:
    40 foot diameter is 125.6 feet circumference.
    100 foot length is 12560 square feet
    1000 foot is 125600 square feet.
    125600 divided by 48 is 2616.6 cubic feet.”
    So want say 7oo feet and 1/16th inch walls. 2616.6 times .7 and and 1/4 of 1/4″
    walls: 457.9 cubic feet of iron/steel/aluminum. On earth cubic foot steel weighs:
    489 lbs. Times 457.9 is 223,913 lbs. On Moon: 37,318.8 lbs.
    And it could be 1/32th probably rather than 1/16th.
    Though fabrication would be difficult. One could probably make iron rods on fairly easily on Moon, so make framework and use a liner- say same stuff used for balloons.
    And rather than try to try stuff off walls, blow vapor into container.

  49. George Turner says:

    Well, I was thinking of laying the tube horizontally. I just ran some rough numbers for expanding SSME exhaust to about 20 feet, and the pressure should be roughly 0.145 psi (well, actually I’m using some RP-1 numbers that shouldn’t be too far off). A 20 foot diameter Kevlar tube 20 miles long, able to hold that amount of pressure, would weigh about 2,100 kg, and it would be ridiculously thin. So most of the mass would be in the inflated structures that hold the tube round, straight, and level, along with supporting the vehicle during the launch run, plus some amount of micrometeroid, UV, and thermal protection. But even if you up the weight 10 fold or 100 fold, it seems like an easily deliverable amount of mass that would pay for itself in just a few launches.

  50. Hop David says:

    I daydream of mass drivers from Shackleton or Whipple Crater. One of the things that vexes me though is a route to EML1 or EML2. To get to these places you’d want the line of apsides of the lunar orbit to be collinear with the line passing through the earth and moon centers. Also you’d like the transfer orbit to be coplanar with the moon’s orbit about the earth.

    But when perilune is on the north or south pole, line of apsides is at right angles to the moon’s orbital plane. Apolune will be either above or below the moon’s orbital plane.

    The best compromise I can think if is to launch to a 1.6 km/s LLO when the rail coincides with the 0º/180º latitude line. Then when the ship on LLO reaches the equator, do a .8 km/s burn for injection to a trans EML1 or 2 orbit.

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