Repost of an Idea

Some years ago I did a post on using Lunar fuel to raise a sub-orbital vehicle to Earth orbit. One of the comments by sjv linked to a similar concept that had been done several years earlier but much more professionally by a far more qualified person. The recent FH flight and the more Lunar focused interest at this time makes the idea more relevant than 10 years ago when I blogged it or 14 year ago when Dr. Walthelm published his work.

What makes it more relevant is the possibility of orbiting a hundred tons with one reusable F9, three hundred with one reusable FH, and both without expending an upper stage. The Blue Origin offerings won’t be far behind if the capability comes to pass. BFR payloads to the four digits. The expendable industry could be mostly extinct in a decade or so except for niche one offs. If there is any compelling reason to get tens of thousands of tons into Earth orbit, this could create an early profitable Lunar export.

This is the link to Dr. Walthelms concept.

This is my original post.  Comments are better than the post.

If I got the link wrong, this is the post.            Earth launch for heavy vehicles currently involves lifting a lot of propellant to lift a lot less vehicle to lift even less payload. One of the frequent criticisms of suborbital flight is that it only uses a small fraction of the energy required to reach orbit. While that argument has some serious validity issues, it would be nice to be able to pop up a suborbital vehicle and hand off a payload to something else that took it to orbit. Mass ratio required would drop by a factor of 5. While it would be nice, it may be a while before some magic tech makes it possible.

Tethers are the frequent solution suggested for making this happen. Unfortunately current tech seems to be that your suborbital vehicle would need to be traveling at mach 15 or so to match velocities with a rotovator. While this will be a major breakthrough when it takes place, it still requires a serious performance vehicle to make the rendezvous. The suborbital vehicle for that mission will have to be fairly aggressively designed if it is a single stage. The mass ratio is about half that of an orbital vehicle with serious TPS still required. It will have to ride the tether around for a few orbits to get home, or land far enough down range that getting home is another logistics problem. As better tethers become available, these problems will slowly get better until a true beanstalk becomes possible.

I’m not aware of any other feasible technologies for doing the job of an advanced rotovator.

With airless aerobraking propulsion, there is a possible solution.  A chunk of lunar LOX launched into a near earth reentry trajectory will be at over 11 km/sec as it makes a near approach 100 miles up. If one pound of this impacted a suborbital pop up vehicle that had no significant horizontal velocity it would deliver an impulse equivalent to an Isp of 1,100+. If that pound vaporized and rebounded at random from a heat shield, it would deliver equivalent of another Isp of 550. So each pound of lunar volatiles would have an effective ‘Isp’ of 1,650 while the suborbital vehicle is motionless earth relative. As the vehicle gathered momentum, ‘Isp’ would drop as a linear function of less impact velocity of each succeeding LLOXball. By the time the suborbital vehicle was pushed to orbit, ‘Isp’ would be down to about 460 or so. Lunar regolith aerogel was suggested for the airless aerobraking. If feasible, that would solve several problems with the concept.

It works out to about 1.5 times as much LLOX as vehicle to make the push to orbit. A one ton upper stage with heat shield would need about one and one half tons of LLOX impact to push it to orbit. The size vehicle it would take to get that one ton inert upper stage into position is in dispute by the various people that build actual hardware. The old V2 would have used 4 tons of vehicle and 9 tons of propellant. There are at least a half a dozen credible newspace companies that believe they can beat that with a vehicle that flies daily or more. The list of less credible is somewhat more extensive. The list of companies that can place a ton in orbit without help is fairly long, and fairly expensive.

If a firm can just match the old tech and get an upper stage boost from the moon, then a ton to orbit will be considerably cheaper than is currently possible. This is a 14 ton earth GLOW and 1.5 lunar volatiles per ton to LEO. Heavy lift is the field that would make this pay. A modern expendable design for this purpose would have a mass ratio of about 2.5 and a dry mass of less than 10%. A 3,000 ton GLOW (Saturn5 class) would get 900 tons in orbit in one shot with help from 1,350 tons of lunar volatiles in intersect trajectory.

If it becomes desirable to get a lot of large payloads from the earth surface into space, this might be one path for doing it. If a suborbital craft can fly often, then it could launch large payloads once a day as it phased with the moon launched trajectory paths. It would be cheaper to use lunar raw material to facilitate earth launch than to manufacture finished components on the moon for the short term of a few decades. If SPS became economically desirable, this is a technique that could help make it possible to launch millions of tons of earth built products into orbit.

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I do construction for a living and aerospace as an occasional hobby. I am an inventor and a bit of an entrepreneur. I've been self employed since the 1980s and working in concrete since the 1970s. When I grow up, I want to work with rockets and spacecraft. I did a stupid rocket trick a few decades back and decided not to try another hot fire without adult supervision. Haven't located much of that as we are all big kids when working with our passions.

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8 Responses to Repost of an Idea

  1. N/A says:

    So trying to sail your way to orbit via wind drag effectively?

    Kinda reminds me of those orbital debris removal gas puffer concepts.

    The killer issue is maintaining a tight grouping of gas from the moon to LEO. One possible workaround, is solid oxygen slugs during transit that are melted near time of use via laser. No hardware on the fuel, self disposing eventually due to solar heating, though no guidance makes terminal trajectory control approaching your popup spacecraft problematic.

    You can’t expect your spacecraft to hop around trying to chase a wiggling virtual firehose of gases. Maybe spacecraft based lasers to burn off the edges of the slug to provide small puffs of control.

    But from an infrastructure perspective, saving up lunar mass slugs for a rotovator counterweight strikes me as a more appropriate use of mined materials for transportation infrastructure. As the counterweight mass goes up, you can extend the tether farther down, and can pick up larger payloads.

  2. johnhare john hare says:

    I think many concepts need to be reevaluated at a low level from time to time as conditions change.

    Your comment suggests that you didn’t read Dr. Walthelms description or understand the objections to a rotovator with current technology.

  3. peterh says:

    How much of the gas packet would usefully push the payload, vs. fly past?

    A probably half baked alternative, a current loop magnet in a highly eccentric orbit, the payload threading the loop and getting a kick via magnetic induction. Ion engines on the kick loop make up for the drag and correct the orbit.

  4. Paul451 says:

    “or understand the objections to a rotovator with current technology.”

    I know I didn’t. Whose rotovator concept required docking at mach 15?

  5. Paul451 says:


    “A probably half baked alternative, a current loop magnet in a highly eccentric orbit, the payload threading the loop and getting a kick via magnetic induction.”

    If I’m picturing this correctly, it sounds like you’re two thirds of the way to an orbital ring. Might as well finish the job.

    “Ion engines on the kick loop make up for the drag and correct the orbit.”

    Ion engines are a bit rubbish for anything but a circular orbit. (Or a solar orbit.)

  6. Paul451 says:

    Oops. /blockquote.

  7. N/A says:

    I read Dr. Walthems concept description, which specifically requires a swarm of robots to manage fuel delivery after lunar launch. In many ways this is a non-relativistic form of massbeaming pellet propulsion.

    There is a non-consequential cost to having to provide (and recover!) a swarm of microtugs (the robotic spacecraft swarm described is merely a fleet of tugs positioning and deploying fuel cargo). Using unjacketed frozen solid slugs of oxygen provides a largely fire-and-forget approach for the lunar launch of fuel, and you avoid sandblasting your spacecraft with failed swarm robots that could potentially puncture your shield/engine nozzle (though there is still the non-trivial risk of oxygen slugs not laser vaporized striking). There is also the considerable risks of have a close flyby of swarm robots at high relative velocity while you are chasing the fuel stream as it spreads out in its final gaseous form.

    (Ground) laser vaporization of solid oxygen fuel slugs cuts the robot swarm out of the equation, at the cost of reduced control over gas dispersion. Something that increased the gas capture area in the vein of a plasma bubble/magsail/e-sail, perhaps something similar to the MSNW magnetoshell aerocapture concept might help here. In some ways you are effectively opening a parachute and having the wind carry you up, so a bigger parachute/umbrella helps.

    Small scale rotovators do have a tip speed problem that can only be addressed by faster horizontal speed of the popup launcher, but HASTOL derivatives have consistently shown possible vacuum rendezvous horizontal ground speeds that are not hypersonic (and would have similar gating requirements for rendezvous as the swarm robots doing final gas vaporization and avoidance maneuvers).

    Inverse aerobraking (aeroacceleration?) is certainly interesting as an alternative propulsion concept in the stable of methods used to “pull up” payloads from the bottom of earth’s gravity well, but feels like it has similar problems to an unbomber tug. You are expending gaseous propellant resources mined from the moon, which could be used elsewhere for propulsion/life support, compared to sending sintered regolith mass slugs to a rotovator to build up its counterweight over time. Seems inefficient from a systemic transportation infrastructure perspective, and presupposes a mass launcher infrastructure on the moon. As an emergency propulsion method for heatshield equipped spacecraft which lack propellant, it might have merit as it is externally supplied.

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