Early Testing and Demonstrations on the Depots and Rotovators

Doug Plata has done some detailed comments on the suggestions, ideas, and differences from his ideas that my posts laid out in minimal form. His comments deserve better than I can do in a little comment window. Probably better than I can do in a post, but I won’t admit that part in public. It is my opinion that these technologies would speed up Lunar development, expand the possible scope of operations, and cut the cost in the process. They do carry risk though, which is Dougs’ objection, along with getting permission to use them in the first place.

I did a post on my views on the Lunar hoverslam landings. This one is how I see the depot and rotovator getting underway. I think initial proof of concepts and bringing the TRL up to snuff should be done on company internal resources without fighting the Federal funding battles.

The only real reason depots are not in use now is that there is no real demand for them. A comment on one post noted that propellant in orbit was as valuable as dirt. A bitingly true statement as long as there are limited missions beyond LEO, and few satellites share orbits that would make depots useful. A constant flow of material to and from Lunar orbit changes the situation with many vehicles taking the same path.

I see the initial depot flights as secondary payload technology demonstrators. Often flights to LEO are at less than payload capacity of the launcher. Either the upper stage carries a small second spacecraft, or it makes a rendezvous with another vehicle. The upper stage docks(berths?) with another vehicle and transfers propellant to it. They separate for a while and then hook up and transfer propellant back. Operating as a secondary payload on a stage that is expendable anyway should have the possibility of being a fairly economical mission. This would give a chance to solve propellant settling and transfer in (off?) the real world. Several missions could be flown for relatively low operating costs until the company is comfortable with the transfer techniques and has the boil off data for a few configurations. Then start flying more ambitious missions that do need some help until it is an accepted practice. There is too much information out there on depots to justify me going long on the subject.

Rotovators are far more risky. The payoff is also very high. The Lunar rotovator alone would offer major savings to a serious development operation. The ability to return material from the Lunar surface to an Earth bound trajectory without propellant, engines, or tanks would make it attractive even without the ability to intercept cargos from Earth for   a soft landing without fuel. The TRL is very low for tethers of any kind in space with rotovators having no test data at all.

I suggest the rotovator  demonstration unit be a secondary payload with the minimum mass that can demonstrate the principles.  This mission would be the rotovator itself, whatever auxiliary equipment is needed to make it work, and a bunch of expendable small spacecraft with the only function being thrown and caught.

The rotovator is lightly spun up when orbit is reached testing deployment and system dynamics. The initial target velocity is that which brings the tip thrown  vehicles to a 15 orbit per day instead of 16 of the base vehicle. This brings the small vehicle back to rendezvous in one day if all the calculations and results work out. It is to be expected that most of the little test spacecraft will be missed and lost early on. Perigee would be kept low enough that missed ships would reenter in a matter of days to avoid creating more orbital debris. It would be a risk that there would not be enough of the little ships to establish success and possibly no captures at all on the first rotovator mission. Further rotovators would be sent out as secondaries  until accurate slinging and reliable captures were expected instead of experimental.

After initial proficiency is reached at the 15/16 orbits, velocities are increased to 14/16 and 13/16 until the 1,600 m/s target is reached that would validate a Lunar rotovator. Then one is sent to Lunar orbit as a working system. If the 1,600 /s units were successful enough in Earth orbit some would remain to pick up suborbital ships to sling them most of the way to GTO or TLI with the rockets relighting after the rotovator boost.

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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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About johnhare

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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13 Responses to Early Testing and Demonstrations on the Depots and Rotovators

  1. Andrew Worth says:

    The surface based lunar sling would work better than an orbital system that would require on-board propulsion or the continual balancing of up and down payloads.
    Such a system would be located at one of the Lunar poles firing packages into LLO or beyond.

  2. Paul451 says:


    (Some re: this post, some relating to previous posts or comments.)

    Re: Depots.

    The amount of excess fuel in routine LEO satellite launches that happen to be in the same orbital plane, altitude, and window as the depot infrastructure will be trivial. I can’t see you’ll save many (or any) dedicated fuel launches.

    In which case, in the early stages, depots don’t make a lot of sense. You will be launching prop in a vehicle (tanker) the same size as the lander, so you might as well skip the depot and just dock the lander directly with the tanker. Depots only make sense when you are launching many low-cost tankers per depot over many months before launching the main mission vehicle.

    Bigger savings comes from developing a reusable upperstage, hence reusable tanker. (Plus if the upperstage can re-enter and land, and can serve as a lunar lander, then it can serve as a crew re-entry vehicle.)

    Re: Rotovators.

    Rotovators have such low TRL than I wouldn’t mention it at all in connection with a lunar proposal. Yes, if it works it’s awesome, and becomes a go-to technology across the solar system, so I’m not objecting to a separate research program, just that it makes no sense to talk about it now in connection with a lunar program. It’s a wholly separate subject.

    LEO variable gravity facility vs Lunar centrifuge:

    The only possible savings from a LEO variable-gravity-facility is if Doug can skip his centrifuge module in his inflatable lunar base. If the VGF shows that extra g-load is still required then he still needs to develop the lunar centrifuge. And if (as you’ve said to Doug) you aren’t saying to wait on the VGF results before starting work on the lunar development, then Doug still needs to design his base on the assumption that it’ll need a centrifuge. So Doug isn’t saving on development cost for the base+centrifuge. At absolute best, if VGF isn’t expensive, and if it isn’t delayed and beats the lunar base to orbit, and if it gives clear results, and if those results remove the need for a lunar centrifuge, then Doug merely saves on the costs for delivering the already developed lunar centrifuge to the moon.

    It’s highly doubtful that the development, launch and operational cost of the VGF will be cheaper than the mere transport cost of the lunar centrifuge. So in net, you haven’t saved anything.

    Look, I’m a fan of the concept of a LEO-VGF, but if someone is funding a centrifuge on the moon, why not use that rather than try to chase extra funding for the LEO-VGF?

    Re: ISRU propellant vs cheap launch.

    If your mission goal is the lunar surface, then having ISRU propellant on the lunar surface makes a bigger difference than refuelling in orbit. For example (using Merlin), if you bring fuel from Earth for the return journey, then you need 16 tonnes of propellant in low-Earth-orbit for every tonne of ship+cargo. If you refuel on the surface, you only needed 6 tonnes of propellant in LEO. It takes 10 tonnes more propellant to end up with 1 tonne of fuel on the lunar surface. (Earth-sourced depots in lunar orbit don’t change that, they just divide the load. Totals are still the same.)

    Merely having locally sourced fuel on the moon (not in orbit, just on the surface) reduces your needed number of launches nearly 3 fold. Even at low launch prices, that’s not to be sniffed at.

    Lunar-fuelled depots/tankers/whatever in LLO or L1 makes less difference; if Earth launches were reasonably priced, the benefits wouldn’t be worth the cost of developing lunar ISRU. But if you’re refuelling on the surface anyway, and you already have tankers in the same vehicle-family as the lander, why not?

  3. matterbeam says:

    I have to agree with Paul in this one. Rotovators are a completely unknown technology, and entire space programs have been cancelled with less innovation in them.

    As for the ISRU vs cheap launch question, I believe it is only a question of how much extraterrestrial activity you expect to have. Low activity, and reusable tankers will readily put the tons of propellant you need in LEO. High activity, and the cost of ISRU will be offset by the cheaper propellant per ton.

    For example, assuming Hydrolox propellants with 450s Isp and all dry mass absorbed within the payload:

    If you want to send a 10 ton payload to LLO from LEO, using a 2 ton tug, you would need about 4.5km/s, the you’d need to send back to tug with 4.5km/s to LEO to pick up the next payload. The cost of propellant in LEO is $500/kg, and the cost of propellant produced by ISRU is $X/kg.

    If we use ISRU to refill the tug so that it can return to LEO, then bring up another 10 ton payload, we would need 4.5km/s for 12 tons dry mass, then another 4.5km/s for 23.3 tons dry mass. The total propellant required is 21.3+41.2: 62.5 tons. The cost is $62500X.

    If we instead refill the tug in LEO and use minimal ISRU, we’d need 4.5km/s from only 2 ton dry mass, so the tug arrives empty in LEO. We’d need 3.54 tons of propellant from ISRU, and 21.3 tons of propellant from LEO. Total propellant load is 24.8 tons. The cost is $12400000 + $3540X.

    If we use only propellant launched from Earth, we’d need 3.54 tons of propellant to bring back the 2 ton tug from LLO, plus 43 tons of propellant to send out a 5.54 ton tug plus a 10 ton payload. The total is 46.54 tons of propellant, which costs $23250000 to launch.

    We work out that if lunar ISRU can create propellants cheaper than $3054/kg, then partial ISRU is the best option, and if it can create propellants cheaper than $372/kg, then full ISRU is the best option.

    I hope this sort of calculation, with your own set of numbers and additional levels of detail (such as the additional 1.6-18km/s cost of landing on the Moon) will better inform the comparisons between ISRU and other options.

  4. john hare says:

    Tanker preferable to depot. I can live with that.

    You may notice that I did suggest rotovators as a separate development to be used as soon as ready.

    The variable gravity facility in LEO would be much cheaper than getting the same data on the moon. It would start developing data much earlier than the Lunar unit which would extend the baseline by several years. The VGF in LEO could launch in a year if a serious party was actually after the information. The centrifuge on the moon is about a decade out with Dougs’ plan.

    It is not ISRU vs cheap launch, it is both.

  5. DougSpace says:

    I agree with Paul & Matterbeam that rotovator is sufficiently uncertain that it should not be used in making the case for lunar development initially. After we have a cost-effective transportation system to the Moon (e.g. FH-Xeus) then we can attempt harvesting lunar ice for propellant which makes the system elements reusable. Ice harvesting at propellant levels not only significantly reduces the mass necessary for launch but the water and organics make up most of the consumables needed to be launches. At that point, we can afford to try any number of things. But let’s get the cost-effective transportation system first using the Moon as the initial, self-filling propellant depot.

    When do we need to know the artificial gravity prescription (AG Rx)? My concern with anybody doing a LEO VGF is not the cost (so much) but the impression that we need to know the results before we send people to the Moon. We don’t need to know the AG Rx for gestation, childhood, or even adult health when crew first arrives on the Moon. We can follow the biomedical indicators of the crew and ship them back if / when necessary.

    We need to clear out the things that might get in the way of sending crew to the Moon sustainably, sooner than later. That includes depots, VGF, DSG, international partners, sortie missions, LLO power utility, rotovators, NTR, and even Resource Prospector (RP). In the case of the RP, we should still do it but not wait for the results before starting to develop the lander.

  6. John hare says:

    Andrew, you may be right. Especially considering development.

  7. peterh says:

    A surface based sling could be very useful getting payloads into orbit. A rotovator can work delivering payloads both directions, and works best if payloads going down can provide the momentum needed for payloads going up.

  8. john hare says:

    Somehow I interpreted Andrew’ thought as a surface based rotovator that could learn to catch payloads as well, but I could be mistaken. My thought on it being less expensive to develop was that a subscale unit could be inexpensively developed on Earth with almost sonic capabilities. Then it could be deployed in LEO as a cubesat dispenser to different orbital planes as the tech is learned. As the tech improved, more difficult planes could be reached including near GTO trajectories. Then the surface mount unit could be delivered to start sending Lunar material nearly to orbit though it would need an orbital catcher or some propulsion on the material. Finally, catching inbound material if that proves feasible. On the way, another unit in eccentric Earth orbit could economically send small probes to NEOs.

    Knowing that even a 600m/s sling could send prospectors to a 1,000 km radius on the Lunar surface could partially finance the rest would be helpful.

  9. Andrew Worth says:

    There’s a frozen lunar orbit at 86 degrees, my thoughts were that the surface sling could both fire to and catch from a station in that orbit.

  10. Landis’s paper “Analysis of a Lunar Sling Launcher” points to the possibility of lunar sling launch to LEO or to Mars with existing materials – no need for carbon nanotubes. http://home.earthlink.net/~geoffrey.landis/Lunar_Sling_Launcher.pdf

  11. john hare says:

    Interesting paper there. He gives 2,500 kg tether mass with Spectra for the 1680 m/s case which is 10% of what I mentioned for system mass. Buckytubes would have a tiny mass. For a system in Lunar orbit, I think it is fair to suggest that 25/1 is a reasonable system/payload mass. I think the ability to capture inbound materials would be as valuable as sending outbound units, though Landis does not discuss this possibility.

  12. gbaikie says:

    “Lunar-fuelled depots/tankers/whatever in LLO or L1 makes less difference; if Earth launches were reasonably priced, the benefits wouldn’t be worth the cost of developing lunar ISRU. But if you’re refuelling on the surface anyway, and you already have tankers in the same vehicle-family as the lander, why not?”

    The significant element shipping lunar rocket fuel to lunar orbit is to increase the amount lunar rocket fuel sold- increasing your market share.
    Or significant problem of mining lunar water and making rocket fuel is selling enough of it per year.
    If you brought rocket fuel from Earth to lunar surface, that would have a high cost and therefore need to have high price. And if brought rocket fuel to low lunar orbit from Earth, that could a lower cost, and therefore could lower price as compared to rocket fuel shipped from Earth to lunar surface.

    Suppose you were just making LOX on lunar surface. In that case one need to bring LH2 or methane or whatever from earth to lunar surface. One would need to make a lot lunar LOX and at cheap price if all you doing was selling LOX to lunar surface.
    It’s “possible” that you could import earth LH2 to lunar surface and use lunar LOX plus earth LH2 and ship lunar LOX to low lunar orbit and sell lunar LOX at lunar orbit.
    Because you using only 1/6th of rocket mass needed which is the imported LH2.
    But with making rocket fuel from water, you have the hydrogen so you don’t need to import Earth hydrogen to lunar surface.
    But you could just export lunar LOX to lunar orbit and import LH2 from Earth to lunar orbit.
    When split water the mass ratio is 1 part H2 to 8 parts O2. And need 1 to 6 for rocket fuel giving a surplus of LOX. Plus with any kind of other mining on the Moon- the mass of Moon is 40% oxygen- so going to have surplus of O2 from that activity.

    So like Earth one will have cheaper oxygen than compared fuel that burns with it on the Moon. It’s also cheaper to make liquid oxygen compared to liquid hydrogen.
    So with Earth the price difference is LOX being 10 cent per kg compared to LH2 of about $5 per kg: 1 to 50. With the Moon I generally think it would at at least 1 to 4.

    Or if lunar LOX is $1000 per kg, Lunar LH2 is + $4000 per kg. Or in terms of
    7 kg of rocket fuel: 6000 + 4000 = 10,000 / 7 is rocket fuel at $1428.50 per kg.

    So if instead was importing LH2 from Earth, say Lunar LOX was $500 per kg and Earth LH2 was 10,000 per kg: 3000 + 10,000 = 13,000 /7 is $1857.14 per kg of rocket fuel. Or if Earth LH2 was 20,000: 3000 + 20000 = 23000 / 7 is $3285.71 per kg of rocket fuel.
    But assuming there was lunar LOX and LH2 and assuming price was 1000 and 4000.
    I could ship lunar LOX to lunar orbit for 1000 + say 3000 per kg to ship it, or sell it
    for 4000 at low orbit and with Lunar LH2, it’s 4000 plus 3000 per kg to ship it.
    So Lunar LOX at low lunar orbit: +4000 per kg and Lunar LH2 for +7000 per kg.
    If shipping from Earth, one might be able to beat the $7000 per kg LH2, and have harder time beating the $4000 per kg price of LOX.
    Or the price of lunar rocket fuel doesn’t need to be cheaper than earth shipped, rather it has to be competitive with earth shipped rocket fuel- and Lunar LOX is more competitive [can be cheaper].

    In terms of L-1, lunar LOX and lunar water can be competitive- assuming there is need of water is L-1. And if going to Mars, people need water for the trip [shielding and drinking and cleaning].
    So roughly speaking the cost of lunar rocket fuel at lunar surface can be competitive with earth shipped rocket, but has to [quickly] become much cheaper.
    Or roughly speaking Lunar water has to start around $500 per kg and LOX around $1000 and LH2 has to be less than $10,000 per kg. And say within 10 years might be 1/2 these prices.

  13. gbaikie says:

    –Or roughly speaking Lunar water has to start around $500 per kg and LOX around $1000 and LH2 has to be less than $10,000 per kg.–
    In terms of gas O2 might be $900 and H2 might be $8000 per kg.
    Of course there problem of volume of storage- but generally speaking whether storing
    liquid or gas that cost you want to avoid. So you don’t want to store a lot of liquid or gas.
    So say, 10 tons or more of anything other than perhaps, water, for more than 1 month, is costly.

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