15 thoughts on “The Cis-Lunar Gateway”

  1. A refuelable lander can be its own depot and so break the chicken / egg dilemma. If it provides enough demand, then it can lead to commercial depots in LEO and at L1/L2. If it doesn’t then such depots aren’t needed. A gateway station is still nice even without a depot. In fact, refueling at L1/L2 is nice even without a gateway (or SLS/Orion) and that should leave a lot more money for actual propellant launches. And since these can contribute towards cheap lift, that’s what we should focus on.

    In short, what’s wrong with the plan is not that it doesn’t have depots in it (it shouldn’t), but that it does have a gateway, an HLV and Orion in it and no lander.

    1. And you need more than one type of lander, I would think. An upscale LM to get astronauts and equipment to the surface, probably a custom lander that is its own factory, and a much more optimized single-stage lander that can deliver fuel from the surface, up to the depot, and then return, all in a single-stage mission.

      By my rough delta-V calculations, assuming an achievable 20:1 mass ratio, hypergolics should be 33 to 36% efficient at delivering fuel to the depot, and LH2/LOX would be a little more than 50% efficient. (Based on 2500 m/sec each way)

      I don’t know if it’s a bit useful, but light gas guns (LOX/LH2 artillery) could make 2500 m/sec and could be landed as a unit (as opposed to building a much longer maglev mass driver). But the fuel container (oil barrel?) would have to be built like an artillery shell to survive firing, and I’m not sure they could be efficiently returned. It also means the crew at L2 would be chasing down little fuel tanks all day long. Offsetting the poor mass performance, however, is the fact that the empty shells would be very useful as radiation shielding and later as construction materials.

      1. It’s been a while since I did the sums, but I could never get the numbers to work out for export of propellants with hypergolics. Importing the necessary hydrazine and nitrogen made it uneconomical compared to just bringing it from Earth. That doesn’t mean ISRU wouldn’t help, just that it’s not good enough for exporting lunar oxygen. It would still be good enough for reducing the amount of NTO you’d have bring with you.

        There’s an old study by a company called Eagle Engineering or something that shows export of lunar LOX all the way to LEO could be done with LOX/LH2 even if you had to bring the LH2 from Earth.

        I’m still in favour of starting with a hypergolic lander, but you do have to be aware of the limitations.

  2. Much of the argument against L2 applies to L1 as well. One thing the article got right is the need for a LEO depot first. Put a general purpose ship in orbit with fuel transfer capability and you’ve got that, with radiation protection.

    The second depot, another GP ship, goes in lunar orbit.

    When a red dragon lander is doing practice runs to the surface of the moon we will begin to get a better idea of how to do things. Actually, that other lander is one of the things that other companies should be building in competition with SpaceX.

    1. A LEO depot isn’t terribly useful without an L1/L2 depot, while the reverse isn’t true. Having both would be great, but if you can have only one, it should be an L1/L2 depot. If you can’t have even that, then mere spacecraft refueling at L1/L2 is enough. All the rest, including a gateway, is just nice to have.

      1. Lagrange points have both advantages and disadvantages. It can not just be assumed they are the best place to put a commercial business. Looking only at missions starting from EML1 misses a bigger picture.

        From earth you’ve got to go through LEO before you can get anywhere else. It takes more energy to get to the lagrange point than to bypass it. This makes the LEO business case easier for more businesses.

        Apollo never went to EML1. Why? Because there was no advantage. LEO to moon is 5.93 m/s. Including L1 it is 3.77+2.52 or 6.29 which isn’t much but it adds up.

        EML1 depot is a good idea, but not necessarily the first place to go. Get fuel transfer a simple reliable procedure in orbit first.

        2. I should have qualified that: they are probably the best place for exploration missions, both manned and unmanned. Nevertheless orbit raising from LEO to GTO / GEO is of course a very possible commercial market once you have a cryogenic LEO depot. But for exploration not having an L1/L2 depot is very inconvenient because of problems of orbital mechanics.

          Total delta-v is not the only consideration, the size of the individual hops also matters for the required size of your transfer stages and the required thrust level. For instance DCSS could get Orion to L1/L2 with enough propellant to get back, but couldn’t do that to LLO. Delta-v-wise Lagrange points are enormously strategic locations. Have a look at Hop David’s blog post Inflated Delta Vs, it shows how much more easily you can travel between the edges of various gravity wells.

          Then there are orbital mechanics issues like phasing, nodal regression, plane changes and energy efficiency that favour using Lagrange points. The thermal environment is better too and multiple staging points allow the use of complementary forms of propulsion such as SEP that can greatly improve effective Isp.

          The issue has been studied in depth and Lagrange points are of strategic importance. LEO depots on the other hand are also important, but as an addition to L1/L2 depots.

          As for making fuel transfer a simple reliable procedure in orbit first, that has already happened. It is counterproductive to insist on cryogens first, especially if so much is riding on early exploration, namely early cheap lift.

          We already have the tools (existing EELV-class launch vehicles, hypergolics, SEP) we need for very extensive exploration of the solar system. The only real obstacle is the fact that launch prices are so high. Doing exploration with propellant transfer as soon as possible would allow us to attack that problem as soon as possible.

          To first approximation it’s the only thing that matters.

          1. That chart you link to has sold me. If you can go to mars on a 1.1 km/s budget rather than 5.7 you’ve just save about a billion dollars in fuel each time for a six passenger general purpose ship. Upgrade the 23mt BA330 to something like a 40mt BA700 with much upgraded life support that could be launched using a Falcon Heavy and per passenger cost goes way down.

            I’ve got to revise some of my cost estimates. This becomes a real game changer. Making the case for going to mars now much stronger.

    2. I’ve suggested an an L3 depot might be more useful than an L1 or L2 depot, simply because most managers would rather pretend the moon isn’t there. From L3 you can’t even see it.

      1. An L3 depot would be in a Lissajous or Halo orbit, right? And can’t you see the moon from either of those orbits?

        Additionally, L3 is occupied by Contra Luna — you know, the moon directly opposite Luna where Comrade Leonov took his first (goose) step. I read all about it on the Communism Explained blog.

  3. @Ken:

    Bear in mind that by itself this only solves the delta-v per hop problem, not total delta-v and IMLEO. You now get to use heavy shielding without having to lift it out of a gravity well only to descend into another and then back and you don’t need a monster transfer stage that won’t fit on an EELV. But the flip side is that this only takes you from the edge of Earth’s gravity well to the edge of the Mars gravity well.

    You still need to get you crew to EML1/2 before departure and to LMO after arrival. But the good thing is that you need only a small and light ferry craft to do so, instead of your massive MTV.

    You also need to get lots of propellant to EML1/2 and a Mars Lagrange point. This is where SEP and ISRU come in. SEP already exists in a form suitable for prepositioning storable propellant, ISRU would need more work. But SEP alone can make a major difference since propellant represents such a significant fraction of IMLEO.

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