Friday, July 22, 2016

Long range plans of all sorts

(Note: the following is one of various posts that were copied here from the forum for the sake of preserving the early days of the project. It was originally posted there on Feb 28th, 2016.)

I added lots of stuff to the mission timelines over the last few days, and today tossed in a bouquet of ideas of many sorts. I am on the Space Show on Monday afternoon (2pm PST, 5pm EST, 10pm GST), and wanted to broaden the menu as much as i could in advance. So, i hashed out very, very brief outlines of missions going out far into the future, out into the times when the Moon will be a boomtown with extraordinary scope. The timeline roughly goes to 30 years or so after the first mission lands, accelerating sharply in development speed as it goes. I think this needs to be expected, as by the time the current timeline ends, robotics will be highly advanced. Most labor by then will be executed by independent machines acting almost entirely without supervision or interference, and very little of the mass of those robots will come from Earth. It is only the briefest of outlines of those missions, but it gives a sense of the giddy potential there.

Something to note. I named a few of the installations further out, big things like space stations deserving of a name. Those names come from the family names of people who have contributed to Moonwards so far. This is one little way i can recognize these people, do a little something for them that might actually be seen a lot in the future if things go well. Thus we have Brendberg Station for Sigvart Brendberg, Lynch Spaceport for Brian Lynch, Garcia Lunavator for my husband Aldo Garcia, and Holder Lunavator for me, because hey, we're a pair, and so that the pattern is maybe more clear if people are missing it. As time goes on, i'll give names in the same way to various things in the project. Of course this is only a taste of how in the future credit markers will be on everything. I thought about calling them 'plaques', like how buildings and monuments have plaques talking about their creators.

And now to explain this lunavator thing. Basalt fiber cable is something the colony should have little trouble manufacturing in bulk early on. There is a lack of good documentation regarding the strength of such cable, but the little there is suggests this stuff makes great rope. Wikipedia says its ultimate tensile strength is 4.8 GPa, compared to 5.8 GPa for Zylon, and 3.8 for Kevlar. That's really high. It is a preliminary number and must be treated with caution, but not so much caution that i can't get a little starry-eyed over it. At any rate if basalt rope doesn't cut it for this, further out something else will. Might be harder to make on the Moon though, for a while.

This document, from Robert Hoyt of Tethers Unlimited, looks at a really great version of what you could do on the Moon with an orbiting tether on page 9. The whole document makes delicious reading, an excellent treatment of tether launch systems. I love me a good tether. The paper was done under grant from NIAC, the NASA Institute for Advanced Concepts, so its quality is high. The idea is a tether with a total length of 200 km makes clever use of a counterweight and a station on the tether that travels its length in order to manage angular momentum. This way the tether can be made to meet the surface of the Moon at essentially no relative speed, pick up or drop off a payload, and swing back up into space. And it can do that again and again. Its capacity to move payload is high. It will orbit the Moon once every 90 minutes, so if everything is clicking, it grabs a payload once every 90 minutes. If what it can handle is 1 ton, then it can launch 16 tons every day, day in and day out. Even better, there is an extremely clever way to transfer momentum between the tether and the Moon so that the Lunavator doesn't need to expend propellant to adjust its orbit. It just takes a break every now and then to focus on swinging itself up to the orbital speed it needs.

200 km doesn't seem so very long if you are making basalt rope in really large quantities locally. Sure maybe a Lunavator made of basalt cable would need to be much larger than one made of more ideal materials, but if it can be made of basalt, i like the idea anyhow. Not only would it mean you don't have to spend payload space on an awful lot of cable, it means you can do continuous maintenance on the complex using your own resources. The tether tip velocity to pick up a payload is 0.75 km/s, much better than the 1.68 km/s tether tip velocity needed for a sling launcher to put something in lunar orbit. If a payload is released at that speed, then it has to add on the 0.93 km/s it needs to stay in orbit before it hits the ground. But it has a fair bit of time to do that, meaning big crude rocket engines made on the Moon would be good enough. This gives the Moon a way to ease in to both tether manufacture and rocket manufacture. I even pondered the idea of giant liquid-oxygen-based resistojets as a way for lunar robots to build rocket motors that are good enough to get the job done, albeit inefficient and crude. Otherwise known as simple and reliable. Or maybe more conventional engines from Earth could be made beefy enough, with such a light workload, to work for several hundreds of trips. They could fly back to the Lunavator to be captured, set down on the surface, and reused.

As the Lunavator tether is thickened and reinforced, and experience is gained, it can start hefting payloads at full lunar orbital velocity, or at lunar escape velocity. It can start doing this with small payloads and work up to larger ones. It would allow ion engines alone to be used for stuff that doesn't happen quickly, ones that stay in space and get refueled and serviced there. It is a way to launch lots of mass, which is the only way to do the really interesting things. It is the way the Moon will build its first space station, on a shoe-string budget compared to what it would cost an Earth enterprise to do it. A really nice space station that people would pay lots of money to be able to use.

You see, the Moon is an excellent example of how things change once you reach a tipping point. The Moon goes from being a money pit to a boomtown really quickly once you are able to do the right things - things that are based on known technology and well understood physics. A sense of the possible is all we need.

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