The Moon itself isn’t very homogenous. So its gravity well is quite lumpy. There are really no long-term stable lunar orbits at any altitude. They all require comparatively aggressive station-keeping.
But you’re right that any attempt at a selenosynchronous orbit fails. Due to the Moon’s very slow rotation the synchronous orbital radius is so large that the orbit falls outside the Moon’s Hill Sphere. And is therefore wrecked by Earth’s gravity. Here’s a decent cite with further cites
The component parts of a tether system aren’t exactly in a synchronous orbit although the whole is so, at least in a flexible wobbly sense. Given that distinction, I don’t think we can easily rule out building one on the Moon. But its dynamics would have some “interesting” features requiring “interesting” engineering. For particularly vexing values of “interesting”.
Would there be any point in building a space elevator around the moon, though? There’s no atmosphere and the gravity well is relatively shallow, what advantage would an elevator have over a magnetic accelerator?
I guess one might build an elevator with the cable running through the L1 point, which IIRC is about 60Km from the moon… not THAT much more in the big picture than the ~36 Km for geosync? Might even be possible with current materials?
But then the economic question comes back in spades: how much demand is there for moving large amounts of material on or off the moon?
Yeah, my off-the-cuff “put one on the Moon” isn’t very likely either. The Moon is actually a much better use-case for a mass driver. Or, if water is truly abundant, Just use a nuclear pile and a giant water bladder on a frame. Heat the water to steam, and shoot it out the bottom. Low ISP (270 or so), but no cryo fuels, no pressurized tanks, no fuel production required. Just melt water, filter it, and heat it.
It would work. Mind you, it wastes water, but it seems like every day that goes by we are finding more water on the Moon.
But thats getting far from the topic of Starship vs space elevators. Or just Starship.
Will it reach orbit this year? I’d probably give 2-1 odds that it will. Will it successfully come back from orbit this year? That’s a more difficult question, as we haven’t seen how it would have handled re-entry so far. But I’d say probably not. The booster will likely make a soft water landing and maybe even be caught by the chopsticks this year, but I’m guessing we’ve got a few flights before they can bring a Starship back in one piece.
Probably. Plus we can almost certainly build a railgun accelerator with known technology and materials.
And the working parts are on the ground, which has to simplify maintainance.
By the way, I inadvertantly omitted a factor of 1000 in an earlier post about L1 & geosync distances… in case anyone noticed!
Not just political pork, but an entire industry built on big government contracting. The big space contractors aren’t lean, can’t make fast decisions, and don’t like to take risks other people aren’t paying for.
Imagine if the Starship program was run by NASA. Every subsystem, every purchasing decision would have a political element. They would be forced to work with existing contractors. An explosion of Starship would proably have resulted in months of hearings and hand-wringing, along with attempts to shut it all down by politicians who have other plans for the money. A novel idea like catching a rocket with chopsticks instead of paying the mass penalty of landing legs would likely have been laughed out of the room.
And all along the way, political opponents of the program would be looking to undermine it, delay it, cut funds, etc. Every setback would result in politicians running to their favorite news person with tales of woe. Starship would be supported by the people who voted in the current administration, and opposed by many others purely for political reasons.
Apollo worked because the entire nation was behind it, the agency was new, and everyone was focused on the mission. It was basically a startup mentality. And the requirements were clear from the very start: Put a man on the moon, and get him back safely. Everything else was subordinate to that, so you didn’t get the creep and bloat of later programs.
Then the shuttle came along, and it had no real defined mission. So it became a white elephant that was going to have to do everything, and any engineer will tell you that when you have a project with loosy-goosy requirements like that, it’s going to expand and run over budget and over schedule and won’t perform well.
The Shuttle was a beautiful machine, and it worked. So we were stuck with it and its $35,000 dollar per kg launch cost. That crippled space for a generation and in part was the reason for the ISS - to give the Shuttle program a justification. The ISS then turned into its own white elephant, and between the two programs ate up a significant chunk of NASA budget with not much in return.
I think it’s amazing that the development cost of Dragon AND Starship is probably less than or close to what Boeing has spent getting their Starliner capsule working, and it still hasn’t flown a real mission.
Actually, I think the Shuttle was a horrible mistake: there was a feeling that we had to be ‘doing something’ to keep the myth of a US space capability sort of alive. But it was never useful for any real space travel or exploration.
Something can be a beautiful machine and still be a bad idea. See: Concorde.
That’s what I meant. The shuttle was magnificent, so it had lots and lots of support that kept it flying. Politicians loved being associated with it, especially if subcontractors were in their state (and NASA made sure that subcontractors were spread all around the country to maintain political support.) But the Shuttle sucked at its job of providing cheap space access. Throwaway rockets were 1/3 the cost of the ‘reusable’ Space Shuttle.
There’s actually not much in Starship that couldn’t have been done in the Shuttle era. The RS-25 engines are overly complex and expensive, but they are in the same category as the Raptor: a large, reusable, high-ISP rocket engine. The DC-X demonstrated powered landing decades ago - then was cancelled because it didn’t have friends in Washington and threatened programs that did. Stainless Steel being better than composites in this application was discoverable by anyone.
The problem was lack of vision and a space program dominated by risk-averse governments who see the space program as a way to funnel money to their subcontracting constituents. But also just the massive, distributed bureaucracy of NASA crippled their ability to move fast and take risks. Successes like JPL are disconnected from the massive bureaucracy around space launch development.
The Shuttle contained some fabulous engineering. But it was still a bad idea executed with too many compromises. Just about every component displayed some part of compromise brought on it by political forces. Starting with its size, and it just got worse from then on in.
The Shuttle was designed in a different world when it came to computational tools. It was designed when supercomputer was equivalent to Cray, and it did at least get some benefit from early use of machines like a Cray-1. The modern world of design has been transformed by access to what amounts to science fiction levels of compute, compared to the 70’s; bringing with it extraordinary capabilities in structural, thermal, aerodynamic, and most importantly, mixed mode coupled analysis. Hard to know how much, but there must be a significant boost in confidence found in the modern design process that enables the fast paced design iterations possible now. Back when the shuttle was designed, things were frozen as late as possible, but still too early, and iteration often just wasn’t possible. Which was how the Saturns were designed. Evil problems just had to be engineered around at any cost. But they had clear goals, and one of them wasn’t to be cheap. The Shuttle had few clear goals, one of them was to keep a lot of constituents employed.
To be fair to the Cray, those were 64 bit floating point ops, and it could keep up that sustained rate of flops all day. Something that took a very long time for ordinary processors to catch up with. It was a pretty miraculous beast.
Caches? we don’t need no stinkin’ caches. (Or famously - parity.)
Heck, on a gravity well as shallow as Luna’s, even old-fashioned chemical rockets are easy. To launch a pair of astronauts and all the life support they needed into lunar orbit, all we needed was something the size of a small car.
The only problem is that propellant is not readily accessible on the Moon. You can bring it with you, but that doubles the required delta-V, and thus squares the mass ratio. You can make propellant with water, in principle, but obviously water is not in great abundance in general, and not necessarily available where you want to land. It may be possible to build a hybrid rocket that burns, say, aluminum and oxygen, but those aren’t well-researched and are less efficient than liquid fuels. You can maybe get the oxygen from the Moon and bring your own hydrogen, which is much less massive.
Aluminum-Ice propellant is pretty good, and all of it is abundant on the Moon.
I don’t know if you are up on what’s going on woth lunar water, but it’s looking to be pretty abundant. It’s not just in the permanently-shadowed craters, but present in the regolith above and below about 45 degrees latitude.
Furthermore, it may be really easy to extract. Lunar regolith is very easy to microwave to remove water:
In other news, the Chang’e-5 lander found significant amounts of hydrogen in the regolith at 45 degrees latitude, and when the samples were brought back they easily extracted water from it. Estimates are that the regolith around the poles (not in the shadowed craters) has anywhere from 500-700 ppm of water in it, and as the link above showed, hitting it with as little as 250 W or microwave energy will extract the water.
In yet more water news, glass beads from old volcanic eruptions appear to absorb water:
These glass beads are drier in the center than at the edges, showing that the water wasn’t there at formation, but migrates in after the solar wind creates water on the surface. These things can only hold water for 15 years, and the estimate is 10^14 kg of water. That means at least that much water is created on the moon every 15 years, which means that water on the Moon is a renewable resource!
Here is a good reference to all evidence for water on the Moon to date:
They are talking about bright, sunlit regions not just dark craters. Apparently, the water forms from the interaction of the regolith and the solar wind, then transports below the surface where is is protected from the sun, and may be adsorbed into volcanic glasses in the regolith. If that’s true, then the amount of water in the regolith at high latitudes is much higher. And again, it can all be extracted simply with microwaves.
So we’re talking about hundreds of millions of tons in the polar craters, large amounts in the regolith around those craters at the poles, a water cycle that may produce more than 10^14 kg of water every 15 years, and possibly a lunar-wide hydrated later under the regolith with huge amounts of trapped water.
Finally, the existence of pyroclastic volcanoes in the past suggests there are still reservoirs of water and other volatiles in pockets in the lunar crust and mantle, of unknown size.
A nuclear thermal rocket using water in a bladder can achieve an ISP of around 200, which isn’t great for modern fuels, but for the Moon or interplanetary flight it’s fine.
An NTR with water fuel requires about 280 tons of fuel per ton of payload. So if we were launching 10 ton payloads from the moon to lunar orbit, that’s 2800 tons of fuel. But if the shadowed craters have, say 500 million tons of water in them, that’s more than a million flights. For the number of flights we could expect in a near-future scenario, water use is trivial in terms of depleting the moon.
But it may not be trivial in cost - depends how much it costs to extract it. And water being renewable on the moon changes everything, If those 500 million tons took billions of years to form, then it’s precious. If 10^14 kg of water are created on the moon every 15 years, then game on.
There are certainly reasons to be optimistic, but there are quite a few steps between the present day and nuclear-thermal rockets with lunar ISRU-extracted water propellant. Thanks for the links, though; I hadn’t seen all of that.
We will need a lot of electricity for these proposals. I have a new idea for this: a wire run all the way around the moon.
You just need one wire, because it already forms a loop. It doesn’t have to go around the equator; you could pick a higher or lower latitude to make it work.
Say you can drop 100 tons of aluminum wire with Starship. That’s 7,500,000 meters at 2.5 mm. That would go around the moon at +/- 45 degrees.
The total resistance is 43 kOhm. Which is high, but not unreasonably so. If the total voltage potential was 1 MV, then you could send an amp down the wire, and extract up to ~950 kW. Which is decent for a smallish base. This could of course be scaled up further.
The one-wire system eliminates the problem of lunar night. Half the wire is always illuminated, so you can just build a bunch of solar stations and space them around the ring. Each solar station would only add a smallish voltage to the total. Say, 50 stations, which each need to add 40 kV (since only half are illuminated).
It’s safe as long as the wire isn’t broken. You don’t even need any super-advanced regulation equipment; if there is no load, you just let the current rise until it’s naturally dissipated by the wire itself. You can bury the wire or whatever to reduce damage where people are present.
It’s trivial to tap into the wire at any point. You attach two electrodes and cut the wire between them. The voltage will rise until it hits the maximum, but you can stop it at any point by bypassing some of the current. You can manage the load by changing the effective resistance, so that a certain voltage appears across the terminals.
