And interesting end-of-Shuttle post from Karl Schroeder (at Charles Stross’s blog):
A couple of years ago I sat down to lunch with a prominent astronaut, a Shuttle commander and space station veteran. We talked about space development and alternative paths to what NASA has actually done since 1970. I told him that what I’d been waiting for ever since Skylab was a variable-gravity research station, because it hadn’t taken us long to accumulate lots of evidence that lack of gravity is bad for the human body, and because lower gravity was the only physiological variable for the Moon, Mars and other possible destinations that we couldn’t currently test for. It’s also one of the most important; a variable-gravity station could tell us whether unaltered humans could live long-term on Mars, for instance. The astronaut asked me how I would be build this station, and I said, “Rotate two booster modules, one habitable, linked by tethers.” Much like Skylab, and very simple to construct.
He shook his head. “Tethers in space,” he said, “break.”
I blinked at him. “Well, if they break, you build ’em stronger, make ’em out of something else, or you use a number of them.” I didn’t quite say, “This isn’t rocket science,” but really, it’s basic engineering.
He shook his head even more vehemently. “Tethers in space,” he snapped past gritted teeth, “break.”
I had no reply. I had been watching him; he became visibly tense every time the conversation moved away from strict NASA doctrine.
If accurate, it does sound like a strange exchange. It’s like saying “rocket engines blow up” in response to a proposal to launch a rocket. Clearly, tethers have broken in space, but it’s a bizarre logic to thereby infer that all tethers in space will break. If you put in the proper structural safety factor (and aren’t heating it up by running a current through it electrodynamically), of course there is no reason that a tether need break. Being an astronaut is no guarantee of being a good engineer (or even necessarily logical). And many of them, of course, don’t have an engineering background.
[Via Chris Gerrib]
Tether’s in space break because they’re designed with a safety factor slightly less than one. If I designed a space tether I’d use a cable that could actually support the load with a little bit of bouncing going on, instead of using a piece of piano wire or fishing line. It’s not rocket science, it’s yo-yo bolo science.
The other reason they break is erosion from micro-meteorites. I forget the exact numbers, but a zero tension tether a few mm across will break in a few days, I think.
Not that you couldn’t design around that, but you would need to give it shielding and redundancy just like every other part of the space vehicle / station. That certainly complicates things.
Not so strange. This is basically a religion we’re talking about. Here the dogma involves engineering, but the more usual and far more harmful cases are the dogmas involving fantastic economics.
Sorry – I just noticed that I didn’t talk at all about this, but tether length is also a factor. I was looking into space elevators / rotors, so those were much longer. The numbers may be better for something short.
Can a tether be a rigid structure or must it be some sort of wire/cable?
A “rigid tether” has another name — a strut. Tethers are flexible by definition. That is, it has strength in tension, but none in compression.
Why are so many sci-fi writers such liberal / democrat goofs? I love good sci-fi stories but give me RAH over Charlie Stross any day.
Some of the comments at Charles Stross’s blog make me want to beat my head against a wall.
Some NASA expert say a tether will always break, so it must be so. All proven applications from engineering (even stone age engineering), common sense, and logic are therefore refuted.
They might make some progress if they’d stop treating the tether as an afterthought that has to fit spooled up in a glove compartment. Here’s a Star Fleet Academy thought experiment: Put two Gemini capsuls at opposite ends of a main support cable from the Golden Gate Bridge. Is the cable going to snap?
Moving on, try using 5/8″ steel aircraft cable. It weighs 855 pounds per thousand feet and has a breaking strength of 47,000 pounds. Use a bundle of ten cables 2000′ feet long you’ve got 470,000 pounds breaking strength, a cable weight of 17,000 pounds, and a diameter that will generate Mars gravity at 0.75 RPM. Using an entire Space Shuttle payload at both ends would still give it a safety factor of 20.
They look at the hazard of meteorites to tethers in this paper, it’s surprisingly high for (tens of thousands km long) lunar tethers to L1/L2, proportionately lower for smaller tethers.
http://www.niac.usra.edu/files/studies/final_report/1032Pearson.pdf
(2.4mb)
Easy solution… plan on your tether breaking. Use two with a strut in the center to keep them apart. The chance of both breaking at the same time is significantly reduced. I don’t have to mention that either tether should be rated to handle the whole load?
Between fixed struts and tethers you also have air beams.
Being an astronaut is being a civil servant. Need we say anything more?
Good point Trent. Didn’t you have a great post on your site about an artificial gravity lab in orbit?
Dunno how great it was, but the gravity gradient phenomena is interesting to me. http://quantumg.blogspot.com/2010/12/non-rotating-artifical-gravity.html
BTW, there’s a simple solution to this problem. Leave the astronaut in Houston and just fly the tether.
Tethers, struts, and air beams oh my.
Thanks for the infos.
My suspicion is it’s not the certainty of a cable breaking that bothered that astronaut but the thought of what would happen after the break. I.e., two objects — one at least a manned spacecraft — would go flying off in different directions. At an unpredictable time, in unpredictable directions, likely on trajectories from which they could not be retrieved, likely without sufficient food and air and fuel for manuevering to make recovery possible. Likely there are studies…
I.e., an “I don’t want to think about this anymore” recollection might well lead to an “I don’t want to talk about this” moment.
And no doubt that’s because he’s never run the numbers and found that tangential velocity is minor compared to the typical delta-v requirements of spaceflight.
Indeed Trent. I think my little 2000′ aircraft cable Mars gravity suggestion would leave you with a 34 m/sec delta V.
Funny, the tethers used on Gemini to create artificial gravity didn’t break, but then that was the Apollo generation when engineers were ENGINEERS who used slide rules and drafting tables, not wimpy computer simulations. 🙂
Hmm. What’s the delta-v capability of an orbiting shuttle or Soyuz capsule that’s intended for future use? E.g., suppose you’ve set up your orbital test with something like a Soyuz or Gemini spacecraft tethered to a pile of sandbags by some cable of reasonable length…
Assume you’re shooting for a simulated 1-gee environment in that Gemini, and that the rotation period is one minute (60 seconds). Ballpark, I calculate that the cable length is 1800 meters, and that the tangential velocity of the spacecraft is about 100 meters/second. Half the rotation rate, and the velocity is halved, which is a step in th rigfht direction but at some coriolis forces enter in. I.e., the astronaut doing experiments gets sick, and your data analysis gets a bit hairy.
Anyhow, assume you’re an ISS astronaut and the word goes up that “The rope just broke and Lori and Charlie are flying off way out of the ecliptic at one tenth KPS! We got to save them!” Which on-board rescue device would you choose to deploy, how quickly do you suppose folks at JSC or Moscow would concur, and how far would Lori and Charlie have flown by the time you reach them? Bear in mind that neither Shuttle nor Soyuzes were really set up to handle much on-orbit manuevering.
Not saying some sort of rescue scenario is always impossible, but it’s possible to imagine some that are quite dicey.
@George Turner
Great thought experiments!
@mike shupp @ 5:47 pm
Buncha sissies! All L&C need are a half dozen Payload bars and several 20oz Mountain Dews (19 tsp of sugar per) and they’ll be fine until we figure it out.
1800′ of tether is a lot of tether, but we should still do it. Acceptable risk and all.
With the small delta-V you’d get from a Lunar or Mars gravity tether, rescue wouldn’t be much of a problem because it still wouldn’t be enough to cause a deorbit, and the module would easily compensate because it has to have enough delta-V reserve to stop spinning in the first place. As in my example, a 20 to 1 safety factor on the tether doesn’t even impose much of a weight penalty on the system. If it breaks it probably means someone is firing nuclear warheads at it, so there would be much bigger things to worry about.
One of the very odd things about the discussion following that post is that half of the commenters seem to think that non-rotating tether systems are being considered for the design. You know, the ones tens of kilometers long that extend toward or away from the center of the Earth while the spacecraft orbits.
…try using 5/8″ steel aircraft cable. It weighs 855 pounds per thousand feet and has a breaking strength of 47,000 pounds. Use a bundle of ten cables 2000′ feet long you’ve got 470,000 pounds breaking strength, a cable weight of 17,000 pounds, and a diameter that will generate Mars gravity at 0.75 RPM. Using an entire Space Shuttle payload at both ends would still give it a safety factor of 20.
Bingo. And to get in and out of the experiment module, you wouldn’t have to de-spin the system. Put an unpressurized docking station at the barycenter, and use a small, light funicular to travel up and down the tether cables between the hub and an airlock on the “top” of the research module.
George Turner:
“the module would easily compensate because it has to have enough delta-V reserve to stop spinning in the first place.”
That’s a good point, and I should have thunk of it and didn’t. Mea culpa.
OTOH, I’ll stick to the idea that brought me here: The astronaut’s rejection of tethers, as described by Karl Schroeder, seemed more related to a psychological defense mechanism than to simply parroting NASA dogma.
I agree with Mike Shupp. I think the astronauts are remembering Gemini 8 more than Gemini 11. It’s not just delta-V but the ability to use that delta-V for recovery before the sudden change in rotational speeds overloads your senses. The astronauts are over simplifying the issue, and so doing, not considering the mitigation capabilities. For instance, flight software in the aviation world has advanced (since Gemini) to handle dynamic loading faster than pilots. I use paratheticals, because those advancements are not so new anymore.
My personal issue with tethering is a slight variation of Mike’s original hypothesis. You have your station rotating now, and your ready to resupply. How do you dock? Do you expend propellant to stop the spin and then resupply and refuel? Or do you inact the incredibly stupid seen from “Armageddon” where you (first spin up the space station then) try and match rotational speed and then dock.
Leland, I think docking at the axis of a spinning station might be trickier than commonly thought, since the exact center of the rotation will change slightly everytime somebody flushes a toilet or adds a new piece of equipment to one of the modules. The station is only going to be rotating at 1 to 2 RPM, so the docking target will usually be wandering in a small, slow circle. That may introduce complications because we like to dock very slowly, following careful procedures. That problem could be mitigated if the station’s docking port had an extra degree of freedom or two so it could move itself to the exact axis of rotation.
A more obvious problem, as you and other’s mentioned, is moving from the axis down to the modules. A finally, axial docking limits the number of attached capsules to two, one on each end of the axis, until you add lots of complexity to move the capsules out of the way once they’re docked.
I’m sure it’s been covered in the sci fi literature all over the place, though I can’t think of a specific example, but a completely different solution with a spinning toroid is to just land on the inner surface. For example, for my 2000′ Mars example, instead of a tether we spend a fortune and launch a ring with hab modules hung underneath and an inner surface that acts as a conventional runway.
Instead of docking up at the axis, a small shuttle type craft approaches the center of rotation (coplanar with the ring) and then starts moving out toward the ring edge. As it approaches the ring edge, it aligns itself to touch down on the flat runway surface that is spinning underneath at about 100 fps (70 or so mph). The shuttle is still in zero-G, so it can approach very slowly, watching the “deck” sliding by underneath. It can either push outward with thrusters to get good contact with its landing gear, catch a wire, or use an electromagnet to try and pull it down.
Then it applies brakes and starts spinning up (in inertial coordinates), which to the astronauts would just be a touchdown and braking, with G forces slowly building up to the station’s level. To the crew it would look and act almost like landing on the deck of an aircraft carrier, but the runway is 6000′ feet long, scrolls along, and then repeats in an endless cycle.
The vehicle could use electric motors on the main gear to taxi to an apron area that’s cantilevered out to the side, where it stops, either making a final docking like airlines do with a boarding ramp (except perhaps from underneath), or the astronauts pile out and walk in though a door. To takeoff, just taxi anti-spinward and accelerate to ring velocity, just like taking off, at which point the craft is in free-fall and can maneuver clear.
The advantages are almost unlimited flight capacity (instead of two at a time), a double use of shuttle landing gear (which is otherwise a pretty crazy thing to carry into orbit), making first contact with robust and shock absorbing landing gear instead of complicated docking mechanisms, the inherent ability to taxi around and park, and a method that’s visually and mentally familiar to any aircraft pilot.
The disadvantage is the cost of building such a structure.
landing gear (which is otherwise a pretty crazy thing to carry into orbit)
Ah but landing gear on a lander (enabling movement after landing) is not such a crazy thing on other rocks as well.
Tether snapping in orbit is no big deal if you align the spin such that snapping just changes the orbit a bit. Might even be a cheaper way for a polar orbit than using fuel?
It’s a different problem for interplanetary trips though. Reason enough for supplies to include many kegs of beer (for emergency propulsion of course.)
For some good work in redundant tether design, check out the Hoytether.