Elastic Launch Loop (Space Elevator)

JoshNH4H · 2013-11-24 17:28:18

Although Tom's proposed launch loop has very significant disadvantages, I do have to agree with him that it is a big plus that you don't have to do any hypersonic docking. Although I suppose hypersonic docking isn't any better or worse than any other kind when there's no atmosphere around.

Terraformer · 2013-11-24 17:50:03

We do "hypersonic docking" everytime the space station is resupplied.

Due to the way the rocket equation works, cutting the velocity required by a couple of km/s represents a massive advantage.

JoshNH4H · 2013-11-24 17:55:01

The difference between this kind of hypersonic docking and the kind that spacecraft do with the ISS is that docking with the ISS is much less time sensitive, whereas if you don't hit this one exactly right you have ailed to achieve orbit.

Also, your momentum still has to come from somewhere; you'll need to have a thruster of some kind on your skyhook

Tom Kalbfus · 2013-11-24 19:21:53

Terraformer wrote:

We do "hypersonic docking" everytime the space station is resupplied.
Due to the way the rocket equation works, cutting the velocity required by a couple of km/s represents a massive advantage.

Do you want hypersonic aircraft on the surface of Mars as colony modules?

Terraformer · 2013-11-25 04:24:04

Huh?

Tom Kalbfus · 2013-11-25 06:15:10

I just wonder how your going to fit a Mars hab inside a hypersonic aircraft so it can fly into space and meet up with an orbiting skyhook, knowing that the Hab is about 10 to 15 feet in diameter, that hypersonic aircraft will have to be huge, bigger than a concord I imagine, and it will have about 3 minutes to open its cargo doors into space, release the hab to hook up with the skyhook, close its doors again and then reenter the atmosphere, if it does not succeed, it will have to try again the next time the sky hook is heading its way.

Terraformer · 2013-11-25 07:28:27

You don't need to launch more than a few Mars habs. Colonists aren't going to be taking their habitat with them... once we've got the basic infrastructure set up, launches will be for stuff like people and computer chips. Nothing that's particularly bulky.

Tom Kalbfus · 2013-11-25 19:12:39

Oh, but that's the whole point of having a sky hook, so you can lift things that are too big for rockets. You will need a container for your cargo anyway. Obviously the hypersonic jet is not going to Mars, so something inside it will. I mean a sky hook is big, if your going to build it, you should design it to lift the heavy stuff, so you will probably want to lift 50 colonists at a time, and those colonists will need something to live in on their journey to Mars. Unlike a launch loop, a sky hook presents limited opportunities to lift something into space, the advantage is you don't need to climb any cables, it takes just minutes to fling something into space, with a space elevator it could take days to climb to the top!

JoshNH4H · 2013-11-25 22:53:49

I don't see why a skyhook has an inherently larger lift capacity than any other way of getting to orbit. A skyhook could be built with pretty much any cargo capability.

Tom Kalbfus · 2013-11-26 07:50:56

Lets just say there is only a limited number of skyhooks you can put into orbit at any one time, after a certain point they start getting in each others way, so I say the sky hooks we do put in orbit ought to be pretty hefty, so we have to think of what's a useful payload we'd want to take to Mars? I think a single hab at a time complete with everything the colonists need to survive the interplanetary journey and all the tools they'd need to survive on Mars, so I'd say the skyhook ought to be able to fling a complete and fully functional hab on its way toward Mars. If bigger skyhooks are build, smaller skyhooks need to be taken out to make room for them.

JoshNH4H · 2013-11-26 10:02:04

Actually, a skyhook can have an arbitrary size. It's limited primarily by what you consider to be an acceptable centripetal acceleration. If you're okay with 8g, and your skyhook is in LEO, and your launcher imparts no horizontal velocity beyond that provided by the Earth, it only needs to be 650 km in radius. You could fit quite a good number of them, in that case, especially if you throw them in polar orbits timed so that no skyhook crosses the pole at the same time as another skyhook.

If you're okay building rockets with 4 km/s (which as Terraformer and I detailed in this thread, enables easy reusability), your orbital velocity should be somewhat more than 2.5 km/s, but I'll use 2.5 to be conservative.

Anyway, in that case the skyhook radius is reduced to 315 km. If we up the delta-V to 5 km/s, it goes down to 200 km. Even in a plain old equatorial orbit, you could fit 200 of them. If each can handle a payload of 5 tonnes per, and each can accept a payload once per hour, that's a total mass of 24,000 tonnes per day.

Terraformer · 2013-11-26 11:11:47

I'm still not understanding why you want to keep launching habitats to Mars. Are you not planning on your Martian colony having any manufacturing ability? No reusable spacecraft shuttling between the two planets?

I really, really don't think we're going to be launching more than a few tonnes at a time, once we've got enough infrastructure in place that it can bootstrap. Large colonist launches excepted, of course. But I don't think we're ready to colonise anywhere if we'll need to import more than a few high tech items from Terra more than a few years down the line.

Void · 2013-11-26 11:24:19

You guys have started talking skyhooks. You seem to be talking about an above atmosphere skyhook process.

I have a question. If you are going to have a loop or elivator, or spinning tether (My preference) with a hook to grab hardware, why could you not also grab atmosphere?

When hooking hardware, I presume that the hardware and hook must have a point in time of virtual zero differential speed.

When grabbing atmosphere a small amount of differential speed could be tollerated.

In order to keep your grabber from burning up, you either do not dip too deep into the atmosphere, or you maintain a small enough differential speed by spinning the grabber appropriatly.
What good is grabbed atmosphere? Oxygen, Nitrogen, maybe some Argon, and other trace gasses? To be used in chemestry, or life support, or propulsion, Chemical propulsion or condensed to liquid or ice and expelled with a linear magnetic device.

Maybe you will show me that this is not a good application, but I am inclined to think that it could be quite useful. Sorry for the diversion from the hardware hook though.

Last edited by Void (2013-11-26 11:26:09)

JoshNH4H · 2013-11-26 12:55:45

Not at all; In fact I would quite agree that atmosphere grabbing would be useful. I just don't know what the volume required would be. I suppose if you could condense the oxygen to its liquid form it would be extremely useful for Earth orbiting depots. Since Hydrogen is about 15% of the mass of H2/LOX fuel, even if you're only refueling with LOX in LEO that's a great savings indeed.

Good idea with the linear impellers, too. I' also suggest a multi-armed "starfish" design for the skyhook, so that you can take payloads more often.

Tom Kalbfus · 2013-11-26 20:47:46

There are other places you can get oxygen and nitrogen other than in Earth's atmosphere. Venus for example has more nitrogen in it's atmosphere than Earth does, a full 3% of its 92 bar atmosphere. Venus is a good source of oxygen too. If one is going to build a habitat in Venus orbit, then Venus' atmosphere would be a good source for those two gases, the oxygen will have to be separated from the carbon dioxide of course, then you get the carbon too, carbon is a good construction material, one can make carbon nanotubes out of it and very large rotating cylinders, the only thing Venus lacks is water, but one can make water with hydrogen and oxygen, with hydrogen being the lightest gas, a little of it can go a long way, a kilogram of hydrogen makes 5 kilograms of water if you have the Venusian oxygen. Venus has just about every other element you could want for a space colony. What if a skyhook were to reach all the way down to the bottom of Venus' atmosphere and scoop up some regolith and rock, flinging it into space for collection by the space colonists? The skyhook would just dip into the atmosphere briefly so it would have plenty of time to cool afterwards. The Venusian regolith could then be processed for useful minerals.

Last edited by Tom Kalbfus (2013-11-26 20:49:46)

JoshNH4H · 2013-11-26 23:03:46

Tom Kalbfus wrote:

There are other places you can get oxygen and nitrogen other than in Earth's atmosphere. Venus for example has more nitrogen in it's atmosphere than Earth does, a full 3% of its 92 bar atmosphere. Venus is a good source of oxygen too. If one is going to build a habitat in Venus orbit, then Venus' atmosphere would be a good source for those two gases, the oxygen will have to be separated from the carbon dioxide of course, then you get the carbon too, carbon is a good construction material, one can make carbon nanotubes out of it and very large rotating cylinders, the only thing Venus lacks is water, but one can make water with hydrogen and oxygen, with hydrogen being the lightest gas, a little of it can go a long way, a kilogram of hydrogen makes 5 kilograms of water if you have the Venusian oxygen. Venus has just about every other element you could want for a space colony. What if a skyhook were to reach all the way down to the bottom of Venus' atmosphere and scoop up some regolith and rock, flinging it into space for collection by the space colonists? The skyhook would just dip into the atmosphere briefly so it would have plenty of time to cool afterwards. The Venusian regolith could then be processed for useful minerals.

While everything you've said here is true, it will also not be relevant to space exploration for several decades. In the near term, it's going to be very important to set up a fueling infrastructure in the near-Earth space. When people talk about in-space fuel production, they're mostly talking about the ice deposits that have been discovered at the lunar poles. But I really like the idea of getting the Oxygen from Earth's atmosphere.

In order to keep the skyhook in orbit, it needs to burn some kind of fuel. But the reaction mass to do this can actually come from the cargo gathered.

The Earth's atmosphere is 21% oxygen. If we use a skyhook whose orbit is primarily at 300 km but dips down to 150 km, it will have a total diameter of 300 km. The bottom must have a velocity of 7.8 km/s relative to the center of mass. This means that the remaining 79% of the atmosphere needs to be ejected from the opposite end at 10 km/s. This is high, but not unbelievably so. Some variation on VASIMR could do it, if you've got the power to handle it.

I think the skyhook principle works better for payloads that can meet it with zero relative velocity, but it's still an interesting concept.

Tom Kalbfus · 2013-11-27 13:48:15

Its not exactly clear what the "near term" is, I don't think a Skyhook will be built next year or in the coming decade, I know there are plans for the SLS, which is basically a 21st century version of the Saturn V rocket. If we go to Mars in the near term, it will probably at first be in a vehicle like that, but after that, when we seriously consider colonizing the planets, it may make sense to build a skyhook to facilitate mass colonization or the planets. I think colonizing Venus will at first involve building orbital habitats around Venus to take advantage of the greater abundance of solar energy there.

In any event, don't all sky hooks fling objects at speeds greater that the escape velocity? If you had one that was orbiting at 8 km per second and it rotated so that when it touched the ground, the bottom part is moving at 0 km per second horizontally, the top portion would be moving forward at 16 km per second, which if I'm not mistaken exceeds the Earth's escape velocity of 11 km per second. So anything flung off from the surface of the Earth would go into orbit around the Sun. in order for that not to happen, you'd need a suborbiter traveling at just over 5 km per second, the sky hook would then fling it to just under 11 km per second so it would end up in a highly elliptical orbit around the Earth, so it seems to me flinging entire habs to Mars would make sense, rather than flinging parts of habs and assembling them in Earth orbit.

JoshNH4H · 2013-11-27 23:56:01

What's clear to me is that we'll be playing around with fuel depots in LEO and tethers long before we're doing anything of note on Venus.

By the way, if you climb the tether to its center you'll be moving at exactly orbital velocity.

Tom Kalbfus · 2013-12-01 19:16:12

True, but a tether spins faster than you can climb to the center, there is value in the speed at which it flings stuff into space, a space elevator by contrast requires a vehicle to climb up to the top and then be released, this takes time and there is a limit to how much payload it can handle, unless you can get a vertical maglev elevator.

JoshNH4H · 2013-12-02 00:05:54

Absolutely agreed. Of course there's always a limit, but sykhooks are something to be considered. In terms of technological and monetary investment, I would say they fall somewhere between SSTO and Space Elevator, with the payoff in terms of reduced per-launch payload also falling somewhere in that range.

Spaniard · 2013-12-02 09:40:41

Any planetary ring is huge. But if this is forgotten, it's more easy to build a simple circular ring aroung Earth, at LEO, but rotating synchronous with planet. Of course that's suborbital, but a ring is solid and it wouldn't fall. And with simple buildings reach this place. The need of strong materials are lower than for a classic space elevator.
And, because the less height, the building could be slightly conical instead of cylindrical to reduce the pressure.

Last edited by Spaniard (2013-12-02 09:41:19)

JoshNH4H · 2013-12-02 09:42:41

What's your reasoning to say that its easier to build a non-orbiting ring?

I would imagine that the requirements in terms of compressive strength would be very high, to the point where buckling is a severe hazard.

Tom Kalbfus · 2013-12-02 14:42:54

I think he means the outer skin is not orbiting, it is fixed in relation to the Earth, on the inside is a maglev track, the outer skin is actually a maglev car that is counter-rotated against the orbital, or superorbital motion of the maglev track. The excess centrifugal force of the track in addition to its orbital velocity is enough to support the non-orbiting skin of this track. One can hang down cables from the outer skin of this track, or one can move the ring off center so that part of it rests on the surface of the Earth. One way to do this might be to lower cables from the outerskin of the track and then reel one end in so that it comes in contact with some high point on Earth's surface, and one simply places an outer maglev track on the outer skin to accelerate payloads to orbital velocity, this is another version of my Launch Loop idea. If the track does not stretch, that is if it moves at a constant velocity, then there will be an excess of centrifugal force at the higher altitude, perhaps the tensile strength of the track itself can take care of some of this. I think if the loop is backed up by carbon nanotube ribbon, that might be sufficient to hold the track together.

Tom Kalbfus · 2013-12-02 14:47:09

JoshNH4H wrote:

What's clear to me is that we'll be playing around with fuel depots in LEO and tethers long before we're doing anything of note on Venus.
By the way, if you climb the tether to its center you'll be moving at exactly orbital velocity.

I think if private individuals can get into space and can finance the construction of space colonies, some might find reason to locate in orbit around Venus, perhaps to establish their rights as "Venusians" so they or their descendants can lay claim to the planet below at some point in the future. It is after all the second largest terrestrial planet in the Solar System, and that's got to be worth something.

JoshNH4H · 2013-12-02 22:07:41

Actually it sounded like he was describing a ring that simply supported its own weight. What you're describing is of course possible, and actually sounds like we're getting back to your original premise for this thread.

I decided to try to calculate just how strong a material would have to be to make a stationary orbiting ring a reality. To do so, I assumed that the ring would reach equilibrium when the energy required to compress the material becomes greater than the energy gained from it falling deeper into Earth's gravitational field.

The energy required to compress the material is:

Ec=(.5 E ε^2) * (A*L)

Where Ec is the energy required to compress, E is the Young's Modulus (200 GPa for Steel), ε is the Strain, ρ is the density (7,800 kg/m^3 for Steel), A is the cross-sectional area, and L is the total length around the circumference. Using similar variables, the energy from gravity is:

Eg=(ρ*A*L)*g*ε/pi

Where g is Earth's gravity (Assumed constant). Setting the derivative of these two equations with respect to ε equal to each other, and recalling that the compressive stress S is equal to Eε, we find:

S=ρ*g*L/pi

For a height 200 km above the Earth, where g is 9.2 m/s^2, the compressive force would be almost 1 TPa (That's 1e12 Pascals!). I know of no material with compressive strengths that even approach this value. For Steel you're looking for something more in the region of 500-1000 MPa, which is 1000 times too low. If you use Silicon Cabide (density of 3100, compressive strength of 3900 MPa) the compressive strength is only exceeded by a factor of 100! The point being that for a non-orbiting ring you need supermaterials every bit as much as you do for space elevator.

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#26 2013-11-24 17:28:18

Re: Elastic Launch Loop (Space Elevator)

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#29 2013-11-24 19:21:53

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