h a l f b a k e r yI think this would be a great thing to not do.
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How about reducing the acceleration necessary to get up out of the gravity well by accelerating more gently over a longer distance? With a Gauss gun 1,600 km long you only need an acceleration of 2g (2.24g when you add gravity at right angles to it) to get up to an 8km/s low Earth orbital velocity.
The
Gauss gun doesn't have to be absolutely straight - in fact, if it follows the curve of the Earth (preferably around the equator or thereabouts), by the end of the acceleration, your weight just generates your centripetal acceleration (surprise surprise). At the low speed end, it can curve a lot more than that, if geography makes that more convenient.
The high speed end wants to be as high in the atmosphere as you can get it - climbing up a very shallow ramp reaching maybe 12km above sea level. Supported on those infamous tethered balloons (and again, with balloons here and there part way up the tethers to support the weight of the tethers...)
If you can tolerate 3g for long enough, you could cut the length of the gun down to 1100km, and so on.
Okay, this is a high capital cost project. You'd only do it if you're really serious about putting a lot of people into space. But at least it doesn't use any fairytale technology, like fibres with a strength:weight ratio beyond the capabibilities of interatomic bonds.
It might well want a fairytale quantity of helium. I think we might have to use hydrogen - not as inherently dangerous as some folks think, but definitely something to be careful with.
You'd presumably have much shorter ones for putting up most of the hardware. You might start with those, and work up.
Superspeed spacecraft
Superspeed_20spacecraft
lots of interesting annos [xaviergisz, Apr 16 2007]
[link]
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Thanks for the link, [xaviergisz] - interesting. |
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No need for the ramp at the end. You'll burn up before you get to it. |
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Can one adjust the speed imparted to a projectile by a railgun? I am suspicious. |
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By the way, [Cosh], it looks like the railgun ideas are grouped in the electromagnetic weapon category, so evern though this is not a weapon, you might as well relocate this idea to be with its more martial brothers. |
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Maintaining the necessary structural integrity and alignment on a 1600 km (even 1100km) tube is a nontrivial problem. Simple temperature change, tidal effects, etc. are all going to affect this. I shudder to think what the result of wind shear would be on the floating end of this (which is the most original part of the idea). Even building it up a mountain, which is the most common approach in science fiction and in feasibility studies is not a simple civil engineering problem. And most of those are considering much shorter 10g catapults. |
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How fast can you actually go without
burning up because of air friction? (Bun for
sci-fi coolness.) |
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Sorry - didn't mention that the idea is that this Gauss gun is supposed to be enclosed, and pumped down to a reasonable vacuum. Not sure how to replace the plastic bag over the end really quickly after each vehicle bursts through it, or whether simply to let a continuous stream of vehicle act as pistons pumping it out all the time. |
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Hitting air at the pressure at 12km up at 8km/s isn't going to present insuperable engineering problems, especially if you're going up at a reasonable angle so you'll be out of atmosphere reasonably quickly. It's considerably less thermal stress than experienced on re-entry with current systems. |
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Of course you then need rockets to change your orbit to one that doesn't intersect the Earth's surface again - but very little deltaV would be required compared with a full rocket fired ground launch. |
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Oh, and [MechE] - I know. It's not just the length, it's the velocity of the vehicle. You can't let the track make the vehicle deviate from a smooth curve of huge radius, or you get enormous lateral forces on both track and vehicle. And you have to have an active magnetic levitation track for a vehicle going at that speed, with some pretty stunning quick reacting electromagnets - you certainly can't use wheels... |
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//you certainly can't use wheels//
ummm.... maybe you could, at least for some of the time you're at the bottom of the atmosphere and subject to turbulance...for another idea I calculated that a 10' diameter wheel would have to spin at 15k rpm to reach 5,000 miles per hour. For existing comparison, the Thrust SSC (Land Speed Record holder) which broke the sound barrier, has tyres 4' in diameter. Not sure how far along the track you'd have to be, but you could use wheels for a fair distance to minimize stability problems. |
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I had a suggestion for this idea on its own page, but I'll eliminate that and just summarize the point here since the main idea is already under discussion. |
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We can accelerate the payload in an evacuated tube, mostly on or under the ground, then the last 100km or so starts pointing up and exits a mountainside. There are 4km mountains in southern CO with a good 1500+km of land to the west that you could use for the main tube length; if you go outside the US one could get higher, topping out at 8.8km for Everest. I'm not sure if tethered balloons can provide enough stability, but you can always increase the final altitute by at least a few hundred meters with a simple set of towers. |
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Now the trick is getting the craft from the evacuated tube into the air, without having air rush into the tube again, or damaging the craft. My idea on this is to use a sheet of plastic reinforced by thin wires that are laced into it and suspended from a ring or posts outside of the end of the tube. The wires can be nanotubes if available, Kevlar otherwise. These are very strong in tension, so a fairly small amount of material is enough to hold back the fraction of atmospheric pressure involved. When the pointy-nosed craft hits this plastic surface, it punches right through it, and never hits anything hard. Then a replacement covering sheet is slid over the tube opening, and whatever air got in for a few seconds is removed by continuously-operating pumps. |
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Numerical support for this idea: if the opening is about 20m2, the total pressure on it is 20*(.5)*100,000 Pa, = 10^6Pa; if nanotubes can carry a load of 5*10^10Pa/m2, then you'd need 1/5 cm2 of them; say, 2000 wires of 1/10mm2 cross-section, volume 200cm3, weighing 300g. For Kevlar, multiply this by 10 or so, but that's still not much. A smooth 20 ton metal wedge going at 9km/s may just slice through it like butter. |
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/the last 100km or so starts pointing up/
/A smooth 20 ton metal wedge going at 9km/s/ |
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it is going to be hard to get something like this to change direction. |
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Two words: "ISS" and "ricochet". |
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this is either a really deep hole, a space needle or a elbow at the end that has G's in excess of what you are discribing. I had a gauss gun idea myself that got erased for some reason. also you have to have rocket to insert into orbit otherwise the G's go way up to the requirement of excape velocity. because any orbit starting in the atmosphere will otherwise intercect the atmospere again. |
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People can be helped to endure 100Gs |
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[scottinmn] //the total pressure on it is 20*(.5)*100,000 Pa, = 10^6Pa// sp. total force ... 10^6N. You have calculated the force normal to the sheet; the tension experienced by a non-rigid sheet itself will be greater than that, and will depend on its curvature (rising to infinity for a flat sheet). |
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[Mercury] It's been a while! Cosh i Pi also mentioned the need to change the orbit. //100Gs// Are you thinking submersion in a breathable liquid? |
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it appears you remember my erased idea post. Too bad I can't remember why. Even if we can't get people up there this way it is a great idea perhaps a necessary step for building a needle. I think I decided that we would engineer the liquid based on blood/hemoglobin. Maybe it was the magic handwaving. perhaps it was baked or already thought of instead of presented as a means of making the rail/gauss gun workable for people. |
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