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#11
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As most respondents have pointed out, something in orbit has a huge
amount of kinetic energy that has to go *somewhere*. Either you spend that much energy slowing down with rockets, or you dump it into the atmosphere as heat. There is another possibility that hasn't been mentioned, though: you could transfer that energy to something else in orbit. I'm thinking of a momentum-exchange tether (http://www.tethers.com/MXTethers.html), probably of the rotating variety. Here's how it would work: Space travellers in orbit zip around at, say, 17000 kph. Also in orbit is a large mass connected to a very long, strong tether, rotating something like a wheel as it orbits, so that the high end (away from the Earth) is moving much faster relative to ground than the low end (closer to the Earth). At the high end, the tip of the tether, is travelling at 17000 kph, but at the low end its ground-relative velocity is only (say) 10000 kph. So, our space travellers wait for the tether to be in the right position at the high end, when it's travelling at the same speed they are, and then hook their craft to it. It swings them down, and they unhook at the low end. Presto, they're now travelling at only 10000 kph -- which is less than orbital velocity at that altitude, so they're going down, but they're doing it much more gently. Even larger or stronger (more rapidly rotating) versions of the tether could of course have even more benefit -- even dropping the ship down stationary with respect to the Earth. Where does all that kinetic energy go? Into the tether system, of course. It rises up to a slightly higher orbit. How much its orbit changes depends on the mass of the tether system compared to the ship; ideally it would mass a lot more, so its orbit wouldn't change much. But here's the really cool part: the tether system acts as a "momentum bank". That energy imparted to it from the ship can be used again to haul the next ship up to orbit. On launch, the ship only has to attain the speed of the slow end of the tether, i.e. 10000 kph in my example above. Then the tether imparts the rest of the energy needed to fling it up into orbit. Its own orbit is reduced as a result, of course, but it gets some of that back when it drops the ship back down. What energy is lost to atmospheric drag, mass ejected from the ship, etc. can be made up for in more leisurely (and efficient) ways, such as electrodynamic propulsion. So, a rotating tether helps with the two biggest problems we have today: getting to orbit, and getting back down. A spacecraft that by itself is only capable of suborbital launch and reentry, can nonetheless reach and leave orbit safely with the help of the tether system, and we're no longer wasting huge amounts of energy both ways -- much of it is simply being banked and reused instead. Best, - Joe ,------------------------------------------------------------------. | Joseph J. Strout Check out the Mac Web Directory: | | http://www.macwebdir.com | `------------------------------------------------------------------' |
#12
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Nobody has addressed what seems to me an obvious solution:
- While orbiting at 17000mph or so to dip down into the upper atmosphere - the very edge -then deploy a large parachute similar to the modern sport chutes that forms an airfoil. These things generate lift, just like any other airfoil. It seems to me that an inclined plane should still generate lift, even when the medium is a very thin gas - or even independant molecules. Then just skip along in the upper atmosphere for a long time (maybe as much as a full "orbit" changing angle of attack - slowly slowing and dropping into thicker air enough to balance temperatures and lift. If things get too hot, set the chute to a low drag configuration, and use the lift to lift up for a bit, then drop back down when you can. You still have a lot of kinetic energy to dissipate, but the grossly increased time will allow it to be lost by several mechanisms to the atmosphere and to radiation. This may not be a solution for the huge mass of the shuttle, but a future "X-Prize" orbiter that weighed only a few thousand pounds could use it. |
#13
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OK if the shuttle is going the same orbital velocity required to get at into
orbital velocity. Then cannot it be slowed down while in orbit where you would not need a massive amount of energy to slow it down from 22700mph if done over a few days which I take it is its approximate speed while in orbit Could this work? And if it was slowed down could they not use parachutes to keep it from reentry problems wrote in message oups.com... Someone please tell me why spacecraft are designed to reenter the earth's atmosphere at high speed. The answer is really quite simple when you think about it. Slowing down from orbital velocity requires exactly the same change in speed as attaining orbital velocity. It is entirely possible to slow down with rockets instead of air resistance, but the ISP of those rockets would have to be basically the same as is required to get into orbit. You know the Space Shuttle, with that large tank of fuel and those two huge boosters? All the power from those boosters and that fuel is used to accelerate the shuttle to orbital velocity. Sure, it's possible to slow the shuttle down a lot so that it would enter the atmosphere at a leisurely 200kts, but doing that would require the same power as is required to get it into orbit in the first place. So basically we're talking about having the shuttle in orbit with a large, *full* external tank at least. Getting the shuttle into orbit with a large, full external tank would require three times the amount of thrust required to put the bare shuttle into orbit. So just imagine the shuttle sitting on the launch pad with not one but three external tanks, and six external boosters. That's on the order of magnitude of what would be required to get it into orbit with the fuel to brake out of orbit. That's a larger stack than anything that anyone has ever launched. That's much larger than the Saturn V or the Russian Energia. It's much too large to be practical. And of course there are other considerations, like keeping all that fuel cooled for the duration of the mission. It's really just not a workable idea. Has anyone modeled the idea of unfolding some large wings to add a lot of surface area This is similar to the idea of a ballute. http://en.wikipedia.org/wiki/Ballute It's certainly helpful, but for a full reentry in less than one orbit you still need a heat shield. Slowing down more gently in the very high atmosphere, as you're suggesting, results in a ballistic trajectory that brings you down into the lower atmosphere before you can bleed off enough speed to no longer need the heat shield. Another idea, that I don't know enough about to speak to, is to drop down into the atmosphere and then pitch up so that you fly out of the atmosphere like a rock skipping on a pond. You're still on a sub orbital trajectory though, you don't fly off into space, you come back down into the atmosphere and repeat the process. This idea was employed by the X-20 Dyna-Soar. |
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#15
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Icarus wrote: How much could the Shuttle decelerate with its own engines, given an unlimited supply of fuel, before it encountered significant atmospheric drag? I was thinking that if they built a (relatively) cheap and cheerful rocket for the sole purpose of getting rocket fuel into orbit, the Shuttle could use that for its descent, and the same orbiting refueling stations could also be used for spacecraft leaving Earth orbit, making (say) missions to other planets cheaper and quicker. Let's look at it this way. Suppose that the space shuttle could just "magically" lose all of its orbital speed in an instant. The pilot throws a switch and, presto, the spacecraft no longer has any horizontal speed. What happens next? What does happen is that the only component of motion remaining is the ship's downward acceleration towards Earth; It falls like a rock. That's no good, because this alone will also subject the ship to a significant air speed with high temperature. (Not as high as with the re-entry trajectories that are actually used. But high enough to be a problem still.) So you see it's not strictly a problem of having enough fuel to slow down. Slowing too rapidly can actually be counterproductive. The spacecraft needs a way to travel slowly both horizontally and vertically. It might be conceivable at least hypothetically to deliver a large tank of propellant to the space shuttle, which could then retrofire its motors to slow it waaayy down horozontally. But to keep it from falling too quickly vertically would require a high *upward* thrust for the entire trip down. There's just no way to do that. The mass ratio required would be impossibly high. Of course, as far as the Shuttle is concerned, the easiest solution would be just to make its heat shield 100% reliable... Probably the best thing would be to abandon the whole space shuttle paradigm of carrying both cargo & people all in one package. It'd be much safer, much more reliable, to send them up separately on conventional rockets. The Russians have been doing it this way for decades, and it works very well, and more cheaply. They built an entire space station (in fact, several space stations) by boosting the components and then rendesvousing crews with them later, in the Soyuz. The Soyuz is a very highly optimised configuration for a disposable orbital ferry. It's designed so that the ablative heat shield is a minimal size, to keep its mass down. It works nicely. I think that in 45 years there's never been a genuine failure of an ablative heat shield, either Russian or American. -Mark Martin |
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In article ,
Ron Webb wrote: - While orbiting at 17000mph or so to dip down into the upper atmosphere - the very edge -then deploy a large parachute similar to the modern sport chutes that forms an airfoil. It's been proposed, actually. These things generate lift, just like any other airfoil. It seems to me that an inclined plane should still generate lift, even when the medium is a very thin gas - or even independant molecules. Correct. The rules are somewhat different up in the region of molecular flow -- where the molecules are indeed pretty much independent -- and at hypersonic speeds, but lift is still available... at the usual price of drag. (See below.) Then just skip along in the upper atmosphere for a long time (maybe as much as a full "orbit" changing angle of attack - slowly slowing and dropping into thicker air enough to balance temperatures and lift. Alas, here you propose a numerical impossibility. *It can't be done.* When you buy a certain amount of lift, you pay with a certain amount of drag. And the L/D ratio of reasonable shapes is not that good at hypersonic speeds in molecular flow. If you're getting enough lift to hold you up, you are *not* decelerating very slowly and gradually. Oh, initially, yes, because at just below orbital speed you don't need much lift... but the situation doesn't stay that good for very long. At half orbital speed, "centrifugal lift" is only 1/4 as strong, and aerodynamic lift must do most of the work, and that means you're getting a *lot* of drag and slowing down rapidly. In fact, when you study the details, it turns out that the large surface area of something like a parafoil doesn't really make any difference to how *quick* reentry is. That is determined almost solely by the L/D ratio of the shape, and there are real limits to how good that can be. A large surface area does buy you something: you decelerate earlier, in thinner air, and the heat is spread out over a larger area. This lowers temperatures and makes materials problems much easier. But things still happen just about as quickly. -- "Think outside the box -- the box isn't our friend." | Henry Spencer -- George Herbert | |
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Anni wrote:
Then cannot it be slowed down while in orbit where you would not need a massive amount of energy to slow it down from 22700mph if done over a few days which I take it is its approximate speed while in orbit Could this work? No, it won't work, because "orbital velocity" is the velocity you need to stay in orbit. If you slow down, your orbital altitude drops. For a vehicle like the shuttle, that means the vehicle ends up in the atmosphere. And you still would need a lot of fuel to slow down a significant degree. Henry Spencer wrote: It wasn't quite that simple. The original ideas for metallic thermal protection were complicated, Hey, hey. No fair blocking my rosy-tinted 20/20 hindsight. They required very thin layers of exotic metals that were difficult to work with and tended to be brittle; Where's the requirement for "very thin" layers come from? Just weight requirements alone? Mike Miller, Materials Engineer |
#18
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Henry Spencer wrote:
In article .com, wrote: Another idea, that I don't know enough about to speak to, is to drop down into the atmosphere and then pitch up so that you fly out of the atmosphere like a rock skipping on a pond. You're still on a sub orbital trajectory though, you don't fly off into space, you come back down into the atmosphere and repeat the process. Skipping doesn't really help with the fundamental problem, that there isn't *enough* aerodynamic lift available to stay up in the thin air where To elaborate - aerodynamic lift doesn't work nearly as well at orbital speeds. Gliders can get lift/drag of 40. A reentry L/D of 5 is astoundingly good. And as lift on average has to be the same as the weight of the vehicle, then drag (which creates heat) has to be 1/5 of this value on average. This doesn't change the total heating if you go deep enough to accellerate upwards at 2G, so you can skip. deceleration is gradual. Sure, the heating is concentrated in brief periods, but so is the lift -- there is no net gain. There may be gain though. Some proposals have been to skip, and dissipate the heat over the whole vehicle while it's back in space. However, nobody has come up with numbers saying that this is astoundingly good. It turns out that shedding heat from a heatshield that's glowing white-hot is really efficient, and other schemes tend to get a bit messy as they have to absorb and store or radiate at low temperatures the heat. |
#19
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"Anni" wrote in message ... OK if the shuttle is going the same orbital velocity required to get at into orbital velocity. Then cannot it be slowed down while in orbit where you would not need a massive amount of energy to slow it down from 22700mph if done over a few days which I take it is its approximate speed while in orbit Could this work? No. It would take far too much fuel. Far more than could be carried aboard the shuttle. The shuttle may appear weightless because it is in freefall, but it still retains its 100 tons of mass. Momentum is mass times velocity and 100 tons of mass times 17,500 mph (or thereabouts) is a hell of a lot of momentum and would use an entire external tank's worth of fuel to counteract. And if it was slowed down could they not use parachutes to keep it from reentry problems? The problem here is that the parachutes would be useless until the shuttle was deep into the atmosphere. If the shuttle slowed down sufficiently it still drop like a rock and continue accelerating until it entered the atmosphere at excessively high speeds - far too high to deploy a parachute. Besides, try, for a moment, to imagine the stresses on parachutes trying to slow down a supersonic 100 ton space shuttle. Imagine the size and weight of the parachutes involved. |
#20
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Anni wrote:
OK if the shuttle is going the same orbital velocity required to get at into orbital velocity. Then cannot it be slowed down while in orbit where you would not need a massive amount of energy to slow it down from 22700mph if done over a few days which I take it is its approximate speed while in orbit Could this work? And if it was slowed down could they not use parachutes to keep it from reentry problems I'm having trouble reading that, but I think this is the core of your question: "while in orbit can it be slowed down over a few days?" The answer is no. This is one of those things that's very easy to explain with a diagram, but very difficult to explain with just words. Imagine a nice circular orbit. If you turn your spaceship so that it faces in the direction you're going, and you fire your engines, that's called a prograde burn. Prograde means, "in the direction you're going." If you turn your spaceship so that it faces opposite the direction you're going, and you fire your engines, that's called a retrograde burn. Once again, imagine that nice circular orbit. Pick a point on the circle. That's where your spaceship is. If you make prograde burn at that spot, do you know what happens to the orbit? Nothing at all happens at the spot where your spaceship is. Instead, the part of the orbit on the opposite side of the planet moves outward. If you make a retrograde burn, nothing at all happens to the spot in the orbit where you made that burn. Instead, the part of the orbit on the opposite side of the planet moves inward. This is rule number one of orbital maneuvering. When you make a prograde or a retrograde burn, nothing at all happens to your orbit at the spot where you made the burn. Instead, the other side of your orbit moves in or out. And that gets us to the reason why you can't slow down gradually over several days. Once you slow down enough that the opposite side of your orbit touches the atmosphere, you *will* hit the atmosphere in exactly one half of one orbit. At the altitude that the shuttle orbits, it only takes 90 minutes for a complete orbit, so once it slows enough to touch the atmosphere, it is coming home in 90/2 minutes. End of story. It doesn't have several days left to slow down gradually. Once it touches the atmosphere that's it. If you had a lot of thrust, you could make a very powerful retrograde burn and lower your speed as much as you want, all the way down to zero if you wanted to, but you have to do all of that in one half of one orbit. Like I said, this is very easy to understand if someone can draw it on a chalkboard. |
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