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#12
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Could a bullet be made something that could go from orbit to Earth's surface?
"Alain Fournier" wrote:
Scott T. Jensen wrote: "Doug..." wrote: Since you're adding energy to the bullet's orbit perpendicular to its orbital direction, you're not going to raise or lower its *overall* altitude. Oh, I see. Hmmm. What if it was fired from a geo-stationary (fixed over a location on Earth) orbit straight down at the Earth? It would enter an elliptical orbit. The bullet would never get close to Earth. Please elaborate. I don't see why it wouldn't hit the Earth. Scott Jensen -- Got a business question, problem, or dream? Discuss it with the professionals that hang out at... misc.business.consulting, misc.business.marketing.moderated misc.business.moderated, and misc.entrepreneurs.moderated |
#13
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Could a bullet be made something that could go from orbit to Earth's surface?
Scott T. Jensen wrote:
"Alain Fournier" wrote: Scott T. Jensen wrote: "Doug..." wrote: Since you're adding energy to the bullet's orbit perpendicular to its orbital direction, you're not going to raise or lower its *overall* altitude. Oh, I see. Hmmm. What if it was fired from a geo-stationary (fixed over a location on Earth) orbit straight down at the Earth? It would enter an elliptical orbit. The bullet would never get close to Earth. Please elaborate. I don't see why it wouldn't hit the Earth. It is moving around the Earth at a speed of a little over 3 km/s (it goes around Earth once every 24 h at a distance from the center of the Earth of about 42 164 km). Most riffles have their bullets exit at less than 1 km/s. So no matter what direction you shoot the bullet still has a substantial part of its original velocity. Enough to stay in orbit. Don't let yourself be fooled by the fact that a geo-stationary satellite stays always above the same point of Earth, the satellite is still moving around Earth at high speed. As the bullet goes down towards Earth its angular velocity will go up, because with the same speed you can go around Earth in less time when you are closer to Earth and because of the way orbital mechanics works, when you fall towards Earth you gain some speed. So even though at first you stay above the same point over Earth while you go down you will start moving east. Alain Fournier |
#14
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Could a bullet be made something that could go from orbit to Earth's surface?
"Alain Fournier" wrote:
Scott T. Jensen wrote: "Alain Fournier" wrote: Scott T. Jensen wrote: "Doug..." wrote: Since you're adding energy to the bullet's orbit perpendicular to its orbital direction, you're not going to raise or lower its *overall* altitude. Oh, I see. Hmmm. What if it was fired from a geo-stationary (fixed over a location on Earth) orbit straight down at the Earth? It would enter an elliptical orbit. The bullet would never get close to Earth. Please elaborate. I don't see why it wouldn't hit the Earth. It is moving around the Earth at a speed of a little over 3 km/s (it goes around Earth once every 24 h at a distance from the center of the Earth of about 42 164 km). Most riffles have their bullets exit at less than 1 km/s. So no matter what direction you shoot the bullet still has a substantial part of its original velocity. Enough to stay in orbit. Don't let yourself be fooled by the fact that a geo-stationary satellite stays always above the same point of Earth, the satellite is still moving around Earth at high speed. As the bullet goes down towards Earth its angular velocity will go up, because with the same speed you can go around Earth in less time when you are closer to Earth and because of the way orbital mechanics works, when you fall towards Earth you gain some speed. So even though at first you stay above the same point over Earth while you go down you will start moving east. Please bear with me on this. I'm trying to understand the mechanics of what you're saying. I naturally agree that even at geo-stationary orbit, the rifle would also be orbiting at a great speed. However... The bullet is fired directly down at the Earth. Rounding things to make it easier, let's say it is traveling at 1 km/s, from geo it is 42,000 km from Earth's center, Earth's radius being 6,000 km (yes, I know I just shrunk the Earth but I'm going for simplicity here so bear with me), and thus the bullet has to travel 36,000 km to hit the equator. That means it will take the 1 km/s bullet 36 seconds to strike IF it can go down. There is no real drag on the bullet until the last hundred miles. There will be some gravitational pull but that's pulling it down as well. I do not see how it would then go into orbit and not enter our air and, saying it is made of the right stuff, hit the surface. Scott Jensen -- Got a business question, problem, or dream? Discuss it with the professionals that hang out at... misc.business.consulting, misc.business.marketing.moderated misc.business.moderated, and misc.entrepreneurs.moderated |
#15
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Could a bullet be made something that could go from orbit to Earth's surface?
"Scott T. Jensen" wrote:
[...] bullet has to travel 36,000 km to hit the equator. That means it will take the 1 km/s bullet 36 seconds to strike IF it can go down. hmmm, 36000/1 = 36? Mayhap we need to track a decimal point a wee bit better.... /dps |
#16
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Could a bullet be made something that could go from orbit to Earth's surface?
"dave schneider" wrote:
"Scott T. Jensen" wrote: [...] bullet has to travel 36,000 km to hit the equator. That means it will take the 1 km/s bullet 36 seconds to strike IF it can go down. hmmm, 36000/1 = 36? Mayhap we need to track a decimal point a wee bit better.... Oops! *laugh* Yes. Sorry about that. Scott Jensen -- Like a cure for A.I.D.S., Alzheimer, Parkinson, & Mad Cow Disease? Volunteer your computer for folding-protein research for when it's idle. Go to http://www.distributedfolding.org/ to sign up your computer. |
#17
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Could a bullet be made something that could go from orbit to Earth's surface?
Scott T. Jensen wrote
Please bear with me on this. I'm trying to understand the mechanics of what you're saying. Don't try to understand orbital mechanics using the normal concept of speed and distance, like you know them from everyday travelling situations. Orbital mechanics are often counter-intuitive in this regard. If you want something in circular orbit around earth to enter the atmosphere, you must change its velocity so that it enters a elliptical orbit where the lowest point is inside the atmosphere, say at 100km just to keep the numbers simple. The most effecient way of doing this (i.e. requiring lowest change of speed) is to "break" by altering the velocity directly opposite the current velocity wrt to earth. If you are in a normal prograde equatorial orbit the orbital velocity vector is pointing east and you therefore need to change the velocity towards west, which means you want to shoot your rifle towards west if you want the bullet to go low. However, beside direction, your bullet also need a certain minimum speed in order to be able to reach the atmosphere. The higher the orbit the more speed is required. For instance, at 300km altitude the bullet only needs around 60 m/s, but at 36000 km altitude (GEO) the required speed is around 1500 m/s. So depending on the gun, it should be able to re-enter a bullet at least up to some fairly high altitude, but probably not so high as geostationary altitude. The math is as follows: If you are in a cirular orbit at altitude h, you are r = h + R from the center of earth (with R = 6378 km) and you are moving with respect to the center of earth at a speed of v = sqrt(k/r), where k = 398601 km^3/s^2 is the gravitational constant for earth (also called GM). Now you want to change your orbit into an elliptical one with the highest point at radius r, and the lowest at s = 100km + R. In such an orbit you would at the highest point need an orbital speed of w = sqrt(k*(2/r-1/s)) which means that you must change your speed with dv = v - w in order to change from the circular orbit to the elliptical orbit. If you use the numbers as I have written them above, then the speed unit will be km/s. Regards, -- Filip Larsen |
#18
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Could a bullet be made something that could go from orbit to Earth's surface?
"Scott T. Jensen" wrote in message
... The bullet is fired directly down at the Earth. Rounding things to make it easier, let's say it is traveling at 1 km/s, from geo it is 42,000 km from Earth's center, Earth's radius being 6,000 km (yes, I know I just shrunk the Earth but I'm going for simplicity here so bear with me), and thus the bullet has to travel 36,000 km to hit the equator. That means it will take the 1 km/s bullet 36 seconds to strike IF it can go down. There is no real drag on the bullet until the last hundred miles. There will be some gravitational pull but that's pulling it down as well. I do not see how it would then go into orbit and not enter our air and, saying it is made of the right stuff, hit the surface. Keep in mind it's still moving "sideways" with it's original orbital velocity. So, 1,000 seconds later it's also moved eastward several thousand km. And as a result it "misses" the Earth. It starts moving "up" again until it reaches its perigee and then starts falling "down" again. Of course it's moving "eastward" the entire time so it keeps missing the Earth. Scott Jensen -- Got a business question, problem, or dream? Discuss it with the professionals that hang out at... misc.business.consulting, misc.business.marketing.moderated misc.business.moderated, and misc.entrepreneurs.moderated |
#19
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Could a bullet be made something that could go from orbit to Earth's surface?
Filip,
Thanks for the explanation. I think I'm getting it now. I want to slow down the bullet's speed and the way to do that is fire it backwards. The lower the orbit, the slower this backfire needs to be. Have I've gotten it yet? Scott Jensen -- Got a business question, problem, or dream? Discuss it with the professionals that hang out at... misc.business.consulting, misc.business.marketing.moderated misc.business.moderated, and misc.entrepreneurs.moderated |
#20
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Could a bullet be made something that could go from orbit to Earth's surface?
Scott T. Jensen wrote:
Filip, Thanks for the explanation. I think I'm getting it now. I want to slow down the bullet's speed and the way to do that is fire it backwards. The lower the orbit, the slower this backfire needs to be. Have I've gotten it yet? Scott Jensen You are getting close. You are right that the best way to de-orbit is to fire backwards. But you are wrong about the lower orbit needing slower backfire. The lower the orbit the faster the orbit, so you need a bigger backfire. This is offset by the fact that if you are in a low orbit you only need to get the orbit a little elongated for it to intersect Earth's atmosphere. So in a very low orbit you need only a small push to de-orbit, in a very high orbit you also need only a small push to de-orbit (but this time it has to be more precise if the push is too strong you will end up orbiting in the other direction or even escaping, if the push is too weak you will will stay in orbit just change the shape of the orbit). It is if you are somewhere between very low orbit and very high orbit that you need the biggest push to re-enter. Alain Fournier |
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