Thread: Speed in space
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Old September 19 2012, 12:43 AM   #10
Christopher
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Re: Speed in space

smiki wrote: View Post
Anyway, when it comes to laser/particle-beam sail technology, there's a significant problem with one thing. If it is to be used as a viable method of transportation from point A to B, you're gonna have to have the same kind of technology at your destination, as a means of decelerating from the speeds you reach, otherwise you'd just zoom pass the destination and keep on going...
Not really. For lasers, Robert L. Forward proposed a compound sail that could split apart so that the outer portion could then reflect the laser backward onto the inner portion and decelerate it; see his paper here. And for magnetic sails, remember what I said about a ramjet's magnetic field creating drag against the interstellar medium? A magsail ship would have a built-in braking system, and could use the destination star's magnetosphere to brake against.


MadaBidyoni wrote: View Post
A.
ok so not near to light speed & not 90%.
70-80%
& lets say we will find a way to do it without a fuel tank & get the energy from outside in some way...
will we get to... say... 70%?
Depends on what technology you use and whether it's a crewed craft or a robot probe. Most cutting-edge propulsion proposals are looking at 10-30% of lightspeed as an optimistic goal. However, a particle beam could potentially accelerate a lightweight sail probe to a much higher percentage of c.

I recommend you track down the book Centauri Dreams by Paul Gilster, or do an archive crawl on his blog of the same name. They have material on advanced starship design concepts.


B. lets make it simpler. why a comet can't get to 70%? it just accelerating & accelerating (no friction)...
Comets are not under thrust. They're big chunks of rock and ice orbiting the Sun on eccentric orbits. When they come close to the Sun, the heat evaporates their volatiles, which are blown outward in a tail by the solar wind and radiation pressure. This leads a lot of people to the false conclusion that comets are some kind of fireballs roaring through space like rockets and leaving fiery exhaust trails behind them, but that's about as close to reality as the Moon being made of green cheese.

Also, as I said, the amount of fuel involved is prohibitive. If you did mount an engine on a comet, feed in all its ice, and blast it out the back of the engine -- heck, even if you extracted the hydrogen and used it to fuel a fusion rocket -- you'd run out of fuel long, long before you reached any significant fraction of c. Again, it's very difficult for the human mind to comprehend just what a huge velocity the speed of light is. It's fast enough to cross the North American continent in less than the blink of an eye, or to go around the entire Earth eight times in one second.



The fastest speed a human-built spacecraft has ever achieved is about 70 kilometers per second, which is a measly fortieth of a percent of lightspeed. And that was a probe launched inward toward the Sun, so it got added acceleration from the Sun's gravity.
oh... wasn't voyager the fastest?
Apparently not. Here's a Centauri Dreams post about it.
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