escape velocity

that is concert but you need a high speed if you want to put a satelite in an stable orbit m*g = m*v^2/R v = sqrt(g*R) = sqrt(M*G/R) or the satelite will come down again if you want to escape the earths orbit you need a speed so that the kinetic energie exceeds the potential energie of the gravityfield as it goes to infinity m*v^2/2 = int(R -> +oo, m*g(r)*dr) and g(r) = M*G/r^2 v = sqrt(2*M*G/R) another thing is that the explosion velocity of high energiefuel is large it is more efficient to go fast and in the end it comes all down to the dollar

The velocity needs to be relatively fast after takeoff. One way to think about it is this: the slower you travel, the longer your rocket is having to "fight" gravity. If you launch and travel straight up at, say, 100m/s, it would take over half an hour to reach even the modest altitude of 200km. Firing an engine with enough force to accelerate at

1g for 30 minutes would require exotic fuels (or nuclear reactions) we can only dream of. The rocket is MUCH more fuel efficient by travelling rapidly to orbit.

Also, most rockets don't reach escape velocity. However, you are correct in that rockets don't have to travel at escape velocity to leave earth. They could just slowly climb in altitude until they've reached an altitude where the current velocity exceeds what escape velocity would be at that point. But then again, you're using fuel very inefficiently.

As far as being safer, going slow really wouldn't be. What exactly do you think would be safer? Slow=more time for something to go wrong. Holding everything else constant, the combustion temperatures in a large rocket engine are the same as they are for a small rocket engine. Getting to orbit fast leave less time for something to go wrong (i.e. thermal failure).

Hope that helps, Dave

Dear ajay:

Correct. There are more-or-less serious discussions of a "space elevator" going on now.

Not necessarily. For example with an elevator, your structure and yourself spend a lot of time passing through an area of space at low speed, while debris passes at orbital speed. If it passes, great. But it may not.

David A. Smith

The way orbital spacecraft (like Discovery, hopefully) take off has nothing to do with escape velocity. That's the velocity which, if you could achieve it at the instant you left the Earth's surface, and if it's direction was pointed exactly away from Earth's center, would allow you to coast outward forever, without further acceleration, and without ever being drawn back into Earth's gravity well. And it's more like

11,000 m/sec.

What rockets really do is accelerate from a standing start, on a trajectory that becomes tangential to an Earth-concentric circle, and increase speed gradually till they reach orbital velocity of about 7,700 m/sec. How hard/fast they accelerate is determined by the velocity they need to achieve, and the time/distance they want to spend achieving it. You could go straight up at 5 miles per hour, if you had enough time and fuel; but then you'd still need to make a 90 degree turn and accelerate to orbital velocity. Otherwise, you'd have to keep "standing" on your rocket exhaust forever, to keep from falling right back down again. Or, you could take a long, slow upward spiral, maybe doing several sub-orbits before actually getting into space; but that would mean way too much flight inside the Earth's atmosphere, where air friction would make a mess of your plans.

So current spacecraft do the best balance of up and sideways, and the best mix of acceleration and fuel efficiency (with allowances made for the max forces that hardware and passengers can stand). They don't escape, or come even close to it. That's a whole different matter.

KG

I don't know if this is a good analogy but we'll soon find out. Imagine being somewhere with no gravity where you have a ball that's attached to the surface with a long rubber band. To make it so the ball doesn't come back to you and go away from you forever you'd have to throw it hard enough to break the rubber band. That means it would have to have enough acceleration when you threw it get up to speed with enough momentum to break the rubber band (gravity). In another scenario with gravity imagine shooting a rifle into the air. The bullet would continue to go up till it lost all its momentum at the end of its arc and return to earth. A more powerful gun would make it go higher but I don't think you can make one powerful enough for the bullet to escape.earth's gravity, the bullet has to have on board power.

Dear Henry Kolesnik:

Its OK, but still an analogy.

*velocity*. The rubber band provides the acceleration, your "throw" could be approximated by an impulse.

Rail guns are really close to having the speed. You'd need to fire a couple of rounds to ionize and decrease the density of the air, then the third round could likely escape. Rails guns don't yet have the necessary cycle rate (either).

You can escape Earth's gravitation and travel no faster than 1 meter/second. You just have to have either a "space elevator", or lots of fuel.

David A. Smith

Henry Kolesnik wrote: > I don't know if this is a good analogy but we'll soon find out. > Imagine being somewhere with no gravity where you have a ball that's > attached to the surface with a long rubber band. To make it so the > ball doesn't come back to you and go away from you forever you'd have > to throw it hard enough to break the rubber band. That means it > would have to have enough acceleration when you threw it get up to > speed with enough momentum to break the rubber band (gravity). In > another scenario with gravity imagine shooting a rifle into the air. > The bullet would continue to go up till it lost all its momentum at > the end of its arc and return to earth. A more powerful gun would > make it go higher but I don't think you can make one powerful enough > for the bullet to escape.earth's gravity, the bullet has to have on > board power.

The rubber band analogy doesn't work too well. A rubber band will exert MORE force on the ball as it's stretched longer and longer. Gravity works the other way. The farther you are from its source, the less effect it has.

KG