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2003-Jun-25, 11:47 PM
Heres the question,

Why do rockets etc need to be going at 'escape velocity' to get away from earth?

I mean, when you look at the shuttle or any rocket during launch, to start with they are going pretty damn slow, but they are still going up, so if it kept putting out just enough thrust to keep moving upwards at that same speed (assuming it had enough fuel or something and kept upright) and with gravity getting weaker the further up you go, why should it need to go at the escape velocity to get away when its doing it already at a crawl, not to mention the weaker gravity making it easier and easier?

ToSeek
2003-Jun-26, 12:10 AM
Heres the question,

Why do rockets etc need to be going at 'escape velocity' to get away from earth?

I mean, when you look at the shuttle or any rocket during launch, to start with they are going pretty damn slow, but they are still going up, so if it kept putting out just enough thrust to keep moving upwards at that same speed (assuming it had enough fuel or something and kept upright) and with gravity getting weaker the further up you go, why should it need to go at the escape velocity to get away when its doing it already at a crawl, not to mention the weaker gravity making it easier and easier?

Probably because you'd run out of fuel before too long and then fall back to Earth. I'm sure one of our rocketry experts can supply a more technically accurate explanation.

Glom
2003-Jun-26, 12:40 AM
I mean, when you look at the shuttle or any rocket during launch, to start with they are going pretty damn slow, but they are still going up, so if it kept putting out just enough thrust to keep moving upwards at that same speed (assuming it had enough fuel or something and kept upright)

Bingo! Rocket fuel does not come cheap. From Earth, it takes a lot of fuel to launch even the smallest payloads. If you add more fuel, then you need to add fuel to lift the fuel etc. In theory, if a rocket could keep thrusting, then it wouldn't need to worry about parabolic orbits, but then you're talking about so much fuel, that you'll be lifting bloody Phobos into orbit. Assuming it had enough fuel is a no small assumption.

Why make it so much more difficult when you can let gravity do the work for you?

I should note that with the development of nuclear propulsion, there is talk now of a more proactive attitude to space travel. More thrust, less coast.

daver
2003-Jun-26, 01:07 AM
Heres the question,

Why do rockets etc need to be going at 'escape velocity' to get away from earth?

I mean, when you look at the shuttle or any rocket during launch, to start with they are going pretty damn slow, but they are still going up, so if it kept putting out just enough thrust to keep moving upwards at that same speed (assuming it had enough fuel or something and kept upright) and with gravity getting weaker the further up you go, why should it need to go at the escape velocity to get away when its doing it already at a crawl, not to mention the weaker gravity making it easier and easier?

In THE MOUSE ON THE MOON, the Duchy's rocket takes exactly that approach. The physics don't work out too well (the Isp of Pinot Grand Fenwick must be extremely high, which implies an enormous temperature--something your average shower head couldn't handle), but that's not the point.

Hamlet
2003-Jun-26, 01:38 AM
Heres the question,

Why do rockets etc need to be going at 'escape velocity' to get away from earth?

I mean, when you look at the shuttle or any rocket during launch, to start with they are going pretty damn slow, but they are still going up, so if it kept putting out just enough thrust to keep moving upwards at that same speed (assuming it had enough fuel or something and kept upright) and with gravity getting weaker the further up you go, why should it need to go at the escape velocity to get away when its doing it already at a crawl, not to mention the weaker gravity making it easier and easier?

Only rockets that are leaving Earth's orbit need to be going at escape velocity. For rockets or satellites that are in Earth orbit they travel at a slower rate. IIRC, escape velocity is around 11 km/sec and for Low Earth Orbit the velocity is around 7 km/s or about 17,500 mph.

As as been suggested, leaving Earth slowly would be possible if you didn't have to worry about fuel. Unfortunately, this is not the case. The other thing to remember is that for any orbit there is a horizontal as well as a vertical component. Much of the acceleration a rocket provides goes into the horizontal component. If you watch a Shuttle launch you can see that fairly soon after liftoff the Shuttle pitches over from vertical and begins accelerating at an angle that takes it downrange as well as up.

I'm sure some of the other folks here can give a better explanation.

Donnie B.
2003-Jun-26, 02:42 AM
You can accelerate as slowly as you like, but unless you eventually get to escape velocity, you will eventually fall back to Earth (disregarding the influence of other gravity wells, such as the Moon or Sun). It's a straightforward calculation that dates back to Newton.

Think of it this way: if you are at some very large distance from the Earth and motionless w/r/t the Earth, you will begin to accelerate toward the Earth. You can calculate how fast you'll be going when you hit the ground. You can even plug in "infinity" as the distance you start from, and get a finite answer for the speed on impact. That speed -- the speed you'll hit the Earth if you fall from infinity -- is the escape velocity. Run the tape backward (that is, leave the Earth at escape velocity) and you'll come to a stop at infinity. Leave faster than EV and you never stop, never fall back. Leave slower -- say, jump up as high as you can -- and you eventually stop and fall back.

Now, is it possible to get away from the Earth by applying low thrust continually, but never reaching EV? Sure, especially if you take into account the rest of the universe. IIRC, the Apollo missions never reached EV. They just went "up" fast enough to cross into the Moon's gravity well. And they would have gone up without ever coming down, if not for the trusty SPS! 8)

kilopi
2003-Jun-26, 09:36 AM
You can accelerate as slowly as you like, but unless you eventually get to escape velocity, you will eventually fall back to Earth (disregarding the influence of other gravity wells, such as the Moon or Sun). It's a straightforward calculation that dates back to Newton.
No, that's not true, and I think that that is Invader Spleen's point.

Just imagine a small craft that leaves the surface of the Earth at a small rate of speed and doesn't accelerate at all. In order to do that, it has to use a lot of power (imagine a hovercraft just inches above the ground), but as long as it has power available, it does not have to fall back to Earth.

I realize that's a big but, but still, we are talking in hypothetical terms. A space craft could leave Earth going 1 meter per minute, never accelerate, and it would never fall back. It would take tremendous amounts of fuel to do that though.

As others have pointed out, escape velocity is calculated for an object that is quickly accelerated to that speed, and then has no subsequent propulsion. That's actually a more efficient way to go, because the fuel gets burned up early and you don't have to lift it away from the Earth as well.

kucharek
2003-Jun-26, 09:45 AM
When you crawl up at let's say 1m/s, than, sooner or later, you are so far away from Earth's gravitational influence that the escape velocity from this point is less than 1m/s. Then you are faster than escape velocity, you can switch of your engine and will never fall back.

A few points why you should try to get away as fast as possible:
-Every second you are close to Earth, you "loose" some 10m/s of velocity due to gravitational pull.
-The quicker you burn your fuel, the less you have to carry up to great heights. Carrying up something needs energy. Don't waste your fuel to carry fuel out of the gravitationel funnel.

Harald

Iain Lambert
2003-Jun-26, 10:30 AM
Ah, I see I've just been beaten to the point.

For wherever you are, on Earth, another planet, or halfway up in the sky, there is an "Escape velocity", which is defined as the velocity at which you don't need to add any more thrusting from here on out in order to escape local gravity, and can switch your engines off. The higher up you are, the less gravity you're feeling, and so the smaller that is.

Mainframes
2003-Jun-26, 03:24 PM
Ah, I see I've just been beaten to the point.

For wherever you are, on Earth, another planet, or halfway up in the sky, there is an "Escape velocity", which is defined as the velocity at which you don't need to add any more thrusting from here on out in order to escape local gravity, and can switch your engines off. The higher up you are, the less gravity you're feeling, and so the smaller that is.

Isn't escape velocity the velocity at which it will take an infinite distance for a gravitational body to bring you to a standstill relative to that body? (assuming that there no other gravitational influences)

 And is also therefore a function of distance from said body....

kilopi
2003-Jun-26, 03:29 PM
Isn't escape velocity the velocity at which it will take an infinite distance for a gravitational body to bring you to a standstill relative to that body? (assuming that there no other gravitational influences)

 And is also therefore a function of distance from said body....
You also have to assume that there is no additional acceleration of yourself, excepting the negative acceleration of the body.

Mainframes
2003-Jun-26, 03:33 PM
Isn't escape velocity the velocity at which it will take an infinite distance for a gravitational body to bring you to a standstill relative to that body? (assuming that there no other gravitational influences)

 And is also therefore a function of distance from said body....
You also have to assume that there is no additional acceleration of yourself, excepting the negative acceleration of the body.

Should have added that. As ever grapes you manage to fill those little gaps in my knowledge.... :)

Iain Lambert
2003-Jun-26, 03:46 PM
Yes. Thats exactly what I was trying (clearly failing) to say. Thanks for making it rather more clear.

kurtisw
2003-Jun-26, 04:07 PM
[Just imagine a small craft that leaves the surface of the Earth at a small rate of speed and doesn't accelerate at all. In order to do that, it has to use a lot of power (imagine a hovercraft just inches above the ground), but as long as it has power available, it does not have to fall back to Earth.

Just a nit-picky point, but if you are leaving the earth at a constant speed of 1 m/s, then you are accelerating, you just happen to be accelerating at the exact opposite of Earth's gravitational acceleration. This is why it would take so gosh darn much fuel.

crazy4space
2003-Jun-26, 04:13 PM
Now, is it possible to get away from the Earth by applying low thrust continually, but never reaching EV? Sure, especially if you take into account the rest of the universe. IIRC, the Apollo missions never reached EV. They just went "up" fast enough to cross into the Moon's gravity well. And they would have gone up without ever coming down, if not for the trusty SPS! 8)[/quote]
When Apollo reached orbit they did what was called a LOI, lunar orbit injection or insertion. 8)

kilopi
2003-Jun-26, 04:34 PM
Just a nit-picky point, but if you are leaving the earth at a constant speed of 1 m/s, then you are accelerating, you just happen to be accelerating at the exact opposite of Earth's gravitational acceleration.
I don't accept that nitpick. If I am leaving the Earth at 1 m/s, then I will feel the same gravitational acceleration, and in the same direction, as someone standing on the Earth. So, it's not the exact opposite, even if you accept the equivalence of gravity and acceleration. If you use the usual Newtonian definition of acceleration, then clearly a constant velocity of 1 m/s is zero acceleration.

ToSeek
2003-Jun-26, 04:50 PM
Now, is it possible to get away from the Earth by applying low thrust continually, but never reaching EV? Sure, especially if you take into account the rest of the universe. IIRC, the Apollo missions never reached EV. They just went "up" fast enough to cross into the Moon's gravity well. And they would have gone up without ever coming down, if not for the trusty SPS! 8)

When Apollo reached orbit they did what was called a LOI, lunar orbit injection or insertion. 8)

Actually, from Earth orbit they did a TLI, or Trans-Lunar Injection. They did the LOI when they got to the Moon. (Love those TLAs!*)

*TLA - Three-Letter Acronym. Very popular at NASA. ;)

Klausnh
2003-Jun-26, 06:39 PM
I think there is a confusion between the speed and the velocity. If an object is accelerating, it is changing its velocity. Because the object must overcome the earth’s gravity, an object that is rising must be accelerating. If an object has a speed of 1m/s away from the center, then its velocity is increasing because it is accelerating. The object must reach an escape velocity (not speed) to escape the earth’s gravity.
In a graph of velocity versus time, acceleration is defined as the slope, so any object that is accelerating (non zero slope) must have an increase in velocity.
A=dv/dt. If acceleration is not 0, then velocity must increase with time.

kilopi
2003-Jun-26, 06:58 PM
I do agree that there seems to be some confusion. :)

Since we're talking about escape velocity, that's in relation to the Earth reference frame, and I think we're talking about a constant 1m/s velocity, so apparently no acceleration either.

Glom
2003-Jun-26, 07:06 PM
Actually, I think that according to the two-body maths, it doesn't matter about the direction of the velocity, just its magnitude. If a body is on an escape trajectory, a parabolic orbit, then its specific mechanical energy is zero. Hence, 0=˝v˛-µ/r. So if it has the right speed at a given altitude, regardless of direction, the specific mechanical energy will be zero and hence, it will be on a parabolic orbit. Of course, this assumes particles. In reality, if the velocity was directed towards Earth itself, it would crash. But if you avoid that, then any direction will do.

The reason why escape velocities are directed is partly because there is generally a specific direction that the rocket is meant to reach and because escape orbit insertion is cheaper, if it makes use of the velocity already possessed by being a prograde burn, rather than some other burn, which would be more expensive.

kilopi
2003-Jun-26, 07:13 PM
Actually, I think that according to the two-body maths, it doesn't matter about the direction of the velocity, just its magnitude. If a body is on an escape trajectory, a parabolic orbit
But the OP was clearly asking about non-escape velocities, whether something that had, for instance, a 1 m/s second velocity relative to the Earth, would be able to get into space (or "escape"). The answer is clearly yes.

It's just not that practical. For instance, you could climb a ladder to the height of the space station (it's only a few hundred miles), if there were such an incredible ladder, but when you got there, the space station woulb be zooming by at about 18000 mph.

Glom
2003-Jun-26, 07:23 PM
Well, by the time you were done climbing the ladder, you'd be going at 500m/s, yourself, but the station would still be going 7000m/s faster, so it wouldn't really make much difference.

tracer
2003-Jun-26, 07:30 PM
I should note that with the development of nuclear propulsion, there is talk now of a more proactive attitude to space travel. More thrust, less coast.
Talk now?

As in, we already have a nuclear spacecraft propulsion system in actual use, that's been in space, right now? :o

kilopi
2003-Jun-26, 07:37 PM
but when you got there, the space station woulb be zooming by at about 18000 mph.

but the station would still be going 7000m/s faster, so it wouldn't really make much difference.
I guess I should have said 16000 mph. :)

Glom
2003-Jun-26, 07:38 PM
As in, we already have a nuclear spacecraft propulsion system in actual use, that's been in space, right now? :o

Not that I know of, but now that such systems are being developed, there is talk.

Klausnh
2003-Jun-26, 09:20 PM
If you don’t apply a force (ie accelerate, since F=ma) to maintain the 1m/s, you’ll fall back to earth. If you let go of the ladder, you’ll fall back to earth. Either way you need a force (ie acceleration) to go up.
Back to the OP, after some googling: Escape velocity is defined to be the minimum initial velocity an object must have in order to escape the gravitational field of the earth, that is, escape the earth without ever falling back. So a rocket does not need to be traveling at the escape velocity to get away from the earth if they apply a force (accelerate) on the way up. But as long as thrust (ie acceleration) is available on the way up, escape velocity is not necessary to escape the earth’s gravity

kilopi
2003-Jun-26, 09:26 PM
If you don’t apply a force (ie accelerate, since F=ma) to maintain the 1m/s, you’ll fall back to earth. If you let go of the ladder, you’ll fall back to earth. Either way you need a force (ie acceleration) to go up.
I agree with everything you say, except when you equate the necessary force with an acceleration which seems to be non-zero.

Sure, you need a force to counterbalance the force of the Earth's gravity--but if they are balanced, then the resultant force is zero. That is, 0 = ma, so the acceleration is zero too. If your velocity is constant (1 m/s in this example) then of course the acceleration is zero.

2003-Jun-27, 12:41 AM
wow, thats a lot of quick replies, thanks for the response guys , things make a heck of a lot more sense now.

Klausnh
2003-Jun-27, 01:48 AM
If you don’t apply a force (ie accelerate, since F=ma) to maintain the 1m/s, you’ll fall back to earth. If you let go of the ladder, you’ll fall back to earth. Either way you need a force (ie acceleration) to go up.
I agree with everything you say, except when you equate the necessary force with an acceleration which seems to be non-zero.

Sure, you need a force to counterbalance the force of the Earth's gravity--but if they are balanced, then the resultant force is zero. That is, 0 = ma, so the acceleration is zero too. If your velocity is constant (1 m/s in this example) then of course the acceleration is zero.

You’re right of course. I must have had a brain cramp. :oops: Every hill I drove up on the way home I noticed the constant speed and 0 acceleration. :D
I did not understand you’re “I’m surprised” link.???

kilopi
2003-Jun-27, 03:20 AM
You’re right of course. I must have had a brain cramp.
No problem, Klausnh, I just had one of those over on the analemma thread.

I did not understand you’re “I’m surprised” link.???
Old saying, I think that link goes to an old rec.puzzles post.

crazy4space
2003-Jun-27, 04:12 PM
Now, is it possible to get away from the Earth by applying low thrust continually, but never reaching EV? Sure, especially if you take into account the rest of the universe. IIRC, the Apollo missions never reached EV. They just went "up" fast enough to cross into the Moon's gravity well. And they would have gone up without ever coming down, if not for the trusty SPS! 8)

When Apollo reached orbit they did what was called a LOI, lunar orbit injection or insertion. 8)

Actually, from Earth orbit they did a TLI, or Trans-Lunar Injection. They did the LOI when they got to the Moon. (Love those TLAs!*)

*TLA - Three-Letter Acronym. Very popular at NASA. ;)
Oh! I get it trans-lunar, trans-lunar, trans-lunar - makes sense :oops: