Naked Science Forum
General Science => General Science => Topic started by: neilep on 03/04/2008 16:27:02
-
Dear Escapologists,
I'm confused a bit about the nature of escape velocity.....
See this Rocket ?
[ Invalid Attachment ]
A wonderful tribute to humankinds technological advances !...check out the astronauts as they bravely, with confidence and pride, climb on board for their journey to the stars !!
Gawwwd bless em !!
Now this rocket will have to travel at something like 7 miles a second to escape the gravity pull of the planet !!
Now check me out !!
[ Invalid Attachment ]
I'm also boldly being propelled against gravity upwards !!......
so, my question is....(Finally)..why do I need to go so fast to leave the planet ?...why can I not just continue upwards ?...a plane defies gravity!!..when ewe jump ewe do also !.........is it all to do with fuel ?
Help me grasp the gravity of the situation here *le groan*
Ta
-
You don't have to travel at any particular speed, as long as you have enough energy to spare for your rocket engine.
Escape velocity is the speed that you would need on the ground, if you were to be fired from a 'gun'. It would give you enough kinetic energy equal to the amount of gravitational potential you would have infinitely far away from the Earth.
To get into a low orbit, you need less energy than this. But you need to use your rocket motor to push you 'horizontally' or you would fall back the way you went up.
-
The shuttle uses two solid fuel boosters and a large liquid fuel tank full of liquid Hydrogen and liquid oxygen in order to get into orbit. The orbiter itself has a small liquid fuel tank to allow it to maneuver in space.
-
(also)
People (or sheep) jumping on trampolines do not defy gravity; neither do airplanes. They both are just ways of pushing against it. Gravity is a force and it gets added with any other force, such as the force generated by your muscles when you jump up in the air.
-
To elaborate, the escape velocity is the velocity needed, at the earth's surface (assuming no atmosphere as the air friction plays a huge role) and perpendicular to it, so that you would continue away from the earth (slowing down all the time by the earth's gravitational pull) to just get a long way off (mathematically infinite) but have no residual velocity. I was going to put the maths down but it is easier to look at Wikipedia.
The idea involves no atmosphere, no help from the earth's rotation and (of much less importance) no other gravitational effects from other celestial objects.
One of the biggest problems with getting stuff into space is the weight of rocket fuel that only gradually gets used up en route. So having to lift all the fuel to quite a height against gravity means having to carry even more fuel. The energy from the fuel is only just great enough to do the job of lifting itself and all the containment vessels for the fuel, hence the need for multistages which at least dispose of some of the overhead. On the moon, where there is no air resistance, a gun (or more likely an electromagnetic rail gun) would be much more attractive as it could accelerate an object to its escape velocity without having to accelerate unnecessary fuel and its container. Of course the escape velocity is much lower than earth too. This can't be practically done from the earth because the air resistance would slow the craft down again (or cause it to burn up) before it got to any significant height.
Another interesting point is that rocket launches are easier nearer the equator because use can be made of the earth's rotation to give any rocket a head start (like a sling shot). You could think of it as the gravity being slightly less at the equator. There are other good reasons too, especially when putting craft into orbit, which may be more dominant, but it is an advantage that is used where practical.
-
WOW !!
THANK YOU all indeed for clearing this up for me.
I understand.........I always did wonder about this because I always had it in my mind that if I had a ladder long enough (and the right equipment) all I need do is climb it !!....but for some reason I confused ' escape velocity' as prerequisite.
Just reading this quote from Wiki helps to explain my confusion "It is commonly described as the speed needed to "break free" from a gravitational field.".........and this is interesting too "The term escape velocity can be considered a misnomer because it is actually a speed rather than a velocity, i.e. it specifies how fast the object must move but not in which direction."
Thank you Sophiecentaur, Make it Lady, MayoFlyFarmer and Graham D (welcome to the site)
-
To elaborate, the escape velocity is the velocity needed, at the earth's surface (assuming no atmosphere as the air friction plays a huge role) and perpendicular to it, so that you would continue away from the earth (slowing down all the time by the earth's gravitational pull) to just get a long way off (mathematically infinite) but have no residual velocity. I was going to put the maths down but it is easier to look at Wikipedia.
etc
It can be explained by the way that potential energy is defined; it is the work done in bringing an object from infinity to the point of reference (e.g. the Earth's surface). If you get more Kinetic Energy than this, you can 'break free'. If you have less energy, you can still get into orbit, of course. LEO (low Earth orbit) craft need nothing like as much energy as they would need to 'escape'.
There are a lot of issues in choosing how you use the available energy your engines have. Efficiency is very relevant - this has been discussed a lot.
-
Strictly speaking there is no need to achieve any escape velocity to get away from the earth. if you had a craft that was capable of going up at one foot per second and it continued to do this it would eventually escape from the earth. The real problem is that the only way we know of lifting ourselves up like that is by throwing a lot of stuff out the back at high speeds (rocket motor) the slower you rise the less efficient this process is because rising at a steady ift per second you are putting all your energy into avoiding falling back to earth and so you use lts more energy. The best way to do it is to accelerate as quickly as possible.