# Escape Velocity

1. Nov 26, 2006

How Does a Space Ship reenter the earths atmosphere at such a great rate of speed? I understand speed in this way W=F x D. there is nothing in space for the ship to propel itself off to build this great speed.

2. Nov 26, 2006

### Hootenanny

Staff Emeritus
Have you ever considered the change in gravitational potential energy?

3. Nov 26, 2006

### ZapperZ

Staff Emeritus
You forgot two important factors:

1. The ship already has a large speed to begin with. Just think of how fast the ship has to move to maintain an orbit at that distance above the earth. If it is stationary with respect to the earth, it'll fall towards the earth. So using basic mechanics, you can easily calculate the speed that it needs to maintain to use the central force from the earth.

2. When it is coming back down to earth, a radial component of the velocity increases mainly due to gravity. Drop a ball and its velocity increases at it races towards the ground. So if it doesn't have any "lift" or thrusters, it will increase in speed by itself due to gravitational force.

Zz.

4. Nov 27, 2006

### rcgldr

No one answered the last question. The space ship emits part of itself (burnt fuel) at a very high rate of speed. The acceleration of the mass of the fuel to high speed generates a force that propels the space ship forwards, and the spent fuel backwards. The higher the speed of the propellent, the more efficient the rocket engine is (more energy gained per amount of spent fuel emitted).

When launching a space ship, the more final speed you need, the more fuel you need, but the fuel also adds weight. Using solid rocket boosters on the shuttle helps since they are only accelerated to a sub-orbital speed before being detached after they are spent. It took a huge rocket and a lot of fuel to put a man on the moon. The Saturn V weighed 6 million pounds, and started with just 7 million pounds of thrust at lift-off. Again, multiple stages were used, and detached.

To save fuel and costs, rather than use even more fuel to slow down a space ship, the ship is only partially slowed, then relies on aerodynamic drag to do most of the braking, which generates a lot of heat and was the cause of destruction of one of the shuttles. The fastest piloted re-entry speed ever was the Apollo 10 mission where the return was made when the moon was further from Earth than the other mission.

http://www.aerospaceweb.org/question/spacecraft/q0260.shtml

Fastest land based objects are rocket sleds, here's another link:

http://www.46tg.af.mil/world_record.htm [Broken]

Last edited by a moderator: May 2, 2017
5. Nov 27, 2006

### ZapperZ

Staff Emeritus
I did!

If you follow your scenario, if you do a "burn" to increase your speed, you'll never get back to earth from that orbit, unless you turn your ship and point it directly at the earth, which isn't how it is done.

What is done instead is to slow down the ship so that it start falling out of orbit towards the earth. But by doing this, you also pick up a radial component of the speed, meaning you start to fall faster towards the earth simply via gravity. So the ship can simply "propel" itself using the gravitational field.

Zz.