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aerospacepilot · April 12th, 2008, 18:21

First, let me apologize for all the acronyms. The only field I know of that uses more acronyms than aviation is space.

Let me clarify a few things about escape velocity.

If the space shuttle wants to get into a low earth orbit (LEO) of about 300km (that is about the altitude of the international space station (ISS)), it does not need to reach Earth escape velocity. Escape velocity is the speed an object must obtain without constant propulsion if it wishes to escape orbit around the Earth. This is necessary for interplanetary travel.

Now, why don't we just point the shuttle where we want to go?? Why do we need this escape velocity? Why not just keep firing the engines and go where we want?

The nice part about traveling through space is space is a vacuum. There is no friction. If I accelerate to 10km/sec, I will continue traveling at 10km/sec. No need to keep the engines firing to maintain speed. Firing the engines uses fuel. Fuel is weight, and to get weight into orbit costs a lot of money. To get 1 pound of matter into orbit costs about $5,000. So things must be done as efficiently as possible.

The most efficient way to get an object from Earth to another planet is to launch a rocket into a LEO, then do some type of transfer orbit (Hohmann transfer, bi-elliptic transfer, etc...) to get it to the planet.

Launching an object into LEO is basically what you described. You point the rocket up and since the thrust is greater than the weight, the rocket will accelerate and continue to go up until it reaches LEO. Since the earth is spinning, you gain about 0.4km/sec of speed by launching eastbound (that is from Kennedy Space Center (KSC) in Florida. The speed varies slightly depending on latitude)).

Using a Hohmann transfer orbit (generally most efficient) requires two engine burns. The first burn speeds the object up faster than escape velocity. Then once it reaches the desired planet, it will do one more burn to enter the desired orbit around the planet. Aside from these two positioning burns, the engines will not be firing throughout the flight.

If the spacecraft did not initially speed up faster than escape velocity, the rockets would need to be firing the entire time you are in the Earth's Sphere of Influence ((SOI), and that is about 1,000,000km away). Even once you left Earth's SOI, you would have almost no relative velocity, and now you would be stuck falling towards the sun unless you continued to fire your engines all the way to the other planet.

As I am sure you could see, it is MUCH MUCH MUCH more efficient to escape the Earth's gravity by accelerating to escape velocity from a LEO, rather than firing your rockets to fight off the force from Earth/Sun gravity all the way to the planet.

Does this help? Let me know if I can clarify more.

April 12th, 2008, 18:21	#8
aerospacepilot Junior Member Join Date: May 2007 Location: SF Bay Area, CA or Boulder, CO Posts: 168	Re: Escape Velocity First, let me apologize for all the acronyms. The only field I know of that uses more acronyms than aviation is space. Let me clarify a few things about escape velocity. If the space shuttle wants to get into a low earth orbit (LEO) of about 300km (that is about the altitude of the international space station (ISS)), it does not need to reach Earth escape velocity. Escape velocity is the speed an object must obtain without constant propulsion if it wishes to escape orbit around the Earth. This is necessary for interplanetary travel. Now, why don't we just point the shuttle where we want to go?? Why do we need this escape velocity? Why not just keep firing the engines and go where we want? The nice part about traveling through space is space is a vacuum. There is no friction. If I accelerate to 10km/sec, I will continue traveling at 10km/sec. No need to keep the engines firing to maintain speed. Firing the engines uses fuel. Fuel is weight, and to get weight into orbit costs a lot of money. To get 1 pound of matter into orbit costs about $5,000. So things must be done as efficiently as possible. The most efficient way to get an object from Earth to another planet is to launch a rocket into a LEO, then do some type of transfer orbit (Hohmann transfer, bi-elliptic transfer, etc...) to get it to the planet. Launching an object into LEO is basically what you described. You point the rocket up and since the thrust is greater than the weight, the rocket will accelerate and continue to go up until it reaches LEO. Since the earth is spinning, you gain about 0.4km/sec of speed by launching eastbound (that is from Kennedy Space Center (KSC) in Florida. The speed varies slightly depending on latitude)). Using a Hohmann transfer orbit (generally most efficient) requires two engine burns. The first burn speeds the object up faster than escape velocity. Then once it reaches the desired planet, it will do one more burn to enter the desired orbit around the planet. Aside from these two positioning burns, the engines will not be firing throughout the flight. If the spacecraft did not initially speed up faster than escape velocity, the rockets would need to be firing the entire time you are in the Earth's Sphere of Influence ((SOI), and that is about 1,000,000km away). Even once you left Earth's SOI, you would have almost no relative velocity, and now you would be stuck falling towards the sun unless you continued to fire your engines all the way to the other planet. As I am sure you could see, it is MUCH MUCH MUCH more efficient to escape the Earth's gravity by accelerating to escape velocity from a LEO, rather than firing your rockets to fight off the force from Earth/Sun gravity all the way to the planet. Does this help? Let me know if I can clarify more. __________________ http://www.youtube.com/watch?v=oX6pNsQzRy4 Props are 4 boats. Jets are 4 hot tubs. Rockets are for aerospacepilot!