Escape Velocity

frog_flyer

frog_flyer

FredFlyer
I've always heard that in order to escape the gravitational pull of the Earth, the space shuttle has to accelerate to an escape velocity. Under that speed, it would be unable to get out.

If this is the case, why is it so? As far as I'm concerned, even if the shuttle was moving at 1kt vertically, it's still going to get up eventually.

I've read this http://en.wikipedia.org/wiki/Escape_velocity . Anyone have layman's terms for me?

Why can't something just keep trucking along at whatever speed, pushing up the power when necessary to maintain a positive rate of climb?
 
I think the key is here:

the escape velocity is the minimum speed an object without propulsion needs to attain in order to "escape" from gravity.

without propulsion...
 
I have a degree in this, let's see if I remember correctly.

First, the shuttle is not going straight up but rather at an angle in which it increases it's altitude as well as its horizontal velocity, with horizontal being the greater.

It is actually called Delta V, or change in velocity. For every orbit there is a velocity in which you will be able to sustain orbit. The higher the orbit the slower this velocity is. I'm sure you have heard the analogy of the bullet. If you fire a bullet fast enough it will go around the earth, and although it looks to be keeping it height above the ground it is actually in a perpetual state of falling otherwise it wouldn't go around, well that's basically an orbit going fast enough that although you keep falling you are going to fast to loose any altitude and basically stay there.

Now back to the shuttle, remember that the earth is rotating, therefore at the surface you already have an initial velocity, hence the reason we always launch to the East, we use that initial velocity to help us, can't remember that velocity of the top of my head but lets say it 1000 miles per hour for the sake. Second we can calculate how fast we need to be at a certain altitude, shuttle 500 miles, Low Earth Orbit, Roughly 20000 miles per hour or so, don't quote me. So to successfully launch the shuttle into orbit we need to accelerate 19000 miles to the east, if we tried a west launch we would need a delta V of 21000. See the advantage of an East launch.

That's the basics, now why not simply launch straight up, if you think about it even in space large object still have a gravitational pull hence why we go around the sun and the moon around us. If we launch straight up to lets say 500 miles we would simply fall back to the Earth once we get there. we have no momentum to keep us going. That's what the X-prize did a simple up and down, it does not take as much energy to go straight up, but you won't stay there as well.

The key to any orbit, whether circular, elliptical, hyperbolic is the tangental velocity with respect to the objet you are trying to go about. Even when we went to the moon, we just simply didn't point the rocket to it, but first established an orbit about the Earth, Delta V Earth, and then we did a transfer orbit, Delta V elliptical about two large bodies, to the moon and then again established an orbit around the Moon, Delta V Moon, this one actually being a deceleration.

I know this is a little longwinded but hope it helps.

P.s. Tangental velocity is a velocity parallel to a surface.
 
phoenix 23684 said:
If we launch straight up to lets say 500 miles we would simply fall back to the Earth once we get there. we have no momentum to keep us going.
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That doesn't match what my physics book says. In fact, it derives the formula for escape velocity from shooting a bullet straight up. It makes the point that escape velocity is path independent, meaning that it's really an escape speed, rather than a velocity. The wiki article makes the same point. The fundamental reason is that escape speed depends on the exchange of kinetic energy for potential energy, and potential energy isn't changed by any component of velocity tangential to the earth, only the radial distance matters.

The formula for escape speed solves for the speed where kinetic energy becomes zero at a distance of infinity from the earth and it doesn't matter in what direction the projectile moves. (Ignoring air resistance and the rotational velocity of the earth.)
 
tgrayson said:
That doesn't match what my physics book says. In fact, it derives the formula for escape velocity from shooting a bullet straight up. It makes the point that escape velocity is path independent, meaning that it's really an escape speed, rather than a velocity. The wiki article makes the same point. The fundamental reason is that escape speed depends on the exchange of kinetic energy for potential energy, and potential energy isn't changed by any component of velocity tangential to the earth, only the radial distance matters.

The formula for escape speed solves for the speed where kinetic energy becomes zero at a distance of infinity from the earth and it doesn't matter in what direction the projectile moves. (Ignoring air resistance and the rotational velocity of the earth.)
Click to expand...

Let me clarify my previous statement. If you try to put something at an altitude of 500 miles by launching straight up it will simply come back down. I'm taking about orbits, The first post asked why does the shuttle simply not go straight up. That's the reason: (I've always heard that in order to escape the gravitational pull of the Earth, the space shuttle has to accelerate to an escape velocity. Under that speed, it would be unable to get out.)

I apologize for not distinguishing between speed and velocity, yes velocity is a vector and speed a scalar, didn't think we needed to get into such detail. That is why it is called Delta V, cause we give it speed and direction.

This is from wikipedia: the escape velocity is the minimum speed an object without propulsion needs to attain in order to "escape" from gravity, i.e., so that gravity will never manage to pull it back Objects in orbit to include our solar system and the comets have not reached escape velocity, if they had they would not be in orbit. In physics, an orbit is the path that an object makes around another object or a barycenter while under the influence of a central force, such as gravity.

So Escape Velocity/Speed in lame terms not an orbit, any orbit is affected by gravity. In practical application this velocity is never used too expensive, we don't have the resources for this. Even when we go to Mars we are still doing orbit transfers. We never use escape velocity, a silly term of hollywood. Much like there is no such thing as centrifugal force. This is how we do it. http://jwilson.coe.uga.edu/EMAT6680Fa05/Bacon/hohmanntransfers.html

One last thing, believe it or not, but everything that we have orbiting the Earth will eventually come down. If the shuttle or ISS did not do a type of station keeping, by using gyros, reaction wheels, burns, etc., it would fall back. We have satellites in Geo orbits that we no longer use that will come crashing in 1000 years, nice huh, wonder what people in the future will think of us.

My disclaimer: Aerospace Engineer degree with concentration in Astrodynamics, and 3 years as a satellite engineer on GEO birds.
 
phoenix 23684 said:
Let me clarify my previous statement. If you try to put something at an altitude of 500 miles by launching straight up it will simply come back down. I'm taking about orbits, The first post asked why does the shuttle simply not go straight up. That's the reason: (I've always heard that in order to escape the gravitational pull of the Earth, the space shuttle has to accelerate to an escape velocity. Under that speed, it would be unable to get out.)

I apologize for not distinguishing between speed and velocity, yes velocity is a vector and speed a scalar, didn't think we needed to get into such detail. That is why it is called Delta V, cause we give it speed and direction.

This is from wikipedia: the escape velocity is the minimum speed an object without propulsion needs to attain in order to "escape" from gravity, i.e., so that gravity will never manage to pull it back Objects in orbit to include our solar system and the comets have not reached escape velocity, if they had they would not be in orbit. In physics, an orbit is the path that an object makes around another object or a barycenter while under the influence of a central force, such as gravity.

So Escape Velocity/Speed in lame terms not an orbit, any orbit is affected by gravity. In practical application this velocity is never used too expensive, we don't have the resources for this. Even when we go to Mars we are still doing orbit transfers. We never use escape velocity, a silly term of hollywood. Much like there is no such thing as centrifugal force. This is how we do it. http://jwilson.coe.uga.edu/EMAT6680Fa05/Bacon/hohmanntransfers.html

One last thing, believe it or not, but everything that we have orbiting the Earth will eventually come down. If the shuttle or ISS did not do a type of station keeping, by using gyros, reaction wheels, burns, etc., it would fall back. We have satellites in Geo orbits that we no longer use that will come crashing in 1000 years, nice huh, wonder what people in the future will think of us.

My disclaimer: Aerospace Engineer degree with concentration in Astrodynamics, and 3 years as a satellite engineer on GEO birds.
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I am a fellow AE with astro concentration (3 classes till graduating though...can't wait!)

As for the question a lot of people ask, "Why can't something just keep trucking along at whatever speed, pushing up the power when necessary to maintain a positive rate of climb?"...I find that many people (myself included before I started taking classes on it) try and equate the principles of flight with the principles of space flight. For example, if I am in the space shuttle and I want to go faster "why can't I just nudge the throttle a bit and go?" Rather intricate physics becomes involved when dealing with space flight (..don't even start rendezvous situations). It is a really interesting dance between simple Newtonian physics which governs everything on Earth and in space coupled with a very nonintuitive way of thinking. I usually think of myself as a pilot first, so my way of thinking is always governed by flying rules which I now find intuitive. Interesting field though nonetheless!
 
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.
 
Items in orbit have not escaped earths gravity, rather they are going fast enough to continuously fall over the horizon.
Think about a round planet (or moon) that has no atmosphere and is perfectly smooth. If an ball were fired from a cannon fast enough it would always fall over the horizon .. in orbit.

This is a gross over simplification..
 
phoenix 23684 said:
One last thing, believe it or not, but everything that we have orbiting the Earth will eventually come down. If the shuttle or ISS did not do a type of station keeping, by using gyros, reaction wheels, burns, etc., it would fall back. We have satellites in Geo orbits that we no longer use that will come crashing in 1000 years, nice huh, wonder what people in the future will think of us.

My disclaimer: Aerospace Engineer degree with concentration in Astrodynamics, and 3 years as a satellite engineer on GEO birds.
Click to expand...

In an idealized world (the textbook LOL) the satellites would not fall from orbit if it's not perturbed by anything else. The fact that it has angular momentum -which gravity won't change in the two body problem- would keep it going around and around... :)

Disclaimer: I stayed at a Holiday Inn Express last night:D.
 
aerospacepilot said:
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.
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The idea you are brushing up against but not naming is termed gravity loss. To grasp what it is consider a hovering rocket that generates just enough thrust to cancel its weight. It's belching exhaust but not transferring energy from the fuel/oxidizer to its forward motion. The method to mitigate the loss is through minimizing the time spent firing the rocket in a direction aligned with the gravitational force vector. In an ideal world rockets would always be aligned perpendicular to the graviational field or they would behave like artillery shells, giving impulse instantaneously. There is a limit to the practial amount of thrust that can be developed by a rocket engine as well as the limitations that soft cargo (humans) bring, so the artillery shell approach cannot be used. You will see, however, in the shuttle's trajectory that it climbs vertically for a short bit and then tips over to continue its burn perpendicular to gravity. Launch trajectories are a balance between aerodynamic considerations such as maximum dynamic pressure allowable and gravity loss.
 
THe space shuttle never attains escape velocity - only the vehicles that've gone to outer space need that - be in solar-system probes, or the Apollo missions to the moon. The shuttle attains orbital velocity which keeps it in orbit, which is about 17,500mph.. escape velocity I believe is about 25,000 mph which allows the craft to 'escape' Earth's orbit and go to the moon or planets or wherever..
 
kiwi lover said:
THe space shuttle never attains escape velocity - only the vehicles that've gone to outer space need that - be in solar-system probes, or the Apollo missions to the moon. The shuttle attains orbital velocity which keeps it in orbit, which is about 17,500mph.. escape velocity I believe is about 25,000 mph which allows the craft to 'escape' Earth's orbit and go to the moon or planets or wherever..
Click to expand...


Wellll technically, the apollo missions did not attain escape velocity because the moon is in orbit around the earth.

Look, it may seem counter intuitive, but in orbit, the more power you add, the slower you go relative to the surface of the earth. Let's say you're in orbit around the earth at the ISS. You want climb up to your satelite parked in geosynchronous orbit, you don't do what would be the intuitive thing to do, and turn so that your exhaust is perpendicular to the surface of the earth, then blast straight up. What you want to do is burn "prograde" so that you're in the same direction as your orbit. It is best to do this at the peregee (the closes approach of your orbit to earth). As you add power, you're increasing your energy, that elongates your orbit and puts the apogee (the highest point away from earth up where the geosynchronous satellite is if you timed it right). Then when you get up to "apogee," you have to burn so that your peregee is now equal to you're apogee and you're in a circular orbit. Now since you are higher, you will be moving slower relative to the surface of the earth.

-Pat
 
Well a lot of really great posts. Mine will be short and simple. You need a force to overcome -9.8m/s/s (that's acceleration guys) which is gravity. So using F=ma, solve for at least 9.800000~1m/s/s and you can get to space.

As for railguns, or horizontal flight, the magic number is mach 25 where the earth should start to fall away while maintaining a stable pitch. It varies a bit based off of the altitude, but if you can get a cannonball going that fast, it will come back to hit you.
 
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