Escape Velocity and the Atmosphere

[KLE xy]

exiting Earth's atmosphere??

could someone please explain why the shuttle needs such a great velocity to exit our atmoshere? my son would like to know why we could not use a balloon to slowly rise out,I told him that a balloon would pop because of lack of preasure and that got me thinking, if we ever figure out how to beat gravity, could we just float out and avoid the heat from friction?my son is still young and he is asking questions that I have a hard time answering

 

[russ_watters]

You don't need speed to exit the atmosphere, you need speed to achieve orbit.

A balloon can be made to expand (high altitude balloons look almost empty when launched), so they can go quite high, but you can't leave the atmosphere because at some point (something like 150,000 feet) it gets too thin for the balloon to go any higher.

 

[FredGarvin]

To escape Earth's gravitational field, the energy of the object must at least match that of the gravitational potential energy.

After all is said and done, the equation can be represented as:

$$V_e = \sqrt{\frac{2GM}{r}}$$

where

$$V_e$$= Escape Velocity (speed)
$$G$$= Newton's gravitational constant
$$M$$= mass of the Earth
$$r$$ = radius of the Earth (when the rocket is still on the ground)

You can see mathematically why the speed is so high with the mass of the Earth in the numerator.

There's more information here:

http://en.wikipedia.org/wiki/Escape_velocity

 

[DaveC426913]

As Russ points out, the speed is necessary to achieve orbit. But one more factor comes in: at some point the shuttle (and all spacecraft ) will run out of fuel. It has to be in orbit before it does that.

In a perfect world, you might have a spacecraft that has effectively unlimited power. This spacecraft would not have to worry about achieving orbital speeds, and could if it wanted to, travel at 10mph all the way to the Moon and beyond (though it would be dumb to do so).

So, in a nutshell:
- the speed is necessary to achieve orbit.
- Orbit is necessary to keep it up there once it turns off its engines.

 

[Danger]

There is the compromise solution of launching a rocket from a balloon platform in the upper atmosphere. That can cut down tremendously on the fuel load. If that rocket then wants to be orbital, it will still have to achieve the proper speed.

 

[russ_watters]

The vast majority of the energy is in the speed, not the altitude, so it really doesn't help much to launch from a balloon.

 

[Danger]

The vast majority of the energy is in the speed, not the altitude, so it really doesn't help much to launch from a blloon.

True, but my initial thinking was in regards to the launch angle. I might be wrong about this, but it seems that a high-altitude launch would allow a much shallower departure, which would see less fuel required to reach the desired speed. In addition, a geostationary balloon is moving much faster than the surface of the Earth beneath it. That in turn means that the rocket would have less to make up for.

 

[DaveC426913]

In addition, a geostationary balloon is moving much faster than the surface of the Earth beneath it. That in turn means that the rocket would have less to make up for.

Geostationary ballooon??
Best I can think of is a balloon that's in the jetstream. You'd get maybe an extra 150mph out of it.

 

[Danger]

Geostationary ballooon??
Best I can think of is a balloon that's in the jetstream. You'd get maybe an extra 150mph out of it.

I admit that it might be difficult to have such a thing as a geostationary balloon without tethering it. Still, there is a reason that most launch sites are situated as closely as possible to the equator; it's to utilize the angular speed of the Earth as an assist toward reaching orbital speed. A balloon tethered high above the equator would provide just that much more initial speed.

 

[russ_watters]

I admit that it might be difficult to have such a thing as a geostationary balloon without tethering it. Still, there is a reason that most launch sites are situated as closely as possible to the equator; it's to utilize the angular speed of the Earth as an assist toward reaching orbital speed. A balloon tethered high above the equator would provide just that much more initial speed.

The problem is that a balloon isn't really that high above the equator. The Earth's radius is 4,000 miles. A balloon can take you up about 30 miles. So it would add 0.75% to your takeoff speed.

 

[Danger]

The problem is that a balloon isn't really that high above the equator. The Earth's radius is 4,000 miles. A balloon can take you up about 30 miles. So it would add 0.75% to your takeoff speed.

I'm not arguing the math, but if it's so insignificant an advantage, why does NASA launch from Florida instead of New Jersey?

 

[LURCH]

I'm not arguing the math, but if it's so insignificant an advantage, why does NASA launch from Florida instead of New Jersey?

I think Russ is talking altitude and you're talking lattitude, Danger. A balloon at the equator gives no noticable advantage over a launchpad on the ground at the equator. Of course, that's for getting into orbit; for sub-orbital tourist hops like Spaceship One, the balloons altitude is a big help, but latitude ceases to matter.

 

[DaveC426913]

I'm not arguing the math, but if it's so insignificant an advantage, why does NASA launch from Florida instead of New Jersey?

He's got a point there. The difference between Cape Canaveral and Newark is a mere 108mph.

Equator = 0deg = 1000mph
Cape Canaveral = 29deg = 874mph
Newark = 40deg = 766mph

 

[FredGarvin]

They don't launch from New Jersey because they can't afford the teamsters as launch personnel.

 

[KLE xy]

so If somehow I beat gravity and was able to float out what would my ship be made of to protect me from radiation,cold ,space junk? would g-forces effect me in space?

 

[joema]

could someone please explain why the shuttle needs such a great velocity to exit our atmoshere? my son would like to know why we could not use a balloon to slowly rise out...

I strongly suggest downloading and installing the free space flight simulator "Orbiter". It will graphically demonstrate why this doesn't work.

http://orbit.medphys.ucl.ac.uk/orbit.html

Your son can fly his own spaceship (at a slow speed, if he wants) out of the atmosphere and see what happens.

It's easy to use and very educational.

 

[ghost02]

KLE xy, Your ship would be made out of radiation resistant materials made up of lead, paper, foil, and some other material. The cold shouldn't be an issue if your insulated correctly and space junk isn't an issue because you are such a small target. G-forces wouldn;t affect you because G forces come from gravity and there is no gravity in space. [Or neglible amounts of it.]

 

[FredGarvin]

G-forces wouldn;t affect you because G forces come from gravity and there is no gravity in space. [Or neglible amounts of it.]

Oh dear...Do you want to revise that statement? Please? If what you say is true, then how would you explain a person experiencing more or less than 1g on Earth?

G-forces are a generalized term to represent the number of multipliers of Earth's gravity a person/thing is experiencing at any time. Those forces are due to motion. So even if you are in space and you experience an acceleration, you will experience forces.

 

[D H]

Just to elaborate on Fred's post:

The strength of the Earth's gravitational force on an object in low Earth orbit (e.g., the Space Shuttle) is only a bit less than the gravitational force when the object is on the ground. From a classical mechanics viewpoint, it is, after all, the gravitational force that keeps the object in orbit.

People use the term "G-force" as a handy and easy-to-understand unit of force. 1g (times my mass) is the force I feel on my rear end while typing this post. 75 kilogram-meters/second² (or 75 Newtons) is a bit harder to grasp.

Gravity is a very curious force: It is the only force that we can't feel! I am feeling my chair pushing me away from the center of the Earth on my rear end with a force of 75 Newtons. I do not feel Earth's gravity pulling toward the center of the Earth with a force of 75 Newtons. Were I in low Earth orbit, Earth's gravity would pull me toward the center of the Earth with more than 71 Newtons of force, and I wouldn't feel that force at all. I would instead feel "weightless" because my spaceship (with me inside) would be constantly falling with nothing to resist that fall.

 

[ghost02]

Sorry, I didn't explain what I meant by G-forces. My definition of G-forces was wrong! I thought it meant the power of gravity in space. Sorry about the confusion I didn't think about it. So, to clarify, I would feel G-forces in space when accelerating as if I am in a Jet on Earth? [I am only 14 and just got a book on Physics so I am trying to learn about this stuff.]

 

[DaveC426913]

So, to clarify, I would feel G-forces in space when accelerating as if I am in a Jet on Earth? [I am only 14 and just got a book on Physics so I am trying to learn about this stuff.]

You would feel a force when accelerating - either in a spaceship or in a jet - yes.

Note that, in a jet, in level flight, you do not feel a force (except gravity, straight down). This is how you're able to eat the peanuts and drink your highball on a transatlantic flight.

 

[D H]

Note that, in a jet, in level flight, you do not feel a force (except gravity, straight down). This is how you're able to eat the peanuts and drink your highball on a transatlantic flight.

You do not and cannot feel the force of gravity. What you feel in a jet in level flight is the seat of the plane pushing up on you (i.e., the "normal force"), not the force of gravity pulling down on you.

NASA does fly a plane that let's one feel (or not feel) nothing but gravity. It does not fly straight-and-level and is colloquially called the "http://jsc-aircraft-ops.jsc.nasa.gov/Reduced_Gravity/trajectory.html" ". When the C9-B is neat the top of one of its parabolic arcs, the human guinea pigs aboard the aircraft do not have the luxury of an airplane seat exerting a normal force on their bodies. They can fly!

Edited to add
For a mere $3,675 (US), you can now experience "weightlessness" on a KC-135 (the predecessor of the C9-B NASA now uses for reduced gravity flights) courtesy of the http://www.gozerog.com/" .

 

[ghost02]

Thank you for the explination Dave. Paying so much money for 8 minutes of wheightlessness sounds like a large price to pay.

 

[DaveC426913]

You do not and cannot feel the force of gravity. What you feel in a jet in level flight is the seat of the plane pushing up on you (i.e., the "normal force"), not the force of gravity pulling down on you.

While technically you are correct, I think it's confusing the issue. The same can be said for other forces, such as acceleration. If the jet made a hard outside loop you'd not feel the acceleration, you'd feel the pull of the 5-point harness on your body.

So this:

Gravity is a very curious force: It is the only force that we can't feel!

isn't true.

 

[D H]

The fact that the jet makes a hard outside loop does not directly exert a force on your body. The force you feel in this case is exactly the same force you feel when gravity pulls down on you: the normal force exterted by the seat (and the five-point harness).

It is this curious nature of gravity that led Einstein to elaborate on the equivalence principle, thereby setting the groundwork for general relativity. Gravity is a fictitious force in general relativity, and just like every other fictitious force, we can't feel it because it's not real.

 

[DaveC426913]

The fact that the jet makes a hard outside loop does not directly exert a force on your body. The force you feel in this case is exactly the same force you feel when gravity pulls down on you: the normal force exterted by the seat (and the five-point harness).

Right, which is why gravity is not unique (or curious) in this sense. Any acceleration will do that.

 

[D H]

Any acceleration but gravity. A satellite orbiting the Earth is accelerating due to gravity (classical mechanics POV) / not accelerating as it is following a geodesic (general relativity POV). An accelerometer on that satellite will not and cannot measure what gravity is doing to the satellite. It will measure accelerations that result from satellite rotation and true external forces such as drag, but not gravity.

Now let's put the accelerometer in a stationary car on the ground. The normal force on the accelerometer case does not transfer to the test mass, so the test mass moves down, making the accelerometer will register an acceleration of 9.8 m/s² upward. Ignoring Earth rotation, the accelerometer is not accelerating (classical mechanics POV) / in an accelerating frame (general relativity POV). When the car starts moving forward, the test mass will move rearward (and down), making accelerometer will register an acceleration of 9.8 m/s² up plus the car's horizontal acceleration forward. Gravitational acceleration is very different from any other acceleration in a classical mechanics POV. Gravitational acceleration is also very different from any other acceleration in a general relativistic POV, as gravitational acceleration per se doesn't exist in general relativity.

 

[russ_watters]

Dave and D_H, while I appreciate your clarifications and Dave certainly understands what you mean, I think we've gone far enough for the purpose of explaining to a 14 year-old. Let's just let it go with what we have.

 

[KLE xy]

so if lead was used to insulate from radiation ,how thick would it be ?, and what about windows?? what's the point of going if you can't see anything?

 

[ghost02]

Why would you want windows in space? Just use camras. Besides the windows might be sucked into the vacuum because the glass might break. It might even break on liftoff, although I am not entirerly shure, it seems logical that the glass would crack on liftoff. Since you would need a rocket to get into space and I am answering based on that. I do not know how think the lead would be.

DH, when you talked about the accelerometer, would the car be experiancing horizontal forces & gravitational forces, or just horizontal/gravitational forces? Also, for example, when I am in a jet and I am go at an extreame AOA [175 degress] and accelerating, would the forces exerted on my body be gravitatonal?

 

[D H]

The Earth has two main lines of defense against particularly nasty radiation. One is our thick atmosphere. Your spaceship will be above that, so there is some concern. Then again, people living at high altitude have the same problem, and they have that problem for all their life. The incidence of cancer is a bit higher for people who live at very high altitude compared to sea level, but in general they get along just fine. That's because these people are still protected by our planet's first line of defense: The Earth's magnetic field. If you want to read more, google "aurora (astronomy)" and "Van Allen Belts".

Unless your spaceship goes far from Earth, you will still be inside that first line of defense. Radiation and proper shielding against it is a concern for astronauts who go to the Moon or beyond. It is a much lesser concern for people in low Earth orbit. Since you are young, you may be able to be a space tourist sometime in the future and have a spectacular view of the Earth below you through large viewing windows. (The company for I work for is helping with one such project right now.)

 

[D H]

Why would you want windows in space? Just use camras.

Warner von Braun initially thought to make the Mercury capsule windowless. The initial astronauts let von Braun know in no uncertain terms that he had another think coming. Every US human-rated spacecraft has had windows.

Even if we could display a camera view in ultra widescreen 1080-p (or whatever improved video technology we have when space tourism beomes real), it will not have the UMPH factor that looking out a window yields. Anyone can see the video. Only someone who is actually in the vehicle will be able to look out the window. Since those tourists will pay a lot of money to get into space (current rate: a cool $20 million to got to the Space Station), they most certainly will want and will get a window. A very big window.

DH, when you talked about the accelerometer, would the car be experiancing horizontal forces & gravitational forces, or just horizontal/gravitational forces? Also, for example, when I am in a jet and I am go at an extreame AOA [175 degress] and accelerating, would the forces exerted on my body be gravitatonal?

The gravitational acceleration induced by the Earth on any object on or near the Earth is more-or-less the same in magnitude. It doesn't matter if the object is planted firmly on the surface of the Earth, flying through the atmosphere, or just above the atmosphere. That car, and jet fighter, and even a spacecraft in low Earth orbit are all subject to Earth gravity. The object's velocity or angle of attack doesn't come into play. All that matters is proximity to the Earth. The difference in the gravitational acceleration for the car and the spacecraft is a mere 5 percent or so.

Regarding the car: Many forces are involved for a car accelerating down a road. Listed as third law pairs, gravity pulls it toward the Earth and gravity pulls the Earth toward to car; the Earth's surface pushes up on the car while the cars whiles push down on the surface of the Earth, the cars wheels push backwards on the road while the road pushes forward on the wheels, air drag pushes ... You get the picture.

 

[ghost02]

Thank you very much for the explination.

 

[FredGarvin]

Sorry, I didn't explain what I meant by G-forces. My definition of G-forces was wrong! I thought it meant the power of gravity in space. Sorry about the confusion I didn't think about it. So, to clarify, I would feel G-forces in space when accelerating as if I am in a Jet on Earth? [I am only 14 and just got a book on Physics so I am trying to learn about this stuff.]

Ugh. Sorry if my post seemed a bit harsh. I didn't know you were only 14. I hope you are on the right track now.

BTW, it would help if people would fill out their profile information so we can avoid things like this!

 

[mheslep]

could someone please explain why the shuttle needs such a great velocity to exit our atmoshere? my son would like to know why we could not use a balloon to slowly rise out,I told him that a balloon would pop because of lack of preasure and that got me thinking, if we ever figure out how to beat gravity, could we just float out and avoid the heat from friction?my son is still young and he is asking questions that I have a hard time answering

I still like Newton's original thought experiment for the best intuitive feel for escape velocity: fhttp://astro.unl.edu/naap/atmosphere/gravity.html" at greater and greater velocity until finally the shell makes it over the horizon before it 'falls'.