Exiting earths atmosphere?

1. Dec 20, 2007

KLE xy

exiting earths 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

2. Dec 20, 2007

Staff: Mentor

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.

3. Dec 21, 2007

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.

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

4. Dec 21, 2007

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.

5. Dec 21, 2007

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.

6. Dec 21, 2007

Staff: Mentor

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.

Last edited: Dec 22, 2007
7. Dec 21, 2007

Danger

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.

8. Dec 21, 2007

DaveC426913

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.

9. Dec 22, 2007

Danger

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.

10. Dec 22, 2007

Staff: Mentor

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.

11. Dec 22, 2007

Danger

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

12. Dec 22, 2007

LURCH

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.

13. Dec 22, 2007

DaveC426913

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

14. Dec 22, 2007

FredGarvin

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

15. Dec 22, 2007

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?

16. Dec 24, 2007

joema

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.

17. Dec 27, 2007

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.]

18. Dec 27, 2007

FredGarvin

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.

19. Dec 27, 2007

D H

Staff Emeritus
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.

20. Dec 27, 2007

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.]