Expert: Steve Nelson - 6/5/2008

Question
QUESTION: Hi, Steve! I promise my question won't be as esoteric as "quantum gravity", haha. I've been reading a few books from the library, on Physics, Black Holes, gravitational lensing(STILL something I don't understand the mechanics of), but something hit me:
Apollo Rockets and Shuttle Launches go about 20,000 miles per hour, to escape the pull of Earth's gravity, right? Well, WHY do they need to go that fast? What would happen, if, say, an F-16 was specially designed, to fly srtaight up, without the need for oxygen in the air, all limitations accounted for. Why COULDN'T a plane, flying 1,000 miles per hour, or even 500 mph, just keep flying up, to the edge of the atmosphere, and the edge of the Earth's gravitational pull? What would HAPPEN, if a slow-moving object(relatively slow-moving) approached that "zone"?

ANSWER: If you design an F-16 to fly without the need for oxygen, it would be a rocket.  Technically, you could design a rocket to fly up at 1000 miles per hour forever as long as it kept providing thrust, but the fuel cost of doing so would be absolutely ridiculous.  It would dwarf any rocket we've ever made.  These rockets are designed to quickly reach orbital velocity, where their circular motion balances out gravitation, and where they need no further fuel to stay up.  Nothing special would happen if you designed a slower rocket to go that high, it would rapidly accelerate back to Earth and burn up.  The "edge" of the atmosphere isn't very far up, you reach it in approximately 100 km (where the air is so thin we need special vacuum pumps here on Earth to get that low) and the Earth is over 6300 km in radius...so you're just not very high yet relative to the distance (many Earth radii) you need to escape the Earth's gravitational pull.

---------- FOLLOW-UP ----------

QUESTION: Hi again, Steve. Thanks for answering. But I don't think I was as clear as I had hoped to be, about the point of my question. You said: "Nothing special would happen if you designed a slower rocket to go that high, it would rapidly accelerate back to Earth and burn up.  The "edge" of the atmosphere isn't very far up, you reach it in approximately 100 km".
What I REALLY wanted to know is, WHY would it fall back to Earth, if it had the neccesary fuel to reach the "No Gravity Zone"? Why cant a rocket acheive a stable orbit, flying slower than 20,000 mph? I'm aware that the altitude where the AIR is gone is lower than the altitude where GRAVITY dissipates: Air is not part of my question, please excuse me if I gave that impression. Gravity is what I'm presently studying. I'm just trying to understand, if there's a Physical Law, something in Newtonian Physics about gravity, that neccesitates the need for a certain Speed or Acceleration to escape the gravity of a planetary body. I didn't think there was, I've never seen a formula for it. So I don't understand WHY you say "it would rapidly accelerate back to Earth and burn up." Am I being any clearer, this time?   Thanks again, hope you can help.

ANSWER: You don't seem to have calculated the scale of the distance you're talking about.  Use this handy orbital velocity calculator and it'll tell you the velocity you need at the height you're talking about:  http://home.att.net/~ntdoug/UCM2.html  Gravity falls off as 1/r^2, where r is the distance to the center of the Earth.  It never just dissipates.   20,000 mph is required for a low Earth orbit because you're still at nearly Earth's surface gravity at that point.  You absolutely do need to reach *some* orbital velocity to balance out Earth's gravity, probably the exception to this is Lagrange points 4 and 5, which would orbit the Earth at about the same rate as the moon (about 1 km/sec or 2240 mph).  http://en.wikipedia.org/wiki/Lagrangian_point

Outside of the orbit of the Moon, you start to become subject to the gravitation of the Sun.  At that point you get attracted to the Sun instead of the Earth.  Escaping that velocity is even harder, we're going at an orbital velocity of 29.8 km/s (67,000 mph) as it is!

For further physics details, see hyperphysics:  http://hyperphysics.phy-astr.gsu.edu/hbase/vesc.html

---------- FOLLOW-UP ----------

QUESTION: ok, one last question. pretend i had a rocket, that only flew 1000 mph, but had an "endless" supply of fuel. I pointed it straight up. I launch it. what happens? Does it just STOP flying straight up, at some point? If yes, at WHAT point? I misspoke, when I said "gravity dissipates", I know it never dissipates TOTALLY, but at acertain point, things appear "weightless", they float, even though they still have mass. I'm just trying to understand, WHY can't an object CONTINUE to fly at 1000 mph, away from the center of Earth? you said "it would fall away and burn up". Is that EVEN if it had an "endless" supply of fuel? And if the answer is yes, then what will "stop" the object, and at what point? I appreciate your help, I'm really tring to understand.

Answer
When I said it would fall and burn up, I didn't assume you meant an impossible rocket that could thrust forever.

You're setting a condition of 1000 mph that the plane flies, straight up, then sure.  It can go away from the Earth's center (forgetting about other bodies in the solar system for the moment) practically forever.  You're assuming thrust that lasts forever (or waaaay longer than human technology can provide), so if that's your assumption then you're fine.    You're also assuming it can push a rocket at a constant velocity, which isn't generally realistic because as they burn their large quantities of fuel rockets accelerate (because they have to push less and less mass as the rocket gets lighter).

The problem is that that's a ridiculously fuel-inefficient thing to do for practical reasons with a rocket.  And by ridiculously, I don't mean like me trying to balance a pin on its point and on the point of another pin.  I mean like unearthing the bones of a real flying dragon.  

Think of it this way, what's the most fuel-inefficient thing you could do?  Hover.  Getting nowhere, burning fuel forever.  It's an extension of the concept you mention.  You're pushing speeds way down to past what's possible with even some nuclear-powered rocket.  A real rocket that could get off the ground under the weight of the fuel you're talking about (for constant thrust at such low speeds) just isn't possible, that's why it's not done that way.