Space shuttle

[Loosehead]

OK I have a pretty reasonable understanding of the whole heat sheild thing. I know that the orbiting shuttle will have to enter the atmosphere at about 17000mph and that the heat as it slows is caused by friction, they are worried that it will burn up. My question is, why is it necessary? Why can it not simply slow by it's retros above the atmosphere until it slows right down and just falls into the atmosphere at a more managable speed. I know there must be a good reason why not but I can't think of it! Or perhaps I should phone up NASA and..........

[mycatis]

Oh thank you! I really wanted to ask this but was to embarrased, lol Why cant it just go slower? seems pretty obvious to me, though of course I'm not a scientist.

[BigDogsWang]

It is the gravity of the earth that pulls the Shuttle towards it at approx 17,000 mph. In the same way that when it takes off it needs to travel at 17,000 mph to escape the earth's pull. To slow an object down which is being pulled by by the gravity of something weighing 5,972 sextillion metric tons would need the mother of all brakes!

[BigDogsWang]

It is the gravity of the earth that pulls the Shuttle towards it at approx 17,000 mph. In the same way that when it takes off it needs to travel at 17,000 mph to escape the earth's pull. To slow an object down which is being pulled by the gravity of something weighing 5,972 sextillion metric tons would need one hell of a set of brakes!

[BigDogsWang]

I HATE it when that happens!

[jimmer]

slowing down that much would require too much fuel, so they slow down aa little bit with retro rockets then use the atmoshere to slow down further. Great question

 

http://en.wikipedia.org/wiki/Re-entry#Why_active_braking _cannot_be_solely_relied_upon

[Loosehead]

Understand that Bigdogs but at orbital speed it is in "free fall" missing the earth, powered by gravity but not being pulled as in the perpendicular but as in the earth curving away as fast as the fall hence orbit. There are control rockets on the shuttle for manuovering if they slowed it, would it not start to spiral in getting slower and effectively start to fall toward the ground?

[Clanad]

Actually, escape velocity is approximately 25,000 miles per hour...

[Loosehead]

Who said anything about escape velocity?? We are talking about re entry from orbit.

[Clanad]

BDW, stated "In the same way that when it takes off it needs to travel at 17,000 mph to escape..." Perhaps you didn't see that, Loosehead...

[Loosehead]

I did see that but in the light of the question and being one not normally afflicted with pedantry I assumed that he meant to escape the Earth's pull enough to acheive orbit. You are however correct in the escape velocity.

[Gef]

The retro-rockets slow down its orbital speed so that it starts to be pulled down by Earth's gravity. The Shuttle has no brakes and so soon reaches terminal velocity which is why the friction of the Earth's atmosphere heats it up so much. The only way to avoid this would be to use the same amount of power ( two solid rocket boosters plus the main engine) to slow it down as those used to launch it.