At 10:55 AM 2/15/2008, Joe wrote:
How come, going up,, 0 to 17K MPh through the atmosphere, all is fine.. BUT
coming down,, 17K MPh to 0 unless it has protection it will burn up in the atmosphere from friction with the air.
I suspect if you look at the launch profile, you'll find that the passage up through the atmosphere is relatively slow, and that the bulk of the speed increase occurs above where the air density is sufficient to cause problems.
Also, maximum acceleration is towards the end of the main engine burn, due to simple physics. As the propellant is expended, the mass of the vehicle is less, but the thrust remains roughly constant, unless the engines are throttled back to reduce the G forces, as is done with the Shuttle to minimise stress on the crew. This implies that much of the speed increase occurs late in the burn, when you're high enough to avoid significant friction heating.
73 de VK3JED http://vkradio.com