On Fri, 28 Dec 2007 12:27:44 +0000 (UTC), "Oz Lander" wrote:

Bob Noel wrote:

In article ,
"Oz Lander" wrote:

Why does the shuttle have to be travelling so fast to re-enter the
atmosphere?

How do you propose to slow the shuttle down from orbital velocity?

That I guess answers my question then. I was not aware that such high
speeds were required to just stay in orbit. What would it take to slow
the shuttle down whilst in orbit, enough to allow it to re-enter at a
slower speed?

You have to understand what "orbit" is: A balance between velocity and gravity.
Here's a simplified explanation.

Imagine a vehicle 100 miles in space with no velocity. It immediately starts
falling straight down, accelerating at 32 feet/second per second until it hits
the Earth.

Imagine the same vehicle at 100 miles with a horizontal velocity (e.g., tangent
to the Earth) of 1000 miles per hour. It now falls at a slant. But it takes a
bit longer to actually hit the ground, because the Earth is curved... it's
"curving away" from the oncoming vehicle. The vehicle want to travel in its
original direction, but gravity keeps pulling it toward the center of the Earth.
The velocity vector (imagine an arrow pointing in the direction the vehicle is
traveling at any given moment) alters until it intersects the ground, and the
object hits downrange of the release point. Because the Earth is round, that
downrange point is a bit further away, and the time to drop is a bit longer than
the no-velocity release.

But...what happens if you give your vehicle a fast enough speed that it "misses"
the Earth? If you give it *just* enough speed, you're in orbit...the forward
velocity balances the effect of gravity to hold you at a near-constant altitude.

The velocity is critical: If it's too low, the vector will sag downward. If
the velocity vector intersects the Earth, the vehicle will impact. Even if the
vector doesn't dip below the horizon, if the vehicle gets too low, the drag of
the atmosphere will further reduce its velocity...and the velocity vector drops
even further.

At 100 NM, a vehicle in a circular orbit is doing about 25,500 feet per second.
If it slows down just 150 feet per second (a bit more than 100 mph), it *will*
impact the Earth...and the atmosphere only makes matters worse!

The upshot, to a pilot, is that space objects cannot do "slow flight." There's
nothing "holding you up" other than your spacecraft's velocity...if you reduce
velocity, you're going down. There's really only a small range of speed you can
play around before the top of the atmosphere starts slowing you down and lets
the Earth suck you in. Unfortunately, the upper reaches of the atmosphere are
too thin to generate any appreciable lift unless you have very long
wings...which aren't the thing you want, hitting atmosphere at Mach 25.

You can add "lift" to your vehicle to maintain your altitude while it slows, but
there's only one way to do it: Add lift by firing rocket engines downward. This
is analogous to a Harrier transitioning to hovering flight. In fact, if you
could run a Harrier's engines in space, it probably would do quite nicely for a
low-speed return to Earth.

The problem is, this would take a *lot* of fuel. As others have posted, about
as much as it took to put the spacecraft into orbit to start with. The trouble
is, each pound of "return fuel" that you want to put into orbit takes about 15
pounds of launcher fuel to GET it there!

Until we develop antigravity, or highly-efficient engines that can put out the
thrust levels needed to hover, we're stuck with the high-speed reentry process.

Ron Wanttaja