Space Shuttle.

The same amount of fuel that was needed to accelerate it in the first
place, plus lots more to get that extra fuel up.

Actually not the same amount...not even close. You only have to lower
the perigee such that you reenter on the proper trajectory whereas
going up you have to get into a circular orbit

Ron Lee

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

"WolfRat" wrote

They could aero-brake from orbit slower but it would take forever.

Not really. If you slow down gradually, the effect of gravity takes hold,
and you start falling faster. Mush of the speed is picked back up in that
manner.

If they launched an unmanned fuel source, docked and then transfered fuel
it could be done.

You still have to pay to get all of that fuel up there. That is not cheap,
let alone the fueling station cost, and the weight to get that up there.

Keep it simple the new Orion(Apollo on steroids) will work just fine. The
Russians have done a good job with safe and repeated journeys from orbit.

I would have more of a warm fuzzy feeling, if that were true. The Russians
have lost a couple crews on the whole re-entry landing procedure, I believe.
At least one, I am positive.

The Orion will be/should be more reliable, because of more simplicity, and
the fact that it will be above the whole rocket, and the fact that it does
not need to be made reusable.
--
Jim in NC

On Fri, 28 Dec 2007 13:53:06 -0500, "Morgans" wrote:

Keep it simple the new Orion(Apollo on steroids) will work just fine. The
Russians have done a good job with safe and repeated journeys from orbit.

I would have more of a warm fuzzy feeling, if that were true. The Russians
have lost a couple crews on the whole re-entry landing procedure, I believe.
At least one, I am positive.

Soyuz 1 and Soyuz 11. The first due to a parachute failure, the second due to
failure of a valve.

http://en.wikipedia.org/wiki/Soyuz_1

http://en.wikipedia.org/wiki/Soyuz_11

Ron Wanttaja

On Dec 28, 4:05*am, "Oz Lander" wrote:
Just watched a show on the Columbis disaster, and a question came to me.
Why does the shuttle have to be travelling so fast to re-enter the
atmosphere?

Think of it this way. The shuttle isn't moving fast, its landing on a
fast moving object. Landing on the earth is like a carrier landing,
the earth is spinning around and the shuttle has to match to it.

-Robert

WolfRat wrote:
Wolfgang Schwanke wrote:
"Oz Lander" wrote in
:

What would it take to slow
the shuttle down whilst in orbit, enough to allow it to re-enter at a
slower speed?

The same amount of fuel that was needed to accelerate it in the first
place, plus lots more to get that extra fuel up. To avoid all that,
spacecraft use the atmosphere for braking. They've been doing that
since the beginning of manned spaceflight, it's not specific to the
shuttle. They just accept the risk associated with that method as a
tradeoff against the extra complication of carrying those enormous
masses of fuel all along.

Regards



They could aero-brake from orbit slower but it would take
forever. If they launched an unmanned fuel source, docked
and then transfered fuel it could be done.

And put it in what and do what with it?

It took the jettisoned external tanks and engines to provide the
energy to get up in the first place and would take an equivelant
amount of energy to kill all your velocity and energy of position
in orbit to get down slowly.

--
Jim Pennino

Remove .spam.sux to reply.

Ron Lee wrote:

The same amount of fuel that was needed to accelerate it in the first
place, plus lots more to get that extra fuel up.

Actually not the same amount...not even close. You only have to lower
the perigee such that you reenter on the proper trajectory whereas
going up you have to get into a circular orbit

How do you get rid of all the velocity than builds up as you decend
through near vacuum and your energy of postition becomes energy of
motion?


--
Jim Pennino

Remove .spam.sux to reply.

In article ,
"Steven P. McNicoll" wrote:

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

The shuttle does slow from orbital velocity to re-enter.

It slows only slightly, enough to get its orbit to dip into the atmosphere
where it then uses aerodynamic braking. Avoiding the need for
aerodynamic braking was, I believe, the whole point of the original
question.

--
Bob Noel
(goodness, please trim replies!!!)

"Bob Noel" wrote in message
...
In article ,
"Steven P. McNicoll" wrote:

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

The shuttle does slow from orbital velocity to re-enter.

It slows only slightly, enough to get its orbit to dip into the atmosphere
where it then uses aerodynamic braking. Avoiding the need for
aerodynamic braking was, I believe, the whole point of the original
question.


The short answer to your follow-up question is the Orbital Maneuvering
System. The shuttle uses the OMS for both orbital maneuvering and to slow
from orbital velocity for reentry.

In article ,
"Steven P. McNicoll" wrote:

The short answer to your follow-up question is the Orbital Maneuvering
System. The shuttle uses the OMS for both orbital maneuvering and to slow
from orbital velocity for reentry.

I know how the shuttle de-orbits. My follow-up question was directed at
the OP.

In the context of the original question, the OMS is entirely inadequate.

--
Bob Noel
(goodness, please trim replies!!!)