Space Shuttle Roll @ T/O

[kangaroota]

Why does (did) the Space Shuttle roll over on its back shortly after lift off?

Why couldn't they orientate it in that position prior to lighting the touch paper?

How was the roll achieved? Dynamic forces on control surfaces, assymetric thrust or other?

[anti-skid]

**Very simply put**

As far as I know the roll is achieved by the shuttle's
elevons to roll the vehicle due east to gain 'free' speed
from the earth rotation (around 900kph?)

:confused:

[kangaroota]

Thanks A/S

But why do they need to roll it. Why not point it east from the very start?

[ferrydude]

roll is achieved by a combination of Aerosurface (elevons) movement and thrust vectoring. The main engines and the boosters are gimbaled

[Maximum]

The launch pads have been there since Apollo. The Shuttle has to be positioned correctly over the flame trenches on the pad, so this is what dictates orientation before launch.

The shuttle rolls once clear of the pad to head generally east, and then more specifically to a heading for the required orbit (in relation to the equator).

The roll allows for better comms with the ground, makes some emergency manoeuvres easier, gives the pilots a better view(!), and creates a slight negative angle of attack that protects the whole structure and the wings from overloading.

That's how I understand it anyway.:cool:

[411A]

And when it is in the approach/land mode, it uses DLC (ala Lockheed TriStar), and approach/land (autoland), as in Trident, later Lockheed.
All analogue, so i'm told.

Seems to work...ah, good:ok:

[Right Stuff]

The roll is made for two main reasons - as stated earlier it unloads the orbiter somewhat and prevents a structural failure and two, it allows an optimised trajectory.

If you send a launch vehicle straight up, you suffer horrendous gravity penalties but clear the thick portion of the atmosphere quickly, thereby minimising losses due to air resistance. Of course if you launch with a large horizontal trajectory the opposite is true. Therefore, the best solution is to head vertically initially and then start to introduce a horizontal component to decrease gravity losses and improve overall efficiency of the launch platform

[DrSyn]

Just to expand on Maximum's reply: It is not just the Shutlle, but all rocket launches which follow the same procedure. The giant pads at KSC, Cape Canaveral, Korou, etc, are fixed in one place. As no two launch trajectories are exactly the same, every flight has to be orientated to it's correct inclination after lift off.

Even the "circular" cross-sectioned rockets have X,Y,Z axes for guidance and orientation purposes, although it's not easy to see this at first view. The mighty Saturns followed a Roll Program after clearing the tower but not through such a great angle as the Shuttle and therefore not as obvious to the casual observer. Next time you have the oportunity to view an unmanned launch, watch carefully and you will notice the craft rotate about its longitudinal axis shortly after lift off.

Shuttle steering is achieved by a combination of the aero-surfaces and the gimballing of the booster and main engine nozzles, depending on the exact point along the flight path. There was a slightly more detailed answer here (http://www.pprune.org/forums/showthread.php?s=&threadid=72629) last time this one came up. Hope this helps.

[kangaroota]

Ladies and Gentlemen
Thankyou for taking the time to post your very informative replies.
I now know something my F/O doesn't - a first!!!!:D :D :D

[Speedbird252]

Just to add my 10 cents worth, the shuttle roll programme has a few good reasons, and have been covered mostly above.

The main reasons that im aware of are:

Allow the vehicle to achieve negative q alpha -

this removes a large proportion of the stress loading on the wing areas, the weakest link in the complicated structural chain

This also helps the orbiter use the earths gravitational pull to accelerate, the big gain being that it gets faster quicker which allows a heavier payload to be carried into a higher orbit.

It increases the S-band look angles - the data antennae gets a better line of sight back to earth during the entire launch, seperation and entry to orbit phase.

If im wrong I appologise, this is from memory, but im reasonabily sure that I havent completely lost it!

Regards all.

Speedy:ok: