I referred the Roll program in space shuttles. But, I can't understand the mechanism. So, They rotate during the launch in order for proper alignment. I know that only tilting, gimbaling or using additional vernier thrusters would align the space shuttles in proper direction. But how does this rotation of a space shuttle help in alignment? What's the mechanism behind this..?
[Physics] Rotation of Rockets
rocket-science
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Rory Alsop explained why the idea is wrong, but it may originated from the following reasoning.
When a space rocket takes of, it does so vertically. At that time it is fully loaded with fuel and hence its acceleration is slow. When you watch a video of a space rocket take-off, it seems to crawl along the launch tower.
However, in order to achieve orbit, the rocket has to travel 7 km/sec horizontally. To achieve that, after a while the rocket's path starts to curve towards the horizontal. At that point the first stage may already have dropped off and a large amount of fuel has been burned, so the rocket is a lot lighter. Because the acceleration is inversely proportional to the mass the rocket will be accelerating significantly faster at that point. At the same time, because the rocket is now fairly high up, the air pressure has dropped significantly, and the reduced drag also increases acceleration.
Hence, the rocket accelerates faster when it is going horizontally. Somebody could then take that as meaning "faster than upwards as well as downwards"
Edit
Another issue is the is the "dynamic pressure" which is created by the speed and air-drag. Because of this, the engines may not be run at full power until past the "max-q" point. In the case of the Shuttle, the main engines ran at 65% for the first minute or so of the flight. Only then was it throttled up to 100%, increasing acceleration. See http://www.aerospaceweb.org/question/aerodynamics/q0025.shtml
There are several issues with using ramjets on rockets:
The startup problem. Ramjets rely on the supersonic velocity of the vehicle to create the compression needed for the combustion chamber to operate. So you'll need something (traditional motor stage, air breathing engine, etc) to get the craft moving to the point the ramjets can actually function.
Ramjets have relatively low thrusts. They don't weigh much because they have no moving parts, but they don't produce a whole lot of thrust. So a rocket would need a large number of the engines to function.
Ramjets will eventually stop working as the speed gets higher. At some point, the duct upstream of the combustion chamber will not slow down the incoming air enough. This will make the air in the combustion chamber supersonic instead of subsonic. This is what a SCRAMJET engine is used for, but that's a slightly different configuration. So either the ramjet would need to have moving parts to change its geometry to handle subsonic and supersonic flow in the combustion chamber, or you would need to shut down the ramjets and open up some scramjets (which means the rocket would now have 3 different types of engines).
Ramjets, and scramjets, are air-breathing engines. For anything that gets very high in to atmosphere, there isn't much air left. So the engines won't work very well anymore. Traditional rockets carry both their fuel and oxidizer so they work anywhere.
All of that said, there is considerable research going on in the area. The USAF is actively funding research into them, and the Russian military has a pretty cool design for some self-guiding bombs that drop off of an ICBM and fly to their targets using scramjet engines.
Best Answer
The barbecue roll and the roll program are not related.
The former is for passive thermal control when the spacecraft is exposed to the Sun and the latter is a maneuver early in the launch sequence to orient the launch vehicle to the proper heading.
For a number of reasons, the space shuttle launch vehicle needs to be oriented "heads down" (the shuttle is positioned "underneath" the external tank) during most of the ascent. It's explained pretty well here: