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A ride on the Shuttle

by Quinzio
Tags: ride, shuttle
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Quinzio
#1
Mar25-13, 12:28 PM
P: 558
Yesterday I got fascinated by the camera recordings on the Atlantis Space Shuttle.
http://www.youtube.com/watch?v=2aCOyOvOw5c

I have two questions which I cannot give an answer. Maybe some of you may help.

1) Lift off: we can assume the big ship has its center of mass in the geometrical center. The force if applied in the lower end (at the nozzles). This is a really unstable way to move something. In the first seconds of the lift off the wings cannot have any stabilizing effect, I think. Why doesn't the thing fall over ?

2) Back into the atmosphere. Meteorites become balls of fire when they fall into the atmosphere. Why doesn't the same happen to the boosters ?
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SteamKing
#2
Mar25-13, 02:21 PM
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When the shuttle lifted off, the boosters only provided part of the total thrust. The shuttle main engines are also running and are putting out a certain amount of thrust as well. When you combine all of these different thrusts along their lines of effort, then this combined thrust acts along a line thru the c.g. of the shuttle, producing no net torque.

When the boosters are ejected, they are not travelling at the velocity which a meteor might have when it strikes the atmosphere. After four minutes of flight, the shuttle has reached an altitude of more than 100 km, and there is very little atmosphere causing friction to the boosters.
etudiant
#3
Mar25-13, 03:30 PM
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You are entirely correct that rockets pushed from the bottom are painfully unstable.
That is why the shuttle liquid fuel motors are flexibly mounted, you can see them wiggle in the final checkout before the solids are fired and the vehicle released. Achieving the necessary control to adjust the engine thrust vector to keep the vehicle going straight was a major hurdle in rocket development.

Ibix
#4
Mar25-13, 04:19 PM
P: 378
A ride on the Shuttle

Per the Bad Astronomer, you can either tilt ('gimbal') the main engines, or provide attitude control thrusters that fire perpedicular to the main engine and tip the vessel. You use a gyro to monitor the direction you want to go and counter any motion.

Alan Bean apparently described the lunar lander taking off as a 'sporty ride'...
cjl
#5
Mar25-13, 06:28 PM
P: 1,008
Quote Quote by etudiant View Post
You are entirely correct that rockets pushed from the bottom are painfully unstable.
That is why the shuttle liquid fuel motors are flexibly mounted, you can see them wiggle in the final checkout before the solids are fired and the vehicle released. Achieving the necessary control to adjust the engine thrust vector to keep the vehicle going straight was a major hurdle in rocket development.
Rockets pushed from the bottom, from the top, or from any other point are all equally stable (or at least the point at which the force is applied does not impact the stability). That's not to say they are all stable - the location of the applied force is just not a factor. This is a common myth though - even Goddard's first liquid fueled design was impacted by this (incorrect) thinking.

See this wiki page for details.

As for the Shuttle? It (and all other modern large launch vehicles) uses a full, closed-loop control system and engine gimballing to remain stable (it is actively trying to keep itself on the correct trajectory by tilting the engines a few degrees when needed). They do not use aerodynamic controls, as this would only work during a relatively small portion of the launch after the rocket has gained enough speed for them to be effective, but before it is so high that there is insufficient atmospheric density.
etudiant
#6
Mar25-13, 07:43 PM
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[QUOTE=cjl;4321709]Rockets pushed from the bottom, from the top, or from any other point are all equally stable (or at least the point at which the force is applied does not impact the stability). That's not to say they are all stable - the location of the applied force is just not a factor. This is a common myth though - even Goddard's first liquid fueled design was impacted by this (incorrect) thinking.

See this wiki page for details.


Thank you for a constructive clarification.
Quinzio
#7
May19-13, 04:18 AM
P: 558
Thanks everybody for shedding light.

Some doubts remains.
Everyone has experienced at least once that keeping a broom upside down supported by the palm of a hand is an embarassingly difficult task.

Does the lenght of the object makes the task easier ?
So, is it much more easy to balance a spaceship than a broom ?
Introyble
#8
May19-13, 02:22 PM
P: 33
A great question, here is a fantastic engineering series by Nat Geo. This particular episode is about the Space Shuttle. A marvel of physics at a mere 37 million horsepower. Watch this, you will get many answers.

cjl
#9
May21-13, 02:32 PM
P: 1,008
Quote Quote by Quinzio View Post
Thanks everybody for shedding light.

Some doubts remains.
Everyone has experienced at least once that keeping a broom upside down supported by the palm of a hand is an embarassingly difficult task.
That isn't really an analogous situation. Balancing a broom involves a force from your hand which is directed upwards, even when the broom is slightly off-vertical. On a rocket, the engine tilts with the rocket, so if the rocket as a whole starts to tilt, there isn't the runaway instability that there would be with the broom.
etudiant
#10
May21-13, 02:58 PM
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Quote Quote by cjl View Post
That isn't really an analogous situation. Balancing a broom involves a force from your hand which is directed upwards, even when the broom is slightly off-vertical. On a rocket, the engine tilts with the rocket, so if the rocket as a whole starts to tilt, there isn't the runaway instability that there would be with the broom.
This is not correct.
The engines in rockets are flexibly mounted and can wiggle from side to side by several degrees.
Watch a video of a shuttle launch, they usually show the liquid fuel engines starting up and swiveling to check out the system before the solids fire.
That ability to move is essential to allow the desired trajectory to be followed, despite winds and aerodynamic loads.
LastOneStanding
#11
May21-13, 03:24 PM
P: 718
Quote Quote by etudiant View Post
This is not correct.
The engines in rockets are flexibly mounted and can wiggle from side to side by several degrees.
Watch a video of a shuttle launch, they usually show the liquid fuel engines starting up and swiveling to check out the system before the solids fire.
That ability to move is essential to allow the desired trajectory to be followed, despite winds and aerodynamic loads.
I think, from the context, that by "tilts with the rocket", cjl may have meant "tilts at the same time as the rocket does in compensatory way".


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