How do they stop a space shuttle in space??

Could you explain why you think they "stop" a shuttle? Did some one tell you that, or are you confused about what happens when, say, they dock with the Station?

Let's talk about the thrusters for a minute and just leave the stopping behind us, because I think it has been sufficiently stated they don't stop. The thrusters create thrust (hence the name) which is a force. For every force (some former genius claimed) there is an equal and opposite force. So you squirt a jet out of thruster engine and voila, an opposite force acts on the body of the shuttle and moves it away from the direction the jet moves.

You don't really even need a thruster...urine evacuations cause enough thrust (force) to move the shuttle on orbit.

Then finally - there is some atmosphere in LEO (Low Earth Orbit) - the molecules are just few and very far between - but it's there. In fact, there is enough to cause drag - hence the need to do thruster attitude adjustment burns because the drag force messes with the trajectory.

Hope that helps.

Originally posted by enigmania

No, air and gravity is not involved. This is HORIZONTAL force, not vertical. So gravity doesn't apply.

Originally posted by Deaf Alien


No, air and gravity is not involved. This is HORIZONTAL force, not vertical. So gravity doesn't apply.

This is incorrect. There are:

drag forces
gravity forces

which cause an effective "crabbing" of the vehicle as it orbits the Earth...primarily because the primary forces change at different points along the orbit. For instance, on the backside (opposite the sun) of the Earth the drag forces will be greatest and at the sides of the orbit the gravity forces will start coming into play.

reply to post by Valhall


Wasn't talking about that. I was talking about the experiment involving two people on wheeled chairs. When these people push each other, they will go away from each other in opposite directions.

This is a demonstration of third law. Obviously, gravity doesn't apply in this case.

reply to post by Valhall


That still doesn't explain how they would prevent the shuttle from perpetual spin in the opposite direction of the last fired thruster, in a non-friction environment.

I'm not talking about stopping the shuttle in orbit, I'm talking about axis rotation while in orbit.

reply to post by enigmania


You fire a retro thruster ... in the opposite of the rotation until the rotation cancels... they have thrusters for all directions by the way.





[edit on 14-8-2008 by The_Alarmist2012]

Originally posted by enigmania
reply to post by Valhall

 

That still doesn't explain how they would prevent the shuttle from perpetual spin in the opposite direction of the last fired thruster, in a non-friction environment.

I'm not talking about stopping the shuttle in orbit, I'm talking about axis rotation while in orbit.

Well, if you did an attitude thruster burn that put it in a perpetual spin it would mean that you did an incorrect attitude burn. An attitude adjustment burn is not a single jet firing but a combination of jets. And after a burn it DOES continue to drift in the new direction and eventually the navigation system detects an error of expected position that is too great and a new combination of thrusters are fired and so on and so forth.

[edit on 8-14-2008 by Valhall]

Originally posted by Deaf Alien

Originally posted by enigmania

No, air and gravity is not involved. This is HORIZONTAL force, not vertical. So gravity doesn't apply.

Doesn't the force of gravity pulling down on the ball exert some force against me throwing it.

Isn't the force I must use, because of gravity, to lift and throw the ball, the equal force that moves me?

Originally posted by enigmania

Originally posted by Deaf Alien

Originally posted by enigmania

No, air and gravity is not involved. This is HORIZONTAL force, not vertical. So gravity doesn't apply.

Doesn't the force of gravity pulling down on the ball exert some force against me throwing it.

Isn't the force I must use, because of gravity, to lift and throw the ball, the equal force that moves me?

Right. Gravity is a vertical force. It doesn't go anywhere but down, so in my experiment gravity is not involved.

reply to post by Valhall


So, technically speaking, is it possible to fully stabilize a spinning object in space, without having to perpetually correct its spin?

They don't stop, they fly around in orbit. But thrusters work without atmosphere.

Originally posted by enigmania
reply to post by Valhall

 

So, technically speaking, is it possible to fully stabilize a spinning object in space, without having to perpetually correct its spin?

No. Not with the attitude control systems we have now. Maybe in the uber-spacey-future...but not now.

reply to post by Deaf Alien


What I meant was that because of gravity, I have to exert force to throw the ball, and that the opposite of that force, would move me in the opposite direction.

So that would make gravity count in your experiment.

But then I realised my mistake, the same gravity that is pulling down the ball, is also pulling down on me and the chair, so in the end, it gravity indeed doesn't matter.

Well it took some time, but it's finally getting a bit clearer. Still don't get why it works like this though.

Thank you guys for discussing this, I appreciate it.

I'm off to bed now.

edit: I also just realised that I mixed up the experiment you mentioned with a similar experiment where you throw a ball while sitting on a chair with wheels. Sorry. I need some sleep.

[edit on 14/8/08 by enigmania]

Newton's third law: "For every action there is an equal, but opposite, reaction". This applies to space as well.

reply to post by enigmania


Oh, help me, Jeebus!!! (from the Simpsons...)

enigmania...if your are asking a serious question, here's a serious answer. You are in orbit, in a space ship. You fire a thruster that causes you to rotate in a certain direction. THEN you fire thrusters that STOP your rotation.

So, now you have a different orientation than you did before. Nothing happens quickly, in current technology, in these scenarios. You may have seen science fiction movies....they don't show true space flight, as in the Shuttles....

You may have seen footage from the Apollo era....some of it, released to the public, was sped up!! Showing, for instance, the LM Ascent stage docking with the CM, in Lunar orbit. Because the actual process is painfully slow, and delicate. Would you rather watch two hours of a docking sequence, or see it sped up and edited??

Yah know, it's hard to describe the reality, when too much fictional entertainment has polluted the minds of the average person.

An object in LEO will not 'fly around' the Earth....unless acted upon, its orientation relative to its free-fall trajectory, will not change.

So, if the Shuttle is, say....showing its back to the observer on the Earth (imaginary, please) at some point over the US, then 45 minutes or so, later, halfway around its orbit, the back is now facing AWAY from Earth.

The vehicle's orientation won't change, unless acted upon by some force....

I know it's hard to comprehend, because it isn't something you do everyday....

If you can try to think three dimensionally, it will help. An object will have three axes (plural for 'axis')

let's call them X, Y and Z On orbit, nothing will change the spacecraft's orientation about these axes, unless or until a force is exerted.

A picture would be worth a thousand words, right about now....you can look this stuff up....ask your science teacher, or Wikipedia....

Or, email NASA....I don't know how you'd do it, but the answers are out there...go study and learn!!!!

Originally posted by weedwhacker


Or, email NASA....I don't know how you'd do it, but the answers are out there...go study and learn!!!!

Or you could just ask people....as I believe they have and they are getting the answers. I'm not sure why you're talking down to people. I'm assuming you've got some great achievement in life that you believe gifts you the right to have the attitude??? If so, bring your feet back down to the layman's level and talk to people as if you were sitting on a couch across from them and discussing this, instead of smarking at them from behind your LED shield.

Now - back to the subject.

Let's look at the ISS (station) - it flies what is referred to as a TEA (Torque-Equilibrium Attitude). This minimizes the load on the CMGs (contorl moment gyroscopes). What the TEA does is adjust the attitude to minimize:

1. drag during regions when drag is the dominant force on the station,
2. inertial forces when gravity is the dominant force on the station....

and it continues this way for each orbit attempting to minimize the torque acting on the station and therefore minimize the number of times momentum dumps have to occur for the CMGs to prevent gimbal lock. This is about as efficient as we've gotten on attitude control to date. By majority the station operates (if all is going according to plan) with as little attitude thrust as possible and relies on its CMGs.



[edit on 8-14-2008 by Valhall]

reply to post by Valhall


thanks, Valhall....I hadn't thought about gyro-stabilization, as pertains to an orbiting platform...but that makes sense.

When the Shuttle is docked....must change the C/G a bit....but those geniuses think of that.

Actually, I wished NOT to talk down to anyone....it was a valid question, I did my best to answer....using high-school science, and not 'Star Wars science'.

So, thanks for the gyro bit....perfect sense, there....not sure about the conservation of energy problem.....but that's why the ISS needs occasional replenishing of supplies, right?

BUT, doesn't the ISS (or Shuttle) need ot re-orient, occasionally, to control the uneven heating effects, from the Sun?

Originally posted by weedwhacker

So, thanks for the gyro bit....perfect sense, there....not sure about the conservation of energy problem.....but that's why the ISS needs occasional replenishing of supplies, right?

What has to happen occasionally is a reboost, because the ISS is constantly falling, and this is usually done by either a Proton or by the Shuttle when docked to the ISS.

BUT, doesn't the ISS (or Shuttle) need ot re-orient, occasionally, to control the uneven heating effects, from the Sun?

They are on a bit of a natural rotisserie, if you will. There aren't any specific maneuvers (that I'm aware of) to deal with heating as the attitudes they tend to fly even it out per orbit. More importantly is working the drag and inertial effects out over each orbit. The ISS does this on a TEA, but for an orbiting shuttle (not docked to the ISS), I'm not sure they do any torque equalization.

reply to post by Valhall


Thanks, Valhall....one point

About the 'constantly falling' bit....that may confuse some folk.

Maybe your wording will invite a misconception?

ANYTHING in orbit, is constantly 'falling'....as I've mentioned.

That is the nature of an 'orbit'.

But, you already know this.

I was explaining the concept to someone else, who didn't know.

Thanks, though, for your info about the ISS...I in fact, had not considered the use of gyros to orient the ISS....NOW, it makes better sense, based on what few bits I've seen on TV..

In fact, more I think about it, the more I ponder it.

I will think about it, and realize that, to change the orientation of the ISS, all they must do is alter the gyros....

Lots to think about, and I appreciate it.

When the fuel explosively combines in a rocket engine, the rapidly expanding exhaust gases only have one exit......and that's the open end at the bottom. Newton's Third Law states that every action has an equal and opposite reaction, so the upper end of the combustion chamber experiences an equal force to that being expelled at the back. That force lifts the entire launch vehicle off the ground.


[edit on 15-8-2008 by Mogget]