Author Topic: Something I've Never Understood...  (Read 3875 times)

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Offline R. U. Sirius

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Something I've Never Understood...
« on: September 05, 2012, 08:56:23 am »
Maybe someone here can help me out.

How the heck do rockets work in space? I can understand how they would work here on Earth and to get INTO space, but in a vaccuum, there would be nothing to push against. I would think the energy expended would just dissipate without effect. MY BRAIN A SPLODE!

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Offline Art Vandelay

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Re: Something I've Never Understood...
« Reply #1 on: September 05, 2012, 10:05:48 am »
Basically, Newton's third law. Think of it this way. It's not just that the exhaust is being pushed out of the rocket, it's that both the rocket and the exhaust are being pushed apart. It means that not only will the exhaust be forced out of the rocket, but at the same time the rocket will be pushed in the opposite direction. It's the same principle as recoil on a firearm. The force that pushes the bullet out of the gun also applied in the opposite direction on the gun itself.

Hope that helps.

Offline SpaceProg

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Re: Something I've Never Understood...
« Reply #2 on: September 05, 2012, 03:30:27 pm »
I dunno... I think he meant something like:  When the rocket leaves the atmosphere where you have all the ingredients for combustion and Newton's law taking effect, to the vacuum of space where you can't ignite something (?) how do you keep going into space farther than just falling into an orbit?  How is it pushed further once you're in the vacuum?

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Re: Something I've Never Understood...
« Reply #3 on: September 05, 2012, 03:43:28 pm »
I dunno... I think he meant something like:  When the rocket leaves the atmosphere where you have all the ingredients for combustion and Newton's law taking effect, to the vacuum of space where you can't ignite something (?) how do you keep going into space farther than just falling into an orbit?  How is it pushed further once you're in the vacuum?
Because the thrust itself is still pushing against the housing containing it, as well as pushing out into the vaccum. Probably at a bit of a reduced rate, but at high speeds through the air they'd have a similar issue due to a lack of air pressure behind the thruster and we've managed that well enough.

Offline Art Vandelay

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Re: Something I've Never Understood...
« Reply #4 on: September 05, 2012, 09:25:32 pm »
I dunno... I think he meant something like:  When the rocket leaves the atmosphere where you have all the ingredients for combustion and Newton's law taking effect, to the vacuum of space where you can't ignite something (?) how do you keep going into space farther than just falling into an orbit?  How is it pushed further once you're in the vacuum?
If that's the case, then it's simply because rockets carry their own supply of oxygen for combustion. It's jets that rely on the outside atmosphere for their oxygen.

Offline SpaceProg

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Re: Something I've Never Understood...
« Reply #5 on: September 05, 2012, 09:33:27 pm »
We have a long way to go I guess if we want to travel very far away from Earth.   

Offline Star Cluster

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Re: Something I've Never Understood...
« Reply #6 on: September 06, 2012, 12:09:19 am »
Art mentioned Newton's 3rd Law of motion, which is correct, but didn't quite go far enough with the explanation. 

Simply put, Newton's 3rd law is "For every action, there is an equal and opposite reaction."  In rocketry, it makes no difference if there is something to push against or not.  The exhaust of burning fuel is a chemical action.  For that, there must be an opposite reaction, which in this case is thrust.

Actually, it takes less thrust in space to move a rocket than it does on Earth, since the gravity is much less the higher you get.  That's why maneuvering rockets on spacecraft are so small.  It only takes a small, short blast to turn or position an object.  In blasting off from earth, a rocket must create tremendous thrust in order to lift off the ground and reach escape velocity.  The rocket isn't really pushing against anything except gravity.  It's just that the thrust created is far greater than the weight of the rocket itself.

It's much the same way that an balloon will move forward once you release it.  Although it is only air coming out of the balloon, the volume of air is creating thrust against the opening in the balloon which is greater than the weight of the balloon, therefore propelling it forward. 

@SpaceProg--yes, we have a long way to go in rocket technology before humans can go very far away from Earth. 
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Offline The Illusive Man

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Re: Something I've Never Understood...
« Reply #7 on: September 06, 2012, 12:35:06 am »


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Offline Katsuro

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Re: Something I've Never Understood...
« Reply #8 on: September 06, 2012, 04:10:04 am »
If that's the case, then it's simply because rockets carry their own supply of oxygen for combustion. It's jets that rely on the outside atmosphere for their oxygen.

Which is why you CAN have explotions in space. I'm sick of people always pointing out firey-exploding space ships as an inaccuracy in sci-fi movies and TV shows. There's combustable fuel on the ship as well as the oxygen for the crew to breathe.

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Re: Something I've Never Understood...
« Reply #9 on: September 14, 2012, 08:33:36 pm »
If that's the case, then it's simply because rockets carry their own supply of oxygen for combustion. It's jets that rely on the outside atmosphere for their oxygen.

Which is why you CAN have explotions in space. I'm sick of people always pointing out firey-exploding space ships as an inaccuracy in sci-fi movies and TV shows. There's combustable fuel on the ship as well as the oxygen for the crew to breathe.

Yes, but the common mistake is that you hear a BANG. Unless you were on the expldong craft, you wouldn't hear anything as sound does not propagate through a vacuum.
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Re: Something I've Never Understood...
« Reply #10 on: September 14, 2012, 10:22:47 pm »
If that's the case, then it's simply because rockets carry their own supply of oxygen for combustion. It's jets that rely on the outside atmosphere for their oxygen.

Which is why you CAN have explotions in space. I'm sick of people always pointing out firey-exploding space ships as an inaccuracy in sci-fi movies and TV shows. There's combustable fuel on the ship as well as the oxygen for the crew to breathe.

Yes, but the common mistake is that you hear a BANG. Unless you were on the expldong craft, you wouldn't hear anything as sound does not propagate through a vacuum.

I think that's a mistake you have to make.

The audience expects a visual and audio experience from what they're watching.  If an explosion has no sound, the majority of the audience will leave feeling disappointed.

Personally, if I saw an explosion on television, in space, with no sound, I'd wonder if my television glitched out, even though I know sound doesn't travel in a vacuum.  It's theatrics.
« Last Edit: September 14, 2012, 10:25:53 pm by Zachski »
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Offline Yla

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Re: Something I've Never Understood...
« Reply #11 on: September 21, 2012, 03:09:11 pm »
Maybe someone here can help me out.

How the heck do rockets work in space? I can understand how they would work here on Earth and to get INTO space, but in a vaccuum, there would be nothing to push against. I would think the energy expended would just dissipate without effect. MY BRAIN A SPLODE!

Anyone?
My physics professor demonstrated it by sitting on a wagon and throwing a ball backwards. (To general hilarity, the so-generated thrust was not sufficient to overcome static friction. In other words, nothing happened. The lecture hall was in stitches.)

If that's the case, then it's simply because rockets carry their own supply of oxygen for combustion. It's jets that rely on the outside atmosphere for their oxygen.

Which is why you CAN have explotions in space. I'm sick of people always pointing out firey-exploding space ships as an inaccuracy in sci-fi movies and TV shows. There's combustable fuel on the ship as well as the oxygen for the crew to breathe.
Yes, but they look rather different. With no air pressure, that oxygen/combustible has a noted tendency to evaporate rather quickly, and in otherwise same conditions, what makes a fiery explosion in an atmosphere isn't necessarily sufficient (by far) to make a fiery explosion in vaccum. Also, the explosion itself has less counteractors blowing its own fuel away.
The typical fireball also relies on interaction with the surrounding air to have the visual apperence they have.
« Last Edit: September 21, 2012, 03:13:41 pm by Yla »
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Offline Star Cluster

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Re: Something I've Never Understood...
« Reply #12 on: October 02, 2012, 06:34:55 pm »
If that's the case, then it's simply because rockets carry their own supply of oxygen for combustion. It's jets that rely on the outside atmosphere for their oxygen.

Which is why you CAN have explotions in space. I'm sick of people always pointing out firey-exploding space ships as an inaccuracy in sci-fi movies and TV shows. There's combustable fuel on the ship as well as the oxygen for the crew to breathe.

Yes, but the common mistake is that you hear a BANG. Unless you were on the expldong craft, you wouldn't hear anything as sound does not propagate through a vacuum.

I think that's a mistake you have to make.

The audience expects a visual and audio experience from what they're watching.  If an explosion has no sound, the majority of the audience will leave feeling disappointed.

Personally, if I saw an explosion on television, in space, with no sound, I'd wonder if my television glitched out, even though I know sound doesn't travel in a vacuum.  It's theatrics.

Actually, the original Star Trek series did just that.  They would show an explosion in space with no sound just as it should be.  But they would also show a shot from outside the ship when they fired the ship's phasers or photon torpedoes and you could hear them fire.  Yay for inconsistency.
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Offline Joey

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Re: Something I've Never Understood...
« Reply #13 on: November 15, 2012, 04:57:22 am »
We have a long way to go I guess if we want to travel very far away from Earth.
Not really, the only energy required is that needed to achieve escape velocity. By Newton's 1st law, once you're outside of the gravitational pull (no external forces) you'll keep going and going without any fuel required.

The bigger problem is sustaining human life on board the ship for the amount of time needed to go anywhere interesting.
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Offline Katsuro

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Re: Something I've Never Understood...
« Reply #14 on: November 15, 2012, 05:27:39 pm »
Maybe someone here can help me out.

How the heck do rockets work in space? I can understand how they would work here on Earth and to get INTO space, but in a vaccuum, there would be nothing to push against. I would think the energy expended would just dissipate without effect. MY BRAIN A SPLODE!

Anyone?
My physics professor demonstrated it by sitting on a wagon and throwing a ball backwards. (To general hilarity, the so-generated thrust was not sufficient to overcome static friction. In other words, nothing happened. The lecture hall was in stitches.)

If that's the case, then it's simply because rockets carry their own supply of oxygen for combustion. It's jets that rely on the outside atmosphere for their oxygen.

Which is why you CAN have explotions in space. I'm sick of people always pointing out firey-exploding space ships as an inaccuracy in sci-fi movies and TV shows. There's combustable fuel on the ship as well as the oxygen for the crew to breathe.
Yes, but they look rather different. With no air pressure, that oxygen/combustible has a noted tendency to evaporate rather quickly, and in otherwise same conditions, what makes a fiery explosion in an atmosphere isn't necessarily sufficient (by far) to make a fiery explosion in vaccum. Also, the explosion itself has less counteractors blowing its own fuel away.
The typical fireball also relies on interaction with the surrounding air to have the visual apperence they have.

A very fair point. But I've never heard anyone say "explosions in space wouldn't look like that" they just say something like "explosions in space are impossible" which implies they mean they're completely impossible altogether, in any shape or form.