Got into a discussion at work today about the thrusters on the shuttle and was wondering exactly what does the air push against if space itself is a void of anything including mass? in our atmosphere it obviously pushes against the air wich has mass as it can be weighed but if you bottle space its a vaccum? ugh brain hurts. my co worker thinks the air pushing against the jet itself is the push off point but this is a flawed idea in space cause the other end of the air stream coming from the jet is still having to connect with some form of mass to push against right??

[This message has been edited by jetsnvettes2000 (edited 02-02-2012).]

First off they are not air jets. They would be rocket thrusters. They work on the principle of for every action there is a reaction. There is mass to the rocket gasses that is pushed in one direction and the reaction to that is the thrust that propels the vehicle. The rocket gasses do not have to by held back by anything.

There's not a conveyor belt involved here somewhere, is there?

quote Originally posted by MadMark: First off they are not air jets. They would be rocket thrusters. They work on the principle of for every action there is a reaction. There is mass to the rocket gasses that is pushed in one direction and the reaction to that is the thrust that propels the vehicle. The rocket gasses do not have to by held back by anything.

Yes we went over bernoulies principles and newtons as well but, those apply to air wich has mass and gravity. What does the thrust of the rocket engine itself push against say in deep space free of gravity's pull = drag a force to fight against wich is the oppisite reaction of the thrust? kinda like pushing yourself thew water with your hand the resistance you feel is the oppisite of that thrust drag. if you flap your arms in space u dont go anywhere as Alan Shepard found out when he steped out of his capsule but then with a crudly rigged compressed air rig he was able to move.

[This message has been edited by jetsnvettes2000 (edited 02-02-2012).]

To my understanding, it's like the frictionless ice problem.

If you try to walk, you won't move. However, if you start throwing objects (clothes or ice chunks) away from the direction you want to travel, you will begin to move.

The rockets thrust mass that was previously in the ship, away from the ship, and that is what creates the movement.

However, that's just what I've heard. Perhaps Marvin knows--he usually pops in these threads with the right answer haha.

Lol its a funny question but we have had much deeper chats about usless triva like this. Cant ask the great google at work so here I am lol!

Think of it this way: the particles (any particles) ejected from the space ship don't suddenly start moving by themselves. They need to "push themselves of" something. That something is the space ship. And by doing so, they "push" the space ship in the opposite direction. They transfer kinetic energy.

If two people of equal mass stand face to face on ice, and one pushes the other, both will move with the same speed away from the point of origin (assuming the friction between the persons and the ice is the same for both). Now if one of the two was as small baby, then they both would still move away from the point of origin, but the baby would move much faster than the grown-up of course. Same principle with the space craft.

The mistake everybody makes, it that they think of the particles as something connected to the space craft (like arms) and that they push against "something" outside the space craft to push the space craft away. That's not how it works. The particles are being pushed away by the space craft causing the space craft to move.

Sit in a spinny chair and throw a bowling ball.

Like that.

How about something like a helicopter taking off from the moon?

Newton's 3rd Law of Motion: For every action, there is an equal and opposite reaction.

If you propel matter out of the craft in one direction, the craft is pushed in the other. It's not pushing against air, it's pushing against the craft.
Imagine yourself in a swimming pool. If you push off from the side of the pool, are you pushing against the water - or the side of the pool? The exhaust from the thruster is pushing against the thruster as it pushes off into space. Once it exits the thruster, it's job it done. The vacuum in space doesn't matter.

Also remember, Force = mass * acceleration.
The truster exhaust has very little mass, but it's accelerating very, very fast and that generates a force. That force acts against the much larger mass of the craft, giving it a much smaller amount of acceleration.

As was already said, they're not air jets, but an air jet would work the same. They use "Hypergolic" fuels, meaning that when you mix the fuel and oxidizer it immediately combusts and doesn't require any source of oxygen to burn or any form of ignition system. It's a very simple design - just a valve on the fuel and one on the oxidizer. Open the valves and you have thrust.

[This message has been edited by Formula88 (edited 02-02-2012).]

Although others have done a wonderful job of explaining this phenomenon, let's just say it's magic and if I told ya, I'd have to kill ya.

Being the non-violent, passive liberal that I am, I'd prefer to not have to do that.

Disclaimer: No individuals were harmed nor is any harm meant to be implied in this post.

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Ron
The key thing is to wake up breathing! All the rest can be fixed. (Except Stupid - You can't fix that)
Always remember these words of wisdom.

"The Lord must truly love fools, for he made them in abundance."

MadMark,you are right,but, you didn't go far enough with your definition.
Formula88,right on with Newton's third Law :For every action,there is an equal and opposite reaction.
Cliff,good explanation !

quote Originally posted by hugh: MadMark,you are right,but, you didn't go far enough with your definition. Formula88,right on with Newton's third Law :For every action,there is an equal and opposite reaction. Cliff,good explanation !

Oh, I see, you don't agree with the magic theory. OK Bub, now I'm hurt.

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Ron
The key thing is to wake up breathing! All the rest can be fixed. (Except Stupid - You can't fix that)
Always remember these words of wisdom.

"The Lord must truly love fools, for he made them in abundance."

Despite all the ecellent answers, I'm not sure his original question has been answered: What do the gases push against? He knows they don't push against the air, it's not there.

What the gasses push against is the inside of the rocket nozzle. As stated, the ignite and expand. They need to go somewhere, so the go out the open end of the nozzle by pushing against the closed end. The closed end, being attached to whatever you want to move, shoves it in the opposite direction to the escaping gasses.

Jet engines work the same way, by funelling the exhaust out the back, it winds up pushing against the inside of the combustion chamber, and shoving the jet forward.

Prop planes on the other hand, don't work like that.

quote Originally posted by Zeb: Prop planes on the other hand, don't work like that.

Or Helicopters. They beat the air into submission.

quote Originally posted by htexans1: Or Helicopters. They beat the air into submission.

Hey Hey Hey, let's stop that right now. OK, now you're in time out. You can come out when you've realized the falacy of your statement.

OK, I feel better now.

As to the OP, consider a ballon filled with air. The thrust from the ballon is due to air escaping from the ballon, not from pushing the air surrounding the ballon.

There, I said it, now you're on the "hit list". Make your peace with God, family and all others.

Disclaimer: There is no hit list, really, there isn't.

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Ron
The key thing is to wake up breathing! All the rest can be fixed. (Except Stupid - You can't fix that)
Always remember these words of wisdom.

"The Lord must truly love fools, for he made them in abundance."

Let's say that you were working outside the Space Station, and were stranded several meters away (for whatever reason) without a tether on. If you had a tool on your belt, you could just throw it in the opposite direction of the station and it would set you moving toward the station (more or less - in addition to moving, you would probably also set yourself up in a spin). You are imparting force against the tool, which is resisting movement, and that is where the thrust comes from - the tool is, essentially pushing you back, in the opposite direction.

Taken with a thrust engine, the fuel and the oxidizer are added together and they explode. They really don't care where, but they expand. Because a rocket has only one exit, the expanding gas pushes against the entire chamber more or less equally, but, because it can only vent one way, the result is a sort of "unbalanced push," in that the expanding gas pushes the combustion chamber away from the direction it is forced to go. The amount of thrust is based on the specific energy of the combustibles and the duration of the combustion. Since the rest of the ship is attached to the combustion chamber, it, too, moves, based on its mass, the distance of the thruster from the center of that mass and the direction the thruster is pointed.

quote Originally posted by Zeb: Prop planes on the other hand, don't work like that.

So helicopter = no, no atmosphere to pull/push?

[This message has been edited by 2.5 (edited 02-02-2012).]

quote Originally posted by 2.5: So helicopter = no, no atmosphere to pull/push?

In all seriousness, a prop plane and a helicopter work on whats called the "rotating wing theory"

Like a wing gets lift when it goes forward, a rotary wing works the same way, except the "wing" is mounted to a shaft turning it.
The turning of the "wing" (or prop or helo blade) causes the "lift" to occur which provides propulsion or lift depending on it being a helo (lift) or propulsion (prop)

quote Originally posted by 2.5: So helicopter = no, no atmosphere to pull/push?

Yeah, in space, a propellor is pretty useless. Propellors work by pushing the air in th opposite direction. The propellor is attached to the aircraft (or boat) and gets pushed by the reaction to that.

quote Originally posted by Formula88: Newton's 3rd Law of Motion: For every action, there is an equal and opposite reaction.

That's one correct way of looking at it. The momentum (MV ... mass x velocity) of the exhaust gases is "reacted" by transferring an equal momentum to the rocket body: MV_exhaust = MV_{rocket body}.

quote Originally posted by Patrick's Dad: Because a rocket has only one exit, the expanding gas pushes against the entire chamber more or less equally, but, because it can only vent one way, the result is a sort of "unbalanced push," in that the expanding gas pushes the combustion chamber away from the direction it is forced to go.

That's another, equally correct, way of looking at it.

Here's a thought experiment. Consider a sealed rectangular box with high pressure gas inside; the gas presses equally on all surfaces of the box. Now quickly remove one end of the box. At that moment the gas will still be pressing equally on all the remaining surfaces, but at one end there is no longer a wall for it to press against. So we now have a situation of unbalanced forces on opposite ends of the box, and the result is acceleration the box toward the remaining end.

quote Originally posted by Zeb: What do the gases push against? He knows they don't push against the air, it's not there. What the gasses push against is the inside of the rocket nozzle.

Correct. That's why a rocket motor is actually slightly more efficient in space than down in the atmosphere; the air in the atmosphere represents a small "back pressure" opposing the rocket's exhaust.

quote Originally posted by Zeb: Jet engines work the same way, by funelling the exhaust out the back, it winds up pushing against the inside of the combustion chamber, and shoving the jet forward.

Correct. Thrust is equal to the mass of air (per unit time) passing through the engine times the change in velocity (acceleration) of that mass.

If you really want to drive yourself crazy sometime, try to figure out how a pure ramjet works. Since there's no front wall, what do the combustion gases "push" against? (Answer: They push against the inlet air mass, which has a high dynamic pressure due to the speed of the ramjet through the atmosphere.)

quote Prop planes on the other hand, don't work like that.

Nope. They work exactly the same way. They just use a propeller to accelerate a mass of working fluid ... in this case air ... to generate thrust. MV_air = MV _airframe. (N.B. Technically, I should be using dM/dt and dV/dt here rather than just M and V, but I'm trying to keep the notation simple and since d/dt is constant it doesn't affect the qualitative results.)

Note that you can obtain the same thrust by imparting a big acceleration to a small mass of air (M_small x V_big) ... as in a pure jet ... or by imparting a small acceleration to a large mass of air (M_big x V_small) ... as with a propeller.

The techno-geeks among you should now be asking, "But what about conservation of energy?" (MV_in) may equal (MV_out), but (1/2 MV²_in) should equal (1/2 MV²_out) as well. How can both be correct at the same time? Good question ... and the answer is efficiency. Turns out it's much more efficient to apply a small acceleration to a large mass of air (i.e. the working fluid) than to apply a large acceleration to a small mass of air. That's why old-fashioned propellers are more efficient than jets, and why the trend in commercial aircraft design is to bigger and bigger turbofans. It's also why helicopters use big rotors for lift rather than using smaller rotors and/or jet thrust alone.

[This message has been edited by Marvin McInnis (edited 02-02-2012).]

quote Originally posted by Marvin McInnis: . . . Turns out it's much more efficient to apply a small acceleration to a large mass of air (i.e. the working fluid) than to apply a large acceleration to a small mass of air. . . .

Well, yes, but that sure makes it hard to justify asking for MORE POWER, doesn't it, Marvin?

quote Originally posted by blackrams: Oh, I see, you don't agree with the magic theory. OK Bub, now I'm hurt.

I'm willing to go with your magic explanation, Ron, but I keep coming back to the idea that this isn't your specialty like it is for Marvin. I mean, he put lots of math in his explanation and you said, " Trust me." We've been there with Nixon, haven't we?

quote Originally posted by heybjorn: I'm willing to go with your magic explanation, Ron, but I keep coming back to the idea that this isn't your specialty like it is for Marvin. I mean, he put lots of math in his explanation and you said, " Trust me." We've been there with Nixon, haven't we?

Keep in mind that while I've studied helicopters, Ron has actually flown them ... repeatedly. For that alone (and for surviving, I guess) he has my respect.

[This message has been edited by Marvin McInnis (edited 02-02-2012).]

quote Originally posted by Marvin McInnis: Keep in mind that while I've studied helicopters, Ron has actually flown them ... repeatedly. For that alone (and for surviving, I guess) he has my respect.

An Man, now I'm all choked up. Marvin, thanks. The respect is mutual, I assure you.

Now, as far as Heybjorne goes, pftttttt..........................................

Just kidding Mike, you know you're my favorite road hog.

------------------
Ron
The key thing is to wake up breathing! All the rest can be fixed. (Except Stupid - You can't fix that)
Always remember these words of wisdom.

"The Lord must truly love fools, for he made them in abundance."

quote Originally posted by Marvin McInnis: Keep in mind that while I've studied helicopters, Ron has actually flown them ... repeatedly. For that alone (and for surviving, I guess) he has my respect.

Yes, Marvin, but a good part of that is Ron doesn't know any better; you know, like bumblebees.

quote Originally posted by blackrams: Just kidding Mike, you know you're my favorite road hog.

Are you saying I'm FAT?

[This message has been edited by heybjorn (edited 02-02-2012).]

quote Originally posted by heybjorn: Are you saying I'm FAT?

Just between the ears.

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Ron
The key thing is to wake up breathing! All the rest can be fixed. (Except Stupid - You can't fix that)
Always remember these words of wisdom.

"The Lord must truly love fools, for he made them in abundance."

LOL you guys aw man I tell ya after reading this I really need to dust off my aircraft science study books from school, I think since I have left the aviation field 10 years ago I prob have forgot over 80% of what I learned maby its time I go back to the tech for a refresher. Far as the Ram jets oh I totally remember those and the principle of how they work, back when I worked for gulfstream they were toying with the idea of making a SST using a hybrid ramjet, I was given a book by them that has a pic of it so ill share it.



So now I got curious and ooops look what I found my old powerplant book from school: I should have studied harder all the answers and formulas u guys just gave here are right in this book!


a few teaser samples for ya





It is amazing how much you can forget when not in use grrrr so this is what going senile feels like.

Not to mention, while yes space is a vacuum that does not mean it is completely void of atmospheric gases.

quote Outer space has very low density and pressure, and is the closest physical approximation of a perfect vacuum. It has effectively no friction, allowing stars, planets and moons to move freely along ideal gravitational trajectories. But no vacuum is truly perfect, not even in interstellar space, where there are still a few hydrogen atoms per cubic centimeter.

There are still some hydrogen and helium particles floating around out there in space. Not that this really changes "how it works" it still works by pushing away from the enclosed area of ignition, not against the air outside. Just saying there's still some stuff floating around out there, LOL.

[This message has been edited by Khw (edited 02-02-2012).]

quote Originally posted by blackrams: ... now I'm all choked up.

Don't get too choked. [With a nod to PK86:] i'm sure i could fly a plane helicopter no problem.

Well, someone taught blackrams to, MM, so . . .

Lol hmmmm my first flying lesson my instructor took me up in a 172 cessna and told me to put it in a controled stall and as it felt like slush had me kick the rudder and cut the power oh ya I experienced the whole holding ball out in chair effect right there!! ground sky ground sky ground sky burp oh no almost ralphed hehehe! the second time was so fun! I have always wanted to try heli's but ran outa money to even finish my reg pilots license.

quote Originally posted by Marvin McInnis: [With a nod to PK86:] i'm sure i could fly a plane helicopter no problem.

Twist the throttle up slightly, twist the collective up slightly, push the peddles slightly in the opposite direction of the torgue, push the stick slightly forward, increase collective, increase throttle, up, up, and away, right? Swing tail with peddles, right-stick left-stick for banking. Just your basic rotary-wing, right?
Just kindda' guessing from what I seen and done on computer.

[This message has been edited by Boondawg (edited 02-04-2012).]

quote Originally posted by Marvin McInnis: Don't get too choked. [With a nod to PK86:] i'm sure i could fly a plane helicopter no problem.

There is no doubt in my mind you could learn to fly anything you wanted to.

quote Originally posted by Boondawg: Twist the throttle up slightly, twist the collective up slightly, push the peddles slightly in the opposite direction of the torgue, push the stick slightly forward, increase collective, increase throttle, up, up, and away, right? Swing tail with peddles, right-stick left-stick for banking. Just your basic rotary-wing, right? Just kindda' guessing from what I seen and done on computer.

Something like that Boonie but a little more intense when there's real blood and guts involved.
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Ron
The key thing is to wake up breathing! All the rest can be fixed. (Except Stupid - You can't fix that)
Always remember these words of wisdom.

"The Lord must truly love fools, for he made them in abundance."

[This message has been edited by blackrams (edited 02-04-2012).]

The different between air and a vacuum.




Here is a riddle for ya!

If you put marshmallows in a a vacuum it will grow. Why?