How does a rocket work in a vacuum

[Nink]

Amazing! Next you'll be telling me that the Earth is flat. After all, maps are flat, aren't they? And the Sun is hollow, since you can see into the black interior through sunspots.


No, you do NOT need "something to push against". 

Look up "conservation of momentum". 

Mass is discharged very rapidly in one direction out the rocket nozzle. Since momentum is conserved, the rocket itself moves in the opposite direction.

You can prove this to yourself with some bricks and a skateboard.

Hi TK

We know the earth is round though observation, measurements and calculations. Something everyone on Overunity I believe is familiar with.  The properties of the sun as you are aware can also be estimated using a similar process.

Your skateboard example is based on Newtons third law of motion. For every action there is an equal and opposite reaction. If I weigh 100kg and the Bricks weigh 100kg and we are both on 2 different skateboard (Me on one skateboard and the bricks on the other) assuming friction is equal and I push the skateboard in the opposite direction we will both move the same distance, directly opposite each other.  This works because I am pushing on the bricks..   If I weigh 100kg and the Bricks weigh 100kg we will both move 1 meter in opposite directions from the starting point. 

If the bricks weigh 10kg and I push the bricks and they move  1 meter,  since I am 10 * heavier than the bricks (100kg) I will only move 10cm (discounting the additional friction from myself and the weight of the skateboards of course).   If the Bricks weigh 200kg and I weigh 100kg, I would move 2 meters if the bricks moved 1 meter. 

Now lets concentrate the entire system onto a single platform, 1 skateboard as per your example, where the bricks are all on the same skateboard as me, I can throw the bricks off the skateboard and move in the opposite direction. The distance I move will be proportional to the weight of the bricks I throw. Unfortunately there is only one scenario where I can move further than the bricks move. I have to throw more bricks than the combined weight of me and the bricks I continue to hold on my skateboard.  Assuming I weigh zero kg I would have to throw a minimum of 50% of my bricks every time to move the same distance as the bricks I am throwing. So I use 50% of my fuel source every time I move the same distance as the bricks. If I can move 1m the first time I will then move 50cm 25cm 12.5cm 6.25cm etc   

Now this works because we are horizontal to the vertical force of gravity and friction is minimized because of the bearings in the skateboard wheels. 

Picture the scenario of a skateboard heading vertically into space with a pile of bricks 100km above the surface of the earth. Now instead of moving horizontally across the force of gravity we are now trying to move vertically directly in the opposite direction of gravity.  It does not matter how many bricks you throw you will both continue to lose moment at a rate of 9.5m/s^2 so it is physically impossible, irrespective of the amount of fuel you have on board, in the absence of an atmosphere, or something to push against, to ever traverse the remaining 60km and reach a low earth orbit.

[citfta]

Nink,

You are ignoring a very important part of the equation.  It is the mass and the velocity of the mass that causes the equal and opposite reaction.  Have you never fired a shotgun?   If you took the pellets from a shotgun shell and tossed them as hard as you could you would get almost no reaction.  But put those same pellets into the shell and fire the shotgun and you will feel a significant kick against your shoulder.  The very high velocity of the exhaust gasses from the rocket is what causes the reaction which propels the rocket.  It has nothing to do with pushing against something else.  Get back on your skateboard and gently toss the brick off the back and see how far you go.   Then repeat the experiment and this time throw the brick as hard as you can and see how far you go.  Mass times velocity equals force.  A very small mass at very high velocity can overcome a large mass with a lower velocity.

Here is a simple experiment you can do that should help you understand how this works.  Make a box out of cardboard the same size as your brick.  Now standing on the skateboard again throw the box as hard as you can.  How far did you move?  If your theory was correct you should have gone just as far as when you threw the brick because both of them were pushing against the same amount of air.  You actually went a much shorter distance because the box made of cardboard has much less mass than the brick.

I hope this helps some.

Respectfully,
Carroll

[Nink]

Hi Carol

Newtons Second Law of motion is often used to describe how a rocket works. "The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object."

So lets look at the Recoil effect as another method to describe the way a rocket works.   

The process for  recoil of a bullet in the barrel of a gun I believe is as follows
1) An explosion occurs when the the gun powder is ignited. 
2) The explosion heats up the gas in the chamber causing expansion of the gas.
3) The pressurized gas now  builds up in the chamber
4)  The pressure pushes the bullet forward creating a force.
5)  As the bullet is lighter than the combined weight of the person holding the gun the bullet will move forward
6)  The gun will push backwards with a force proportional to the Mass * Acceleration of the bullet moving forward.
7) As soon as the bullet leaves the barrel of the gun both the bullet and the gun stop accelerating
8 ) The bullet will now decelerate proportional to the force of gravity and friction from the atmosphere
9) As the gun is being acted on by a net force (the person holding the gun)  it will stop moving

Number 9 is obviously conditional on being in the presence of atmosphere and gravity.  At 100km high we are no longer in the presence of atmosphere but we are in the presence of gravity. 

This works because we have two major items that were critical to the process. 1) a medium to push against (the bullet)  and 2) a build up of pressure as a result of the presence of that medium to push against. 

On earth the rocket does not push against bullets it pushes against the atmosphere.  In space we do not have an atmosphere to push against. 


As per my example if I had 10,00 bullets (or bricks)  and I fired them from the gun (or threw them from the skateboard) the distance I move as you stated would be directly proportional to my mass and the mass * acceleration of the bullet. So lets do the math.  If I weigh 100kg and a  bullet weighs  10g  I fire the bullet so F=MA the bullet now leaves my gun at an acceleration of say 44000 m/s^2 So the force is mass of 10g * acceleration 44000 m/s^2  or 44 Newtons.

Since I weigh 100kg I will move in the opposite direction in the absence of gravity with an acceleration of 44 / 100kg = 0.44 m/s^2 or 1/100th of the acceleration of the bullet.   If I have 10,000 bullets to fire (I want to keep moving) so my starting mass is 200kg. I have to move 200kg of weight with 1 bullet so now my acceleration is actually 1/2 of this or 0.22 m/s^2

Now the challenge.  Remember I am in a rocket at 100km high and I want to go straight up not left to right.  So I am faced with a negative force of gravity of 9.5m/s^2) so irrespective of how many bullets I fire I am not going anywhere except straight down and acceleration a rate of 9.28m/s^2 until I run out of bullets while I get closer to earth, while the force of gravity continues to increase my downward acceleration to 9.8m/s^2

I believe a rocket actually uses thrust pushing against the atmosphere.  In space we do not have an atmosphere to push against to build up pressure due to the free expansion of gas or bullets to block the barrel of our rocket to build up pressure. 

Based on this taking into consideration the strength of gravity and the mass of the fuel required to "push against" and the inability to build up pressure in the absence of an atmosphere, I do not believe a rocket uses the recoil affect, nor do I believe NASA is actually claiming this is the case.

[Paul-R]

Mass is discharged very rapidly in one direction out the rocket nozzle.

TK, I hope he doesn't discover the Jean Louis Naudin experiment where he has a threaded rod with felt at one end and a hard bolt to terminate the other. When the coiled spring in the middle is released, one end hits the felt and the other end hits the bolt. The system lurches to one side.

[citfta]

Nink

The formula for force is Mass times Velocity.  No where in that formula does it say anything about pushing against something else.  And you totally ignored my suggestion for a simple test to prove this to yourself.  If throwing a cardboard brick shape does not move you the same as throwing a real brick then that proves that pushing against air has nothing to do with it.  As the formula says, it is mass times velocity that gives the force.

Carroll with 2 rs and 2 ls