Gravity powered devices => Gravity powered devices => Topic started by: Nink on January 26, 2016, 04:28:31 PM

Title: How does a rocket work in a vacuum
Post by: Nink on January 26, 2016, 04:28:31 PM
We have all seen it on TV.  Rockets flying past the karman line at approximately 100km high, where we have no atmosphere and traversing the next 60+ km straight up and into space. At 100km gravity is still at around 9.5 m/s^2 but now we have no atmosphere to push against. How does the rocket continue to accelerate until it reaches the 160km + range where it can find low earth orbit.

Basic physics tell us that according to the Joule-Thomson effect or free expansion theory we just violated newtons third law of motion.  Some people will argue that this is an equal and opposite reaction of the force of the fuel leaving the rocket but free expansion means that without anything to push against you will not move. Others say it is "the recoil affect like when you fire a gun", or  "we have gained sufficient momentum so we reach orbit before we stop", or the "we push against the previous stage of the rocket" or "explosion inside the combustion chamber forces energy out of the chamber" or ...

So I am hoping someone can explain to me how a rocket actually works in space and describe the process or perhaps even show the calculations of  how we get passed the karman line then up to and beyond low earth orbit in a rocket. This is completely beyond my comprehension.

The official NASA response "The rocket pushes on its exhaust. The exhaust pushes the rocket, too. The rocket pushes the exhaust backward. The exhaust makes the rocket move forward."   https://www.nasa.gov/pdf/153415main_Rockets_How_Rockets_Work.pdf

Sorry what was that again ?

Title: Re: How does a rocket work in a vacuum
Post by: Paul-R on January 26, 2016, 04:38:55 PM
... but now we have no atmosphere to push against.
The Third law doesn't require an atmosphere or anything else to push against. This is why astronauts doing repair jobs on he ISS finding themselves rotating as they try to tighten or untighten bolts.
Title: Re: How does a rocket work in a vacuum
Post by: Nink on January 26, 2016, 05:16:18 PM
The Third law doesn't require an atmosphere or anything else to push against. This is why astronauts doing repair jobs on he ISS finding themselves rotating as they try to tighten or untighten bolts.

I think we need to park the astronaut gallivanting around the ISS conversation until we work out how they got into space in the first place. Until then  "In space analogies" are probably a waste of time as some people would argue the astronauts working on the space station are actually working in a swimming pool and pushing against water.  I guess only the astronauts and NASA know for sure. Tiny bubbles https://www.youtube.com/watch?v=ErBFJDQOIHg

And yes you do need something to push against. Start swimming while standing on the ground and see how far you move.  You won't go anywhere without a medium that is dense enough to move you in the opposite direction when you push on it.
Title: Re: How does a rocket work in a vacuum
Post by: massive on January 26, 2016, 08:23:13 PM
it would be interesting to see how many sites/sources revert to the Gov/nasa explaination

Title: Re: How does a rocket work in a vacuum
Post by: TinselKoala on January 27, 2016, 12:38:59 AM
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.
Title: Re: How does a rocket work in a vacuum
Post by: Nink on January 27, 2016, 03:45:52 AM
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.

Title: Re: How does a rocket work in a vacuum
Post by: citfta on January 27, 2016, 03:06:25 PM
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
Title: Re: How does a rocket work in a vacuum
Post by: Nink on January 27, 2016, 04:21:17 PM
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.
Title: Re: How does a rocket work in a vacuum
Post by: Paul-R on January 27, 2016, 04:58:22 PM

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.
Title: Re: How does a rocket work in a vacuum
Post by: citfta on January 27, 2016, 05:13:30 PM
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
Title: Re: How does a rocket work in a vacuum
Post by: Nink on January 27, 2016, 05:46:31 PM
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

Hi Carroll

Title: Re: How does a rocket work in a vacuum
Post by: pomodoro on January 27, 2016, 05:47:55 PM
Gases are like balls with space between them. They need to bounce back to the rocket from air collisions to push it according to your theory.Only if the air was solid stiff  and same for the flame gases would this theory work as solids can transfer forces.
In reality the gas from the flames is detached from the rocked because gas molecules have great distances between them. Whether it hits a wall or the air , it has no way of communicating any momentum back to the rocket. Because of its great velocity it will not bounce back from any collisions with the air as only hot exhaust gases directed away from the rocket are near the exhaust.
Title: Re: How does a rocket work in a vacuum
Post by: Nink on January 27, 2016, 06:19:12 PM
Gases are like balls with space between them. They need to bounce back to the rocket from air collisions to push it according to your theory.Only if the air was solid stiff  and same for the flame gases would this theory work as solids can transfer forces.
In reality the gas from the flames is detached from the rocked because gas molecules have great distances between them. Whether it hits a wall or the air , it has no way of communicating any momentum back to the rocket. Because of its great velocity it will not bounce back from any collisions with the air as only hot exhaust gases directed away from the rocket are near the exhaust.

The Gas molecules from the rocket may have a great distance from each other but the molecules in the atmosphere that they collide with do not. In the absence of an atmosphere in the infinite vacuum of space gas molecules will never collide with anything. They will just continue to move outwards. As they do not have anything to collide with there is no increase in pressure inside the combustion chamber or outside in the atmosphere. As the gas molecules heat up and expand they will simply exit the combustion chamber without a force to push against.  This is the free expansion of gas. During free expansion no actual work is done as there is no opposing force so there is no pressure that would result in the creation of work.
Title: Re: How does a rocket work in a vacuum
Post by: lumen on January 27, 2016, 06:29:37 PM
Nink:

The gas IS the mass.
The FASTER you can expel it the harder it pushes the rocket.
The MORE gas you expel the harder it pushes the rocket.

Nothing else is required.

Title: Re: How does a rocket work in a vacuum
Post by: citfta on January 27, 2016, 06:31:42 PM
It does appear that you are really not interested in how a rocket can work in space.  You only want to try and convince others it can't.  Sorry, but I am not going to fall for that and I don't think anyone else will either.  I am done here.

Carroll
Title: Re: How does a rocket work in a vacuum
Post by: Nink on January 27, 2016, 06:35:30 PM
It does appear that you are really not interested in how a rocket can work in space.  You only want to try and convince others it can't.  Sorry, but I am not going to fall for that and I don't think anyone else will either.  I am done here.

Carroll

I agree a square cardboard box the same size as the brick is lighter than a brick so the mass is lower but the air pressure against the brick and the box is the same. I think we are mixing concepts. One is pushing against the brick and ignoring the friction caused by the atmosphere and the other is pushing against the atmosphere and discounting the mass of the object.   We could reverse this and say a large sheet of cardboard that weighs less then a brick is pushed outwards while standing on the skateboard. This will force me to go further in the opposite direction  then if I was throwing the brick of a heavier mass.
Title: Re: How does a rocket work in a vacuum
Post by: Nink on January 27, 2016, 06:44:58 PM
Nink:

The gas IS the mass.
The FASTER you can expel it the harder it pushes the rocket.
The MORE gas you expel the harder it pushes the rocket.

Nothing else is required.

Unfortunately the gas can not be the mass as we are trying to move that very same mass upwards as we expel a small portion of that mass downwards. Your theory is that we are releasing a small portion of this mass and then pushing against this mass in vertical direction directly opposite to the force of gravity. I think we did the math for this under the recoil idea with Yosemite Sam firing bullets at the ground.
Title: Re: How does a rocket work in a vacuum
Post by: citfta on January 27, 2016, 07:21:08 PM
I agree a square cardboard box the same size as the brick is lighter than a brick so the mass is lower but the air pressure against the brick and the box is the same. I think we are mixing concepts. One is pushing against the brick and ignoring the friction caused by the atmosphere and the other is pushing against the atmosphere and discounting the mass of the object.   We could reverse this and say a large sheet of cardboard that weighs less then a brick is pushed outwards while standing on the skateboard. This will force me to go further in the opposite direction  then if I was throwing the brick of a heavier mass.

Without realizing it you now have the concept.  The rocket is not pushing against anything except the exhaust it is pushing out the back.  Exactly the same as you pushing against the brick when you throw it hard.

Carroll
Title: Re: How does a rocket work in a vacuum
Post by: Nink on January 27, 2016, 07:44:53 PM
Without realizing it you now have the concept.  The rocket is not pushing against anything except the exhaust it is pushing out the back.  Exactly the same as you pushing against the brick when you throw it hard.

Carroll
Hi Carroll,

In the example I gave with the sheet cardboard when I push the cardboard against the atmosphere the cardboard pushes back against me as a result of the molecules in the atmosphere on the opposite side of the cardboard. When I push on the cardboard I am leaving the cardboard behind and pushing off the cardboard. If I hang onto the cardboard it will exert the same force when I try to move so I need to either re orientate the cardboard to reduce the size of my force against the atmosphere or get another piece of cardboard and leave the old one behind.

This is all great on earth where we are not in a vacuum.   In a vacuum we have no air for the cardboard to push against.  We push the cardboard out in space nothing will happen if I hang onto it. Even if I re orientate it and repeat I still will not move.  If I throw the cardboard out away from me I will move in the opposite direction of the cardboard but the mass is what will determine the distance I move and not the air resistance created by the size of the cardboar.d

If I push on very same sheet of cardboard in space it will not move me further than a solid brick of the same mass as the sheet of cardboard moves me. Our momentum is determined by the mass of the cardboard and not the air resistance produced by the cardboard.

If the gas is what is causing me to move the gas has to push against something. in the absence of atmosphere there is nothing for the gas to push against.

Hopefully this video will explain why this is not the case.  https://www.youtube.com/watch?v=AfVfsnL-zbo

Title: Re: How does a rocket work in a vacuum
Post by: lumen on January 28, 2016, 12:32:16 AM
Strange.
Title: Re: How does a rocket work in a vacuum
Post by: pomodoro on January 28, 2016, 12:56:20 AM
You have just answered yourself the question as to why there is no back pressure from the air. As far as the work is concerned it went into heating the exhaust gases. The molecules of the gas got hotter and the gas expanded. The expansion in one dimention (the opening of the exhaust) pushed the rocket the other way . If you did press against air, then all of the momentum available will push the rocket and would not be shared with the exhaust.
The Gas molecules from the rocket may have a great distance from each other but the molecules in the atmosphere that they collide with do not. In the absence of an atmosphere in the infinite vacuum of space gas molecules will never collide with anything. They will just continue to move outwards. As they do not have anything to collide with there is no increase in pressure inside the combustion chamber or outside in the atmosphere. As the gas molecules heat up and expand they will simply exit the combustion chamber without a force to push against.  This is the free expansion of gas. During free expansion no actual work is done as there is no opposing force so there is no pressure that would result in the creation of work.
Title: Re: How does a rocket work in a vacuum
Post by: LibreEnergia on January 28, 2016, 01:28:16 AM
F = MA

Relative to the rocket gas (a  mass) is accelerated out the exhaust,  producing a force.

Nothing is required to push against. I can't believe why you'd think any differently.

Title: Re: How does a rocket work in a vacuum
Post by: Nink on January 28, 2016, 01:39:42 AM
F = MA

Relative to the rocket gas (a  mass) is accelerated out the exhaust,  producing a force.

Nothing is required to push against. I can't believe why you'd think any differently.

If we go with the F=MA please help me with the math as I am obviously not able to work out the formula.  What is the mass of the Gas ejecting. What is the acceleration rate of the gas ejecting.  Is this greater than the force of the Mass of the rocket and the deceleration caused by gravity.  Please give me some numbers to work with.  You choose the variables and lets plug them in and see if we can get enough force to be greater than the combined mass of the rocket and the fuel it has to move and also counteract the force of gravity.

1 what is the Mass of the fuel ejected
2 what is the acceleration rate of that fuel ejected
3 what is the total mass of the fuel on board at 100km
what is the Mass * that acceleration so we can calculate Newtons.

We know gravity at 100km high is 9.5m/s^2
We know the mass of the Apollo 11 rocket is  28,801 kg

So if you can provide me the rest of the values lets plug them in and see how far our rocket will move in the absence of an atmosphere to push against
Title: Re: How does a rocket work in a vacuum
Post by: LibreEnergia on January 28, 2016, 02:00:00 AM
If we go with the F=MA please help me with the math as I am obviously not able to work out the formula.  What is the mass of the Gas ejecting. What is the acceleration rate of the gas ejecting.  Is this greater than the force of the Mass of the rocket * the acceleration of the rocket.  Please give me some numbers to work with.  You choose the variables and lets plug them in and see if we can get enough force to be greater than the combined mass of the rocket and the fuel it has to move and also counteract the force of gravity.

1 what is the Mass of the fuel ejected
2 what is the acceleration rate of that fuel ejected
3 what is the total mass of the fuel on board at 100km
what is the Mass * that acceleration so we can calculate Newtons.

We know gravity at 100km high is 9.5m/s^2
We know the mass of the Apollo 11 rocket is  28,801 kg

We don't actually know any of those numbers since we would need to know how the velocity and mass of the rocket varied as it flew. All we know for sure is that the rocket accelerated and continued to do so till it reached orbital velocity.

The mass ejected was the entire mass of the fuel and oxidiser which was about 2.4 million kg. We don't know the dynamics of how that was accelerated, other than the thrust on take off was about 75 mega Newtons. Without knowing the flight profile we cant calculate anything else.

Title: Re: How does a rocket work in a vacuum
Post by: Nink on January 28, 2016, 02:38:03 AM
We don't actually know any of those numbers since we would need to know how the velocity and mass of the rocket varied as it flew. All we know for sure is that the rocket accelerated and continued to do so till it reached orbital velocity.

The mass ejected was the entire mass of the fuel and oxidiser which was about 2.4 million kg. We don't know the dynamics of how that was accelerated, other than the thrust on take off was about 75 mega Newtons. Without knowing the flight profile we cant calculate anything else.

2,400,000 kg of fuel or the equivalent of 800 elephants worth of fuel wow.  Quick search this thing was heavy. You would think there would be tonnes of data available so we can extract the mass of the rocket and remaining stages and fuel when it crossed the karman line.  If we can't get stats for Apollo how about we pick any rocket that flew into space.    Don't rockets go there all the time?  Someone must have that data.

Saturn V
Height    363.0 ft (110.6 m)
Diameter    33.0 ft (10.1 m)
Mass    6,540,000 lb (2,970,000 kg)[3]
Stages    3
Payload to LEO    310,000 lb (140,000 kg)[4][5]
Payload to TLI    107,100 lb (48,600 kg)[3]
Title: Re: How does a rocket work in a vacuum
Post by: LibreEnergia on January 28, 2016, 03:23:21 AM
Let's assume it did need to push off the air or some other thing other than the mass it ejects out the nozzle.

800 elephants or (2,970,000 kg) worth of air having a density of 1.225 kg/m3  would be a cube of air about 140 metres per side.  I'm just wondering how they fit that much air directly under the cross section of the rocket on take off ?  Perhaps it would have been better to put the elephants under the rocket instead, given that elephants have a density approach 1000 kg/m3. That would fit.

Or better still accept you don't need anything to push off, either the ground or the air or elephants . Instead , carry the elephants on the rocket, flinging them out the back fast enough to make it rise.

However, In the interests of animal cruelty perhaps replace the elephants with two liquids that when reacted together produce a gas that can be accelerated out a nozzle (in a way that no elephant can)...

Title: Re: How does a rocket work in a vacuum
Post by: ramset on January 28, 2016, 03:30:28 AM
Libre
Why you hate Elephants ??
Quote
carry the elephants on the rocket, flinging them out the back fast enough to make it rise.

horrible  man ...........

Title: Re: How does a rocket work in a vacuum
Post by: LibreEnergia on January 28, 2016, 03:41:26 AM
Libre
Why you hate Elephants ??
Quote
carry the elephants on the rocket, flinging them out the back fast enough to make it rise.

horrible  man ...........

It's just as well elephants never proved to be particularly adept at moving fast of their own accord and a elephant catapult was deemed impractical early on in the space program.
Title: Re: How does a rocket work in a vacuum
Post by: ramset on January 28, 2016, 03:50:42 AM
Libre
Quote
a elephant catapult was deemed impractical early on in the space program.
---------------------

Wicked people,  looking to the poor Ponderous pachyderm for propulsion short cuts...
Title: Re: How does a rocket work in a vacuum
Post by: Pirate88179 on January 28, 2016, 04:22:21 AM
F = MA

Relative to the rocket gas (a  mass) is accelerated out the exhaust,  producing a force.

Nothing is required to push against. I can't believe why you'd think any differently.

Exactly correct.

Bill
Title: Re: How does a rocket work in a vacuum
Post by: tinman on January 28, 2016, 05:36:31 AM
Good to see you started this thread Nink

As you can see, the answers or reasons why a rocket works in the vacuum of space, are the very same ones I gave you in our email conversation before you started this thread.

I am in the process of making my own vacuum chamber.
It will be large enough to test a small rocket powered car in.

Title: Re: How does a rocket work in a vacuum
Post by: Nink on January 28, 2016, 12:55:28 PM

I still don't have an answer.   I have been told  that F=MA and Carroll and NASA say F=MV in space. NASA do not  say F=MA in a vacuum like everyone else here is saying, NASA says F=MV  https://www.nasa.gov/pdf/153415main_Rockets_How_Rockets_Work.pdf  and then NASA make the bold statement "In real rocket science, many other things also come into play" I have no idea what that statement means and I have no data to model this against.   I think we all fully understand F=MA when we are in the presence of an atmosphere or firing a rocket on the ground with earth to push against but I don't understand how we move from F=MA to F=MV, especially when we have no atmosphere and still have the force of gravity to contend with.

Until we can mathematically model this with actual flight DATA we have no way to prove this.  With the hundreds of rockets launched we should have the starting mass and fuel consumed at every step of the way. This should include the fuel and total mass, height and velocity at the points that the stages were released and the points approaching and directly after the karman line with the data that continues until we are in orbit.  I appreciate that we all say stuff works.  I could say F=MV when we do not have an atmosphere and it can counteract the negative force of gravity as well but that is not going to make it correct.

Unfortunately building a vacuum chamber is of no value for igniting a rocket for multiple reasons.

1) Your vacuum chamber has an internal surface so when you fire your rocket it will push off that surface (molecules push against the side of the vacuum chamber)
2) It is not possible to build an infinite sized vacuum chamber, any vacuum chamber you build will not allow for free expansion of gas
3) The instant you ignite your rocket gas will be released and fill up that vacuum chamber so you no longer have a vacuum chamber

We need to get the actual DATA and the formulas and plug everything into a spread sheet. Unless we can at minimum mathematically model these theories all we have is ideas.

I have searched youtube and I am unable to find a video of a rocket that goes straight up for 160km to space. They all start off going up straight but as soon as they get above a certain height they turn and go into an arc.  I am not sure if this is an optical illusion or something else.  https://www.youtube.com/watch?v=zvMlAqC4bm4
Title: Re: How does a rocket work in a vacuum
Post by: tinman on January 28, 2016, 03:11:15 PM

Quote
I have searched youtube and I am unable to find a video of a rocket that goes straight up for 160km to space. They all start off going up straight but as soon as they get above a certain height they turn and go into an arc.  I am not sure if this is an optical illusion or something else.

Maybe they are going straight up,and only look like they are arcing because the earth is spinning ;)
As your view point is from the spinning earth,the rocket would appear to arc in accordance to that spin. But what if your view point was from a fixed point in space--would the rocket appear to arc then?.

Title: Re: How does a rocket work in a vacuum
Post by: Paul-R on January 28, 2016, 06:02:55 PM

I have searched youtube and I am unable to find a video of a rocket that goes straight up for 160km to space.
Surprisingly enough, that is because that is not where they want to go.

Almost every launched vehicle is on its way into orbit which means travelling to a specific height and to a speed of around 18k mph horizontally. Listen to the RT comms which used to go out live. "Stand by to roll" etc.
Title: Re: How does a rocket work in a vacuum
Post by: lumen on January 28, 2016, 06:25:10 PM
I would suggest you simply buy a toy water rocket and you would quickly understand that a rocket works from ejecting mass.
If you pump it up with only air it will not work well, but fill it with water (larger mass) and then some air and it works very well.

http://www.ebay.com/itm/Toysmith-Water-Rocket-also-includes-2-Fuel-Funnels-and-Launcher-New-/331413428205?hash=item4d29c797ed:g:WB4AAOSwpDdU7ASb
Title: Re: How does a rocket work in a vacuum
Post by: LibreEnergia on January 28, 2016, 09:21:33 PM

I still don't have an answer.   I have been told  that F=MA and Carroll and NASA say F=MV in space. NASA do not  say F=MA in a vacuum like everyone else here is saying, NASA says F=MV  https://www.nasa.gov/pdf/153415main_Rockets_How_Rockets_Work.pdf  and then NASA make the bold statement "In real rocket science, many other things also come into play" I have no idea what that statement means and I have no data to model this against.   I think we all fully understand F=MA when we are in the presence of an atmosphere or firing a rocket on the ground with earth to push against but I don't understand how we move from F=MA to F=MV, especially when we have no atmosphere and still have the force of gravity to contend with.

Have a look carefully at how they state F = MV.  You'll note it is actually written F = M (exit) * V (exit).  They are measuring those quantities as the velocity of the exiting gasses relative to the frame of the rocket.  The mass is being accelerated from  zero to V(exit), so the relationship F = MA is what is providing the thrust.

It just so happens that the mass flow rate MV is equivalent to MA in this situation since the initial velocity of the accelerated mass is zero.
Title: Re: How does a rocket work in a vacuum
Post by: Nink on January 28, 2016, 09:59:51 PM
I would suggest you simply buy a toy water rocket and you would quickly understand that a rocket works from ejecting mass.
If you pump it up with only air it will not work well, but fill it with water (larger mass) and then some air and it works very well.

http://www.ebay.com/itm/Toysmith-Water-Rocket-also-includes-2-Fuel-Funnels-and-Launcher-New-/331413428205?hash=item4d29c797ed:g:WB4AAOSwpDdU7ASb

LOL now if I can just get into space in the absence of gravity and atmosphere to test how it works.
Title: Re: How does a rocket work in a vacuum
Post by: lumen on January 28, 2016, 10:09:52 PM
LOL now if I can just get into space in the absence of gravity and atmosphere to test how it works.

Or if you could reason that the exiting air would push against the atmosphere as well or better than the water and yet the heavier water works better than air, you could understand that it is in fact the exiting mass that causes acceleration and not the fact that it is pushing on the atmosphere.

Title: Re: How does a rocket work in a vacuum
Post by: LibreEnergia on January 28, 2016, 10:24:23 PM
LOL now if I can just get into space in the absence of gravity and atmosphere to test how it works.

Think about a jet boat. They work much better when the jet is exhausted above rather than into the water. If it needed to 'push' on something other than the ejected mass itself this would not be the case.
Title: Re: How does a rocket work in a vacuum
Post by: Nink on January 29, 2016, 12:10:02 AM
Think about a jet boat. They work much better when the jet is exhausted above rather than into the water. If it needed to 'push' on something other than the ejected mass itself this would not be the case.
A jet boat has an unlimited supply of water that it draws from underneath the boat.  It then uses Newtons 3rd law of motion by ejecting the water (equal and opposite reaction)  to travel horizontally across  the force of gravity. The water goes one way the boat goes the other. I get it.

Unfortunately a rocket does not have an unlimited supply of water or fuel or anything to push away from itself to invoke 3rd law of motion and it also has to travel vertically straight up against the force of gravity and not horizontally across the force of gravity. A rocket has a limited  supply of fuel it can burn to generate thrust and that fuel is quickly exhausted. By the time the rocket reaches 100km (if it even reaches that height) and the multiple stages of the rocket that contain the fuel have been exhausted and separated from the rest of the rocket and now with out fuel to push out I still have no idea how it travels the next 60km straight up into space.

As I continue to state everyone has great theories but that is all they are, theories and a theory needs to be proven.

This is basic high school physics. Although NASA may say Don't worry this is rocket science we can quickly verify the facts with data.  WE NEED THE DATA and without the data we are just making stuff up.
Title: Re: How does a rocket work in a vacuum
Post by: tinman on January 29, 2016, 12:30:15 AM
A jet boat has an unlimited supply of water that it draws from underneath the boat.  It then uses Newtons 3rd law of motion by ejecting the water (equal and opposite reaction)  to travel horizontally across  the force of gravity. The water goes one way the boat goes the other. I get it.

Unfortunately a rocket does not have an unlimited supply of water or fuel or anything to push away from itself to invoke 3rd law of motion and it also has to travel vertically straight up against the force of gravity and not horizontally across the force of gravity. A rocket has a limited  supply of fuel it can burn to generate thrust and that fuel is quickly exhausted. By the time the rocket reaches 100km (if it even reaches that height) and the multiple stages of the rocket that contain the fuel have been exhausted and separated from the rest of the rocket and now with out fuel to push out I still have no idea how it travels the next 60km straight up into space.

As I continue to state everyone has great theories but that is all they are, theories and a theory needs to be proven.

This is basic high school physics. Although NASA may say Don't worry this is rocket science we can quickly verify the facts with data.  WE NEED THE DATA and without the data we are just making stuff up.

Like all machines both here on earth,and out in space-->once you run out of fuel,then the force moving that machine stop's. But in space,you have no resistance,and so the machine will maintain it's maximum velocity reached -until it encounters another force that impacts it's motion.

If the rocket ran out of fuel before it reached a point of either orbit or had escaped the earths gravity field,then it would simply be pulled back down to earth,and impact the surface. As they do not do that(mostly :D),then we can assume that one of the two above is true.

Title: Re: How does a rocket work in a vacuum
Post by: LibreEnergia on January 29, 2016, 12:41:01 AM

Unfortunately a rocket does not have an unlimited supply of water or fuel or anything to push away from itself to invoke 3rd law of motion and it also has to travel vertically straight up against the force of gravity and not horizontally across the force of gravity. A rocket has a limited  supply of fuel it can burn to generate thrust and that fuel is quickly exhausted. By the time the rocket reaches 100km (if it even reaches that height) and the multiple stages of the rocket that contain the fuel have been exhausted and separated from the rest of the rocket and now with out fuel to push out I still have no idea how it travels the next 60km straight up into space.

Rockets destined for orbit do not go straight up, except initially.  The trajectory curves over and flys parallel to the earth. Once it runs out of fuel the thrust stops. If it has not reached orbital velocity by that time then it will come back down to earth.

In orbit the rocket is always "falling towards earth". It's just that it's tangential velocity is high enough for it to always miss.
Title: Re: How does a rocket work in a vacuum
Post by: Nink on January 29, 2016, 02:12:59 AM
If the rocket ran out of fuel before it reached a point of either orbit or had escaped the earths gravity field,then it would simply be pulled back down to earth,and impact the surface. As they do not do that(mostly :D),then we can assume that one of the two above is true.

I am really confused when I watch a video of the Apollo Launch with stages ejected and the last height reading is 92.8km before they cut the camera just as they are about to cross the Karman line as it is approaching the edge of the atmosphere. The rocket is a lot lighter now then when we launched and there is a velocity of 2880 m/s but we are only 60% of the way to low earth orbit at 160km where as you say we no longer need fuel to maintain velocity. But how much fuel do we have left to get from that point to escape the pull of gravity  (ignoring the fact I don't believe the propulsion method used to take us to the 92.8 km  point will work past the 100km point as there is no atmosphere).    We also don't need to worry about the fuel to get to the moon, land on the moon, take off from the moon and fly back to earth since neither of us believe that part happened.

Does anyone have all the data  for this trip.  The person calling out the data is different than what is on the display,

Example
+30 seconds syncs clocks Says 30 second shows 30 seconds

but video time  https://youtu.be/F0Yd-GxJ_QM?t=74
at 56 second mark says altitude  2 miles but screen says 5.06 km but that is actually 3.2 miles

now
Says 2195 ft / s
Screen  460 m/s
Convert 669 m/s

Video time https://youtu.be/F0Yd-GxJ_QM?t=127
Says 4000 ft /s
Screen 1040 m/s
Convert 1219 m/s

Video time https://youtu.be/F0Yd-GxJ_QM?t=201

Syncs clocks
Says 3 minutes
screen 3 min 3 seconds

says range 70 miles
convert range 112km

Says altitude 43 miles
convert 69km
screen says 87km

says 9300  f/t per second
convert 2834 m/s
screen 2870 m/s
Title: Re: How does a rocket work in a vacuum
Post by: Nink on January 29, 2016, 03:10:22 AM
Maybe they are going straight up,and only look like they are arcing because the earth is spinning ;)
As your view point is from the spinning earth,the rocket would appear to arc in accordance to that spin. But what if your view point was from a fixed point in space--would the rocket appear to arc then?.

I checked your theory with the voice over data (I ignored the screen data) looks like you are correct.  Earth is spinning at 1,674.4 km/h https://en.wikipedia.org/wiki/Earth%27s_rotation  and at the 3.06 mark we can use the voice data. Quick bit of trigonometry the earth should have rotated 85km at that time math for rocket shows 88km so there could be a slight margin of error on vertical tragectory but I think you are right.  Rocket travels straight up but earth rotates below it. At least until the 69km mark.  What worries me is if the voice data is correct and screen data is wrong we are now only at 69km altitude we still have 91km to go until LOE.

Math works but we need data.

Title: Re: How does a rocket work in a vacuum
Post by: allcanadian on January 29, 2016, 08:22:28 AM
@Nink
Quote
I still don't have an answer.   I have been told  that F=MA and Carroll and NASA say F=MV in space. [/size]NASA do not  say F=MA in a vacuum like everyone else here is saying, NASA says F=MV[/size] [/size]https://www.nasa.gov/pdf/153415main_Rockets_How_Rockets_Work.pdf (https://www.nasa.gov/pdf/153415main_Rockets_How_Rockets_Work.pdf)[/font][/size]  and then NASA make the bold statement[/size]"In real rocket science, many other things also come into play"[/size] I have no idea what that statement means and I have no data to model this against.   I think we all fully understand F=MA when we are in the presence of an atmosphere or firing a rocket on the ground with earth to push against but I don't understand how we move from F=MA to F=MV, especially when we have no atmosphere and still have the force of gravity to contend with.  [/size]

Many times people give a textbook answer however fundamentally they do not actually understand what it means. I believe the word you are looking for is Inertia which is why a mass resists changes in motion.

Imagine you are on a boat and you throw a bowling ball to the left which makes the boat go to the right. Some may say it is the Mass Acceleration or the exchange there of but this does not really explain anything. Others may say it is the conservation of momentum or MV=MV but again this really does not explain anything fundamentally. In our boat example when you throw/push the bowling ball to the left the inertia of the ball resists your pushing force to accelerate it which produces a force in the opposite direction on you thus you and the boat move to the right.

From your question I think you are looking for more and this is a good sign even if most here cannot see the problem. The bowling ball cannot resist anything and it cannot produce a counter-force on you nor the boat and there is no momentum exchange. To do so implies some mysterious force we call Inertia has caused the mass to act on itself in itself which violates the laws of physics. An object cannot act on nothing or act on itself however when we speak of Inertia which is the cause of momentum and a property of mass this is exactly what we are saying hence all the confusion. As I said it is a very good sign that you see the problem and have questioned it.

What is Inertia?. Let's think about this, I am in space and I push on my bowling ball and it resists my pushing force somehow producing a counter force on me in the opposite direction. Some say the ball is pushing off me just as I am pushing off it but this is not correct and two wrongs do not make a right. If the ball had no inertia then it would move and I would not move at all. The property of inertia is not a property of mass but a property of the space the mass occupies. In a nutshell all the electromagnetic waves in the universe which fill every space everywhere act on each individual particle of the object or mass influencing it's motion. This also relates to the supposedly mass-less particles which have no inertia and really confuse the hell out of everyone. What they are actually saying is the particle lacks a property by which they measure the mass which is of course Inertia. A mass may seem to resist a change in motion by apparently acting on nothing or acting on itself however intuitively we know this cannot be true.

This also relates to why Einstein said- nothing happens until something moves, also when something moves the whole universe moves with it...literally. People don't like to talk about Inertia because they find it disturbing. You see if people actually understood what it is fundamentally then we could modify or negate this external influence on matter. A mass with no inertia would take almost no energy to accelerate to any velocity. Obviously this is a real problem for many psychologically which is why Inertia is generally considered a taboo subject.

AC