Author Topic: How does a rocket work in a vacuum (Read 23175 times)

Nink · « **Reply #15 on:** January 27, 2016, 06:35:30 PM »

Quote from: 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

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.

Nink · « **Reply #16 on:** January 27, 2016, 06:44:58 PM »

Quote from: 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.

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.

citfta · « **Reply #17 on:** January 27, 2016, 07:21:08 PM »

Quote from: Nink on January 27, 2016, 06:35:30 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

Nink · « **Reply #18 on:** January 27, 2016, 07:44:53 PM »

Quote from: citfta on January 27, 2016, 07:21:08 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

lumen · « **Reply #19 on:** January 28, 2016, 12:32:16 AM »

Strange.

pomodoro · « **Reply #20 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.

Quote from: Nink link=topic=16367.msg472442#msg472442 date1453915152

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.

LibreEnergia · « **Reply #21 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.

Nink · « **Reply #22 on:** January 28, 2016, 01:39:42 AM »

Quote from: 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.

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

LibreEnergia · « **Reply #23 on:** January 28, 2016, 02:00:00 AM »

Quote from: Nink on January 28, 2016, 01:39:42 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.

Nink · « **Reply #24 on:** January 28, 2016, 02:38:03 AM »

Quote from: LibreEnergia on January 28, 2016, 02:00:00 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]

LibreEnergia · « **Reply #25 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)...

ramset · « **Reply #26 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 ...........

LibreEnergia · « **Reply #27 on:** January 28, 2016, 03:41:26 AM »

Quote from: 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 ...........

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.

ramset · « **Reply #28 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...

Pirate88179 · « **Reply #29 on:** January 28, 2016, 04:22:21 AM »

Quote from: 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.

Exactly correct.

Bill

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Author Topic: How does a rocket work in a vacuum (Read 23175 times)

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