Naked Science Forum

On the Lighter Side => That CAN'T be true! => Topic started by: Justin129246 on 03/12/2017 01:51:21

Title: How does an expanding propellant move a spacecraft?
Post by: Justin129246 on 03/12/2017 01:51:21
Space flight is impossible by every standard that has been used. Now hear me out before totally dismissing the topic outright.

I've been thinking about the propellant system on SIS and shuttles when docking. In searches I've run across several differing explanations of how a pressurized cylinder creates the momentum to change velocity a trajectory once out of the atmosphere.

My problem is explaining how a pressurized propellant initiates a thrust or momentum change when the propellant is allowed to expand at the rate of its release. There is not an energy transfer to create such a driving force.
Secondly since objects become weightless relative to their own self frame of reference the directional loss of mass propelling an object in the opposite direction don't live either. That would be like kicking off of yourself to swim instead of kicking off of water to swim forward. And by those same standards any type of propulsion is just as useless. So what gives? If I'm wrong and I kind of hope I am, I'd appreciate an explanation that satisfies my problems.
Title: Re: How does an expanding propellant move a spacecraft?
Post by: RD on 03/12/2017 02:15:34
My problem is explaining how a pressurized propellant initiates a thrust or momentum change when the propellant is allowed to expand at the rate of its release.
The expanding gas rushing out of the rocket has momentum, (even in a vacuum), that exerts an equal & opposite change in momentum on the rocket : Newton's 3rd law of motion ... https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion#Newton.27s_3rd_Law
Title: Re: How does an expanding propellant move a spacecraft?
Post by: chiralSPO on 03/12/2017 02:59:24
In science experiment trumps theory. You can have the most elegant and self-consistent framework possible, but if it disagrees with what is observed in the real world, it can't be correct.

There are many experimental observations that establish that spaceflight and thrust are quite possible.
~~~
Let's imagine there is an airtight box filled with helium gas. Each of the helium atoms has mass, and a speed that is related to the temperature of the gas (the hotter the gas, the faster the atoms are moving; at 20 C, the average speed of the atoms is about 1200 m/s). The helium atoms are whizzing around within the box, bouncing off of each other, and off the sides of the box. It is the repeated action of the atoms hitting the side that causes what we observe as pressure. Every time a helium atom hits one of the walls, there is a transfer of energy and momentum between the two, but because the atoms are hitting all walls of the box at the same rate and with the same force, the box doesn't move.

If we now open one side of the box (say it's the left hand side), any helium atom that would have hit that wall, instead keeps moving (to the left). Thus all of the  helium atoms that impart their rightward momentum to the wall opposite the open side have no opposing force to keep the box stationary. Eventually all of the helium will have exited the box, heading towards the left, and having imparted all of their rightward momentum to the box.
Title: Re: How does an expanding propellant move a spacecraft?
Post by: Petrochemicals on 03/12/2017 03:28:26
Your not the only one having problems, nasa too apparently genius !

https://www.extremetech.com/extreme/187346-nasa-tests-impossible-no-fuel-quantum-space-engine-and-it-actually-works

The space flight thing puzzled me fo4 a bit too as it has nothing th thrust upon,
But long story short it boils down to the fact that in space there is no drag so a little bit of thrust goes along way, if george cloony throws a spanner at sandra bullok, he moves opposite to the spanner, just the same as if he pushes off the side of the iss, only a smaller ratio. It is the action and equal and opposite reaction thing. With a thruster the fuel mix expantion in ignition and the energy released causes a reaction in the spacecraft. Pressure release with the energy that was on board the space craft leading to a reaction in the craft. Essentially george cloony pusing off himself 2hen he throws a spanner. As to why it doesnt expand wi5hout effect, the engines are sesigned to deliver this thrust in a direction. Its the same as why explosives cause small damage placed on a surface, but when directed the effecr can be great. In effect the force is designed to push off the space craft in one direction.

The other thing is the momentum kinetic energy difference, with no drag a little energy goes along way given enough time.

Ion thrusters, use very litttle energy, but are extremley quick in there velocity, so with no drag, given enough time the acceleration reaches great velocities, where as lots of fuel over short burns is extremley waistful and only achieves low speeds.
Title: Re: How does an expanding propellant move a spacecraft?
Post by: Kryptid on 03/12/2017 05:10:12
Space flight is impossible by every standard that has been used.

A self-evidently incorrect statement.
Title: Re: How does an expanding propellant move a spacecraft?
Post by: evan_au on 03/12/2017 07:47:50
Quote from: Justin129246
There is not an energy transfer to create such a driving force.
The expansion of the gas "releases" potential energy, which gets turned into kinetic energy of the reaction mass and the spacecraft.

Quote
That would be like kicking off of yourself to swim instead of kicking off of water to swim forward.
Not quite.
The spacecraft "kicks off" the reaction mass.

Title: Re: How does an expanding propellant move a spacecraft?
Post by: chris on 03/12/2017 09:30:16
Great question which has got everyone talking!

I was asked this at a school I visited recently.

I approached it from the perspective of Newton's Laws (paraphrased):

1st Law: for something to move or change direction you need a force to be acting on it. Ergo, the spaceship must be feeling a force because it has begun to move in a new direction.

2nd Law: F=ma - the force the rocket feels is equal to the mass accelerated x the acceleration; the force has come from the propellant, so the acceleration of the known mass of rocket propellant released is producing the force, and the rocket accelerates at the equivalent rate; hence ma [rocket] = ma [propellant].

3rd Law: For every action there's an equal and opposite reaction; the propellant pushes on the rocket, so the rocket pushes back on the propellant. So both feel an equivalent force (ma) is opposite directions.
Title: Re: How does an expanding propellant move a spacecraft?
Post by: Bored chemist on 03/12/2017 10:21:49
Space flight is impossible by every standard that has been used.

A self-evidently incorrect statement.
Not strictly "self evident". If you had made that statement 100 years ago  many people would have accepted it as true.

However it is an evidently true statement- in the last 100 years we have produce the evidence that shows the statement to be false.
Title: Re: How does an expanding propellant move a spacecraft?
Post by: Colin2B on 03/12/2017 10:24:04
Great question which has got everyone talking!
If the original question had been phrased as youve changed it, I think it would have stayed in physics area.

The incorrect assumption that the rocket engine needs something to push against seems quite common and often comes up here.
Title: Re: How does an expanding propellant move a spacecraft?
Post by: jeffreyH on 03/12/2017 10:38:11
In science experiment trumps theory. You can have the most elegant and self-consistent framework possible, but if it disagrees with what is observed in the real world, it can't be correct.

There are many experimental observations that establish that spaceflight and thrust are quite possible.
~~~
Let's imagine there is an airtight box filled with helium gas. Each of the helium atoms has mass, and a speed that is related to the temperature of the gas (the hotter the gas, the faster the atoms are moving; at 20 C, the average speed of the atoms is about 1200 m/s). The helium atoms are whizzing around within the box, bouncing off of each other, and off the sides of the box. It is the repeated action of the atoms hitting the side that causes what we observe as pressure. Every time a helium atom hits one of the walls, there is a transfer of energy and momentum between the two, but because the atoms are hitting all walls of the box at the same rate and with the same force, the box doesn't move.

If we now open one side of the box (say it's the left hand side), any helium atom that would have hit that wall, instead keeps moving (to the left). Thus all of the  helium atoms that impart their rightward momentum to the wall opposite the open side have no opposing force to keep the box stationary. Eventually all of the helium will have exited the box, heading towards the left, and having imparted all of their rightward momentum to the box.

This answer is all that is needed to explain why it happens. People really should take this in.