Confused, missing the obvious I guess

[Phantom Mark]

So space......

A dark void which contains no friction or resistance.

With this in mind once you accelerate in a direction there is no resistance to slow you down, hence you carry on in the direction you accelerated in........

Which got me to thinking, there must be some sort of resistance in space, else what would push against for the force to have an equal and opposite reaction to thrust ?

Analogy ? Push your hand out away from your chest.........zero resistance, you just eventually reach the end of your arm, should your arm be long enough you could push it out forever, do it really quickly you will feel some air pressure build up behind your hand, use the same technique and swing your arm across your front left to right........with this in mind you can quickly see how a jet engine or propeller pushes against this resistance to get an equal and opposite reaction.....still with me ?

Now, with this in mind what exactly are the thrusters pushing against in space to get that equal and opposite reaction ?

Answers on a postcard......

 

[Karnak]

So space......

A dark void which contains no friction or resistance.

With this in mind once you accelerate in a direction there is no resistance to slow you down, hence you carry on in the direction you accelerated in........

Which got me to thinking, there must be some sort of resistance in space, else what would push against for the force to have an equal and opposite reaction to thrust ?

Analogy ? Push your hand out away from your chest.........zero resistance, you just eventually reach the end of your arm, should your arm be long enough you could push it out forever, do it really quickly you will feel some air pressure build up behind your hand, use the same technique and swing your arm across your front left to right........with this in mind you can quickly see how a jet engine or propeller pushes against this resistance to get an equal and opposite reaction.....still with me ?

Now, with this in mind what exactly are the thrusters pushing against in space to get that equal and opposite reaction ?

Answers on a postcard......

You are, I assume, referring to the speed cap. That is a gameplay concession enforced by being a multiplayer game over the internet. When speeds significantly exceed WWII fighter speeds (and even those produce noticeable problems) what different players see becomes too divergent for reasonable interactions with each other. In other words, on my screen you are 100km away and on your you are right behind be shooting me in the ass. That, of course, would require very high speeds, but WWII fighter speeds can produce as much as 100m difference in apparent location, though it is more often about 20m.

 

[QuantumTron]

The thrusters aren't pushing against anything, it's the act of your vehicle throwing out reaction mass by way of burning fuel, that is producing the thrust.

Here's an example: Suppose you were out in space with just a space suit just floating around. You throw a 1kg mass. You will now gain some motion away from that mass, in whatever proportion, depending on your mass.

So think of the exhaust from the thrusters as lots of little tiny bits of mass.

 

[Thunderchief]

Now, with this in mind what exactly are the thrusters pushing against in space to get that equal and opposite reaction ?

Answers on a postcard......

Absolutely nothing! the Equal and opposite reaction IS the answer. You throw something out back of your space ship, and you spaceship moves in the opposite direction the same amount, adjusted for mass. it's the same on earth you don't need something to "push against"

Cheers

 

[Cmdr Andy B]

i think you are misinterpreting the 'equal and opposite reaction' dynamic, if we were both in space and i pushed you away i would be moving away from our start position at the same speed as you are. with thrusters in effect the exhaust is the mass you are pushing against to provide the same movement.

edit: ninja'd

 

[QuantumTron]

You are, I assume, referring to the speed cap. That is a gameplay concession enforced by being a multiplayer game over the internet. When speeds significantly exceed WWII fighter speeds (and even those produce noticeable problems) what different players see becomes too divergent for reasonable interactions with each other. In other words, on my screen you are 100km away and on your you are right behind be shooting me in the ass. That, of course, would require very high speeds, but WWII fighter speeds can produce as much as 100m difference in apparent location, though it is more often about 20m.

I thought it was a general question about how thrusters work in space, not this accursed artificial "Friction In Spaaaace", 'because gameplay'

 

[Reidquat]

You apply thrust, you get movement. Action:Reaction. Happy now Mr Newton?

 

[Phantom Mark]

I am in fact not talking about the game at all

I was just sat thinking about thrusters and engines on real space ships

So in effect the practice is that engines of any type are offsetting a mass which comes out as a pushing force, but not reliant on an type of mass or resistance to push against ?

Damn this is deep for half four on a Thursday

 

[Mike Evans]

You are, I assume, referring to the speed cap. That is a gameplay concession enforced by being a multiplayer game over the internet. When speeds significantly exceed WWII fighter speeds (and even those produce noticeable problems) what different players see becomes too divergent for reasonable interactions with each other. In other words, on my screen you are 100km away and on your you are right behind be shooting me in the ass. That, of course, would require very high speeds, but WWII fighter speeds can produce as much as 100m difference in apparent location, though it is more often about 20m.

I think you didn't really read the OP and got the complete wrong end of the stick.

To answer the OP's question, thrusters in space require a propellent that is physically pushed away from the thruster to create the equal and opposite force on the thruster. Obviously the propellent has very little mass compared to the ship so the it'll shoot out of the thruster at a high speed given the force applied to it compared to that same force being applied in the opposite direction against the ship.

So basically you need to bring your own material to push against to get anywhere in space (I'm sure there are exceptions, like the EM-Drive). Jet engines don't need to provide a propellent, it just uses the air to do the same thing; sucking it in then pushing it out at a higher speed).

 

[Vennt]

...with this in mind you can quickly see how a jet engine or propeller pushes against this resistance to get an equal and opposite reaction.....still with me ?

Now, with this in mind what exactly are the thrusters pushing against in space to get that equal and opposite reaction ?

Answers on a postcard......

This is a fundamental misunderstanding about how thrusters / jet engines work.

The reaction inside the thruster produces thrust, period. It doesn't rely on pushing against anything to produce thrust, rather it relies on a chemical reaction (or mixture of chemical and mechanical in the case of a Jet engine).

The "equal and opposite reaction" part comes in when the craft the thruster is attached to, moves in the opposite direction to the thrust produced.

 

[Phantom Mark]

This is a fundamental misunderstanding about how thrusters / jet engines work.

Indeed it is, and you have saved wobbling around the house for the rest of the evening trying to understand it

Thanks to Mike and all the others, one of those things which would have continually bugged me until I found the answer otherwise

 

[Defacto]

I don't think of the spacecraft in elite as using reaction-based propulsion. They surely don't exhibit the behavior one would expect from traditional spacecraft...

I have my own magical wankel-engine in space theory to suspend my disbelief.

 

[AtomicHorror]

Indeed it is, and you have saved wobbling around the house for the rest of the evening trying to understand it

Thanks to Mike and all the others, one of those things which would have continually bugged me until I found the answer otherwise

Simplest way to think about it: You have been abandoned in space with only your pet space rock for company. If you push the space rock away from yourself, you are also pushing yourself off the space rock as you have nothing to brace yourself against, no friction to stop you. You and space rock both move away from each other. You cry tears of grief that you and your pet rock will never meet again, but the tears provide no thrust because they are contained within your rapidly dampening space helmet, providing cancelling forces as they push back against it. Your tears are thus meaningless in the vast emptiness of space.

I might have lost my way a bit in that analogy. Anyway, a spacecraft and the propellant from a thruster push against each other just like you and the rock you abandoned.

 

[Phantom Mark]

So in a nutshell the propelled mass which is ejected from either a Jet or a Thruster are mass, and the very mass which the engine pushes away from.....thus no physical mass in space or in an atmosphere bear any relation to the process.

 

[Zvanya]

So in a nutshell the propelled mass which is ejected from either a Jet or a Thruster are mass, and the very mass which the engine pushes away from.....thus no physical mass in space or in an atmosphere bear any relation to the process.

Exactamundo!

 

[Bald Avenger]

thus no physical mass in space or in an atmosphere bear any relation to the process.

Well the mass in an atmosphere will bear relation to the process through drag coefficient.

 

[Megalon]

we need a "let me google that for you" section.

 

[Queebatron]

If the OP must conceptualize it as the spacecraft "pushing against" something, its engines are doing so against the mass represented by the rocket exhaust coming out of them.

BTW, WRT to the game, I sure hope that the "FA off allows top afterburner speed to not bleed off" isn't going to go away. There has to be at least some concession to Newtonian physics in a vacuum.

 

[Alric]

Here's to space games that have people wondering about physics.
+1

 

[Zvanya]

I sure hope that the "FA off allows top afterburner speed to not bleed off" isn't going to go away. There has to be at least some concession to Newtonian physics in a vacuum.

They have pretty much said that this will change. My hope is the speed bleed will stop at the normal non boosted top speed and not slow all the way to zero. I'd have a tough time swallowing that.