How does propulsion in space work

[Gary C]

Moonhawk said:

The amount of thrust is a function of the mass of the propellant and the speed at which it is chucked out the back.

That's why ion engines are so good - they only use a small amount of fuel - but they chuck it out at very high speed.

But the thrust of an ion engine is really poor (often measured in milli newtons)because the mass is so small, but does have the advantage of consuming very little 'propellant' and in theory can build very high speeds by running for very long 'burn' times.

[Silver Smudger]

Does this help? - In the balloon (or rocket), the pressure is pushing against the inside of the balloon equally until it is allowed to escape at one end so it can then push forward on the inside of the front of the balloon

Edited by Silver Smudger on Sunday 26th February 18:19

[talksthetorque]

Silver Smudger said:

Does this help? - In the balloon (or rocket), the pressure is pushing against the inside of the balloon equally until it is allowed to escape at one end so it can then push forward on the inside of the front of the balloon

Edited by Silver Smudger on Sunday 26th February 18:19

No, because in this example the expelled air also has air to push against.
If it was in space the balloon would pop!

[Dr Jekyll]

talksthetorque said:

Silver Smudger said:

Does this help? - In the balloon (or rocket), the pressure is pushing against the inside of the balloon equally until it is allowed to escape at one end so it can then push forward on the inside of the front of the balloon

Edited by Silver Smudger on Sunday 26th February 18:19

No, because in this example the expelled air also has air to push against.
If it was in space the balloon would pop!

It does have air to push against, but it doesn't need it to have thrust.

[Nimby]

That's all Newtonian propaganda. The truth is that the vacuum of space is trying to suck the rocket in all directions. But there isn't a vacuum at the nozzle end because of the rocket exhaust so it gets sucked forwards.

[Simpo Two]

Eric Mc said:

Another technique used to manoeuver an object in space are reaction control wheels. These have the advantage of not needing any sort of rocket fuel or gas so, in theory, can be used for as long as they have an electricity supply or don't physically break.

Not really propulsion though, just a sort of gyroscope. ie it will rotate around centre of mass but not actually go anywhere.

[Eric Mc]

I didn't say it was a form of propulsion. It's for changing attitude - and is an alternative to using reactio control thrusters.

[Einion Yrth]

Eric Mc said:

I didn't say it was a form of propulsion. It's for changing attitude - and is an alternative to using reactio control thrusters.

Reaction wheels do become saturated over time though, and then require some kind of propulsive system to de-saturate.

[Eric Mc]

That is why the wheels on Hubble were replaced as part of regular servicing.

They only work for certain applications. It depends on the nature of the mission as to whether they are a better alternative to reaction control thrusters.

[Zad]

Another attitude control system I have seen used is magnetorquers. Only on smaller satellites in relatively low orbits though. Feeble torques, but no mechanical parts to wear out.

[Super Slo Mo]

Silver Smudger said:

Does this help? - In the balloon (or rocket), the pressure is pushing against the inside of the balloon equally until it is allowed to escape at one end so it can then push forward on the inside of the front of the balloon

Edited by Silver Smudger on Sunday 26th February 18:19

That's backwards. The action is the act of pushing air out of the balloon or burning fuel out of the rocket. The reaction is the balloon/rocket moving in the opposite direction.

[davepoth]

talksthetorque said:

No, because in this example the expelled air also has air to push against.
If it was in space the balloon would pop!

Nearly there then.

Just after you've started your rocket in space, what's immediately behind the rocket?

Some gas that has just come out of the rocket.

The gas that is about to come out of your rocket has some gas immediately behind it to push against.

[Super Slo Mo]

davepoth said:

talksthetorque said:

No, because in this example the expelled air also has air to push against.
If it was in space the balloon would pop!

Nearly there then.

Just after you've started your rocket in space, what's immediately behind the rocket?

Some gas that has just come out of the rocket.

The gas that is about to come out of your rocket has some gas immediately behind it to push against.

It isn't even that complicated, the gas you've just expelled out of the back of the rocket has mass. In order for it to be expelled the gas has to push on the rocket, which moves in the opposite direction, with equivalent force.
There is no need whatsoever for anything outside the rocket for the gas to 'push' against.

[Eric Mc]

I often wonder if old Isaac is saying to himself "After 300 years, why don't people still not get it?"

[callmedave]

Eric Mc said:

I often wonder if old Isaac is saying to himself "After 300 years, why don't people still not get it?"

I feel your pain Eric

I was in a conversation yesterday with someone regarding the difference in efficency launching from the Equator vs launching from a 'pole'

[talksthetorque]

Eric Mc said:

I often wonder if old Isaac is saying to himself "After 300 years, why don't people still not get it?"

He is saying it out loud in the afterlife pub, and then Shakespeare is leaning over and saying 'I never was nor never will' understand why double negatives are still a mystery too.

[Eric Mc]

Ha - Shakespeare's English could be dodgy at times too.

[Gandahar]

Eric Mc said:

I often wonder if old Isaac is saying to himself "After 300 years, why don't people still not get it?"

To be honest Eric I think Sir Isaac would actually be thinking they did not get it, but at least did not get the plague like in my days and caused problems with my research.

Sir Isaac would teach in this case, not disparage.

Edited by Gandahar on Tuesday 7th March 23:07

[speedtwelve]

A good analogy is wearing a pair of skates and throwing a brick away from you; you'll move in the opposite direction to the throw. As mentioned, same principle as a mass of gas being ejected from a propelling nozzle; it doesn't need to 'push' against anything.

[V8LM]

That's not right. When I fart people move away from me rather than me move forward. Or is this relativity?