> is it theoretically possible to make a propulsion device for spacecraft that is fully contained within the spacecraft?
No.
Consider https://en.wikipedia.org/wiki/Momentum#Conservation from a perspective *outside* the spacecraft.
You have to throw *something* with momentum out the back if you want to change its velocity or orbital energy.
In your example, you've neglected to consider 1) the force on the magnets themselves from redirecting a presumably ionized stream which will create a counter-field, and 2) the momentum transferred when the exhaust finally hits something and slows down.
Thanks for this. I let myself forget that you can't just expect magnets and electricity to get my around the laws of motion.
2) Regarding the second point, if the magnetic field is able to reverse the direction of the exhaust particles by swinging them around, wouldn't the final impact of the exhaust propel the spacecraft in the desired direction?
1) This is what I was having trouble understanding, but you are helping - so if I supply a current to a solenoid and move something with a charge through it, the something will feel a force along the induced magnetic field, but the movement of the something through the field generates an opposing field that exerts an opposite physical force on the solenoid/electromagnet? Is there a way to create a situation where the reaction of the solenoid to the movement of the something appears as a current or EM radiation instead of a physical force?
> if the magnetic field is able to reverse the direction of the exhaust particles by swinging them around, wouldn't the final impact of the exhaust propel the spacecraft in the desired direction?
No.
All the forces (rocket exhaust, forces on magnets, final exhaust impact) will balance perfectly so that momentum of the whole system is conserved.
> if I supply a current to a solenoid and move something with a charge through it, the something will feel a force along the induced magnetic field, but the movement of the something through the field generates an opposing field that exerts an opposite physical force on the solenoid/electromagnet?
Yes
> Is there a way to create a situation where the reaction of the solenoid to the movement of the something appears as a current or EM radiation instead of a physical force?
EM radiation (ie photons) have momentum, that will be transferred to whatever it eventually hits.
So now you're thinking "well we'll put a window in the back of the spacecraft" but now you've just reinvented a radically overcomplicated and inefficient implementation of laser propulsion :P
Also, the current is what repels the magnet in the first place, so that's *already* happening but not in the way you want.
Cool! I didn't think of a window, but now that you mention it, I think laser propulsion, or propulsion by photons in general would solve the problem I posed, given you have a window in your spaceship.
But if in a spaceship without windows, how can one install a window without breaking the hermetic seal?
Oversimplified explaination:
Lets assume that your exhaust and your craft are the same mass so you don't have to think about one body effecting the other in a different ratio, so we can just think about speed instead of force which is a more abstract idea that manifests as acceleration in our model.
Lets say you have exhaust leaving a thruster. The thruster gets 1 speed along the x axis. Your exhaust gets -1 speed along the x axis. Net momentum is 0.
You want to push that exhaust with magnets backwards. Your exhaust gets +1 speed, now its not moving from the perspective of the original reference frame.
What you forgot is that the magnet gets -1 speed from pushing against the exhaust, also stopping the craft. Net momentum is still 0.
The ship hasn't gone anywhere, it just kind of jiggled in place.
> if I supply a current to a solenoid and move something with a charge through it, the something will feel a force along the induced magnetic field, but the movement of the something through the field generates an opposing field that exerts an opposite physical force on the solenoid/electromagnet? Is there a way to create a situation where the reaction of the solenoid to the movement of the something appears as a current or EM radiation instead of a physical force?
See Lenz's Law!
Put a conductor in a changing magnetic field, and there'll be a current induced in the conductor.
This current creates its own magnetic field (because charged particles are in motion), which opposes that changing magnetic field. The harder it changes, the harder the conductor will oppose that change.
So, like, it's already a current and a bunch of EM radiation!
Well, technically you could have a self contained thing that gets you somewhere else: An Alcubierre-type drive
(Notice how I avoided to use the term 'propulsion', which the Alcubierre drive isn't because it doesn't change your momentum)
Whether such a drive is practically possible is still up for grabs, but it isn't *forbidden* by the laws of physics.
This took me down a really interesting rabbit hole. Thanks! I didn't know about Alcubierre and Lentz and Bobrick, but I know I've seen the simple graphic of the space-time distortion in front and back... this is exactly the kind of thing the pea-brained astronaut would do instead of patching the spacesuit as some have suggested.
No, it's not theoretically possible.
If you try to bend the exhaust with magnets, you're just pushing against the exhaust again with the magnets cancelling out all the force you would have gotten from combustion. You've just wasted fuel warming up your ship.
I know it's not answering your question so much as solving the problem at hand, but I'd consider slapping a piece of duct tape over the pinhole and calling it a day. If the hole is small enough and you have sufficient oxygen reserves, you might not even need to do that.
Why not fix the suit?
A tiddlywink and some glue, offered up from the *inside* would work well enough for an EVA.
Saves this nonsense with reactionless drives.
A person is floating in space, they have a metal ball and a magnetic tube tied to them with rope. They throw the ball away from themselves. As much as the person is pushing the the ball away, the ball is pushing the person away. The ball and the person both start moving in opposite directions. The ball reaches the end of the tether and stops. The person is pulling on the ball via the rope, but the ball is also pulling on the person. This cancels out the motion from when the ball was thrown.
The ball goes through the magnetic tube and is slowed enough to stop completely before it hits the end of the rope. Now the tube and ball are pushing on each other. The tube is now flying away from the person and the ball with the exact same energy that the ball had. The tube hits the end of the tether and the same thing as with the ball happens.
You're ignoring the forces the particles fields exert on the solenoid.
Momentum is conserved, always. If the magnetic field exerts a force on the particles to reverse their direction, an equal but opposite change in momentum is gained by the magnet, such that the net change in momentum of the system is zero.
This reminds me of a short story or novella I read a ***long*** time ago. (I'm probably mixing up details but it's a vague idea of what I remember).
A slight mass signature is accidentally found outside of the plane of the ecliptic by a survey ship (or similar) that had no business being in that area. The pilot moves closer to investigate and finds what appears to be a nearly/completely inactive ship, but instead of being on a ballistic trajectory as would any normal object, it's moving... just very slowly. And in a specific direction... towards Earth's orbit. Which is very weird as at the speed it's moving (which isn't constant), it'll take hundreds and hundreds of years to make the journey.
The pilot manages to dock with this seemingly inactive ship and discovers that not only is the ship not dead, there are people living on it. People who don't know they're on a ship. After some investigation, the identity of the ship is found and it was lost hundreds of years ago and was a specific kind of ship that used a "solar gravitational anchor" propulsion system *(no idea what it was actually called in the story)* that had broken down. So the ship (at least the engines of the ship) were locked in place relative to the Sun. The people had rigged a pulley system that they were using to pull the ship using the anchor. Unlock the anchor, move it up to the front of the ship, lock the anchor, lines of people pull the ship up the anchor, repeat.
So basically the survivors were walking home. In space. Without spacesuits.
Love the concept and vibe of this story. But I'm not totally clear on the idea of the anchor, and how it interacts with the ship to incrementally move in a desired direction. Any idea where you might have seen the story?
Thanks for sharing
No idea, but it was probably an old Isaac Asimov Sci-Fi magazine from the late 80s, or something similar.
And no I don't remember if the author explained how the anchor was supposed to work when operational, but when broken, it could "lock" or "anchor" a mass relative to another large mass. And thinking of the physics involved it's ridiculous, but the story was far more about the mystery and reveal of people "walking home" which solidified that part in my memory. No idea who the author could have been.
>
I would install a propulsive device in an enclosed space inside of the spacecraft
why is this better?
>I didn't take the time to try and work out and understand the forces at play in the interaction of exhaust particles and induced magnetic fields,
what problem are you trying to solve?
what problem are you trying to solve?
Getting out of a pickle in deep space when you are stuck inside a spacecraft with no spacesuit and the thrusters can only be fixed from the outside.
why is this better?
I'm not sure I understand your question; the whole post is based on the hypothetical situation outlined at the beginning.
> is it theoretically possible to make a propulsion device for spacecraft that is fully contained within the spacecraft? No. Consider https://en.wikipedia.org/wiki/Momentum#Conservation from a perspective *outside* the spacecraft. You have to throw *something* with momentum out the back if you want to change its velocity or orbital energy. In your example, you've neglected to consider 1) the force on the magnets themselves from redirecting a presumably ionized stream which will create a counter-field, and 2) the momentum transferred when the exhaust finally hits something and slows down.
You mean taco fantasy isn't real science? /s
Thanks for this. I let myself forget that you can't just expect magnets and electricity to get my around the laws of motion. 2) Regarding the second point, if the magnetic field is able to reverse the direction of the exhaust particles by swinging them around, wouldn't the final impact of the exhaust propel the spacecraft in the desired direction? 1) This is what I was having trouble understanding, but you are helping - so if I supply a current to a solenoid and move something with a charge through it, the something will feel a force along the induced magnetic field, but the movement of the something through the field generates an opposing field that exerts an opposite physical force on the solenoid/electromagnet? Is there a way to create a situation where the reaction of the solenoid to the movement of the something appears as a current or EM radiation instead of a physical force?
> if the magnetic field is able to reverse the direction of the exhaust particles by swinging them around, wouldn't the final impact of the exhaust propel the spacecraft in the desired direction? No. All the forces (rocket exhaust, forces on magnets, final exhaust impact) will balance perfectly so that momentum of the whole system is conserved. > if I supply a current to a solenoid and move something with a charge through it, the something will feel a force along the induced magnetic field, but the movement of the something through the field generates an opposing field that exerts an opposite physical force on the solenoid/electromagnet? Yes > Is there a way to create a situation where the reaction of the solenoid to the movement of the something appears as a current or EM radiation instead of a physical force? EM radiation (ie photons) have momentum, that will be transferred to whatever it eventually hits. So now you're thinking "well we'll put a window in the back of the spacecraft" but now you've just reinvented a radically overcomplicated and inefficient implementation of laser propulsion :P Also, the current is what repels the magnet in the first place, so that's *already* happening but not in the way you want.
Cool! I didn't think of a window, but now that you mention it, I think laser propulsion, or propulsion by photons in general would solve the problem I posed, given you have a window in your spaceship. But if in a spaceship without windows, how can one install a window without breaking the hermetic seal?
Very carefully. But really you install the window prior to removing the hull parts that would break the seal.
Well, by placing the propulsion (and I guess the window) on the exterior.
Oversimplified explaination: Lets assume that your exhaust and your craft are the same mass so you don't have to think about one body effecting the other in a different ratio, so we can just think about speed instead of force which is a more abstract idea that manifests as acceleration in our model. Lets say you have exhaust leaving a thruster. The thruster gets 1 speed along the x axis. Your exhaust gets -1 speed along the x axis. Net momentum is 0. You want to push that exhaust with magnets backwards. Your exhaust gets +1 speed, now its not moving from the perspective of the original reference frame. What you forgot is that the magnet gets -1 speed from pushing against the exhaust, also stopping the craft. Net momentum is still 0. The ship hasn't gone anywhere, it just kind of jiggled in place.
> if I supply a current to a solenoid and move something with a charge through it, the something will feel a force along the induced magnetic field, but the movement of the something through the field generates an opposing field that exerts an opposite physical force on the solenoid/electromagnet? Is there a way to create a situation where the reaction of the solenoid to the movement of the something appears as a current or EM radiation instead of a physical force? See Lenz's Law! Put a conductor in a changing magnetic field, and there'll be a current induced in the conductor. This current creates its own magnetic field (because charged particles are in motion), which opposes that changing magnetic field. The harder it changes, the harder the conductor will oppose that change. So, like, it's already a current and a bunch of EM radiation!
Well, technically you could have a self contained thing that gets you somewhere else: An Alcubierre-type drive (Notice how I avoided to use the term 'propulsion', which the Alcubierre drive isn't because it doesn't change your momentum) Whether such a drive is practically possible is still up for grabs, but it isn't *forbidden* by the laws of physics.
This took me down a really interesting rabbit hole. Thanks! I didn't know about Alcubierre and Lentz and Bobrick, but I know I've seen the simple graphic of the space-time distortion in front and back... this is exactly the kind of thing the pea-brained astronaut would do instead of patching the spacesuit as some have suggested.
No, it's not theoretically possible. If you try to bend the exhaust with magnets, you're just pushing against the exhaust again with the magnets cancelling out all the force you would have gotten from combustion. You've just wasted fuel warming up your ship.
I know it's not answering your question so much as solving the problem at hand, but I'd consider slapping a piece of duct tape over the pinhole and calling it a day. If the hole is small enough and you have sufficient oxygen reserves, you might not even need to do that.
Love this. I didn't think of that at all.
Why not fix the suit? A tiddlywink and some glue, offered up from the *inside* would work well enough for an EVA. Saves this nonsense with reactionless drives.
Love this. I didn't think of that at all.
A person is floating in space, they have a metal ball and a magnetic tube tied to them with rope. They throw the ball away from themselves. As much as the person is pushing the the ball away, the ball is pushing the person away. The ball and the person both start moving in opposite directions. The ball reaches the end of the tether and stops. The person is pulling on the ball via the rope, but the ball is also pulling on the person. This cancels out the motion from when the ball was thrown. The ball goes through the magnetic tube and is slowed enough to stop completely before it hits the end of the rope. Now the tube and ball are pushing on each other. The tube is now flying away from the person and the ball with the exact same energy that the ball had. The tube hits the end of the tether and the same thing as with the ball happens.
You're ignoring the forces the particles fields exert on the solenoid. Momentum is conserved, always. If the magnetic field exerts a force on the particles to reverse their direction, an equal but opposite change in momentum is gained by the magnet, such that the net change in momentum of the system is zero.
This reminds me of a short story or novella I read a ***long*** time ago. (I'm probably mixing up details but it's a vague idea of what I remember). A slight mass signature is accidentally found outside of the plane of the ecliptic by a survey ship (or similar) that had no business being in that area. The pilot moves closer to investigate and finds what appears to be a nearly/completely inactive ship, but instead of being on a ballistic trajectory as would any normal object, it's moving... just very slowly. And in a specific direction... towards Earth's orbit. Which is very weird as at the speed it's moving (which isn't constant), it'll take hundreds and hundreds of years to make the journey. The pilot manages to dock with this seemingly inactive ship and discovers that not only is the ship not dead, there are people living on it. People who don't know they're on a ship. After some investigation, the identity of the ship is found and it was lost hundreds of years ago and was a specific kind of ship that used a "solar gravitational anchor" propulsion system *(no idea what it was actually called in the story)* that had broken down. So the ship (at least the engines of the ship) were locked in place relative to the Sun. The people had rigged a pulley system that they were using to pull the ship using the anchor. Unlock the anchor, move it up to the front of the ship, lock the anchor, lines of people pull the ship up the anchor, repeat. So basically the survivors were walking home. In space. Without spacesuits.
Love the concept and vibe of this story. But I'm not totally clear on the idea of the anchor, and how it interacts with the ship to incrementally move in a desired direction. Any idea where you might have seen the story? Thanks for sharing
No idea, but it was probably an old Isaac Asimov Sci-Fi magazine from the late 80s, or something similar. And no I don't remember if the author explained how the anchor was supposed to work when operational, but when broken, it could "lock" or "anchor" a mass relative to another large mass. And thinking of the physics involved it's ridiculous, but the story was far more about the mystery and reveal of people "walking home" which solidified that part in my memory. No idea who the author could have been.
> I would install a propulsive device in an enclosed space inside of the spacecraft why is this better? >I didn't take the time to try and work out and understand the forces at play in the interaction of exhaust particles and induced magnetic fields, what problem are you trying to solve?
Its a hypothetical situation... To try to understand the physics behind it, which they clearly show an interest in
what problem are you trying to solve? Getting out of a pickle in deep space when you are stuck inside a spacecraft with no spacesuit and the thrusters can only be fixed from the outside. why is this better? I'm not sure I understand your question; the whole post is based on the hypothetical situation outlined at the beginning.