Rail gun engine the future of space travel?

[PB666]

1 hour ago, sh1pman said:

Well, a PhD in theoretical cosmology claims that

 

You can combine systems, for example our solar system and systems in our local cluster. However the old addage about extrapolation the Mississippi river does not stick out of the face of the Earth like a fishing pole.

 

[Vanamonde]

Let's get back on-topic, please. Some posts have been trimmed. 

[mikegarrison]

7 hours ago, sevenperforce said:

Using work in its formal sense, of course: the product of force and distance. Apply a force of 10 Newtons to an object across a distance of 5 meters, and you have performed 50 Newton-meters of work. A Newton-meter is, of course, a Joule, and if you measure the change in kinetic energy of the object, you will find it has gained 50 Joules of kinetic energy.

This is a reversible thermodynamic process, because there has been no increase in entropy and you can extract 100% of those 50 Joules of kinetic energy by allowing the object to impinge upon a spring or other energy storage mechanism. All thermodynamic processes which actually perform a net amount of work are reversible with respect to the work they perform. So even though entropy always has its lukewarm fingers in every pie, you can separate the positive-entropy component of a process from the work-producing component of a process. The former is irreversible; the latter is reversible.

I still disagree. The distinction between reversible and irreversible processes is very useful in getting a basic understanding of thermodynamics, and it's also useful as an engineering approximation. But that's what it is -- an approximation. Just like how boundary layers that separate viscous from inviscid flow are really only an approximation.

In reality, there are no reversible processes. Take a real world process like hydroelectric pumping storage. You pump the water up into the storage area by doing work, and then you get the energy back by having the water do work to the pump. Sounds great, and ignoring friction and resistance and evaporation and inductive losses and heating losses and turbulence and ... well the point is it's not actually reversible. It's only mostly reversible. It's theoretically reversible. It's approximately reversible.

[Hypercosmic]

That was me, hot-headed. Good thing I had radiators.

Ignoring the fact that he's trying to make a reactionless drive, this railgun propulsion talk thing reminds me of those 'sandblaster' railguns from the Children of a Dead Earth community. Those things that fire a 1 gram (or smaller) projectile at tens of kilometers per second.

Edited March 14, 2018 by Hypercosmic

[sevenperforce]

16 hours ago, mikegarrison said:

I still disagree. The distinction between reversible and irreversible processes is very useful in getting a basic understanding of thermodynamics, and it's also useful as an engineering approximation. But that's what it is -- an approximation. Just like how boundary layers that separate viscous from inviscid flow are really only an approximation.

In reality, there are no reversible processes. Take a real world process like hydroelectric pumping storage. You pump the water up into the storage area by doing work, and then you get the energy back by having the water do work to the pump. Sounds great, and ignoring friction and resistance and evaporation and inductive losses and heating losses and turbulence and ... well the point is it's not actually reversible. It's only mostly reversible. It's theoretically reversible. It's approximately reversible.

Well, we may be talking past each other. Obviously, there are no reversible processes in real life. But I wouldn't say that reversibility is an approximation. Rather, reversibility is an attribute of isentropic processes. Isentropic processes describe the component of a thermodynamic system which performs work; if work is being performed, then there is a reversible, isentropic process involved, even if entropy is flying willy-nilly left and right.

Reversibility and isentropy describe the underlying cycle which is available to perform work within a thermodynamic system. If you have a thermodynamic system which performs net work, then there is a reversible cycle somewhere in there, even if entropy would bleed off energy at each stage in the cycle. Reversibility exists even though the overall system is always going to have positive entropy.

So, for any reactionless drive, we can ask, "Does it perform work?" If the answer is no, then it cannot accelerate anything and it is useless. If the answer is yes, then there is a reversible, isentropic cycle somewhere in the system, and so that reactionless drive can (in theory) also be used as a reactionless generator. A reactionless generator can be coupled to a reactionless drive to produce over-unity, violating conservation of energy (even if entropy would bleed some of that infinite energy from the cycle).

IIRC, any violation of the laws of thermodynamics and kinematics can be exploited to produce infinite energy. For example, suppose you have a Brownian ratchet capable of extracting work from waste heat without a heat sink. This is a violation of the second law of thermodynamics. Couple that Brownian ratchet to a dynamo, and you can route the waste heat from the dynamo back into the Brownian ratchet, producing over-unity.