Spacesuit Manuvering WITHOUT RCS

[kerbiloid]

They can. But don't like.

Spoiler

 

 

[Starman4308]

6 hours ago, Spacescifi said:

Air is a lot less dense than water so you need more kinetic energy to apply to it to get it to do work for you.

Not really. The astronaut there generated very little kinetic energy, as evidenced by his very slow movement.

He generated plenty of thermal energy, because the conversion from chemical to kinetic energy was very inefficient. Without a large surface area, something like a wingsuit, the human body is hard pressed to transfer much momentum to the surrounding air. While travel in air is typically less energy-efficient, the full story is more complicated than that: it's a matter of engineering moreso than theoretical physics.

Pretty much all atmospheric or water-based engines exist on a spectrum between giving a small acceleration to a lot of fluid, or a large acceleration to a small amount of fluid. The first is more energetically efficient, as while momentum transfer is p=mv, kinetic energy transfer is k=1/2*mv^2. As such, the ratio of kinetic energy to momentum transfer k/p = v/2.

With air, each liter contains very little mass, so atmospheric engines tend towards the high-exit-velocity low-exit-mass end of the spectrum, as you would need a very large propeller or jet engine to act on a comparable amount of mass.

With water, the surrounding fluid is roughly 1 g/mL, making it much easier to engineer a propeller or turbine moving a large mass at a low exit velocity. On top of that, instead of using lift to avoid falling to the ground (necessitating high speeds and high atmospheric drag), ships primarily use buoyancy. They don't need to go fast, so they can optimize their speed almost solely based on water resistance.

The key reason why it's difficult to make atmospheric engines as efficient as water-based engines is simply due to difficulty in getting enough surface area to intake the necessary quantities of air, but that's an engineering restraint, not a law of the universe.

6 hours ago, Spacescifi said:

Air is a lot less dense than water so you need more kinetic energy to apply to it to get it to do work for you.

Not really. The astronaut there generated very little kinetic energy, as evidenced by his very slow movement.

He generated plenty of thermal energy, because the conversion from chemical to kinetic energy was very inefficient. Without a large surface area, something like a wingsuit, the human body is hard pressed to transfer much momentum to the surrounding air. While travel in air is typically less energy-efficient, the full story is more complicated than that: it's a matter of engineering moreso than theoretical physics.

Pretty much all atmospheric or water-based engines exist on a spectrum between giving a small acceleration to a lot of fluid, or a large acceleration to a small amount of fluid. The first is more energetically efficient, as while momentum transfer is p=mv, kinetic energy transfer is k=1/2*mv^2. As such, the ratio of kinetic energy to momentum transfer k/p = v/2.

With air, each liter contains very little mass, so atmospheric engines tend towards the high-exit-velocity low-exit-mass end of the spectrum, as you would need a very large propeller or jet engine to act on a comparable amount of mass.

With water, the surrounding fluid is roughly 1 g/mL, making it much easier to engineer a propeller or turbine moving a large mass at a low exit velocity. On top of that, instead of using lift to avoid falling to the ground (necessitating high speeds and high atmospheric drag), ships primarily use buoyancy. They don't need to go fast, so they can optimize their speed almost solely based on water resistance.

The key reason why it's difficult to make atmospheric engines as efficient as water-based engines is simply due to difficulty in getting enough surface area to intake the necessary quantities of air, but that's an engineering restraint, not a law of the universe.

EDIT: Please let me know if this double-posted: I wound up having to click "Submit" twice.

Edited May 18, 2021 by Starman4308