Space ships and manoeuvrability

jsea-7

The question is why space ships cannot use propellers or conventional airplane jets to move in space?

Stratosphere

There is no air for them to use. Space is a vacuum.

jsea-7

I presumed that. But I also thought that space contained cosmic dust and hydrogen and hellium atoms, therefore it can't be a total vacuum.

Nabeshin

You would of course be correct, but such a dust is so tenuous that any propeller type design would be extraordinarily un-feasible. That is why for practical purposes relating to aircraft and spacecraft design, space is considered a vacuum.

ecao

just imagine those tiny dust particles as micro meteors. Any propellar would be ripped to shreads. Also, there is no where near enough hydrogen or dust to use a propellar let alone a jet engine.

jsea-7

How exactly would a propeller be ripped to shreads?

russ_watters

I don't think it would exactly be ripped to shreds, but objects we put in orbit get damaged - essentially eroded away - over time due to the speed of the particles hitting them.

qraal

Propellers and jets work because the gas (usually air) they encounter is dense enough to have very small mean-free-paths - thus the particles of gas collide with each other within a short distance and can be relied on to behave like a thin "liquid" in which pressure differences are significant.

In space the mean free path between collisions is thousands of kilometres typically, thus particles pass each other easily and essentially exert no pressure. Of course if you could collect the gas from a huge volume in front of you and apply energy to it to expel as a jet, then it would work. That's how a Bussard ramjet works BUT the volumes and energies required are much, much larger than for regular propellers and jets.

Is there the equivalent of a space plane? There might be. A vehicle might be given immense electromagnetic wings that interact with the thin space-plasma and "ride" the currents that fill space, much like a glider can ride thermals and use dynamic soaring to reach high speeds. The wings would need to be very large and probably supported dynamically.