Author Topic: Spaceship "aerodynamics"?  (Read 2883 times)

Offline chutson99

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Spaceship "aerodynamics"?
« on: April 05, 2013, 07:54:00 AM »
OK, so I was thinking about science-fiction spaceships and how their designs have changed over the years:

how they used to be sleek and pointy pretty much across the board, but gradually, since everyone knows that space is a vacuum, they got lumpier, with aerodynamic designs reserved for shuttles and things that may have to enter an atmosphere while deep-space starships became more and more irregular, with stuff hanging off them all over the place. Of course, these are probably to some degree inspired by our real life spacecraft such as Voyager, which are coasting along just fine through the void.

But what about something going a lot faster than Voyager?

Since space is not a perfect vacuum, but has clouds of gas and dust in various regions, would a hypothetical spaceship traveling at a significant fraction of the speed of light encounter enough matter that aerodynamic issues would affect it at high speed?

I have no idea, so I brought it here.

Offline bn

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Re: Spaceship "aerodynamics"?
« Reply #1 on: April 05, 2013, 09:48:48 AM »
good question.
i'll think about it.
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Offline stephako

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Re: Spaceship "aerodynamics"?
« Reply #2 on: April 06, 2013, 06:14:43 AM »
My first intuition would be to say no.

If I remember correctly a main issue in aerodynamics is the formation of vortices and the behaviour of the boundary layer of the gas/fluid. Since the particles are much too far away from each other in interstellar space I doubt that they could interact enough too form a vortex.

Another point I just looked up is the drag equation. So the drag force is F=(density)*velocity^2*geometric factors. The geometric factors are where the aerodynamics comes in. The density of interstellar medium is about 10^6 molecules/cm^3. Air has about 10^22 molecules/cm^3.
So to get a drag similar to air you'd have to go 10^8 times faster than your average aircraft (probably even more, since air is composed of heavier elements than interstellar medium, I would say roughly 10 times as heavy).
With these numbers I would guess you run into relativistic problems well before aerodynamics comes in.


It just occurred to me that a better way to compare the situations would be too look at the respective Reynolds numbers. The Reynolds number is a dimensionless quantity that is used to check if two situations have the same hydrodynamic (or aerodynamic) behaviour (e.g. if you want to build scale models of planes and learn sth about how the real ones behave you adjust the flow to the dimensions of the model until you get the same Reynoldsnumber). In contrast to the drag force above it doesn't assume some specific flow pattern (I think) and includes the viscosity of the medium.
The downside here is that one needs to estimate the viscosity of the interstellar gas. I found a paper that gives an equation for this, but I just took the equation and didn't read where it came from. This would put the viscosity at 4K at about 10^-9m^2/s or roughly 10^-3 times the viscosity of air at 300K. With the estimate of the density above this would mean that the velocity would have to be about 10^14 times higher in space than the standard plane speeds (or cars, it doesn't really matter any more with these numbers). This is much higher than the previous estimate is that it includes viscosity and drag. Using 100 kph as a reference speed this would mean that your spacecraft would need to go much faster than even the speed of light.

EDIT2: I'm not sure how hydrodynamics needs to be modified at relativistic speeds, so maybe there are some effects that make aerodynamics kick in much earlier.
« Last Edit: April 06, 2013, 03:54:07 PM by stephako »
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