# Speed to maneuver with flaps and ailerons in interstellar dust

I know the interstellar space is not empty. It has (among micrometeorites and gas clouds) interstellar dust. And the effect of the dust is more noticeable if the speed of the starship increases (we are talking here about speeds not faster than light). My question is: At which speed could an interstellar ship use the interstellar dust to maneuver through it with flaps and ailerons? (like a plane does with the air).

• Vaguely Related: Link to other question (No actual quantities are discussed) – Magic Octopus Urn Nov 9 '18 at 17:20
• This also is not a dupe, but "lift" may apply here as well as a catch-all for transverse forces Is aerodynamic lift ever useful in rocket flight? – uhoh Nov 9 '18 at 17:58
• I bet the speed is high enough that the stuff being hit just plows through the flaps and ailerons in a flash of gamma radiation. – zeta-band Nov 9 '18 at 18:13
• FYI, an airplane does not maneuver with flaps. Their purpose is to increase the wing's lift and thus lower the stall speed (at the cost of increased drag), and so are normally only used for takeoff & landing. – jamesqf Nov 10 '18 at 3:23

It is not only how fast the airfoil has to travel, but also how large the airfoil must be to even have fluid-dynamics-like behavior. Because of the low density of particles in the interstellar medium, a normal-sized wing will not act as an airfoil. It will be more like bouncing tennis balls off the wing every so often.

The Knudsen number quantifies when fluid dynamics takes over: Kn = mean_free_path / airfoil_length_scale. Knudsen numbers greater than about 10 are in the regime of ballistic collisions, rather than fluid flow. The mean free path in the interstellar medium is huge (about 70 astronomical units). So to get fluid dynamics, you'd need an airfoil of size more than the Sun–Jupiter distance.

What is the difference between the fluid dynamics (low Knudsen number) and the ballistic regime (high Knudsen number)? When you are driving down the road and air flows over your windshield, this is because some air molecules collide with the windshield and then collide with air molecules further in front you, transmitting force forward. In a sense, air molecules several meters in front of you know about your car and begin moving away before the car actually reaches them. High Knudsen number, is, again, like driving through a field of floating tennis balls. They bounce off your windshield, but don't collide with or transfer their momentum to any of the other tennis balls (assuming the tennis balls are widely spaced). You can change your momentum by deflecting them at different angles, but it won't work anything like the flaps and ailerons of a plane.

At relativistic speeds, you might be able to steer by deflecting molecules of the interstellar medium. Even at these speeds, the drag force (and your ability to apply steering forces) will be very low for a normal-sized wing. There's a calculation on reddit for for drag in the interstellar medium at 0.9c. See Russell Borogove's answer for practical issues with utilizing this drag.

• Great answer! I was just coming here to invoke Knudsen but I can't improve on what you have here. – Organic Marble Nov 9 '18 at 19:24
• So short answer: It works great if you're making a Gundam. – Fund Monica's Lawsuit Nov 9 '18 at 20:30
• “field of floating tennis balls ... won't work anything like the flaps and ailerons of a plane” – well, it is different for sure, but I wouldn't say “not anything like”. In particular, positive angle of attack still gives you positive lift. The main difference is that camber doesn't make sense anymore; you'd instead want to make your airfoils as thin as possible. This is actually somewhat comparable to supersonic / hypersonic aerodynamics. – leftaroundabout Nov 14 '18 at 12:03

The density of the interstellar gas and dust medium varies widely, but I estimate that for fairly dense regions (1 million hydrogen atoms per cc), you get about 1 Newton of lift from a meter-square area of wing at about 10% of the speed of light, so the concept isn't totally inconceivable.

The trouble here is that friction with the interstellar medium is a real problem. You're hitting matter at 10% of the speed of light, which is going to produce a lot of heat. It's generally assumed that some sort of non-physical shielding is going to be necessary for high-sublight speeds -- perhaps a big laser to ionize the gas followed by a big magnetic field to push the ionized gas out of the way (or to collect it for a Bussard ramjet).

• Do you think a magnetic solution might work for the OP? Perhaps in addition to shielding, the particles could be deflected to change momentum in an analogous way to an aerodynamic control surface? – wedstrom Nov 9 '18 at 19:58
• Probably, but it seems of limited usefulness. – Russell Borogove Nov 9 '18 at 20:11