Airplanes in Space.

[WestAir]

Some of you are talented in mathematics and science, which is great for answering this random question that popped into my head:

How fast would you have to be traveling in a dense Stellar Nursery or Nebula to achieve lift in a generic aircraft? Due to the obvious lack of gravity, would this mean that a rocket with wings would eventually reach a point where it must fire its thrusters nearly perpendicular to its direction of travel to move "forward"?

Much appreciated to anyone who can do the math or who has knowledge of both aeronautical science and astronomy.

[LordOfTheOrbits]

I can't tell you any numbers, but even the denser nebulas aren't nearly as dense as the exosphere of the earth, in which most of our satellites including the ISS are flying. The solar panels of the ISS are much larger than usual airplane wings, so what you are suggesting is like maneuvering the ISS just with its solar panels.

I don't know if it's possible to archives notable lift in nebulas, but it would probably require speeds of more than 0.1c if it's possible at all.

[Rakaydos]

A related note... the Buzzard Ramjet, that scoups intersteller hydrogen and burns it for fuel, hits terminal velocity (where the energy lost collecting and velocity-matching the hydrogen matches the thrust receved by a fusion reaction) at around .2C

[PakledHostage]

The solar panels of the ISS are much larger than usual airplane wings, so what you are suggesting is like maneuvering the ISS just with its solar panels.

As a point of interest, the solar panels on the ISS do contribute significantly to its orbital decay. Re-orienting the panels into an "aerodynamic" alignment with the direction of travel while in the Earth's shadow (where they don't need to face the Sun) significantly reduces drag on the station's solar arrays.

According to a paper cited in Wikipedia's Night Glider Mode article, carefully orienting the solar panels in this way reduces average drag on the ISS' solar panels by 30% and saves about 1,000 kg of orbital-maintenance propellant per year.

[WestAir]

As long as the fluid medium being traveled has pressure then there is a speed for which an airfoils lift will overcome weight. I'm not certain how that works on a craft with no weight to overcome, however.

[Ralathon]

First of all, you have to realize that stellar nebula are still very VERY sparse. A dense nebula has 10 000 particles per cubic cm, a factor 10¹⁵ smaller than air at sea level. This is equivalent to the best vacuums humans can make.

This means that regular effects that keep airplanes flying (vortex above the wing, coanda effect etc) are completely absent. Any lift you create will be purely due to your angle of attack and even that will be ridiculously small.

Say you hit the nebula at a relative velocity of 10000 km/s (1/30 c), have an angle of attack of 45 degrees and a cross section of 1000m^2. The density of a good stellar nebula is about 2*10^-17 kg/m^3. If we treat the gas atoms as elastic collisions we get a lift of:

(1e7*1000*2e-17)*1e8 = 20 newton. Or to put that in perspective, enough to barely lift 2kg on earth. You would have to take it into account, you are going to deviate a bit. But you won't have to fire thrusters to keep your course. And this is very much a worst case scenario.

[AngelLestat]

yeah, nebulas are far to be what we can call like "dense".

And even if you are traveling at great speed (A LOT) that you cross a volume of gas material compared to a high atmosphere with low speed. It would not work either.

Becouse those gas molecules or atoms, would destroy your ship instead help to change your movement direction.

Maybe with a really big magnetic field you can simulate a wing. But again, the higher G of the manuver (change the angle of your direction in a short time) at that speed will destroy the ship.

[DonLorenzo]

Why would they destroy the ship? Either there's going to be a lot of force, or there isn't. If Ralathon's numbers are correct you can expect a 2N force acting on your ship in those circumstances, I'm having trouble imagining a vessel that wouldn't stand up to that.

[spudman2]

@DonLorenzo

Your craft is moving at 1/30c, which means that gas particles are hitting you at 1/30c. It would be like trying to get lift from the bullets a wall of machine guns is shooting at you. Which is also a thing I really want to do now.

[Ralathon]

@DonLorenzo

Your craft is moving at 1/30c, which means that gas particles are hitting you at 1/30c. It would be like trying to get lift from the bullets a wall of machine guns is shooting at you. Which is also a thing I really want to do now.

Yea but there are only a few million bullets per cubic meter. Atoms are so small that it doesn't really matter how fast they hit you (until you reach some ridiculous speeds and they gain enough energy to ionize things). Sure, they'll penetrate your surface a bit deeper, but it isn't going to rip your spacecraft apart. You simply aren't going fast enough for that.

For example, the solar wind can move up to 1/300th c. But our spacecraft have been subjected to it for decades and they still work just fine.

[Pds314]

The problem with maneuvering an airplane in a nebula is that, unlike sci-fi might have you believe, Nebulae aren't very dense. Even the densist molecular clouds might reach a few million atoms of hydrogen per cm^3. This is a density of something like 10^14 times less dense than air ASL. Likewise, for similar hypersonic flight effects, one would need to move 10 million times faster. So if one had a 100 m^2 control surface or wing at an AoA of 30 degrees, they could get up to 3.5E-13 newtons per (m/s)^2. So if they moved 1700 km/s, a 1 m^2 control surface could get a newton of force. A single newton. The wing would have to be made of thin paper in order to make it apply 1 gee. At Earth sea level, it would only need to move 17 cm/s.

Of course, that is momentum. The story for kinetic energy is very different. 10^-14 kg of H2 moving at 1700 km/s has an energy of 14 millijoules. Not bad, right? What if that much hydrogen is hitting every square meter of your ship every 600 nanoseconds? That is something like 25 kilojoules per m^2. That is only 17 times brighter than sunlight, right? Well.... Yes. But it is in the form of H2, not sunlight. That means it would, for the purposes of anyone on board, be ionizing radiation. And it also means that the wings would probably just molecularly decay and absorb th H2 instead of reflecting the H2, as even crystalline graphene cannot survive hydrogen impacting directly at speeds over 50 km/s unharmed. This means the wings would suddenly be very good ablative thrusters, loosing their entire mass normal to their surface at 20 km/s or so within a matter of seconds. So actually, you'd be experiencing forces similar to those on an older airplane, but only because your ship is a giant ablative reverse thruster.

[Mr Shifty]

@DonLorenzo

Your craft is moving at 1/30c, which means that gas particles are hitting you at 1/30c. It would be like trying to get lift from the bullets a wall of machine guns is shooting at you. Which is also a thing I really want to do now.

https://what-if.xkcd.com/21/

[architeuthis]

As long as the fluid medium being traveled has pressure then there is a speed for which an airfoils lift will overcome weight. I'm not certain how that works on a craft with no weight to overcome, however.

Lift force is always normal to the fluid velocity even if the subject of the force is weightless. This is why it is sometimes useful to add fins to rockets: the lift force is proportional to the angle of attack, thus if something disturbs a rocket such that it is no longer pointing in the direction of fluid velocity a lift force is created. If the lift force's line of action acts through a point behind the center of mass, then this lift will produce a torque that will tend to correct the rocket's angle of attack back to zero. So the fins make the rocket dynamically stable, even though the lift force is not acting in opposition to gravity in this case.

Regarding the OP, it's a really interesting question! Lift coefficients are functions of an object's shape, Reynolds and Mach numbers, and effective roughness ratios among other things. I've heard that nebulas can be quite hot, but I'm guessing that even despite that viscous effects would be basically negligible, and so we would mostly only need to worry about the shape of the object. Let's say some crazed alien art collector of the far future towed an ancient Earth-Human Boeing 747 out to the Orion nebula and mounted it to a relativistic rocket kick stage for the purposes of creating an avant garde installation. The 747 has a wing area of 541 m^2 according to wikipedia, so the velocity needed to generate a single Newton of lift given a 45 degree angle of attack would be 6.12E6 m/s or about 0.02 c.

Edited February 14, 2014 by architeuthis