NASA state "aerodynamic forces are directly proportional to the density of the fluid".

Therefore would it be possible (for a short period of time) to reduce air friction by pumping hydrogen into a 'bubble' around your vehicle? It seems this would have a similar but less pronounced effect than super cavitation.

Air is ~800 times less dense than water, while Hydrogen is ~14 times less dense than air. If the Hydrogen was heated to ~1600 degrees it would be ~100 times less dense than air at room temperature.




  • $\begingroup$ Maybe, but you would need massive amounts of hydrogen, given that you would be moving forward at extreme speed. $\endgroup$
    – Javier
    Commented Sep 11, 2016 at 17:14
  • 1
    $\begingroup$ I don't think this is the same thing as cavitation, if that's what you're thinking about. You still need to displace the air, which is the source of the aerodynamic drag. In cavitation, you create a "pocket" of compressible gas within an incompressible fluid, which is different. $\endgroup$ Commented Sep 11, 2016 at 17:50
  • $\begingroup$ @Javier: I agree that the amount of hydrogen would be an issue. I don't know how thin you could get the bubble in a hypersonic flow. Honeste: I see, super cavitation reduces both form drag and skin friction. I think this would only affect skin friction. $\endgroup$ Commented Sep 11, 2016 at 18:02

1 Answer 1


Yes, it would work. The closest nature has to offer is the slimy substance that fish are covered in. This also reduces surface friction.

Aerodynamic drag has two principal components: Pressure and friction. Pressure is the backwards-pointing part of the integral over all forces perpendicular to the local surface, and friction is the integral over all tangential forces. The hydrogen layer would indeed reduce the tangential forces but would indirectly cause an increase in the pressure component of drag. The volume of whatever is moving through air needs to displace some air momentarily, and by adding hydrogen this volume is increased. The exact amount of the pressure drag increase depends a lot on the lift produced. At low lift and low hydrogen volumes the pressure drag increase should be negligible.


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