4 ms·

I think what Chuck meant was that the speed of sound increases with pressure, so the pressure wave in front of the capsule would cause the transition speed to s

by graeham 14y ago

I think what Chuck meant was that the speed of sound increases with pressure, so the pressure wave in front of the capsule would cause the transition speed to supersonic to be higher. I'm not sure this is a good thing though from a drag perspective, but could be if it keeps the flow under ~0.8 Mach. Definately the complexity of the fluid dynamics at high speeds can't be neglected. http://en.wikipedia.org/wiki/Drag_(physics)#Wave_drag_in_transonic_and_supersonic_flow http://en.wikipedia.org/wiki/Drag_(physics)#Wave_drag_in_tra...

ChuckMcM 14y ago
You're both right, at the boundary layer between the volume of air you're pushing and the tunnel there is going to be a bit of turbulence given the viscosity of the air, within the bubble you may never exceed the mach number for the air around you. Excellent paper put out by Barrett on designing long range sniper rifles [1] talking about the issues have having the bullet not be deflected by leaving the barrel (where even though its travelling quite fast it has yet to exceed the mach number of the expanding gas around it) and the ambient atmosphere (where the air is 'still' to the now speeding bullet). Needless to say the paper lost me in the fluid dynamics of the barrel air mixing with the outer air but they seemed pretty clear that inside the barrel things were pretty linear up to that point. Anyway, what ever it turns out to be I'm sure it will be cool. [1] My Dad is an engraver with an FFL and gets all sorts of interesting things in the mail. Usually to engrave, but the promotional literature is cool too.
ET3 14y ago
The principal of "choked flow" limits flow velocity in a constant section tube to Mach one unless the pressure drop is much higher than 1 atm. The drag force at choked flow would be about 20 tons. Tests show drag in a tunnel with high blockage ratios can be more than an order of magnitude greater than the same shape moving in open air.

by graeham 14y ago

ChuckMcM 14y ago

You're both right, at the boundary layer between the volume of air you're pushing and the tunnel there is going to be a bit of turbulence given the viscosity of the air, within the bubble you may never exceed the mach number for the air around you. Excellent paper put out by Barrett on designing long range sniper rifles [1] talking about the issues have having the bullet not be deflected by leaving the barrel (where even though its travelling quite fast it has yet to exceed the mach number of the expanding gas around it) and the ambient atmosphere (where the air is 'still' to the now speeding bullet). Needless to say the paper lost me in the fluid dynamics of the barrel air mixing with the outer air but they seemed pretty clear that inside the barrel things were pretty linear up to that point. Anyway, what ever it turns out to be I'm sure it will be cool. [1] My Dad is an engraver with an FFL and gets all sorts of interesting things in the mail. Usually to engrave, but the promotional literature is cool too.

ET3 14y ago

The principal of "choked flow" limits flow velocity in a constant section tube to Mach one unless the pressure drop is much higher than 1 atm. The drag force at choked flow would be about 20 tons. Tests show drag in a tunnel with high blockage ratios can be more than an order of magnitude greater than the same shape moving in open air.