Fat Man’s DairyAire

The “Fat Man” rocket is appropriately named as he is a large squat rocket. Like some people’s pets, rockets can resemble the owner. This one is human sized with a M1315 motor.

Looking up the rear of this fellow with a 400mm zoom reveals something interesting. There appears to be vapor condensation trailing the rocket, lit up by the plume. In successive photos it builds as the rocket picks up speed.

According to Toma, my rocket buddy, “it looks like oblique expansion fan aft of the airframe is creating a pressure drop associated with base drag, and the resulting temperature drop associated with that is causing a condensation of the freestream. You can see the streamline of the contrail, until it runs into the expansion plume from the motor nozzle. Likely hits a shock wave from the motor expansion, and is then re-vaporized.”

Roughly translating: The Fat Man is not particularly aerodynamic in the rear, and he makes a cloud ring on his tail.