Base drag. I wondered how those curled-up foam pool noodles flew! I'm not an aero student - or maybe I'm a student of any subject - but I love learning things here.
From
cambridgerocket, a fabulous PDF: The aerodynamic forces are complex phenomena and there are no simple analytical solutions for the coefficients. In the incompressible flow regime the forces can be divided into pressure force and viscous force. Pressure force arises through the stagnation of fluid on the rocket forebody, fins, and any other protrusions, and also through a suction force created by a low pressure region at the base of the rocket where boundary layer separation occurs. Viscous force is due to skin friction between the rocket and the air. ... The base drag on the rocket is the drag due to the low pressure region at the base of the rocket that is caused by boundary layer separation. This drag is estimated using equation (42):
db = max body diameter
dd = base diameter
Cd(fb) = Cd of rocket forebody
From
UiO in Norway:
UiO also had this super shock wave photo!
From the
Flight Dynamics Laboratory, Wright-Patterson AFB, July 1984:
The last formula is based primarily on aircraft, but I thought it was interesting to compare the two equations. I couldn't find a definition of Sref, but S in this document is surface area. Clearly a noodle rocket's Sbase must approach Sref, explaining its high base drag and stability.
Andrew: one day you will be applying for a job. Maybe it's time to learn that HR departments these days are very good a combing the net for posts and breaking through anonymity using outsourced big-data correlations. What company wants to hire someone who's posted pix of themselves snorting coke? Every year the search algos get better, so it's only going to get more transparent. You're leaving embarrassing breadcrumbs, unless you're planning a career in bowling ball design.
So imagine yourself applying at Lockheed or Boeing someday, then after a good interview HR calls and asks "Are you Andrew_ASC?" Whatcha gonna say about your posts? That you were too lazy or arrogant to search the net? That you thought you knew everything in your early 20's? For goodness sake, in less than an hour I learned how noodle rockets have stability and why boat tails are so good (which I use for benchrest target shooting)... and it's all about base drag, not pixie dust.