View Full Version : B/Gs with bottom-mounted pop pods?

05-31-2009, 10:53 PM
Hello All,

I've been ruminating about the intersection between hand-launched gliders and front-motor pop pod boost-gliders, which are the most common type of B/G. The Centuri Swift (see: www.ninfinger.org/rockets/nostalgia/72cen044.html ) is a typical example of this B/G design.

For a glider that one might wish to use as both a hand-launched glider and a boost-glider, the location of its vertical stabilizer poses a possible problem. Since most front-motor boost-gliders have only a single vertical stabilizer (instead of twin vertical stabilizers mounted on the tips of the horizontal stabilizer), the vertical stabilizer must be mounted on the underside of the rear fuselage or tail boom so that it will be clear of the rocket motor's exhaust plume.

While such boost-gliders do fly and fly well, the downward-protruding vertical stabilizer is subject to excessive wear & tear. It is also more likely to snag on rocks or other obstructions on the ground, which increases the risk of it and/or the rear fuselage or tail boom being broken on landing. From an aesthetic point of view, it's just plain funny-looking.

The Vashon Industries cold-propellant boost-gliders (which Estes carried for a time in the 1970s) had a different physical arrangement that allowed the use of gliders with normal tail assemblies, and this configuration could also be utilized in solid-propellant boost-gliders. In these models, the pop pod was an ordinary three-finned rocket with one "missing" fin, which was replaced with a glider. While they looked like parasite boost-gliders at first glance they were not, because the two-finned carrier rockets were not aerodynamically stable without their gliders attached.

The glider was mounted "belly down" on the rocket body in such a way that its wings were even with the rocket's two fins, and much of the glider's airframe was located behind the rocket's nozzle. This provided both aerodynamic stability and "trailing-member stability" (like a skyrocket's stabilizer stick). The Vashon X-13 (see: www.ninfinger.org/rockets/catalogs/vashon/vashon14.html ), Astro-Gnat, and Baron (see: www.ninfinger.org/rockets/catalogs/vashon/vashon16.html ) all had this configuration.

While this arrangement would have a bit more drag during the powered ascent than the common front-motor boost-glider, it might be less prone to weathercocking under power. It would also allow many un-modified hand-launched gliders (and especially catapult-launched gliders) to be used as boost-gliders. (Two low, forward-pointing "L-shaped" wood brackets glued to the rocket in place of the "missing" third fin would fit on either side of the glider's fuselage and hold the trailing edges of its wings during ascent. At ejection, the glider would slide forwards out of the brackets when the rocket body was kicked backwards.)

I hope this information will be helpful.