Ye Olde Rocket Forum - View Single Post - Scrounged-Up Designs 2006

I worked up a RockSim version of the Andromeda using the ringtail function as opposed to the inside tube method [used in the first version]. There is a drastic degraded difference in the performance using the ringtail, at least as shown in the simulations. My conclusion is that RockSim places a much higher drag value on ringtails as a class than it does on the tube itself.

Attached are both RockSim files...

Stability margins are different, even though no changes in the components are made: 5.04 [empty] for the IT version, and 4.56 [empty] for the RT version.

Center of Gravity (CG) locations: 13.3226" IT, 13.3224" RT
Center of Pressure (CP) locations: 18.0688" IT, 18.5643" RT

As you can see, there is a 1/2" difference in the CP location just for changing between an inside tube (IT) and a ringtail (RT).

Note the differences in performance:

B4-4: 351' for the IT version, 193' for the RT version
B6-4: 362' for the IT version, 192' for the RT version
C6-5: 881' for the IT version, 381' for the RT version

I can vaguely remember something from my early RC days in the late '70s, when some of the local fliers had a model with simulated engine nacelles along the body. Someone commented that they had to "block off the passage" near the front opening in order to get the model to fly correctly. I think they were experiencing the same effect I'm seeing with RockSim on these tubes. It may be necessary to "block the passage" with some kind of bulkhead near the front openings of each tube to prevent them from acting as ringtails. The bulkheads would actually produce less drag on the model than the airflow through the tubes.

More on this as I get the first prototype ready for flight...

Additional notes: I made some corrections involving the slots for the ST-16 tubes, adding some "spacers" at the openings to hold their shape during construction. These should be removed before final assembly. A beefing-up of the base around the rear shroud was done to reduce the chances of breakage during assembly. This problem was identified in the construction of the prototype. Also, a change was made with the inner and outer armature cores. I changed the material to a 3/16" diameter spiral tube instead of hardwood dowels. Essentially, this is a length of launch lug material. It lightened up the fins a little. Both attached files reflect these alterations.