Here are a couple things I do know:
- No simulation program - Open Rocket, RockSim, SpaceCad, etc - takes into account the friction generated as the rocket slides up the rail or rod. These codes can roughly account for the atmospheric drag of the lug or rail buttons as the model progresses through its trajectory, but assume that the rod or rail is frictionless, which ain't realistic.
- Calculations done by Tim Van Milligan at Apogee Rockets indicate that rail buttons are more draggy than launch lugs, especially if the lugs are fairly long, and have had their ends modified. So there is going to be a performance hit even if the rod and rail exert the same friction, leaving open the question of whether the simulation programs can model this properly (lugs are in the codes, but rail buttons have to be "kludged").
- Construct a mid power rocket - must be a decent size so that the rail buttons/launch lug do not dominate the drag. It turns out I have the perfect kit - the Balsa Machining 3" School Rocket, which has a pre-slotted 3' diameter tube, laser cut balsa fins, and a nice plastic nose cone that will reduce the amount of time spent finishing the model. All I need to do is add a payload section to hold an altimeter, plus replace the 24 mm motor mount with a 29 mm to provide some needed power.
- Launch the above model several times, alternating between rod and rail pads. Both the rod and rail should be as close to vertical as I can make them, and the launches should be dome quickly, with little time separation. This will minimize the effects of changing weather. Each set of two launches (rod and rail) should be done with motors from the same pack or batch to reduce motor variations. What I am looking for is a trend among the flight altitudes, with either the rail or rod producing consistently lower altitudes. If there is no such trend, I need to try to evaluate whether something is messing with my experiment; if not, then the conclusion is that it doesn't make much difference which type of pad you use.
|The GDTV-1 (Click to enlarge).|