Sunday, May 21, 2017

Which is the smoothest ride - rod or rail?

For the first time, the TARC rules state that the top 100 contestants making it to the Nationals cannot use a rod; however, they can practice and qualify using one. It's difficult enough to achieve a good score with all the variables associated with the rocket, so I can greatly sympathize with any team showing up at Manassas that qualified with their rocket being launched from a standard 1/4" rod. Yes, they can add rail buttons and practice in the month between final score submission and Nationals, but what effect will this change have on the flight performance? Does a rail exert more friction on the rocket than the rod, thereby causing a loss of altitude and cancelling the benefit of its increased rigidity? Or is the rod, with its well known "rod whip", going to cost the model even more altitude? It's important I find out, because a couple of my designs for this year's Geezer TARC barely make the goal with the optimum motor configurations for deployment at apogee, and I need to know if I can go with these motor combos. Duane attributes his loss last year to rail friction (his model was a couple of hundred feet low), and I have no desire to follow in those footsteps.

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").
The above are not very helpful in providing the numbers I need. That means I need to collect data - which means an experiment is in order. After a little thought, I have devised the following scheme:
  • 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.
Here is a screenshot of the OpenRocket design of the modified School Rocket, which I call the GDTV-1 (Guidance Drag Test Vehicle #1). I have already started cutting tubes and have ordered the rail buttons in anticipation of performing the experiment at this year's Southern Thunder (June 24/25). Pegasus Field is already overgrown, so I doubt we will be launching there much until it is mowed in the Fall. This will force me to attend the club launches in Manchester if I want to fly every month.
The GDTV-1 (Click to enlarge).


  1. A better test would be to build 3 rockets, one with lug, one with buttons and one with neither. Each rocket would be adjusted (mass added) to ensure that they are all the same. The same motors, altimeters and recovery are to be used. The rocket with neither lug nor buttons will be launched using 'fly-away' guides for rail and rod. 5 flights each, sequentially, on the same day. Compare altimeter data. Throw a launch tower in the mix to make it interesting

  2. I propose one rocket with both lug and button. This would be used to specifically quantify the friction on the launch rail/rod at liftoff. Other rockets could of course be used to test flying resistance separately. Just a thought.