By Timothy Raneyâ€¦Bald Engineer Guy with Glasses
This time, weâ€™ll talk about how I determined the rocketâ€™s center of gravity and center of pressure. Weâ€™ll also see the result from the first rocket stability simulation. Wow! Even more rocket science!!
I determined the rocketâ€™s center of gravity (CG) by loading it with all its components â€“ parachute, shock cord and its other items. I simulated the motorâ€™s mass with metal weights (121 grams). This mass was on the low side and only considered the propellant weight for a variety of H motors. It did not consider the motor casing. I didnâ€™t have that data at the time. In retrospect, an aluminum alloy motor casing with a spacer and nozzle can weigh ~170 grams. Finding the CG was nothing more involved than finding the point where the rocket balanced on a straight edge, as you can see at right. However, we need a little more data before we can characterize the rocket as potentially stable (or not). Thus, we will discuss the center of pressure (CP). By analogy, just as the CG is a point of balance based on the distributed mass of the rocket, the center of pressure (CP) is the point where all aerodynamic forces act. Determining the location of these two points and their relationship to each other is essential for stable flight. For example, we want â€œpositive stabilityâ€ where the CG is forward of the CP. In other words, the rocket will fly straight.
We definitely do not want to guess where the rocket will fly upon launch. Why? We donâ€™t want to become a target. We donâ€™t want anyone else, cows or the docile llamas at our site to become a target either. The distance between the CG and CP should not be closer than 1.5x the rocket body diameter or 3.42â€ in this instance (1.5 x 2.28â€). However, instead of going through the Barrowman equations to find the CP location, we will run a simulation instead. I just donâ€™t want to do a page of math right now. And I like mathematics very much. Being able to describe something mathematically separates me from the spider under this lab bench. Yes, I am a lazy guy.
However, itâ€™s important to know the Barrowman equations allow us to locate the CP and thus determine the rocket stability. The equations are not inconsequential â€“ go look it up, get some paper, a pencil and your scientific calculator. Folks have uploaded spreadsheets too â€“ make your measurements, enter the data and you get the answer. Go knock your socks off.
Rocket Stability Simulation
In my case, I chose to run the simulation to determine the rocketâ€™s overall stability â€“ the program computes the CP as a matter of course. The CG-to-CP distance in this design was then ~12.4â€ â€“ a good result. I used the â€œOpenRocketÂ©â€ program (v12.09.1) available from www.openrocket.sourceforge.net by S. Niskanen, et al. So, here we go. Entering the data for the simulation went well, just a little slow at first, but not because of any complexity. It was me. I forgot to â€œbuild the rocketâ€ in a specific order. For example, starting with the nosecone, then the forward body tube, the tube coupler and aft tube. When I forgot the coupler tube after completing everything else, I couldnâ€™t go back and add it. So, it was back to the drawing board and start over. In all honesty, there might be a way of inserting the coupler, but it was not apparent to me. Like Iâ€™ve said before, Iâ€™m not the sharpest knife in the drawer. Well, the resulting simulation was fine as seen in the summary slide above, though it showed the parachute deployed when the rocketâ€™s velocity was still ~160mph. Iâ€™m not sure why yet. I must review the data I entered for the flight conditions and the rocket itself. The good news â€“ it showed the rocket was capable of stable flight.
Next time, weâ€™ll discuss payload bay static ports and the more mundane tasks like adding more epoxy to the motor retainer; filing spiral body tube grooves and sanding the rocket. Weâ€™re getting close to the final preparations, honest! Who ever said rocket science was faster than the rocket?
 G. H. Stine & G.W. Stine, Handbook of Model Rocketry (7th Ed.), John Wiley & Sons, Inc., Hoboken, NJ, 2004, pp. 132-133.