By Timothy Raneyâ€¦Bald Engineer Guy with Glasses
Hello again! This time weâ€™ll discuss the initial design process, to include determining the rocketâ€™s overall dimensions and fin design. (Link to Part 1)
The design process used existing literature and will include software simulations to refine the design preparatory to actual fabrication. Part of this effort included a review of existing high power kit rockets. These kits helped me to determine the gross dimensions. Moreover, I chose to use existing [standard] supplies intended for high power rockets. I did not glue toilet paper tubes together or turn my own nosecone. In this case, I selected 2.28â€ diameter phenolic-impregnated cardboard tubing for the body tube and 1/8â€ thick Baltic birch plywood for the fins. The motor mount tube stock was 1.65â€ nominal diameter along with its plywood centering rings. The plastic parabolic type nosecone was designed for the selected tubing. I bought the plywood locally at a craft store (Hobby Lobby – http://hobbylobby.com). The other items were purchased from Public Missiles, Ltd., Jeddo, Michigan (www.publicmissiles.com) – a longtime supplier of high power rocketry supplies.
After mentioning these parts and materials, I suspect itâ€™s a good time to show a diagram of a basic rocket with its parts labeled as shown above. What nice amateurish artwork. Additionally, a great place to further your knowledge is to start at Apogee Components, Inc., in Colorado Springs, Colorado (www.apogeerockets.com). They are a tremendous educational resource â€“ tutorials and newsletters on every facet of model rocketry. Moreover, they are a major supplier for everything you would need for building rockets â€“ from kits or scratch-built. Before we leave this topic, the National Aeronautics and Space Administration (NASA) is another great resource, particularly their “Beginner’s Guide to Rockets” (http://exploration.grc.nasa.gov/education/rocket) with the basics and more advanced material. Letâ€™s continue with some of the actual design work and initial construction, shall we?
After selecting the materials, I decided to do the fin design since I needed to know this information before cutting the slots in the aft rocket body tube. Any math needed to design the fins is based on the rocketâ€™s diameter (2.28â€ here). In the case, I decided to use the clipped delta fin planform â€“ itâ€™s easy to layout and cut. However, itâ€™s one of the best designs from a low-drag standpoint. More about reducing drag later. The basic fin design and the equations are shown at left. As noted, I did not use the fin thickness equations, but here they are in case youâ€™re interested: Troot = 0.1 x Croot (max); Ttip = 0.1 x Ctip (max). Yes, the â€œTâ€ is for thickness.
Overall Rocket Dimensions
I mentioned doing a little research by examining several kit plans to determine the overall dimensions. So, given the 2.28â€ body tube diameter mentioned above, the rocket will consist of three sections â€“ a 36â€ long aft section; the 18â€ long forward section for the payload bay and recovery parachute, along with a 10â€ long parabolic nosecone.
Given these calculations, I cut the 3.5â€ long slots in the 36â€ long body tube (right). The slots began 0.75â€ from the tubeâ€™s aft end. Yes, rockets use nautical nomenclature: forward for front, aft for back or rear; port for left and starboard for right. Arrr matey. I cut the slots using a 0.125â€ diameter tungsten carbide 2-flute end mill in the milling machine â€“ spindle speed was 2580rpm.Â The tube was held by a lathe chuck on the rotary table positioned horizontally to enable cutting of three (3) fin slots 120 degrees apart. With one end of the body tube held by a 3-jaw chuck centered on the rotary table, the other end was allowed to rotate within a lathe steady rest. This method worked fine. Who says I canâ€™t use a lathe steady rest on the milling machine? You are only limited by your imagination and the desire to perform a machining function safely and efficiently.
Next time, weâ€™ll discuss fitting the fins to body tube, designing the expulsion charge baffle system and other cool details. Hope to see you then!