Thursday, October 4, 2007

Camera Rocket, v02

I've been thinking about the next flight of the camera rocket. It didn't take me long to come to the conclusion that I wanted to do some thing different, either a larger motor or a different camera orientation. Initially I gave more thought to the camera, because there really wasn't much room left in the aeroshell for a larger motor. Then one weekend while I was out in the garage I came across the motor casing from the first firing of my N-class motor. That firing suffered a breech in the insulation and a burn thought near the bulkhead. I added more insulation and at the next test had a successful firing. While this motor is much too long for this aeroshell, it turns out that diameter would fit if I removed a tiny amount of material from the heads of the retaining screws. I played around with the numbers for a while and decided that I could use the bulkhead, nozzle, and a shorter section of the casing in a workable 3 grain configuration. I was planning on using the same nozzle as the larger 6 grain motor, which would have resulted in a very low Kn or grain surface area/throat area. My plan was to increase the burn rate by adding a catalyst (iron oxide). I wouldn't have gotten the chamber pressure up to the initial design of 1000 psi, but I figured that I would be able to get it up to around 600 psi. While performance would suffer some, the overall performance should have been fine. It didn't take me long to decide on fabricating a new nozzle though. The deciding factor was weight though, not performance. The new motor would already weigh more because of the increased propellant load, so using the existing nozzle which is fairly heavy wasn't seeming like such great idea any more. The rocket has flown very well in both of it's previous flights but adding a lot of additional weight to the tail end didn't seem like a good idea. I decided to fabricate a new nozzle, well essentially a new motor. I decided to stick with the 7:1 expansion ratio to reduce the weight of the nozzle and simplify the machining. You can see a comparison between the new motor design and the previous motor that powered this rocket.
While they appear to be very similar in size the new motor should provide a 33% increase in total impulse. Once the nozzle was fabricated, I was pleased to find out that it actually weighs less than the nozzle from the 3.5" motor. Unfortunately, the shorter nozzle also means that the boat tail is going to be shorter, but I am considering extending it beyond the nozzle exit a bit. The previous flight got to 14,400' and I'm hoping this flight can get close to 18,000'. I am planning on adding another on board camera, or adding a mirror to the existing camera bay to shoot straight down. Maybe I'll do both.

Here are the basic designs of the new motor and the previous motor.

The new nozzle design.

I did a 3D model of the nozzle. This is the first time that I've done this and it was very helpful to be able to rotate around the piece before starting to actually cut metal.

Here is the finished nozzle. It looks just like the 3D model.

This illustrates the basic layout of the motor in the camera rocket (well minus the camera). When I finish the boat tail and decide exactly how I'm going to lay out the cameras for the next flight, I'll do a new illustration.

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