The Thanksgiving break has led to quite a bit of conceptual work on the design of the rocket. I'm working on CAD models, which I did at first for the hell of it, thinking it was probably overkill, but it's really helped me to determine how I'm going to put the flipcam at the base of the rocket, since space there is really tight. I've also come up with an alternative GPS solution that isn't as full featured as what I had before, but gets the job done and does it for hundreds of dollars less. I've also been throwing around ideas for the parachute system, and coming up requirements for it.
This is a bottom-up view. Here you can see the latches I'll be using to secure the camera. The black circle you see on the left is the lens of the camera, which appears to be blocked by the gray piece (it won't be in the final product, of course), which is supporting the red piece. What's happening here is that the rockets, which are attached to the blocks of wood sticking out slightly, are supporting the red piece. Then, the wood-colored piece, which holds the camera, gets inserted through a large hole in the red piece, and secured to the red piece through latches. The goal here is to be able to start the camera recording and stick it into the rocket right away with little effort.
It probably just looks really cluttered though! I'll post pictures during construction and hopefully those will make more sense.
Doing the CAD for that part was useful in figuring out how to get the camera into the bottom quickly and easily. Why is there a camera in the bottom you ask, and not looking at the side? Well first of all, there will be a side-looking camera as well. The bottom camera is to see what's going on with the rocket when the engines ignite. I think that a view looking down, using the ground as a reference, will help figure out which way the rocket is going while under power.
Ultimately what are the requirements for GPS? I want to know what maximum altitude the system reached, and I want to be able to find the damn thing after it lands. So initially, it seemed logical to buy the high-altitude rated GPS chip and a radio modem to send down the data.
Here's the catch though, the radio modems cost HUNDREDS of dollars. So instead I've come up with an alternative solution from reading about other balloon projects. Using a GPS-equipped cell phone, I can send down location information shortly after liftoff, and when it comes back down. I won't have positioning data for some time, as it's above the range of cell towers or above its rated altitude (whichever is lower, probably the cell tower range), but as long as I can find it when it lands, I don't really need to know where it is when the cell phone data isn't coming through.
So total cost then becomes the GPS chip ($40), the Arduino ($30), and the GPS cellphone, which can be had for around $50. So the total cost becomes ~$120 for the GPS system. Much easier on the wallet than 370 (Arduino+GPS+Radio).
I've been racking my brain over this one for a while, but I think I've got what's maturing to be a viable solution. The problem is the parachute can't be deployed the whole time, since that will undoubtedly mess up the trajectory of the rocket when it launches.
So I've come up with an idea to hold the parachute close to the nose cone (which is another TBD piece) using releasable pins. When the rocket detects it is falling (or some preset number of seconds after the ignitions command) via GPS or an accelerometer, the pins will release and the parachute will be able to deploy fully. Of course, the rocket may be in an awkward position when the pins deploy, so I've got a "backup" solution.
The backup is the fins you see in the lead picture (3 deployed, one stored). They'll be attached to the rocket tube via a piece that essentially converts a round surface into a square one, to which they'll be attached with hinges of some sort. What I like about it is that it's a passive system. As the rocket falls, the fins will deploy naturally via aerodynamic forces, and likewise they fold naturally as the rocket rises. Their motion will be restricted with strings so that they don't simply rip off.
The whole system was sized to give a terminal velocity of about 20 m/s at 1km altitude. That's still pretty fast, but it's better than freefall. The only issue with the sizing right now is that I assumed a Cd of 0.8, which is probably in the ballpark, but you'll never get accurate numbers when you assume a Cd. Finding one is possible, I have the resources in terms of wind tunnels to figure it out, but I'm not sure if that's the best way to spend my time. If I'm finished with construction at the start of finals weeks (which is less than 2 weeks), then maybe, but otherwise it's not likely I'll refine that number until flight.
So there's a lengthy and rather detailed update, although not as much progress as I would have liked. It's like my manager at my internship this summer told me, I have good attention to detail, but I lack in dependability - sticking to something. Still, I'm really enjoying work on this project and as winter break looms so does a lot of free time which I can happily spend working on this :)