Thursday, October 25, 2007

Nozzle From Recent Static Test

After cleaning up the motor after last weekends static test I noticed distinctly copper coloration to the nozzle. I have noticed this to a lesser degree in the past few static tests that I've conducted. I attributed it to the copper tubing that I have been using for the pressure gauge. It didn't really make sense because that tubing is upstream of the exhaust flow so it seems like any copper from the plumbing would be pushed towards the gauge, not the nozzle. There are two significant differences between this test and the others. The first is that in this firing the nozzle appeared to get much hotter than the other nozzles that I have noticed this copper coloration in, the other is that in this nozzle there appears to be little globules of copper colored material that have been driven out of the surface of the of the nozzle. the thing that I can't figure out is that these nozzles are made of 12L14 steel, which is made up of Iron, Carbon, Manganese, Phosphorus, Sulfur, and Lead. So where does the copper coloration come from?

Monday, October 22, 2007

Static Test

This past Saturday I participated in a sugar propellant class at the Friends of Amateur Rocketry (FAR) facility. Brent Dougherty did a nice job presenting a general overview of how to safely work with and cast sugar based propellants. Thanks to the FAR guys Kevin, Mark, Ted and the others for letting us use the site. They have done an amazing amount of work on the site, including a massive vertical test stand (I don't remember what it's rated for).

As part of the class I fired an L-class KNO3/Sorbitol demonstration motor. The firing went well and good data was collected. This is a simplified set-up from what I've used in the past and it worked well. I'm planning on making some adjustable steel brackets that mount with bolts rather than the cargo straps. Here is the set-up:

In the picture taken just after the firing you can see the nozzle glowing red hot, once I got the motor torn apart and cleaned, everything appeared to have survived the firing just fine. The gauge measuring thrust began to register data about a tenth of a second or so before chamber pressure began to be recorded. I suspect that the pressure port was briefly clogged by the igniter initially. After tearing the motor apart I found a glob of melted plastic that used to be the soda straw that held the BP.

Saturday, October 13, 2007

Terrapin Nozzle

One of my all time favorite small sounding rockets has to be the Terrapin. Click here to see my earlier Terrapin post including pics and launch footage. Developed at the University of Maryland in the Mid '50s, it is a study in simplicity. It was a two stage vehicle that used existing motors, fins welded directly to the motor case, a zero length launcher, and had tremendous performance. One of the cooler pieces of Terrapin hardware has to be the nozzle from the booster. The booster was a modified Deacon motor. Here is a photo of another one of my favorite small rockets that used two Deacon motors for the initial boost. I have been doing some 3D modeling recently so I did a model of the Terrapin nozzle. I found a drawing of the Terrapin in Small Sounding Rockets which is a great book with a lot of drawings and photos. I wish the illustration quality was a little better, but I highly recommend it to any one interested in rockets/rocketry. The Terrapin nozzle appears to have been of all steel construction. It also appears to have had an O-ring seal between the nozzle and the casing liner and another between the nozzle and the casing. Retention appears to have been through a number of small roll pins (not shown in renders) I would love to see what one of these nozzles looked like after firing. Any additional info., photos, or corrections are welcome.
Anyway, here are the renders:

Thursday, October 11, 2007

Nozzle Cross Section

Here is a render of the new nozzle for the camera rocket. This render shows the much shorter that usual convergence section, this was done in an effort to reduce overall weight while still keeping the rest of the nozzle fairly "beefy".

Monday, October 8, 2007

New Motor

Here is a 3D model of the new motor. The insulation is not shown so that the grain assembly can be clearly seen. This motor uses a ring of screws to retain the nozzle and bulkhead that are not yet shown.

Here is a view inside the casing showing a propellant segment and the insulation.

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.