Cosmo Rocket Project

With static testing of the new MB-2 motor under way. I decided to start working on a rocket for the motor to be used in. With the MB-2 motor being a high K or low L class motor, it is a great leap in power over any previous rockets I have flown. In fact, the MB-1 motor with 1/2 of the impulse of the MB-2 motor, has yet to be flown, even though it has been static tested 5 times.

Here is a starting list of my projects in a project:

Dual deployment will be needed. With a software estimated maximum altitude of over 11,000 feet, I'll need dual deployment or the rocket may drift to Canada.
Even with dual deployment, I want redundancy. If I go with dual timers on the drogue chute, and one timer on the main chute, it means I will need to initiate three timers. Three pull wires is out of the question, so it's time to build a g-switch for launch detection. I'll use a pull wire on the drogue chute timer, and the g-switch for the back up drogue timer and the main timer.
I would like a better tracking system, and some sort of data during the flight would be interesting too. With that in mind, I decided to try an FRS radio in the rocket. Another EX friend offered to build a directional antenna for FRS signals. If it works, what a great way find a rocket!
I need to design and test dual deployment. I'll split the body between the fin can section and the payload section. The drogue chute will be deployed from the split airframe and the main will be deployed from the payload section.
I would like to redesign my fins, both the attachment and the fin material. The rocket sims out at mach 1.2, so strong fins will be in order. I use aluminum angle brackets to hold the fins on now. The problem is, the angle bracket doesn't position the fin perpendicular to the body tube. All the fins are at the same angle, and it hasn't been a problem in the past. But it looks funny, and if I can come up with a better attachment method I will.
With such a heavy motor I'm going to have stability issues. I did a quick check of the center of gravity with the MB-3 fully loaded in an existing rocket airframe. The rocket has dynamic stability, but does not have static stability. Static stability isn't required, but it would be nice to have it in case of a mishap. I think I will end up making the body longer, and that will help. I suppose by the time I get all my electronics in the payload section that will help some too. In the end, I can size my fins according to the degree of stability I want.

G-Switch

This was something I had been intending to do for some time. A simple g-switch to turn on my timer at liftoff. I had a very small toggle switch, so I measured the force required to move the switch from off, to on. The force was 95 grams. So, if I wanted the switch to activate at a launch force of 4 g's, I would need 95/4=23.75 grams of weight. That's not so bad. I'm sure I could have found a switch that required less force, or I could have used a more complex lever system to use less weight, but I like to keep things as simple as possible.

Above is a picture of the completed g-switch.

The toggle switch is on the top left. I cut some small angle aluminum to make a bracket to hold the switch. I used the same aluminum to make a pivot arm that holds the weight and moves the toggle switch. I had to cut out some aluminum so the arm would not catch on the bolt holding the toggle switch bracket.

The weight is a slug of lead I had. I thought about casting a nice little cylinder of lead, but instead saved the time and just cut the lead slug with an old chisel. After it was cut and weighed, I trued up the lead with a hammer. I drilled and then tapped a small hole in the lead so I could bolt it onto the pivot lever.

The small spring was salvage from a dead VCR (don't throw them out!), the spring is there more to keep the lever in place after the switch is activated. There is not a great force from the spring acting on the switch.

Here is a closer look at the switch.

Once I had everything together I used the incredibly scientific method of flinging the switch upward while holding it in my hand. It takes a nice fling to activate the switch. The bad thing is, if you were to drop a rocket even just a foot or so, the resulting force would activate the switch. So, a master power switch is needed to keep the power off until the rocket is prepped and on the rod ready to launch.

After I was happy with the way the switch was working. I used epoxy on the backside to secure the nuts and the switch bracket to prevent them from shifting.

Here is a picture of the g-switch mounted to my electronics bay module. On top is the switch, below it is the timer and the battery.

I decided to do a slam test of the switch and electronics. The electronics module is made from 1/2" plywood, so it's pretty tough. The bottom of the module has a large eye lag bolt to facilitate removal from the body tube. I had everything hooked up and the battery on. Instead of a deployment charge I had an intact bulb in the deployment charge socket. The timer also has a piezo buzzer so you know when the timer is active.

I quickly thrust the entire module into the air, as expected the g-switch activated and the buzzer confirmed the timer was running. I then slammed the whole module onto a 4"x4" board laying on my work bench. I repeatedly crashed the module into the wood, after 12 seconds the timer timed out and the light bulb lighted. The 4x4 was badly dented from the eye bolt crashing into it. The slam test was a bit aggressive, but I wanted to know how everything would react to severe g-forces. Satisfied, I'm ready to do a launch test using the g-switch.

Update: The switch has been tested in Launch Test 38, and worked flawlessly.

FRS Radio

I mentioned earlier that it would be nice to use an FRS radio for tracking. Another EX friend is going to try building a directional antenna. If it works, what a great, inexpensive way to find a lost rocket. Even in poor conditions you should be able to get a mile on the ground out of them, and straight up in a rocket the signal should be good for many miles.

Almost a year ago I bought a second pair of FRS radios on sale for $10. That was too cheap to pass up. My other pair are better radios, and do have better range. But, I wanted a very simple set. Most radios don't allow continuous transmission, or even reception for that matter. They will only transmit for 30 seconds or so, and will turn off completely after so many minutes. The second set, the cheap ones, will transmit as long as you hold the button in, until the batteries die. Just what I wanted!

Even if we don't get good directional tracking out the radio. I wanted it for another reason. Simple data acquisition. With the radio transmitting, I should hear the piezo buzzer when the timer starts its timing cycle. I should also be able to hear deployment charges going off. It should be interesting to hear what's going on inside the rocket.

Here is the FRS radio mounted to the other side of my electronics module.

This was really a no-brainer. Four screws and the thing fell apart. I screwed the radio to the board using the existing holes in the radios board. A small plywood block was used on the bottom to support the high end. I cut the top half of the battery case off, and screwed it to the module as well. I couldn't see any reason not to reuse the battery case. Even the cover still fit. I did have to solder the power wires to the board, but that was all. The little wire antenna was hot glued at the top to keep it from bouncing around. The speaker was removed and the volume turned all the way down and hot glued in that position.

Update: The FRS radio was test flow in Launch Test 38, the radio worked very well.

Update: A new directional antenna has been made and tested.

Above is a picture of the newly made directional antenna for the FRS radio.

Here is a close up of the connections to the driven elements of the antenna.

The antenna boom was made from a 1.125" hardwood dowel, the elements are 1/4" aluminum rod. The wooden dowel was drilled through with a 1/4" drill and the elements hot glued in place. I salvaged some old 50 ohm cable and soldered connectors to it, then the connectors were screwed to predrilled holes in the driven elements. The two driven elements are separate pieces, and needed to be kept apart, so I cut a small plastic disk to use as an insulator.The other end of the cable was soldered directly to the PC board of the radio, after removing the rubber duck coil antenna, the shield was soldered to the radios ground. I also had some fittings in my parts drawer, so I installed a coupler so the radio could be removed from the antenna.

Initial tests at close range weren't very good. And that was really expected. The signal is so strong that it doesn't matter what direction the antenna is pointed, the signal is strong.

So I sent Bill out in his car, and told him to stop somewhere with the transmitter and let me find him. I got a good idea of the direction by slowly rotating the antenna, and listening for the strongest sound. It didn't take me long, as Bill was about 1/3 mile away and in his car.

For the next test, I told Bill to go a mile or two away, and leave the transmitter in a ditch somewhere. I waited about 5 minutes and went after him. There was no signal at all, so I started driving in a circle with a one mile radius, at about one mile east I picked up the signal, so I went south and lost the signal. Then I went north, and didn't pick up the signal again. Then I went east again and picked up the signal. I got within about 100 yards of the transmitter, but could not actually find the transmitter, the signal was again too strong to zero in on.

For the last test of the day, I left the transmitter home. And just went for a drive about a mile away. We stopped at different points to see how well we could pinpoint the signal location. It went pretty well at that range.

Conclusion: This should work fairly well. It's cheap, and it will give a good idea of a rockets location from a distance. For short range locating, an attenuater on the antenna may be needed. An audible locator would also be a great asset. A tone loud enough to hear from about 150' should do the trick. A FRS radio with a headphone plug would be nice, you really have to listen for subtle differences in the tone to get a good heading. A receiver with a signal meter would be nice, but I'm not going to go out and buy one just yet. But a more sensitive receiver with a signal meter would make locating a transmitter a breeze.

Channel No. (MHz)

1................... 462.5625
2................... 462.5875
3................... 462.6125
4................... 462.6375
5................... 462.6625
6................... 462.6875
7................... 462.7125
8................... 467.5625
9................... 467.5875
10.................. 467.6125
11.................. 467.6375
12.................. 467.6625
13.................. 467.6875
14.................. 467.7125

Thanks to Jeff for doing the math on the antenna design. Here is the link to Jeff's 462 MHz Yagi antenna. It's a doc file format.

Material Strength Testing:

Next on my list of things to do was to test the strength some of the fasteners I use, on the materials they are used with. Now, any real engineer would cringe at the crudeness of my tests, but they do give me some simple and valuable information.

First was #6-32x1/2" machine screws. I use these to bolts on my fin angle brackets, and many other odds and ends places in the rocket. This test was simple, drill a single hole through my 3" PVC pipe I use as a body tube, then add a nut on the other side and push on the bolt sticking out past the nut. I ran the bolt from the inside of the pipe out, with the nut also on the outside of the pipe. The 3/8" or so of bolt sticking out was pushed on with a wooden block. The whole thing was on a digital scale so I could read how much force I was applying to the bolt.

In real world use, there would be a larger surface the bolt was going into, and I would use a washer to displace the load on the other side. But I was curious if the PVC would hold, or elongate, or fracture from the force the small bolt placed on it. The bolt was totally unsupported, and I assumed the PVC would crack with a fairly low force. Ambient temperature was about 75 F.

Here is what I found:
At 70 pounds the PVC would flex to the point I would notice it. There was no permanent deformation.
At 95 pounds the PVC flexed, and there was minor permanent deformation.
125 was the maximum load I could achieve, and the PVC again held with only minor permanent deformation.

Conclusion: The PVC is stronger than I thought. I was sure I would be able to tear the small bolt head through the PVC, or it would shatter. I use three of these to hold the fin angle brackets on, so they should survive huge aerodynamic forces.

Next I wanted to test the shear strength of regular old 1/2" A/C interior grade plywood. The screws I used were (2) #8x1/2" Pan Head sheet metal screws. For this test I used a plywood disk that was cut to fit inside the 3" PVC pipe. I inserted the disk into a 5" length of PVC pipe, drilled holes through the pipe 180 degrees apart, then screwed the screws into the plywood until flush with the outside of the PVC pipe. The screws were inserted into the middle of the plywood thickness.

To test the strength. I used a 4" length of 2" EMT on the plywood disk, then placed a block of wood over the EMT to push on (stand on actually). The whole thing was then placed on a scale to read the force applied.

Results:

I slowly added pressure, at about 210 pounds I heard some cracking of the wood. At 220 the plywood failed at one screw and broke through.
Upon disassembly, the PVC showed slight indention at the top of the screw heads, but no elongation of the holes, or permanent deformation.

Conclusion: The strength of the plywood is about what I expected. I typically use (6) of these screws, only longer at 5/8", on my small rockets thrust ring. The longer the screw, the greater the shear strength of the wood. For the Cosmo rocket, I am using an epoxy glued sandwich of 1/2" and 3/4" plywood, with a total of (12) of the 5/8" screws, additionally, the thrust ring is held in by the glued and screwed PVC coupler. So my thrust ring would have at least 2x the required strength, even if I only used just the 1/2" screws.

Body Tube Separation Test:

Another aspect of the project that needed addressing was the separation of the rocket for dual deployment. A standard BP charge would be used to separate the rocket between the upper and lower body tubes. I'm not thrilled with the prospect of holding the rocket together with nylon screws, but it does seem the simplest solution, and is commonly used in HP rocketry.

My first test used (2) #6 nylon screws 180 degrees apart. The only problems was, I used an old body tube and upper stage that only had about an 8" long compartment. In my rocket I will have a compartment length of close to 20", so the pressure would be much higher in the shorter compartment. I used my small launch controller to initiate the separation charge.

5,4,3,2,1 ignition...Pop! The upper body tube flew off the lower section and flew about 20 feet in the air. It looked good, but, after closer inspection the lower body tube that had the coupler section in it was missing about a 3 square inch area. Interesting, the thickest part of the PVC was the part damaged, it was double layered at the coupler area. I suppose the double thickness of PVC did not allow as much expansion, and that caused the area to crack.

The next test used the same configuration, only with an 18.5" long compartment and I used (3) #4 nylon screws. Ignition, pop! Again, the upper body tube sailed about 20' high. This time, all three screws sheared perfectly and there was no damage or visible deformation of the PVC. I like the idea of three screws better, it should allow less flexing of the coupler area. I also have made the coupler longer. A full 5" of the coupler extends into the upper body tube. This is to help support the joint and put less stress on the nylon screws.

I'll do one more test on the ground. Then I'll do a low altitude flight test to see how the parachute deploys from the air frame.

I moved the new electronics module section to the electronics bay page. The new content is at the bottom of the page.

Notes:

Rocket weight completely loaded with propellant: 18.8 pounds.

Rocket weight no motor: 8.92 pounds

Total Motor Weight with Propellant: 9.88 pounds

Empty Motor Weight: 5.2 pounds

Propellant Weight: 4.68 pounds

Center of Gravity: 76" from tip of nose cone

Body Tube Length: 102.75"

Nose Cone Length: 6"

Overall without motor: 9' 3/4"

Center of Pressure (CP1): 89.5"

Center of Pressure (Aerolab): 84" to 86"

Calibers of Stability: 2.46:1 to 4.15:1

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