"Faster Buddy, Faster!" Build Thread

Those sound like VERY low charge weights. In 3 and 4" rockets I typically use 2 gram charges. Your "charge space" assumption is where I think the issue comes from. You need to pressurize the entire tube to clear the laundry out, not just the small bit of empty space. As the chute and other items compress, the space to pressurize gets larger, and the PSI drops.

Faster motors should do well, i missed what the actual flight motor was here.

As I mentioned - for me it was not about the amount of BP but more about how well it was contained while it burned. Containing it better produced a more effective separation for the same amount of powder. So if, for example, your charges on the day were more loosely contained that the ones you ground tested with, that would explain the failure to separate.

DizWolf said:

Those sound like VERY low charge weights. In 3 and 4" rockets I typically use 2 gram charges. Your "charge space" assumption is where I think the issue comes from. You need to pressurize the entire tube to clear the laundry out, not just the small bit of empty space. As the chute and other items compress, the space to pressurize gets larger, and the PSI drops.

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This doesn't explain the successful ground test vs failure in flight. I agree his weights sound low - I think he has an extra zero in there.

Zebedee said:

This doesn't explain the successful ground test vs failure in flight. I agree his weights sound low - I think he has an extra zero in there.

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Probably. Though I wouldn't consider a four foot separation successful. I agree packing was likely the issue. Also he's using smokeless powder, not BP. Tight packing is even more critical then.

No, the weights and charge space are correct. I have a unique setup in which the charge is contained in a small sealed space between the av bay and a bulkhead seated against a seal and centering ring. When the charge goes off it pushes the bulkhead against the seal containing the charge away from the laundry (I'm doing tests to see if I can get rid of the nomex which takes more space than the chutes), once the pins shear and separate the rocket the shock cord pulls the bulkhead and chute out of the body. In testing with a well packed chute this setup seems to work well, whether the whole system will work in practice is yet to be seen. However, the initial problem of a failed seperation shouldn't be effected by this unless the bulkhead didn't seal well though it is unlikely and I saw no sign of blowby.

An interesting design. I'd worry about getting enough of an ejection to ensure everything comes out, the centering ring for the bulkhead impeding the chute from exiting, or the bulkhead jamming in place.

Smokeless powder is fickle. It needs to be tightly packed or it doesn't generate the sharp spike in pressure. .05 grams is really, really small. I think you need larger charges.

Ravenex said:

I was unfamiliar with this mode of instability, I would bet the rather small fins and the fact that the fins are near a transition could have caused this. My question then is how will this instability mode be effected by larger motors which would reduce my stability margin. It seem that since this condition is caused by the fins needing a high angle of attack to do the job that more speed of the rail may not help, is that correct? Some of the motors I planned to fly this on will get it off the rail faster, but my ultimate plan was to fly an L265 which won't actually get of the pad any quicker despite high top speed and altitude.

As to the ejection issue, the charge space in my rocket is approximately 2" dia x 1.2" long. This gives a piston area of 3.142 in^2 and a volume of 3.77 in^3. I used three 3/32" ABS rods for the shear pins which has a shear strength of 3000 psi or 62 pounds to shear all 3 pins. This equated to just under 20 psi needed to shear the pins. Using the equation found here https://www.info-central.org/?article=303 I calculated that I would need .038 grams of black powder, from the information found here https://www.alaska.net/~aleckson/rockets/smokless.html this would suggest I only need 0.013 grams of the smokeless powder we were using. I did ground testing with .025 grams and the shear pins sheared but the rocket separated weakly. We then increased the charge to 0.35 grams and the rocket separated about 4-5ft. For the actual flight the charge was increased to nearly .05 grams. Another thought that occurs to me, is that these spaces aren't vented and the main charges can be heard to go off very close to each other and even combined they didn't do anything, not even shear the pins, not sure how that happened.

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answer to First Question. Fins are wings. Wings increase their lift with angle of attack until they stall and then they do little. The angle of attack is the vector angle cause by the cross-wind and the axial velocity. When the crosswind velocity is 25% of the axial velocity, the angle of attack is tan-1 (0.25) = 14 degrees.
Angles of attack > 14 degrees may cause the fins to stall and your rocket could loose aerodynamic stabilization. https://www.nar.org/wp-content/uploads/2014/08/launchsafe.pdf

Answer to Question 2. Your charge weight requirements for BP are too low so I think you did something wrong in the calculation of the amount of BP required for ejection charges. Look how much power is used in standard motors. Estes 18 mm motors use 0.5 grams of BP. Pro24 motors use 0.6 gram BP. Pro29 motors use 1.2 grams BP. Pro38 motors use 1.3 grams of BP. Pro54 motors use 2.0 grams of BP.

Bob

bobkrech said:

Answer to Question 2. Your charge weight requirements for BP are too low so I think you did something wrong in the calculation of the amount of BP required for ejection charges. Look how much power is used in standard motors. Estes 18 mm motors use 0.5 grams of BP. Pro24 motors use 0.6 gram BP. Pro29 motors use 1.2 grams BP. Pro38 motors use 1.3 grams of BP. Pro54 motors use 2.0 grams of BP.

Bob

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Looking at his design, he's created a small sealed chamber above the chute and under the av bay, and wants to blow the av bay/payload off and have it pull out the chute. I understand trying to keep the chute away from hot gasses, but I think it creates a few more issues, and getting a reliable pressure out of such a small charge is problematic.

Last weekend, April 18th 2015, I was able to fly this rocket with Michianna Rocketry in Three Oaks, MI and successfully completed my L1 and L2 flights.

I pre-prepped the majority of the rocket and arrived at the field with everything but the charges and motor installed. The wind was extremely calm and I wanted to get my L1 in the air first thing, unfortunately I had forgotten the washer plate for my Aero Pack 54/38 adapter. Luckily, DAllen was willing to lend me a LOC motor adapter which worked fine and I built and loaded up an Aerotech I327 Dark Matter motor. Fully loaded the rocket came in at 7 lb 8.4 oz and was expected to reach 3150 ft. The rocket flew beautifully and both chutes deployed perfectly. After a long recovery walk to the south west we I had the rocket back unscathed, strangely the on board camera had stopped recording. The stratologger recorded a peak altitude of 2870' AGL and a peak velocity of 430 fps.

[video=youtube;zM8NxeKV6yA]https://www.youtube.com/watch?v=zM8NxeKV6yA[/video]

I took my L1 test and ate lunch and then began preparing the rocket for my L2 flight. I assembled the CTI J295 Classic, which is so easy it should be cheating, and loaded the rocket for flight. By this time it was early in the afternoon and the winds were picking up. The flight was simulated to go to 8000 ft, the field had some significant patched of trees, and I was having trouble getting a lock with my Eggfinder. But I had set out with the goal of getting both certs and I wasn’t sure when I would get my next chance so I brought the rocket to the pad. The boost was beautiful with only minimal wiggle from the winds and out of sight. I did not hear the small drogue charge and after searching the skies for what seemed like forever with no sign of it we heard the main fire and one of the Michianna members caught a glimpse of it just as it touched down ½ mile away in the opposite direction from where we were looking. Everything was recovered intact for a successful L2, though the on-board camera failed to record. The Stratologger reported 9088’ AGL and a peak velocity of 950 fps.

[video=youtube;G_oMZvRAxpg]https://www.youtube.com/watch?v=G_oMZvRAxpg[/video]

Ravenex said:

I began making parts! Luckily my ex-boss gives me free access to his production level equipment. I attempted to laser the bulk of the plywood and fiberglass pieces, but the edges charred to much on the ply and the laser wouldn't cut the thicker, yellower plywood. I have decided to make all of the rest of the pieces on the CNC router this weekend.
View attachment 186679 Masterwood 4-Axis 5ftx10ft CNC wood router. 18000 rpm 10HP spindle, 14 tool changer.
View attachment 186680 Epilogue Legend 36EXT laser. 36" x 24" 120 watt
View attachment 186676 Forms for laying camera hood on upper body tube cut on the cnc router.

4) I finished the fixtures for machining my tubes on the mill at work. This will allow me to get to 6 sides of the rocket for my 3 fin design, and I can add more pin holes to allow 8 sided machining for 4 fin designs. I'm planning on starting the tubes tomorrow.

View attachment 186682 Airframe tube in fixture rings
View attachment 186683 Fixture rings.

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Ravenex said:

View attachment 186784 Cutting Tubes on the horizontal band saw, this think cuts amazingly straight and accurate. All the lengths were scribed with a height gauge on a granite table.
View attachment 186785 Milling the tubes square and to length
View attachment 186786 Drilling holes

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I'm insanely jealous of both your access to tools and your obvious facility in using said tools. Also, I have got to learn to use some CAD software. Very cool project. (I haven't read to the end of the thread yet, but I see there are launch videos.)

My father, Dan Schwartz of python rocketry, has been painting my rocket for me. The fumes give my wife a migraine and I haven't been able to get to his house to do it myself. Looks great.This weekend we will apply the vinyl stencil and paint the lettering and graphic the same green color. Then a few coats of clear and some sanding and polishing.

DAY-em! that's a nice paint job.

We applied the paint mask vinyl and painted the graphic. I think I much prefer paint making instead of permanent decal. A few layers of clear and it'll be ready to prep for the next flight. I also want to say how much I love the duplicolor engine enamel paint one coat and the graphic looks great and we were able to peel the mask after less than an hour and it was dry.