Derecho - The Rebuild

[Clusterphiliac]

Hey all,

Last year I had a build thread for my Level 1 certification project, the Derecho. Scratchbuilt, 3" diameter, about 54" tall...

Well, to make a long story short, I done goofed. Today at Chili Blaster 7, I screwed something up, and it came in ballistic from an estimated 2500 feet.

The result was my first ever "shovel recovery." The rocket was almost completely destroyed. Photos are available here. In summary:

• The nose cone embedded itself into the ground for its full 12" length, requiring a shovel to remove. The cardboard coupler tube I was using as a shoulder was pushed about 3" forward into the nose, along with the entire avionics bay and possibly part of the upper airframe. Interestingly, the polypropylene was bent and folded outwards, and had almost no cracking. I suspect that heating from the Pueblo sun got the plastic hot enough to soften it.
• The cardboard upper airframe section, predictably, disintegrated. A small piece remained attached to the nose; the rest stayed with the Blue Tube coupler. The tube was attached to the coupler by three plastic removable rivets. These did not shear, but the holes were so deformed that the airframe slid backwards on the coupler, smushing its end against the lower airframe.
• The lower airframe also came apart where it wasn't held together by the coupler or centering rings. It crumpled right down to the forward ring, making repair by cutting away damaged tube impossible.
• The fincan itself was mostly intact, apart from one fine, which was severely cracked. The other two fins were completely undamaged, so this was likely due to the rocket bending and allowing the fincan to slam into the ground at high speed, landing on the fin that broke.
• The avionics bay built into the nose cone did not react well to being rammed into said nose cone. Basically every single piece broke, and it was so well embedded that I had to cut the nose cone apart using a bandsaw to look inside. Basically, the tracker bay got smashed into the end of the nose, and was crumpled at the other end, the weld nuts popped free of their mounting ring, pushing the allthread rods backward, the altimeter holder tubes crumpled, the bulkheads were shattered, the charge wells popped off, etc.
• When I saw the state of the nose cone, I though the Raven 3 altimeter would be in tiny pieces. As it turned out, it was mostly together. The terminal block on the end bent, the capacitor was bent in the direction of impact, and the micro USB data port was forcibly removed (possibly due to being hit by the capacitor). A trace on that side of the board was also peeled up; it looks like it used to be attached to the data port.

The immediate cause of the accident was obvious from initial inspection of the wreckage: neither the apogee nor the main ejection charge (in a Cable Cutter) had fired. One severed charge well still had the dog barf and unburned e-match in it, although the black powder had spilled out on impact. Both e-matches were later successfully fired using a drill battery.

Upon reviewing the Raven manual, I believe the cause of the failure is user error: I forgot that the Raven uses low beeps to indicate insufficient voltage or no charges detected, and did not correctly interpret the beeping. I remember it beeping out a battery voltage of 4 volts on testing the day of the launch, and I think it beeped out 4 volts on the pad, which indicates that the secondary cause was incorrect wiring of the charges. The custom-made avionics bay had not been flight-tested before. During construction, I tested the resistance between the Raven terminals and the avbay terminals and found it to be low, but I may have missed a wire or it may have come loose.

I will be rebuilding this rocket, and further posts in this thread will discuss design changes.

At the moment, my main question is whether there is any chance of recovering the data from the Raven to confirm the cause of the failure. I do not yet know whether it will power on, but as mentioned the data port was torn off. I don't trust my soldering skills on something that small, but if I sent it to Adrian is there a possibility that he could repair it enough to retrieve the data?

 

[dixontj93060]

Unfortunate mishap. Rebuild almost seems like a new build; well I guess you do have the fin can. I assume you had done electronic deployment with a cable cutter a few times before, did you use the Raven as your altimeter then?

Not sure of the status on Raven repairs right now as Adrian has been scarce here since taking a start-up job a few years back, others may have had recent dealings with him.

 

[Clusterphiliac]

Basically this will be a new build under the same name, with mostly the same design but a few updates.

This was only my second DD flight. On the first flight I was using a Featherweight 29mm Avbay kit, which worked, but the magnetic switch was a nuisance to get aligned. The accident happened with a custom-made 29mm E-bay made out of 1/16" plywood. As mentioned, this setup was not flight-tested, and I suspect that it's where something went wrong. Next time, I'll be testing that the avbay/altimeter setup works properly with live e-matches before the rocket gets within 50 feet of a launch pad.

Anyway, more on the avionics later. I have a couple questions about structural upgrades.

First of all, I want to reassure everyone that I'm not the kind of idiot who tries to overbuild to the point where a rocket can lawndart and come out without a scratch, and ends up making a bunker buster. However, I would like to be able to confidently throw any available motor at the rocket without worrying about it shredding, and I'd like it to be more resistant to hard landings and "hangar rash." While I was away from my college over the summer, somebody who wasn't courteous enough to leave a note somehow bumped the rocket with some sharp-cornered object, putting a good 1/4" divot in the leading edge of a fin. This is visible in the picture of the fincan: the forwardmost damage was actually there before launch; the same fin just happened to be the one to hit the ground hardest in the crash. I've also found that drilling holes through cardboard tubes for venting, shear pins, and electronics access can be awkward and the holes tend to get smeared.

So, although the airframe overall proved pretty strong, I've decided to upgrade to Blue Tube for the rebuild. The fincan also seems to be very sturdy, but as mentioned the plywood fins were easily damaged by handling of the rocket.

So, I have a couple questions.

First, should the 1/4" thick plywood fins be sufficiently strong and stiff to withstand flights in the Mach 1-1.2 range on high-thrust J motors without adding fiberglass? Second, if so what is a good way of improving the surface hardness of the fins to reduce accidental damage? Sanding sealer? Soaking in epoxy? Papering the fins?

If the fins are not strong enough for large J motors, I'm thinking of going up to 1/8" G10. However, I've heard that cutting G10 is very hard on tools, and I don't want to be responsible for a trail of ruined sawblades and such in my college workshop.

I currently have access to a laser cutter (makes short work of plywood, but almost certainly not G10), CNC mills (I could easily buy a carbide endmill for the job, but I've heard that abrasive glass dust can get in the machine's cooling system), a bandsaw (which I could buy a bimetal blade for, but carbide blades are a bit pricey), and a cordless dremel (I can buy cutting wheels, but I'd prefer not to do the whole job with it because charging the battery every ten minutes is a PITA).

What would you recommend as an inexpensive way of getting fairly precise fins? I will also need to make 1/16" G10 fins for another project, so even if I go with plywood for Derecho I'd like information on cutting G10.

Finally, while designing the entire rocket to survive a ballistic recovery is silly, is it reasonable to attempt to make the nose cone and avionics bay stronger so that the expensive electronics can survive such an event, or would the g-forces be so high that they would be destroyed anyway?

The plastic nose cone actually didn't fare that badly; the tip was the same shape despite being driven into relatively hard soil. What really seemed to do the damage was the avionics bay being forced forward into the nose cone, as well as possibly the top of the airframe tube. Ideally, what I'd like to have happen is for the forward airframe to absorb most of an impact's energy, acting as a crumple zone for the booster. For this to work, I'd also like the nose cone and the coupler used as a shoulder to be stronger than the airframe, so the tube would disintegrate around it. Is this at all reasonable, and would switching to a fiberglass shoulder with the plastic nose cone help?

 

[Clusterphiliac]

Update: I decided the best approach would be to just use a Blue Tube coupler for the nose, and use G10 fins on bandsaw.

But, a quick detour away from that, and to Project Build a Better Avbay:

When I initially designed Derecho, I knew I wanted to be able to quickly swap out altimeters between high-power rockets, because there's a strong possibility of me someday showing up at a launch with five different HPR birds. The initial solution was to build a modular avbay that I could put the 29mm avbay kits from Featherweight Altimeters into. Basically this thread, but with a nosecone-mounted avbay and cable cutter: https://www.rocketryforum.com/showthread.php?24581-Modular-Raven-AvBay

Long story short, I used the 29mm kit for my cert, and found that the coupler was a bit awkward to get aligned, the nuts were a bit irritating to keep properly tightened, and I had no way of keeping the magnetic switch aligned with the airframe, so powering the altimeters up had a significant "wave a magnet over it and pray" component. Also, the 29mm kit didn't have a built-in charger. The 38mm did, but I couldn't fit two of them into a 3" airframe. So I decided to make my own tiny 29mm modular avbay (with Blackjack and Hookers). Basically, I forgot about the space taken up by fasteners and designing lasercutting slots oversized, and still didn't have a good way of keeping it aligned, but now the screw switch I used was even more sensitive to alignment. These were eventually fixed with a lot of futzing around with a dremel and a file. The other problem was that I ****ed it up and crashed the rocket. Note to self: test wiring with actual e-matches before a launch, and bring a copy of the Raven manual to the pad so I don't forget what the beeps mean.

So, on to the third version. Well... the drop-in ones were kind of a pain to keep aligned, but I don't want to totally use a normal sled design since I still want to be able to swap out altimeters easily. So what I came up with is what I call a "Quick-Change Sled." If the previous design was a ghetto version of the Featherweight Avbay kit, this is a ghetto PowerPerch. Everything is attached to a 1/16" plywood sled that also has a bulkhead with a terminal block on it so the only wires I have to attach are the e-matches. This all screws onto a bigger, sturdier sled that's actually a permanent part of the rocket. It's pictured here in high-quality "I literally just threw this together this evening" Sketchup to shuffle around component positions before doing a serious design in SolidWorks. It's extremely compact, allowing me to squeeze redundant altimeters in a 54mm airframe.

Unfortunately, it has a couple of shortcomings. First, the switch has to be mounted in a kind of weird location to put it close to the outside of the airframe (which seems to make it much easier to see where I'm putting the screwdriver to turn it on), and one of the mounting screws would have to be kinda tucked under the wires going from the altimeter terminal block to the main terminal block. Also, actually fitting two of them into a 54mm rocket leaves almost no room on the bulkhead to actually attack the shock cord, and nobody friggin uses two altimeters in a 54mm rocket, nor will they until some madman makes a 54mm M motor.

On the other hand, the drop-in 29mm looks like it can be made more convenient with a few tweaks, such as gluing a piece of string onto the bulkhead to pull it out of the rocket, accounting for fasteners and standoffs, and actually lasercutting a little piece of wood to keep it aligned instead of kludghing one together with a dremel. So, at this point I'm leaning towards keeping the drop-in design (shown as an actual Solidworks render, minus fasteners and a couple of other tiny bits of wood that will help hold the battery in place). Also note that multiple sets of mounting holes will be made to make this version compatible with the EggTimer Quark as well as the Raven.