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Discussion in 'High Power Rocketry (HPR)' started by snrkl, Mar 20, 2018.
Hmm. Interesting idea. Hadn’t thought of that.
Well, I have no evidence, but that sounds like a very good hypothesis.
I think it’s the most likely...
It also tells me that the fin can is built like a brick **** house - it won’t break.. unless it does, in which case something very very bad has happened.
Besides, if anything is going to die, it will be the 3mm aircraft ply.
20:20 hindsight being what it is, I think my one regret with this bird will be that I didn’t use 3.2mm g10 for the fins.
Oh well, live and learn. My LRV suggested it, but I was nervous about adding to many new materials all at once.
Ebay switch band gluing...
Prepped with 80 grit and wiped with isopropyl.
Thank goodness for the cooler weather - I got a little more working time - it’s hard to put a 10cm x 32cm layer of epoxy on a cylinder quickly without touching the epoxy or touching something with a finger that has epoxy on it!!
The clamps worked well as a make shift jig to hold the whole thing while I worked.
Other than the motor retainer, there’s nothing left to glue.
I’m thinking of doing the retainer last after paining. Haven’t decided yet.
Now I have to figure out:
- how the hell to mount the sled onto the ebay all threads,
- how to mount and operate a missileworks rrc3 (I bought the LCD and USB attachments while I had the money) and
- how I’m going to arm this thing.
I have two rotary switches and imagined using one living flush mounted in the switch band. Then reality set in and I realised I need to be able to get the sled in and out.
I also need to mark and drill:
- Shear pins on upper recovery for nosecone / payload
- Holes for nylon push rivets to attach lower half of payload semi-permanently to the ebay
- Shear pins for eBay to fin can
- pressure sampling holes in the switch band
- pressure sampling holes in the nose cone (optional additional altimeter and an eggfinder mini to go in the nose bay I built.
- determine if pressure relief holes are needed in fin can and payload bays, and if so, where and how big.
Plan is to use 2/56 nylon shear pins. I’m tempted to look at brass bushings to encourage clean separation, otherwise I’ll just drill and thin CA the phenolic sections.
I also like the little bumps people use to ensure correct alignment of the eBay and upp/lower halves. I’m tempted to use some leftover ikea wooden dowel pegs on the intersection with the switchband, both on the eBay, and matching 1/2 holes in the upper and lower airframe to match...
You can get lengths of CF rod from places like local hobby shops and eBay. I use either that or sometimes steel dowel-pins in larger rockets.
First off, let it be known that these are challenges I intend to face in the near or not-so-near future but have not faced yet. So take the following for what it's worth.
It may not be too late to apply a layer of cloth and epoxy over the fins and blend into the airframe, if you want to take that on. Consult someone who knows what s/he is talking about on that (or don't bother).
(I haven't done it on rockets, but I do have experience designing wire harnesses for satellites, so...)
You can mount the switch to the band and attach long wires.
If the board has wire terminals attached, that's best. Just leave the ends of the wires stripped, and tinned or not tinned as the terminal block manufacturer recommends. All else being equal, tinned is preferred.
If not, put a connector on the other end of the wires. Attach the mating connector to the electronics board or to another set of wires that go there. Directly on the board is better if you can do it, as this avoids having the weight of the connectors moving around in the middle of the wire run.
Then when you want to remove the sled, there's enough slack in the wires to pull it part way and get access to the terminal block or connectors.
I've used 1/4" launch lug material epoxied to the bottom of the sled to go over 1/4" allthread. The sled butted right up against the two bulkheads. That gave a nice solid connection. For the altimeter, use some #4 machine screws (or whatever suits the altimeter) through the sled. Nylon spacers work nicely to stand the altimeter off the sled. If you want to get fancy, you can probably use the little brass standoff screws that you see in computers. You'd still want to isolate them with plastic washers.
If you're using Schurter switches, I've had a lot of trouble breaking switch leads while sliding the sled in and out. A disconnect is really helpful here. I also settled on just mounting the switches to my sled and having extra-large air sampling ports so that I could get a screwdriver in to them. That won't work for key switches though.
We look forward eagerly to your updates. It's been three days and I'm getting a bit nervous, irritable with the family, distracted at work, that sort of thing. What's new?
I’m working on the eBay layout at present. I’ve decided to try to surface mount the rotary switch and add a wiring harness with a molex snap connector.
I’ve also hit a snag with recovery. I misordered the shock cord, and only ordered 3m of 25mm Kevlar and not the 12m I need.. my LRV is out of stock at present.
I’m pondering whether I bite the bullet and just order harnesses from onebadhawk. Need to look at costs and shipping to timbucktoo first.
I’m also thinking through Drogue sizing. I’m not sure if the 18” drogue that makes OpenRocket happy is what I should go for, or the 9” that my LRV suggested would be adequate.
I’ll write up the OR sims later and let people weigh in...
You could try ship's chandlers/boat supplies place for the necessary cord. Not sure if they have Kevlar, but they do have really nice shock cord. Probably depends on the business. Try your local ones and see what they might have.
Also, fishing equipment. You won't find kevlar (I think) but you'll find spectra line. Heavy enough? I dunno, but it might be worth a look.
Spectra line is very strong but it’s even more susceptible to melting than nylon. It melts at 164°C compared to nylon’s melting point of 256°C or Kevlar which decomposes between 427-482°C.
All good - LRV doesn’t have stock on the website but has a stash of 1/4” Kevlar that I just ordered 12 meters of.
He also had an 18” PML chute in the delivery that arrived yesterday, so all is well.
As luck would have it, asking yielded some 1/4” launch lug cardboard tube that I’m going to use for sliding the eBay onto the all thread.
That's a great idea using the clamps as a jig!
Thanks.. necessity being the mother of... *something something...
Ebay Design.. It's pretty simple, but I like to plan before I cut / drill...
Here is the plan... pink are holes / cuts... Blue are zip ties... Nothing fancy, but a little mechanical security for power/switch/ematch wires...
That gives me this as a cutting / drilling template:
Me being.. well... me, means that once I have cut it all, I will likely print the design with components and glue that onto the ebay sled so that I can remember everything when I eventually totally forget what I am doing when I decide to fly this rocket again in a year's time.. (I learned my lesson the hard way with a few of my LPRs... I didn't document some stuff that when I eventually forgot it and had something weird/bad happen on a flight, I took the time to document it in my flight log...)
The overall layout looks good to me, i.e. the locations of the components, the locations of zip ties, the short length of wire between the molex and the first tie on that pair. There are some details I see that you may want to think about.
The slots for the zip tie passages are pretty close together, leaving only a small width of material between; what's the sled made of? The spacing is what it has to be, but if the sled is weak, like balsa, the ties are liable to bull through.
The one spot with two ties next to each other, the battery wires and arming switch wires, leaves a really tiny strip in between; you might be better off with a single tie holding all four wires.
Many people like to use a skinny tie or some other method of securing the battery connector to the battery cell. Consider doing one of two things:
Push the slots for the lengthwise tie closer together so that it is really snug against the battery and connector, or
Offset the tie from the centerline so that it is clear of a centered skinny tie.
All that said, it's still good. Remember that nit picking this stuff is my job, so I really can't help it.
I usually use a single cable tie and sticky velcro between the batteries and sled. The velcro takes the shear forces and the CT stops it peeling away.
Ok. Plans for today:
- Glue bulkhead sections together.
- epoxy eyelets into bulkheads and Epoxy coat outside face.
- Source ejection wells (pvc end caps seem less than ideal for 1g of powder, so I’m planning to head to the hardware store to look at copper caps and pipe.
I’ve drafted up plans for where alignment keys, vents, shear pins and hold rivets will go on the eBay. I’m going to use small sections of soft drink can aluminium epoxied to inside of PML phenolic tube coupler to ensure good shear.
- scribe up the vent/shear/rivet and key alignment holes
- drill holes (no drill press so slow and small to start with)
Feedback was appreciated in sled design, I’ll be taking some suggestions on board (Velcro isn’t practical with 9v disposables) but expect updates design with conjoined zip hold down sections.
Also need to pull trigger on final decal print order, which means I need to update source files to give scale properly and rename layers to make the vector for outside cutting easy for LRV...
Erm could you use a pvc T pipe section and pvc end caps for more volume? Or like automotive rubber vac line for bp charge?
Remember you can tack the wires to sled using CA glue if you want. That is my preferred method. You don't need to glue the whole length
Ok - lots of measuring 8 times and drilling once today..
First up: alignment keys:
Used a 6mm dowel left over from an ikea flat pack. I knew they’d come in handy one day!
Without a drill press, trying to drill half the switchband was a wash, so I just drilled the holes as close to the SWband as I could.
Inserted and epoxied the dowels and left 2mm inside the coupler:
I’ve started drilling smaller lead-in holes first and I’m liking the results. Wish I’d done that sooner with other builds.
Next was making the matching notches in the fin can and the payload bay tubes with a round hand file. I had a brainwave 1/2 way through the second notch and used a piece of 6mm all thread - much faster and perfectly sized...
Carefully cut off the excess with a hobby saw, then used the dremel sanding drum to flatten
I’ve got static ports * 3 aligned at 120° with one hole on the rail side (so my deadpool decal is not perturbed!)
I’ve got 4x 4mm plastic rivets to bolt the payload tube to the fwd of the eBay - these are aligned with the fins.
I’ve got 3x 2mm holes for fincan shear pins that are inverted from the three static ports. I opted to use some aluminium can epoxied to toughen up the ID of the PML phoenolic Tube where the shear pins will go:
Once the epoxy cures, I’ll drill the shear pin holes. I’ll likely also thin ca the coupler outside as well.
Then I will repeat with 3x shear pin holes for payload to NC...
Igniters and last bits of recovery gear are on the way from my LRV now and I’ve scrounged up 3G of 4fBP leftover from other flights so that will let me do 1.5 ground tests at home. If it isn’t successful, next ground tests will need to be done at the club field as I don’t yet have an explosives license that lets me buy BP...
I usually drill the holes for the dowel pins with the airframes mated, then the drill centers easily. I start with small drills and work up too.
I looked at that and with my limited bench space and no drill press, this seemed the next best thing. Next build I’ll get Blake to drill before he cuts the tubes I think... ;-)
I just use a cordless drill .
So do I - let me explain better: I only have a small bench and my ability to have the rocket assembled on the bench is limited. I tried, and was worried I couldn’t keep the rocket steady while drilling... so not so much the lack of a press, but the lack of space to put the bird in a vice or equivalent to prevent it moving or separating while I drilled - I wasn’t confident.
Your success in these endeavours has buoyed my confidence to try that next time...
Your pins and notches turned out nicely, so you obviously did the right thing if you weren't comfortable. For another time, how about leaving the bench and placing that spiffy stand you made in the driveway (or wherever there's room and a steady surface). And as for the rocket not separating, that's a job for masking tape. Tape can be right up to the hole for holding force exactly where you need it; just apply the tape then drill through it.
(Now me, I'd move heaven and earth to do it on a drill press, because I am THE WORST at hand holding anything. My motto is "Measure twice, cut once, curse lots, do it over.")
Last of the shear pins done. Drilled, sanded, CA’d, re-drilled, re-sanded, re-ca’d and done. You can see the little aluminium patches here:
Bought some copper tube and end caps to make charge wells. I don’t have anything for brazing, so I opted to epoxy the tube to the cap. Spent some time making the lip as smooth as I can to prevent sharp ematch wires getting cut. Backed with a tee-nut and loc-tited... I’ve put a dollop of epoxy into the base of the well - hoping this helps keep the base of the BP towards the middle.
Tested out something I saw on an epoxy table build, where the builder hit the epoxy with a blow torch to degas it a little on the surface and prevent floating bubbles. I tried it on the base of the charge wells - they are so narrow that there were epic bubbles. It worked a treat!
I’ve also put an ematch wire lead hole through the bulkhead - plan is to use blu-Tak to seal the hole, and a zip tie around the well to keep the wires in short order.
All up, I’m pretty happy with the charge wells.
PML H.A.M.R retainer went on too - JBWELD took an age to set, and I had epic runs - not sure why, but it didn’t do this the last time I used it. I’m thinking it might be the temp being low...
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