Darkstar Series Build

[mtnmanak]

The new booster tube Tim sent me was in great shape, but still needed some slot cleanup. In the photos below, you can see the slots were about 1/4" short. Used a tube marking guide to draw a line around the tube at the front and rear of the two sets of slots and cut out the necessary material with a dremel and file. Pretty quick work, but necessary prep before starting the build.

 

[mtnmanak]

Okay, before we get to some of the more interesting stuff (like the ebay and nosecone bay), let's get the normal build out of the way.

One difference in this build compared to the previous DS builds I did is that I am using one of Teddy's bridles from Onebadhawk. Teddy makes way better kevlar products than I could ever do on my own, especially for a bigger rocket. For this build, Teddy recommended he sew the bridle straight onto some 5/16" welded eye-bolts. Never one to argue with Teddy's wisdom on recovery items, I agreed. So, unlike the previous builds where I carved a slot in the forward CR and epoxied the strap right to the motor tube, this needs just a slight bit more planning.

I measured the CR locations, then found the center line on the forward CR to drill the holes for the eye-bolts. I drilled the holes on my drill press and, when I fitted the bolts, I realized the washers that Teddy sent were just a bit too wide for the CR. He recommended I grind them down, but, luckily, I had some nice smaller ones from McMaster-Carr that fit the CR perfectly and were a bit thicker. So, I JB welded the underside hardware and tacked the forward CR to the MMT with CA. I am not so worried about the top of the hardware right now because I will pour an epoxy dam in front of the CR later.

One note - I also used some Loctite on the bolts. Overkill, I know, but the main point I wanted to make is that you really should wash your hardware with acetone before you bond it (with either JB Weld or Loctite). The hardware comes with oil on it to prevent corrosion and that oil has a tendency to make the bonding agent less likely to stick.

Once the CA was cured, I added some 5 min epoxy on top of the CR to hold it in place.

 

[Banzai88]

Those 5/16" eyebolts are perfect for applications like you're doing. I've done it a few times and always been amazed at just how slick it comes out!

 

[mtnmanak]

Couple house keeping chores before we get the MMT tacked in and start gluing up fins:

First, need to get the hole in place for the rear rail button. I have been shifting towards using Rotaloc Adhesive Mount Nuts from McMaster_Carr for rail button mounting. For a bigger rocket like this that will often have to go to the away cell, I am using the 1/4"-20 adhesive nuts with a large flange:

https://www.mcmaster.com/98007A029/
You can get the nuts in a bunch of different thread and flange sizes. The 1/4"-20 mount allows me to easily switch between Unistrut and 1515 buttons (I do not plan to need 1010 buttons on this rocket). For smaller rockets, I use one of the 8-32 sizes to switch between 1010 and 1515. One mount, multiple buttons.

One issue, though, is that these adhesive nut flanges are large and flat and the tubes are curved. I gently bend the flange out in a vise until it fits nicely along the tube. I am not gluing these in right now, but it makes it easier to drill the 1/2" hole and fit the nut before you install the MMT.

For the fins, I am not making actual glue wells like I do in my 8" and larger rockets, but I am planning to inject a fair amount of epoxy at the fin root, so I drilled some 1/8" holes along the bottom to allow the epoxy to have a more mechanical grip.

With those tasks done, I tacked the MMT into the booster tube. At this point, the only glue point is the front CR. None of the other CRs are in place yet. The rear CR is just there as a placeholder to keep the MMT centered while the epoxy on the front CR cures. Some holes and zip ties make it easy to remove the rear CR.

 

[mtnmanak]

With the MMT tacked in, I tacked in the forward fins using 5 min epoxy on the leading edge of the fin tab and the fin tab root. Main intent here is to ensure the fin is securely tacked in and the edges are sealed so I can inject a robust pool of epoxy later. I do have some fin jigs I CNCed for another 6" diameter project, but once I get to 5" diameter and larger rockets, I find it easier and quicker to just ensure the tube is level and plumb, then clamp it down and then you can be reasonably sure the fin is straight if it is line up in the slot properly and plumb. So far, this method has worked well in a number of rockets and I have not had an issue with misaligned fins. For smaller rockets, I usually use my guillotine jig.

 

[mtnmanak]

Once the forward fins were securely tacked in, I epoxied in the CR that goes right behind the fins and then injected a shallow pool of epoxy behind the CR. 60ml of West 105/205. Once that pool cured, on the forward end of the fins, I drilled and inserted 3 x 1 inch long #8 screws just above the forward CR and poured a 100ml epoxy dam in front of the CR. The screws ensure the epoxy has more mechanical grip on the body tube. The epoxy dam also covers and locks in the eye-bolt hardware.

In the past, I have struggled with using long flexible tubing to inject epoxy that far down a tube. The tube wants to curl up and get epoxy all over the place. I realized the solution was to simply tape the tubing to a long piece of dowel. Tape the tube to the dowel and then tape the tube to the syringe. Then I just pour the epoxy in the syringe, plunge and refill until all the epoxy is used up.

 

[g.pitts]

I really appreciate your detail-orientation. It's often the "little things" that bite us, and you cover those well in your build technique.

 

[mtnmanak]

I drilled 1/8" holes next to each fin, levelled the rocket and injected 30ml of epoxy to the root of each fin.

 

[tim cubbedge]

Great thread!

 

[mtnmanak]

I epoxied in the forward CR for the rear fins and then adhered the Rotaloc nut to the frame before installing the rear fins. For this, I first tack the nut in with a bit of 5 min epoxy and attach the rail button to keep the nut tight and centered while the epoxy cures. Then, I take the button off and slather on a healthy bit of JB weld. I go ahead and cover the whole thing, threads and all. I get some of the JB weld up in the threads and also fill in the gaps the nut leaves on the outside of the body tube. This may seem counterproductive to fill in the thread portion, but, as much as I like these Rotaloc nuts, I have found their weak point to be the weld connection of the nut to the baseplate. It can and has snapped off. JB weld is easy to drill out and tap, so I just fill it all in and then drill it out and tap it later. The 1/4"-20 screw then engages not just the nut, but the JB Weld as well.

 

[mtnmanak]

Got the rear fins tacked and internal fillets injected.

 

[gerbs4me]

Great build so far.

I love how big the motor mount is

 

[mtnmanak]

Once all the internal fillets were cured, I mixed up a batch of epoxy thick with colloidal silica and generously filled in the spaces internally the injections did hit near the aft end and proceeded to smear a bunch of epoxy on the tubes and end of the fins to adhere the rear CR in place. I cut some notches on the forward side of the thrust plate to give the JB Weld a little more to hold onto, then mixed up a healthy batch of JB Weld. Smeared it all over the aft end of the rocket, installed the thrust plate, and held it in place by inserting an Aerotech 98mm case. I used the rest of the JB weld to make a small fillet along the outer edge of the thrust plate.

 

[rfjustin]

I think we need to discuss the elephant in the room....

I sheepishly have to ask, are the rear fins on backward? My OCD is trippin'

 

[mtnmanak]

I think we need to discuss the elephant in the room....

I sheepishly have to ask, are the rear fins on backward? My OCD is trippin'

Wow. You know what? They absolutely are on backwards. The wrong edge is tapered. That is how they came from Wildman and I didn't even notice. I was kind of rushing this build a bit because I was hoping to fly this next weekend and didn't even notice the wrong edge is tapered.

I am pretty confident it will fly fine like this, but I guess we'll find out!!

 

[rfjustin]

Wow. You know what? They absolutely are on backwards. The wrong edge is tapered. That is how they came from Wildman and I didn't even notice. I was kind of rushing this build a bit because I was hoping to fly this next weekend and didn't even notice the wrong edge is tapered.

I am pretty confident it will fly fine like this, but I guess we'll find out!!

It'll be fine! Best of luck on the first flight. I have a maiden voyage planned for next weekend as well.

 

[mtnmanak]

It'll be fine! Best of luck on the first flight. I have a maiden voyage planned for next weekend as well.

Thanks, good luck on your flight as well - praying for good weather for all of us!

 

[mtnmanak]

Finishing up the major work on the booster - drilled and tapped the rotaloc nuts for the rail buttons. Can now securely switch between Unistrut and 1515 airfoil.
Also, got all the external fillets in place.

 

[mtnmanak]

OKay, going to need to speed this build up if I hope to fly it this weekend.

Got some housekeeping done tonight - tapped in 2-56 shear pins for the booster and 4-40 pins for the payload bay. Didn't get a picture, but also epoxied the switch bands to the couplers and drilled and installed 6-32 PEM nuts to hold the ebay to the booster and the nosecone ebay to the nosecone. Also Dremelled in all the necessary witness marks. And drilled the switch holes on the Ebay and the vents holes - all done on my drill press.

 

[thzero]

That tap guide is a good idea.... need to whip one up, but too bad don't have any clear filament

 

[mtnmanak]

That tap guide is a good idea.... need to whip one up, but too bad don't have any clear filament

I got the idea for these particular tap guides from a video Adam Savage did:



I did the same thing he did - I just cut blocks of acrylic out on my table saw and then drilled the tap holes out. Probably took about 20-30 min to knock out 4 or 5 tap guides. I kept one flat, then sanded the bottom on some of the others to match common tube sizes. For that, I just wrapped some 60 grit sand paper around an FG tube and put a little elbow grease into sanding the contour. I probably could have CNCed something more elegant, but sometimes the simple solution is the easiest and quickest.

The acrylic itself is super cheap. A 1" thick piece of cast acrylic from TAP Plastics is about $10 for a 2" x 6" piece. That should net you at least 5 tap guides.

https://www.tapplastics.com/product/plastics/cut_to_size_plastic/acrylic_sheets_cast_clear/510

 

[gerbs4me]

tapped fiberglass...interesting. How well do they threads hold up?

 

[mtnmanak]

tapped fiberglass...interesting. How well do they threads hold up?

I have tapped holes into maybe... 20 or 30 FG rockets? Maybe more? So far, none of the holes have failed.

To be fair, we are mostly taking about nylon shear pins here. Not a lot of stress on the threads.

Generally, if I am going to need to put a metal screw into a rocket, I use a PEM nut.

 

[tsmith1315]

I use a PEM nut

Just tightened into place or do you use adhesive?

 

[mtnmanak]

Just tightened into place or do you use adhesive?

Adhesive. I use PEM nuts adhered to the frame for a lot of functions. Things like: Connecting the Ebay to the payload bay, connecting the nosecone to an AV Bay (if you want to be able to the remove the AV Bay), camera housings, etc. I have even used PEMs to hold tube fins to a body tube.

Basic procedure I use is to drill the hole ever so slightly smaller than the PEM, use a fine file to make the hole perfect (this also has the benefit of cleaning up the fiberglass "fuzzies" when you drill a hole), epoxy the PEM in place and then attach what ever you are trying to connect while it cures so the PEM cures perfectly in place.

@rfjustin used a lot of PEMs on his current build - check out his thread and the photos: https://www.rocketryforum.com/threa...x-3-which-one-needs-built-first.167169/page-3

Checkout posts 85 and 91 - what do you have in there, Justin, about 13 PEMs ?

I love tapping things and using PEMs - just feels better as a "maker" when you can securely use a screw!

 

[tsmith1315]

I love tapping things and using PEMs - just feels better as a "maker" when you can securely use a screw!

I agree 100%, and like to be able to remove pieces for service or changes. Thanks for taking a minute to respond, I know you're busy getting things done before the weekend!

 

[mtnmanak]

Now that the booster is pretty much complete, I moved on to something (arguably) more interesting - the Ebays. I spent a butt-load of time this weekend ("butt-load" is a thoroughly scientific measure of time equal to 264,747,794,976,000 oscillations of a caesium-133 atom) cutting, sanding, filing, soldering and gluing to get the Ebay in shape the way I wanted it.

Basic config is an Eggtimer Proton for the primary computer and a Missile Works RRC2L for backup. I decided to go with separate batteries for the logic and deployment on the Proton. I love the eggtimers, but those WiFi models suck down a lot of juice. I have had a Proton drain a 2S 7.4V 400mAh LiPo in less than two hours. Considering I have had times when a rocket could easily sit on the pad for 90 minutes (happens a lot at big events like Red Glare), I wanted to make sure I could vary the logic battery as necessary and keep the deployment channel "pure". So, I will use a LiPo for the Proton logic and an Energizer Lithium 9V for the deployment charge. The RRC2L will get an Energizer Lithium 9V.

Started out by CNCing the bulkheads out of aluminum and tapping the holes for the terminal blocks and the charge wells. Then I CNCed all the sled parts - the sled itself, the end brackets and a switch bracket out of 3/4" basswood.

I cut out some aluminum angle and drilled holes to attach the pieces to the sled brackets. I used 4-40 bolts and nylon lock nuts to attach the aluminum brackets. I lined the brackets up on the sled and secured the sled to the brackets by drilling holes through the aluminum and sled and securing with 4-40 bolts and lock nuts.

I reinforced the wood bracket by "painting" epoxy on the section that would be drilled for switch holes. Once cured, I measured the holes for the switches and drilled them out on my drill press. I am using screw switches from Binder Design. They have a little notch in the switch housing, so I needed to elongate one end of the hole with a Dremel, then the switches fit perfectly. I am only using 2 switches right now, but I drilled a third placeholder in case I need to add a switch later or if I want to add an indicator light, beeper, etc. I will epoxy the switches in later, once everything is tested.

 

[mtnmanak]

I know there are folks who don't like terminal blocks, but I am trying something new (for me) from McMaster-Carr this time around. These are some pretty industrial looking terminal blocks. They have very secure screw-down terminals, made of some heavy duty plastic, you can get some cool marker strips to label the terminal and you can get covers to help protect them from the black powder charges. Also, you can get them as small as 2-circuits and as large as 20-circuits if you need to do some crazy large amount of air-start/cluster/staging from one terminal block. I am looking forward to trying them out. Here is the link for them - the marker strips and covers are in each line after the link for the terminal block itself:

https://www.mcmaster.com/catalog/terminal-blocks-11/
The other nice thing about McMaster-Carr parts is they give you the schematics on the site which make it easy to layout the part in CAD for CNC. Even better, if you use Fusion 360, you can download the part directly from McMaster-Carr using the Fusion 360 interface and place the part on your CAD model. Using that method, I was able to have the holes for the terminal block perfectly positioned when I CNCed the bulkhead and then I tapped the holes for 8-32 screws and easily secured the terminal blocks to the bulkheads. While I was at it, I had also tapped the holes for the screws that came with the Binder Designs charge wells, so I screwed those in as well. I secured all the bolts on the other side of the bulkhead with nylon lock nuts.

 

[Banzai88]

Interesting approach. I've recently gone the opposite direction, back to direct wiring the ematches to the flight computers and passing wires though a hole in the av bay and plugging it with a well nut.

 

[mtnmanak]

Interesting approach. I've recently gone the opposite direction, back to direct wiring the ematches to the flight computers and passing wires though a hole in the av bay and plugging it with a well nut.

I built a Nike Smoke like that just recently and it works great. On these bigger builds, it just makes it easier for me to use terminal blocks. Especially for rockets I plan to fly multiple times in a day. If all I have to do is change out the e-matches and refill the charge wells to get back to flight ready, it saves a lot of time.

However, I very much acknowledge the risk with terminal blocks. I have had those cheaper white plastic ones completely get blown apart!! These definitely look sturdier, though. For all my terminal blocks, one thing that has made them much better to use is a healthy covering of green frog masking tape over the whole block. Since I started doing that, even the white ones no longer break on me. On the subjective "big" vs "small" rocket question, though, these would be very large on a 4" diameter bulkhead. Without the marker plate, they measure about 1.5"L x 0.75" wide. Two of them would take up a lot of real estate on a 4" plate with all the other stuff that needs to get on there. I think they could work, but once you get smaller than 4", I think they would be impractical.