L3 rocket update

I managed to get some work done on my L3 rocket today.

The centering rings' ODs were about 1/32" too large. I used a dremel and a sanding block to sand them down. I also drilled and mounted the U-bolts in the forward centering ring.

Marked the hole locations first. The X markings are to remind me to not line up the
U-bolts with fins because the front fin tabs will butt up against the back of this ring.

CA used on all nuts for anti-vibration.

I completed most of the nosecone modifications today. As mentioned in the last post, the goal is to be able to adjust the mass in the nosecone easily. I cut the bottom off of the nose cone and sanded it. I'll be using a ~5" diameter, 1/8" thick plywood bulkhead to brace the 1/4-20 threaded rod that will be glued into the tip. I drilled a 1/4" hole in it for the threaded rod and sanded a chamfer in the outer edge so that it would sit in the cone better. The threaded rod was cut to length so that an inch or so would stick above the bulkhead for attaching an eye nut for shock cord attachment. I also drilled some small holes in the bulkhead and superglued in some thread so that I could pull it out. This bulkhead will not be glued in place; instead, it will be held in place using nuts and washers. This allows me to add and subtract mass (in the form of 1.25lb lifting weights) from the nose tip be sliding weights on or off the threaded rod. The weights will be held in place using large washers and more nuts.

I then marked and drilled holes for 3/16" wood dowels to go through the nose tip. These will help hold the threaded rod and the epoxy plug in the nose.

I had to dremel these because I didn't get them straight the first time.

Markings for the second set of holes. I realized the lower
(original) markings were too close to the other holes.

Both sets of holes drilled. I also sanded the inside of the nose
 tip to help with epoxy bonding.

Supergluing washers to nuts. Probably should of bought flange nuts.

After many test fits, the final locations for the nuts were set with CA.

5 minute epoxy + silica was used to glue the dowels in.
These will be cut and sanded flush later.

Inside view before potting with epoxy

Washers help hold the threaded rod in.
Camera didn't resolve threads for some reason.

For potting the nose tip, I used 3x pumps of West Systems 105 resin and 206 (slow) hardener mixed with about a tablespoon of colloidal silica. I carefully poured it all into the tip, avoiding the threaded rod. I then shook/tap/hit the nose to get the epoxy into all of the crevices. After that, I poured a few ounces of steel BBs into the tip and repeated the settling exercises. After that, I put the bulkhead in and placed the nosecone upside down in a piece of body tube. It takes a few hours to cure, so I'll check it tomorrow. After this, I need to cut the dowels, sand and fill the tip, prime, and paint it with high temperature BBQ paint.

Left to do:

• Finish nose cone
• Purchase a bunch of stuff
• Make motor mount
• Install fins
• Filleting 
• Build altimeter bay
• Minor stuff
• Final primer coat, sanding, then painting
• L3 paperwork

L3 rocket update

I haven't done much fabrication. I installed the AeroPack 98mm retainer on the rear centering ring. This was kind of a pain in the ass. I tried the method in the instructions on friend's centering ring last year. It suggests holding the retainer aligned with a motor casing, marking the holes, then drilling. The problem is that getting the marks for directly in the middle of the holes is pretty much impossible. This time, I CAD'd the AeroPack retainer and the centering ring, assembled them in CAD, made a drawing of the assembly with hole center marks, printed that out, cut out the center, and taped it to the centering ring. I then punched the center of the holes and drilled them. It came out much better this time, so I did my aft centering ring next.

The holes match up very well.

Screwing in the thread inserts.

Tapped over retainer to prevent epoxy from gluing the retainer to the centering ring.

My friend has a fiberglass aft centering ring (the one I messed up the first time around). The instructions suggest tapping the holes, but I don't think the fiberglass will hold threads well. We'll be using steel nut plates glued to the back of the centering ring to provide threads.

I also cut the motor tube to length and sanded it. Because phenolic tends to fray after awhile, I treated the ends with CA. I did the same to the two tube couplers (I did this to the body tubes earlier).

I did some modelling Modelling and Design updates. I took out the 12" section of body tube and one of the couplers from the stretched version of the rocket (which is the one I'll be using for my L3). The 12" section will be used instead of the 36" section for the scale (non-stretched) version.

Updated CAD

Because my Dad wanted to buy a CTI 5G 75mm case, I've chosen either a CTI M1400 or M1401 as the cert motor, and updated the OpenRocket simulations accordingly.

Stretched

1/2 Scale

It predicts about 7500ft on the M1400.

Avionics bay design: The avionics bay end caps will have an airframe bulkhead and a coupler bulkhead glued together. Each end cap will have an eyebolt for recovery system attachment (see below). Two 1/4" threaded rods will run through the avionics bay and end caps to link the two end caps together and provide rails to mount the electronics sled. Two altimeters will be mounted on the electronics sled. The end caps will have high temperature screw terminals and charge holders glued to them. The vent hole(s) will be drilled through the upper body tube section and into the avionics bay. The av bay will be secured to the upper body tube section with 4-40 stainless steel screws. Overall, a pretty classic design.

Nose cone design: While running OpenRocket, I realized that, if I wanted to be able to use the same nose cone for both the stretched and non-stretched versions, I would need to make the nose weight adjustable. I'll show pictures of this later when I actually make it, but the plan right now is to epoxy a 1/4-20 threaded rod into the tip of the nose cone with a eye-nut on the end of it to attach the shock cord. I'll mix steel BB's and chopped carbon fibers in with the epoxy to increase weight and strength respectively. I'm also planning on drilling through the tip of the nose in a few places and running wooden dowels through in order to help anchor the tip/threaded rod in place. It'll make more sense once I post pictures of the process. Weight can be added to the 1/4" threaded rod as required.

The recovery system will be as follows: All connections are done with 1/4" quicklinks unless otherwise stated. A Kevlar Y-harness connects to two 1/4" U-bolts in the forward centering ring. The Y-harness connects to a ~20ft long Kevlar or tubular nylon shock cord. The drogue parachute (~48") is attached to a loop in the shock cord about 1/4 of the way from the nose/payload section. The other end of the shock cord attaches to a 1/4" forged, galvanized steel eyebolt in the avionics bay. There is an identical eyebolt on the other side of the avionics bay. This connects to a shock cord similar to the previously mentioned one. The main parachute (Sky Angle Cert-3 XL) attaches to the shock cord about 1/4 of the way from the nose cone. The nose cone will have 1/4" forged, galvanized steel eyebolt in it that attaches to the other end of the shock cord. Nomex cloth protectors will be used to shield the parachutes and shock cords from the ejection charge gases.

Hardware acquisition: I got two PerfectFlite altimeters for Christmas, so that covers the electronics. I managed to find some of the hardware I needed from Home Depot (U-bolts, threaded rod, etc.). The rest I'll get from McMaster.


Time outlook looks abysmal...I kinda have a thesis I need to do. This rocket may not get finished for another year :( .

Left to do:

• Purchase a bunch of stuff
• Sand centering rings
• Make motor mount
• Install fins
• Filleting 
• Build altimeter bay
• Minor stuff
• Final primer coat, sanding, then painting
• L3 paperwork

Fins

I got around to sanding the fins. I started by marking a line for the ~10 degree bevel on both sides and a line down the center of the edges I was going to bevel. The real Phoenix has a bigger bevel on the leading edges, but I decided against that for convenience. I attempted to use a palm sander first, but realized that it would take way too long that way. I then tried to use a hand plane. That mostly worked, but it took a long time and the plywood tended to rip instead of slice off nicely. I finally decided to try the vertical belt sander. I didn't want to use one originally because one slip up and the fin is ruined, but it ended up working out. I finished them with a sanding block. I ended up sanding the LE of the tab of the rear fins because there wasn't a good way to avoid that on the belt sander; I didn't do the finish sanding on the tabs of course.

They didn't come out perfect, but good enough. Total time sanding was probably about 10 hours. I can't wait until I have a shop with a CNC router. That would have made this so much easier.

Since the last post, I also purchased centering rings, bulkhead plates, giant rail buttons, and a nose cone from Wildman. I hope I can get this done by mid-January, but it may not happen. Next semester is going to be insanely busy.


Left to do:

• Cut motor mount tube
• Make motor mount
• Install fins
• Filleting 
• Build altimeter bay
• Minor stuff
• Final primer coat, sanding, then painting

L3 Rocket

Purpose: document the build and flight of my Tripoli Level 3 certification rocket

I've always liked the Phoenix AIM-54A missile. I had an Estes kit of it when I was little that I put a 24mm motor mount in it. I flew it probably close to 50 times before it died. I've seen a few larger scale Phoenix missiles, but I've never seen a 1/2 scale kit, so I decided to custom build one.

I got the specs from here. Using stock 7.5" ID, 7.75" OD phenolic tubing results in a 0.517 scale, which is close enough. Given that, the body tube length should be about 63", and the nose cone should be slightly less than 3 to 1. Though it's not ogive in real life, 3:1 7.5" ogive nosecones are commercially available, so I decided to use one and make the body tube slightly less than 63" to compensate for the slightly too long nose cone. I decided on a 3ft long, 98mm motor mount and 1/4" thick 12ply wood fins. I'm building it super tough, so it should be able to go supersonic (large M or small N motors).

The rocket was designed in OpenRocket and Solidworks.

Solidworks model of stretch 1/2 scale Phoenix

OpenRocket model of Phoenix

OpenRocket model of Stretched Phoenix

OpenRocket is predicting ~7000ft for the stretched (dual deploy) Phoenix on a medium M. It predicts ~2000ft on a medium K for the non-stretched Phoenix. Both configurations will use about 4lbs of nose weight. This is due to the stubby and spread-out nature of the fin area, causing the CP to be further forward than usual for a rocket this size.

My dad gave me two 48" long, 7.5" phenolic tubes, two 12" couplers, and 48" of 98mm phenolic motor mount tube. He also has (had) tons of fiberglass, so he let me use it. This wasn't my first time making composite rocket tubes, but I didn't have all the supplies I was used to (MIT Rocket Team, DBF had vacuum bagging supplies we used to make tubes with...I was spoiled) at dad's house, so I had to do a basic wet wrap. I measured and cut two wraps of ~10oz 0-90 E-glass and two wraps of ~4oz 0-90 S-glass for each tube. I used medium cure time West Systems epoxy and wet out the FG as I was laying/stretching it around the tubes.

Setup for layup. Katie's L3 kit is in the background

Lots of fiberglass

Finished layup

Heated cure

Each tube went from 3lb pre-glass to ~5lb 12oz post-glass. Final OD was ~7.75". The surface finish was terrible (no peel ply or release, no vac bag, etc. ), so I had to do a ton of sanding and filling (two coats of automotive spray primer and filler). You can see the result in some of the pics below. Total time spent on the tubes was probably 20-30 hours. 

If I had left the couplers stock, I ran the risk of coupler failure (breaking in half), which is common in these large rockets during high accelerations. So I fiberglassed the insides with 2-3 layers of 10oz glass. Balloons were used to hold the glass against the insides, then popped after curing.

 

I wanted to do 1/8" G10 fins, but I do not have access to a CNC router or large enough laser cutter here. Custom fins would have been ~$400, which is crazy, so I bought a $50 sheet of 12ply plywood from aircraftspruce. I penciled the lines using a cardboard stencil and checked them all with a ruler. 

I then cut them out with an oscillating saw. I tried to sand them with a palm sander and sanding block, but quickly realized it really needs to be done with a belt sander, particularly the beveling. I'll finish them later. I'm going to build a jig for the FIT shops's belt sander and do the beveling there.

I slotted the tube next using a Dremel and reinforced cut-off wheel. I made two slits lengthwise per slot, then broke the material in the slot out using needle nose pliers. Fiberglass is brittle, and the thin/small amount on the ends of each slot made breaking it out easy. I then finished the ends of the slots with a 1/4" sanding drum attachment.

All tubes done

All tube ends were sanded smooth, treated with thin CA, then sanded smooth again to prevent phenolic wear.

Left to do:

• Buy plywood centering rings, bulk head, nose cone, altimeters
• Cut motor mount tube
• Finish sanding fins
• Make motor mount
• Install fins
• Filleting 
• Build altimeter bay
• Minor stuff
• Final primer coat, sanding, then painting

The fins will have internal and external fillets. The rear two centering rings will not be installed until the set of fins they contact are completely done so that the volume they enclose can be filled with expanding foam. The shock-cord mount will be a pair of U-bolts in the forward centering ring. Altimeter bay will be classic rods + sled construction. I haven't picked the altimeters yet. The goal is to be finished by December.

Next time I build a big rocket, I will probably order pre-glassed tubes unless I can figure out a way to get a much nicer surface finish. I'll get them pre-slotted, too, unless I build a nice router tube slotting machine/jig. I want a CNC router and laser cutter in my home shop eventually, so that will make fins significantly easier.