L3 Winter Build Thread - 3/4 Scale PAC-3 Patriot

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Yep. I'm at that good feeling phase. Once I'm done building, it will be replaced with the anxiety phase. Will it buckle on thrust? Will the retainers hold? Is it stable enough? Deployment? Will the hardware and shock cords hold? Ahhhh!

Yup. Enjoy the honeymoon! :grin:
 
Yep. I'm at that good feeling phase. Once I'm done building, it will be replaced with the anxiety phase. Will it buckle on thrust? Will the retainers hold? Is it stable enough? Deployment? Will the hardware and shock cords hold? Ahhhh!

When in doubt: double everything! (You don't really need it to go Thaaat high do you?):eek:
 
When in doubt: double everything! (You don't really need it to go Thaaat high do you?):eek:

I don't prefer going very high anyway. It's more fun for me to see the whole flight. I usually target 2500 - 6500 ft. for most HPR flights. Plus, I don't have to chase the rocket down to far.

This one is simming to fly about 6500 ft. at the moment at mach 0.75.
 
Thanks a perfect altitude for that size rocket. You will be able to see everything, any higher is harder to see imho. My L3 was about the same size and at 7K I could barely see it separate at apogee and see shiny spots as it fluttered down under drogue. Any further and would've just seen a puff of smoke but not the components.

The rocket looks great - man that's a lot of work to that point, great job so far. Those dry fits really help spur you on!
 
Easter weekend means I didn't get much done, but I did manage to set up my fin guide. Since I see quite a few posts from people asking about how to make fin guides, I'll detail how I generally make them.

I haven't yet had a need to build one from plywood or fiberglass plates. Go to Home Depot or any other major hardware store and pick up a 2'x2'x1" high density foam insulating panel called Foamular. https://www.homedepot.com/p/Project...x-2-ft-R-5-Insulation-Sheathing-PP1/203553730 This stuff is pretty rigid and holds up well as a fin guide. Plus, it's easy to cut using a heavy duty snap blade utility knife. It's only $5-6.

I start by putting my tube on top of the foam board and I use a pencil to trace the outline of the tube. Also, I mark the spots on the foam where my fin slot lines are on my tube. That marks where I want to cut out the fin guides. From there, I use a rules to connect the fin markings and draw a straight line. That gives me good fin alignment.

I measure out the right distance from the circle for fin height and mark that off. I go over the outline and fin template a second time with pencil to cut in deeper and darker to clearly show where I need to cut out.

The results...

20170415_150612.jpg

From there, it's a simple matter of cutting with the utility knife. I use a heavy duty snap blade knife so I can have a long enough blade to get through the 1" foam.

20170415_152203.jpg

After cutting out, a quick check on my airframe shows perfect alignment.

20170415_152655.jpg

This trace and cut method works best when you have 4 fins since you can draw a straight line from fin to fin on the template. If I have 3 fins, it's a simple matter of doing the same tracing. However, to draw the fin lines, first find the center point of the circle and draw from there to each fin marking.

If I have a smaller rocket, and the whole profile fits on letter paper, I'll use the templates from https://www.payloadbay.com/index.php?page=Tools&action=FINGUIDES About the largest model that fits there is a 4" tube with 4" fins. Make sure the template aligns with your fin slots before cutting the guide.

Hopefully, tomorrow I'll be able to start the fun part of putting in the fins.
 
Fins are in!

I had some time last night, so I was able to epoxy in my fins. Here's how I usually set up for fin insertion.

20170420_190835.jpg

I use JB Weld for the root attachment to the motor mount tube since it doesn't run as much as my other epoxy, Rocketpoxy, and it is a good higher-temp epoxy.

Everything is measured and numbered. Before insertion, each fin is numbered and measured for insertion into a correspondingly numbered fin slot. I use a caliper to check depth to the motor mount tube. All four fin slots were 50.5 mm +/- 0.25 mm from outer frame to the surface of the motor mount tube. I couldn't ask for a more centered tube there. All the fins were then checked for depth from the tip of the fin to 7" down (the planned fin span) and then 50.5 mm to the root to contact the motor mount. A couple fins needed ~1 mm sanded down for a perfect fin.

Since my fins are 0.5" inches thick, I had enough room to reach in with a craft stick to directly apply the JB Weld to the motor mount tube. Normally, I have to use the "double butter" technique where I put epoxy on the fin root, insert to put the JB Weld on to the tube, remove, and repeat; doubling up the buttering of epoxy. Here, you can see the JB Weld generously applied directly to the tube. Also (not seen) since the centering rings are exactly spaced at the leading and trailing edges of the fin slot, I put JB Weld on those as well since they will be in contact with the fins.

20170420_193700.jpg

After buttering up the motor mount tube, I applied a generous layer of JB Weld to the fin roots.

20170420_193716.jpg

The fins were inserted and temporarily held in place with duct tape until all four were in and ready for the fin guide to hold them in place overnight.

20170420_194218.jpg

Since I use foam board for my fin guide, I cut the template span so that it's a tight fit and has to compress the foam very slightly to fit. This gives a tight fit that holds the fins pressed firmly against the motor mount tube. And since the template slots are cut to the exact width needed, the template can't wiggle or rotate at all; keeping the fins all perpendicular to the body without any eyeball aligning or guess work.

20170420_194231.jpg

This morning, after a 12 hour cure, I pulled off the fin guide and checked the fins. They're all straight and tight with no wiggle.

Now I have to decide. Do I put Rocketpoxy for the internal filets? Or to I put in foam? With a 12" fin root, I'm concerned about getting a good internal filet without having to add a boatload of epoxy (and weight).

Anyone ever had good results putting in Rocketpoxy on such a large internal filet and getting it to run down the length without adding a ton?
 
I think you answered your own question. Foam weighs a lot more than epoxy, so if weight is your concern, go with fillets. I like to leave my aft CR or thrust plate off and use a dowel for internals. Never been a fan of drilling holes and injecting -- no visibility, no control over the run, lots of luck required. But if you have to, you have to.
 
I think you answered your own question. Foam weighs a lot more than epoxy, so if weight is your concern, go with fillets. I like to leave my aft CR or thrust plate off and use a dowel for internals. Never been a fan of drilling holes and injecting -- no visibility, no control over the run, lots of luck required. But if you have to, you have to.

Normally, I also like to either cut the tube and pull out the fin can or leave off the aft rings. However, with this model, the fins being higher up means I'd have to cut too much off the tube to pull it out. Also, with the reinforcing stringers, I couldn't leave off the aft rings. So I'm stuck with drilling and injection. The guesswork is minimized since I have a borescope camera where I can look inside to check the work. At $8, it's a great purchase.
 
Also, per recommendation from one of my TAPs, I'll paint the leading edges of the fins with laminating epoxy to make sure there is no issue with plywood delamination during flight. I'll tackle that tonight.
 
Math time! How much foam would I need to fill each fin can quadrant? I'm good with approximation, so I'm rounding things to keep it simple.

I need the volume of the fin can that can be filled. Since there's a motor mount in it, it's a hollow cylinder.

Hollow cylinder volume = pi * h * (R2outer - r2inner)
Hollow cylinder volume = pi * 12" * (4"2 - 1.93"2)
Hollow cylinder volume = 463 in3

That's the whole fin can. Divide that by 4 for the quadrants. I'm ignoring the volume taken up by the 1/2" fins in there to keep it simple and directionally correct.

Each quadrant = 116 in3

US composites urethane foam (per their website) is 2 lb per ft3 or 2 lbs per 1728 in3.

Each quadrant = 2 lbs foam * (116 in3 / 1728 in3)
Each quadrant = 0.134 lbs foam -or-
Each quadrant = 61 g foam

The whole fin can will then have ~240 g foam (less when taking out the space occupied by the fins). Just over 1/2 a pound.

I'm assuming it will take me ~20 g epoxy to put in filets. Each fin has two filets, so I'll use 160 g epoxy, or just over 1/3 a pound.

Epoxy will save 80 g at the cost of the ease of foaming.

Decisions, decisions... Is it worth an extra ~0.2 pounds to have the more sure contact of fin-to-tube when foaming?
 
You did some great math to come up with your foam weight, but "assumed" the weight of the fillets? Also, I generally go with four fillets per fin.

intfillets.png
 
Oh, BTW, if you foam, make sure you put your external fillets on first and let them harden. Otherwise the foam compresses out through the fin slots and is a pain to clean up.
 
Oh, BTW, if you foam, make sure you put your external fillets on first and let them harden. Otherwise the foam compresses out through the fin slots and is a pain to clean up.

Been there. Made that mistake when learning foaming with MPR. AND, I didn't check it after pouring, so the foam hardened. :facepalm:

I've never done internal filets on the backside of the airframe. Since that point is the center of a level action, it would represent the weakest point to have a filet. The filet at the end near the motor mount would be the best place to hold the whole level in place. To me, it adds weight without much strength. However, my experience is limited to up to L2 models at max speeds of mach 0.8.
 
I posted a poll awhile back to see if folks do the airframe internal fillets, or just the MMT side. It was split, with people giving good reasons for both. While I agree that the MMT fillets are critical, it just seems to me that the combo of topside and inside fillets at the fin slot has to add strength. The more epoxy, the more the force on the fins is absorbed. But I can see why you might go without them, especially if you are injecting.
 
I would inject. If you go that route, however, I would use something thinner than rocketpoxy, something like leftover laminating epoxy, with a little filler mixed in, like milled carbon or glass fibers. Too long of fibers mixed in or to thick of epoxy and you could have issues injecting... I know that from experience.
 
I finished all the fins and reinforcement this weekend. I decided to go with foaming the fin can since I wanted to make sure it was all tight and solid. I used US Composites expanding urethane foam with an expansion ratio of 25-30:1. I went with my initial calculations on how much was needed and added a few mL of foam to make sure the space would fully fill. By my calculations, I needed 42 mL of each part. Luckily, I had a pack of 60 cc and 100 syringes handy, so I used those for measurement and injection.

20170428_213058.jpg

Before injecting, all the fins were taped off to seal gaps and prevent foam from leaking out. The injection hole was also surrounded by tape to make cleanup easier.

20170428_212051.jpg

I squirted 42 mL of each part into the 100 cc syringe and mixed vigorously with bamboo skewers for 30 seconds until thoroughly mixed. Each injection was let to sit and cure for an hour before rotating the rocket and moving on to the next foam injection. The foam expanded all the way into the space and spilled out of the hole an amount roughly the size of a golf ball. Cleanup was done with paper towels and acetone.

The fin can was left to sit overnight and harden. After hardening, I re-installed the fin jig to make sure no fins shifted under the force of the foam. Everything was still square.

Next up were the surface attachment of the forward stabilizing strakes. Before epoxy attachement, I roughed up the body surface with 80 grit sandpaper around the line where the fins will be epoxied on.

20170430_152932.jpg

To keep the strakes in line with the fins, I clamped angle iron to the fins and the strake. The strakes were epoxied on using RocketPoxy. I used just enough to form a small filet of excess epoxy after the strakes were pressed on. These filets were smoothed out with a gloved finger wetted with isopropanol.

20170430_155548.jpg

Each strake was left to cure 2 hours before moving on to the next one. Once cured, I will go back over and add proper filets with an ~1 cm radius.

After foaming and adding strakes, another dry fit of the lower half of the rocket.

20170501_065842.jpg

What's left:

The fin and strake filets
Drilling for all the screws and shear pins
Drilling for the pressure and sample vents
Payload sled adapter
Nosecone weights
Attaching cords/laundry
Filling/sanding/priming
Painting

Pretty sure I'll be done well before MWP in the fall. :p
 
I spent the last few weeks secretly restoring an old, rotted out, busted up bench that my wife's deceased mom used to sit on with her deceased favorite Aunt. She thought it was thrown out due to the fact it was falling apart.

I learned a lot about rotten wood restoration along the way. :p

Plus, my wife cried when she saw it for Mother's Day, so uh...mission accomplished?

Ok. Back to rocket building stuff!

This week will be drilling out all the holes for pins, screws, and ports.
 
Ok. I have a question about the proper port sizing for the payload bay and the barometric sensors within.

For my altimeter ports, I planned on following the sizing charts from Vern Knowles' site. His charts don't quite match my size. However, he was kind enough to include the equations. From his site, the equation is:


DN = 0.02216*DT*sqrt(L/N)


DN is the diameter of the static port holes
DT is the inside diameter of the payload bay
L is the payload bay length
N is the number of holes


For symmetry, I'm planning on 4 static port holes. This gives me the following:


DN = 0.02216*(7.75" inner diameter)*sqrt(22" length/4 ports)


DN = 0.4" static ports

So, I'll drill 4 x 0.4" static ports into my switch band and payload bay?

In talking with one of my TAPs, he feels 4 x 0.4", even for a 22” long payload bay is a lot. The concern is loss of airframe integrity. The question is how scalable the above formula really is. His personal feelings would be to put in 3 x 0.25” holes, but said it would be a good idea to pick people's brains.

For those of you who've built larger birds, what ports did you drop in for an 8x22 payload bay? Is the formula really scalable?

If it is scalable, and I just go with 3 holes, the formula calls for 3 x 0.47" holes. :-O
 
Great to hear the "Winter Build" is coming along nicely!

There's a thread titled "New Build, Have Questions" in the HPR section that had a good vent hole discussion regarding the VernK formulation.

I think it errs on the side of "slightly bigger holes are better than holes that are slightly too small", but you're right, almost 1/2" holes do sound pretty big.
 
I'm getting .5" drill bit for 3 holes and a 15/32 bit for 4 holes...it does seem like a lot of volume to account for. I would look into where dhbarr is going and see if you can sub-divide the AV bay and lose some internal volume.
 
8" rubber ball is an easy way to take ~5k cc's off the market, plus self-sealing and reversible.

Bonus points for using one on each end as vibration dampers ^_^
 
If the worry really is structural integrity, reinforce between the holes. Though, I doubt you are going to compromise an 8 inch airframe with a few half inch holes... At least until you start approaching Mach, larger holes than necessary won't hurt you aerodynamically much either, or mess up your altimeters readings. But if you are buying bits anyway, drop to one close to the size the equation calls for, and don't oversize. As others have pointed out, the equation is a bit conservative already.
 
Last edited:
I went with four 1/2" holes in my Formula 200 switch band. Plus, two more for the switches, but those are filled (with switches). Anyway, six 1/2" holes in the switch band has not caused me any problems.
 
I went with four 1/2" holes in my Formula 200 switch band. Plus, two more for the switches, but those are filled (with switches). Anyway, six 1/2" holes in the switch band has not caused me any problems.

Ahhhh, I probably went with three 1/2" holes. I just ran my calcs, and for four holes I got .41; for three holes I got .47. I honestly can't remember if I did three or four. Probably three with two more for the switches.
 
Four half-inch holes sounds like overkill IMHO. Just going by feel I would think four at 1/4" more reasonable, but I don't have calcs to back that up. Is there any fluid dynamicists out there that can chime in?

What is your expected altitude and accelerations? That affects how quick the air needs to exit too.
 
For my PR 8" Madd Max, my AV bay is 16" long x 7.75" id. I used three 3/8" holes. If you have a switch band, that will add strength back to your airframe where you poke the holes through... So, for what it's worth... I wouldn't have any problem with three 5/16" or even 3/8" holes. But 1/2" sounds huge...
 
Thanks for the thoughts. I'm going to go with 4 x 3/8" holes that go through the switch band into the payload bay. There's a lot of reinforcement there, so it shouldn't cause structural issues. Plus, I'll soak the holes with laminating epoxy after drilling them out for extra strength.
 
Back
Top