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

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Well, if it makes you feel better your far cheaper than my L3 by over $600
 
But yours is at least $1200 prettier than anything I could ever hope to build.

Very kind, thank you. I had big help from many in the rocket hobby industry that's for sure.

Really enjoying your build and the behind the scene work your doing. Any thoughts on where / when / what your going to launch it?
 
Very kind, thank you. I had big help from many in the rocket hobby industry that's for sure.

Really enjoying your build and the behind the scene work your doing. Any thoughts on where / when / what your going to launch it?

Thanks! I love the science and experimentation part of it. It appeals to my scientist background.

I'm targeting MWP in November for the cert launch; that's the closest event that would likely have a TAP present to witness it. I'm also eyeing Walnut Grove in the summer as a possibility. Or maybe Mini MWP in May....

I might do a shakedown launch on a CTI L2375 in the summer. Not sure about that since I don't have long-term plans for a 4 grain 75 mm CTI case, so I'm not too motivated to purchase that.
 
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Thanks! I love the science and experimentation part of it. It appeals to my scientist background.

I'm targeting MWP in November for the cert launch; that's the closest event that would likely have a TAP present to witness it. I'm also eyeing Walnut Grove in the summer as a possibility. Or maybe Mini MWP in May....

I might do a shakedown launch on a CTI L2375 in the summer. Not sure about that since I don't have long-term plans for a 4 grain 75 mm CTI case, so I'm not too motivated to purchase that.

You've certainly started some great technical threads. v/

Well darn. When L3 time comes, I'm in almost the opposite situation. I think there are 3 TAPs in my local club, but the nearest two L3CC members are 3.5-4 hours away (one in another state)

Do you have a longer case that can adapt down with a spacer? Hmmm.....does CTI do spacers that big?
 
You've certainly started some great technical threads. v/

Well darn. When L3 time comes, I'm in almost the opposite situation. I think there are 3 TAPs in my local club, but the nearest two L3CC members are 3.5-4 hours away (one in another state)

Do you have a longer case that can adapt down with a spacer? Hmmm.....does CTI do spacers that big?

The Aeropack 98>75 adapter is just a set of rings. You can put them on any commercial 75 mm case to fit inside a 98 mm motor mount tube.

https://www.apogeerockets.com/Build...e_Motor_Adapters/Aeropack_75_98_Motor_Adapter
 
Oop, my mistake. I assumed you had a longer 75mm case (adapting 6grain to 4 or similar). If that's not the case, then you're probably right. buying a 75mm case just to test out would be pretty expensive.
 
Both Aerotech and CTI make spacers for 75mm motors to fly shorter reloads in the same case. You can go up to 2 grains shorter. If you ask around at your club someone might be willing to loan you a case.

I never did a test flight for mine. It took two tries, so I kind of wish I had. I also never kept track of the cost on mine, but the rocket itself was relatively cheap. I got free cardboard tubes and glassed them myself. The nosecone I turned on a lathe (which I no longer have access to) out of wood I didn't have to pay for. The altimeters were from my other rockets, and I borrowed a case for both flights. I have my own 75mm hardware now...
 
Mark,

I have a AT 75/5120 case that I've used the L2375 motor in with no problems. You're welcome to borrow it at Mini MWP if you want. (standard borrow rules apply :wink: ). Just let me know.
 
Mark,

I have a AT 75/5120 case that I've used the L2375 motor in with no problems. You're welcome to borrow it at Mini MWP if you want. (standard borrow rules apply :wink: ). Just let me know.

Thanks for the offer. I need to see where I am on the build and if I'm free for mini MWP when it gets closer.
 
The weekend = more progress. I have the two largest sections of my airframe complete, so I wanted to work on the coupler and payload bay since they will be made from the same tube. My airframe has an inner diameter of about 7.8" and no smaller Quiktubes match that perfectly. I had to make my own coupler.

I've had success in the past using the same method seen in the Apogee Rockets tutorial.

[video=youtube;iH3HesegWv0]https://www.youtube.com/watch?v=iH3HesegWv0[/video]

To cut my Quiktube to be used for the coupler, I took another 8" OD tube and drew a straight line using angle iron. Using my trusty Dremel with the diamond cutter, it was quick work to slice along that line and open up the tube. After the coupler was inserted and marked where I need to remove material, another pass with the dremel gave me the right size.

Time for the epoxy fiberglass. After cutting the tube to the right diameter, I hold it in place with some duct tape on the outside and a long strip of adhesive fiberglass tape on the inside. The inside is laminated with 4 strips of 6 oz fiberglass cut wide enough to cover two wraps.

This was left to cure overnight.

20170129_175209.jpg

After curing, I peeled off the outside duct tape and filled the seam with bondo and sanded it smooth after a one hour cure time. The final product was tested for fit in the airframe, and I have a nice, tight fit all around.

20170129_174634.jpg

I'll use this tube to cut off a section for the payload bay and also another section for my coupler.

I also got around to sanding smooth the inside ends of the airframe tubes. Reinforcing them with laminating epoxy left the peeled inside ends a bit jagged and swollen. I sanded these until the fin can would fit inside again. I also tested the thrust plate for fit. It nice and snug all around with no gaps.

20170128_190457.jpg

Next up is to fiberglass the final airframe section and then I start on the fins.

Coming together slowly, but nicely.
 
Didn't do much last night. Cleaned up the edges of the coupler. I tested the full 48" length for fit inside my 48" airframe tubes. Perfect fit with no gaps at all. If you look closely, you can see the seam on the bottom right where I sealed the coupler with fiberglass tape (faintly more opaque strip).

20170130_211244.jpg

I'll cut this to make the coupler and the payload bay.
 
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Here's a question I sent to my TAPs on fin strength vs impact force. I'll toss it out here to the masses for educational purposes. I enjoy technical discussions; especially when they lead to design theory and design decisions.

My current weight measurements have the rocket weight at 15.0 kg after the nosecone is separated by pilot chute. I'm looking at a 12' main from Rocketman to bring it down at 15 fps (4.6 m/s).

If I look at the force on impact:

E = 1/2 m v2
E = 0.5 * 15 * (4.6^2)
E = 158.7 N = 36 pounds force

Note that this doesn't take into account the gradual slowdown when impacting soft dirt. Let's assume I hit a rock. :p

My concern would be if the rocket lands on one corner of a fin. Can it take a 36 pound thump?

My Nike Smoke-like fins would have an approximate contact surface area of 1/8" x 1/4" (3.175 mm x 6.35 mm = 20.2 mm2). Note that is a really crude approximation if I assume the fin corner takes the impact.

I came across an excellent resource on plywood strengths.

https://www.forestindustries.fi/mediabank/887.pdf

It has detailed calculations and reference tables on plywood. There is also a simpler rule of thumb on load limits:

If there are high loads over a small contact area, compression perpendicular to face of plywood could be critical. In most practical cases the following values can be used.

Birch plywood 9 N/mm2
Combi plywood 5 N/mm2
Spruce plywood 4 N/mm2

Using this general guide, my load limit approximation would be:

20.2 mm2 * 9 N/mm2 = 181 N (or 41 lbs)

Yikes! That's way too close to the 36 lb landing force when hitting hard dirt or a rock (*cough* or a car).

So the numbers theory tells me that I'd need to reinforce the fins to make sure they can survive a landing. That gives me another parameter to consider in fin design:


  1. Fin size to give proper stability
  2. Fin weight
  3. Fin flutter velocity limitation
  4. Fin strength on landing impact

And I have calculations and theory behind each one, so that makes me happy.


 
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You may want to review your calculation. You used the equation for energy, which has units of Newton meters or Joules, not Newtons.

A better calculation which would give you Newtons is to calculate F = mass * acceleration, where you substitute for acceleration by solving for the change in velocity over the distance you expect the fin to move the rock when it strikes it, which is just going to be a guess. I would expect may a centimeter or about 0.01 meter.



Steve Shannon
 
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You may want to review your calculation. You used the equation for energy, which has units of Newton meters, not Newtons.

A better calculation which would give you Newtons is to calculate F = mass * acceleration, where you substitute for acceleration by solving for the change in velocity over the distance you expect the fin to move the rock when it strikes it, which is just going to be a guess. I would expect may a centimeter or about 0.01 meter.



Steve Shannon

Yeah. I took the slow down distance to be 1 to oversimplify the equation which isn't right.

F s = E
F = E / s

The point of impact would be up to 360 lbs then.
 
I'm also not convinced about the rule of thumb. I think a better solution would be to apply Euler's Buckling theory to a flat plane. Should be much higher strength. But the impact still worries me until I suss out the numbers.
 
Yeah. I took the slow down distance to be 1 to oversimplify the equation which isn't right.

F s = E
F = E / s

The point of impact would be up to 360 lbs then.

No, E= 158.7 Joules, s = 0.01 meter, therefore F = 158.7/0.01 = 15,870 N, or 3,566 lbf. for the force of impact.

In order to have a force of 360 lb. you would need to move the rock 10 centimeters or 4 inches.

Moral: don't hit a rock.

Moral 2: always do dimensional analysis.

Don't let this demoralize you.


Steve Shannon
 
No, E= 158.7 Joules, s = 0.01 meter, therefore F = 158.7/0.01 = 15,870 N, or 3,566 lbf. for the force of impact.

In order to have a force of 360 lb. you would need to move the rock 10 centimeters or 4 inches.

Moral: don't hit a rock.

Moral 2: always do dimensional analysis.

Don't let this demoralize you.


Steve Shannon

Thanks! I got confused with Joules = Newtonmeters. I'm a molecular biologist, numbers are hard. :p

Assuming I avoid rocks and put a 4 inch dent into the dirt, I'm still looking at approximately

158 J / 0.1016 m = 1555 N (~350 lbs).

I think a 1/8" tip of plywood won't take that impact unreinforced.
 
Also, my calculations are incorrect in that they assume the rocket is perfectly rigid. The fin and the rocket would flex, bringing the impact force down.

Steve Shannon

That's where I try to use Axial compression calculations and Euler's buckling calculations for my approximations. I've only fleshed out a calculator for columnar bodies when determining how tough a rocket tube is using my online tool:

https://drive.google.com/open?id=1cGEeEXgq9ScOMsaF5nFxY3m08tIn-kY2F8sNV7NQd8k

I'm sure it's not perfect, but it at least gives me numbers for sanity checks assuming I have straight flights. I need to wrap my brain around the effect of varying angles of attack for it.

Thanks for the corrections. I like learning stuff.
 
Change the fins so the butt end of the motor hits first.
Nothing should be coming down sideways.

Thanks! I have it set up that way. The fin root is 13 cm from the end, and they're Nike Smoke shaped fins, so the tips are even further up from the end. And you're right, it's super likely that the aluminum butt thrust plate will hit. I'm just paranoid. I always over worry and over think stuff.
 
Thanks! I got confused with Joules = Newtonmeters. I'm a molecular biologist, numbers are hard. :p

Assuming I avoid rocks and put a 4 inch dent into the dirt, I'm still looking at approximately

158 J / 0.1016 m = 1555 N (~350 lbs).

I think a 1/8" tip of plywood won't take that impact unreinforced.

Fred is right. I think that your rocket will hit on the retainer first. When you hang it from the shock cord what does it look like?

I fly on a rocky field. It's amazing what one plate sized rock can do to a fin, but it's just as amazing how dirt parts to allow penetration. If your rocket does hit fin first, it won't travel four inches before something else hits and the force distributes itself.
Still, 1/8 inch is very thin plywood for that size rocket. I used 3/16 inch G10 for my L3.


Steve Shannon
 
Fred is right. I think that your rocket will hit on the retainer first. When you hang it from the shock cord what does it look like?

I fly on a rocky field. It's amazing what one plate sized rock can do to a fin, but it's just as amazing how dirt parts to allow penetration. If your rocket does hit fin first, it won't travel four inches before something else hits and the force distributes itself.
Still, 1/8 inch is very thin plywood for that size rocket. I used 3/16 inch G10 for my L3.


Steve Shannon

It hangs straight down off the shock cord and kevlar bridle, so I'm probably ok. I'm probably overthinking out of an unnecessary abundance of caution. I've had tons of MPR to HPR launches and never broken a fin.

The 1/8" is for the very tip of the fin. It's tapered from 1/8" up to 1/2" in a Nike Smoke fin profile.
 
It hangs straight down off the shock cord and kevlar bridle, so I'm probably ok. I'm probably overthinking out of an unnecessary abundance of caution. I've had tons of MPR to HPR launches and never broken a fin.

The 1/8" is for the very tip of the fin. It's tapered from 1/8" up to 1/2" in a Nike Smoke fin profile.

I think you'll be just fine.
Better to over analyze than screw the pooch.


Steve Shannon
 
The weekend brings more progress. Gotta finish fin beveling, but here's the first dry fit of the frame, fincan, coupler, and one of the fins for fit. It's all a tight fit and nice and straight. Waiting on the 4:1 nose cone from Python Rocketry, but I'm looking at about 12' for it all.

20170204_155822.jpg
 
Were there any tricks to slotting the tube? I'm really enjoying this thread.
 
Were there any tricks to slotting the tube? I'm really enjoying this thread.

Thanks! I enjoy updating build threads and having lively debates on my ideas. I try to put in a lot of build theory and details so others can hoefully learn something. I know I rely heavily on the build threads to educate myself.

I'll show my slotting technique in the next post. Working on putting down my thoughts now.
 
I got a lot done this weekend. It all revolved around finishing up the booster section and getting ready to epoxy it all together this week.

I got all the fins cut and sanded to have identical dimensions. I used sheets of 12 mm Finnish Birch plywood marked off with 8" roots, 6" tips, and 7" semispans with fin tabs to reach the motor mount tube.

Ack...measure twice, cut once. No idea how my marks got off track, but luckily I double checked.

20170203_220837.jpg

After cutting out the fins, I clamp them together and give a quick sanding pass to make sure they are all identical dimensions.

20170204_133526.jpg

This Finnish Birch plywood is 9 layers of alternating ROCK HARD wood. It takes forever to sand. It's quite a bit more dense than Baltic Birch and way more stiff. I couldn't bend the wood in any direction when testing a small scrap.

Unfortunately, my scraps didn't allow me to align the outer grain perpendicular to the tube, but it doesn't matter since it's alternating layers and 1/2". If using thinner plywood for fins, it's really best to align the outer grain perpendicular to maximize the number of layers in the correct orientation.

Also, I had one scrap of Baltic Birch to compare weight. A quick cut to make a 5th fin and comparison weight:

Finnish Birch fin: 550 g
Baltic Birch fin: 443 g

Wow. 24% more dense. That aligns with what I read that it is 20-30% stronger. Worth it.

Next up was cutting the fin slots. I have a pretty repeatable technique that give me straight slots that doesn't need fancy jigs or routers; which I lack. First up, is what I call "squaring the tube". I have a board on the floor that is perfectly leveled off. The floor has tape to show me where to put the board when I need it. Otherwise, it's up and out of the way.

I put a tube on the board and use a simple string + washer taped to the top of the tube to check the tube's straightness. If the end of the tube isn't even, the string won't hang parallel to the tube. I check it in 4 spot. I got really lucky with these Quiktubes in that they were all absolutely clean, level cuts on the tube ends. Now that I know I can stand the tube up level, I pick one point on the tube and draw a light pencil line with angle iron to get a straight line. On these large, long tubes, you can get the angle iron a degree or two off parallel. So I put the tube back on the board and check the line against the string.

After making sure the pencil line was good, you can see here (hard to see the strings) that the marker line I put on is perfectly aligned with the plumb. I know I have a good reference line that is perfectly aligned. A good, level tube makes life so much easier.

20170206_064701.jpg

Starting at that reference line, I wrap a length of butcher paper around the tube. I put hash marks across the paper to ensure I can put it back together straight. I use a razor to cut the paper off down where it is on top of the reference line. Total paper length is measured and divided by the number of fin slots I need.

My paper was 640 mm long, so I need to draw lines at 160 mm intervals (640/4).

20170131_214711.jpg

I tape the paper back on with the ends at the reference line again. Back to the angle iron to draw the center lines here the fin slots are. To make sure my center lines are all also perfectly in line with the tube, slide the paper sleeve up to the top of the tube along the starting reference line. Each fin slot line should still align with the paper. That's how I know all 4 slot center lines are parallel to the tube.

Measure fin width and thickness, and draw lines where the slots should be cut. The bottom border line starts where the second fin can ring ends. This was measured along with the height of the aluminum thrust plate height that goes into the tube.

20170203_205427.jpg

I use my trusty Dremel with a diamond cutting wheel. It goes through fiberglass like a hot knife through butter. The wheel likes to run in a straight line, so I don't need any special guides. I start with a light first cut to give an engraved line. The light cut lets me make corrections before I cut deep.

20170203_210024.jpg

Then it's a simple matter of going around and cutting along the lines.

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After cutting out the fin slots, I slid in the fin can and thrust plate along with the fin blanks to check for fit. The fit was perfectly straight and tight.

20170203_220512.jpg

Then on to beveling. To start, I make quick cuts with a box planer to get somewhat close to what I need. Then it's on to using my palm sander. This rock hard plywood takes forever to sand down, and I'm using 40 grit extreme removal sandpaper. I've had good luck with my technique of pressing the fins on a spacer and then holding the sander level. The glue lines between plies make a good reference line. If I keep the line spacing even, I know I'm sanding at the same angle along the length of the fin edge. The slightest deviations make the line bend significantly. The end results came out nice, but took forever.

You can see here that the spacing between glue lines is pretty even along the length and also similar width compared to the opposite fin edge. So I know it's a consistent sanding all around.

20170205_154825.jpg

I also got around to putting on a reinforcement block of wood to screw my 1515 rail button into.

20170203_204156.jpg

Next up is the critical part of epoxying in the fin can and the fins. First, I'll need to cut a fin jig from some cheap 1/8" plywood. Hopefully, I'll get to it the week since I'm busy this weekend.
 
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