Build Thread: LOC/Precision 3" Iris 38mm. "The Comeback Kit". First build in 20 years!

[Orien]

Got a delivery this morning. I could just stare at it all day. My attraction to this rocket's rear end is borderline unhealthy...

 

[new2hpr]

I was thinking maybe you were going to keep the IRIS proportions and cut off the back of the airframe by the length of the tailcone. Then keep the end of the rocket (tailcone and fin trailing edges) all flush. Might be worth thinking about. Either way, that tailcone is dead sexy.
-Ken

 

[Orien]

I was thinking maybe you were going to keep the IRIS proportions and cut off the back of the airframe by the length of the tailcone. Then keep the end of the rocket (tailcone and fin trailing edges) all flush. Might be worth thinking about. Either way, that tailcone is dead sexy.
-Ken

I’m kind of mad at you right now. I had the centering rings and the tailcone and the motor mount tube all perfectly measured and in place although not glued yet… Now you’ve got me thinking I might have to start all over! I don’t know about the whole length of the tail cone, though. Then it’s into the fin tabs quite a bit. Maybe cut it back to the aft centering ring?

Although there’s something to be said for that clean, unobstructed shiny tail extending out the rear. Plus I’ve been told this rocket is overstable, so maybe this will help?

 

[richP]

I’m kind of mad at you right now. I had the centering rings and the tailcone and the motor mount tube all perfectly measured and in place although not glued yet… Now you’ve got me thinking I might have to start all over! I don’t know about the whole length of the tail cone, though. Then it’s into the fin tabs quite a bit. Maybe cut it back to the aft centering ring?

Although there’s something to be said for that clean, unobstructed shiny tail extending out the rear. Plus I’ve been told this rocket is overstable, so maybe this will help?

Don't know if you've gotten to Simulating it in Openrocket just yet, but would be a good time to start.

 

[Orien]

Don't know if you've gotten to Simulating it in Openrocket just yet, but would be a good time to start.

Simulations? OpenRocket? I was just going to aim the pointy end up and the fire blower part down.

Seriously though, I know I need to get into that. I've downloaded the software but haven't tried learning it yet. Is there a steep learning curve for a first time user?

 

[richP]

Not very steep at all. You could get halfway there by just using getting a copy of someone else's Iris project, and just editing some of the specs to reflect your build. If you need one, just lemme know.
In my opinion, of all the things that I've really been impressed with since re-entering the rocketry world, the ability to accurately and easily simulate flights is right up there.

 

[thzero]

Simulations? OpenRocket? I was just going to aim the pointy end up and the fire blower part down.

Seriously though, I know I need to get into that. I've downloaded the software but haven't tried learning it yet. Is there a steep learning curve for a first time user?

Did you weight everything before you built? That makes it easier to do a very detailed simulation. Otherwise, you'll probably need to use the default iris from loc or others, and then use the CG overrides once the model is built (with all components in launch including engine) and you know the CG. Good practice anyways.

 

[Orien]

Did you weight everything before you built? That makes it easier to do a very detailed simulation. Otherwise, you'll probably need to use the default iris from loc or others, and then use the CG overrides once the model is built (with all components in launch including engine) and you know the CG. Good practice anyways.

Did I weigh everything? What am I, a drug dealer? This is getting complicated…

 

[OKTurbo]

The tailcone isn’t scale, but it shouldnt affect the CP very much. The stock center of pressure is just behind the leading edge of the fins. I suspect the tailcone would move it just smidge further aft. If your CG is a couple of body diameters ahead of that you’ll be fine. If it’s not, you add some weight in the nose cone... but I can’t imagine that being necessary.

A while back I built a 4” Iris. I show the cp at about 6:30 mark in this video.

 

[Orien]

With the tailcone here, the structural components can be assembled! I look back at this vid and smile at my younger self's naïve optimism...

 

[thzero]

Every step you typically learn something new. Or you read something and decide to try it.

 

[Nytrunner]

Tailcones generally move the CP forward and need to be accounted for. When you modify or deviate from the stock build, simulations become very important (especially as a new rocketeer playing with mid or high power rockets)

There are several Openrocket tutorials at the link below.
https://hararocketry.org/hara/resources/trajectory-analysis-tools/

 

[Orien]

Tailcones generally move the CP forward and need to be accounted for. When you modify or deviate from the stock build, simulations become very important (especially as a new rocketeer playing with mid or high power rockets)

There are several Openrocket tutorials at the link below.
https://hararocketry.org/hara/resources/trajectory-analysis-tools/

I agree 100%. I fully intend to dive into the software and run simulations. I figure I can adjust CG once I know the CP by configuring the rocket accordingly. For example, if my CG is only off by a little, then the single-deploy (with a chute reefing device, perhaps) would put the recovery system in the forward payload. If it needs more weight forward, use the avionics bay. Even more, add the camera. This should be interesting!

And thanks for the link. Watching the Open Rocket overview now.

 

[Orien]

So here's what happened...

 

[thzero]

I always put the chutes, etc. as far down the tube as possible in the sim so that its 'worse case scenario'. Honestly with 4 fins as big as the Iris' and the length, I really doubt you'll have much trouble.

 

[Orien]

Success! Thanks to Neil_W for the great tip!

How I fixed the warped fins:

1. I sanded all the filler off, then sprayed both sides with a little windex.
2. Then I clamped them down as pictured to a granite cutting board.
3. Next I placed them in front of a space heater and cranked it to High.
4. I checked it after 15 minutes, and it was damn near fixed. So I flipped it over for a little longer and it's perfect!


While I was at it, I sanded ALL that cursed filler off the other two fins as well and put two coats of CA on every fin. This morning I sanded them all smooth and filled any imperfections.

Standby for a video update of this process during the build. I pulled an all-nighter!! (an "all-nighter" is anything past 9:30 pm for me). I'm having trouble going from phone to 'puter to YouTube. The You Tube uploads take HOURS! I hope you guys appreciate the HD!

 

[OKTurbo]

Tailcones generally move the CP forward and need to be accounted for. When you modify or deviate from the stock build, simulations become very important (especially as a new rocketeer playing with mid or high power rockets)

There are several Openrocket tutorials at the link below.
https://hararocketry.org/hara/resources/trajectory-analysis-tools/

I’d agree if you keep the total body length the same with a tailcone, but in this case he is adding to the stock body tube. The extra tailcone would move cp aft a slight amount. A run thru Rocsim would be more precise in knowing where the cp will end up.

 

[Orien]

Finished Fin Can:
It looks like there's way more excess than there is. It's just that dark gray JB Weld smearing as I wipe off the excess. I ended up using the regular JB Weld instead of the Kwik Weld. I liked it's consistency better. And while it takes a long time to "set", it gets sticky enough to hold it's shape in just a couple minutes. I had no problem doing the internal fillets all at once with no running.



Here's the completed fin can with all the fins sanded down smooth to 400 grit:

 

[Orien]

I’d agree if you keep the total body length the same with a tailcone, but in this case he is adding to the stock body tube. The extra tailcone would move cp aft a slight amount. A run thru Rocsim would be more precise in knowing where the cp will end up.

I was confused about that. Thanks for the clarification. I couldn't understand how adding surface area would move the CP forward. But if I was removing surface area by cutting in the tail cone, CP goes forward. Got it.

 

[thzero]

And it moves the CG aft; that is a pretty big mass.

 

[Orien]

And it moves the CG aft; that is a pretty big mass.

Of course. But I'll get it figured out.

 

[Orien]

Late-Night Update! (It was like 8:45 PM!)



Motor Mount/Fin Assembly Installed!

Prepping the fin can:


Centerline for the rail buttons:


Interior epoxy spreader thingy (I went kinda deep so the centering ring had to push through it a little way which built the fillet up):


Epoxy and CA at the aft centering ring got her locked into place. The CA helped me get the tube tight against the centering ring where I had cut it previously to get the fin can in. The four sections wanted to flair out off the centering ring and keep it out of round. That Gorilla Super Glue Impact Tough Formula is awesome. Thick enough to get into gaps, but workable. I even coated all my fins with two coats of it!


Built Ford Tough! A final application of JB Weld over everything to really lock it together as one solid unit. I was careful to keep the rear edge of the airframe where the tailcone shoulder sits free of adhesives.


Ooooh, Mama!


Baby's got some junk in the trunk now, that's for sure. Well, I can always make her longer!

 

[Tractionengines]

I need to ask something. (I have not used a tail cone before so please correct me if wrong, I just saw this in another thread )
When you "set" the position of the MMT did you have a motor in the MMT, so the retainer (tail cone) will seat against the end of the body tube with a motor.
The thrust ring of some motors caused there to be a gap between the tail cone and end of body tube. There are differen't spacings for different motors, so you need to size for thickest ring, then use spacer rings, if using a motor with a narrower ring.
Does this apply to the tail cone you are using?

 

[Orien]

I need to ask something. (I have not used a tail cone before so please correct me if wrong, I just saw this in another thread )
When you "set" the position of the MMT did you have a motor in the MMT, so the retainer (tail cone) will seat against the end of the body tube with a motor.
The thrust ring of some motors caused there to be a gap between the tail cone and end of body tube. There are differen't spacings for different motors, so you need to size for thickest ring, then use spacer rings, if using a motor with a narrower ring.
Does this apply to the tail cone you are using?

I did not have a motor in it. No one I talked to about these, and I asked about them extensively, never mentioned anything like that.

 

[Orien]

Fillets are looking sweet!

 

[Tractionengines]

[ Deleted, did not apply. ]

 

[Tractionengines]

[ Deleted, did not apply. ]

 

[Tractionengines]

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[Orien]

Still need to figure out the av bay and recovery system, but she's ready for final touch-up sanding and then primer and paint!

 

[Orien]

Think I figured out cross-linking threads.
Post 143 & 145 initial fit:
https://www.rocketryforum.com/threa...-any-glass-kit-build-starts.45666/post-444514https://www.rocketryforum.com/threa...-any-glass-kit-build-starts.45666/post-444541
Then re-work reason and fix:
https://www.rocketryforum.com/threa...-any-glass-kit-build-starts.45666/post-445070https://www.rocketryforum.com/threa...-any-glass-kit-build-starts.45666/post-445083

That’s a lot of reading and clutter in my build thread to describe a solution to a problem I’ll never have. This only applies to retainer ring motors. Can you delete this please?