Inter Stage Coupler

[Bruiser]

I am working on my first mid power two stage designed for composite motors. I have a question about the bulkhead for the inter stage coupler.



So in the design above you can see I am working with 3" tube. I started with a four inch coupler (because that's what available). It will slide 1" into the sustainer. Then there is a 1.375" piece of body tube to act as a shoulder. That leaves 1.625" (mistype of 2.625 in the drawing) of coupler to slide down inside the booster. Then I need a bulkhead to put an eye screw in for the recovery system. I had thought about putting it aft end of the coupler as pictures in red but then I thought that might leave the upper end of the coupler flimsy so thought about putting it in the lavender position but that would leave the aft flimsy. Maybe I need a bulkhead in both positions or should I just put the 1/4" bulkhead in the middle?

-Bob

 

[Cameron Anderson]

Recessing sustainer motors into the airframe can be done, but I've never had it work well, and lots of people avoid it (there are a ton of potential issues). Also, a 1" piece of coupler doesn't provide for a lot of support as the ISC. I've found that shorter couplers bind more and have issues separating cleanly in addition to not having the length/strength you will want from your ISC.

 

[mikec]

+1. Don't recess the motor and let the coupler go into both first and second stages at least one diameter (so it needs to be longer). One bulkhead at the bottom is fine. If you use TTW fins on the second stage (recommended) then their fin tabs will be partial so the ISC coupler can insert.

 

[Bruiser]

The drawing isn't quite to scale, just a quick representation. Both stages will be using the Estes 29mm retainers and the aft edge of the retainer will be even with the end of the body tube.

I could add on to the coupler bottom and make it longer so that it does go into the booster 1 cal like most nose cones but I run into a problem with the sustainer. The fins are 4" tall and the forward 1" or so is tapered. With the coupler going into the sustainer that only allows a fin tab that is 2" tall. If I were to make the coupler go in the sustainer 3" there would be no fin tabs. It's a scale rocket so I can't make the fins taller to compensate.

Is this a catch 22? Thru the wall fins or coupler length? What is needed most? I don't have it here so I can't measure it to be absolutely sure but the Pro Series booster I use on my Majestic is right at 1" and the Majestic is a 2" rocket. It is my favorite two stage out of the few I have. That's what I was basing the 1" on. Maybe 1/2 cal is ok?

-Bob

 

[David Schwantz]

Here is a Mach 1 Black Hole converted to a 2 stage. 1st pic shows joint between stages. 2nd shows Aeropack retainer for booster. 3rd shows fin slots in ISC. 4th fin slots. 5th Aeropack retainer for upper stage. 6th ISC bulkhead. 7th shows retainer, TTW fins and tube for igniter wires. If you slot the ISC for the fins tabs, you still get TTW fins and have a longer ISC. Maybe this can give you some ideas.

 

[Bruiser]

Ah, slots in the coupler. That'll allow both ttw fins and a longer coupler. But you'd have to move the aft centering ring up. Most of the kits I've built have the aft centering ring pretty close to the end of the body tube. How far in can it safely go on something like this. It's mid power (29mm F and G motors) and the body tube is heavy cardboard as is the motor tube (Estes pro series parts). The centering rings are 1/8" ply. Recessed 2" ok? 3"?

David, pic 6 is pretty hard for me to make it out for sure but it looks like the ISC bulkhead is at the aft end of the ISC. Is that so? Also it appears that the ISC has some really thick walls aft of the fin slots???

-Bob

 

[David Schwantz]

Nope, look closely and you can see the fin slots in the ISC, so bulkhead is set in 3" or so. Also the thick walls are actually the inside of the MMT and the forward CR. She is a 2.6" bird with 38MMT.

 

[Bruiser]

So your booster has no recovery system? It's the booster/ISC all in one? Like the pro series booster with a tumble recovery?

The ISC on mine serves both as a stage coupler and as a "nose cone" for the booster. It should go: booster ignites, rocket goes up, booster burns out and drag separates with the ISC from sustainer, sustainer ignites and if booster has not completely separated it separates from ISC/booster, sustainer accelerates away, booster motor ejects ISC/parachute and floats to the ground, sustainer burns out, ejection charge blows out nose cone and chute, JLCR opens at 400 feet and sustainer floats to the ground. At least that's how I have it in my head

Thanks for showing me your Mach 1
-Bob

 

[mikec]

I'm not following your fin dimensions. Can you post a drawing?

Slots in the ISC coupler might work, but it's easier to do this in fiberglass than in cardboard and have it be rigid enough.

Another option might be to use rods instead of a coupler to connect the stages, I've never done this but many have.

 

[David Schwantz]

OPs first post did not state material. But should be able to come up with something that will work. And my booster will have streamer recovery. Although small, just enough to spot in the grass. Small hole will get drilled for cord and streamer goes in 2nd stage and just gets pulled out. Sounds like your chain of events should work. Good luck. And yes, booster and ISC all one piece.

 

[Bruiser]

Best I could up with quickly in Paint Fins are 1/8th ply



I found a 6" coupler and ordered it. It will allow me to insert the ISC 2 inches into the sustainer by making 1" notches to accommodate the ttw wall fins. I would recess the aft CR in the sustainer 2.125". The 6" coupler will allow the ISC to slide in the booster 2.625".

How does that sound?

Still have my original question . Where should I place the bulkhead inside of the ISC?

-Bob

 

[David Schwantz]

Forward bulkhead of ISC needs to be inset far enough to clear upper stage motor. And you do need to lower upper MMT. Think of how you are going to retain the motor in mount. Lower ISC bulkhead also needs to allow boost motor to be inserted. Have you thought of hollow bulkheads? they still will supply stiffness for ISC and give attachment point. But they will also allow motors to slide into them if clearance is a problem.

 

[mikec]

Put one bulkhead at the bottom of the ISC.

You could still have fin tabs in the beveled part of the fin, though I admit they would be hard to sand and perhaps not as strong.

I assume this is a "dumb ISC" and the electronics is elsewhere.

 

[Bruiser]

Yes, I am planning on building a compartment in the side of the sustainer like this one Apogee has for the Sandhawk:


I want to start it between the fins and go up. The rocket is not very long and the area for the parachute is only about 7 inches long. I was thinking of the area in the black rectangle area for the compartment and hatch


-Bob

 

[Bruiser]

Coupler woes continue...

The 6" long coupler I ordered did arrive. It doesn't fit inside the body tube well at all. The posted measurements of the coupler and the body tube suggest that it would be perfect but it is very snug. I sanded the outside edge of the coupler and the inside edge of the body tube to eliminate any ridges and if I line it up perfectly the coupler will go in very snuggly for an inch or so. I don't think I want to try sanding the outside or peeling a layer off to get a good fit because after I coat it with something to protect it (thin CA, epoxy, WBP; haven't decided) I'll probably need to sand it to fit again and I'm not sure there will be enough "core" material left.

I am thinking of making a custom fiberglass coupler. I know I can make the form out of some body tube and centering rings. I see that WalMart has Bondo branded fiberglass cloth that is .20 mm thick in stock and I have Z-Poxy laminating resin. So I need to figure out if this fiberglass cloth and resin is up to the task and then I need to figure out what size to make the inside form. Form size will depend on how many wraps of cloth I need and the thickness each wrap will be. Is there anyone reading that can help me with this?

Thank you,
-Bob

 

[kalsow]

Here's a good starting point:

 

[mikec]

The posted measurements of the coupler and the body tube suggest that it would be perfect but it is very snug.

Soak it in thin CA and sand it down until it fits.

 

[Bruiser]

Ok Mike, you empowered me. I stripped of a .005 inch layer of paper then soaked the coupler in thin CA. I gave it a light sanding to smooth it out and it fits very nice now. The remaining cardboard is .034 thick.

-Bob

 

[Ez2cDave]

Make sure that your coupler into BOTH the Booster and Upper Stage are AT LEAST 1 caliber. Otherwise, you risk "offsetting" the airframe during boost, possibly resulting in structural failure.

Dave F.