4" to 3" Minimum Diameter Interstage Coupler Build

[AllDigital]

I've been meaning to post this for six months, but things got busy. My son and I did a 4" to 3" minimum diameter two stage last year and we couldn't find a lot of plans that worked for us, so we crafted our own based on others input and designs. Our rocket was hacked together using two existing rockets in the garage, so it is loosely based on a Wildman 4" Extreme fincan and a Madcow 3" G3 Color (extended/modified) sustainer. We flew it three times with Sugar motors using a custom 4" six grain motor casing and a 3" Loki 76/6000 motor case.

Our design objective was to keep the interstage coupler (ISC) as simple as possible and allow full use of our "stock" sustainer and booster rockets with little modification.

The overall design assumed the primary flight computer was in the sustainer, head-end ignition on the sustainer motor, a BP separation charge triggered by small wires running on the outside of the sustainer. The sustainer motor extended four inches out of the fincan to provide a stiff connection with the ISC and booster. The sustainer had both a drogue and a main, but the booster only used a single main chute.

Interstage Coupler:

For the interstage coupler we used three standard tubes and a custom 3D printed "fairing". The advantage to this approach was that all the tubes and the centering rings could be ordered "stock" without a lot of fabrication or machining required. At the core of the ISC was a 12" piece of 3" tubing. The sustainer motor mount only went 4" into the tube, but the full 12" gave us a lot of rigidity in the design. That 3" core was seated inside a 4" airframe with centering rings, just like you would for a typical motor mount. The top four inches of the 3" tube were outside the 4" airframe, so a custom 3D printed angled fairing could slide on (see link to file below). The 3D printed fairing was coated in resin (west marine) to give it additional strength. A permanent bulkhead was glued in the center of the 3" tube to separate the separation charge from the booster altimeter below. Another (removable) bulkhead was added at the aft end of the 3" tube with the booster altimeter sitting in the compartment ahead of it. That bulkhead provided the eye bolt and separation charges for the Boosters main chute.

The ISC was connected to the booster using a piece of 4" couple tube (the third tube). It was screwed in on top to the ISC and shear pinned into the booster below.

A few other considerations:

- To charge separate or not? We wanted fin alignment off the pad, so we decided to have a very tight friction fit, but we also didn't want to drag the fincan longer than we needed, so we decided to do charge separation. That required small wires coming down from the avBay in the sustainer into the charge well of the ISC. We also put a 3D printed cap into the nozzle of the sustainer motor, so the charge wouldn't damage the grains (see file link below). Ultimately, we used a little bit of aluminum tape to hold them together (overriding the friction fit) to ensure the charge wires did not disconnect.

- For sustainer motor retention we put an internal coupler stop for forward retention and a threaded rod through the eye bolt for reverse retention. The rod at the forward end of the motor went through the sustainer airframe and was held in with "upside down" launch lug threaded screws on each side. That worked perfect for "reverse retention".

- The ISC could have been shorter, but we had a lot of electronics, including a high power radio and a dipole antenna, built into the ISC avionics bay. We also provided more length to fit a bigger parachute. This wasn't setting any records, so weight was not our primary consideration.

-Mike












3D printed fairing: https://www.thingiverse.com/thing:5183489



Nozzle cap: https://www.thingiverse.com/thing:5183528

 

[James Kobel]

Did you get to fly this beast yet? Or do you still have some work to do on it

 

[AllDigital]

We flew it three times last year. All three launches on sugar motors. The last flight we got the sustainer to about 28K feet. On AP it would likely go to about 40-45K feet.

 

[James Kobel]

We flew it three times last year. All three launches on sugar motors. The last flight we got the sustainer to about 28K feet. On AP it would likely go to about 40-45K feet.

Where are you flying this? California or at black rock?

 

[AllDigital]

Where are you flying this? California or at black rock?

California. At FAR.

 

[Neutronium95]

Why did you choose to run wires for the stage separation charge, instead of using something in the booster to fire it? Running wires down the side seems to defeat the purpose of HEI.

Otherwise this looks like a really cool build.

 

[AllDigital]

Why did you choose to run wires for the stage separation charge, instead of using something in the booster to fire it? Running wires down the side seems to defeat the purpose of HEI.

In our situation it was really out of convenience of using a single flight computer for all the staging activity. Our flight computer could more accurately detect motor burn out and fire the separation. Running the lines on the outside doesn’t create too much drag, but they are at higher risk for failure, especially going down to the booster. Any hard twist of the sustainer could have disconnected them. That said, I’m sure lighting the sustainer would have separated everything, but it would have been late. Our set up actually had two pairs of micro wire going to the booster. The second gave us a positive radio report of separation. So through a single computer we got transmitted events to the ground for 1) call for separation, 2) pyro loss of continuity, then positive confirmation of 3) physical separation.

We could have run the sustainer igniter wire down the outside also, but with a separation charge down there it was way too risky. HEI uses much more secure and reliable wiring.

 

[Chad]

We could have run the sustainer igniter wire down the outside also, but with a separation charge down there it was way too risky. HEI uses much more secure and reliable wiring.

I have a little 3" to 2" (H-to-G) two stage that i'm using to experiment with before building a larger version since i can fly it at my local club's monthly launches ( max alt is 4.5k feet ). The wiring for the sustainer igniter is a PITA, can you describe how you did the HEI setup?

I really like your ISC design, I did something similar with a 3dprinted reducer. Keeping it short enough to not crowd the booster recovery is a challenge. How are you doing booster recovery, single deployment? In my POC airframe I'm just using a rocketman streamer since it doesn't go out of sight. For the sustainer i'm using a chute release on the main and keep all the electronics in the nosecone. For the large build, i'll probably have a chute release in the booster and sustainer to simplify however the larger parachutes may make that not feasible.

PS here's my build, i've only been able to fly it about 3 times and have yet to have a succesful sustainer ignition but am more confident for a launch in a couple weeks. I think i had my tilt lockout too tight for this airframe and a couple other issues.
https://www.rocketryforum.com/threads/3-to-2-two-stage-scratch-build-should-fly-in-may.172357/

 

[AllDigital]

The wiring for the sustainer igniter is a PITA, can you describe how you did the HEI setup?

My sustainer was using a Loki 75mm case and a Loki "smoke" bulkhead. I drilled the top of the bulkhead, ran 18awg wire through the hole, coiled the wire in the smoke chamber, and then potted it with JB weld. After the JB weld I put a heavy layer of RTV. I attached two igniters to the 18awg wire for redundancy.

In my case, the flight computer was in the center avBay of the sustainer, so I just had an extra pair going through the aft bulkhead of the avBay to light the sustainer. My booster was on single deployment with a GPS signal (and a long walk) and the sustainer was configured as a traditional dual-deploy with center avBay. The aft drogue separation in the sustainer would pop open the small JST connectors going to the HEI in the motor.