Gluing BP motors for a cluster?

[SolarYellow]

I had the silly idea of doing a 7x13 cluster in a small BT-60 rocket. As in it would be cool to launch a Baby Bertha with seven A10 motors. Should get off the rod in a hurry and be a relatively low-cost way to explore the technical challenge of clustering. The availability of the A10-PT means I could even tailor how many of them had ejection charges to get the right amount of punching out the laundry. Or might make it a booster with a bigger single motor in the sustainer lit by one -0T with a charge directing tube nested into the sustainer nozzle and surrounded by six -PTs . I spend enough time with OR that I don't have any worries about working out the stability well enough, and then actually swing testing before anything flies.

I just tried mocking up a few different configurations with tubes and motors, and the only thing that looks like it works is the motors in a BT-60 coupler with no individual MMTs. Would it be considered kosher to just glue seven cardboard BP motors together and then glue them into a chunk of BT-60 coupler to make a cluster "cartridge" that would be loaded into the BT-60 tube as if it was a regular motor? Understood there'd need to be some plugging of the gaps between them to contain the ejection charge/ensure clean separation of a booster stage, etc. Would use epoxy to avoid risk of the water from yellow glue affecting the burny parts.

I have found the docs regarding clustering over at Narham, but if this is addressed in there, I didn't see it in the quick scan I did. Plan to read and digest fully and more carefully later.

Would an RSO look at me like the idiot I probably am and kick me the heck off his range, or would he give it a thumbs-up if it looked well done and I could explain it well? Would this be considered a permanent, non-reversible "modification" of the motors requiring L2 cert?

 

[jmasterj]

Seems like it should be fine. You're just attaching things to the outside of the motors.

 

[Steve Shannon]

If I were the RSO I’d have no problem with it as long as you have blocked the passages between motors so the recovery system works.

 

[BEC]

...and be aware that at least some A10s have a really strong ejection charge. Also, with only the choice of booster, plugged or 3 second delay, chances are that a Baby Bertha going up on a set of motors that has just a hair more total impulse than an Q-Jet D20, that delay is going to be kind of short.

I agree with Steve — I'd probably let it fly at one of my launches.

 

[Cape Byron]

...and be aware that at least some A10s have a really strong ejection charge.

The charge seems really 'hot' as well. Some of my minimum diameter 13 mm birds get blistered paint from one A10-3T launch.

 

[Rocketjunkie]

I have a scaled down Ultimate with a BT-60 airframe and 7 13 mm mounts, BT-5 tubes will fit with some distortion of the BT-60. I have 3 in line tubes going through and the other 4 are short and stop before the top CR. Use A3-4 or A3-6 motors in the through tubes and A10-Ps in the rest.

 

[mh9162013]

I noticed the ejection charge of an A10 was weaker than the A8-3. So when people say the A10s have a powerful or hot ejection charge, I presume they're comparing it to other 13mm engines and not making the statement in absolute terms?

 

[Daddyisabar]

These RSOs are really kinder and gentler! My old RSO always used extreme prejudice. A seven motor cluster immediately received a stink eye and a thorough inquisition on all aspects of the launch, right down to the cleanliness of each electrical connection.

 

[jmasterj]

@SolarYellow have you thought about how you're igniting these? 7 igniters? Or a small flash pan? @Rocketjunkie how do you ignite yours?

 

[SolarYellow]

Still researching that. Have been thinking 7 enhanced igniters. No solid plan locked in.

The idea came about as I was trying to make up an excuse to add another 3-inch build to my list and realized a 7x24mm cluster would be an interesting challenge. Thought it would be a good idea to work out the kinks on something smaller and cheaper. Expect to have a spare Baby Bertha around after some other hijinks, so it’s a natural vicim to experiment on.

 

[heada]

If you have loose BP, flash-pan can work and is fun and exciting. If you don't want to use flash-pan, I'd recommend looking at these

https://electricmatch.com/rocketry/see/23/6/bp-rocket-starters
Much longer leads and lower current requirements than Estes igniters so easier for the controller to fire them all at the same time. $0.73 each isn't too bad either

 

[Alan15578]

Back in the early '70s I build and flew a scale model of the Honest Ivan. It was scaled to BT-60, and flown with seven B3 motors mounted in BT-5 tubes. These tubes were enclosed in the special tube from the Saturn V K kit that was slip fit around the BT-60 tube. It was topped off with a CMR egg capsule. All 7 motors ignited with a 12V relay launcher. Unfortunately, I had neglected to fill the gaps between the motor mount tubes, and as only the Russians could say; the flight was a glorious success.

I would not fly a 7 A10 cluster today. There have been too many reports on TRF of A10 catos. A3s would be better.

If I was RSO, I would certainly look at you as an idiot for gluing motors together, but if it was for a non-contest flight I might still allow it.

 

[mntanabob]

I had the silly idea of doing a 7x13 cluster in a small BT-60 rocket. As in it would be cool to launch a Baby Bertha with seven A10 motors. Should get off the rod in a hurry and be a relatively low-cost way to explore the technical challenge of clustering. The availability of the A10-PT means I could even tailor how many of them had ejection charges to get the right amount of punching out the laundry. Or might make it a booster with a bigger single motor in the sustainer lit by one -0T with a charge directing tube nested into the sustainer nozzle and surrounded by six -PTs . I spend enough time with OR that I don't have any worries about working out the stability well enough, and then actually swing testing before anything flies.

I just tried mocking up a few different configurations with tubes and motors, and the only thing that looks like it works is the motors in a BT-60 coupler with no individual MMTs. Would it be considered kosher to just glue seven cardboard BP motors together and then glue them into a chunk of BT-60 coupler to make a cluster "cartridge" that would be loaded into the BT-60 tube as if it was a regular motor? Understood there'd need to be some plugging of the gaps between them to contain the ejection charge/ensure clean separation of a booster stage, etc. Would use epoxy to avoid risk of the water from yellow glue affecting the burny parts.

I have found the docs regarding clustering over at Narham, but if this is addressed in there, I didn't see it in the quick scan I did. Plan to read and digest fully and more carefully later.

Would an RSO look at me like the idiot I probably am and kick me the heck off his range, or would he give it a thumbs-up if it looked well done and I could explain it well? Would this be considered a permanent, non-reversible "modification" of the motors requiring L2 cert?

I have had a fair amount of success with competition cluster rockets, and this would be my recommendation:

• peel or sand down T5 tubes to ~ half wall thickness. (seven will then fit in a T60 tube.)
• cut out the segments of the two centering rings and build up the motor assembly ahead of time then glue the whole motor mount assembly in the T60 tube as one unit. ( I would laser cut the centering ring parts but you can use the template below to hand cut the segments. )
• friction fit the motors, or retain all 7 with a single retaining washer screwed into the centering ring. (see the retainer I did for cluster altitude at naram60 published here: https://nar.org/member-only-reports/rd/NARAM-61/N61-Designing_Cluster_Rockets_for_Altitude-Zurek.pdf)
• twist all of the starter leads together - do not try to hook up 7 sets of clips.
• dip the starters in Rocketflite ClusterFire dip( I use this on all of my cluster rockets and have yet to have a motor not light.)
• clip the leads to the launch pad to allow a foot or better of movement on the rod to allow time for all of the motors to light before the clips are pulled off.
• make sure you have enough battery to light all 7 starters.
• have someone take good pictures ;^)

Good Luck!

 

[dhbarr]

I would probably just stuff glued wads of paper in to seal ejection gases rather than trying to cut and align all those fiddly crescents

 

[SolarYellow]

I have had a fair amount of success with competition cluster rockets, and this would be my recommendation:

Awesome post, thanks!

 

[Steve Shannon]

I would probably just stuff glued wads of paper in to seal ejection gases rather than trying to cut and align all those fiddly crescents

I would put one end of the cluster into clay and pour some epoxy into each gap, not much, just enough to seal the gaps.

 

[SDramstad]

I would light it with a flash pan. 7 igniters are a major pain

 

[BABAR]

Gorilla polyurethane glue makes a good adhesive and should expand to fill the gaps, in fact the problem is not using too much that it extrudes out the front and the back..

Pics and video of flight requested!

 

[DaHabes]

If you have loose BP, flash-pan can work and is fun and exciting. If you don't want to use flash-pan, I'd recommend looking at these

https://electricmatch.com/rocketry/see/23/6/bp-rocket-starters
Much longer leads and lower current requirements than Estes igniters so easier for the controller to fire them all at the same time. $0.73 each isn't too bad either

The MJG BP igniters are good, but if you want to avoid the cost, just buy a jar of Testor's Silver paint and dip your Estes igniters in them. Cheap. Works great. I use them for my NCR Cluster Duck all the time. If you really want to make sure, dip the igniters and then sprinkle on a wee bit of loose BP. Huge, immediate flare-up.

 

[DaHabes]

I have had a fair amount of success with competition cluster rockets, and this would be my recommendation:

• peel or sand down T5 tubes to ~ half wall thickness. (seven will then fit in a T60 tube.)
• cut out the segments of the two centering rings and build up the motor assembly ahead of time then glue the whole motor mount assembly in the T60 tube as one unit. ( I would laser cut the centering ring parts but you can use the template below to hand cut the segments. )View attachment 549661
• friction fit the motors, or retain all 7 with a single retaining washer screwed into the centering ring. (see the retainer I did for cluster altitude at naram60 published here: https://nar.org/member-only-reports/rd/NARAM-61/N61-Designing_Cluster_Rockets_for_Altitude-Zurek.pdf) View attachment 549662
• twist all of the starter leads together - do not try to hook up 7 sets of clips.
• dip the starters in Rocketflite ClusterFire dip( I use this on all of my cluster rockets and have yet to have a motor not light.)
• clip the leads to the launch pad to allow a foot or better of movement on the rod to allow time for all of the motors to light before the clips are pulled off. View attachment 549665
• make sure you have enough battery to light all 7 starters.
• have someone take good pictures ;^)

Good Luck!

I like the retainer design.

FWIW, I prefer individual leads for each igniter. I built a simple cluster breakout box. That way, each igniter will get full voltage (in my case, 12V). With my Cluster Duck, six leads around the airframe, the 7th up the middle.