Mixed BP and APCP clustering

[Dugway]

It looks like I may actually get my Sirius Saturn V done in time for Hellfire. I built it as a cluster with the central 38mm surrounded by four 24mm motor mounts and I plan on flying with the recommended I284 and four E12-0s. I don't have a lot of experience clustering a mix of BP and APCP motors and I'm a bit concerned with what happens if the center motor doesn't light. One idea I have is to physically tie the rocket down with a cotton string, or maybe mono filament, such that the thread passes directly under the center nozzle. When the center motor lights, the thread burns through, and away we go.

Has anyone ever tried something like this? If E9-0s were still available I wouldn't worry so much, according to Rocksim the rocket would only move a few inches, with the E12's it's predicting 7+ ft.

 

[Arsenal78]

I wouldn’t risk it in a kit that expensive. Throw some single use 24mm Aerotech motors in there instead. Someone said not too long ago that if you put a 0 motor in backwards and light it (no clue if it’s even possible), it produces lots of smoke but no thrust.

 

[tomsteve]

seems that for that motor combo, having the BP motors airstart shortly after movement from the I284 is detected would be the safest-other than going to the APCP motors recommended by arsenal

 

[Dugway]

I wasn't going to use electronics, just to keep it simple, but there is plenty of room in the top end. And a nice conduit to the aft end disguised as an internal launch lug that I wasn't planning on using. Airstarting the 24mm's does open up a lot of possibilities.

 

[Bat-mite]

I wasn't going to use electronics, just to keep it simple, but there is plenty of room in the top end. And a nice conduit to the aft end disguised as an internal launch lug that I wasn't planning on using. Airstarting the 24mm's does open up a lot of possibilities.

There's no "simple" way to do it. BP and APCP ignition times are different. Even with two APCP motors that are exactly the same, you may get a different ignition time based on grain age, grain density, etc.

As noted above, you should select motors such that either the central motor OR the outlying motors are sufficient to safely get the rocket off the pad, and airstart the rest based using an accelerometer.

 

[DeeRoc29]

The simple way is the old fashioned way. Light the AP motor then airstarts the BP outboards with fuse. This was common practice in HPR in the 80's and 90's.

I was just looking at this old issue of HPR this morning. Photo is of an all-AP cluster but you get the idea. I've lit Estes F15's this way around AP motors in my 7x29 rocket.

 

[gldknght]

seems that for that motor combo, having the BP motors airstart shortly after movement from the I284 is detected would be the safest-other than going to the APCP motors recommended by arsenal

I think this is a very cool idea.

 

[tomsteve]

There's no "simple" way to do it. BP and APCP ignition times are different. Even with two APCP motors that are exactly the same, you may get a different ignition time based on grain age, grain density, etc.

i flew my old thoy nighthawk on many different motor configurations and can say this held true for me. flew it once on 3-G80's and had one come up to pressure at the end of the launch rail. i flew it on quite a few other combos- sticking with blue thunder and white lightning motors- and had the occasional ignition time variation

 

[MClark]

The simple way is the old fashioned way. Light the AP motor then airstarts the BP outboards with fuse. This was common practice in HPR in the 80's and 90's.

I was just looking at this old issue of HPR this morning. Photo is of an all-AP cluster but you get the idea. I've lit Estes F15's this way around AP motors in my 7x29 rocket.

I did this quite often. Thermalite (not thermite) fuse in Teflon tube burns very fast so there is little delay. I was using a 29mm and 4 D12's.

 

[Johnly]

I used the same technology as Mark with Fat Boy I modified that had a 24mm center motor mount and 6 outboard 18mm, except I didn't put the Thermalite in teflon tubing.
Not using tubing slows down the burn rate of the Thermalite, so the when the F39 burned out, the C6-0 motors lit. Since it's unlikely that you will be able to find Thermalite, your next best option would be to contact some fireworks people and see if you can get some Quickmatch from them. The UK TARC teams often use Quickmatch frequently to cluster BP motors.

 

[Dugway]

I'm lucky to have a few inches of Thermalite. I use it to augment igniters only when a motor absolutely refuses to light.

 

[heada]

It isn't thermalite nor quickmatch but might as well be.

https://www.skylighter.com/collections/firework-fuse/products/quick-fuse-white-gn1207

It burns at 0.1s to 0.4s per foot. 20 ft roll is $15. Cut lengths so that the flame coming out of the APCP motor lights the fast fuse and then transfers flame to the BP motors. Use a normal igniter for the APCP motor. As always, check with the RSO before starting something like this and since it burns so fast, make sure the pad area is completely clear of flammables. I've seen the fuse fly through the air a few feet when not contained.

 

[Dugway]

I just ordered some of the quick fuse, so that will be a possibility if it gets here before the end of the month. No worries about nearby flammables, this is getting launched on the salt flats. How do you keep the fuse inside the nozzles of the BP motors? Just use Estes plugs? Maybe hot glue? It seems like the central motor lighting could be pretty energetic and just blow the fuses away before lighting them.

I really like the idea of airstarts, since I already have the electronics. The problem is getting to the top of the internal launch lug. This would have been really easy during construction, but now not so much. I'd have to drill a hole about a foot inside of the 4" diameter blue tube. Maybe with a sharp chisel??? Then thread wiring from the bottom of the rocket, up the lug, and then somehow get it to come out of the new hole. Hmmm.

 

[cerving]

Maybe you can make a wiring tunnel outside the airframe... aerodynamically it might balance out the rail buttons.

 

[heada]

The way I used fast fuse on BP motors was to bend about 1/4 inch of one end over 180 degrees and tape that over the BP nozzle opening such that the fuse was in contact with the BP grain. I then rotated the motor so that the other end was directly below the APCP motor. It is stiff enough that it doesn't need to be supported. The fuse will light with the slightest flame so even a strong chuff might do it so make sure you have a good igniter in the APCP motor. Once the fuse starts, it's near instant to the BP motors. When you get it, cut off a foot or so, using a brick to hold it down, test it out. You'll be amazed.

 

[Dugway]

Sounds good. The fuse has shipped and is only two states away so hopefully it will get here in time.

 

[kenstarr]

I wish I could be there to see it fly. I have a very similar configured rocket I've finally finished after over 2 years. I missed hellfire last year and I won't be there again this year!
https://www.rocketryforum.com/index.php?posts/1767290