Cluster booster for Rip-Roar

[Wayco]

Over the years, my two stage Rip-Roar has gained some weight. After the last rebuild, it weighed in at 12.1 oz empty. The last repair was a new MMT in the sustainer with plywood Cr's and epoxy. I modified it to fly "E" motors, and with an E9-4 the rocket was 15 oz., well over the max liftoff weight for the booster D12. I flew it anyway and it wallowed off the rod. So I tried to make it lighter with a D12-5 and took out the nomex pad and 30" chute, which I replaced with the original plastic chute and dog barf. Weighed in at 13.9 oz when I flew it again, with slightly better results, but still slow off the rod.
So I'm adding another motor to the booster. I ordered some BT-70 from BMS and a couple of 2x24mm CR's to fit. Cut a couple of MMT's 7" long so I will have room for a small 12" chute in one of them. The original booster was tumble recovery and was prone to cracking fins, so with the added weight, I figured a small chute would be in order. The main concern I have is the gap staging. With the booster motor directly in line with the sustainer, it always lit, but now it will be offset by half a motor diameter. When I cut the one MMT, I sliced a small wedge out of one end, and used metal tape and epoxy to direct the hot gas toward the center and upper motor, but not sure if this will work. I will also vent the gap between them to help the ignition process. Here is where the build is now:

As you can see, I added a part of a champagne cork with an eyelet to protect the chute from the sustainer motor. The big question is how much gap do I need between the top of the MMT's and the upper motor? I figure the hot gasses and particles from the booster will act like shotgun pellets and spread out as they move up, and this might help get the upper motor lit, but I'm just guessing.... This ain't exactly rocket science here, but anybody have any ideas that could help?

 

[[POW]Eagle159]

Its probably best to have none, or the shortest possible gap between the tube and upper engine. On my gap stage rockets the Bt-20 upper stage tube goes a little inside the Bt-50 booster tube...

Yours looks good! I would think a one inch gap would be fine.

 

[kjohnson]

I mentored a TARC team a while back that flew a clustered booster to a single motor sustainer. They started with a cone made of card stock and aluminum foil to direct the staging heat and burning particles, but later flew without out it. They had an inch or two between upper and lower stage motors.

kj

 

[Wayco]

Thanks guys, that gives me hope that mine will work as is. The original booster had about two inches between the upper and lower motors and it always worked. With an eight inch booster body tube I should have about an inch gap.

 

[Marc_G]

I think you'll be fine but I have one caveat:

What if the boosters don't fire simultaneously? Suppose the parachute one pops first... and the parachute locks the hot gasses/particles from getting to the sustainer before the other booster motor burns through?

Have you considered using a delay engine in the tube that has the parachute, to avoid this possibility?

 

[Wayco]

That's been my plan all along, D12-0/D12-3 to an E9-6. Or I might try the new E12's if I can find them locally....