I’m working to determine the limits of indirect staging (sometimes known as gap staging): how far apart can the booster and sustainer motors be, and still get reasonably reliable ignition? I built STB-1 (Staging Test Bed #1), pushing the first stage out to about 40 cm, staging an Estes D12 to a B4. The first flight worked perfectly, though there was some interesting tip-off shortly after launch and an odd sort of corkscrew in the flight path.

The second flight wasn’t nearly as successful: the sustainer failed to light.

The booster motor mount tube has four vent holes, roughly 3mm diameter, about 4cm or so from the top. These vent into the airframe, with vent holes drilled into the middle and after centering rings (BT-60 airframe). I’d thought that, perhaps, the failure’s cause was clogged vent holes in the middle centering ring. Post-flight dissection showed that not to be the case.

The sustainer’s motor mount had been dislodged and shoved well forward in the airframe. I couldn’t tell if this happened on impact or during staging—I would guess on impact, though. There was a small hole in the forward centering ring, though, to provide clearance for the sustainer’s engine retaining hook; after the second flight, it looked to me as though that hole had grown. It’s possible that the stages are too far apart for reliable ignition, and it’s possible that the hole in the forward centering ring allowed enough pressure to vent forward that the stages separted before sustainer ignition. (That forward centering ring is too far forward: it should not be impinging on the sustainer’s engine hook.)

Perhaps more testing to come.