An idea, perhaps helpful.
as a static at home test, do NOT tape the motors together but go ahead and put the friction fit tape on the forward and the rear motors.
put a SINGLE wrap around the nozzle end of the sustainer motor, remember their will be a circumferential half wrap left at separation. This can unintentionally “friction fit” the booster to the sustainer motor.
put the forward motor in The sustainer.
hold the rear motor in position and slide the booster over it until it slides over the extruded tail end of the sustainer motor, including the tail end tape wrap.
set the rocket on the work table nose up.
the “coupling” should be firm enough to prevent any bending or lateral motion ,but loose enough so that if you abruptly lift the sustainer, the booster stays on the table.
if it is too tight, you can put TWO LAYERS of cello tape around the base of an EXPENDED motor casing and gently “force” it into the forward end of the booster to dilate it a bit so it will be loose enough, or you can play with thin CA and sand the internal diameter so it fits loosely over the sustainer motor with tape.
also, when you actually FLY only use one wrap of cello tape to hold the motors together, any more and the overlapping extra tape again may unfortunately friction fit the booster to the sustainer.
when I design non-gap staged boosters, I cut the booster tube length 1/8” longer than initially needed. I cut a 1/8” section of motor mount tubing (usually just the same BT-20 or BT-50 I am using for a minimum diameter Rocket.). I cut a snip out of it so it rolls up inside the booster, and I use white glue (paper to paper, slow to dry) and tuck it just inside the tail end of the booster. (I do the same thing to make engine blocks on regular rockets, for LPR it’s all you need, i haven’t had a LPR motor blow through one yet.). Anyhoo, this acts as a “backstop” to prevent the booster motor from ejection from the booster at staging. It is SHORT enough that it doesn’t cause Krushnic effect and usually doesn’t make igniter placement much harder.
Recommend use lowest viable thrust motor in sustainer for first flight, can even use adapter to step down a motor diameter, like substituting an A8-5 (or 3) for a 24 mm mount, or an A10-3T for an 18 mm mount.
for first flight boosters, use the lowest motor that thrustcurve.org or your sim of choice says will get the whole STACK off the rod or rail at a good speed. Can usually get away with subbing a B6-0 for a C, B should get you off the pad as well as the C.
If you have a lighter 24mm, especially with a smaller and lighter booster Motor, C11-0 can sub for a D. If thrustcurve puts the thrust as borderline, go with the bigger motor, as multistagers tend to weathercock and thrust is generally your friend here.
two reasons to go small on first flight.
first, Safety. Even with kits, and especially with scratchers, in an untested rocket sometime meconium happens and staging is significantly off vertical. In the unfortunate circumstance where the rocket decides to go cruise missile and depart the reservation, er… range, I’d rather have it on an A8-5 than a D12-7.
second, with the exception of staged saucers like Frick’n’Frack or Twin Factor, mulitstagers logically tend to go higher (ask
@Ronz Rocketz about this
.). You have two parts to track, and booster has no streamer or chute so much hard to track or find than regular chute or streamer rockets. And because they tend to weathercock, unless there is zero wind they tend to track into the wind, and often the degree of off vertical tracking greatly exceeds the winds ability to float it back to you. So the later the staging and the later the ejection, the further away you are gonna walk.
Wishing you two straight vertical trails and two short walks!
