You could always put spacers/standoffs between the rail buttons and airframes
so that the buttons are far enough out to clear the coupler. 1/4" isn't that
far.
-Payton
---- On Fri, 11 May 2018 08:25:55 -0700 terry@xxxxxxxxxxxxx wrote ----
Hello all,
I'm hoping you can give my son & I some feedback on our 2-stage rocket
design. The short version: I'm worried it will not sit down all the way
on the launch rail, and thus will have a higher probability of launching
not very straight. This is kind of a long email but please bear with me.
We have 2 LOC Precision 3" rockets (an IRIS and a Black Brant X) that we
used for our HPR Level 1 certs. Rather than buying a real 2-stage
rocket kit, we said "let's just stack them on top of each other!" What
could possibly go wrong? :)
The attached photos show the setup. The lower half of the BBX is on the
bottom i.e. it's the booster, and balanced on top of that is the entire
IRIS. Each has a main chute only, and each will use an I-180W motor or
similar, using the motor's ejection charge to deploy the chute. The
booster stage will be ignited by launch control and the upper stage will
be ignited by a MissileWorks PET2+ timer. There's also an EggTimer TRS
in the payload bay of the upper stage.
So far so good. Now we come to the problem with the rail. We took a
blue tube 3" payload pay and extended it with another section of blue
tube 3" coupler. This we stuck between the halves i.e. down into the
BBX lower half and up the aft end of the IRIS. It fits but the
coupling point between the two rockets was not strong enough IMHO and
would probably bend & detach during the booster stage acceleration. In
the LOC design, the aft motor mount centering ring is only 1/2" from the
aft end, so the blue tube coupler goes only that 1/2" up into the IRIS.
I don't trust a 1/2" of blue tube to hold up 5 pounds of rocket under
acceleration!
But, we have a 3D printer and when you have a hammer, all things look
like nails, right? So we 3D-printed a custom coupler -- it's the white
thing in the attached photos. Basically it's a solid ring with 4
"fingers" sticking up which go between the fins of the IRIS, holding it
in a loose slip fit, enough to keep it upright during the booster stage
but letting it pull loose during 2nd stage ignition. The solid ring
part is epoxied to the payload bay / coupler section and the fingers
point upward, holding the IRIS.
After all this was assembled I realized, much to my dismay, that I had
made a serious miscalculation. Since the ring is solid and about 1/4"
thick, it would hit the launch rail. Or more to the point, our combined
rocket would not be able to slide all the way down the launch rail when
getting ready to launch. It would stop where it hit the coupler ring.
Now my question is (finally, I get to the point) -- is this a problem?
Will we will have enough rocket on the rail for it to launch OK or do we
need to redesign?
The ring is 42" from the bottom of the combined rocket i.e. the aft end
of the BBX. The result is that only the BBX lower half would sit on the
rail; the ring would sit on the top end of the rail and the entire IRIS
would be above the top end of the rail. The BBX lower half has 2 rail
buttons at 12" and 32" down from the ring. So, it would still have 2
rail buttons on the rail but there would be the whole IRIS (about 5 feet
tall) above the rail. The total length of the combined rocket is about 104".
What do you all think? OK to fly or potentially too unstable and dangerous?
We could re-do the coupler section to not be a solid ring and thus let
the combined rocket slide down on the rail all the way. What we'd do is
have a cutout for the rail, and rearrange the "fingers" so there isn't
one right in the center between the fins on the rail side. the drawback
of this is that the ring would not be as strong and the fingers, not
being evenly spaced, would not provide as much stability as our current
design.
Comments? Suggestions?
Thanks everyone for your help!
Terry McKiernan
