Couldn’t you just notch the Coupler Piece enough to clear the rail, but not
completely through the material?
-----Original Message-----
From: roc-chat-bounce@xxxxxxxxxxxxx <roc-chat-
bounce@xxxxxxxxxxxxx> On Behalf Of Terry McKiernan
Sent: Friday, May 11, 2018 8:26 AM
To: roc-chat@xxxxxxxxxxxxx
Subject: [roc-chat] 2-stage rocket trouble with launch rail?
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
