Any reason why you couldn’t just put the rail buttons on standoffs, like they
do with Nike-Smokes?
- Greg
On May 11, 2018, at 11:25 AM, Terry McKiernan <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
<2-Stage Rocket.jpg><2-Stage Rocket Coupler.jpg><2-Stage Rocket Coupler
Attached.jpg>
