Thanks everyone for the suggestions. Sounds like the rail button
standoffs are the simplest solution, but I might also notch the coupler
like Rich said, or maybe both to make sure it stays clear of the rail.
I'll think about adding an ejection charge to separate the stages. I
have a 2nd PET2+ that I could put in the interstage payload bay, or I
suppose I could use the 2nd pyro event of the PET2+ that's already in
the upper stage.
Thanks again
Terry
On 5/11/2018 9:08 AM, John Coker wrote:
I like the idea a separation charge. If the motor start pushes out the booster, it will coat it with soot and may even char it. (I had this happen to the two booster stages of the Comanche 3 upscale. All stages were 4" diameter and so the boosters didn't drag separate.)
John
On Fri, May 11, 2018 at 9:04 AM, Kurt Gugisberg <kurtgug@xxxxxxxxx <mailto:kurtgug@xxxxxxxxx>> wrote:
Yep, standoffs for the rail guides. I do have some doubt about
the 3D printed interstage coupler working. Personally, I think
that half inch of coupler, if it is a tight fit, may work just
fine by itself. If you have a separation charge to blow it apart
then then you don't have to worry about it being too tight. Or
just let the motor starting up separate it.
Kurt
On Fri, May 11, 2018 at 8:25 AM, Terry McKiernan
<terry@xxxxxxxxxxxxx <mailto: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
