No, I don't have a separate parachute deployment charge as a backup. I
guess I could add that since I have both a PET2+ and an EggTimer TRS in
the upper stage. The PET2+ does have separate batteries for its power
and the igniter power, so it should not brown out, but I suppose I could
wire an ejection charge to the TRS as a redundant option.
Hmm, this rocket is getting more complicated by the minute :)
Terry
On 5/11/2018 8:16 PM, Andrew Wimmer wrote:
Usually motors take long enough to light that if you put your separation charge on the same channel as the motor, the separation charge will get the first stage clear in time.
I may have missed it, but do you have a separate electronic backup on the upper stage parachutes? Always a good idea in the not unlikely chance the second stage doesn't light. Even better if it's a separate unit from staging as sometimes electronics will brown out and malfunction when they light the motor (if the igniter wires short before the electronics shut off the current, bad things can happen)
-Andrew
Sent from my iPhone XcB
On Fri, May 11, 2018, 8:10 PM Terry McKiernan <terry@xxxxxxxxxxxxx <mailto:terry@xxxxxxxxxxxxx>> wrote:
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
