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> 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>
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
