Discussion in 'High Power Rocketry (HPR)' started by Bat-mite, Apr 17, 2019.
That's a lot of holes
sure is. What's up with that?
If your compartment is 51" long and 7.75" ID I work out around 4.5g of BP as what I would use as a starting point. Your 8g canisters might be ok I think.
[EDIT] Just rechecked my numbers with the correct formula. 18g would be my starting position. Sorry for the confusion.
Good luck with the project. I lit an O3400 recently and it was a nice feeling .
Would you mind showing your numbers? What psi? I use three #8 shear pins on my big rockets. What shear strength are you using?
Two for the 3/4" switches and three for vents, as I described in my post. I don't understand what is confusing about that?
diameter x diameter x length x 0.006 = grams BP. From the Mark Canepa book. Sizes in inches, BP in grams. Pressure I usually never bother with as it can be a bit variable, depending on setup. Ground test before flight .
Apologies for the confusion in post #63.
I usually use two M3 nylon shear pins.
Got some paint on.
Went over Scott's house last night, and it took us an hour to get the stuck coupler out of the booster extension. Once it was out, a slide test revealed that it is fatter on one half. Since its end is to be permanently epoxied into the extension, problem solved. I will smear it with a thin layer or epoxy and pound it back in. That sucker won't go anywhere!
The other side will be bolted onto the booster, and it slides in no problem.
I've been busy! The wife and kids have been away a few days, and with the holiday and all, I got a lot done.
The AV bay is finished. (Sled and electronics will be shared with my other two big rockets.)
The booster has been drilled for #10-24 screws into the coupler, and #8 shear pins drilled and tapped. The outside hole is a 1/16" larger for easier alignment.
The nose cone is finished, tapped for #8 shear pins. More pics after I remove tape.
Dry fitted the pieces just for grins. That's only half of it!
It’s coming together nicely!
Thanks for posting the progress pics.
nope, aint nothin wrong with that!
very well done
I am really struggling to get a grip on how tall this thing is going to be!
Be sure you read Robert Galen’s article What Barrowman Left Out. http://argoshpr.ch/joomla1/articles/pdf/sentinel39-galejs.pdf
You might have a longneck rocket there that might be butt heavy with an O motor.
The gist of the article is a longneck rocket requires the CG to be a bit further forward
for stable flight. I built a longneck Wildman rocket and had a bit forward CG as suggested and had
a bit of concern with the “smallish” fins and long length. A Loki L1400 pushed it to 10k and it went like
a bat out of Hades as straight up as can be.
As it turns out, the sustainer is so long that it could hold a 54mm Loki M, the CG is still acceptable but there
is no place nearby where it wouldn’t bust the waiver. Best of luck with your project. If you’ve already read the article, my apologies for my presumptiveness. Kurt
Thanks for the read. My sim shows the stability margin as pretty big with the O motor. I don't know if you read the whole thread, but the motor will be a six-foot long 98mm job. Because the motor is so long, its weight is spread out almost to the middle of the rocket. I would be more nervous with 6" O, which is why I'm not going in that direction.
With that amount of cash outlay, Ken & Curtis would have been immediately informed of the error and replacement parts would have been requested, on their dime !
If the parts were unusable, no doubt. But I just moved the extension from the front to the back, and after thinking about it some, I like it better this way.
You were fortunate that it worked out, this time.
Great. Hope the flight turns out well.
Got some trim on.
Once I get clear coat on this upper half, it will be flight-ready.
And you may ask yourself, why isn't he building the booster? Ah, just saving the hardest for last. It's coming.
Just a little update ....
I clear-coated the upper half (shown above), and started work on the booster. I widened the fin slots and designated a fin for each slot. I checked fit of the CRs into the booster tube, and found that one of them is perfect, and the other three are a little too big. Will be sanding down. I measured for rail buttons and marked off the holes. I drilled through the four CRs for threaded rods (#10-24), and for two U-bolts (5/16").
I decided that the included 30" MMT was just too short, so I ordered a four-footer from Ken, which is shipping out today. Once I have that, I can check fit on the CR centers to the MMT, and start sanding everything.
I'll add some pics when I actually have something interesting to show.
Interesting, what were your hesitations with the 30" mount?
30" should be perfectly fine for a 4" motor in a fiberglass rocket. My understanding is there is not much, if any, torsional forces on the casing. It is almost entirely vertical, unless the nozzle is misaligned or if there is a CATO, which the length of the motor mount would not help with. A longer motor tube will also just add a lot of mass in the rear, which could cause more problems with tail weight. Then again, the motor is pretty heavy. Thrust Curve says an O3400 is 37 pounds, and this isn't an altitude bird so some support might be a good idea for the sake of overbuilding.
Maybe you could make another short motor support up higher (around 12" long is what I am thinking) with two centering rings if you want extra support for the motor case, this would save a bit of mass from the extended motor tube but provide the leveraged support on both sides of the motor cases Cg. It provides the same amount of support as a longer tube but saves weight.
Disclaimer: I in no way mean to pretend to say I am in any way near as experienced as you or have ever attempted an O motor build, this is just what comes to my mind when I think about lengthening the motor mount.
Having a six-foot long, 40-pound motor only supported at the lower 42%, added to the need to pressurize 2453 cu. in. of tube for the drogue charge seemed too risky. By going with a 4-foot MMT, I'm supporting 67% of the motor and pressurizing 1604 cu. in.
Thanks. Those are good ideas. I have room in my stability margin, and the little bit of extra weight in the rear shouldn't be a problem.
Extending to reduce the internal volume does make practical sense. Heck I have a crazy long stuffer tube in my L2
I'm still unconvinced that 10 more inches of mmt is value-added from a structural standpoint (especially considering how solid you build). Through the wall fiberglass is very strong
Yeah, it was a toss-up. Another, even bigger consideration was that with the 30" MMT, I could not reach far enough into the booster to connect/disconnect the quick links for the drogue harness. I considered just attaching the Y harness while building and then never removing it, but with the 48" MMT I can connect and disconnect with ease. This is going to work best for me, I think.
That rocket is looking good!
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