Author Topic: Super Astron Cobra - when one motor isn't enough  (Read 10502 times)

Offline RickB

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Super Astron Cobra - when one motor isn't enough
« on: November 21, 2015, 10:06:34 PM »
I just started cutting tubes for this:


It'll be a 3" upscale of an Astron Cobra.  My intent, for the time being, is to just fly it on BP motors and with that in mind, it might be more of a midpower rocket.  But, it'll have three 29mm mounts, so...

Anyway, parts are starting to come together for it.  The nose cone and transition are coming from Sandman shortly.  I'm undecided if the fins should be light ply or if basswood will suffice.  But in the meantime, I can at least start on the motor mount.
Rick Barnes

Offline RKeller

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Re: Super Astron Cobra - when one motor isn't enough
« Reply #1 on: November 21, 2015, 11:06:27 PM »
nice! with 3 29mm mounts I would go ply on the fins.
Riley Keller
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TRA#15415
L1 25 Oct 2014 Estes MDRM CTI H133
L2 25 June 2015 Rocketry Warehouse DYOK, CTI L1030RL 14.7k ft mach 1.57

I'm addicted to building stuff, rockets are just a way to get high

Offline RickB

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Re: Super Astron Cobra - when one motor isn't enough
« Reply #2 on: November 22, 2015, 09:42:41 AM »
That's what I am thinking, I just have to find some pieces large enough.  That's another drawback to the basswood as well; it doesn't seem to come in sheets large enough for fins this big.
Rick Barnes

Offline RKeller

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Re: Super Astron Cobra - when one motor isn't enough
« Reply #3 on: November 22, 2015, 10:24:59 AM »
I've been using 1/8" ply from michael's with good results. And with a 40% or 50% off coupon its dirt cheap.
Riley Keller
URRG
TRA#15415
L1 25 Oct 2014 Estes MDRM CTI H133
L2 25 June 2015 Rocketry Warehouse DYOK, CTI L1030RL 14.7k ft mach 1.57

I'm addicted to building stuff, rockets are just a way to get high

Offline RickB

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Re: Super Astron Cobra - when one motor isn't enough
« Reply #4 on: November 25, 2015, 10:24:30 AM »
I've started epoxying up the motor mount.  I notched the centering rings to accept the fin tabs, since with three tubes and four fins, i can't really glue the fins to the motor tubes readily.  Or, at least not squarely.  I think I'm going to use 3/16" ply for the fins; for as large as they are, i don't think 1/8" will be stiff enough.



Rick Barnes

Offline RickB

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Re: Super Astron Cobra - when one motor isn't enough
« Reply #5 on: November 28, 2015, 11:00:19 PM »
I wound up buying 3/16" thick ply at Dan's to make the fins. 


And I made an ejection baffle, with the reasoning that I ought to build in as much protection from three ejection charges as possible.  Since I didn't find a ready made kit for Estes 3" tube, I built one from a coupler and bulkheads ordered from BMS:





And, the baffle installed:


Also worth noting - the completed motor mount.


The three rings in place, and the three tubes are siamesed together with epoxy.  With this design, I need to have the rear centering ring in place to install the fins, so I won't be able to apply internal fillets.  But with the fins epoxied into the notches and against the motor tubes, it should be plenty strong.

Rick Barnes

Offline dbmccann

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Re: Super Astron Cobra - when one motor isn't enough
« Reply #6 on: November 29, 2015, 12:59:31 PM »
Could inject internals, but I think you'll be plenty strong without them.  I've stopped doing it on most builds now, and just use huge externals.

Offline RickB

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Re: Super Astron Cobra - when one motor isn't enough
« Reply #7 on: November 30, 2015, 04:27:45 PM »
Maybe, though I'm not sure how you'd gain access.  But like you said, I don't think they're needed.  The fins should have plenty of contact length inside and out, notch engagement and external fillets.  Anything more is overkill anyway, IMHO.  Even with three Estes F15's in it, the total impulse is still only a little less than 150 N*s, so its still in the upper end of the "G" range.  To me, that doesn't warrant being overbuilt.  Of course, if I ever try to cluster three 3-grain motors in it, it would be a different story.  But that's not really what I'm looking at here.
Rick Barnes

Offline RickB

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Re: Super Astron Cobra - when one motor isn't enough
« Reply #8 on: December 02, 2015, 08:42:28 AM »
I got the motor mount epoxied in last night.  Now, I suppose the next big thing is to cut out and prep the fins...
Rick Barnes

Offline RickB

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Re: Super Astron Cobra - when one motor isn't enough
« Reply #9 on: January 02, 2016, 07:19:55 PM »
I finally got back to working on this.  After sanding/filing to fit (each one had a different tab shape due three motor tubes, but four fins), I got them epoxied in last night and today:


Finished assembly:


I tossed the chute & shock chord inside to check CG.  To my surprise, its actually where its supposed to be.  Hopefully, filing and painting won't change that much.
Rick Barnes

Offline RKeller

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Re: Super Astron Cobra - when one motor isn't enough
« Reply #10 on: January 02, 2016, 11:04:56 PM »
that thing is awesome. hard to get a sense of scale until you see it above the counter top. nice!
Riley Keller
URRG
TRA#15415
L1 25 Oct 2014 Estes MDRM CTI H133
L2 25 June 2015 Rocketry Warehouse DYOK, CTI L1030RL 14.7k ft mach 1.57

I'm addicted to building stuff, rockets are just a way to get high

Offline RickB

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Re: Super Astron Cobra - when one motor isn't enough
« Reply #11 on: January 14, 2016, 04:09:04 PM »
I know what you mean, I keep that it looks huge when up to this point it, it only existed on the 10" screen on my tablet.  You never really appreciate the scale in the design phase, not until you see it for real.  It isn't actually larger than my Leviathan (which is taller and 3" its whole length), but the really wide fin spread really add to it's presence, make it look big.  At this point, I'm just waiting on weather for painting, which probably means springtime.  I doubt I'll have it for the first launch in April, though I guess there's nothing wrong with an unpainted test flight...
Rick Barnes

Offline RickB

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Re: Super Astron Cobra - when one motor isn't enough
« Reply #12 on: May 02, 2016, 08:59:28 AM »
Finally an update.  I got this painted over the weekend.  Well, over the last two weekends.  Now it just needs a few decals, rail buttons and to have the recovery harness hooked up.

Rick Barnes

Offline Greg Young

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Re: Super Astron Cobra - when one motor isn't enough
« Reply #13 on: May 02, 2016, 09:14:46 AM »
That's a great paint job on your upscaled Cobra Rick!
Can't wait to see it fly!
Greg

Offline Bob Krech

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Re: Super Astron Cobra - when one motor isn't enough
« Reply #14 on: May 02, 2016, 01:31:33 PM »
Don't forget to cover the tops of the ejection charges of the BP motors with a piece of masking tape just in case all 3 motors do not ignite on launch.  There is always a chance that if one does not ignite on lift-off, the heat of the ejection charges from the other motors can cause the unignited motor to burn backwards, and that can ruin your day.

Bob