URRG

General Rocketry => High Power => Topic started by: RickB on November 21, 2015, 10:06:34 PM

Title: Super Astron Cobra - when one motor isn't enough
Post by: RickB on November 21, 2015, 10:06:34 PM
I just started cutting tubes for this:
(https://c1.staticflickr.com/1/731/22897789370_733a73b1a9_b.jpg)

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.
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: RKeller on November 21, 2015, 11:06:27 PM
nice! with 3 29mm mounts I would go ply on the fins.
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: RickB 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.
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: RKeller 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.
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: RickB 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.

(https://c1.staticflickr.com/1/616/23264385466_536611a29e_b.jpg)

(https://c2.staticflickr.com/6/5714/23290525335_f08c0cdbf9_b.jpg)
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: RickB on November 28, 2015, 11:00:19 PM
I wound up buying 3/16" thick ply at Dan's to make the fins. 
(https://c2.staticflickr.com/6/5751/23089404700_f4ddf11f7b_b.jpg)

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:
(https://c1.staticflickr.com/1/622/23221793172_f88aa01376_b.jpg)
(https://c1.staticflickr.com/1/782/23034272790_ddebc00fc6_b.jpg)
(https://c1.staticflickr.com/1/769/22962146759_4d1606d88a_b.jpg)
(https://c2.staticflickr.com/6/5813/23230000552_89a1437c4c_b.jpg)

And, the baffle installed:
(https://c1.staticflickr.com/1/661/23017212519_403f077a94_b.jpg)

Also worth noting - the completed motor mount.
(https://c1.staticflickr.com/1/763/22709957894_654428e7be_b.jpg)

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.

(https://c1.staticflickr.com/1/681/23017216809_87600ecca9_b.jpg)
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: dbmccann 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.
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: RickB 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.
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: RickB 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...
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: RickB 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:
(https://c2.staticflickr.com/2/1479/24027651162_f3a82284a4_c.jpg)

Finished assembly:
(https://c2.staticflickr.com/2/1475/24109616176_2fde7291fe_b.jpg)

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.
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: RKeller 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!
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: RickB 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...
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: RickB 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.

(https://c2.staticflickr.com/2/1504/26770868395_061ed34e6d_b.jpg)
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: Greg Young 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
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: Bob Krech 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
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: RickB on May 02, 2016, 02:43:52 PM
That's a great paint job on your upscaled Cobra Rick!
Can't wait to see it fly!
Greg
Thanks, Greg.  I expect this will be a Potter-only (non-Geneseo) flyer...
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: RickB on May 02, 2016, 02:46:45 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
Interesting though, Bob.  I hadn't considered that will previous clustered rockets.  How, though, would the ejection charge of one motor light the top of a previous unburned motor, it there's a clay cap at the top of that motor?  Am I missing something?  It seems like the cap would provide better protection than a piece of tape...
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: Bob Krech on May 02, 2016, 04:39:21 PM
It normally does, but a small fraction of the motors can have a thin or porous clay cap and therein lies the problem.

Bob
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: Greg Young on May 03, 2016, 08:21:31 AM
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

.. not to mention your rocket! :)
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: Larry Weibert on May 18, 2016, 10:37:11 PM
Some things in life you just don't think about till someone tells you. That is a great tip thanks Bob
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: RickB on June 27, 2016, 02:17:47 PM
I finally made the first flight of this rocket on Saturday at URRF3, when the winds were relatively calm.  All three E16-6 motors lit (after multiple continuity checks on black-tip ignitors), and it made a good flight.  The only issue was that my altimeter didn't get a good reading; I'm still learning to use it properly.  However, the Chute Release did it's thing, and it landed close by.  I was able to set it up on the pad just before going on duty as Low Power manager, so I was able to watch it launch fairly up close.  The landing was within the low power pad area as well.  I'll post some photos once I upload them all to flicker...
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: RickB on June 29, 2016, 12:50:39 PM
I finally made the first flight of this rocket on Saturday at URRF3, when the winds were relatively calm.  All three E16-6 motors lit (after multiple continuity checks on black-tip ignitors), and it made a good flight.  The only issue was that my altimeter didn't get a good reading; I'm still learning to use it properly.  However, the Chute Release did it's thing, and it landed close by.  I was able to set it up on the pad just before going on duty as Low Power manager, so I was able to watch it launch fairly up close.  The landing was within the low power pad area as well.  I'll post some photos once I upload them all to flicker...
First flight photos:
(https://c2.staticflickr.com/8/7438/27981872155_738a560502_b.jpg)
(https://c2.staticflickr.com/8/7301/27702052900_e248dd9f73_b.jpg)
(https://c2.staticflickr.com/8/7311/27947574846_302e83b055_b.jpg)
(https://c2.staticflickr.com/8/7087/27880083702_e4a8eae1cb_b.jpg)
(https://c2.staticflickr.com/8/7369/27947570856_6ec3cd1ca8_b.jpg)
(https://c2.staticflickr.com/8/7512/27368906464_d99cf3dfd3_b.jpg)
Title: Re: Super Astron Cobra - when one motor isn't enough
Post by: RKeller on June 29, 2016, 12:57:40 PM
love that last shot! very cool flight Rick!