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Build thread: 1 x 54mm + 2 x 38mm in a 5.54" airframe

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Tim51

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So I finally launched Clear Air Turbulence yesterday, after a series of cancellations due to freakish weather over the past few months. Tracker was an Egg Finder 'classic'. Apogee was 3215'. As stated earlier, the original plan was to fly on a J449 core with two I55 outboards igniting at 3.5s, but there was some concern over whether two long burn motors with a relatively heavy rocket would lead to the rocket arcing under power, and landing outside the range, so I swapped the I55's for 2 x H225s, and reprogrammed the mDACS for airstart ignition at 2.9s, immediately after the J449 burnout. At first sight it seemed everything had gone perfectly - straight boost, good dual deploy recovery. The obligatory DRP:
DRP.jpg


And a nicer one:
On the pad.jpg


Conditions were ideal with wind at around 2mph. The initial boost:
J449 burn.png

Airstarting took longer than expected, but produced a satisfying throaty roar. I was surprised how loud even an H motor can sound when the noise is coming from over head - and I say 'an' deliberately, because it became clear afterwards that only one had ignited...

H225 burn.png

Apogee event. I used a Loc Angel 24" drogue
Booster sep.png



And here's the main (Rocketman 6') inflated after deploying at 800':

NC and Main.png


Why exactly only one of the two H225s ignited is still something I'm looking at. At first I thought perhaps one of the motors had come up to pressure before the other, ripping out both wires.. or was there too much power drain on the 9V battery to ignite both ? Curiously the nozzle cap was still on the unburned motor when it landed, albeit somewhat melted, but the igniter wire (finger loop and all) had been whipped away. You can see in the onboard footage they broke away at apogee.

The second problem was internal damage to the MMT CR/ Bulkhead. I admit I was liberal with the BP, and used 4g. I had to remove the motor charges anyway, relying on electronics, and I added a bit extra to all my charges from those. I'd also shear pinned the booster section, which was necessary because of the extra weight of 3 motors, and I thought I had calibrated correctly. However the resulting punch broke the CR:

MMT CR damage.jpg


This is going to be tricky to fix. Normally I would simply saw the airframe below the CR, and use a coupler to connect a new section of airframe. However, the outboard 'nosecone' fairings extend beyond the damaged CR point, and would make this difficult. I will try - I hope to fly this again, this time using a larger core motor, with the I55s, hopefully at IRW in Scotland in late August.
 

mbeels

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Thanks for the update, I had been anticipating some launch photos. Did the boost continue to be straight even with one outboard motor?
 

Tim51

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Thanks for the update, I had been anticipating some launch photos. Did the boost continue to be straight even with one outboard motor?
Yes more or less. No sharp turns, just a very slight wobble. I suppose the combination of the big fins and the short burn kept it going upwards.
 
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Tim51

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Luckily, the damage to the forward CR was relatively easy to repair: I realised I could poke a dowel up through one of the outboard motor mounts and push the CR straight, before reinforcing with segments cut from a spare CR, carbon fibre strips, filler and epoxy. It ain't pretty, but it's proven to be solid:
20190818_120536.jpg


That done, I flew this project again at the Scottish Aeronautics and Rocketry Association's excellent annual International Rocketry Week over the weekend, this time with a CTI K630 serving as the core motor, and two I55 outboards:

Clear Air Turbulence 250819.png


This time both outboards lit, all from one 9V battery. I'd reprogrammed the RRC3 using the mDACS to light at T+2.2s, whilst the K630 was still burning, but there was a satisfactory short interval between burnout of the core and the two outboards coming up to pressure. Here it is back on the workshop bench at the IRW basecamp after the flight:

20190825_215851.jpg


A few observations on the flight, and this design as a whole:
1) The repaired CR held up fine.
2) The altitude was less than predicted. The rocket weathercocked into the wind whilst the outboards were burning, sending the rocket arching down range, and giving a total altitude of just over 5200' (I'd expected around 7000+). Luckily, once it was under drogue, the prevailing winds blew the rocket back in the opposite direction (main deploy was 800') so it was not a particularly long walk.
3) More seriously, the payload bay partially zippered (again). This seems to be due to the hefty weigted NC, which is clearly a design weakness. I have a longer section of Loc 5.5" tubing handy, so I'll re-think this problem over the winter, and probably try make a new stronger one with a kevlar / CF wrap rather than just the thin layer of 200gsm FG.
However, as I said earlier in the thread, my approach to this project was to view it as a kind of 'test vehicle' to learn from, and I feel this has been a really worthwhile project in developing my experience and knowledge. It's a fairly complex project that I can tinker with and refine as I progress.
 
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Tim51

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Why not bring the NC down separately?
Thanks - good idea - that's certainly an option worth exploring - there's probably room for two chutes in the payload bay. As I said I'll probably not fly this particular project again this season so I'll give that some thought over the next few months.
 

Tim51

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Finally had some time to download the RRC3 data from this second flight, with a recorded altitude of 5319':
Clear Air Turbulence RRC3 26 Aug 2019.PNG


By way of comparison this was the data from the earlier flight in July, (J449 core, only one of the two H225's lit):

RRC3 7 July 2019.PNG

Interestingly both flights fell well short of predicted sim altitudes, which is a first for me, since I find OR usually under estimates, but only by a relatively small amount.
 

Tim51

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Just an update on this - one of those rockets where you build it then learn the idiosyncrasies of flying it... to counteract the tendency to zipper, at the beginning of this year I built a new payload bay, this time Loc tubing wrapped in Kevlar:

New payload.jpg


Have to say I don't plan doing that again in a hurry (either laying up Kevlar or repainting the chequerboard...) but whilst I was doing a bit of a refurb I also keyed all the sections of the rocket for ease of alignment. During the original construction I hadn't quite realised just how tricky it would be shearpinning the sections together without alignment guides. I also made an anti zipper device comprising of a foam ball wrapped in nomex, threaded onto the main shockcord. The rocket finally flew again 11 days ago, this time on a K445, airstarting 2 x H225s. I'd originally planned to airstart some 38mm 3G motors but the bottom of the cloud deck was around 6000' so the 2G motors were the right choice, and being White Thunders they gave a nice roar as they kicked in. The averaged apogee from the two altimeters was 5132'. I was very pleased with the flight, all of which was visible. Definitely one to remember:

Clear Air Turbulence 18_10_20.jpg


For this flight I also used a smaller drogue and a slightly larger main (8' Rocketman chute)

CAT recovery 18_10_20.jpg


Padding up I realised I'd forgotten my aluminium tape so used some electrical tape I found in my jacket pocket instead. Not ideal but it worked - a few scraps were still on the thrust plate and the edge of the airframe when it came down.

20201018_133430.jpg


The anti zippering measures worked fine and the keying has certainly made prepping alot easier. After the various running repairs and reinforcements it's a heavy tank of a rocket, but as a project it's been a great learning experience, and it's great fun to fly. [Flight pictures courtesy Pete at H.A.R.T. Rockets].
 
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