Back in Black - 2.26" / 57mm semi-scratch build

I've called this project 'semi-scratch' because I won't be making the NC from scratch. The project is emerging from a combination of experimentation and learning new skills, questions I've been asking for a while on TRF, and thinking through the various parameters I have to work with - field size, opportunities to fly high and fast (limited) and my current L1 certification. Some of the parts have been garnered as the result of initial experiments with a range of materials new to me – mainly Carbon Fibre. Where it seems to be ok, my aim is to incorporate the results of these try-outs into the design, the aim of which is build a robust 2.26” diameter rocket capable of withstanding repeated Mach transition flights. As much as possible, I’ve been designing the rocket around the projected capabilities of four and five grain I motors. After he saw me unrolling the CF fabric, my son suggested the name Back in Black in reference to (what for him) is the ancient folk song by those wandering minstrels going by the name of AC/DC.

The OR design I have come up with is reasonably conservative in shape – 120 cm / 48” long airframe with four clipped delta fins, and drogueless dual deploy. The field I normally fly in will not accommodate a supersonic flight. A CTI I540 should push it to just over Mach 1
I've previously posted my OR file under a post entitled 'Transatlantic arrival'.

Some time ago I joined 2 lengths of Estes BT70 tubing I had spare with a PML coupler using Gorilla wood glue. As an experiment, I decided to give the result a single wrap of two part epoxy and some fibreglass mesh which I had lying around. The mesh was of interest to me as it's sometimes used as a substrate in house building, and is more or less impossible to rip by hand. I regularly checked there was no bending of the tube using an angle iron. The result had very slight flex but was clearly a lot stronger. After filling pin holes with Isopon body filler and sanding, I have given this tube a second wrap of 2/2 twill 3K CF from Easy Composites here in the UK. Easy Composites CF systems are relatively odourless, which is great. The procedure involves mixing and applying a two part black basecoat epoxy directly to the object to be wrapped, and then setting aside for three hours to go to tack. Whilst this is in progress I measured and cut a sheet of CF, using strips of folded over masking tape at the edges to avoid fraying. Once the base coat was at the tack stage, one simply lines up and roll the tube. Two coats of clear top coat is then brushed on, allowed to cure, before more progressive sanding and clear coating. The result seems to be a very strong, rigid airframe, which I'm now in the process of reducing in length (to save weight), slotting and sanding.

That is going to be one fine-looking, tough rocket.

Igotnothing said:

That is going to be one fine-looking, tough rocket.

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Thanks - I certainly hope so.

The internal diameter of the BT70 tubing is somewhat wider than the OD of the LOC coupler I am using as the basis for the avionics bay. So I've been building the latter up until the two make a snug fit using layers of brown paper gum strip. My bay design includes a switch band, so I've also been building up by a similar technique, using alternate layers of good quality, carefully cut and measured cartridge paper and aliphatic resin wood glue (Gorilla). I used epoxy diluted with isopropyl, to initially seal it, but Gorilla glue applied with a fine brush then gently heat cured in the boiler cupboard seems to work very nicely. To secure the payload bay to the coupler I ordered some neat little hex drive M3x8 steel countersunk machine screws, and secured them with tee nuts. The backing for the tee nuts is stacked squares of mount board secured with epoxy. After drilling and countersinking the holes I tidied them up with my needle files. The final picture in this post shows a dry test fit with the RW NC. Naturally I was concerned whether the two would mate ok but they fit fine.

Although I haven't posted for a few weeks, I've been busy refining the AV bay design and build, so this post is something of a catch up.

The AV bay design I've come up with utilises a single offset alu all thread, with a nylon washer and eye nut at each end.

My recipe for homemade charge wells comprises the screw cap end of key-chain pill cases secured to brass tubing using JB Weld. I drill a small hole at the base, and work it into a slot shape with an oval section needle file to accommodate the igniter. This has worked ok before. I've made stepped bulk heads from laser cut disks of birch ply, then drilled holes for the little keychain 'post' to fit into. These are secured with more JB Weld.

Once I'd cut and installed the alu all thread, I realised the was the potential for quite some amount of twisting, so I added a locking male/female mechanism consisting of little alu threads and alu tubes epoxied at each end, to lock the bulk heads into position and stop them rotating. As the eye nuts are both holding the bulkheads in place and serving as the connecting loops for the recovery train, I was aware the releasable eye nut could easily rotate free during descent, so my proposed solution to this is to stop it rotating by looping a thick cable tie around it and a small section of alu tubing secured to the bulkhead, as shown.

I've made the AV sled from a piece of carbon fibre fabric layered with epoxy. This will probably need some stand off insulation as I realise the carbon fibre is a conductor. There's enough room on there for a rechargeable 9V battery and a PerfectFlite Stratologger CF.


Any thoughts or criticism welcome...

Fins and CRs laser from 3mm birch ply. The fins will be laminated with CF. The rocket will have four fins, but I like to get a few spares cut just in case.

I've decided to build fin pockets using balsa dams which I will sand and shape to sit between the middle and aft CR. These will be added once I'm sure of the thickness of the laminated fins.

I've laminated the fins with CF, and sanded them smooth. Love the slate like smooth quality. As always I've made slightly more than required (in this case 6) in order to select the best. After close inspection, one of those in the picture was designated a '2' (reject) rather than a '1' because I noticed I'd messed up the bevelling..

Having established the finished thickness of the fins, I've now constructed fin pockets on the MMT. I used balsa, sanding them to shape with a section of MMT tubing covered so they sit flush on the MMT, and sanding to the correct angle to maintain consistency in the slight gaps between the fin tab and the fin pocket dam wall. The balsa was drilled to allow epoxy to seep in and through.

As my son pointed out, the whole thing looks like a cardboard lightsaber...

Some shots of motor mount installation. Having neatened up the slots with my rotary cutting tool, I aligned the pockets with pencil lines on the tube and corresponding marks on a strip of masking tape. As this is my first time making fin pockets, I used 90 minute epoxy to give me plenty of time to watch and check it wasn't somehow slipping out of whack - and correct if it did. I tensioned and taped up the shock cord to keep it clear and ensure that it didn't get caught in any epoxy that was forced through as the MMT was inserted.

The back end fillet was made using epoxy mixed with chopped carbon fibre strands.

That's quite a motor mount assembly! Looking good

Nick@JET said:

That's quite a motor mount assembly! Looking good

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Thanks - yes, I'm pleased so far. The fins are a good fit and make a satisfying 'click' noise when I dry fit them, with just enough gap for the epoxy.

Whilst working on the MMT and fin pockets I came to some decisions about the Nose Cone and Coupler. In the shipment they sent me, Rocketry Warehouse had supplied one 24cm length of coupler tube as part of a larger order. I've cut this piece into two, using the shorter 4" piece to make the coupler. I want to leave the NC space as free as possible, and also to allow the coupler to be be detachable from the NC to allow the future placement of a tracker.Originally I'd had a vague idea of fitting a length of alu all-tread running the length of the NC, from the screw in tip to the coupler bulkhead. This utopian ideal crashed against the rocks of reality when I realised several obstacles: (1) I've only been able to find Alu all thread (or 'threaded bar' as it's called here) in metric sizes, and is incompatible with the thread in the RW/Proline NC and (2) I really didn't like the idea of having metal rod running the whole length of the NC, both for weight concerns, and because it would potentially present problems for electronics or transmitters should I want to fit them later.

The bulkhead is laser cut and drilled to take a short stub of M5 alu all-thread, with a lock nut and washer on one side and an eye-nut bonded with JB Weld on the other. The idea is to allow this to rotate freely. I pre-sealed the outward face of the bulkhead with epoxy, within the pencil circle line, leaving the outer portion bare to allow the coupler/bulkhead fillet to wick in fully.

I fitted the NC to the cut length of coupler, then drilled two holes at each side, wide enough to take M3 tee-nuts. The holes in the NC were then countersunk. The tee nuts have been bonded on the inside of the coupler with copious amounts of JB Weld.

Pictures of the assembled NC/Coupler assembly, with counter sunk hex drive M3 screws inserted flush with the sides of the NC.

Tim51 said:

I've laminated the fins with CF, and sanded them smooth. Love the slate like smooth quality. As always I've made slightly more than required (in this case 6) in order to select the best. After close inspection, one of those in the picture was designated a '2' (reject) rather than a '1' because I noticed I'd messed up the bevelling..

Having established the finished thickness of the fins, I've now constructed fin pockets on the MMT. I used balsa, sanding them to shape with a section of MMT tubing covered so they sit flush on the MMT, and sanding to the correct angle to maintain consistency in the slight gaps between the fin tab and the fin pocket dam wall. The balsa was drilled to allow epoxy to seep in and through.

As my son pointed out, the whole thing looks like a cardboard lightsaber...

Click to expand...

The more I use them the more I like fin pockets, they are so much easier than internal fillets (when needed).

rharshberger said:

The more I use them the more I like fin pockets, they are so much easier than internal fillets (when needed).

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Agreed. Thanks for your earlier advice on your fin pocket thread back in December, by the way, which I'm following here. I think after this experience I'll use them again on future builds wherever possible. I'm going to add a slight external fillet between the fin and the exterior of the airframe, of course, but I hope this will allow me to keep that to a minimum. I've started fin-setting now. I've given the fin tab a generous coating of 90 min epoxy on all sides and edges, and worked them back and forth a little before seating it,hopefully this will help pump the epoxy through the pre-drilled holes, and wick into the raw balsa.

Although the fin pockets make for a reasonably snug fit when inserting the fin tabs, I hadn't routed radial grooves into the CRs at either end of the fin can. I decided to work on each fin individually, and align by eye and angle, rather than using a foam board guide to fix all four at once. This was more a personal choice in terms of gut feeling than for any rational reason - I guess just working with what I know, as it were. I have a pretty steady hand and eye so it worked ok for me. There's no need for external fillets as such, but I used a small amount of epoxy infused with graphite powder, slightly diluted with isopropyl, to smooth over small pits irregularities in the seam between fin and airframe. I've used the same recipe to fill pin holes and slight depressions in the CF wrap during progressive sanding, and also on the leading and trailing edges of the fins. This is cosmetic of course - the brief encounter with very low +Mach speeds will not require ablative as such.

I like the hard glossy black surface created by the epoxy+graphite powder mix so much I decided to paint the Av Bay bulkheads with it also.

Tim51 said:

After he saw me unrolling the CF fabric, my son suggested the name Back in Black in reference to (what for him) is the ancient folk song by those wandering minstrels going by the name of AC/DC.

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First off love the build and your ambition, I really appreciate it when I see someone stretching their abilities and growing their skill set.

As for the name NOOOOOOOO :wink: darn your son beat me to it. I had Mark at stickershock do up the sticker-set below back in 2014. It is for my Madcow Super DX3, which was my first HPR that I picked up, sadly in May of 2013 and still sitting in the bag with a motor mount built and a BT with the spirals filled.

mpitfield said:

First off love the build and your ambition, I really appreciate it when I see someone stretching their abilities and growing their skill set.

As for the name NOOOOOOOO :wink: darn your son beat me to it. I had Mark at stickershock do up the sticker-set below back in 2014. It is for my Madcow Super DX3, which was my first HPR that I picked up, sadly in May of 2013 and still sitting in the bag with a motor mount built and a BT with the spirals filled.

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Very nice image! No worries - the 'Back in Black' moniker was more suggested as a title for the build thread, which my son was pushing me to do. I won't be putting that name on the side of the airframe - so the name Back in Black remains yours (on condition you do a build thread here on TRF about your DX3 very soon :wink

Thanks for the encouraging words about the build. I wasn't familiar with the Madcow DX3 - strangely my design for this build evolved out of an original plan to CF wrap a LOC Vulcanite.. (I don't believe MC rockets are that common here in the UK, although I stand to be corrected if any UK flyers know different). Anyway, it looks stunning and I look forward to seeing it take shape.

Tim51 said:

Very nice image! No worries - the 'Back in Black' moniker was more suggested as a title for the build thread, which my son was pushing me to do. I won't be putting that name on the side of the airframe - so the name Back in Black remains yours (on condition you do a build thread here on TRF about your DX3 very soon :wink

Thanks for the encouraging words about the build. I wasn't familiar with the Madcow DX3 - strangely my design for this build evolved out of an original plan to CF wrap a LOC Vulcanite.. (I don't believe MC rockets are that common here in the UK, although I stand to be corrected if any UK flyers know different). Anyway, it looks stunning and I look forward to seeing it take shape.

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Well as much as I would like to lay ownership on the name, it was Mark from stickershock who came up with the it and I would be very surprised that it hasn't already been used at least once...not that it really matters.

As for a build thread, I have thought about it many times, but I am such a sporadic builder based on my availability and by the time I think about taking pics, cropping pics, and doing up the write up, I am usually onto something else. But I do appreciate seeing what others do and it has bee so valuable in learning the hobby...there are a lot of really good builders and ideas on this forum.

My delay in building the DX can be summed up as ADD. The rocket came with a 38mm MMT, which I built with the CRs and retainer, then I said I want to have the 54 MMT for more options, so I got the parts for that and built it with the CRs and retainer. Then I said hmm I should replace the fins with G10 so I ordered new fins. Of course logically next I then "needed" to reinforce the BT so I got the GLR glass sock. However I couldn't just roll that on and be done with it, I "had" to do it under vacuum which required a mandrel, then I "needed" to...you get the idea.

Sadly the rocket is still in the bag with a sanded and filled nosecone, a spiral filled BT (but not sanded) and a MMT assembled, oh and can't forget the decals, I got the decals done, well I paid Mark to anyway.

BTW where are you from in the UK? I have a lot of relatives there as I was born in Bedfordshire, but 10 months after my parents immigrated to Canada.

My delay in building the DX can be summed up as ADD. The rocket came with a 38mm MMT, which I built with the CRs and retainer, then I said I want to have the 54 MMT for more options, so I got the parts for that and built it with the CRs and retainer. Then I said hmm I should replace the fins with G10 so I ordered new fins. Of course logically next I then "needed" to reinforce the BT so I got the GLR glass sock. However I couldn't just roll that on and be done with it, I "had" to do it under vacuum which required a mandrel, then I "needed" to...you get the idea.

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Yup - I know that feeling! I think it's partly a result of what we all do - i.e. learning as you build - so even with meticulous planning beforehand, better ideas occur once a build has already started. After I'd fixed the motor mount on this one I suddenly wished I'd fitted it with a wiring conduit for airstarts so that at some future time (and once I'd got the certification) I could use this as a sustainer mounted on a larger, 3" or 4" booster... but i guess there's no answer to that except the old adage about life not being a deer hunt and there's no use trying to get everything in one shot.

BTW where are you from in the UK? I have a lot of relatives there as I was born in Bedfordshire, but 10 months after my parents immigrated to Canada.

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I've been in London for the majority of my life now. My family are originally from the towns around Manchester

An update snapshot of this build: the progressive wet sanding of the airframe is still a work in progress, in between other non-rocketry commitments, but it's getting there.

I've yet to wire up the Av bay and decide on a switching mechanism, but there's enough room to install a schurter rotary cam switch so I'll probably go with that.

The off centred all thread Av bay design allows enough space for a schurter rotary cam switch. Over the past few days I've bored out a countersunk hole, primed and sprayed the switch band, and installed the wiring from the interior to the terminal blocks on the bulkheads.

I was surprised how small the static ports are. This is a 6" x 2.14" bay, so the Stratologger CF instructions call for 4 x 0.025" / 0.63 mm holes. I've spaced these evenly around the switch band. They're hardly visible in the photo.

It's been a while since I posted on this project but it was happily nearing completion - as the pictures show, I'd finished the CF wrapped airframe with a blue roll pattern and rattle can of clear lacquer. However, trouble occurred the other day when one of the terminals on the shurter switch snapped off whilst I was loading the electronics... The switch is epoxied in already, so is impossible to reach or re-solder...Hmmm...

Looks like I may have to re-think the whole design of the AV bay.

One option I've thought of is to use a snap-switch and pin design, but (as I understand it) the size of the static ports for this size of AV bay would make it impossible to use the preferred four static port design. I'm concerned that using just one static port (as a snap switch access hole) rather than four would make the altimeter more vulnerable to sudden high altitude gusts and false readings.

Man sorry to hear that. How about drilling the switch out and replacing?

Nick@JET said:

Man sorry to hear that. How about drilling the switch out and replacing?

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Yes I was mulling that option over as I fought back the tears of frustration but I was concerned the drill might crush the coupler tube (basically paper soaked in epoxy) before it cut through the broken shurter.... but hey, nothing to lose now so I might as well give that a try.

Another possibility: I have an extra length of profusion coupler which came with the RW NC, which fits the stepped bulkheads nicely, and over the weekend I made a possible switch band by slicing a section of 2.1" Loc tubing I'd wrapped in CF... it looks neat but adds extra weight, which is getting critical once I add the tracker. Hmm....

Then a third option - I was thinking maybe a length of phenolic coupler was an alternative but wanted to determine the precise OD before ordering some..

It's been a while since I updated this thread. Following the mishap with the broken schurter switch, I've spent some time re-thinking the avionics bay for this rocket to accommodate a snap switch. I decided to construct a sled from birch ply with a hoop which would hold the switch over the sled, allowing one of the static ports in the switch band to act as the access hole for the brass rod switch pin. Although in the previous post I'd considered using a section profusion FG coupler, but I opted for a section of PML phenolic instead.

Hi Folks

A suggestion for your switch challenge. You might want to drill it out - using a forstner bit that matches the size of the outer diameter of the switch. Then , when you have the switch cut to the diameter of the inside barrel, use a regular drill bit.

You might want to fill the switch itself with epoxy, so you have a place to exactly centre the bit in the switch - to get you going on the right path. : )

Cheers

Red Phenix said:

Hi Folks

A suggestion for your switch challenge. You might want to drill it out - using a forstner bit that matches the size of the outer diameter of the switch. Then , when you have the switch cut to the diameter of the inside barrel, use a regular drill bit.

You might want to fill the switch itself with epoxy, so you have a place to exactly centre the bit in the switch - to get you going on the right path. : )

Cheers

Click to expand...

Thanks for your input on this. Pleased to report it's now well aligned and working.

After a long hiatus due to life, hospitalisation and whatever over the past year, this project finally took to the skies last Sunday 2nd April at EARS in Cambridgeshire. This was a shakedown low 'n' slow flight on a CTI H120 RL to check everything worked, including the custom designed, scratch built Av-bay. I'd ground tested it in February and used 1.2g 4F BP in each charge well, which was a little vigorous, but everything's a tight squeeze in this airframe. The NC was pinned with two nylon sheer pins, the booster friction fit with a little more blue painters tape coated in epoxy. Launched about 10 degrees off the vertical into the breeze, apogee was 2502'. I flew it drogue-less with the main deploying nicely at 700'. A satisfying run-through - everything seems ok for more adventurous flights soon.

Excellent photos courtesy Pete Barrett / Hornchurch Airfield Rocket Team hartrockets.co.uk, + screenshot of SLCF data.

A rather sad postscript to this build: I flew this 5/5/18 on a CTI J330 with somewhat alarming results. I'm still trying to work out what happened. As the sequence shows, shortly after leaving the rail the rocket veered sharply one way then the other, doing a pronounced shimmy. The winds were slight, and in the direction of the first veer (you can see the windsock in the first picture). As the rocket ascended it righted itself, so that the last few seconds of the ascent were smooth straight line. The mystery then deepens: despite the almost perfect conditions (we could see the drogue charge smoke at apogee) visual contact was lost during descent, and despite extensive searches in the surrounding fields it remains so. Unfortunately the tracker would appear to have failed, so no data. OR simulation (attached) suggests it would have entered the transonic region (Mach 1.06). So what happened..?



As ever, I'd welcome any informed explanations based on similar launch experiences.