Build Thread - Long Burn 129

[DaHabes]

So, now that I finished the E-Bay, it became apparent that because more than half of it extends into the airframe, given the nose cone challenges, the body tube is going to be too short to fit in the laundry. Plan B, cut out a longer airframe out of Blue Tube (again, in inventory) and recut the fin slots. With BT, I cut the lengthwise cuts first, but since it is so hard to cut, I cut the slot in the middle and pealed the ends up. This separated the ply's and made cutting the ends out much easier. Again, xActo Z blades to the rescue.

 

[DaHabes]

The next step was to install the lower rail button. I need to do this before installing the MMT as it site between two of the fins. I like to use Apogee's rail buttons because the come with weld nuts and machine screws and not wood screws. I'm not fond of screwing something into the centering rings, if possible. I don't want their structural integrity compromised in any way. The Apogee buttons are also countersunk so the machine screws are flush to the top to the rail button. It's a cleaner look. I just use 5-minute BSI for this since there are no stresses to worry about.

 

[DaHabes]

Next up is the MMT. I picked up a tip in TRF from one build thread on using epoxy dams for the TTW fins. I used to leave the back CR off and epoxy the fins to the MMT from the arse end of the rocket, but that's a PITA, I never get an even spread front to back and I tend to use too much epoxy because it's not easy to spread those internal fillets. The epoxy dams let me dd the epoxy from the fin slot, use the exact amount, and get consistent application from front to back of the fin tab.

The finished MMT is shown below. I cut wooden cocktail stirrers to the right width and length and tack them to the MMT and CR's with CA.



The forward CR gets dstopped by the rear rail button, so I Dremel it down so it can slip past it.



I next dry fit the MMT into the airframe and make sure I have a good fit, sanding down the CR's and dams as needed.



Next, I add the rear CR to the MMT and epoxy the Aeropack retainer to the MMT with JB Weld. I create a fillet between the rear CR and the retainer.



Last task on this end is to run a bit of epoxy on the inside edge of the MMY between the epoxy dams and push the MMT into place, checking to make sure the epoxy dams are centered properly on the fin slots. Let dry.

 

[DaHabes]

Next step is to epoxy the top of the forward CR. This isn't easy in a 38mm airframe. Since the fin can is going to be rock solid with the TTW fins and epoxy dams, I just need an epoxy dam on top of the CR to anchor it in place. To accomplish this, I taped two paper straws together that would reach down to the CR and ran laminating epoxy down the tubes to create the epoxy dam.

 

[DaHabes]

Next step was to install the fins. I used West Systems epoxy with just a bit of colloidal silica in the mix to thicken it slightly, filled the epoxy dam with the epoxy, pressed in the fin (which squeezed out an excess epoxy, either to the inside of the MMT or at the external fin joint - I wiped off the external stuff), then clamped the fins with my Badass build guide. I think I did one fin at a time, but could easily have done all three with this method.

 

[DaHabes]

Next step was to install the upper rail button. This was also a bit of a pain because of the small airframe. It took a couple of iterations. I first drilled the hole, then created a tool out of cocktail stirrers and masking tape, taped the weld nut to the tip, with doubled over tape, ran the weld nut down to the hole, stuck the screw into the hole to grab the weld nut, then screwed it in a bit, pulled it tight with a pliers and used the tool again to run a bead of 5-minute epoxy over the back of the weld nut.

 

[DaHabes]

Next up, external fin fillets. Standard procedure - determine size of fillet (I do 5% of the root chord, generally), mark lines with carboned paper and your fillet tool of choice, mask, mix and pour the epoxy, spread the fillet, pull the tape, wait. Repeat. Repeat.

 

[DaHabes]

Almost done. Time to wire up the altimeter sled to the terminals. Normally, I prefer to use ferrules on my connections. Tinned wires can still fray or get wonky over time and as I will need to be able to detach the sled from the sleeve, I need solid connection points that will last. (The beauty of using this design is that one sled with altimeter and finder can be used in multiple rockets rather than being dedicated to a single rocket - the sleeves come in 38mm, BT-60, 54mm and 65mm. I have also designed custom bulkheads for 3" and 4" rockets, so I can literally use on altimeter and finder combo in about a dozen rockets I've designed).

Alas, as the pic below shows, the ferrules stick out too far and won't go into the nose cone. The tolerances are that tight. Tinned wires it is for this bulkhead. Bigger birds will use ferrules.

 

[DaHabes]

Build complete.

 

[James Kobel]

I agree the G8 is too low of thrust. That engine is really intended for boost gliders. The G12ST sims out OK in my build with low wind speeds. How many calibers is the mod above? I think you ought to be able to cut the body tube down quite a bit. Mine is coming out to 28" long (just enough room for the parachute).

I'll post my Rocksim file after I finish this build thread.

Yea, I think the model was well over stable just so we had no issues with stability. I like the idea of having just enough room for everything. It takes some careful designing and engineering. I think a good goal would be trying to reach 10K feet!

 

[James Kobel]

Build complete. View attachment 502046

Where are you going to fly it?

 

[David_Stack]

Nice design and build Mike!

Question, I note that the fin leading/trailing edges have no airfoil/taper work done. Do you find it easier to shape those surfaces post-install? I've never attempted to do so and would think it a 'handful' to manage the airframe while shaping fins, but then again that is only supposition...

 

[DaHabes]

Final step. Ground testing.

My goal here was to use a JLCR because this rocket can hit 6,000' on an H13ST. 38mm is a big maybe when it comes to using a JLCR. In this case, because this is a high-powered rocket with 29mm motors and a fairly heavy e-Bay, it requires (per sim) a 21-24" parachute to come down at a reasonable speed. It was also not going to possible to fit in a nomex protector, so a lot of dog barf was in order.

BP estimators came up with .83g of BP, so I planned to test with 3/4 g and move up. I used a TFR 21" TM parachute for the test.

First test - failure. The parachute hung up at the top of the airframe. Because the parachute area is pretty short, the dog barf did not fully protect the parachute and it took some damage (sigh).

2nd test, 1g - failure. Same result, parachute is thoroughly toasted.

3rd test, 1.5g, blow it up or blow it out - failure. Same result, parachute is not thoroughly roasted.

Conclusion: JLCR is not feasible, it is just too tight. Dog Barf is non-starter, the laundry area is just too small.

This bird is going to require a really tight packing parachute. It is going to require a nomex protector that completely engulfs the parachute.

Spherachute Ultralight to the rescue. I packed a 24" UL in a 6x6 nomex blanket and rolled it ultra-tight. The fit was good. I did add some dog barf above the nomex just in case there were any leaks. I tested using 1.25g of BP.

View attachment LB 129 Ground Test #4 - Success.mp4

Success. Full extension of the shock cord, very energetic, parachute had no damage. This is a good result since the UL's melt if any hot gasses get near to them (don't ask me how I know, you already know how I know). I can probably get by with a bit less BP and may retest when the weather warms up. It's winter in the Midwest and I almost froze all sorts of appendages to run this test. At his point it can fly.

It will require a really calm day to launch, since there is no JLCR (at least it has a tracker). I might even be able to paint it before it flies.

 

[DaHabes]

Nice design and build Mike!

Question, I note that the fin leading/trailing edges have no airfoil/taper work done. Do you find it easier to shape those surfaces post-install? I've never attempted to do so and would think it a 'handful' to manage the airframe while shaping fins, but then again that is only supposition...

I decided to leave them un-tapered. I have jigs to use to do the taper, but I have found that when I do that to plywood, the edge gets very weak. It either requires not going to a full taper and doing final rounding by hand (ugh) or using CA on the leading edge, but I have done that before with caried success. The leading edges often get nicked up, regardless.

Because I was potentially pushing Mach 1 on high thrust motors with this design, I decided to leave them square. This sims out to between Mach .97 and Mach 1 on a H195NT depending on starting altitude and temperature. I expect it won't go above Mach .85 or .9 with the un-tapered leading edges, but it will be instructive none-the less. Such is life.

I have a similar scratch design (2.2", 29mm MMT) with un-tapered leading edges. I intend (at some point) to build an identical airframe with a 38mm MMT and tapered leading edges (they will use the same nose cone and E-bay). Then I can basically do a side-by-side comparison of tapered vs. un-tapered (adapt the 38mm down to 29mm). I intend to find out just how much difference a tapered leading edge makes.

So much to do, so little time...

 

[DaHabes]

Where are you going to fly it?

My main club is Michiana in SW Michigan. If I don't get a calm wind day this spring there, I may try a QCRS launch this summer or wait until the fall.

 

[teepot]

I use a long hemostat to place the rail button nuts in. It's nice because it locks the nut in it's jaws.

 

[Buckeye]

Build complete. View attachment 502046

This looks like a heavy build - thick fins, lots of metal and plastic. Is the 484g in the RockSim file the final weight? My similar build, based on the LOC Magnum Park Flyer, is 100 grams less, including paint and decals. Even so, the liftoff on the H13 was a little iffy, even with 3m of rail and near zero wind.

Dual deploy is often not worth the trouble on little rockets like this. They drift a long way before the main chute is deployed anyway. Since you need very low winds to get this guy in the air with good stability and low weathercocking, drift is also likely low. I went with altimeter apogee deploy, 0.3 grams of BP, the stock 15" cheapo chute, and a tracker. It came down on frozen ground a little hotter than I liked at ~29 ft/s, but survived unscathed. Next time I will up the chute to 18"

 

[DaHabes]

Yeah, it's heavier than i would like, but it still Sims out OK. It's not dual deployment, it's Apogee deploy with a secondary charge in case of a bad e-matxh or the base charge is too small. I considered using an Eggtimer Apogee, but it really won't fit in a 38mm nose cone. I also wanted a tracker since it is a small rocket and will go well out of sight. This was a design challenge with a lot of compromises needed. It is what it is.