MAC Performance Firestick XL Build

[Buckeye]

Lots of MAC build threads out there. I will add another. This is the 3" Firestick XL with 54mm motor mount. Other add ons for this build:

54mm nose cone payload bay sold by MAC
Slimline retainer
Rail buttons
40" Recon chute

The signature feature of this rocket is the tail cone, which is simply a chopped off Pinnacle nose cone with fin slots. I never did a tail cone before, and this rocket just looks sexy to me. I also like the four fins for improved stability, and also because most of my fleet are 3-finned.

Here is the obligatory dry-fit picture. As mentioned by many others, the canvas-phenolic parts are really nice, and they fit perfectly.

 

[Buckeye]

Since epoxy doesn't like plastic, I put extra effort with the Dremel and knife to scuff up the tail cone where the centering rings will attach.



Since I never did a HPR tail cone before, it took me a while to get my head around the assembly process. The MAC instructions were not very helpful here, as there were no pics and the centering rings in my kit looked different than the ones described. The key was to think backwards: the MMT loads from the front of its parent tube, not the back, like traditional builds. There is also a tapered ring to match the slope of the tail cone. The next two pics show how the MMT and rings get oriented in the tailcone, first dry fit then the final result.



All joint construction is with Bob Smith 15 min or 30 min epoxy. I left room for the Slimeline to attach later. Also, I glued in all the rings right now. I don't plan on any internal fillets for the fin tabs, as there is very little working room between the cone and MMT.

 

[Buckeye]

Fins were scuffed up where the fillets will be applied later.

I cut out a fin template from cardboard using the payload bay tool. Root edges were double buttered with 30 min epoxy and tacked into place through the tail cone. I inserted the whole assembly into the slotted body tube for extra alignment as it dried.

I always like to test the 8 different ways the template fits in the fins and then number them when I find the best orientation. Also, it was key to wipe off any stray epoxy from the plastic tail cone, so that it doesn't inadvertently get glued into the tube, yet. That is the next step.

 

[Buckeye]

I selected a piece of 1/2" copper pipe as my fillet tool (5/8" outside diameter). I inked the pipe and dragged it through the fillet joints, marking the tangent lines. I scuffed up these areas some more, and applied masking tape to the lines.





I used Rocket Poxy for the fillets. A half scoop from a plastic teaspoon of resin and hardener was plenty for one set of fillets. This sequence worked well for me:

Mix epoxy. Wait 15 min.
Spread epoxy in fillets, Wait 5 min.
Pull fillets with pipe dipped in alcohol. Remove excess on the tape from possibly dripping back down into fillet. Wait 15 min.
Remove tape.
Touch up as needed.



I am pleased with the outcome of the fillets. I get better at this with every build.

 

[Nytrunner]

I like your description of the boat tail construction. Come L2 time, I'm replacing the LOC cruisers ogive with an elliptical and considering turning the nose into a slotted boat tail to make it look like more of a tomahawk.

 

[Buckeye]

This is a nifty accessory to add a payload compartment to the nose cone. Again, the written assembly instructions confused me a bit and lacked pictures. Maybe this post will help others.

Cut off the base of the cone.




Glue in the ring in the step where the cone meets the shoulder. You can see the ring in place when held to the light. The front of the payload tube gets glued into this ring.




Assemble the payload tube, which includes a nice threaded PVC coupler and a thin wooden ring that locks in to the nose cone. The following pictures show how everything fits together.




I did not yet glue the payload assembly into the nose cone, because I may add some nose weight when the build is complete. Preliminary OR simulations show some brief negative stability leaving the rail when loaded with a heavy motor and some cross wind.

 

[BDB]

Looking good. I'm a sucker for boat tails too.

 

[Mustang67]

looking good!

 

[Buckeye]

I opted for the Slimline retainer, with the idea that its low profile won't mess up the boat tail look too much. I bought the classic version with snap rings.

I also got the 38mm adapter. This accessory is not so "slim" in my view. It is heavy and uses more snap rings to secure itself in the parent tube. I may cut off some of the long, 38mm tube to save weight. I think Aeropack's adapters are more elegant and efficient.

 

[AdAstraPerAspera]

Could an Aeropack adapter fit in a Slimline retainer?

 

[Buckeye]

Could an Aeropack adapter fit in a Slimline retainer?

Not sure. I'll keep that in mind.

 

[Buckeye]

I am working up some stability calculations. So far, I have a preliminary build (no paint, still some glue to do) weight of the following:

(Sorry for the mixed units, that's how I think.)

mass = 2313 g
CG = 40.875"
Mach 0.3 margin in Open Rocket = 3.87 calibers

For a worst case scenario, I use the following simulation:

motor = K513FJ
wind = 10 mph
launch guide = 6 ft

I then look at the stability at launch guide departure, which is usually the critical moment:

RASAeroII = 0 calibers
OpenRocket = -0.8 calibers

I am thinking of adding ~100g of nose weight to at least bump up the RAII number into positive territory. I followed this rule on my WM BH38, and eliminated some squirrely trajectory coming out of the tower.

Anybody else feel the need to add nose weight to the Pinnacle nose cones in the MAC kits? They are lightweight, for sure.

 

[Steve Shannon]

What happens when you add the extra mass in your simulations?


Steve Shannon

 

[Buckeye]

What happens when you add the extra mass in your simulations?


Steve Shannon

Well, at ~0 calibers stability at the launch guide, RAII won't even run any further.

With the added mass, the launch guide stability is:

RAII ~ +0.3 calibers (RAII will bark at you if stability is less than 2 at any time!)
OR ~ -0.5 calibers

and the flights continue as normal. The poor stability at launch guide departure is very brief, just a few tenths of a second. Nonetheless, this is the critical time in the flight.

 

[Steve Shannon]

What's the speed off the rail?
What if you use an eight foot rail? You've got plenty of thrust to weight ratio.
Where do RAII and OR place the Cp? With motor, where do you you actually get the Cg?
Based on what you've said so far I would add enough nose weight to get somewhere between 1 and 2 actually measured calibers of stability, but no more. I'm not a big fan of massively overstable rockets.

Steve Shannon

 

[Nick@JET]

I've flown mine on J449, K445, K515 and you will have plenty of speed off the rail with a K513!

 

[Buckeye]

Bear in mind I am looking at marginal scenarios. 10 mph wind. I chose 6' as rail length because you never know what you may get at a club launch, and also the effective guide length is only the length where both buttons are engaged. An 8 ft rail may only have 6 ft or less effective length.

Speed off the rail is ~73 ft/s which does exceed 4x the cross wind rule of thumb (~60 ft/s). However, RA and OR still show stability concerns off the rail. This plot shows the negative stability until t=0.4.



The following table was computed earlier in my build and not the same as the numbers above, but the trends are still the same. The last column shows the iffy stability off the rail, especially OR.



The instability is very brief and the rocket recovers normally in the simulation. So, is this really a problem? I have one, maybe anecdotal, observation of a wobbly flight that simulated with negative initial stability. I added weight and the next flight was much better. I tend to trust the simulations.

No way I am gonna get 1-2 calibers margin at lift-off. That would require adding pounds of nose weight. Right now, I am shooting for enough weight to put RA and OR at least north of zero.

 

[soopirV]

I have a more tactical question, although I hope you get your confidence up around stability. With the nose-bay solution presented by MacPerformance, are there any features to prevent the cap from just unscrewing itself? I've been thinking about them, but have not purchased yet- I am contemplating either sinking two small eye-screws- one in the cap, one in the ring affixed to the nose, and wiring them together to prevent any rotational energy from building up, or, countersinking a set screw or key screw(whatever its called when the screw engages facing surfaces instead of just impinging on the flatness of one).

 

[Steve Shannon]

Bear in mind I am looking at marginal scenarios. 10 mph wind. I chose 6' as rail length because you never know what you may get at a club launch, and also the effective guide length is only the length where both buttons are engaged. An 8 ft rail may only have 6 ft or less effective length.

Speed off the rail is ~73 ft/s which does exceed 4x the cross wind rule of thumb (~60 ft/s). However, RA and OR still show stability concerns off the rail. This plot shows the negative stability until t=0.4.

View attachment 311304

The following table was computed earlier in my build and not the same as the numbers above, but the trends are still the same. The last column shows the iffy stability off the rail, especially OR.

View attachment 311325

The instability is very brief and the rocket recovers normally in the simulation. So, is this really a problem? I have one, maybe anecdotal, observation of a wobbly flight that simulated with negative initial stability. I added weight and the next flight was much better. I tend to trust the simulations.

No way I am gonna get 1-2 calibers margin at lift-off. That would require adding pounds of nose weight. Right now, I am shooting for enough weight to put RA and OR at least north of zero.

You're looking a lot deeper than I ever have. I've really only considered the static margin of stability and that's where I meant to try for 1.5 to 2 calibers of stability. Based on your table I would not have hesitated to fly it, but I've never been too bothered by a little tail wag as long as I kept going upward.
I've learned from what you're doing. I'll have to look at this a little more in depth.


Steve Shannon

 

[Buckeye]

I have a more tactical question, although I hope you get your confidence up around stability. With the nose-bay solution presented by MacPerformance, are there any features to prevent the cap from just unscrewing itself? I've been thinking about them, but have not purchased yet- I am contemplating either sinking two small eye-screws- one in the cap, one in the ring affixed to the nose, and wiring them together to prevent any rotational energy from building up, or, countersinking a set screw or key screw(whatever its called when the screw engages facing surfaces instead of just impinging on the flatness of one).

Good point. No, there are no features to prevent the MAC payload cap from unscrewing itself. I like both your ideas. However, the unscrewing scenario would only possibly happen after the main deploys. Probably not a lot of nose spin in that last few hundred feet.

 

[Buckeye]

You're looking a lot deeper than I ever have.

Guilty as charged!

I first started looking at this launch guide stability issue when I was working on 38mm MD and a custom-made launch tower for a personal altitude record. I wanted perfection on that flight, and it didn't happen. She wobbled a bit and took off down range (with the wind!) at about a 30 deg angle. So, I took a deeper dive into the simulations, fixed the negative stability off the rail, and got a better flight the next time.

This Firestick is a sport flier for me, so yeah, a little wobble will be fine.

Previous discussion here:

https://www.rocketryforum.com/showt...ive-stability-at-launch-guide-clearance-in-OR

 

[Buckeye]

So, I agonized over the nose weight for a week and then finally did it. I added 80 grams of split shot + epoxy. I scuffed up the nose tip, and two dowels are engulfed in the epoxy to help retain the slug. I will cut off the ends of the dowels and sand smooth after it dries.

 

[Buckeye]

Electronics bay in the coupler. Three (3) #6-32 button head screws were drilled and tapped to hold the bay in the payload tube. I really like these screws.



Raven3 + Power Perch on a sled by Additive Aerospace.



I also did some ground testing this weekend. 1.25 grams of BP for both drogue and main. Two, #2-56 nylon screws for shear pins in the nose cone.

 

[Buckeye]

Rail button configuration. The sloping tailcone would not be good for button alignment, and the plastic didn't seem like a good mounting material. So, the aft button is on the end of the body tube. The middle button is screwed into a centering ring.

I like to put a 3rd button on the av-bay or switch band. This one is less permanent, as it is held with a nut on the inside.

View attachment 314472

 

[StevenB]

Could an Aeropack adapter fit in a Slimline retainer?

If it helps: I frequently use a 29mm Aeropack adapter in a 38mm internally threaded Slimline retainer with zero issues.

 

[AdAstraPerAspera]

I installed a threaded Slimline on my Firestick XL, but I haven't gotten an Aeropack adapter to try in it. I've been wondering the same thing myself. I think I'll be buying an adapter soon. But I need other stuff to finish the build first.

 

[AdAstraPerAspera]

Okay, I finally got the adapter, and it fits inside the Slimline retainer just fine.

 

[AdAstraPerAspera]

I also talked to Mike today, and he's selling his own snap ring retainers that are slightly lighter than the Slimline ones, and look really nice.

 

[Buckeye]

Update. I flew her naked back in April for a shake down flight on a 38mm I motor. 1700 ft altitude and everything looked good, except for the Raven. The Raven was wacked, and I covered that in another thread. Got a replacement unit from Adrian to try next.

I spent the last few weekends painting, which I absolutely hate, but she came out pretty nice. I went with an Ohio State football uniform theme. Black cleats - grey pants with racing stripes - scarlet jersey - grey helmet complete with buckeye leaf awards!