MAC Performance RainMaker 54 Build

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I just realized as I was looking at the bell mocked up pics that the fins were hung up on the masking tape I wrapped on the tube and not the fins. Well at least not until the pics below, which is much closer. There is still a bit to either notch or trim but these pics are closer to the final look. I am really leaning towards this look I just need to feel a bit better about the potential issues with the recessed retainer design.

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If you're looking for opinions...I think the bell looks downright awful. Completely ruins the streamlined look of the rocket and makes the boat tail seem useless.

Worst case scenario: nest a retainer in the boat tail. Best case scenario: retain from the fore end or friction fit.
 
If you're looking for opinions...I think the bell looks downright awful. Completely ruins the streamlined look of the rocket and makes the boat tail seem useless.

Worst case scenario: nest a retainer in the boat tail. Best case scenario: retain from the fore end or friction fit.

Dan feedback and subjective opinions are always welcome. Plus I respect what you have to say, even if it is 180 degrees from my position as I enjoy different perspectives. I remember a thread a number of years ago where there were quite a few objections to the use of the bell retainer, which I went ahead with anyway on my 4" Madcow Tembo.

While I won't go so far as to say that the bell retainer makes the tailcone useless, I would agree that there is a price to pay. Obviously it would introduce more mass where I don't need it, as well as drag including some marginal base drag, but I can live with that. After-all this is a somewhat unique rocket to begin with and a bit of a contraction, which was/is a part of the appeal to me.

It is a carbon fiber airframe and not just a thin walled one but one that could handle many more times the amount of potential stress. It is also relatively and reasonably optimized for performance, however it is not a MD but instead a 54mm airframe with a 38mm MMT. Bottom line for me it is a rocket to have some fun with. I already have several relatively serious rockets and some even more serious performing designs coming into production starting this winter.

So the introduction of some additional turbulence, mass, and base drag is not really a big part of the decision for me, but the potential for the Krushnic Effect is. To qualify that. As I understand the Krushnic Effect, it is not a 100% on or 100% off, it can rob some, or all performance. If it robs 10%, in this case, then I am good with it, but if it robs much more than that then I am not. In the case of my sim vs actual result with my Tembo it was off around 40% (if I remember correctly) and that has me concerned. Now in fairness I have one flight on that so my result is hardly definitive, however my sims have been consistently and remarkably accurate, with initial sims within 10% and adjusted up to 5%. This is why this one sim/result stands out.

In truth at the end of the day I like the look and I am okay with the trade off, and I am trying to find a way to use it which is the purpose of this discussion. I would be very interested in hearing from anyone who has used this retainer, especially if they have sims and actual results to compare. Barring that as you can obviously see I am trying to figure out ways to mitigate the issues, by being open to losing a bit of mass and shortening the bell.

At this stage the gap between the aft edge of the Loki hardware and the Bell retainer is 1.035" or 26.28mm, which I believe is the potential problem dimension for the implications of the Krushnic Effect. Possibly someone here understands this phenomenon better than my limited understanding and can educate me a bit on it.
 
If you want a real measure of the effect, you need to fly the same rocket with and without the bell retainer, on the same motor. Ideally, you would even fly multiple motors both ways. You'd need to pop the thing off or epoxy it on between tests, and fly with the weight of the thing added somehow when flying without. Maybe you could attach one with set screws instead of epoxy to run back to back tests.

Sims are always wrong, good sims are only off by a small amount.
 
There is no way to simulate Krushnic effect with our basic hobby software, but a 40% discrepancy would point in that direction, assuming all else is good. 40% is a lot to screw up in a simulation, so something else is at play. You could build scale models and fly them or fire them on a test stand. A lot cheaper to burn C6's in such an experiment.

Personally, the bell retainer has no business behind a boat tail. Defeats the purpose. After plopping down a couple hundred bucks on this kit, I would want to ensure it performs top notch.
 
If you want a real measure of the effect, you need to fly the same rocket with and without the bell retainer, on the same motor. Ideally, you would even fly multiple motors both ways. You'd need to pop the thing off or epoxy it on between tests, and fly with the weight of the thing added somehow when flying without. Maybe you could attach one with set screws instead of epoxy to run back to back tests.

Sims are always wrong, good sims are only off by a small amount.

I realize sims can and are more often off than on, however my sims have been within +-10% for the first flight then adjusted using the baseline of the first flight and they get more accurate from there. Obviously this could have just be an off sim, which would be unfortunate considering the questions it left me with.

I agree that the best way to prove this one way or another is to do a real world A-B test with use a +- 10% variance factor. However removing the retainer is not practical as once these things are on good luck in getting it off.

On a side note I have read so many posts where people claim that they use heat to remove their retainer that was glued on with JB Weld. Personally I have tried this 3 times with no luck. Once on a 29mm GLR Slimline Qwick-Lok retainer, a GLR 3" to 54mm Ogive Tailcone retainer, and an AeroPack Quick-Change retainer, and all of them had to eventually be cut off. In the case of the GLR Ogive Tailcone I had to cut the retainer into bits to get it off. But I digress.

I think a more practical real world test would be two rockets on the pad at the same time with the only difference being the retainer. Or just two sets of static tests with a load cell may do the job, one with a retainer attached one without.

More opinions....the bel looks seeeeexy!

lol I think I know where you stand on this one Jarrett. I don't disagree I have been staring at the mocked up fincan over the last day and it grows on you the more you see it.

My game plan is to do the following, shave .2 off the .6 forward neck, and rough up the outside neck for adhering to the JB Weld. This will remove possibly 5ish% of the mass and whatever reduced mounting surface should be negated with the additional mounting surface of the outside being roughed up. At the same time I am going to have a machine shop remove .25" off the Bell. This is just an arbitrary number where I am hoping that it may mitigate any negative effects but still retain the Bell look. This may result in another 5ish% reduction in mass, so overall I may shed 10%. I will do a before and after weigh-in to confirm.

Finally I will mock it up once again then decide from there if I want to go with the re-anodizing process, which as I understand removes a bit of the surface then adds a bit. My concern with this would be the tolerance of the threads on the retainer part. If the machine shop can do their part neatly and if the exposed aluminum is not a problem then it may just end up looking good the way it is. I will evaluate it when I am there and solicit advice from someone with experience on the re-anodizing process, and I know just the guy to ask...Riley!
 
There is no way to simulate Krushnic effect with our basic hobby software, but a 40% discrepancy would point in that direction, assuming all else is good. 40% is a lot to screw up in a simulation, so something else is at play. You could build scale models and fly them or fire them on a test stand. A lot cheaper to burn C6's in such an experiment.

Personally, the bell retainer has no business behind a boat tail. Defeats the purpose. After plopping down a couple hundred bucks on this kit, I would want to ensure it performs top notch.

I think trying to research the effect a bit deeper is required before I commit to the Bell, hopefully Google will yield some results. I hear you on not messing with an expensive kit but I also think that this kit is unique enough to consider this retainer. Plus I know Mike gets a kick out of seeing what people do with his creations.
 
I know a pretty simple way to get a retainer off ;)

booster return not included in this service.
 
Personally, the bell retainer has no business behind a boat tail. Defeats the purpose. ...I would want to ensure it performs top notch.

+1. And the reason you should move the whole MMT forward and completely hide the retainer so as to not disrupt the gains provided by the boat tail.

From the Ether...
 
lol tough crowd.

Over the last couple of days I talked myself into the Bell mainly becasue I was staring at the mocked up fincan. Today I mocked up the slimline just to stare at it over the next couple of days and see if I still feel the same way, even though I pretty much know what I am going to do at this stage.

I did say this would be a fast build, for me :)
 
Michael the Slimline looks very nice. I have one in a similar setup on one of Mike’s, MAC Performance, Firesticks. Mike made a kit up for me with the same fiberglass tail cone.

Best of luck with the build.

Bob
 
Michael the Slimline looks very nice. I have one in a similar setup on one of Mike’s, MAC Performance, Firesticks. Mike made a kit up for me with the same fiberglass tail cone.

Best of luck with the build.

Bob

Thanks for the input and well wishes

I'm not digging the bell, use the slimline... :)

You know you're going to need one to finish off that Aerobee 150 properly.
 
I think trying to research the effect a bit deeper is required before I commit to the Bell, hopefully Google will yield some results. I hear you on not messing with an expensive kit but I also think that this kit is unique enough to consider this retainer. Plus I know Mike gets a kick out of seeing what people do with his creations.

Michael, When we did the RainMaker 54 boat tail I left everything long and tight so that the builder with a nip and a tuck or forward retainer could configure it so pretty much any retainer setup could be done.
when the 3" RainMaker comes out around the first of the year it will come with our 54mm FastBack retainer. The retainer will be recessed half its length into the boat tail. Personally.....I like the Slimline.
 
lol tough crowd.

Over the last couple of days I talked myself into the Bell mainly becasue I was staring at the mocked up fincan. Today I mocked up the slimline just to stare at it over the next couple of days and see if I still feel the same way, even though I pretty much know what I am going to do at this stage.

I did say this would be a fast build, for me :)

And, of course, the right answer is *use the one that will make you want to fly it the most*. If it's the bell.... put on the bell.
 
when the 3" RainMaker

You had me at when

And, of course, the right answer is *use the one that will make you want to fly it the most*. If it's the bell.... put on the bell.

Either retainer or friction fit this baby is going to get some use. I just had a thought, if I use the Bell I could name it Hell's Bell.
 
Everyone has different opinions and its gonna be bad ass no matter what!!!

But my 2 cents

Bell looks ridiculous on boat tail
But that just me its not my rocket
 
Way to go Mike Crupe! A 3” RainMaker will be awesome. Three inch rockets are the Goldilocks diameter. Not to big to transport and store, and not to small to see in flight. Plus you have such a large assortment of motors to select from.

Guys I’ve used Mike’s 54 mm FastBack retainer. It works great.

All the best,
Bob
 
Time to start gluing stuff together.

Seeing as I have been staring at the fincan I will start there.

First up the top CR for the MMT. My standard practice is to sand the MMT where the CR will adhere to using 80 to 100 grit, I used 100 because that is what was on the top of the pile. Sanding perpendicular to the length of the tube I sanded where the CR will be, as well as to the top edge of the MMT.



Next I sanded the top and bottom of the CR. The CR is made from the canvas phenolic and this one is pretty thick, this rocket is built like a tank.

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I used 120 for this.



Because I removed the kevlar harness, I masked off the groves that the Kevlar harness slides through. Feeding the Kevlar harness through these holes afterwards may be a bit challenging, due to the tight tolerances, however I have done it before. I could have left the harness attached and glued it up in place. However the way I glue up my CRs is by sliding the CR past the point where it will be then applying an even bead of glue around the tube at the bottom of the roughed up area. Then I work the CR up the tube while spinning it and creating a nice even bead around the top of the CR, as well as working the epoxy in between the CR and MMT. Because of this I didn't want to deal with the epoxy building up on the Kevlar harness as I spun it around.



And tape removed, as well as a bottom fillet applied, which you can't see. The reason I roughed up the MMT above the CR to the top of the tube was to apply a thin layer of epoxy on that area. This is just an added measure to deal with a worst case if the CR fillet cracked, remember this is the attachment point for the recovery harness. Considering I am using ES6209, I highly doubt the fillet would crack, but if it did the epoxy on the MMT would be enough to prevent the CR from moving.



This will sit aside for the next 24 hours, then I will slide the the second CR on the MMT, as well as the sand the CR and where it will sit on the MMT. However before I glue it up I am going to tack the fin roots down, and use the top CR and retainer to position everything. I am doing it this way so I can get access to the inside of the tailcone from the top for internal fillets, then once done I will epoxy the top CR in place.
 
That is a pretty cool Axe rocket and the fact that you used the Bell helps me get a bit more comfortable with it. Tell me did you sim the rocket, was the sim and actual close? I have only used this retainer once and it was a 38mm silver one, same as yours. My sim was way off from the actual, to the point where I can't get it out of my head that the Krushnic Effect played a role. This is why I may shave a bit off the Bell if I go with it, just for a bit of margin and the added bonus of weight loss.

I saw no degradation in performance with the bell retainer. Krushnic Effect as I have seen it described is pretty much a black-white, on-off condition. Until you flip into the secondary expansion chamber mode, I'm not sure you'd see much measurable difference. But in any case, it is pretty easy to see if you are going to have a problem. Measure the exit diameter (of retainer) and then measure the depth of the nozzle back from the exit (of retainer). If depth>diameter then you are at risk, if not, nothing to worry about. From the picture you show here, it looks like exit diameter>>final nozzle depth (once nozzle is placed in motor).

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Would this then be more of an issue with Loki motors where the nozzle exit is farther forward? I had a suspicion already that that might be the case...

To clarify, are you talking about the nozzle exit diameter, or the bell exit diameter?
 
Would this then be more of an issue with Loki motors where the nozzle exit is farther forward? I had a suspicion already that that might be the case...

To clarify, are you talking about the nozzle exit diameter, or the bell exit diameter?
I do not believe that this is manufacture specific. My one flight using this retainer was on a CTI 38mm reload. I think it was a 3 gain but I can't recall the specific motor right now. I mentioned it in a previous thread so searching krushnic should yield a result.

From what I have read, and it sounds like Dixon has read the same information, the nozzle geometry plays a role in the effect. So it is likely reasonable to assume that different motors (assuming different nozzles) with the same hardware may differ in how this plays out.

In my case the rocket was a Madcow Tembo, which is a relativity short squat rocket so base drag may play a role. I am only theorizing in this because if you read about the effect, pressure differentials seem to be at the heart of it. Regardless I think it is safe to say that it has a few moving parts.

NAR published an interesting article on it, which comes up in a Google result. Somewhat of a technical read but not heavy and comprehensible. My take on this phenomenon is that it could use a bit more research and that the general rule of thumb is general at best.

I have some ideas on how to improve mitigating the effect, that I will likely experiment with at some point. That is for anyone interested in the bell look but has a concern over its impact on performance.
 
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I called out Loki only because the are the one functioning manufacturer of snap ring motors. Snap ring motors typically have the entire nozzle recessed inside the case.
 
So after careful consideration I decided to go with the GLR Qwik-Lok Slimline retainer.

This was not my first preference as the look of the Bell grew on me over time, although out of the gate the Slimline was my first choice. However as much as I like the look of the Bell I believe that I may live to regret using it, but I know I won't regret using the Slimline. The fact is I know far too little about the Krushnic Effect, even though I read whatever I could find about the phenomenon in a couple of days and even made some inquires outside TRF resources.

At the end of the day no one was able to provide a firm answer and remarkably it appears that this particular effect could use a bit more study. As I indicated I have some ideas on how to mitigate the effect, or at the very least use it with more confidence, which I may do at a later date.

So on with the build. Over the last two nights I continued working on the fincan assembly. The first thing I did was to pass the Kevlar Y harness back through the holes in the top CR, and re-insert the pins. This was slightly challenging but very doable. The reason I assembled the top CR this was was because I did not want to create a stiff, weak point due to the epoxy that may harden and crack over time. I still need to dab a small bit of epoxy on the pins so they don't fall out. Next I sanded the lower CR on it's top and bottom, as I did with the top CR. Then I mocked up the assembly so I could mark where the CR, fins, and retainer would eventually be located. Once I was confident in my measurements I disassembled everything and prepped the tube with 100 grit. Where the CR and retainer attach I sanded perpendicular to the length of the tube, and where the fins attach I sanded along the length of the MMT. Then I slid the CR back on.



One advantage of the slimline retainer for this build that I had not considered is that I can insert it assembled onto the MMT into the tali-cone from the top and it slides right through. This will allow me to assemble things in a different order than I had originally pictured. The gap between the retainer and the aft opening of the tailcone is almost zero, it fits very well with no play at all. However the MMT is significantly smaller than the ID of the retainer. To fix this I took some masking tape and applied it to my cutting board which allowed me to use the grid pattern to cut even rows of tape that I stacked 7 high times high times by 3.



This gave me three masking tape spacers that I put on the MMT 120° apart.



I then test fit the retainer and it was slightly sloppy. I want the fit slightly tight but not overly, otherwise when you slide the retainer on you can slide the tape around. So I added one more piece of tape per spacer, making up 8 layers.

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Next I mixed up a batch of JB Weld and applied it to the tube in between the tape spacers at a thickness that would fill the entire cavity and squeeze out the excess, but not too much excess.



This was a good night for me as I got it right for once and there was almost no excess or mess to deal with. This was done last night and it was left overnight to set up.



This evening I started sanding the tail-cone on the inside where the fin-slots are for the internal fillets.



Then I cleaned up the aft of the tailcone from where I ground it off, using some 2000 wet sandpaper. I did this because once I assemble the MMT and fins, sanding the aft end would not be possible. I also made sure that the internal sanding that I had previously done on the aft end was adequate for when I adhere the retainer as the assembly goes together. The plan is to rough up the retainer part that slides under the tail-cone and epoxy it to the tail-cone.

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Up next will be more prepping/sanding of the fins using two different grit sand papers, a heavier grit for where the fillets will be and a finer grit just to sand off the shine. I like the shine but I will be clearing the rocket with something possibly just polish, but I want it to be a bit more uniform. Also to sand will be the area on the outside of the tail-cone around the fins, again this is where the fillets will go. I will be marking this off using my carbon paper method. Then I will be onto tacking the root of the fins to the MMT and this fincan may be close to complete, except for fillets. The fillets will be done after I glue the fincan to the booster tube.
 
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