LOC MMAS - Modular Motor Adaptor System
In the RNWS post, it may seem like I was bashing on LOC a bit and I want to be clear that I am not. You get an incredible amount of high quality parts in a LOC kit at a very reasonable price. The one thing I will kind of bash LOC for (sorry guys!) is their instructions. They make this WAY more complicated than it needs to be.
Essentially, as you can see in the MMAS parts photo below, what you get is a “mother tube” that you will install very much like any Motor Mount Tube assembly in any other rocket. Those 3 big huge Centering Rings go on the outside of the mother tube and they are clearly labeled “FWD”, “Mid” and “AFT”. Probably the only tricky part of this is lining up the “mid” and “aft” rings properly with the fin slots and fins, which we will talk about in a later post. But, at the end of the day, this part of the system shouldn’t look a whole lot different than other MMTs you have installed.
LOC instructions talk about either gluing up all the CRs and then installing the assembly or fully gluing up the CRs and fins outside the booster then cutting the booster to install the complete fin/mother tube assembly. Personally, I am not a fan of either of those methods. I like to install the forward and mid rings in the proper place, then install the assembly in the booster, then install your fins, then do all your internal fillets, then install the aft ring to seal it all up. I will illustrate this in further posts, but you should approach this portion of the MMAS in whatever way you are comfortable and used to installing MMTs on other HPRs.
They do provide a small “thrust ring” (that thin ring in the photo below), which is completely ignored in the instructions. It is designed to go inside the mother tube at the forward end to help prevent the adapter assembly from busting through and destroying your rocket. Picture an engine block in an Estes kit MMT and it is something similar.
The modular parts that hold the actual motor did present some interesting challenges. For this post, I am building out an MMAS for 7.6” LOC Bruiser, so I will go through the “stock” build of the 75mm motor adapter that came with the kit, but I will also show how I CNC’ed my own plywood parts to replicate the 1x54mmx6x29mm, 5x38mm, and 3x54mm adapters and that I went ahead and built an extra 75mm adapter to overcome one significant shortfall in the stock build.
Having said all that, the adapters are nothing more than two CRs around one or many motor mount tubes. That’s it, really.
The “collection of parts” issues I have seen in other LOC builds with came up again.
The rings were universally mis-sized compared to the various tubes – by a factor of 1/8″ in some cases. That is a lot of sanding to do on round discs that you need to try and keep round.
In the photos below, you can see that the ID of the large “mother tube” CRs was significantly smaller than the tube. I did peel the glassine layer off the mother tube, but I had to get pretty aggressive with a drum sander to get them to fit on the tube, even without the glassine.
At the opposite end of the scale, the ID of the 75mm inner mounting tubes was much larger than the MMT – about a 2mm gap in in the diameter. Clearly, I did not remove the glassine on that tube.
The Bruiser fins were also a solid 1/16″+ thicker than the slots in the MMAS mother tube CRs. I widened the slots, but since they needed to be widened significantly, you need to keep them centered so the fins aren’t misaligned.
Finally, the fin slots in the booster tube weren’t even close to the correct width. They length of the slots was fine, but it looks like they cut them for thinner plywood. It took some careful Dremel’ing to get them wide enough for the fins to fit.
Parts being a bit tight or loose are normal issues you deal with in a build, but the measurements being so far off suggests they have generic parts that they throw in the bag. The MMAS CRs are a perfect example. They probably cut the slots to the thinnest plywood they use on 7.6″ rockets and will expect the builder to adjust things if thicker plywood is used in a particular kit. Be ready to sand inner and outer diameters of discs and various slots.
The other issue I had with their tee nuts is that they used ones that have teeth that are also longer than the thickness of the wood, so they tend to break through the other side and it is difficult to get the nut in evenly. Heck, the teeth are longer than the barrel!
So, I replaced them with higher quality tee nuts that have the exact barrel length of the wood thickness and teeth that go about half way through the wood. In the picture below, the LOC tee nut is on the left and the ones I used are on the right. In the picture it looks like the LOC one is the same thickness as the wood and the other ones are short, but that is due to my poor picture taking, not reality.
With the properly sized tee nut, it is easy to use a vice to get a clean, even insertion of the tee nut and ensure you get a good result.
Once you have all the parts sanded and modified to fit, to get the mother tube rings spaced correctly, the easiest way to do that is use the fins. There should be about 1/8″ of the mother tube sticking out of the back past the AFT ring to give you something to add a fillet later. The trick is getting the fins straight. I used the notches in the aft ring to mark the mother tube for the left and right limits of the fins and then extended the lines all the way down the tube. That way, when you line up the fins in the CRs, you can see if they are straight. In the picture below, you should be able to see what I mean.
Once I had the Mid ring where I wanted it, I tacked it in with some CA and accelerator (in the photos, the black stuff is medium thick CA).
I also put the U-Bolts on the FWD ring and locked the nuts in with some red Loctite (the nuts will also be epoxied later). I positioned the FWD ring about an inch from the top – mainly, I didn’t want the U-Bolts to be too far forward or too far back and I want to have a good amount of space for epoxy. I tacked the FWD ring in with CA as well.
Then I put healthy epoxy fillets on the aft side of the FWD ring and the foreword side of the MID ring (basically, on the “middle” area that won’t be exposed when I install the assembly in the booster). I let all the epoxy cure before continuing).
At this point, the AFT ring is not glued on – it is just there as a spacer.
I am not going to go over all the steps of the fin installations and booster build, that is in my Bruiser Build Thread, but for the purposes of the MMAS discussion, at this point I placed the mother tube assembly in the booster and inserted the fins to ensure everything was lined up and in the proper place. Once I was comfortable with the fit, I placed a fillet of 5-minute epoxy around the top of the FWD ring and the booster and mother tubes. This is not structural – at this point I am merely trying to tack in the mother tube assembly firmly and seal off the edges so I can pour an epoxy dam on top of the FWD ring later.
Next, I tacked each fin in with 5-minute epoxy and let each one cure before I moved on to the next. I then installed the rail button weld nuts. At this point, the AFT ring is still not epoxied in, so I could get the internal fillets in place.
Before the internal fillets cured, I slathered up the ends of the fins and tubes and placed the AFT ring in place and sealed it on with a healthy coat of the same epoxy, taking care not to get any epoxy in the tee nuts (I taped off the tee nuts while spreading the epoxy).
I built the included 75mm motor adaptor stock. To measure the position for the aft ring (the big one with the 4 holes that secures the motor adaptor to the AFT ring on the mother tube assembly), I inserted a 75mm case and determined placement of the ring based on how well the retainer clip fit.
Then I roughed up the MMT and epoxied all three rings on. For the aft centering ring, there doesn’t seem to be any need to be too precise, so I epoxied it a few inches up from the rear ring. For the forward CR, if you plan to install the thrust ring discussed earlier, you should position the forward MMT CR so will sit right against the thrust ring.
Once the epoxy was cured, I had to sand the OD of the CRs until I got a good fit, then it worked well.
If you are using a 75mm Aerotech case, no problem. The stock clip retainers work fine.
The Loki cases are 76mm and the MMT is barely 75mm. No way I am going to be able to shove a Loki case in this MMT.
The bigger issue is if you try to use an Aeropack 54mm-to-75mm adaptor with the clips. This is not secure because the rear retainer adaptor just sits on the the case closure and the clip screws are way too short. I tried it with some long screws and some spacers, but it was not very secure. I would like to use single 54mm motors, so I am going to build the 75mm adaptor again with an Aeropack retainer.
If you plan to use Aeropack adaptors in any of your MMAS tubes, I would recommend some other retainer system than the clips provided.
Since the build of the 75mm adaptor using stock parts was not exactly what I wanted and I also wanted other options, I CNCed the parts for a new 75mm mount, a 3 x 54mm mount, and a 5 x 38 mount. I used an Aeropack retainer on the 75mm, Giant Leap Slimlines on the 5 x 38mm, and Z-clips with a center all thread for a large fender washer for the 3 x 54mm.
