Mach 1 rocketry BT-60 version of the Speed Demon, 38mm MMT.

[swatkat]

I know there's been a few build threads on this rocket already, and it's a sweet bird, but I wanted to add my version of the build as I've made a few changes to the kit that others may find useful.
First the kit, picked up for 20% off (I didn't bother trying my luck for25...) during the sale and it made it to my door about 10 days later with my Exiter (see low power build thread) which is currently sitting in primer in the garage, and a 2.6 AMRAAM which I haven't started yet. (Wanted a smaller version to my 4" PML). I've built two other Mach 1 kits prior to this batch, the prototype Portia and prototype full-sized Speed Demon. Both are great kits and exhibit a ton of quality. Steve does a great job in pre-fitting, and this kit was even better than all the other ones to date, I'll explain why below. First, the packaging. Simple, with a full color photo. All the fins came taped together and the smaller portions of the kit are in a separate bag in the larger bag.

 

[swatkat]

You note above that I ordered the 38mm mount, making this a minimum diameter kit. The kit normally comes with a 29mm mount, but I already have a BT-60 Madcow kit with the 29mm and Dual Deploy, so had that one covered already. Oh yeah, I forgot to mention above, I also built a Bt-60 Spektor, but ordered it with a 24mm mount for our SARG field that has a 1500' waiver. Anyhow, ordering with the 38mm mount means you get a stuffer tube and no rail buttons. I'll be 3D printing a conformal rail guide here tomorrow for it. One thing to note in this photo, Steve went the extra mile and combed the root of the fins, even adding little eye hooks for Kevlar braid... more on that later. I weighed all the pieces and planned out my attack, then spent the next hour modding an open rocket file that Steve provided to match my intentions. It is posted on another thread in this forum.

 

[swatkat]

Next was the obligatory visual inspection and bath. Things looked perfect, no missed cuts, clean slots, really fine work. Knowing that I'd want to fly with motor and electronic eject in some configurations, I opted not for the plugged closure/eyebolt/anchor to shock cord method that was covered in Eric's build, but rather cut an inch off the 7" A/V bay and will use it as a coupler ring to sandwich in the Kevlar in the body tube. My standard sleds are 6" anyhow and this particular kit didn't come with threaded rod or hardware, and I've got a bunch of 170mm M4 threaded rods... you can see the cut 1" piece in the photo.

 

[swatkat]

Knowing that I would need aft retention, I decided on the standard aeropack. I normally 3D print my own, but aluminum can me made much thinner and stronger and for this application, I needed thinner. The OD of the tubing is 42mm, OD of the aft closure is around 44, and the AP retainer is 49. So yes I'm adding some drag at the tail end. In the sims it lowers my max altitude by 3-8% depending on the motor.

 

[swatkat]

Next it was planning out the DD and cutting nomex. The kit includes what appears to be about 600 lb Kevlar, which is perfectly fine for this application. I will drill and reinforce the plastic nose cone as I never trust the plastic...

 

[swatkat]

Next was threading the Kevlar reinforced fillets. Since this kit is minimum diameter, there isn't much adhesion available at the root of the fin. In addition to the normal scuffing on the stuffer tube and the root, I used Steve's expertly routed hooks to weave some 300 lb Kevlar line that will serve to both strengthen the bond with the epoxy, and also add more surface area for bonding to occur.

 

[swatkat]

I then drilled and prepped the A/V bulkheads, running a single M4 threaded rod down the middle, adding two 600 lb Kevlar loops at each end and leaving a single hole for the e-match to pass through to a charge well.

 

[swatkat]

Next it was masking time for the fillets. I was able to easily just set the fins in their intended slots and tack them in with BSI Maxi Cure. Steve's cuts were perfect I only needed to align them, and then clothespin and popsicle stick them in. After taping things off I did the full fillets with JB Weld. They will dry tonight in front of a space heater, then I'll smooth things out tomorrow with epoxy putty. I also forgot to mention that I did round and slightly bevel the fins while cutting the A/V bay... More tomorrow.

 

[Lloyd Leichentritt]

swatkat, For the MM in the body tube, how did you attach it, what type of epoxy? I saw another thread were they took a hair dryer to evenly fill the gap between the motor mount and Body tube. My concern for me would be if I did it like that I would have epoxy in the slots for the fins then I wouldn't get a good adhesion for the fins.

 

[Normzilla]

swatkat, For the MM in the body tube, how did you attach it, what type of epoxy? I saw another thread were they took a hair dryer to evenly fill the gap between the motor mount and Body tube. My concern for me would be if I did it like that I would have epoxy in the slots for the fins then I wouldn't get a good adhesion for the fins.

I would think you'd apply epoxy to the outside of the stuffer, slide it into the tube, then clean off the excess that's stripped off on the aft end. This should keep the excess from the forward opening.

I would think...

 

[swatkat]

I actually used 30 min BSI as it was thin enough. I was worried that JB would be too thick and things would stick before I wanted The stuffer tube really doesn't take any load, the aft closure on the motor case will hit the screw ring on the retainer, transferring the load to the body tube.

 

[Lloyd Leichentritt]

I haven't received my kit yet, I was hopping to have it today for the before the Thanksgiving Holiday.

 

[swatkat]

I'll get some more photos up tonight on progress, with the cooler weather, it's taking the epoxy much longer to dry.

 

[swatkat]

Just did some sanding and filling tonight. Rain and cold have made it slow progress on the drying part of the build. Started in on 3D printing the sled for it though, 19 more hours. I'll post pics.

 

[swatkat]

Turkey Break and rain slowed the progress a bit, but I managed to get a fair amount accomplished today. Fillets are sanded and filled with CLEAR 5 min. BSI, then followed up at the end with bondo. They may look rough in the photos, but they are really very smooth. It'll show when the primer goes on...

 

[swatkat]

Used the 1" sleeve I cut from the A/V bay to act as a retainer for the shock cord. It was too tight a fit, so I sanded the sleeve quite a bit to allow the Kevlar to pass through. Worked really well.

 

[swatkat]

Wanted to make sure the retainer allowed for the biggest motor I intended to fly, and that I'd still have enough room left for laundry. I planned on cutting a 3/4" switchband from the top tube, and with a 6" A/V bay that meant that I'd have 2 and 5/8" in the booster tube of the A/V bay, so things will be a pretty tight fit. Should be OK though as I'm running a small drogue. I decided the 38/720 case was where I was comfortable, the 1080 left about 1.5" of room Test fit and located the retainer accordingly.

 

[swatkat]

Switchband cut from the upper body tube and attached now to the A/V bay. Epoxy drying and I'll update tomorrow.

 

[Lloyd Leichentritt]

Look what showed up today, still waiting on the AV Bay. I'll be using the Min-Retainer for a 30/720 (still have to get) and need to figure out how to set that up and install it.

 

[swatkat]

Awesome, here are some progress photos from yesterday. The rocket has since been bondo-filled for the fillets and is now in Primer. If you are making a switchband I'd suggest offsetting the ring a bit to allow for more space in the booster tube. Top tube has plenty of room. With a 38/720 it's a tight fit, even for a drogue.

 

[swatkat]

Tight fit with nomex and 9" drogue. (24" main)

 

[swatkat]

Coming in pretty light though at 15.3 Oz. My A/V sled and electronics is 2.53 Oz. and I figure paint will be 1.2-1.5 Oz with the primer and a single coat + clear. Target is to come in fully loaded, but w/o motor at ~19Oz.

 

[swatkat]

I must have missed uploading a shot of it in primer, but I used Duplicolor Primer, and then shot one double coat of Galaxy Blue to match the bigger brother Speed Demon that I've already completed. This is just the first coat, and it looks much better, despite shooting in 60 degree temps and 85% humidity. One more light coat of blue, then a clear and I'll be set for putting on the Aeropak retainer. Fillets came out nice in the end.

 

[Lloyd Leichentritt]

Knowing that I would need aft retention, I decided on the standard aeropack. I normally 3D print my own, but aluminum can me made much thinner and stronger and for this application, I needed thinner. The OD of the tubing is 42mm, OD of the aft closure is around 44, and the AP retainer is 49. So yes I'm adding some drag at the tail end. In the sims it lowers my max altitude by 3-8% depending on the motor. View attachment 367156

Since I'm going to be using the Min-Retainer the will have the motor tube flush with the body tube. Can you tell me how you epoxied the motor tube. What type did you use. II have heard to use up to 3 different types bases on what your doing. I had also read that someone used JB Weld then used a hair dryer to help the flow of the epoxy around the tube but with this tight fit I don't think that would work. I have Rocketpoxy and 15 and 5 min regular epoxy.

 

[swatkat]

Since I'm going to be using the Min-Retainer the will have the motor tube flush with the body tube. Can you tell me how you epoxied the motor tube. What type did you use. II have heard to use up to 3 different types bases on what your doing. I had also read that someone used JB Weld then used a hair dryer to help the flow of the epoxy around the tube but with this tight fit I don't think that would work. I have Rocketpoxy and 15 and 5 min regular epoxy.

For the motor tube, I actually used 30 min BSI. It isn't what I typically use for MMT, but JB is thick and I didn't want to risk getting the tube stuck before it made it all the way in. Since the fillets and roots are JB weld I figured I'd have enough support/adhesion. They were scuffed up really well with 60 grit and there's the Kevlar thread in the neat eye hooks that Steve made!

 

[swatkat]

Conformal Rail Guide attached, ready for flight the night before!

 

[swatkat]

Beautiful day for a launch (or two).

 

[swatkat]

 

[swatkat]

Note in the photo above, I opted for Aluminum tape for min diameter. Worked perfectly!

 

[Lloyd Leichentritt]

Swatkat, Do you think 15 min epoxy will work for the MMT? I can get 30 min if that will work better. What to you think would work better...