Dual Deploy Modified PML Miranda - Build Thread - L2 Certification?

[Mendal]

I've never done a build thread before so I thought I would try my hand at it.

My plan is to take a PML Miranda and convert it to a Dual Deploy rocket. I am currently L1 Certified, since this rocket has a 54mm MMT I may use this as my L2 rocket.

The standard kit is a 3" rocket with Quantum Tube airframe and payload sections. The default kit uses a piston ejection system. I will be ditching the piston, (however I have figured out how I could switch back to the piston if I decided to fly single stage motor recovery, more on that later maybe)

This rocket included PML's quick switch motor mount which allows you to fly the rocket on 54mm, 38mm, or 29mm motors. So I had to decide how I was going to retain the motor. I was looking at PML's HAMR motor retainer and it occurred to me that the biggest disadvantage of the system was the largest motor I could put in it was a 4 grain motor because the quick switch system retains the reduced tubes by a fitting at the forward end of the MMT. This fitting blocks a long motor from going in past the fitting. At this point I decided to go with an aero pack 54mm retainer and aero pack 38mm adapter. This also gives me flexibility in future 54mm rockets. All that said I do have to say that PML's quick switch is a cool system and I would have used it if not for the fact that it blocked the longer motors.

The default kit used a single 1/2" nylon strap glued to the side of the MMT 4" past the forward centering ring. The other end of this strap normally attached tot he piston. I modified this by cutting a second notch in the forward centering ring 180 degrees from the first attachment. This kit included a welded D-ring, I threaded this onto the nylon strap and sewed it in place at the center of the strap with my trusty speedy stitcher. I then slid a piece of heat shrink over the stitching to protect it. The two ends of the strap were then epoxied to the sides of the MMT. I now have 2 points of failure on my recovery harness. A shock cord will be attached to the D-ring by way of a quick link and then attached to the bottom of the AV bay. I have not yet decided if I will fly a drogue chute.

D-ring attached to recovery harness.

Recovery harness epoxied with to the MMT with rocketpoxy died black

The harness presents two points of failure, if one side were to fail the the D-ring would remain attached tot he second strap sewn together.


The coupler AV bay will fly without a switch band. I plan to attach the AV bay to the payload section with mechanical fasteners (likely machine screws into t-nuts in the AV bay. For electronics I will either fly an eggtimer or an eggfinder TRS. I may also invest a eggtimer quantum so I can activate via wifi, we'll see where I end up on that.

That is all for now, I'll keep you all updated as the build progresses.

Thanks
Mendal

 

[Handeman]

Not exactly how I would do it, but I think you are going to have a great L2 rocket! I'll be interested in how you modify the PML kit.

Good Luck.

 

[Buckeye]

Good luck with your build and L2, though you now see the limitations of stock PML kits.

Piston: Thermal contraction of the Quantum tube can cause the piston to stick in the tube on moderately cold days. You are in Minnesota, so good thing you ditched it. Else, you would be sanding the hell out of the piston.

Kwick Switch: Nice idea, but limits motor length as you found out. Also, the plastic threads are brittle, and can easily break. The phenolic adapter tubes, too. Both happened to me, and I am easy on my rockets.

Quantum Tube: Note that this plastic tube is not rated for very high aero forces. See the PML motor chart where strengthening is recommended for the vast majority of L2 motors in the Miranda. I also had bad luck on landings that cracked the Quantum tube near the fin joints. The fins and glue are far stronger than the tube!

Honestly, time and technology have passed by PML. Based on pure anecdotal evidence in this forum, there seems to be many fewer PML builds these days. PML probably sees this as well, and they are upping their game to fiberglass and carbon fiber.

 

[Mendal]

Sorry for the delay in posting, I have been hard at work on the rocket. I have made decent progress,

For starters, I got the air frame sanded, this took more effort then expected as the coupler was a pretty tight fit, I need this to slide easily out during separation. I have a feeling I'm going to need sand paper on hand at the launch site.

I then placed a nice heavy bead of rocketpoxy up inside the tube and pushed the motor mount in. This created a nice fillet at the forward centering ring. I did account for the AeroPack Retainer and left the MMT extended to attach later.

I left the rear centering ring loose so it could be removed for doing internal fillets.
The recovery harness came out perfect that it allows me to connect the shock cord.

While the epoxy dried on the MMT I sanded the fins, at this point its well worth mentioning that I really hate sanding!
400 grit and roughed the glue/fillet area with 80 grit

Next up I tacked the fins using the double dip method I used my guillotine fin jig to align the fins.

I also created anchor points for the rail buttons, I had planed on placing one before attaching the rear centering ring.

You may noticed I said had planned... and I'm one to admit when I make a mistake. In the process of doing the internal fillets it turned into a bit of a mess, the space was very tight and I was getting epoxy all over the place. I decided I need to get the rear centering ring in place or I was going to have a ton of clean up work on my hands. I realized right after I pushed in the rear centering ring that I forgot to drill and mount the block for the rear rail button. Oops. at this point I might attempt to carefully drill into the rear centering ring and use it as the anchor point for the rear rail button. Anyone have any thoughts?

Coming up next is the beginning of converting the coupler into a AV Bay. I ordered a 2 each bulkheads and coupler bulkheads from PML. I was actually impressed that they shipped very quickly and were packaged very well. They will however need more sanding... UGH!

Thanks for reading I hope its not too many photos.
Mendal

 

[Mendal]

Good luck with your build and L2, though you now see the limitations of stock PML kits.

Piston: Thermal contraction of the Quantum tube can cause the piston to stick in the tube on moderately cold days. You are in Minnesota, so good thing you ditched it. Else, you would be sanding the hell out of the piston.

Kwick Switch: Nice idea, but limits motor length as you found out. Also, the plastic threads are brittle, and can easily break. The phenolic adapter tubes, too. Both happened to me, and I am easy on my rockets.

Quantum Tube: Note that this plastic tube is not rated for very high aero forces. See the PML motor chart where strengthening is recommended for the vast majority of L2 motors in the Miranda. I also had bad luck on landings that cracked the Quantum tube near the fin joints. The fins and glue are far stronger than the tube!

Honestly, time and technology have passed by PML. Based on pure anecdotal evidence in this forum, there seems to be many fewer PML builds these days. PML probably sees this as well, and they are upping their game to fiberglass and carbon fiber.

Humm I had thought about adding another centering ring... I've also thought this rocket might be a good candidate to foam the fin can. I have also extended that thought to wonder about glassing the tube, but I'm not sure you could glass over the QT...

That would be more as a practice for future things to come in rocketry...

Mendal

 

[Handeman]

A lot of folks don't have many good things to say about PML, but I've had a PML Callisto since 2003. It has 39 recorded flights since 2008 after I had my L1 but during those first 5 years, it and a BD Thug were my only real MPR and it would fly 3 or 4 or more times every launch. I know it had 6 flights when I took it to my first club launch in 2003. I would think it has 100 flights and counting.
Once you have the piston sanded down for the coldest weather, you don't have to sand it again, just keep the tube clean and the powder residue out. I still fly the Callisto with the piston and original chute. Sanding the piston probably applies to the couple/av-bay also.

The only thing, don't fly PML QT in below freezing weather. Quantium tubing is PVC and it doesn't like cold. The one time I did that the BT snapped in half when it hit the ground in 26 deg weather. I bought a coupler from the vendor that day and had it ready to go by the next launch. That was 3 years ago. It's had 14 more flights, half on 29mm 3 & 4 grain H motors so it was an easy fix.

Good Luck

 

[MaxQ]

Nice build thread.

I've never flown a PML kit but they have been around a longtime...w/ lots of satisfied customers.
I think in the beginning people liked the PML QT because you could get a decent build and good finish quickly...of course we learned a few things over time.

If you follow the suggestions posted here about the QT (on the PML website - motor recommendations), it could be a nice reliable rocket for you.

I'll be bringing my first PML/QT build to my local club next month....a very old kit - Quantum Leap with several mods.
We'll see how it goes.

I actually laid FG tip to tip on those thin G10 fins and it seems fine.
However, Wildman Tim told me not to take it out in freezing temps or it will delaminate.