Public Missiles Ltd. Mystic Build

Got my mystic kit this evening, i got the mmt installed by the time i ran out of epoxy. I will probably finish it this weekend or whenever i remember to go get more epoxy. here are a few pictures of the kit and the hamr 38mm retainer and the 29mm hamr adapter.

the only other thing about the rocket that is keeping me from moving forward is trying to decide whether or not to use the supplied copper lugs or go ahead and get some rail buttons. the only reservation about the buttons would be sending a screw through the tube and how it would affect the piston. anyone flying the pml piston system with rail buttons that could chime in with some advice i would really appreciate it. Thanks for looking ill update this thread whenever i get moving on with the rest of the build

Rail buttons won't interfere with the piston if they are put into the centering rings or at least within the range of the mmt.

I would ditch the lugs and use buttons..As Bill said attach the buttons to the centering rings..Or if you get the forward one too far forward and use the piston, assemble it so when inserted in the rocket it is uoside down from the directions(acting as a cup) this will give you more room to pack the cord and chute on top, and it resting on the rail button screw shouldn't effect deployment...Just make sure the piston slides in and out of the tube smoothly and freely..Personally though, I would do away with the piston and use a good kevlar cloth chute protector..

I'd use rail stuff. However, instead of rail buttons, I'd use rail lugs. On my PML Io, I used PML's linear rail lugs. They ain't cheap, but they are worth it. They have a lot of surface area and two hole for screws. On my Io, I screwed and epoxied the lower lug on, and used epoxy alone to attatch the upper rail lug. The two screw hole "nail" the lug on via the epoxy.

:2:

Michael

I would personally scrap the Piston system. I have had a couple issues with them and the 3rd being what you have already noticed with the buttons. If the temperature gets cold when you launch, you may find yourself sanding like a mad man to get the piston to slide freely enough. I had a chute not come out completely because of this. I ended up removeing 4 inches of cracked tube. Then after you sand the hell out of the piston, the temperatures will come up and the piston will be extremely loose.
The other problem is that after you assemble the piston to the shock cord, you will realize that the nylon shock cord is getting burnt. In time the nylon will break and you can't fix it. There are 2 solutions. 1. If you haven't assembled the shock chord to the MMT, buy tubular kevlar(I use 1/2" but that is overkill and you could get by with 3/8"). The other solution is to use a nomex sleve that fits over the nylon shock cord to protect it(if you already have your piston assemble this will be a problem.) I ended up cutting the piston out and using the nomex Shock cord cover (I think I got it from Apogee), I sew a d-ring in the end of the shock cord where the piston was, then connected a nomex shield for the parachute and used the standard long nylon cord.

If you don't use a piston you don't have to worry about problems with the rail button.

Just my 2 cents worth

John

I suspect you could use Acme Rail Guides, they are surface mount, so there is no chance of interfering with any internals.

G.D.

If you mount a rail button above the upper CR, I would stick with the piston. The heavy nylon cord won't catch on the small screw, but a chute protector, or shroud line certainly can. The piston will stay above the screw for the button and not cause any issues.

Another method I've seen used is to drill holes in the brass lugs and mount the buttons to that. There are no holes drilled in the BT. You can remove the buttons to use the lugs if you need to.

Handeman said:

Another method I've seen used is to drill holes in the brass lugs and mount the buttons to that. There are no holes drilled in the BT. You can remove the buttons to use the lugs if you need to.

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I do like this idea, the brass is thick enough to tap a few threads into. i would have not thought twice about buttons if i had a longer bt, but the mystic is fairly short making internal realestate while using the piston extremely valuable. the idea of the surface mounted rail guide also interested me pml has a set that can be used with or without screws. regardless of what lug style i go with i know im keeping the piston, had i not wanted it i probably would be building an loc v2 , so no protrusions into my bt imo is acceptable unless they were extremely low on the tube. but i think honestly with this being a such a short kit that i could safely use the included lugs.

i also like the pistons design, and may be wrong about this statement but ill put it out there. I believe that even if the piston was sanded to fit a cold tube the amount of wiggle room/ slop on a hot day shouldnt affect its operation, if it is a little loose it is still blocking the harmful gases from the chute as the ejection charge would blow directly into the cone. the idea of putting some type of sleeve onto the nylon cord attaching the piston to the mmt makes perfect sense and i will be doing that.

I am really enjoying the build on this rocket. its my first kit that has actually challenged me to think out of the box and away from the instructions that were included. Even step 2 had to vary from the instructions due to using the hamr retainer only by 3 mm but that could have been 3mm to ruin the build/ require me to purchase a different retainer system. thanks for the input everyone and keep it coming i need everything you can give me.

gagreen said:

i also like the pistons design, and may be wrong about this statement but ill put it out there. I believe that even if the piston was sanded to fit a cold tube the amount of wiggle room/ slop on a hot day shouldnt affect its operation, if it is a little loose it is still blocking the harmful gases from the chute as the ejection charge would blow directly into the cone. the idea of putting some type of sleeve onto the nylon cord attaching the piston to the mmt makes perfect sense and i will be doing that.

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You and my oldest son are two of a kind:wink:he agrees with you totally and he uses the pistons all the time. My opinion is probably more of a personal preference and not necesarily a correct one, but that's what makes this hobby fun. I am glad you will be using the nomex shock cord protector, my son swears by them.

I love the PML kits, and have had excellent luck with them. In fact I did both level 1 and 2 on PML kits. I have even heard of Level 2's being done on a little PML I.O.

I think you'll enjoy the kit.
Good luck

John
John

well got the fins in and the bottom cr epoxied in the piston system is done and the body is sanded and waiting for a decent day (above 19 degrees) to put it in primer. sorry i didnt get pictures of the fin install but it was pretty simple, filleted the inner part of the them to the mmt and body tube with the help of a popsickle stick. I am waiting to install the motor retainer till it is painted. now the question goes to what makes it fly. i had been thinking of going with the cesaroni pro 38 3 grain case, but it seems that aerotech reloads are more abundant and available. My issue with the at motors is that i do not know which 38 grain case to go with. i see them for sale used on yard sale alot. if anyone knows what the equivalent to the pro 38 3 grain case is could you please let me know. thanks ill get some pictures up of it later tonight.

38 360 case is a 3 grain case . :wink:

Yes, the Aerotech 38/360 is a 3 grain motor
38/480 - 4 grains
38/600 - 5 grains
38/720 - 6 grains
38/1080 9 grains
38/1320 6 grains (six double length grains - same as 12 standard, but with less total burn surface when starting)

You really have to compare motors and impulse rather then grains when comparing the two brands. The CTI 5 grain motors are a mix of I and J motors and all but one has more the 600Ns while Aerotech 38/600 5 grain are all I motors and at or below 600 Ns. The Aerotech 38/720 6 grain motors have a mix of I and J sizes that are more comparable to the CTI 5 grain loads.

I haven't looked at the physical sizes of the cases, but suspect you will find a difference there too.

All the loads for the CTI 3 grain have more total impulse then any of the Aerotech 38/360 loads. The Aerotech 38/480 4 grain would compare more closely to the CTI 3 grain loads.

Obviously one manufactures grain size doesn't match any other manufactures. You really need to look at the impulse of the motors, not the number of grains.

I highly recommend NOT using a piston with PML's plastic tubing. It does not take much heat for the tube to deform enough to get the piston stuck in it. This happened with my MR-1, and I had to use a heat gun to carefully heat up the tube to pull the piston out. Of course, the tub was no longer perfectly round, so I had to scrap the piston.

I've been using a Callisto with a piston for 8 years now, summer and winter and I've never had a problem. The piston is sanded down enough to use in cold weather. It's loose but very usable in the summer.

Thanks for the engine explanations, really helped out a ton. I had planned on ordering my motor casing but after talking to a few people locally i decided to wait and see what the vendors have on hand at local launches and probably pick up a case locally.


As far as the great pml piston debate, i cant put a whole lot of experienced info into it as i have never flown it. My thoughts and a few people that i have talked to, is that sanding it down after letting my tube sit outside in 11 degrees or less for a few hours to an operable easy moving state should allow it to work smoothly. I can see how only a light sanding would cause some issues with it sticking as the quantum tube "shrinks" around it. My thinking is as long as the piston seats nicely over the mmt and strap attaching it to the piston it will not allow harmful gases to degrade the chute but will freely move up the bt. Again all theory and what i got after speaking with a few guys who use the piston. I can see the cons to the system and thanks to everyone whom has posted up their issues with it, those warnings have not fallen on deaf ears and i am trying integrate all the failures of the system into a workable solution.

Had this kit built but very much in the nude in the cave for a while now so over the last two days i broke out the rattle can and masking tape. It will be flying this saturday at thunderstruck. I am pretty happy with the paint job not so happy with how my phone took the picture lol.

gagreen said:

Had this kit built but very much in the nude in the cave for a while now so over the last two days i broke out the rattle can and masking tape. It will be flying this saturday at thunderstruck. I am pretty happy with the paint job not so happy with how my phone took the picture lol.

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The chevrons are a nice touch!

May I copy this one to the gallery?

JAL3 said:

The chevrons are a nice touch!

May I copy this one to the gallery?

Click to expand...

Absolutely TY

Flew the mystic today at Thunderstruck 2011, H123, 15mph to 23mph gusting. Wasnt a perfect day to fly, but this rocket flew straight with a slight very slight weather cock. I didnt fly an alt on it but it was cooking and only a spec in the sky when the chute popped perfectly at apogee. I reefed way up on the shroud lines and still had a 3/4 mile hike to recover. Got the rocket back in perfect shape only a few scratches on the nc and walked away a level 1.


I have to thank the guys at red arrow hobbies for the great deal on my cert motor and supervising the motor assembly. They even came out to the pad with me to make sure i didnt have any problems. Great guys. Everyone really at thunderstruck did a great job and was extremely helpful.

Congrats !