How to install a timer in my Thumper?

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Well-Known Member
Jan 25, 2009
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Greetings all,

I will be getting a customized Polecat Aerospace Thumper Jr (5.5" x ~40") kit next weekend at ROCStock, and I want to know how I will/should install a timer into this to control my option for 4x 29mm airstarts. The motor config is 1x 38mm and 4x 29mm. I am only going to use up to G class motors in the outboards (at least that's the plan at this point). I will also be buying a XAVIEN XDRST-2 Dual-Stage Timer a little later down the road, so I'd like to know how I could go about installing the timer, and not have it get cooked by ejection charges. From what I've learned in my experience, I have two options:

1) Mount timer in a tube inside the *large* nose cone.
PRO: I won't have to cut a nasty hole in the side of the tube.
CON: I might run into recovery difficulty with burnt e-matches getting tangled in the 'chute.

2) Mount timer between two centering rings.
PRO: Easy access for arming the timer and igniters.
CON's: Nasty hole in side of tube, will have to buy bulkheads/rings and such to protect timer from gasses, will have to make bulkplates to mount timer to, etc. (like Doug's Magnum)

Are there any other options that I'm not considering that I should be, any articles I need to read?

Thanks all,
I don't 'specially like the sound of cutting the glassed tube either.

I have done this one: Mount a tube inside the chute bay and mount the electronics in a coupler. The wires can be run next to this tube in a separate small tube (BT-5) that runs thru all the rings. Well, I didn't use the conduit tube, that was on yet another project.

Haven't tried this, but...if you have room - Cut a hole in the bottom ring to accept a tube and mount the timer in a long coupler.

Something like this was written up in an HPR mag: run the wires in a separate conduit on the outside of the rocket from nose to butt end.

Make sure you have positive retention on all options
If I were you, I would:

A. not cut a hole in the fiberglass tube

B. Put another 38mm tube next to the main one, and put a coupler in there with the timer in it-I've done something similar to this before and it does work.

C. If you don't like option B, put it in the nose and have a small tube running up the side of the chute bay, then attach micro or alligator clips to the wires so they release when it ejects-I haven't tried this yet, but will be sometime next week

D. Skip airstarting the 29mm motors. Put H's in them and light them all on the ground, then airstart that centeral 38mm J:D

E. use Thermolite to light them

F. Way too small, it would be underpowered :)

G. Skip those to, use H motors intead;)

H. 4 should be good enough:D
You could do something like the retrofit AltiTube set up on the Smokin' Rockets website. My concern with putting it in the nose is: how do you arm it and verify that you have continuity on the circuits? Drilling nose cones accurately is not fun, let me tell you. I'd say your best bet is either to do the altitube setup or to cut a hatch in the side of the rocket, between two outboard tubes, and place in a timer board. It's not *that* bad. John Coker did this with his Minie Magg, which is a similar rocket:

Good luck. That Thumper JR. is a fun kit to play with!
Well...I'm not planning to allow the 29mm's to eject into the body - they'll be plugged. I'm considering attaching a 38mm tube to the inside edge of the tube, then making a sled/plug for the timer, having the e-matches running through a brass conduit down to and into the motors. The fun part will be trying to attach hook up the motors and keep the e-matches away from the central flame. I'm going to talk this over with my dad - he's good at this kinda stuff!

Yeah, brass tubing better then BT-5...brain farts, they happen...
I put a removable rear ring in my Thumper (7") and mounted the alts on it to fit between the fins when the ring was inserted. I drilled and countersunk a hole inthe forward ring to take a wellnut. Hotglued the wiring harness in the wellnut. Worked great. Less room in your Lil' Thumper, but you might make it work.