Sunday, February 14, 2016

Repair day...

Probe-18 fin can, showing some of the damage (click to
Back on September 5, my scratch built Probe-18 rocket suffered damage on its maiden flight. The
payload section, carrying two altimeters, snapped back just after ejection, whacking one of the basswood fins, thereby causing a partial failure of the lower part of the body tube (held on by double glue joints, my fins do not come off - they may break, or the body tube fail, but they will not rip away). At the time, I was not sure if the mishap was caused by a "shot gun" ejection charge of the B6-4 motor - which seemed to be indicated by the buckling of the tube just above the motor mount - or if the shock cord was too short (2 rocket lengths), or a combination of both. At any rate, the rocket was consigned to the repair box when I returned from the HARA launch, where it has lain for 5 months.

The Probe-18 before its 1st voyage (Click to

Today's weather was cold and dreary; perfect for staying indoors and doing some rocket stuff. I spied the Probe-18, and after once again inspecting the damage, decided it was to time to repair this little beauty. The lower body tube was pretty much a lost cause, but everything from the base of the launch lug forward was in great shape; I therefore decided to cut away the fin can (the lower 3 3/8" section). This was easily accomplished by wrapping some card stock around the body tube to serve as a cutting guide - a couple of passes of the hobby knife was all it took to get a nice clean cut. The payload section and upper part of the tube were set aside so I could start building the new fin unit, which was assembled from a scrap piece of body tube and a piece of basswood. The spiral seams of the tube were filled with thinned Elmer's Fill N Finish, and I glued a 1.5" length of BT-50 coupler into the forward end of the unit so that it could be mated to the rest of the rocket, and also to provide a bit more protection against energetic Estes ejection charges. I also lengthened the shock cord by several more inches beyond that of the original, to reduce the chances of the payload section hitting the body at ejection. Construction is now finished; now comes the finishing, with fillets, sanding sealer, primer, and paint.

Will have to wait for better weather for the sealer, primer, and paint. Hopefully that will come towards the end of next week.

The new fin can, awaiting finishing (Click to enlarge).


  1. Hey Bill. Was curious about your technique for attaching fins. Care to elaborate?

  2. It's pretty much straight out of the Handbook of Model Rocketry - I use Aileen's tacky glue and spread a line of glue along the fin's root edge, which is the pressed onto the fin attachment line. I then remove the fin, wait about three minutes, then run another line of glue down the fin root and reattach. The fin will instantly grab to the body tube and will stay in place until dry. Quoting G. Harry Stine: “…when gluing porous materials such as paper or wood you coat both surfaces with a layer of glue or bonding agent and let both surfaces dry. Then coat both surfaces again and join them together. The first coat of glue on both surfaces penetrates the pores of the material. The second glue coat is then free to join with the first coat and with the second coat on the other surface. A double glue joint will be so strong that the materials will break or tear before the glue joint turns loose.“