Radical Rocketry's Thunderbird 3 builder's kit: build thread

[prfesser]

Ever since I saw Thunderbirds on TV, around age 12, TB3 has been my favorite rocket of all time (Saturn V is a close second). I was delighted to find that Radical Rocketry had come up with a builder’s kit. BigMacDaddy seems to be apologetic that it’s “merely” TB3-inspired but it’s clearly far better than anything I could have done. And when one considers that there were several different models of TB3 used in the show, and that paint schemes and proportions vary from one model and one web image to the next, and the best drawing in Spaceship Handbook is stated to be the average of several images…well, semi-scale is pretty much "scale" in this case.

This thread is posted in Mid-Power, because although the rocket can fly on a low-power D12, I expect to use E motors, possibly a baby-F. BigMacDaddy's detailed instructions are here.

The builder's kit includes nose cone, central "cooling fins" segment, aft closure, pod "nose" cones and tail cones, 2mm plywood fins, and a bunch of other parts that make this an accurate rendering. You supply BT70, BT60, and BT50 tubes and recovery system.


The parts are beautiful and well-thought-out. The aft closure includes a motor retention ring, bayonet-style rather than threaded; nice touch. The fit of tubes and nose cone to the parts is impeccable; slightly tight in a few places but not overly so; anyway, that’s why the FSM created sandpaper The nose and tail cones for the pods are for BT50 tubes so that 18 mm motors could be adapted therein. I may want to try that in the future with a second kit, but it might require a lot of nose weight with three 18mm motors back there.

There are shallow ridges on some of the parts, longitudinal and about 2 mm wide. I’m going to leave them in place rather than filling them (except on the nose cone) because (A) they’d be hard to fill and sand, and (B) I think it makes the rocket look as though it was actually constructed from ridged, rigid metal.

The two printed parts on the right---cooling fin unit and transition to the BT70---were one part when I got the kit. It split apart at the top of the transition while I was handling it too roughly. No worries, I CA’d the parts back together and added three small strips of fiberglass-epoxy to the interior (all you can see below is the yellow of the epoxy but the fiberglass is there) to ensure that it would stay together in case I dropped it half a dozen times. The way the rocket is designed should make it very difficult to break again at that joint, once all parts are glued together...but I am A Jedi Master at breaking my rockets…


Tube cutting: Six tubes are to be cut. The MMT is BT-50, cut to 7¾” (but measure your setup to ensure the correct length!). About 1/8" projects above the cooling-fin unit, to permit a nice fillet. According to proportions in The Spaceship Handbook drawing, the BT70 should be 2 9/16” long.

BigMacDaddy built his model with a longer BT60, and I agree that it looks better that way and gives more room for the recovery system. I cut mine to 6½”. There's a fair bit of leeway in how long you cut the BT50s for the pods. Mine are 3½” long.

Next step: fill spirals, sand, fill...you know the drill. Nose cone was given a coat of thinned one-part auto body putty and sanded smooth.

Shock cord mount: there are two holes drilled in the upper end of the cooling-fin unit. You’re supposed to thread a thin Kevlar cord through those holes. The Kevlar I have on hand is braided, 2mm wide and too thick to fit. Enlarging the holes would leave a rather thin gap between them, possibly subject to breakage, so I drilled two new holes about 10mm apart.

Had a dickens of a time getting the fat Kevlar through the holes until I CA'd about an inch of one end, then cut at a sharp angle to leave a point. Pair of tweezers. Easy. Tied a fat stopper knot at the end, with a drop of CA to keep it from ever untying.



The plan is to make several partial assemblies and paint them before final assembly. Masking after complete assembly would, in my hands, guarantee a lousy finish. (I didn't think this through as well as I should have...more on that later.) Anyway, the aft closure, BT70, cooling-fin unit, MMT, BT60, and nose cone have been assembled and will be painted. The nose/tail cones for the pods, and aft fins will be assembled to the pods and painted. The forward fin airfoils will be painted black then glued onto the (filled and painted) forward fins. Finally the forward fin units will be glued to the pods, slots will be cut in the BT70 and BT60, and the rest assembled.

Next up: assembly/painting of central parts.

 

[Cape Byron]

So good to see a build thread on this kit.

 

[prfesser]

Assembly and painting of central parts: The aft closure, BT70, cooling-fins unit, and BT60 were epoxied together. The nose cone needed a little masking tape for a snug fit. The whole thing was given a coat of flat black.

The cooling fins were then covered with foil. I didn't tape the edges of the foil because the forward part of this assembly will be covered by the retrorocket ring anyway. And any slight overspray at the bottom will be covered by the gloss black that will go on those fins. The rest was sprayed gloss orange. It's not as smooth a finish as I'd have liked but that's okay.

I *should* have filled the joints at both ends of the BT70. There's a nice black line at both ends. Oh well, it'll be a 10-foot finish, I guess.

After the orange has cured for about a week or so, I'll mask the orange and spray the cooling fins black.

I can see that I'll have a little difficulty with fillets on the fore and aft fins. Fillets don't stick well to paint...but I'll see if the solution I have in mind will work.



Next up: Start work on the forward and aft fins.

 

[prfesser]

Question for anyone: is there a set of decals available for TB3?

And a general comment: motors with a built-in retaining ring will need to be friction-fitted. The 3D printed retainer is not deep enough to handle the aft closure of a 24/40 motor casing. Not a big deal.

Fins and pod tubes: Fins were CNC-cut so there were tabs to cut through and slightly rough places. All edges were sanded and leading/trailing edges rounded slightly. Fins and pod tube grooves were filled and sanded.



At first glance the aft fins appear symmetrical---looks like you could flip them top-to-bottom---but there’s a very slight difference. The distance between the edge of the TTW tab and the corner of the fin is about 6mm at one corner and about 9mm at the opposite corner. If one fin is flipped relative to the others, that fin—and the pod and forward fin attached to it—would be about 3mm forward (or aft) of the other fin units. Unless you cut the slots differently from one another. Not a huge difference but it’s noticeable. There’s no upside-down or right-side-up, at least as far as I can tell, just make sure they’re all oriented the same.

 

[Cape Byron]

Question for anyone: is there a set of decals available for TB3?

This is about as good a reference I have. A pic of the original 'blueprint'.

 

[Tobor]

 

[BigMacDaddy]

Thank you so much for sharing the build thread -- looks great!

Snaking Kevlar through the little holes is always a bit of a pain. I use wood glue on the tip of the Kevlar like you did. I also randomly got a pair of 12" long tweezers from someone that was giving them away and I am so glad I did. That extra length makes it so much easier for a number of reaching challenges inside long NCs or body tubes.

One note: That rear tube is a BT-80.

 

[prfesser]

Well, I stalled out for a while but finally got the gumption to continue what I started...and do something drastic to TB3, which I was hesitant to do.

I cut off the BT-80 tube, cut through the MMT, and removed the aft closure.


Rationale: I am painting most of the parts before assembly, The original plan was to glue fins to the pods and the body tubes with interior fillets. With external fillets I'd have had to paint afterward, and on a scale of 1 to 10 my masking/finishing skills are around forgetaboutit.

As can be seen here, the BT80 had adhered very well indeed to the aft closure. Spent half an hour working with a safe-edge file to file off most of the epoxy and paper---I still need to sand off the remainder. But as you can see the BT80 peeled cleanly from its forward end. And once I'd cut through the MMT, the short piece remaining in the aft closure peeled off cleanly too. Very curious, as all parts were sanded the same and I used the same epoxy.

I'll cut a piece of BT80 about an eighth of an inch shorter than before. It will be epoxied at the front but the aft closure will NOT be glued in place, just used to center the MMT in the BT80 properly. After re-painting, the fin slots will be cut. Without the aft closure in place I'll be able to glue/fillet on the inside of the BT80. Once the fins are glued in place, the aft closure will be glued back on.