Semroc Nike-Tomahawk

[Bill]

I bought the new Nike-Tomahawk kit with a recent Semroc order.

As usual, it was high quality. The only disappointments, if you can call them that, are the lack of a thrust ring, easily remedied with a dip into my parts stash. Also, the shock cord was surprisingly too short. With the very long Tomahawk sustainer, a lively game of "clackers" can be expected with the insufficiently long stock cord. Again, easily solved with stuff I already have. The only other criticism I can offer is that step 20 on installing the motor mount says to not stop insertion of the mount until it is in the correct place, but does not elaborate exactly what that correct place is: tubes even or the end of the motor hook even with the outer body tube. Luckily, the diagram for a later step showed the latter to be the case.

I was pleasantly surprised to see that the fins are to be built up with a wooden skeleton and a cardstock skin. Honestly, I was not looking forward to sanding a tapered fin with a diamond cross section out of a plank of balsa. Progress on my Arcas kit is on hold awaiting my feeling enough confidence to take that step. I eagerly await the good folks at Semroc to do the same engineering work with the other common Nike fin, the double wedge, in a future offering.

As small and intricate as the parts of the fin skeleton are, I greatly appreciate the beautifully laser-cut parts in basswood.

A model of this size (BT-60 booster and over 3 feet overall length) screams for a 24mm motor mount. I am going to stay with the stock 18mm mount this time so as not to risk overpowering those built-up fins, though I reserve the right to build another with that upgrade at a later time...


Bill

[rocketguy101]

I am going to stay with the stock 18mm mount this time so as not to risk overpowering those built-up fins, though I reserve the right to build another with that upgrade at a later time...


Bill
You shouldn't have any problems with built-up fins...soak the cardstock with CA after you've assembled the fins. Pictured are fins for a semi-scale Terrier-Sandhawk I built from some mailing tubes. The Terrier fins are built up from balsa and cardstock, this rocket flies on F42-4 motors all the time....

[ghrocketman]

While the cardstock fins make the correct scale fin profile without sanding, I HATE cardstock for fins.
I would prefer diamond-profile machine tapered fins.
Short of that, I would prefer plain non-scale airfoil but scale SHAPE basswood fins.
One can guess which fins I used on my Semroc/K29 Saturn 1B fins. NOT the cardstock ones !

[jeffyjeep]

I made the cardstock fins on mine also, but it seems that the rocket would have to come down on the softest grass (or be caught) to prevent damage to the fins.

[ghrocketman]

jeffyjeep-
My sentiments exactly.
WAY too flimsy/prone to damage for my taste.
I have to pick up a sheet of basswood to build mine....out of all the Semroc kits I have built/obtained, this is the largest complaint (the FEW others I have are the built-up S1B fins (but include a solid balsa option), and a few shock cords that have been far too short) I have against ANY of their kits....the 'built up' cardstock fins on this kit.

The biggest pet-peeve I have against any manufacturer is the woefully short shock cords...Semroc is not bad, but most others are HORRIBLY short. Whenever a manufacturer comes up with what they see as a suitable shock cord length, they should INSTANTLY DOUBLE that length and it should be about right. Pass the cost along, I don't care, as then I don't have to substitute or lengthen something.

[jeffyjeep]

I'm a little off topic with this, but this is my starting procedure on most kit builds:

1) slice open kit's bag
2) remove and discard supplied shock cord
3) replace shock cord with a new elastic fabric shock cord that's at least 2 times the rockets completed length

[foamy]

Eh, card stock covering over built-up balsa or basswood? Maybe it's just me, but I don't have a problem with that. I wouldn't think it'd fold or break on ya. I have the conventional fibre fins on the Excalibur and they've held up just fine.

I'll have to check my kit and see what there is to see.

[zog139]

Eh, card stock covering over built-up balsa or basswood? Maybe it's just me, but I don't have a problem with that. I wouldn't think it'd fold or break on ya. I have the conventional fibre fins on the Excalibur and they've held up just fine.

I'll have to check my kit and see what there is to see.


I have used this method with very good success. I have also used 1/64th ply for the skin on mid-power applications. When using the ply I give it one coat of primer, then paint.

[ghrocketman]

I like the 1/64 ply idea. Seems like a sensible minimum for ALL power levels just to avoid 'hangar rash' alone.
Throw the cardstock where it belongs...in a 55 gal. drum "Burner Barrel" and BURN it.

[Doug Sams]

I like the 1/64 ply idea. Seems like a sensible minimum for ALL power levels just to avoid 'hangar rash' alone. I've done this before. It's a great way to make a mid-power fin. In fact, it can be used on H-I-J birds, too. Maybe even higher power than that.

But I don't dismiss papering fins, either. A little copy paper works great on 3/32" balsa.

I'm not as wild about card stock on fins, but I've used it for many other rocketry applications, such as transitions. (I've used 1/64" ply for that, too, but it can be tricky - it's challenging to prevent a flat spot at the seam)

Where I've gotten to regarding fin materials is thus: I try to pick the materials which will give me the best combination of strength and ease-of-finish while avoiding excess weight. Keep mind, the strength needs are dictated by the mission - ie, by the anticipated impulse/power range. I've tried lots of different stuff, but many times the result was too much finish work or negligible added strength (...Lite-ply comes to mind).

In short, if papering the fins makes them easier to finish and stronger without adding lots of weight (relative to the rocket's overall size), it can be a helpful solution. For sure, the 1/64" plywood is easy to finish. And, as zog139 notes, one coat of primer pretty much does the trick for filling the grain.

I've found, where I break the most fins, is due to cross winds on the ground. The rocket may only be falling at 2 feet/sec, but a 20mph cross wind will certainly cause it to hit much harder. And, it seems, the fins tend to get caught at more vulnerable angles when they hit the ground carrying a lot of lateral speed.

In this case, the bad thing about making the fins stronger is that the landing damage can be transferred elsewhere on the rocket, sometimes making it even more difficult to repair than fixing a broken fin. I've seen kinked body tubes, for example. So sometimes it's better to leave the fin alone and let it be the "crumple zone" for the rocket.

Doug

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[Bill]

I have also used 1/64th ply for the skin on mid-power applications. When using the ply I give it one coat of primer, then paint.


1/64th ply instead of the cardstock would have the strength without much weight penalty.


Bill