Semroc - Defender {Kit} (KV-60)

Contributed by Chan Stevens

Construction Rating: starstarstarstarstar
Flight Rating: starstarstarstarstar_border
Overall Rating: starstarstarstarstar_border
Diameter: 1.64 inches
Manufacturer: Semroc
Skill Level: 2
Style: Clone, Cluster
Semroc Defender

Brief:
This is a reproduction of a Centuri design dating back to 1967 featuring a cluster of three 18mm motors crammed into a stretched BT-60. It takes its inspiration from a smaller brother called the Recruiter and the much larger Saturn 1b. If you like clusters, you’ll love this one in terms of sheer performance and design.

Construction:
Craig’s earlier review detailed out the parts list on this one and was accurate. Semroc quality is A-1, featuring Euclid tubes and their own homespun cones and transitions.

The instructions are very good, spanning about 30 steps over 5 pages. This one is best built in phases rather than counting on knocking it off in an afternoon, and I wound up spending about 6 hours on it plus finishing time, which is pretty significant for this one. I would probably rate this around a 3 on the 5-point skill-o-meter as clustering is not for rookies plus there are some challenging fit and finish aspects.

As Craig noted, the first construction step involves gluing together the “fuel tank” tubes and before doing this you’ll want to think through your paint options. One option (Craig’s suggestion) is to paint the tanks before assembly (gloss black) as well as the fin strips a gloss white. Another option is to wait until everything is together and mask/paint per instructions. Either way, you’ll still have some tricky masking. I pre-painted the tanks (glued together first, then painted the 3-tube assembly) and the fin strips, but there are 3 fins that wind up overlapping the fin strips that need white paint plus 3 more fins that get black paint, which meant I had to mask over the black tubes and somehow try to prevent overspray down into the lower BT-60. Having done it that way, I’m inclined to think it’s less of a pain to follow the standard plan and mask off the white fin strips, hitting the tubes and 3 fins with black later.

Once the 3 tubes are bonded together, you then slip a BT-60 over them. It’s a very tight fit and will flatten a bit on 3 sides as a result. At the other end, you tack on a centering ring that also serves to block off some of the exposed tube/exhaust area, which helps the rocket survive multiple flights. The Kevlar® shock cord is anchored in this joint, well protected from exhaust gasses.

The upper BT-60 slips over the centering ring, completing the main body assembly.

There are a total of 6 fins with two slightly different patterns used. Pay careful attention to them as the difference is subtle--one set has a little overlap tab and is designed to extend beyond the lower BT-60 to bond to the fin strips; the other set rests flush with the forward end of the lower BT-60. Alignment is handled via a marking guide template in the instructions (it works OK but I generally prefer a wrap). Before bonding the fins though, you need to slip in 3 fin strips between the fuel tank tubes (these are the white strips in the photo). This is also a very tight fit and winds up stretching the flattened sides of the lower BT-60 back out to a fairly circular shape. Again, think about the paint plan before doing this... Once these are in place, you then bond the 6 main fins.

There is a pretty straightforward payload section on this also, consisting of two tubes and transitions. Note that the upper tube needs to be packed full of clay for weight/stability.

Finishing:
As noted, painting is a bit of a pain on this but can be worth the effort. I pre-painted the tubes black and fin strips white, masked off the tubes, then painted the whole rocket a couple of light coats of white primer. I followed up with 2 coats of gloss white, then masked off the white for painting the 3 black fins and upper payload section.

The decals provided are excellent, covering 3 roll/checker patterns and a bold “Defender” logo/name. The cover art also shows a couple of black trim stripes at the transition seams, which I accomplished using a fine tip Sharpie.

I wrapped up with a couple of coats of Krylon clear coat for protection.

Construction Rating: 5 out of 5

Flight:
First flight was at the National Sport Launch in Muncie, Indiana and wound up being the closing flight of day 1. I hastily loaded the 3 tubes with B6-4s to beat the check-in deadline but forgot to take my clip whip out to the pad. No problem--as closely packed as these 18mm tubes are, I was able to twist all 3 igniters together for a proper cluster prep. I managed to get all 3 lit, and the rocket absolutely screamed up to well over 800 feet.

Craig’s review noted he was a bit early with -6s, and I’ll say my -4s were way too early, as this rocket was still climbing at a good clip.

As an aside, talking to Carl and the Semroc gang at NSL, it seems this can fly OK with two of three motors lighting, but one would not be enough to get it stable off the rod so make sure you prep carefully including packing a little wadding in the tops of each motor to prevent accidentally lighting one in the air off the ejection of others if it didn't catch on the pad.

Recovery:
The kit comes with two 12” chutes for some reason, which are probably an effort to protect the 6 fins. I didn't want the long walk so I just packed one, which was fine despite the fact that my finished model weighed in at 2.8 ounces versus the kit spec of 2.2. You might consider swapping out an 18" if worried about a hard landing.

Flight Rating: 4 out of 5

Summary:
This is a very cool cluster rocket, nice looking, and stylish. Be sure to fly in a big field though, as it will soar on you.

Overall Rating: 4 out of 5

Flights

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