Just a little bit left to do on the XC before putting it aside for painting weather. Next!
For NARTREK silver I need to build and fly a 3- (or more-) motor cluster rocket. What I’ve decided to do is to clone the original 3-motor cluster — or at least the earliest such kit I’m aware of: the Estes Ranger, K-6.*
The Astron Ranger was introduced in the 1963 catalog and continued until 1971. (There was a Ranger in the catalogs from 1986 to 1989, but that was just Estes recycling a name; the rocket was entirely different.) For those who’ve come into the hobby since then, which emphatically includes me, but who think the Ranger looks familiar, that’s because it gave rise to the most enduring Estes rocket of all. To quote Vern Estes:
Big Bertha has always been my personal favorite. Big Bertha was an offshoot of the 3-engine cluster Ranger. We had a local rocket club called the Astron Rocket Society that met in the garage behind our house. We were all building rockets one evening and I decided my rocket would be the Ranger but with only one engine. I decorated it with decals of nice looking ladies and called it the Big Bertha. I wasn’t sure a single engine would provide enough power for a safe liftoff. Later when the countdown reached zero Bertha lifted gracefully into the blue sky leaving a white smoke trail behind. To me, its slow liftoff was an emulation of the birds lifting off at the cape.
The Big Bertha is pretty much the same shape and size as the Ranger but, as stated, has only a single engine. In addition it breaks at the nose cone, while the Ranger breaks in mid airframe, with the forward 7″ of the body tube acting as a payload section. The Big Bertha was presented as a free plan in the April/May 1963 Model Rocket News and in 1965 was offered as a kit, K-23. (There was no Estes catalog for 1965, and K-23 first appears in the 1966 catalog, but various online sources such as the Estes Kit List begun by Tom McAtee give its year of introduction as 1965.) The kit number later became 1223 and later still 1948… under which number it last appeared in the catalog (with no interruptions) in… well… 2015! It’s been in production continually for 50 years, a feat nowhere near matched by any other kit.
From 1972 to 1982 there also was the Mini Bertha (TK-3, later 0803), a BT-50 downscale of the Big Bertha. A BT-80 upscale, the Super Big Bertha, was offered 1989–1993 and again briefly in 2000. Finally, the Baby Bertha has been in the catalog since 2002. It’s essentially a truncated Big Bertha, with a BT-60 body tube only about 7½” long versus the Big Bertha’s 18″, but otherwise about the same.
I could make something Ranger-like by taking a Baby Bertha kit, trimming back the body tube a little bit and using it as the payload section, and adding an 11″ piece of BT-60 for the main body tube. Add two motor tubes to the one in the kit and that’s it.
It’s not quite Ranger-like enough for me, though. Two reasons. First, the nose cone. The original Ranger had a long elliptical balsa nose cone, BNC-60L; so did the Big Bertha. But in later years the Big Bertha got an injection molded plastic cone, PNC-60L, and later still the blow molded PNC-60MS which was inherited by the Baby Bertha. I’m not particularly religious about plastic versus balsa in this case, but the PNC-60MS isn’t quite the same shape and size as the BNC-60L. It’s a little shorter. If it were all I could get I’d use it, but in fact Balsa Machining Service sells a BNC-60L reproduction, and at a reasonable price.
The second reason is the fins. The Ranger, Big Bertha, and Baby Bertha all have fins that look pretty similar in shape and size, but stack them (or scans of them) up and you find the Big Bertha’s fins are a little longer than the Ranger’s while the Baby Bertha’s are a little shorter. This is probably less visible than the nose cone difference and it wouldn’t bother me a lot, but there’s another issue…
On the copy of the Ranger instructions sent out in the NARTREK Silver package (they send instructions for the Cobra and Scrambler, too, but I like the Ranger better) someone wrote in ‘3/16″‘ next to the BFS-30 part number for the balsa sheet, and when I looked into it I somehow thought the part number was BFS-60 which, sure enough, is 3/16″ thick. I was surprised but decided it made sense. After all, quoting from Estes’s Technical Report TR-6: Clustering Techniques (1963 version — same year as the Ranger’s introduction), “The best fin material for cluster rockets is 1/8″ thick balsa sheeting (BFS-40)”. The 1967 version goes into more detail:
Since a cluster rocket will usually be heavier than a single engine model, it is apt to land harder. In addition, the forces acting on a cluster model’s fins in flight are greater. The result is that the cluster model will need extra strong fins. Big fins should be made stronger than small fins. Because of this one-eighth inch thick balsa sheet is the most popular fin material for cluster birds. Fin stock thinner than 1/8″ can be used, but it should be reinforced for best results. Two reinforcing methods are commonly used: Self-adhesive paper (#PRM-1) can be applied to both sides of the fin or strengthening ribs can be glued to the fin, parallel to the root edge and spaced evenly along the fin…
So with those big, swept-back Ranger fins, 3/16″ seemed perfectly reasonable. By the way, until I read that passage I had no idea papering fins was an idea that went back that far.
But no, the part number was BFS-30, which was 3/32″! And no, the instructions don’t talk about paper laminating or ribs of any kind. Strange! Even stranger, when the kit version of the single motor Big Bertha came out, its fins were 1/8″! Perhaps the warnings given in TR-6 were based on sad experience with the Ranger, but if so, I’m not aware they changed the fin thickness or the instructions for the kit to reflect this newfound wisdom. Maybe they did? But not in the copies I’ve found.
Anyway. With 52 years’ hindsight I think I can comfortably assert 3/32″ balsa isn’t optimal. If I were going to use Baby Bertha fins I’d certainly paper them, at a minimum. And how about basswood? How about basswood, papered? How about 1/2″ fiberglass/carbon fiber laminate, through the wall… no, wait. If nothing else I don’t think four TTW fins would work out very well with three BT-20 tubes in a BT-60 airframe…
Papered basswood it is, then. 3/32″ I think should be okay, in that case. And with no Baby Bertha nose cone or fins, there’s not much point in using a Baby Bertha kit for this. I’ll build from unkitted parts instead.
* I also will be building another cluster rocket, modified from a single motor kit, for a club launch contest — more on that soon. But the NARTREK people want you to start with a kit or proven design, and that’s probably a good idea. So, Ranger first.