Friday, December 31, 2010

Review: North Coast Rocketry (by Estes) - SA-14 Archer XL

The NCR Archer was a sport scale kit that resembled the RIM-67A Standard Missile. I really love the styling and this remains one of my favorite rockets.  It is a big 4" kit that was intended to fly on the NCR/Estes F62-4 and G70-5 "Darkstar" motors (the G70 was never released).  These were slightly smaller than standard 29mm motors which meant that, without modification, this rocket couldn't fly on those standard motors.  This review will describe not only the original, but how mine has evolved.

[This is a work in progress.  I need to search for more photos, especially of the early version, and try to recall more construction details.]

Kit Construction:

The Archer kit included two pieces of relatively thin walled 4" tubing, a coupler, four pre-cut plywood main fins, four 1/4" balsa strips, plastic centering rings with an integral motor retainer, an almost-29mm motor tube, a Gorilla Mount steel shock cord mount, a wide elastic shock cord, a rip-stop nylon 'chute, and a blow molded nose cone.

The main differences between this Estes era kit and my pre-Estes Big Brute were the non-standard motor mount assembly, the main fins were through the wall, and the nose cone was no longer the Ace model.  The non-standard motor mount was a negative, however, the plastic rings weren't so bad.  The fins upgrade was a positive, and the cone change was neutral.

I was lucky to have a standard 29mm motor on hand so I quickly discovered that it wouldn't fit in the motor tube.  I talked to the people at the hobby shop where I bought it but they were clueless.  I didn't want to return it so I acquired a piece of standard tubing and ground the plastic rings to fit.

I built the rocket with 20 minute epoxy.  It was probably the 2nd time that I'd used epoxy on a rocket.  It's been a long time but I seem to remember fiddling with the motor mount/fins to get them in right.  The Gorilla Mount fit through holes in the upper CR.  I really loved the Gorilla Mounts.  They did a great job of protecting the elastic cords.

The ends of the balsa strips were cut at an angle and rounded off.  Despite being balsa, these have never suffered any damage.

The stock lug was 1/4".  Along the way, I added 3/8" lugs and rail buttons to provide more flexibility on where to launch an less flexibility on the launch guide.

Finishing:
I painted the Archer flat white and have always wished that I has applied a dull coat.  The kit came with a nice decal set.

Initial Flights:
The first four flights were on two F62-6's, an Econojet G38-4, and an Econojet G35-4.  The delay on the F62's was long and the G delays were short.  In one case, a fin popped loose but was easily repaired.  The next flight, also on a G35, didn't work out so well.   I usually bring a stand-off to hold my rockets above the blast deflector, but forgot it this time. So, the rocket sat flat on the deflector. I thought I might get some singeing or exhaust deposits on the rear of the rocket but was lazy and left it as is. BIG MISTAKE! Instead of going up, the rocket sat there in a large fireball. After the fire was put out, the rocket slid nicely off the rail - no evidence of binding. I since received the following explanation - thanks Scott!
Sounds like a classic Bernoulli lock. The Archer is 4" diameter which gives a base area of just over 12.5 square inches. The G35 has a peak thrust of slightly over 17# (for an instant) which then decreases to around 12-12.5#. The Archer itself weighs 2#, which leaves around 10lbf to lift the rocket... but if the pressure under the base is reduced by as little as 1psi, it will never take off, due to the difference in pressure between the atmosphere and the enclosed volume at the aft end. The high-speed exhaust creates a partial vacuum in a similar manner to a venturi vacuum generator.



Repair #1:
The body tube used to go to the base of the fins. If you look closely at the photo above, you'll see grass where the tube used to be. The lower plastic centering ring was toast. The ply fins were fine after some light sanding. I replaced the bottom section of body tube, the motor mount, and added a ply ring.

Another Flight, Another Problem:
On it's return to flight, the Archer went up on another G35-4.  Great boost, no chute, landed in the launch circle; broke fin.

Repair #2:
This time I cut off the lower section and totally rebuilt it. The fins were removed and re-used. I replaced the ply rings, added Tee nuts for motor retention and substituted mailing tube for the NCR tubing. The mailing tube wasn't a perfect match but it was close enough. I attached the new fin can to the Archer with a coupler made from more of the mailing tube (sectioned and rejoined, etc.). I then decided to foam the fin can for added strength. The 2-part foam over expanded and distorted the lower tube so it doesn't mate evenly with the upper tube. I tried to smooth this transition with filler, but it still looks a little funky. It looks fine from a distance, so I lived with it.  Finally, I added a 1/4" Kevlar leader between the Gorilla Mount and the elastic shock cord.

More Flights!
In its latest incarnation, the Archer has flown successfully 12 more times on four G80-7's, a G64-4, two G79-6's, a G77-4, an H128-6, and three H165-6's. I think I like the baby H motors the best for this model! And, the elastic cord is still going strong.