Saturday, January 29, 2011

Reviews: Two scratch stand-off scale models

The Little Joe I is stand-off and the Talos is quasi-scale (i.e. even less accurate).

Little Joe I

This is a very stand-off scale model of the Little Joe I. It is 2.6" in diameter, has a 29mm mount, and is based on a custom-turned Mercury capsule that was donated by an anonymous guest for the Nekkid Rocket Photo Contest. On the real Little Joe I, the body is larger in diameter than the capsule. In mine, the capsule fits the body tube.

Construction:
The following are the major components used:

* 2.6" phenolic tube, 8.75 long (Giant Leap)
* 29mm phenolic motor mount (Giant Leap)
* Two centering rings (Giant Leap)
* 1/8" tubular Kevlar®, 10' long (Giant Leap)
* Swivel (Giant Leap)
* Scrap bungee for NC shock cord attachment
* 3/16" plywood fins (Michael's Crafts)
* Rail buttons (railbuttons.com)
* Two T-nuts (Home Depot)
* Lead shot

This was a straightforward build: 4FNC with through-the-wall fins. I got the measurements for the body tube and fins from Rockets of the World (ROTW).

The fins are thick. Not that they had to be, but I had the 3/16" ply and delusions of beveling them heavily. I slotted the fin slots using a Dremel. To make sure I didn't chip the end of the phenolic tube at the slots, I inserted the rear centering ring. Next, I attached the front ring to the motor tube, with the end of the Kevlar® shock cord going through the ring. I used 5-minute epoxy throughout. The motor tube was then installed with the rear ring being dry-fit. The latter was then removed and the fins were glued in. Here I ran into a problem that I seem to keep making: I didn't check the alignment of the shock cord which was epoxied to the motor tube. Of course, it ended up perfectly aligned with a fin slot! The fix was easy, as I notched the tab on one of the fins so that it cleared the shock cord.

I made small external fillets and thick internal ones with Superfil. I also filled the spirals at this point. Finally, I attached the rear ring and the rail buttons. In order for the rail to clear the lip on the cone, I used oversized screws and an extra center spacer between the button and the tube.

For the nose cone recovery connection, I epoxied a small section of scrap bungee into the deep core that the builder drilled for me. I also added enough lead shot to make RockSim happy. With the heavy wooden cone, I wouldn't have thought any would be needed, but the thick fins and motor mount worked against its stability.

For the first flight, I did not build an escape tower. However, I since built one out of bamboo skewers, thin dowel, and a spent 13mm motor. A true scale purist may see the next mistake I made--I scaled the tower based on a 3" body vs. the actual 2.6" tube! I also have painted it per RotW, but haven't added decals.



Flight and Recovery:
I flew the Little Joe on a 24mm F39-6 in my virgin Rouse Tech casing. I used a little dog barf, a chute protector, and a nylon chute. The flight was fairly fast, and ejection was just a little late. Recovery was fine. I am contemplating a G64 next.

Summary:
This is a sturdy but heavy rocket. As long as the phenolic stays away from thing like irrigation pipes, it should survive even hard landings. It is barely scale-like, which is good enough for me.


Talos

This is a almost-kinda-semi-scale model of the Talos missile. When I converted my 21st Century Toys F-104 for flight, I had the jet engine turbine component left over. I found it fit a 2.125" mailing tube perfectly and realized it (almost) resembled the Talos nose cone. The real Talos is 2-stage, but mine combines both stages in a single stage 29mm model.

Since this is made from non-standard parts, I assume no one will ever build it. However, it can be used as yet another example of how to use scrounged parts.

Construction:
Parts list:

* Jet engine turbine extracted from from a 21st Century Toys F-104
* 2.125" mailing tube with telescoping inner tube
* Lead shot
* Eye-bolt
* 8/32" bolt with 2 nuts
* balsa fin stock
* buttons from railbuttons.com
* tube from First-Fire igniter
* 29mm motor tube
* Giant Leap centering rings (two)
* 3/16" Kevlar®
* Wood glue, 5 minute epoxy


I based the length of the body tube and the fins dimensions on a scale drawing by Pete Alway. The dimensions can be found in my RockSim file (in the title header of this review).

The bottom TTW fins went on first and then I sequentially attached each set using a ruler and clamps to make sure the fins were aligned with one another. It sports both a 1/4" lug and rail buttons. The motor mount is unremarkable with 2 rings, a tube, and a single bolt as an attachment point for a motor retainer.

The plastic 'cone' has a central hole and a 1/16" deep ridge that fits perfectly in the 2.125" mailing tube that I'm using. This ridge needed to be augmented to make sure the cone stays on. To form the shoulder, I cut a short piece of cardboard tubing that telescopes into the main tube. I also cut a circle of foamboard that fits in this shoulder and also inside of the ridge on the cone. The eye bolt passes through the foamboard ring and is epoxied into the aforementioned hole. I could have added lead shot and epoxy into the tip of the cone, however, on ejection this mass would pulling against the eye bolt/cone bond. Instead, I added the weight to the inside of the shoulder. Thus, it surrounds and is bonded to the neck of the eye bolt. It took ~6oz of nose weight--about the same as the rest of the rocket.

Finishing:
I painted it with a combo of Rustoleum day-glow orange spray, Krylon X-metals metallic primer, and Testor's metallic green and yellow brush-on. Basically, 'use what ya got'. I also used some black vinyl pinstriping. I added some toothpicks to the front of the body tube to look like the antennae on the real thing.



Flight:
The first flight was on an F39-6. The boost was lazy with lots of coning. I wasn't sure whether there was a simulation error or there was something amiss with the motor. The rocket should have been stable prior to leaving the rail. Anyway, it survived for a second flight.

I decided to submit the review before the second flight since I've started building again and don't know when this will fly again.

Warning: Do not assume my RockSim is correct. If you build a Talos, independently determine the proper CG/CP relationship!

Summary:
I thought this was an interesting way to reuse an otherwise useless remnant from my Starfigher PMC. I think it looks cool considering my choice of color schemes.