Saturday, March 19, 2016

Astron Sprint XL (Clone) - Finished!

I'm taking a break from my series on repairs, simulations, etc. - all the good stuff I got from the failed flight of the Quest Magnum Sport Loader - to show off my new rocket.

After months of working in tiny increments on a bunch of rockets, I finally finished one - the Estes Astron Sprint XL.

This BT-60 (1.374 inch diameter) rocket features a long elliptical nose cone, a tail cone or boat tail, which helps to reduce aerodynamic base drag, and elliptical fins - which are said to reduce induced drag - more on this another time.

Part of what held me back for so long was being unable to paint. Finally, I built a small spray painting booth, in about an hour, for next to nothing, from a cardboard box. It works really well, and I'll write up the build soon.

This is actually a clone of the Estes kit. I had started the kit itself about a week before I started this one. But since this was my first time building a rocket with elliptical fins, rather than straight-edged fins, and since I like to sand most of my fins into airfoil shapes, I decided I needed a practice run before I built the kit.

I simply used the balsa sheet from which the kit fins came to trace some "practice fins" onto some scrap balsa.

This has convinced me that it's a good idea to save the balsa sheet from some of your kits - if you need to make a new fin, you have a template already made. This is especially helpful if you have a rocket with unusually-shaped fins.

Turned out that sanding an airfoil into an elliptical fin requires a little practice, but isn't that hard once you've had a couple tries at it.

The leading edges of the fins are rounded, and the trailing edges tapered. When I build the
kit, I will probably have the sharp, tapered edge extend all the way to the tips of the fins.
Since I'd made a few good fins, I decided not to waste them. Why not build the whole rocket and try to make it as nice as possible? Flying on an E9-6 motor, the rocket may go very high - well over 1,000 feet. And considering the luck I've had with keeping my rockets on the field lately, I figured it might be nice to be able to lose one and still have another at home.

Luckily, the nose cone/tail cone combination is part of the new Estes BT-60 nose cone pack you can buy. I accidentally bought six packs of these - I was trying to buy a pack of nose cones which recently went out of production, and most vendors I ordered from simply hadn't updated the photos on their websites. Rather than raise a fuss or have to mail anything back, I just kept the nose cones I received, knowing I'd use them eventually.

Since this was a clone of a kit I was already building, I tested out a few techniques. Rather than filling the wood grain on the fins with carpenter's wood filler (CWF), as I often do, I tried an old-fashioned model rocketry solution: sanding sealer.

I get a little frustrated with CWF sometimes. I often get warped fins or pock marks from air bubbles in the filler, which I only notice after I've painted the rocket.

The sanding sealer was pretty good, and there is barely any wood grain showing on the final rocket.

The tail cone is plastic, and as such, there is a seam where it joins the body tube.

I sanded the overlap of the plastic until it was an exact match for the airframe tube. But once I applied primer to the rocket, I could still see the joint. I thought about how I might conceal that joint, and decided to try medium thick CA (cyanoacrylate - a super glue which comes in different viscocities for modelers). I ran a bead along the seam and wiped off the excess with a paper towel. Then I sanded it as smooth as I could. It was hard to tell if it had worked after sanding it. But once I did a second coat of primer, it was clear it had worked really well. Primer helps you see the imperfections before you paint.

You can't see the joint.

I was really pleased it worked.

The Astron Sprint XL is such an attractive rocket! I spent a lot of time on it, and managed to get it very smooth, going so far as to sand off tiny imperfections in the paint, then polish and wax the rocket before adding decals.

The decals actually came from a damaged kit. I ordered the Astron Sprint XL a while back, and when I opened it, the kit was half smashed. The vendor kindly replaced it for me, and I sent the damaged kit back for return to Estes - but I must have forgotten to put the decals back in the package, since I found them in the Rocket Room recently. I decided to put them to good use.

It's good to have finally finished a project. I'll soon have finished the Estes Goblin as well. Then, it'll be time to prime and paint my larger Estes Pro Series II rockets - in time to make an attempt at a Level 1 high power rocketry certification flight attempt.

For that, I'll have to build a bigger booth.

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  1. Wow, the finish on this rocket looks top notch! Very impressive with the smoothness and clean lines!

    1. Thanks, Scott. I took extra time to sand every flaw I could find in the primer, and I think using the spray painting booth also helped. For one thing, I painted indoors, so I didn't have problems with wind (or bugs).

      I also wet sanded off a few bumps in the finish (flaws from the paint can, I think), and then used Turtle Wax polishing compound, followed by some brand of car wax (I forget which brand). The build was going really well, so I decided to see how good I could make it look. I'm pretty happy with it.

  2. Nice post! I'm setting up a similar paint booth myself in my carport to avoid wind.