Monday, February 20, 2017

Current Projects #3 - Back on the Horse (Lots of Words and Lots of Pretty Pictures)

Admittedly, I've been sitting on this one for a while. Longer than I thought.

* * *

In December, I made a bad decision up in Maine. It was a windy day, the final launch of the season for me, and not good flying weather. The field up there is enormous, and perfectly flat. Nevertheless, it was cold and the wind was about umpteen miles per hour.

But we had rented a car, driven all the way up there, and I wouldn't get the chance to fly again until April. I had what the astronauts used to call Go Fever.

I had made a couple flights already with iffy results. The first was with my Quest Quadrunner, a four-motor cluster rocket.

The Quadrunner struggled off the pad and arced over without gaining much altitude. I thought it was due to the wind, which certainly was a factor, but when I got the rocket home, I discovered that one of the four motors failed to ignite.

Though it is charred from blowback, the motor at the bottom did not fire.

A seven-second delay was too long for this flight. The nose cone popped off and chute opened when the rocket was perhaps 8 feet in the air or less. There was some fin damage.

My next flight was on the scratch built Ceres B booster, from the book Make: Rockets: Down-to-Earth Rocket Science by Mike Westerfield.

It carries a camera payload and can fly on just about any 24mm diameter motor I choose - black powder or composite.

I intended to make two flights with the Ceres B. The rocket had previously only flown with an Estes E9-6 black powder motor, to an altitude of 764 feet.

I was planning to fly once on an E15 and once on an F12, both AeroTech composite motors, and take the rocket to a higher altitude.

The E15 flight went just fine until landing. Then the wind grabbed the parachute and dragged the rocket a good 75-100 feet through the sand. I had to run to chase it down.

Tracks left from the fins. I could tell where the rocket had
flipped over in the sand, as the tracks shifted positions.
Other than some cosmetic damage to the payload section (chipped balsa in the camera window), the rocket survived quite well, despite collecting some soil samples on the way.

I was ready to give up flying for the day after those two launches. But here's the issue: While the E15 is a single-use motor - just pop it into the rocket and fly - the F12 is a reload. You have to assemble it.

Left: AeroTech F12-5 reload kit - 3 motors per kit. Right: AeroTech Reloadable Motor Systems (RMS) casing.

Now, before I start rambling all about composite motors and reloads, etc. (I just deleted three whole paragraphs, due to that rambling), I'll just say this: you don't want to leave an assembled AeroTech reload just sitting around all winter and then try to use it months later. It has rubber O-rings and greased parts in it, and after a while, those parts can get compressed, and fail to spring back. When that happens, you can get hot gasses leaking where you don't want one, and the motor is likely to CATO - basically, the motor blows through the casing and can destroy the rocket.

On recovery, the Ceres B shock chord and chute got so tangled, and my hands were so cold, I couldn't possibly fly it again that day. It would require some major detangling at home.

But I had this motor. It was the first time I'd ever built an AeroTech reload. I couldn't just store it over the winter and use it in spring. And I wanted to know if I'd built it right (AeroTech reloads are generally more complicated to build than Cesaroni reloads. Cesaroni and AeroTech are the two most popular brands of reloadable composite motors).

The completed motor. The red cap on the lower left is the nozzle end. The red cap on the upper right is the ejection charge.
This is why I flew Ceres B on the E motor first. I wanted at least one good flight from in case my F motor had a CATO due to incorrect assembly.

So, I found myself in what seemed like a dilemma - on the one hand, I didn't want to fly any more, and risk losing or damaging one of my nice rockets. On the other, I had this unused AeroTech reload, and I needed to do something with it.

In retrospect, I suppose I could have taken the motor home, disassembled it, soaked the propellant in water to destroy it, and thrown it in the trash (that's how to properly dispose of a motor you don't plan on using - for example, if it's damaged and probably not safe to fly), and cleaned the grease off the casing. Call that motor a loss.

That's not what I did. Since this was my first time using an AT reload, it did not occur to me that this would have been a smarter thing to do.

Instead, I decided to fly the motor. One of the rockets I'd brought with me was one I was pretty proud of - my clone of the Estes Astron Sprint XL.

A very simple looking rocket, but I had gotten the paint about as perfect as can be expected. It was so shiny and smooth!

You may well ask why I would then risk flying this rocket in high winds. My reasoning is that, while I really love the way this rocket came out, it was basically built from scratch. I have the parts to make it over again, and in fact, it was only made as "practice" for the kit, which you can see unfinished at the top of this post, and which I started building over a year ago. I reasoned I could always build this one again, pretty cheaply, if, say, it got damaged by dragging through the dirt like Ceres B.

I installed the F12 motor into the Astron Sprint. There was some wiggle room between the aft end of the rocket and the motor hook, so to prevent the motor sliding back and forth during flight and ejection, I taped it in place. Not pretty, but I figured it should work.

Composite motors are more economical on a per-flight, total impulse basis. What I mean is that you get more power per flight per dollar than you would with black powder motors. A 3-pack of F12 reloads costs about $30, or about $10 per motor, which isn't bad at all, considering that these are F motors.

The casing, however, cost about $40. I didn't want to lose the casing, so I packed the chute using the Jolly Logic Chute Release, and set it to release the parachute at about 200 feet, if I remember correctly. This should have kept the rocket on the field, where I'd be able to recover it.

The Maine Missile Math and Science Club flies from a beautiful field in Berwick, Maine. It's a giant turf farm, with acres and acres of perfectly flat ground covered in short, soft grass. It's nearly like walking an a giant putting green - the grass is that soft. Flights to 10,000 feet are permitted there, and the landowner is happy to have us there. It's perfect for rocketry.

A mile and a quarter long by nearly a mile wide - a great launch site for New England

The Launch Control Officer counted down from five and pushed the ignition button - and the Astron Sprint XL took off like a shot! The F12 is a longer-burning, lower-thrust motor, but for a lightweight model rocket like the Aston Sprint XL, it's got plenty of kick. The rocket climbed a tower of thick black smoke, arced into the wind and nearly disappeared from sight.

At apogee, the nose cone popped off, and the rocket began to tumble. Despite having the parachute tightly held shut by the Chute Release, the wind took the rocket quickly down the field. At around 200 feet, the Chute Release opened, but the parachute was stuck shut - which, considering the wind, was a blessing.

However - and this is the heartbreaking part - despite the fact that the rocket stayed on the field, and the fact that I never lost sight of it completely, it landed in the one bad spot in the middle of all that beautiful turf. There is one long, narrow patch of swampy brush, and the Sprint landed somewhere in there.

I carefully kept a bead on it with my eyes, but when I got to the edge of the swamp, my heart sank a little. It can be easy to walk off course when following a missing rocket, but I knew I was on the right path when I found the one clue - a bit of blue Quest recovery wadding, which I nearly always include in my fireproofing. It must have fallen out of my Sprint as it passed over the edge of the swamp.

Hoping it had perhaps overshot the swamp, I ran around to the other side, but it was no use. I would have to trudge through shoulder-high reeds, thorns, scrub, etc, hoping to locate the downed rocket.

I walked into this, and it got pretty rough in there.

I nearly twisted an ankle a few times, got thorns in my leg, got my foot pinned in some underbrush... And the rocket could have been anywhere. I could have walked right past it several times.

After about 45 minutes in there, I realized I may never find it. I'd have spent more time looking, but it was getting late and dark, and very cold and windy, and the launch was winding down.

So, I lost the Aston Sprint XL clone. But worse - I lost the AeroTech casing and the Jolly Logic Chute Release - a loss of about $170 in one flight, not counting the rocket!

All this brings me to what I've been working on recently.

* * *

In September, Chris Michielssen wrote an inspirational blog post on the Model Rocket Building blog entitled Jake Parker - Finished Not Perfect. It spoke to me.

At the top of this post, you see the beginning stages of a build of the Estes Astron Sprint XL kit. I posted that on my Current Projects page - in December 2015! I only recently realized I had started this simple build that long ago!

The clone - the one I lost in December - was essentially "practice." I like to streamline my fins and sand them into airfoil shapes. It's something I don't find to difficult on a standard fin with four straight edges. But with an elliptical fin, I wasn't sure how to do it or where to begin. So I traced the kit fins onto some balsa, cut out some copies, and started practicing.

Here is a first attempt.

The leading edge is simply rounded.

The trailing edge is beveled, but the bevel doesn't go too far into the fin cord.

A cross-section, seen from the tip edge

Pretty basic, not perfect, but I was able to do it pretty quickly without too much difficulty. It seemed like sanding a better airfoil - and getting all the fins the same - should be feasible. So I tried doing a set of three, with a longer beveled trailing edge.

Marking a guide line down the root edge

The guide lines will help me keep my beveled edges even - so I hope.

A tip from the Model Rocket Building blog - medium CA - cyanoacrylate - on the fin edge, and
a bit of black marker will aid in keeping my sanding straight on the outer edges of the fins.

Guide marks for the beveled trailing edge

I've sanded a little closer to the center on one side of the fin than on the other.
The guide line shows me where I need to even things out.

Checking the evenness of the beveled trailing edges. They'll get sanded down further to nearly a sharp edge.

Once I had three fins which looked the same, I figured I had the technique down. And the fins looked so good, I decided I'd build a whole rocket around them. The current Estes BT-60 nose cone package you can buy includes the nose cone and tail cone from the Astron Sprint XL, so it would be simple to just make another one.

The nose cone pictured at the top is the Aston Sprint XL cone. The tail cone is attached.

I figured I'd end up with two of them, finishing around the same time.

Putting the shaped fins on the rocket. The tail cone was used to install the motor mount to the correct depth, and
was added later, once I had sanded off a slight overhang and filled in the seams from the molding process.

Once I got going, I spent a lot of time and care focusing on the clone. It paid off - the rocket looked beautiful. But the fact that I'd done so well on the clone made me hesitate on the kit - what if I messed up the fins? What if the paint job wasn't as nice? I'll work on it next week, I kept thinking.

This is what I've found. As I've gotten better at building and finishing, better at making my rockets look nice, I sometimes hesitate. I'm not so experienced that I can consistently get great results. I usually get pretty good results, sometimes really good, and sometimes horrible - especially with paint and sanding.

I finally decided I had enough half-built rockets lying around, and how hard could it be to airfoil those fins again, anyway? Besides, I could always make more.

Not only that, but while the Astron Sprint XL clone looked really pretty, it actually wasn't that great a flyer. It rolled really badly! (For rocket n00bs: roll is where the rocket spins around its vertical axis. Most model rockets do it a little. Some do it a lot. It's nice when you build one that doesn't roll at all - the goal for me with most rockets.)

Not only did it roll, but on its first flight, it rolled and wobbled. Such an odd flight! It meant that the center of gravity was not in line with its vertical axis. I wish I had video of that flight, because it was so unusual.

So, I did something when building the Sprint to make the rocket spin fast on every flight. It may have been unevenly sanded fins - one potential pitfall of trying to streamline them. I think a likely culprit was that I got both launch lugs on slightly crooked, causing uneven airflow and inducing roll.

The point is that the rocket looked pretty, but it was far from perfect - it was still just practice. There's nothing wrong with trying to improve your skills and make better and better rockets, but you actually have to build them and accept that you will make mistakes. That's always been a little tough for me.

The Astron Sprint XL needed to be finished, and I needed to start with those fins.

So, I did it, and it wasn't that hard, and they turned out looking great. I did these ones a little different than the last ones.

While on the clone fins, I beveled the trailing edge and merely rounded over the leading edge, on the kit fins I decided to do what I guess it's appropriate to call an elliptically rounded leading edge. That is, instead of simply rounding the edge, I beveled it to a narrower thickness first, then rounded that. This was in an attempt to give the fins a more "teardrop" cross section, appropriate for an airfoil. It's what I tend to do on my rockets with straight-edged fins, when I decide to airfoil them.

Guide marks at specific distances from the root edge

Crossing those guide marks with more marks a specific depth from the edges

Connecting the points where the guide marks cross. For most fins, I'd use a straightedge to mark a guide line.
For these, I was able to use the leading and trailing edges of the fins to connect the points.

Double-checking that the guide lines were the same distance from the leading edge.
I had done the same thing for the trailing edges, which you can see here are already sanded.

Once I got the trailing edges beveled, I gently rounded off any sharp corners left from sanding.
You can see the fin changes from flat to beveled in a nice, even curve.

I find guide lines important when airfoiling fins. Also important - checking my work against other fins.
In this case, I was lucky. I had an extra set from a kit which was smashed in shipping. The fins survived.

Once the leading edges were beveled to a narrower thickness, I rounded them over with a small piece of 220 grit sandpaper.

Leading edges

Trailing edges

Not exactly "teardrop-shaped," but close. A sharper trailing edge (left) and an elliptically-rounded leading edge (right).

Tip edge view
That's where I am at the moment with the Aston Sprint XL.

Speaking of fins, I'm also playing around with trying to shape fins on a scale model, by building them up and sanding them down. Here's a sneak peek.

This is just a first study. It's a work in progress, a rough draft of sorts. When I finish this little experiment, I'll talk about it on the blog.

Editing on the video series on building the Quest Superbird has gotten bogged down, so I've put that build on hold until I can sort out the video I have. But I've completed five videos so far. Click here to check them out if you haven't seen them.

A fun modification I've decided to try is this:

That's an Estes Crossfire ISX with a drag-reducing tail cone. I'm doing it more to have something that looks cool rather than to reduce drag, but that's the idea.

The tail cone is made from a leftover nose cone from an Apogee Components' Avion kit, from last summer's Rocket Camp. One advantage of building rockets with kids is that there are inevitably some left over parts.

Because this is a modification from the Crossfire's original design, it will require me to double check the stability before flight. I'll surely have something about this on the blog in the future.

Apart from that, I'm saving up to replace my AeroTech casing and Jolly Logic Chute Release. I hope to have more successes than failures to write about this year.

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  1. I really enjoy reading your blog. Your story about your lost Astron reminds me of something my friend said when we were thinking about getting GPS trackers: "We spend a bit of money, and try really hard to lose our rocket. It makes sense that we spend a little bit more to help when trying and find it."

  2. Nice build. What is the name of the (Black and Red) tool you used to make guide marks at specific distances from the root edge?

    1. Scott: It's a sliding gauge.

  3. OMG... My wife has one for sewing... Two hobbies meet as one.

    1. When I first saw one, I wondered what it was called and where to find it. I spotted one at a fabric store, for less than a buck - then later realized I had one from years before. I'm not sure what I used it for back then, but it's a tool I use for most rockets now.