raket

Ken announced on Monday that he was heading for 60 Acres on Wednesday morning to fly some rockets, and hoping the weather and soccer camps would cooperate. I ignored his email at first — I usually can’t take time off work on such short notice — but on Tuesday I noticed my meeting schedule had lightened up considerably. So I stayed up until midnight Tuesday, hastily re-rigging rockets that had been under repair and making all the other preparations so that I could join him mid-day Wednesday.

Wednesday morning was cool and cloudy but by midday it was in the mid-70’s (warm for Seattle), humid and downright sweaty when the sun came out. We set up in the northwest corner of the field, hoping the wind — which proved particularly unpredictable — would be coming from that direction. Shortly before I arrived around 11:15, Ken had his first CATO but happily, for all the flame and an aborted flight, no real damage was done to the rocket. He did, however, catch it all on a shaky video.

By the time I arrived, I’d suffered my first casualty as well: a fin on my Baby Bertha had pulled away from the rocket’s body, most likely on the bumpy ride to the field. Ken loaned me a little superglue and after some delay caused by my own lack of focus and a game of musical chairs with the groundskeepers mowing the field, I finally got the Baby Bertha off the ground: my personal tradition to help settle my nerves before moving on to the bigger stuff.

That flight, however, identified the real issue for the day: the ground itself. Out here in the Pacific Northwest, launch season normally runs from October to May: the rest of the year the fields are full of soccer players and/or cows. October to May is also the rainy season, so the fields are not just empty: they’re sodden and spongy from unrelenting soaking they receive. Your rocket descends a little too fast or gets caught by a gust and swings fin first into the ground? No problem: the soft, damp earth will absorb the blow, sparing the rocket from much damage.

In mid-August, as I discovered, it’s a different game. The fields are dry, hard and unforgiving. Hit them any harder than it takes to bend a few blades of grass and you’re risking some real damage, as the Baby Bertha’s second broken fin of the morning demonstrated. Danger wasn’t (generally) to be found in the air today: the landings were the thing to look out for.

My second flight was an Estes Red Nova that I’d modified to fly as a two-stager. Its flight up was great, with the characteristic cork-screwing right as it staged, and the descent appeared “nominal” as people who like to use the word “nominal” say … until the booster thonked front-end first straight onto that hard earth right by the launch pad and crumpled the tube that couples it to the second stage in flight. Then, inspection of the second stage revealed another cracked fin: again a victim of a landing that would have been fine in the winter but was too hard after several months of almost no rain.

Okay, well, if the low-power stuff isn’t going well then it’s time to use more power.

First up was the ever-reliable Estes Doorknob: a great flyer and my most-flown rocket. It went up straight as a pencil on an F15-6 motor (four times more powerful than the booster that got the Red Nova off the ground) and descended nicely too. Despite the parachute not fully opening, it survived impact on landing. The backend of the Doorknob is stout.

Next was the STM-012, which uses an altimeter and tiny flight computer to control the descent by firing off two black powder charges: one to separate the rocket at apogee so it tumbles earthward quickly but not ballistically (straight down), and a second maybe 300 feet above the ground to actually deploy the parachute which slows the remaining descent. That system is usually used in bigger, higher-powered rockets that go thousands of feet in the air (the STM-012 tops out at about 1200 feet), but I used the STM to teach myself the basics of “dual deploy” as it’s called. The idea is to reduce the distance the rocket drifts during the descent by delaying deployment of the parachute until the rocket is closer to the ground.

I was, however, already feeling disorganized — I’ve been carrying around a lot of anxiety and it kind of got in the way when I was trying to focus — and only felt more so when I realized I’d packed the wrong engines for the STM. I didn’t have any that I could fly it with! Ken offered to loan me an “E”-class motor, but I’m not fond of them: the black powder models are prone to catastrophic failures, and the composite versions punch the rocket off the pad like a bullet which I personally don’t enjoy very much. Oh well: the STM was one of the few rockets to survive the day unscathed … because it never flew.

With that, I took a deep breath and started preparing the stars of the show: an Aerotech Arreauxbee-Hi and an LOC IV.

Last March, on the same field, the Arreauxbee flew with a defective engine. The charge that should have deployed the parachute failed to ignite, resulting in the Arreauxbee — a slender, 4 ½ foot long, 24 oz rocket — plunging ballistically downwards from about 1700 ft up and burying itself nose-first in Jolly the Horse’s pen, in the horse farm adjacent to 60 Acres. Jolly was unharmed. After excavating the rocket with the help of a farm hand with a shovel, it was evident that the nose cone and payload bay were destroyed and some cosmetic damage extended further down the rocket body.

I rebuilt the forward end of the Arreauxbee over the summer and equipped it with a dual-deployment rig — like the STM — so it could descend safely even if it flew again with an engine that turned out to be defective. Today was its first flight in its new configuration.

I loaded the Arreauxbee with a “G”-class motor — twice as powerful as the one that had lifted the Doorknob — wired the deployment charges and their igniters to the flight controller, and slipped the rocket onto Ken’s rail launcher. Rail launchers use an extruded length of aluminum rail to support the rocket during lift-off, and are much more rigid than the steel or aluminum rods used for smaller rockets.

The Arreauxbee cleared the rail normally but about 50 feet in the air started cartwheeling sideways, engine exhaust spewing in all directions, until the engine finally burned out and the rocket belly-flopped onto the grass a hundred or so feet east of the launch pad. The flight computer dutifully triggered both deployment charges as the rocket lay on the ground, which drew some laughs.

But what had happened? Usually when a rocket cartwheels like that, it’s a sign that it is inherently unstable. A rocket without an active guidance system — like virtually every rocket flown by amateurs — relies on “static stability”, meaning that its center of gravity — the point that it rotates around when subject to force — is forward of its center of pressure (CP), which is roughly the center of the area the rocket presents to side winds. If the center of gravity (CG) is too close to, or even aft of the CP, the rocket is unstable and will tumble or worse during flight.

But in this case — and I double-checked in the field — at launch time the CG was well forward of the CP and would only move more forward as the engine consumed its fuel.

I did note, however, that the engine had also failed to fire its ejection charge … just like the one that had caused the rocket to bury itself in Jolly’s pen a few months earlier. My thought now is that it was from the same lot as the engine that failed in the spring, with an additional defect that caused the thrust to be off-center or canted away from the rocket’s center line. Either would account for the tumbling behavior: 40 pounds of thrust in the wrong direction is too much force for the fins to correct.

So, I tried again with another engine of the same type, but from a different pack and maybe a different lot. This time the Arreauxbee soared: reaching about 1500 feet altitude. There were two loud “whomps”: first when the programmed deployment charge went off, then when the engine’s own deployment charge fired. The Arreauxbee tumbled downwards in two pieces connected by a long elastic cord to about 400 feet elevation, when the second programmed charged blew out the nose cone and the bright yellow parachute right behind it, allowing the rocket to drift gracefully to earth. Clearly, the first flight had been an anomaly: with a working engine, the Arreauxbee performed beautifully.

Finally it was time for the LOC IV. I refer to this rocket as “Barely Legal” because it is right at the edge of what I’m allowed to fly both in terms of weight and engine size. I was hoping to get a certification to fly larger rockets at the end of the summer, but cancelled my plans so for now the LOC IV on a “G”-class motor is the biggest thing I can put in the air.

On my first attempt to launch it, the igniter spat itself out of the engine, rather than igniting the engine itself. I prepared it for flight with a spare igniter, and went through the countdown. This time the engine gave a huff, a puff, and then belted out a brilliant red flame as it accelerated the LOC IV — a four foot long, four inch diameter, three pound rocket — off the rail and a thousand feet into the air. The rocket reached apogee and then — to my momentary apprehension — dropped sideways like a log out of the sky until the ejection charge finally blew and deployed a big hot pink parachute to carry the rocket safely to earth.

On recovery, all looked good until I noticed that one the rivets that holds the coupler in place between the top and bottom tubes of the rocket had deformed its hole in the body. On closer inspection, each of the three rivets had ripped through an inch or so, lengthwise, of the heavy cardboard coupler. The ejection charge had gone off with such force that the cord holding the two halves of the rocket together had reached full extension before the forward end of the rocket had lost momentum and the sudden shock had caused the rivets to be ripped through the coupler. It’s an easy repair, but I think the LOC IV will be receiving a longer, elastic cord before it flies again.

That was mostly that. The Doorknob flew one more time and — of course — cracked a fin when its parachute failed to open.

Flying in summer is a rare treat here, and I really needed some time hanging out with a friend and just having fun. While I was a little bit chagrined by the toll the hard earth imposed on my smaller rockets, I was also very satisfied by the Arreauxbee’s first post-repair outing. Looking forward to getting many more flights from it and the LOC IV over the next year or two.