Saturday, February 22, 2014

I get some data during TARC practice.

Today was a beautiful day, albeit somewhat windy. Around 1, Duane, his daughter Katie, and I gathered up some rocket stuff and headed out to the pony farm in Harvest. The owner there often lets us fly on his field, and it is almost perfect for TARC birds. When we arrived, Nate Pahman, teacher par excellence, and his Liberty Middle School kids were already set up and prepping to fly. Members of the John Paul II High School team also showed on the field, so there were TARC birds taking to the skies regularly throughout the afternoon. Due to the wind, the rockets had trouble achieving altitude - 801 feet was the best of the day (825 is the goal). There were also the usual "oops" stuff - kicked motors and a nose cone not secured to the payload section, resulting in eggs falling out of the sky and the loss of the nose cone. A typical TARC practice day…

However, today was different - at least for me. Even though I came armed with my trusty stop watch in the event of a qualification attempt, I had also brought a rocket to fly. Not any rocket, mind you, and not my normal fun flights. Today's flights were serious - I was going to do some science!

Back in 1978, Centuri Engineering (by then in the twilight years of its existence) released a rocket outfit called the Power System, which was designed to grow rocketeers from beginner to advanced, teaching concepts such as clustering and multistaging along the way. It wasn't a bad idea, and of course young adult Bill had to have one, especially when I learned that one of the rockets - the RX-16 - featured interchangeable motor mounts that enabled you to convert the model from single motor to cluster power. My original RX-16 has long departed for rocket heaven, but I was so impressed with the design that I cloned one a few years back. It has 3 interchangeable motor mounts - a single 18 mm, dual 18 mm (just like in the original set), and a 24 mm mount for D/E motors (not in the original set). So, today, when I wanted to do science, I called upon the reliable and versatile RX-16.

The goal today is related to my previous post - I wanted to chuck something (to be discussed later) out of the RX-16 at ejection, time its fall, and compare the calculated altitude from the fall time to the altitudes reported by two altimeters in the payload section. Why two? Well, one of the altimeters is the Jolly Logic Altimeter II, which measures not only peak altitude, but also ejection altitude (important for my purpose), motor burn time, various coast times, acceleration, max speed, and descent rate. A lot of data to display on a tiny LED screen! The other altimeter, a Perfectflite Alt WD/15K, is much older and now out of production, but is considered the "gold standard" of small rocket altimeters, having been used in TARC competitions for many years. Unlike the Altimeter II, the Perfectflite can upload its flight data - sampled at 0.1 second intervals - to a computer via a USB connection. It rode in the same bay as the Jolly Logic to act as a check on the upstart.

The downside of all this science gizmo stuff is that the additional weight turned the RX-16 into a pig. I used a rocket app on my iPhone to determine that a B6 would have the rocket claw its way to just 80 feet; a C6 was a little better, with altitudes projected around 250 feet. So, I decided to start the day with flights using Estes C6-3 motors. The C's would do nicely, because D's would send the rocket to about 600 feet; there was a wind, and I was paranoid about losing two altimeters, having already lost one to the cursed rocket eating trees near this field several months ago. Seeing $70 in electronics hanging 50 feet above your head is very painful…

Both rocket flights went well, if you discount the fact that both flights landed in the water-soaked grass ditch near the pads. Fortunately, my birds are painted and sealed, so a quick wipe with a tissue removed the mud and crud. The Perfectflite altimeter beeps out the peak altitude on landing, and I entered this bit of data and the information from the Jolly Logic display into my iPhone's Notes app; I am finding it handier than paper and pen to record short amounts of stuff. 4 PM arrived soon after my second flight, at which point everyone packed up and headed home.

Back at the apartment, I wasted no time grabbing the computer interface kit to the Perfectflite altimeter so I could upload the data to my iMac. Soon the neighbors almost surely heard a stream of curses, as I discovered the altimeter software would not work under Mountain Lion, and that there were no updates due to the product being obsolete. However, it would work under Windows 7 - imagine that! I muttered a few more expletives as I hauled my PC laptop out of the closet, plugged it in, and started the 10 minute Microsoft boot process. Once this was completed, I installed the altimeter software and downloaded the data from both flights into the PC. I then transferred the files to a USB stick so I could analyze the flight information on my Mac. The less contact I have with Microsoft, the better.

Here's the flight data from this afternoon, in graphical form (my RX-16 is shown in the upper right corner):
Flight Data from today's flights. Click here for larger image.

The points are the data from the Perfectflite altimeter; while the horizontal lines indicate the peak altitudes as reported by the Jolly Logic Altimeter II. On the first flight, the agreement was surprisingly close, with the Perfectflite giving a max altitude of 297 feet and the Altimeter II 298 feet. Agreement was not so good on the second flight, with peak altitudes measured at 268 and 277 feet, respectively. Although this looks big, it only amounts to 9 feet, which is about 3% - well within NAR standards.

The dashed lines show fits to the rocket's descent profile as reported by the Perfectflite. According to it, the RX-16 descended at 10.6 mph on the first flight, and 12.0 mph on the second (as is evidenced by the slightly steeper slope). The Altimeter II reported descent speeds of 10 and 11 mph, about 1 mph slower than the Perfectflite. A small difference, but the fact that both speeds were slow is nagging. I will have to make more flights and collect more data to see if this trend persists or if it is simply an artifact of having only two flights.

Looks like I will get a chance this coming Saturday, weather permitting. Who knows? I may actually find the courage to fly the RX-16 with a D motor.

Monday, February 17, 2014

Falling objects and the (largely) unsuccessful search for simple altitude measurements

Between the birth of model rocketry in 1958 and the appearance of small barometric altimeters in the 1990's, there was no small degree of thought and debate given to measuring the altitude of model rockets, especially for competition purposes. Since the beginning (and for the competition purists, even now), optical tracking using two or more theodolites was deemed best, as it could measure altitudes to better than 5%. However, it had/has a few issues:
  1. Equipment fairly difficult to construct and bulky (tripods, etc)
  2. Measured altitude is that at ejection, rather than apogee. This is because it is easy for a ground observer to spot the ejection at a distance, especially if colored tracking powder (chalk) is used to create a small "cloud" when the rocket separates.
  3. The personnel operating the trackers must be adequately trained, else there will be numerous "lost tracks", due to losing sight of the rocket or erroneous altitude/azimuth readings.
The above are not minor issues; in fact, it was proposed in the late 1970's that altitude events be dropped from international FAI rocket competitions because of them. Altitude events were "just too hard to do." After some debate, the proposal was voted down, but many competitions here in the U.S. and other countries featured only duration events (which required just a couple of stop watches) because of the difficulties associated with altitude tracking. There had to be a better way (and with modern altimeters there is, despite the misgivings of a few aging competition rocketeers). In the early 1970's, a group of rocketeers began to think that timing a small falling object ejected from the rocket might be the much sought after Holy Grail of altitude determination.
The Triple-Track Optical Tracker by Trip Barber, Model Rocketeer, February1979,  pp. 8-10
The first order theory behind a falling object in air is pretty simple, and can even be solved analytically without resorting to computers. Imagine for a minute, that you are like my friend Eddie, who likes to jump out of perfectly good airplanes. As you fall out of the sky, there are only two forces acting on your body - gravity, which pulls you downward to the very hard ground below, and drag induced by your motion through the air, which tends to push you up. We all know what the force of gravity is - it's your weight. Drag is a little more tricky, as it depends on your speed (if you ain't moving, there is no drag), your area (the more of you there is, the more drag), and your shape. We can express this drag force in mathematical terms as

where Cd is the dimensionless drag coefficient (related to shape), A is the area of the falling body in the direction of motion, and V is the speed of fall.

So this is what happens when you fall out of the plane. Initially, your vertical speed is zero, which means the drag is zero, and gravity accelerates you downward. As it does so, you pick up speed, which causes the drag to increase, which in turn causes your speed to increase more and more slowly. At some point, the drag force equals your weight, at which point the net force on you is zero. We know from good old Newton's laws of motion that an object with no net force will move at a constant speed, so your falling speed stops increasing. This speed, which we call terminal velocity, is the key to using falling objects for rocket altitude measurements. If I can find an object that reaches its terminal velocity quickly, say within a few feet, I can determine the distance it fell (which is roughly the same as the rocket altitude when the object was ejected) by simply measuring the time of fall and multiplying by the object's terminal velocity. Much simpler than the fancy trig involved in optical tracking, and I can use the same stop watch used for duration events.

But what object do I use?

That is the topic of a future post - stay tuned :)

Side note (because I know you are going to ask): Terminal velocity for humans in a random posture is between 117-125 mph. Some experienced skydivers can assume a bullet-shaped position, which can increase their terminal velocity to over 200 mph. Why they would want to do this is beyond me.

Sunday, February 16, 2014

A TARC Afternoon...

Team America Rocketry Challenge (TARC) pits teams of middle and high school age kids across the country in a contest to be the best in achieving a specific challenge involving mid-power rockets. This year, the challenge is to fly 2 raw Grade A hen's eggs to 825 feet and land them safely - not even cracked - on the ground 48 to 50 seconds after the rocket leaves the pad. It's a pretty tough goal, but we have 9 registered teams in the Huntsville/Madison area who are striving to be one of the top 100 teams who will fly at the national finals in northern Virginia in May. The prizes consist of thousands of dollars to the winning teams and their schools, and a trip to an international airshow (Paris or Farnborough) for the numero uno team. Number 1 will also represent the good ole USA in a match against the champions from other countries who have similar contests.

Today I received an email from club member Duane, who is the mentor (according to the team, his title is actually "benevolent dictator") of the Falcon Rocketeers, which is a TARC group of kids from various schools in the area. They were going to take advantage of today's fine weather to make their first qualification flight (teams are allowed 3; the best two scores are summed to determine which teams make the nationals), and wanted to know if I would be the NAR observer for today's attempt. I responded affirmatively, and 3 PM found me and Duane meeting up with the team at the field next to McDonnell Elementary in Huntsville. The sun was shining, the winds were moderate, and the temperature was in the comfortable 50's - a good day to fly!

The Falcon Rocketeers have two TARC rockets ready for flight. The primary entry is a stubby BT-80 pink rocket with unusual forward swept fins they call "Pink Floyd". The backup rocket is the same design, but slightly heavier and with a green paint job, hence the name "Green Hornet". The rocketeer in me is pleased that the team has given their birds names and personalities rather than simple "TARC rocket 1" and "TARC rocket 2". It would be much better to go to nationals with Pink Floyd than a nondescript number, don't you think? As it turned out, Pink Floyd saw all the action today, while Green Hornet lay patiently in the car. I hope he will fly at a later date - such creativity must be allowed to take to the air.

The first flight of the day was not a TARC flight, but was turned in by a nicely painted Estes Alpha on a C6-5. Belonging to one of the team members, it achieved a height well beyond that capable of landing in the modest field and thus drifted deep into the line of rocket eating trees to the east. A shame, but I had a feeling. One of my fundamental axioms of rocketry is that the odds of losing or damaging a rocket are directly proportional to how nice it looks. This one was nice indeed, so I figured - but hoped otherwise - that it was doomed.

The Falcon Rocketeers launched Pink Floyd on a test flight, in which he achieved an altitude of 803 feet and a duration of 50.3 seconds - a 23 score. TARC is scored like golf - a perfect score is a zero, meaning your rocket hit the altitude and duration marks (825 feet, 48-50 seconds) exactly. So the higher the score, the worse you did. A 23 is pretty respectable for the first flight of the day, and so the team decided to go for broke and attempt a qualification flight. Pink Floyd was once again readied for flight and placed on the pad. Before leaving the pad, the team clasped hands in a small circle around the rocket and had a short prayer. I watched in approval - in TARC, you need all the help you can get!

Pink Floyd did not disappoint his team - he rose to an altitude of 815 feet and landed in 49.995 seconds for an outstanding 10 score. The young lady team member's hands were shaking as she removed the eggs from the rocket; even one cracked egg would result in a disqualification, rendering the score moot. After a careful examination, I pronounced the eggs intact, and cheers erupted. Qualification attempt 1 was a success, and the Falcon Rocketeers had one very good score to send into TARC headquarters!

The day had been kind so far, and the team decided to build upon the momentum and attempt a second qualification flight. Pink Floyd was readied again (I was beginning to feel sorry for the Green Hornet over in the SUV) and placed on the pad. The prayer circle was repeated, a countdown was given, and Pink Floyd once again took to the air. Up, up, up he went… then the seconds ticked away as we waited for the chutes to be ejected… 2 orange parachutes out and open - yay! Hold it… what's that? Something is falling with no chute!

Something was indeed falling - the heat from numerous ejection charges had weakened Pink Floyd's nylon shock cord, which finally broke under the stress of parachute deployment. The sustainer body tube - motor and all - was falling freely, while the egg capsule and altimeter descended safely under the twin chutes. Fortunately, the sustainer fell sideways and so drag slowed its fall just enough that there was no damage upon impact with the ground. However, the TARC rules are very clear - the rocket must descend as one connected unit, and so I was forced to disqualify flight number 2. Very disappointing, but it turns out that perhaps it was for the best. When the eggs were removed, one was significantly cracked, which would have disqualified the flight even if it had been perfect in performance. A sad ending to an otherwise great day, but the Falcon Rocketeers know that they still have one more qualification attempt and are determined that it will be a good one.

I have no doubt it will be so. This team is good!

But I would like to see lonely and forgotten Green Hornet fly just once, even if it is on a practice flight. He deserves that much.

Friday, February 14, 2014

The Big E is having a sale!

Throughout the month of February, Estes Industries (the Big "E", the godfather of model rocketry) is offering daily specials on its website (Click here to see). Normally there are 4 - 3 launch sets, and the rocket du jour, usually discounted anywhere from 60-70% off list. Yesterday, it was the MIRV; a few days ago, the Photon Probe. So today I fired up the web browser expecting to see another bird discounted, hoping that it would be the Alpha, so I could pick up a few for students. A quick click, and I was there…

But no Alpha…

Try 31 - yes, that's right - 31 rockets and starter sets at deep discounts. Among these were 4 Pro Series II rockets (Estes mid power line), normally priced at $60, now only $25. I was in sticker shock heaven!

But I am an adult, so I must show restraint - no more than $50 could I spend, including the $10 shipping. Still managed to get 5 rockets for under $40.

I am happy.

Ya'll ought to check it out. Good deals on rockets!

Thursday, February 13, 2014

In the beginning...

Way back in 1968, my 10 year old self was enjoying a carefree summer morning, and, as was typical in those days, I found myself down the road at a friend's house. Even then, I was a noted nerd, famed for having my nose always stuck in a book, and he said he had just built something I was sure to like. He then pulled out of his closet the most beautiful rocket I had ever seen, all sleek and science-fictiony, with a white body, blue nose, and a black engine pod (An Estes Mars Snooper, as I was soon to learn). I was impressed, to say the least, and the feeling of awe only increased when he explained that the model could actually fly, and showed me the parachute and how the motors were placed. I wanted to know more, so he handed me an Estes catalog and went outside to ride bikes with other neighborhood kids.

For the next hour, I was lost in the world of rocketry. I read about how model rockets work, the motor classification scheme, and how to launch them. I drooled over the various kits - not only was the Mars Snooper something I wanted, but the Astron X-Ray, with its clear plastic payload section begging for insect passengers, was also a must have. By the time my friend returned, I was hooked - I came back several more times that week, each time asking for yet another glance at the catalog. I had to have a rocket, I had to get some motors, and I had to launch it!

Trouble was, rockets were not cheap in those days (nor are they now). A kite could be had for a dime; a quarter made a rich kid. Most rockets were 75 cents or more, the same cost as a 3 pack of motors. A buck fifty was a lot of money for a kid back then, not easy to come by. Fortunately, my birthday was coming up, and I asked my dad if he would take me to the local hobby shop/toy store (A&M Hobbies in East Ridge) and buy me a rocket and some motors. He agreed, with the proviso that I did not catch the back yard on fire (the summer had been dry, the grass was kinda brown, and I was notorious for incinerating green plastic army men with whatever flammable substances I could lay my hands on). The deal now struck, I waited anxiously for my birthday.

Saturday morning (August 3), dad informed me that he would take me to the toy store after lunch, which, for first time in my life, made me wish that Saturday morning cartoons would hurry up and finish, because that would mean it was time to go. The world of Hanna Barbera was left in the dust of the old man's Chevy Impala as we headed to East Ridge to purchase my first space vehicle. Space Ghost and the Herculoids had nothing on me - I was going to have a real FLYING rocket! Visions of an Estes X-Ray, complete with grasshopper passenger, danced through my head during the 20 minute trip. Upon arriving, I leapt out of the car, racing through the doors to the science section in back. I was vaguely aware of my dad yelling that he would meet me at the register in 15 minutes - not a minute more.

It was not hard to find the rockets - they were at the back of the science section, bagged kits hanging from the wall. But, to my great surprise, there were no X-Rays, no Mars Snoopers, no Alphas; in fact, there was not a single Estes kit in sight. The kits were all made by a company called Centuri, which I had never heard of. Confused, I looked them over - Lil' Hercules, Javelin, Payloader II, MX-774. The names were foreign, but I knew I had to select one. Visits to A&M were rare, and I was not going to squander this opportunity. There would be a rocket leaving this store with me!

I liked the looks of the Javelin, which was advertised as a good "beginner" kit; being a beginner, this sounded good. Plus the price was right at the limit - $1.50. I wanted the Payloader II, which had a clear plastic payload section, but it was a dollar more, which would have busted the budget. Pulling a Javelin down from the wall, I went over to the counter where the old man was waiting. I asked the clerk for a pack of 1/2 A motors, which were stored behind the counter and a bag of chute wadding. The total cost: $2.60, before tax.

I spent the rest of the day building the Javelin - cutting out the balsa fins, sanding the leading and trailing edges round, gluing the fins and launch lug to the body, installing the shock cord, and making the parachute. The next day - Sunday - was my birthday, and it would be spent visiting both sets of grandparents. Not that I minded; like every kid, I was spoiled rotten by them. Not even a new rocket could dissuade me from gifts of money and my favorite foods. Painting would have to wait until I got back. I was, and am, not very creative - the paint job I applied Sunday evening was simple gloss black. 2 coats, some runs, allowed to dry overnight.

Monday was launch day, and I woke up wrestling with a minor problem. In order to launch the Javelin, I needed a launch pad and an electrical system to fire the nichrome igniter. The launch pad was fairly easy - I straightened a wire coat hangar to serve as the launch rod, which was inserted into a nail hole in a piece of 15" x 15" pressboard. I knew from reading the Estes catalog, I would need something with a decent current to fire the igniter; flashlight batteries were not going to cut it. Wire was in abundance at our house, as my dad was into electronics (TV/radio repair), but a power source eluded me. I finally hit upon the idea of using my model train transformer. If it could move my heavy cast iron train on the track, I figured it would have no trouble igniting my rocket motor.

This then is my first launch. Imagine the back door to our house standing wide open. The model train transformer is plugged into an outlet near the door, and a pair of 15' wires snake outside into the back yard, ending at the improvised launch pad. The Javelin sits on the pad, parachute packed, igniter loaded into the motor. I twist the bare ends of the wires around the igniter, walk back to the door and connect the other ends to the train transformer. Looking around, I give a short countdown - "5", "4", "3", "2", "1". "0!", slam the transformer swtich to max, and watch as the Javelin streaks skyward. Up, up, up it went, maybe to an altitude of 150 feet. There was a pop, and the black and yellow checkboard parachute deployed, allowing the rocket to descend gracefully in the morning breeze. I still vividly remember that first flight - the thrill I experienced seeing the Javelin fly, the relief when the chute deployed, and the smell of sulfur when I recovered the rocket and brought it back for its next flight. 46 years later, the flights continue.

And that, patient readers, is how I became a rocketeer.