Saturday, July 26, 2014

A new scale project

This week I travelled to D.C. to present at a conference on spacecraft anomalies. I flew back yesterday, but before I caught my flight out of Dulles, Joe Minow and I paid a brief visit to the part of the Air and Space Museum located very near that airport (it was his idea - a very good one, I may add). The place is pretty awesome, filled with all sorts of planes including the Enola Gay, a Concorde, and a beautiful, beautiful Pan Am Clipper. There was also some space stuff, located in the Space Hanger. The big draw there was the Space Shuttle Discovery; seeing it evokes sadness when I realize that the magnificent machines are no longer flying. There were also some rockets and missiles on display, and I was drawn to the inert round of the U.S. ASM-135 anti-satellite missile. Back in September 1985, one of these was launched from the belly of an F-15, taking out Solwind P78-1, an gamma ray satellite orbiting at 555 km altitude. Even though successful, the program was cancelled in 1988 - yet another government head scratcher (why cancel a successful and obviously important program?). The ASM-135 looked like it could be easily scaled down to a flying rocket, and I took some pics to establish the dimensions and markings.

ASM-135 anti-satellite missile on display at the Air and Space Museum (click to enlarge).

Another view (click to enlarge).

Information plaque on the ASM-135 (click to enlarge).
Today, I measured the rocket pieces (in pixels), and tried to work out a scale. The rear has a smaller thickness, and it turns out the BT-60, ST-13 body tubes are perfect for the top and bottom parts of the rocket if I make it a 1/13th scale model (The original is 214 inches in length). The scale being established, it was fairly easy to create the rocket in Open Rocket - it should fly pretty well on a standard Estes 18mm motor. Only problem is going to be the nose cone, which will have to be a custom job.

Open Rocket 1/13th scale design of the ASM-135 (click to enlarge).
Time to call my friend Gordon (The Sandman)…

Monday, July 21, 2014

A very successful Southern Thunder

This year's Southern Thunder was successful by many standards - good turnout, good food (Thanks Boy Scouts!), and a decent amount of revenue for both HARA and Music City Missile Club. We rely on this launch to generate funds for the coming year's expenditures - launch pad repairs, new rails and batteries, fire extinguisher replacements (used fairly often lately), and miscellaneous odds and ends. I am happy to report that the take for Southern Thunder 2014 was better than last year, ending the downward trend we had experienced over the past few years. We need it - the high power pads can use a bit of work, and fixing them ain't cheap.

ST2014 also saw a near record number of launches - our president, Dan Cavender, shared the numbers with us at the July meeting. There were a total of 483 commercial rocket flights, using 488 motors. In addition, Sunday saw 11 research flights, in which home made motors are flown under the Tripoli Research Association safety rules. That's one of the neat things about Southern Thunder - this year, both national organizations "had their day". The launch was conducted under National Association of Rocketry (NAR) affiliation and safety rules on Saturday, and operated under Tripoli auspices on Sunday. All rocketeers, regardless of affiliation, are welcome at our launches; there's room and sky enough for all.

Anyway, back to the numbers. Dan gave a breakdown by motor type, which could be listed in a table, but I am a visual sort of person. Therefore, I made plots:
Motor count for Southern Thunder 2014 (Click to enlarge).
In the above, the green curve represents Saturday, the orange (which looks beige or purple depending on whether it is above green or blue) Sunday, and the blue bars are the 2 day total. The count is broken down by impulse class (1st letter in the motor type, like the A in A8-3), with the number at the top of the bar giving the total for motors of that class. A few things of note:

  • There were 268 motors burned on Saturday, and 220 on Sunday. There was a surge in mid power (E-G) flights on Saturday, whereas Sunday showed a gradual decline with impulse class.
  • C motors were the most popular on both days (I guess a lot of low power guys wanted to "turn it loose" in that big field).
  • Looking at the pie chart in the top right corner, low power flights (A-D classes) account for 47% of the total. Not surprising, given their relatively low cost and ease of prepping the rockets.
  • G motors were slightly more popular than A motors (which surprises me). A fair number of mid power motors were burned - 158, or 32% of the total.
  • With 45 launched, H's were the most common high power motor. The numbers of I, J, and K motors were about the same, in the mid-teens. At 2, L's were the least popular motor, and there were 6 M motor flights (4 on Saturday, 2 on Sunday). High power, including the research flights, made up 21% of the total - very respectable, given the cost of high power.

The above motor count shows just how great Southern Thunder was, and the numbers really don't do the launch justice. I'm hoping that next year's launch will be even better, and that ya'll will come out, fly with us, and grab a bit of the Manchester sky for yourself.

Thursday, July 17, 2014

Tweaking the Mark 1

The more I thought about the Mark 1's design, the less happy I was about the fins. I like my rockets to be stable without needing weight up near the nose, and those 3 small fins kept coming to mind, kinda like that "Let it go" tune from Disney's Frozen (Disney is truly the evil Empire - stay away from their movies or you will have those medleys dancing though your head for days, morning until night). Even though OpenRocket said the rocket was stable - with the egg and altimeter - I kept having bad feelings about the margin. So I finally broke down and tweaked the design a bit by adding a fourth fin. This increased the stability to 2 calibers fully loaded, which brought some comfort to my tortured mind; I only wish getting that stupid song out of my brain was as easy.

While I was tweaking, I replaced the thick balsa coupler with a thin plywood bulkhead/cardboard tube coupler combo, thereby shedding some weight. The improved Mark 1 design shows 80 feet play room with just 2 D12 motors. If I watch the weight, I may be able to pull this off with just those motors rather than going for broke with the full 2xD12, 2xA10 cluster. That's good, because igniting 2 motors is easier than lighting 4. The big unknown here is how much weight the primer/paint will add.

The tweaked Mark 1 (click to enlarge). I is much happier!
I have started construction of both the Mark 1 and 2. Tubes and fins have been cut, motor mounts and shock cords installed. Tonight I shaped the Mark 1 fins - you can see them in the pic below along with the other parts (the Mark 2 fins are lying off to the left). I hope to have the assemblies completed and in finishing by the end of the weekend.

Mark 1 parts.
Off to watch "Catching Fire" now - maybe a little gratuitous violence can drive Elsa from my mind.

Sunday, July 13, 2014

The old guys take on TARC!

Work travel has been kicking my rear lately - Finland, Houston, and soon Washington. It has definitely put a damper on rocketry activities, but August should see a return to a more normal routine. I certainly hope so, because I have been challenged.

A couple of weeks ago, Duane challenged me, Nate, and Woody to a rocket contest. We are to build rockets capable of meeting this year's TARC goals and fly them for the first time at a launch in mid August; no practicing allowed. The dude with the best score wins the bragging and swaggering rights. In case you don't know or remember, here's a quick summary of the 2015 TARC challenge:

  • Fly 1 raw hen's egg (weight of 2.1 ounces) to 800 feet. Altitude is measured by a Perfectflite APRA or PNUT altimeter.
  • The rocket must be at least 25.6 inches in length and weigh less than 650 grams (22.9 ounces) fully loaded.
  • The payload section containing the egg and altimeter must separate from the sustainer. It must descend by parachute, whereas the sustainer can use any safe means of recovery.
  • The payload section must land the egg safely (no cracks) 46-48 seconds after first motion on the launch pad.
  • Rockets are restricted to less than 80 newton seconds total impulse (F class or less).

Not easy, but I think I can win this. As a matter of fact, I have already come up with 2 designs. Neither have names (yet) - right now, they are known as Mark 1 and Mark 2. The names will come as I build.

The Mark 1 is a BT-70 (2.17" diameter) based rocket powered by a cluster of 4 motors - 2 24 mm and 2 13 mm. Standing 30" tall, it features 3 smallish fins to minimize turning into the wind. Stability is good at just over 1 caliber when loaded with 2 D12's and 2 A10's. As you can see from the Open Rocket simulation below, I should be able to make the 800 foot target with just 2 D12-5 motors - IF the weight is as expected, the drag coefficient is right around 0.75, and the motor thrusts are nominal. Too many things need to be right, so that's why I have the 2 A10's. These little motors provide just enough thrust to buy me a margin of an extra hundred feet in altitude, which hopefully will compensate for higher drag and/or lower thrust.

Open Rocket simulation of the Mark 1 (click to enlarge).
Open Rocket depiction of the Mark 1 in flight (click to enlarge).
The Mark 2 is a more conventional (i.e., non cluster) design. It is a minimum diameter (1.8") bird powered by a single 24 mm composite motor, like an Aerotech E15 or E30. Open Rocket shows that I have over 200 feet of altitude margin, so I will probably have to add a little weight to this puppy if I choose to fly it. Yep, you heard right - I intend on building both of these rockets and will select the model that will fly against the others on the day we launch. Never hurts to have choices.

Open Rocket simulation of the Mark 2 (click to enlarge).
Open Rocket depiction of the Mark 2 in flight (click to enlarge).
The next few weekends are going to see a lot of rocket building.