More TARC musings...

I keep thinking about TARC - specifically, my rocket's performance in Geezer TARC. I wanted to understand why the simulation programs were giving way too high altitudes, while Thrustcurve.org and the iPhone app were much closer to the actual performance. So I reweighed EggTu, made sure the Rocksim weight matched and forced the simulation to use the "standard" drag coefficient of 0.75 rather than the computed one. This gave a peak altitude that agreed very closely with those provided by Thrustcurve.org and the app, and I was able to match the FlightSketch altimeter data pretty well by reducing the rocket drag coefficient down to 0.70.

Rocksim/Altimeter data comparison (Click to enlarge).

So my takeaway is that both OpenRocket and Rocksim computed too small drag coefficients for this particular design - why, I don't know. At least Rocksim enabled me to verify this by allowing the user to override the program value. With Rocksim more or less dialed in, I was then able to run additional simulations using other F motors and discovered that the F30 I used was not the motor I should have chosen. It turns out that the best motor would have been the F23FJ-7, also a Fast Jack. This motor sims peak altitudes a bit over the mark, whereas the F30 is always under - yet another example of why you should throughly study and sim your TARC design. The proof, of course, is in the flying, and I shall have to launch EggTu with a F23 in the near future - once a) I repair the fin broken at Duane's demo launch last Saturday, and b) when I can get some F23 motors.

It also occurred to me that I need to gather data with a TARC rocket so that I can demonstrate a few of the math things I have been posting on the blog. I need to work out a better design process, as I am getting tired of Duane kicking my can in Geezer TARC. I think I can kill both of these birdies with one stone, and that stone's name is Bob. A very conventional TARC rocket, Bob features a BT-80 payload section and a BT-70 sustainer, with power provided by a single composite F motor. He is a bit more robust than my standard Geezer TARC builds, featuring thru-the-wall trapezoidal fins to minimize breakage - gonna have to make several flights to gather enough data. I did many sims of the design; once again Open Rocket and Rocksim gave high altitudes using the default drag coefficients. I plotted things out to make sure that I have the best motors in my stash - turns out I need a total impulse around 52 newton seconds, which makes the Aerotech F20 and F23 my prime choices. F20s I have, so I placed an order with BuyRocketMotors.com for some F23s - should get them in about a week.

Bob design motor analysis (Click to enlarge).

Time to start building Bob...

The factor that many TARC teams ignore

Duane spent a few hours this afternoon giving a couple of new TARC teams much needed practical experience in launching mid power rockets. Naturally, he had to fly the rocket that propelled him to Geezer TARC glory. It was configured as before, loaded with the standard Mayer TARC motor - an Aerotech F32. I have no doubt he expected to show the assembled teams how close he could come to the TARC goals of 835 feet max altitude and 41-44 seconds duration.

But it didn’t happen.

Much to his surprise, the rocket, which was only 5 feet off the mark in October, soared much higher, to 955 feet!

While talking to him on the phone, I flashed back to the many instances TARC teams have complained that they couldn’t repeat the previous practice’s performance. Everyone acts puzzled, but they shouldn’t be. One should not expect the altimeter in a TARC rocket configured exactly the same to always give the same altitude. If you read my blog post from March of last year, you know one reason why.

It has to do with temperature -  model rocket altimeters compute altitude assuming the temperature is 59 degrees. If it isn’t, the altimeter will not spit out the true altitude, but what it thinks the altitude is. This altitude will be low or high, depending on the temperature. If it is colder than 59 degrees, the altimeter will think the rocket went higher; warmer temperatures will give a lower reading. In NAR competition, flights made in altitude events have the altimeter heights corrected for the temperature, but in TARC, you are stuck with whatever the little beastie beeps or flashes out. No corrections allowed.

So let’s look at the performance of Duane’s rocket on its two flights. After his October 8 Geezer TARC flight, the Perfectflite APRA beeped out 840 feet. The temperature on the field at that time was 81 degrees - hot for October! Today’s flight went to 955 feet; the temperature at Pegasus field was a cool 36 degrees. If we apply the formula in my March blog post to get the “true” altitudes, we obtain values of 876 and 913 feet, respectively.

This means that:

1) Duane’s rocket is performing fairly consistently, with the actual difference in altitudes being 37 feet - not the 115 feet given by the altimeters! 

2) His rocket is also “hot”, as the corrected altitudes are above the mark. Need to add some weight.

So temperature can be a big deal! The graph below shows the altimeter readings as a function of temperature for a rocket that hits exactly 835 feet when the temperature is 59 degrees. This means that TARC teams must add or subtract weight depending on temperature. If the day is cool, weight must be added. If the temp is 60 or higher, weight must be reduced. If the weight is left the same, the altimeter altitudes will vary from practice to practice, and much frustration will arise.

Click to enlarge.

This is just one factor influencing altitude, but it is one that is often neglected. Many teams adjust for wind speed; very few take into account temperature.

Reckon they oughta?

Finishing off some builds

As the pandemic wore on and my motivation waned, three unfinished rockets - a Boyce Aerospace Redstone missile, a Skylance from an Estes rocket plan and a Glencoe Jupiter-C plastic model conversion - caught my eye every time I looked at my workbench. Things got to the point where I couldn’t stand it anymore and resolved to finish them. First up was the Boyce Redstone - I had already spent beaucoup hours sanding the 3D printed parts smooth, so it was a simple matter of glueing them together and applying primer and paint. The former was quickly done, but I had no olive drab paint, so I had to order some. While waiting for it to arrive, I primed the Redstone with Krylon gray and white primers. Then came gloss white followed by the olive drab, which went on nicely (I really like Tamiya paints, even if they are a bit pricey). For markings, I used Stickershock vinyl decals made for this model; however, after application, it was obvious they are a bit on the large side when compared to photos of the actual missile. No matter - it’s a semi-scale model, after all.

Boyce Redstone compared to the Estes Mercury Redstone 
(Click to enlarge).

Next was the Glencoe Jupiter-C. One of the easiest plastic models to convert to flight, it was built following Harry Stine’s article in the November, 1969 issue of Model Rocketry. The decals were provided in the kit and I used the wonderful Tamiya tape to mask off the roll patterns - worked great! 

Glencoe Jupiter-C on the pad (Click to enlarge).

Both these models have scale fins - the Estes Mercury Redstone fins are way over sized, as you can see from the picture - and so required a lot of nose weight. The Boyce Redstone required so much that I am a bit afraid to fly it, fearing that the 3D printed base of the nose cone may not take the stress of ejection. Don’t want heavy things falling out of the sky, but I suppose I will have to risk it soon, as my models do not just sit on a shelf. I flew the Jupiter-C back in December, powered by an Estes B6-2. It flew straight as an arrow, but the Estes plastic chute did not open, resulting in a hard landing that broke Explorer 1. It is now sitting on the workbench awaiting repair.

The Jupiter-C lifts off (Click to enlarge).

And the Skylance? I put it in a corner so I would not have to look at it. Hopefully, I’ll finish it soon - it deserves that much.

Geezer TARC 2021

The TARC Geezers ready their rockets (Photo by Patrick Morrison- Click to enlarge).

I'm back.

Maybe... The pandemic has dragged on and on, and my motivation really began to suffer several months ago. Things are still sucky out there in the world, but my guilt over neglecting the blog has now got the edge over my depleted morale. So I'm going to try to restart blog posts. We shall begin with a summary of this year's Geezer TARC competition.

As you may know, this year's TARC challenge was to fly two eggs "lying on their back" to 835 feet and bring them safely back to the ground in 41- 44 seconds. The two eggs were going to mean a fairly heavy rocket, but them being mounted horizontally was also going to require at least a 2.5 inch tube for the payload section, thereby adding more surface area/drag. "Gonna need F impulse" I thought as I started designing.

I should have listened to my instincts. But we'll come to that in a minute.

I didn't take very long for me to design EggTu - the fairly conventional rocket used a 29 mm, BT-60 sustainer with upscale Alpha fins joined to a BT-80 payload section housing the eggs, Apogee egg cushions and a Firefly altimeter. The sims showed plenty of margin, even suggesting it could make 835 feet on an E30. I was little bit skeptical, so I ported the design over to Rocksim - it gave similar results with a standard drag coefficient of 0.75. My doubts laid to rest, I moved on to building the rocket, installing the 29 mm motor mount "just in case" (very easy to adapt down to 24 mm motors).

Annotated EggTu design (Click to enlarge).

Weather and other things got in the way of the normal September launch, so the Geezer TARC flights had to wait until the October HARA launch in Woodville. The night before, I was once again nervous about using an E30 motor; the rocket seemed too heavy for an E to power it over 800 feet. So I entered EggTu's mass and diameter into my iPhone's rocket app, which spit out 700 feet on an E30. Great... I then went to thrustcurve.org and used that site's altitude calculator. It too gave a peak altitude around 700 feet for an E30 motor. I was in a fix - the two sophisticated sims said 840 feet and the two simple ones indicated 700. What to do?

October 9 was beautiful day for flying in Woodville, with comfortable temps and blue skies. There were four of us competing in Geezer TARC - myself, Duane, Vince and Doug. Doug was a first timer, eager to try his hand against the "veterans" (if there is such a thing in Geezer TARC). Vince's rocket was the usual Frankenstein, featuring an Estes Omega sustainer and a BT-80 payload section. The BT-60/BT-80 adapter was fashioned out of styrofoam, and it looked weak - very weak. My comment to him - which proved to be prophetic - was that I didn't think it would take the flight stresses.

Vince proudly displays his ill-fated Geezer TARC rocket
(Photo by Patrick Morrison - Click to enlarge)

You can guess what happened - Vince's rocket made it to about 160 feet before the adapter broke and the rocket fell out of the sky. Max altitude of 158 feet and a duration of 9.38 seconds, giving him an 803 score. Miraculously, the eggs survived.

Duane's design was the opposite of mine - his rocket featured a 3" diameter sustainer adapted down to a BT-80 payload section. Powered by the Mayer TARC standard F32 motor, it was very robustly built. To be honest, I considered it the better design - not only were the fins shaped to minimize breakage, but the larger diameter sustainer meant he could use standard rail buttons instead of the bulky 3D printed rail guides I had to purchase from Apogee. Elegantly simple.

Duane's Geezer TARC rocket awaits launch (Photo by Patrick Morrison - Click to enlarge).

And it performed fabulously. Duane's rocket soared to 840 feet, only 5 feet above the goal, and was down in 41.76 seconds. A 5 score - hard to beat, even at the TARC Finals.

Doug's rocket raised some eyebrows - not only were the fins very, very small, but he also used a Pringles can as the payload tube. It left the pad and almost immediately flew nearly horizontal, reaching a peak altitude of of 177 feet. The parachute deployed and he managed to get 24.36 seconds duration before the Pringles can touched ground. Score - 724. Now that he has some experience, I'm sure Doug is going to be a contender in future years. At least he now knows not to put too much faith in Open Rocket.

Doug's Geezer TARC rocket begins to arc over
(Photo by Patrick Morrison - Click to enlarge)

A lesson I have yet to learn, as I decided to go with the E30 in my flight. EggTu flew straight as an arrow, but only reached a max altitude of 678 feet. Naturally, the duration was also short, only 38.56 seconds. This resulted in a miserable 2nd place score of 167.

EggTu descends on its two 15 inch parachutes (Photo by Patrick Morrison - Click to enlarge).

It was over. Duane had won Geezer TARC for the third time.

And he deserved it - his design was clearly superior to any of the others, including mine.

Vince got the Flying Pig award for the worst flight. Doug's rocket may have had small fins, but at least it did not come apart.

Just to check things out a bit more, I reflew EggTu a month later at Pegasus field, this time with an F20. It reached 801 feet and was down in 41 seconds - a 34 score. Not good enough to beat Duane, but a damn sight better than its performance on the E30.

Moral of this tale - Trust your instincts!

Duane the winner! (Click to enlarge)

Vince with the "Flying Pig" (Click to enlarge).