Tuesday, September 24, 2019

Making sense of Geezer TARC...

As mentioned in a prior post, rain forced the early termination of the Geezer TARC launch, leaving both me and Duane with an unflown, untested entry. We decided to rectify that situation this past Saturday, and 10 AM found us setting up pads and gear at Pegasus. We were joined by a few unexpected flyers - Matt, a friend of former HARA member Woody Bevil, was already there at the field with his two daughters, and they had rockets. I had my trusty tripod low power pad and controller with me, so it was a foregone conclusion that those rockets would soon head up into the bright blue sky.

Ignition of the Gnome's motor (Click to enlarge).Apogee Apprentice clears the rod on a B6-4
(Click to enlarge).
One of the young ladies was the first to fly - her Estes Gnome performed well on a 13mm 1/2 A motor, but required a little searching to find it in the tall weeds (Hopefully the city will mow Pegasus again sometime in October - the flora is getting a bit wild). Her sister was next, and an Estes B6-4 powered her Apogee Apprentice to a respectable altitude, after which it gracefully floated down to earth on a parachute. Matt had a very nice Semroc Omega, which flew twice in single-stage mode on C11-5's. They had a good day - 4 flights, with 3 perfect recoveries.

Semroc Omega on a C11-5 (Click to enlarge).Omega under parachute (Click to enlarge).
Which is more than I can say for those of mine and Duane...

First up was the rocket Duane had flown at the Geezer TARC launch on September 14, this time loaded with an Aerotech reloadable E28-7. I had passed along a tip based on data I collected at the Geezer TARC launch, which was to lighten the rocket down from 447 grams and consider a different motor. He listened, as the rocket weighed in 19 grams lighter and the E28 had 6 more newton seconds total impulse. This made all the difference in the world - peak altitude was 815 feet, very close to the 800 foot goal. Quite a difference from the dismal 665 feet apogee from the previous Saturday. Unfortunately, the shock cord attachment pulled loose, causing the sustainer to tumble to the ground and the payload section to slowly descend under the parachute. It took about 10 minutes for Duane to locate the sustainer in the weeds; fortunately, there was no damage.

Duane's rocket clears the rail (Click to enlarge).
My turn was next. Heeding the lesson from last Saturday's fiasco, I had removed the kevlar protector from Oeuf and loaded conventional chute wadding. I did a very close check of the igniters in the two Quest D16-6 motors - this would be my very first composite motor cluster flight - and placed the model on the rail. The clips were connected, the count given, and Oeuf shot up the rail, riding two beautiful pillars of black smoke into the sky. I was relieved that both motors fired, but the relief turned to concern as it appeared that Oeuf had performed a little too well - the altimeter would report an apogee of 873 feet. Then concern turned to horror as the parachute failed to deploy. It dragged behind the rocket, fluttering enough to slow the rocket down to 26 feet per second - slow enough to result in no physical damage but fast enough to break the egg (which was wrapped in saran wrap this time). Inspection revealed that 3 shroud lines had ripped away from the Apogee nylon parachute, despite being reinforced with CA as per instructions.

Oeuf under composite cluster power! (Click to enlarge).
Duane and I were having serious recovery problems...

The final flight of the morning was Duane's unflown entry. It too was loaded with an Aerotech E28, and had the best altitude of all the flights - 793 feet, just 7 feet off the mark. This time the recovery system worked perfectly, making for a nice end to our hour long launch.

Duane's 2nd rocket starts its journey (Click to enlarge).
After the Geezer TARC launch, I was looking for a way to see if I could make sense of the data taken during the launch. As you may know, the peak altitude of a rocket is influenced by 3 primary factors - total weight, impulse of the motor, and the frontal area. There are other variables to consider, but these are the big three. I had no measurements of the rocket diameters, but I knew that they all ranged from the diameter of a TARC egg (1.8") to that of a BT-70 body tube (2.2"), resulting in a 50% variation in area. This is significant, but not factors of two or larger, so I figured I could ignore it for my simple analysis, which involved plotting peak altitude versus (total weight divided by motor impulse). I reckoned that the area variation would show up as scatter in the data points.

Here's the result:

Simple Geezer TARC altitude analysis (Click to enlarge).
Note the correlation - Vince's FrankenBertha comes in at a low 6.4 grams per newton-second, so it goes way too high, and Duane's boat tail rocket with 13.3 grams per newton-second comes in too low. There appears to be a sweet spot (highlighted by the cyan band) between 10.5 and 11.4 grams per newton-second that puts the rockets at the right altitude. And, as expected, there is scatter in the data, in part caused by the neglect of the variation in frontal areas. Still, it's somewhat informative, and may provide a good way to check the built TARC rockets to see if they are "in the zone" with regard to weight and motor choice.

I'm glad something useful came out of that rainy day and those broken eggs.

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