Wednesday, March 31, 2021

Learning more about altimeters...

After the flight of the Shell Shocked on March 13, I got to thinking about the data provided by the Flight Sketch Mini it carried to just over 300 feet. The day had been warm for mid March - 71 degrees - and I wondered how far off the altimeter altitude was from the actual value. I knew there would be some difference because

  • All hobby altimeters use the same mathematical model to convert pressure to height, and
  • They all assume a temperature of 59 degrees (15 degrees Celsius) in the model. Warmer temps mean that the altimeter readings are low, but flying on a cold day gives readings that are too high. It is a simple matter to calculate the correction by using the formula

    where T is the temperature in degrees Celsius.

So computing the altitude the Shell Shocked actually reached was very quick. 71 degrees Fahrenheit = 21.9 degrees Celsius, giving a correction factor of 1.024. The Shell Shocked actual peak altitude was 1.024 x 319 = 327 feet.

Easy peasy. Can do it on an iPhone calculator...

However, I soon wondered what the model used by the altimeters was; I figured it was a pretty simple model, probably based on the U.S. Standard Atmosphere. A bit of internet searching proved me correct - the bottom two lines in the below slide are the basic equations. The altimeter measures pressure, so all you have to do is solve the equations for altitude in terms of pressure and Voila! You have the math model used to calculate altitude by the altimeter. The only difference is that the 2116 in the pressure equation is replaced by the pressure measured by the rocket on the pad. This gives you the altitude relative to your pad, which is what you want.

Please ignore the primitive English units - I particularly hate mass measured in slugs; it is just wrong!

So I had the model, and me being me, I had to check it out by taking the altimeter pressure numbers, setting the temperature to 59 degrees, and seeing if the altitudes I calculated matched the altimeter's. Sure enough, they did.

Altimeter readings versus model calculations (Click to enlarge).

Immense satisfaction. Putting in the actual temperature of 71.4 degrees (21.9 Celsius) produces a plot like

Altimeter readings corrected for the actual temperature (Click to enlarge).

Peak altitude is 327 feet, matching the hand calculated correction.

I love math...

Sunday, March 28, 2021

Of rockets and Girl Scout cookies...

A couple of weekends ago, Duane and I flew a few rockets at Pegasus field. We were both needing a launch fix, and Duane was eager to try out the new wireless launch controller he's building for HARA. So Saturday, March 13 saw us setting up the new equipment, down to the warning lights used to indicate that the pad was active. I have to give Duane major snacks - from my perspective, the launch system worked flawlessly, even handling a drag race with ease. Of course, he, being an engineer, came up with a list of things he wanted to tweak. At any rate, HARA will soon have a setup where we no longer have to snake long cables out to the pads - a major step forward!

And now for the rockets...

First up was Duane's Cherokee-D, which did the "as-usual" great performance on an Estes C. Pegasus can't handle Cherokee flights with D motors - the bird simply soars too high. As it was, Duane's model drifted a bit downwind on the C.

Duane's Cherokee-D gets moving (Click to enlarge).Duane chasing it down (Click to enlarge).

The maiden voyage of my Estes Pop Fly was next. Released by the company in 2007 and discontinued in 2008, the rocket consists of a foam/cardboard/plastic "baseball bat" upon which you place a foam "baseball", which pops off at ejection. According to the 2007 catalog, someone is supposed to catch the ball before it hits the ground while the rest of the model comes down by parachute. I was kinda dubious of this one, but it flew very well on a C6-3 - nice stable flight and the ball came off at ejection as advertised. However, no one ran to catch the ball; us old guys watched it descend to the ground and picked it up later. At my age, life is very much a least energy equation.

Pop Fly heads up the rod (Click to enlarge).Pop Fly under parachute (Click to enlarge).

3rd off the pad was my Estes Nike Arrow. This rocket is obviously a SPEV (Spare Parts Elimination Vehicle) kit - seems like Estes may have had a superabundance of Gnome parts, because that's what the upper part of this rocket is. It is definitely not a replica of any member of the venerable Nike series. Anyway, the model was also making its first flight, propelled by an Estes A10-3T.  The Nike achieved a nice altitude, arced over at apogee, and then... nada. Niente, zero, zilch, zip, nil, nothing - there was no separation. I didn't even hear an ejection charge. The rocket core sampled the Pegasus earth, crumpling the upper section and burying the small BT-5 nose cone so deep Duane had to dig it out. Damage is repairable (I'll cannibalize a Gnome kit for the silver upper tube) but the post flight inspection showed that the ejection charge did not fire - the clay end cap was still in place. Later on, I dutifully went to motorcato.org (yes, there is such a site) and filed a MESS (Malfunctioning Engine Statistical Survey) report on the bad A10. Complete with a picture, mind you.

Duane followed my Nike Arrow flight with a launch of his Estes Make-It-Take-It rocket. No longer produced by Estes, the Make It Take It's were packaged in bulk (24 in a box) for schools, groups, and special events. They were basically an Alpha 3 with a different color scheme - same parts, same build steps. As you might expect, the Make It Take It put in a nice flight.

Estes Nike Arrow on an A10 (Click to enlarge).Duane's Estes Make It Take It (Click to enlarge).

Launch #5 was another maiden voyage - that of my Estes SLS. The released version of the prototype I flew back in 2019 for the Apollo 50th anniversary, it is a RTF (Ready To Fly) model - no building, just stuff in a parachute and motor and go. However, I had decided that the first flight of this model would not use an Estes motor - I wanted some real fire and noise. So I popped in one of the new Aerotech "White Lightning" Q-Jets, a C18-4W. As I have mentioned, motors with the White Lightning propellant produce a brilliant white flame and a nice amount of noise. The C18 in my SLS did not disappoint. The rocket shot off the pad faster than a hound dog chasing a raccoon, producing a beautiful exhaust and a satisfactory sound as it streaked into the sky. The parachute deployed near apogee and the model safely touched down on the ground - I was very happy with the first flight of my SLS!

My SLS scoots on a White Lightning Q-Jet
(Click to enlarge).
Coming down under parachute (Click to enlarge).

Duane's 3" BMS School Rocket was next. He decided to live dangerously with this flight, loading one of the notoriously CATO-prone Estes E9's into the model. Both of us expected the rocket to blow into pieces on the pad, but the E9 did what Estes designed it to do. The model lumbered off the rod (low thrust to weight), arced over, and deployed the parachute. Duane had won his throw of the dice and I was left with some pretty standard launch sequence shots.

The School Rocket was followed by Duane's Estes Mega Mosquito, another one of those "I wish Estes had not discontinued this" kits. Textbook flight on a D12.

Duane's BMS School Rocket risks death riding an
 E9 (Click to enlarge).
Textbook flight of Duane's Mega Mosquito
(Click to enlarge).

#8 off the pad was my Estes "Shell Shocked". Introduced in 1998, the Shell Shocked was a rebranded Estes Omloid (first appeared in 1993). Both featured a huge, screw together egg capsule and plastic fin unit. The Shell Shocked was going to make my NARTREK Silver payload flight, but it would not carry an egg - a FlightSketch Mini altimeter would be the payload. I had cobbled together an altimeter "holder" out of a short length of BT-20 body tube and a CR-2070 centering ring - the fit was perfect! Using a push pin, I also punched three holes into the lower part of the egg capsule to allow proper air venting for the altimeter. The C6-3 powered flight went well - the FlightSketch recorded a peak altitude of 319 feet, in fairly good agreement with the 338 feet predicted in the kit instructions. NARTREK Silver payload flight completed - on to scale!

Estes Shell Shocked on a C6-3 (Click to enlarge).Shell Shocked under chute (Click to enlarge).

Flights 9 and 10 were a drag race of mine and Duane's Astrocams. This was intended to check out one of the features of the wireless controller, but I was also excited about the prospects of getting some nice launch shots and a cool video or two. Both models were powered by B6-4's, and my Astrocam left the pad a smidgen ahead of Duane's. The models almost crossed paths a few feet off the rod, and I was very, very, very eager to see the videos, especially from my camera, which should show Duane's rocket just beneath it. The Astrocam was plugged into my computer's USB port as soon as I got back to the apartment and I opened the video folder to look at what I expected to be a masterpiece. NOTHING was there. Crushed, I reviewed in my mind the moments just before the flight, and sadly realized what had gone wrong. I did indeed turn the camera on, but forgot to start it running by pressing the button again. After a little self-deprecating profanity, I called Duane to ask him to send me a copy of his video.  He informed me that there was no video, that he had also forgotten to activate the camera.

The Astrocams clear the pad (Click to enlarge).Almost colliding (Click to enlarge).

I felt a little better... At least I knew there was another dummy on the field that day <evil smirk>. Note to self - next time, bring the camera instructions and read them before flying the model. Might get a video if you do that.

Duane's last flight of the day was that of his red and white TARC rocket powered by an Aerotech F reload. I shy away from reloads - I don't have an organized mind, and the odds of me screwing something up in the assembly process is high. Being "Mr. Checklist", Duane doesn't have this problem, so he flies reloads all the time. However, about 50-100 feet off the pad, his rocket suddenly deployed the parachute and numerous pieces of the payload section fell to the ground - the nose cone hit just a few feet from my chair. Obviously something went wrong, but an inspection of the motor casing showed it to be in good shape and undamaged. Go figure...

Duane's TARC rocket clears the rail
(Click to enlarge).
The smoking rocket descends - Red circles mark
some falling stuff (Click to enlarge).

By this time, we had some company on the field. Chuck stopped by to watch and Doug and his family arrived with a few of their rockets to fly. Duane and I had flown everything we brought, so the pad was theirs - A Nova Payloader took to the sky soon after their arrival. It was followed by a D12-powered red and black Estes V-2, which drifted periliously close to the roof of the Blue Origin building - Have to watch altitude when the wind is out of the east at Pegasus. An Estes Star Hopper flew next, streamer deploying at apogee. The last flight of the launch was Doug's Estes Wizard, painted in the modern catalog decor. 

Doug's Nova Payloader goes up into the blue
(Click to enlarge).
Doug's V-2 lifts off (Click to enlarge).

Chuck watches the Star Hopper (Click to enlarge).The motor in Doug's Wizard ignites
(Click to enlarge).

But the best part of the day occurred after the launchings - Doug's daughter is a scout, and she had a generous supply of Girl Scout cookies in the back of their vehicle, just waiting for hungry old rocketeers to buy them. We handed over some cash, and I happily left the field with 5 boxes of my favorite flavors - I'm down to 1 box (Tagalongs) as I write this post.

Chuck was absolutely right when he said there were at most 2 servings in each box of Girl Scout cookies. They go very quickly.