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...

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.

Picking up where I left off...

Back in the 70's, a lot of young rocketeers were members of the Estes Aerospace Club (EAC - see this post for more details). There were a couple of EAC specific kits (the Viper and Pegasus), a newsletter, a membership certificate, and - best of all - an iron-on EAC logo. As you advanced through the EAC, you received various colored "thrust bar" iron-ons, which you added to your T-shirt. This shirt became your badge of rank, and the rocketeers in my neighborhood wore theirs to every launch, showing off their rocketry prowess. You were ok if you had the blue thrust bar. Magenta wearers were scoffed at, and those with just the basic gold were ridiculed. I personally wore the green - the red was unattainable, as Camrocs, Cinerocs, and Transrocs were well beyond the means afforded to me by my allowance.

Estes Aerospace Club levels and requirements (Click to enlarge).

The EAC faded away as the many years passed; then, back in 2004, I discovered that the National Association of Rocketry had something called NARTREK, in which you advanced levels similar to the old EAC (except Estes kits were not required). NARTREK intrigued me... First, there was the name - nothing with 'Trek' in it could be bad - and second, it had been around for a very long time, since 1978. I wondered how I could have not known about it, given that it was prominently featured in Model Rocketeers of the late 70's. I guess I was so wrapped up in college, I neglected to properly read my rocket magazines. If I had, I would have known not only of the program's existence, but that the NARTREK name was not just NAR joined with last half of Star Trek, designed to appeal to us nerds. It actually stands for something, as you can see from the logo:

Well, ok, so there's a little Star Trek mixed in...

The NARTREK creators also did not like the program being compared to the EAC. NARTREK, they said, was designed to help NAR members advance their rocketry skills, preparing them to design safe rockets and to compete in competitions; it was not a gimmick to get young rocketeers to buy more kits. In the words of Chas Russell, the first honcho of NARTREK:

Model Rocketeer, December 1978, page 7 (Click to enlarge).

"ROCKET ON!" How could I not want to be part of a program like that?

And so I obtained my Bronze and Silver packs, and began my voyage through NARTREK. Bronze level was super easy - all you had to do was:

• Make a 60-second parachute duration flight with a kit.
• Make a 30-second streamer duration flight with a kit.
• Fly a 2 stage kit.
• Fly a kit with a D or larger motor.

Did all the flights in a single launch, filled out the forms, attached the 4x6 color glossy photos printed on my spiffy ink jet, and mailed the materials to NARTREK Base (ok, so there's a little more Star Trek). My Bronze certificate soon arrived, along with little bronze NARTREK logo decals to put on my rockets (badges of honor, I suppose).

My Bronze certificate (Click to enlarge).

I started Silver immediately - did the boost glider flight in February 2005 using an Estes Dragonfly (which was a very nice glider, BTW) and followed through with a cluster flight the following weekend (3 B6-4s in my Semroc Goliath). I was rolling!

Dragonfly on the pad before its NARTREK Silver flight (Click to enlarge).

Semroc Goliath stuffed with 3 B6-4s for a NARTREK Silver flight (Click to enlarge).

And then I just stopped...

Don't remember why - probably got sidetracked into some other part of rocketry (TARC maybe?). Anyway, I did not do the other 2 Silver flights (payload and scale) - NARTREK simply went out of my mind and stayed gone until this week, when I ran across my NARTREK packet in a pile of some old rocket papers. I had recently seen some posts on YORF about other rocketeers wanting to finish NARTREK, so I got to thinking that I ought to do the same. But 16 years had gone by - would I be allowed to pick up where I left off, or would I have to start back at the beginning? So I emailed George Scheil, the current NARTREK coordinator. He responded quickly and let me know I could pick up right where I left off, as if it were yesterday. He also informed me that I still had to mail in the paper forms with the photos.

I can't remember the last time I printed photos - probably back in 2004 for my NARTREK Bronze. Talk about nostalgia!

So I have added NARTREK Silver and Gold to my goals for 2021. Time to finish what was started many years ago. I have even located some old 4x6 sheets of glossy photo paper.

NARTREK - something every rocketeer ought to do, even if it takes them forever. Check it out here!

Estes culls the herd...

The 2021 Estes catalog is now out - there are some pretty cool things in there, like an upscale, 24mm powered Mars Snooper (super excited about this!), a rather strange looking addition to the Space Corps line (the DARC-1) and the Antar, a new member of the signature series based on a non-flying model G. Harry Stine built back in the 1950's. The Antar looks awesome - I am eagerly awaiting this kit! Lots of goodies from Estes, and you can hear about them (and great offerings from other rocket vendors) in the NARCON Manufacturers' Forum posted here.

G. Harry Stine holding the Antar (Click to enlarge).

I am so excited about the new things in the Estes catalog that I never notice the discontinued stuff. Never notice, that is, until they become unavailable. However, folks over at Ye Olde Rocket Forum have pointed out that Estes has dropped over 20 kits this year, which seems like a pretty big number! So a few days ago I did a little research and put together a list of the discontinued kits (there's also one on YORF) - 23 in all:

This year's discontinued kits (Click to enlarge).

Some of these are not going to make rocketeers happy - the scale kits are quite popular with old  rocketeers like me. I kinda get the V-2, as there are a lot of these offered by other vendors, in all sizes. However, the Black Brant 3, the Nike Smoke and the Nike Apache are surprises, especially since Estes has been prominently advertising their line of scale models over the past year. I suppose sales of these kits were not so great, and ultimately we have to realize the Estes is a business. Poor sellers have to go bye-bye in order to make money.

So, in the midst of our celebration of the new arrivals, let us take a moment to mourn the dead. They will be missed...

NARCON solves a mystery!

NAR's annual convention, NARCON, started on Friday afternoon and finished just a couple of hours ago. As usual, it was great - lots of informative and entertaining talks, good keynotes, socialization, and news from rocket vendors. But this year was special - COVID had eliminated the possibility of an in person NARCON, so the conference went virtual. This may have initially been met with trepidation, but the meeting was absolutely superb - not one iota decrease in quality - and best of all, a record 500+ people attended (as opposed to 100-150 for an in person NARCON). Not only was the attendance 4 times greater, but the virtual format had the advantage of eliminating one great problem experienced by past NARCON attendees. There are 3 parallel tracks, and you often had to make agonizing choices between two interesting talks held at the same time. This year, the conference vendor was making videos of the presentations available to attendees as soon as they were over, so you could go back at your leisure (up to a month from now) and catch up on the talks you missed. How cool is that!

Part of Saturday's NARCON schedule

So, what were the highlights? Well, I loved the manufacturing forum announcing the forthcoming goodies and the two keynote talks (on Osiris-Rex and commercial spaceflight). There were excellent video tours of LOC Precision, Kennedy Space Center, and the rocket capital of the world, Estes Industries. Among the talks, there was an outstanding video presentation by James Duffy on kit bashing common kits into works of art, some great tips on making and using decals by Randy Gilbert, and how to use and care for common rocket altimeters by Bernard Cawley. Because of the packed schedule I missed several good presentations, and I will be looking at those over the coming days. 

Alphas in WWII fighter livery (Click to enlarge).

Estes Explorer Aquarius with tanks painted to resemble freight cars (Click to enlarge).

And let's not forget Ed LaCroix's proposed logo for this year's NARCON...

Bernard's talk on altimeters solved a long standing mystery that has had me puzzled over several years. My altimeters often reported a sharp dip in altitude at ejection, indicating that the compartment with the altimeter was getting pressurized by the ejection charge. I figured it may have been due to too large vent holes in the payload section (altimeters need to sense the outside air to work properly, and vent holes are the easiest way to accomplish this). So I made my vent holes smaller, but the dip remained, showing up in flight after flight. I had resigned myself to living with it, but in his talk Bernard pointed out something that was so obvious I was kicking myself for not thinking of it.

Balsa is a very porous wood.

I use balsa couplers at the rear of my payload sections to connect them to the rest of the rocket, and so the ejection gases were flowing through the balsa into the compartment, spiking the pressure. The vent holes quickly allow the pressure to drop, but not before the altimeter records the spike as a sharp decrease in altitude. Simple, and you fix it by sealing the balsa - easiest way is to coat the coupler in wood glue. That will block the ejection gases. So I will soon start sealing the balsa couplers on my rocket payload sections, starting with the RX-16.

NARCONs are good - even old dogs like me learn a thing or two.

What to do when OpenRocket no longer works...

Some of you may know that I switched from Windows/PCs to MacOS/Macs several years ago - I got tired of all the issues with drivers, the "blue screens of death", the endless stream of updates, and the relentless virus/malware attacks. However, this change came with costs - the first being that Macs are very expensive compared to the equivalent PC in terms of horsepower. There is also less software available, especially in the engineering area, and you pretty much sell your soul to Apple. Microsoft only wants to own your computer's operating system - Apple takes the whole shooting match, hardware and OS. But I will say that things work very well the vast majority of the time - practically no system crashes, and the software that is available works smoothly and has a "better polish" than the Windows equivalent. I have had no problems with software from the Apple App Store; it's only the stuff one downloads that causes the fits.

Which brings me to OpenRocket...

On all my past Macs (including my MacBook Air laptop), OpenRocket has worked flawlessly; it even worked fine on my new Mac until yesterday, when I upgraded the operating system from Catalina (10.15) to Big Sur (11.1). I did the usual reinstall of Java you have to do when you perform such an upgrade, checked to make sure the Java was working, and then fired up OpenRocket as a test. That's when I saw this:

Well, crap.

I spent the next couple hours googling and trying things with permissions and stuff - nothing worked. My frustration mounted, and I began to understand the feelings of TARC newbies who struggle to get OpenRocket working on their computers. I had often shrugged off their issues by telling them their Java installs were not right, and that if Java was working, there would be no problems with OpenRocket. Well, darn it, my Java was working and the stupid program would not run! I can tell you, I will be much more sympathetic in the future.

For a few minutes, I thought OpenRocket's performances on the screen of my shiny new Mac were a thing of the past, and that I would have to run it on my laptop (which I am not going to upgrade to Big Sur anytime soon!). Then my searching revealed this page, in which someone had posted links to OpenRocket run times and executables compiled for different operating systems. I downloaded the MacOS version, and voila! OpenRocket once again graced the screen of my Mac!

Big sigh of relief.

However, this whole experience once again drove home the fact that major updates to our computer operating systems, driven by security and technology advances, are just as, if not more, frequent than software updates. This is especially true for hobby programs like OpenRocket, which hasn't been updated since 2015, and also for altimeter drivers/codes like those from PerfectFlite. Keeping these legacy codes running often involves hacks or bypassing OS security features, and eventually things reach a point where even that is not enough. Catalina did that to me, as 32 bit codes no longer work. Unlike Microsoft, which provided a fair amount of backwards compatibility, Apple simply forced developers to upgrade their programs to 64 bits. So now I have to find a Windows box to download data from my Perfectflite PNUT. This is the major reason I have largely switched to Bluetooth altimeters like the FlightSketch, as small vendors like PerfectFlite focus on the Windows platform, which is over 90% of the market. Macs are an afterthought, if there is a thought.

I understand that we can expect a new version of OpenRocket sometime in the foreseeable future, which would make me quite happy. It's a great program and needs to be maintained/upgraded.

Back to designing rockets...