Same thing, every year...

I thoroughly enjoy being a TARC mentor; working with the teams is great, and I am thrilled to be part of the whole experience that is Team America Rocketry Challenge. However, mixed in with the enormous amount of good, there is always a little bad - small annoying things that crop up every year, things you try to eradicate through workshops, coaching, and cajoling. But no matter how hard you try, they appear year after year, like great cosmological constants of the universe. Last minute qualification attempts are one of these, but the one I want to focus on in this post is especially vexing to me - the tendency of most TARC teams (and mentors) to not understand the limitations of the Open Rocket or Rocksim simulations.

Every year, I join the teams at Pegasus during their practices, and every year someone always complains that their rocket is not reaching the altitude predicted by the simulations, falling short by as much as a couple hundred feet. Such a huge mismatch must be due to some cause, and according to the team, it is usually a bad batch of motors or lots of drag on the rail, which everyone knows is not accounted for in the simulations. And nearly 100% of the time, the team is wrong - it ain't the motor, and it ain't the rail.

It's the sims - they put too much faith in the altitude predictions.

Open Rocket and Rocksim are fabulous tools, and they are absolutely the things to use in designing hobby rockets in this day and age. However, both programs make some simplifications and assumptions, with the net result that they almost always overestimate the altitude the rocket is capable of achieving on a specific motor. I can prove this with a specific example - my Geezer TARC entry for this year, the Eggsploder. Here is the altimeter profile of its flight from September of last year:

Eggsploder PNUT altimeter data (Click to enlarge).

You will note that the peak altitude is around 840 feet, 40 feet off the 800 foot goal (not bad for the first flight, eh?). This is the actual performance; now let's pull up the Open Rocket simulations:

Open Rocket simulations for Eggsploder (Click to enlarge).

Take a gander at simulation 15, which is an exact match to the weight of the rocket as flown under zero wind conditions. It predicts a peak altitude of  976 feet, 136 feet higher than that of the actual flight. Simulations 16 and 17 are for wind conditions of 5 and 10 miles per hour; even they are over 75 feet higher than actual.

So Open Rocket over predicts... What about the other guy on the block, Rocksim?

Rocksim simulations for Eggsploder (Click to enlarge).

Note the last simulation - 1000 feet! This is what Rocksim predicts if I let the program calculate the drag coefficient, which is the default when you create a rocket in the software. This is not too different from the Open Rocket results (~30 feet difference) and still way higher than the model actually flew. However, Rocksim allows you to manually adjust the drag coefficient (You can somewhat do this in Open Rocket by changing the roughness of the surfaces, but this is very crude). This enables you to match the sim to the altimeter readings, which is what I tried to do in the first two sims. Sim #1 is for a drag coefficient of 0.63, and simulation #2 is for a drag coefficient of 0.65; both these numbers are higher than the Rocksim determined drag coefficient.

Bottom line - If I choose a motor combination for Eggsploder that gives simulated altitude of 800 feet, I would be very unhappy with the performance, as the rocket would struggle to make it to 700 feet, well short of the goal. This is why, year after year, I always tell the teams to sim high - you can always add weight to bring the altitude lower, but it is very hard to raise the altitude without changing to a more powerful motor, which can mess with stability (bigger motor = more weight in back = less stable).

So I write this post, hoping that next year's teams will read it and pay attention. Maybe next year will be the year.

But I'm not going to get my hopes up...

Decals and wraps in Open Rocket

Centuri Payloader from 1962 catalog (Click to enlarge).

HARA has received an email from Levi's (the clothing company) looking to do a late 1950's/early 1960's period photo shoot involving model rocketry in Huntsville. So yesterday, I started inventorying my retro clones and noted that I had plenty from the 1970's, but very few from the earliest days of our hobby. This realization planted my carcass in front of my computer, where I began a search of the early catalogs on Ninfinger's site, looking for some models that could be quickly cloned. The 1962 Centuri Payloader - a shorter, skinny ancestor of the popular Payloader II - caught my eye; simple 3FNC with a clear plastic payload section and white decor with a black roll pattern. Easy peasy, and there was a Rocksim file for this model posted on the Rocket Reviews web site.

However, I did not like the fins in the Rocksim simulation - they seemed to have too great a span, so I went back to the catalog image and made a few measurements, assuming a ST-7 (0.759" diameter) and a rocket length in the vicinity of 15". I then created a simulation in Open Rocket, which looked to be a much better match to the catalog, and dashed off a quick order to eRockets for the parts. This ought to have made me happy, except that a plain white rocket in Open Rocket is, well, blah. So how do I get that roll pattern into the sim?

Past attempts at incorporating decals into Open Rocket had failed miserably, leaving me to think those that could were possessed of some great arcane knowledge not dispensed to ordinary mortals. But last night a Google search turned up this thread on The Rocketry Forum, the second post of which was like reading the Rosetta Stone. All you have to do is:

• Use a paint program (MS Paint, Pixelmator, etc.) to create an image wrap for the body tube. This wrap should have dimensions length x (pi x diameter), scaled by a suitable factor to permit easy drawing. In my case, the body tube was 10" long, so the clipboard should be at least 10 x (0.759 x pi) or 10 x 2.38. This is way too small, so I converted to centimeters and multiplied everything by 50, yielding an image size of 1270 x 303. This is usable, and soon I had it drawn up in Pixelmator (see below). Rotate it 90 degrees, save it to a JPG file, et Voila! You are done with the hardest part.

Centuri Payloader wrap in Pixelmator (Click to enlarge)

• Now open the simulation in Open Rocket, select the body tube and click on edit.  Choose the appropriate color for the body tube (white in my case) and then change the "Repeat" option to "Clamp Edge Pixels". Under Texture, select From File, find and select the image wrap file, then click OK. Switch the view in Open Rocket to 3D Finished and you should see your rocket's body tube nicely decorated with your wrap,

Finished Payloader design in Open Rocket (Click to enlarge).

There's more stuff in the thread dealing with fins, but this was enough to get me started. I am pretty pleased that my designs and sims now will be a lot less plain!

Centuri Payloader in Open Rocket's Photo Studio (Click to enlarge).

Back to NARCON...

This past weekend I was in Houston for the National Association of Rocketry's annual convention, NARCON. It's a two day affair, filled with talks, vendor exhibits, and over a hundred rocket geeks - the perfect venue for an old geezer rocketeer like me to hang out and take in this sport we call rocketry. James Duffy and the Texas sections (rocket clubs) hosting the event did an outstanding job organizing and running things, and I can safely say that everyone present thoroughly enjoyed themselves. NARCONs are held in cities that sport some sort of space-related attraction, and this year's convention was held at the Hilton across the street from Johnson Space Center and just down the road from Space Center Houston. This presented mucho opportunity for the participants to experience some space program history, and many did. Who can pass up a chance to see one of the majestic Saturn V's or to climb into the Shuttle carrier aircraft and into a mock Shuttle cockpit?

James Duffy gives tips on how to build a scale Little Joe from scratch (Click to enlarge).

So what was special about this year's NARCON? There were many good talks; Trip Barber gave a presentation with tips on how to win at TARC and there was an excellent status on the Museum of Flight's G. Harry Stine collection. However, there were three that stood out in my mind:

1) Joe Barnard of BPS.space thrilled the audience with how he developed his thrust vector stability kits for finless model rockets. It was kinda like watching the old Wild World of Sports - there was the idea, the agony of defeat (over and over) in the beginning, then finally the thrill of victory. Joe's stuff ain't safety code legal, but it is cool as the dickens and the audience was incredibly stoked by his presentation. Check out his website for some neat video and details.

Waiting for John Beans' talk to begin (Click to enlarge).

2) John Beans of Jolly Logic gave a presentation on what he calls "Rocket 2.0." He pointed out that we have been using pyrotechnic charges to recover hobby rockets for 60 years, and that deployment charges, while fairly reliable and of extremely low weight, still have a big issue - we use delay time as a proxy for deployment at a specific altitude. Mess up the delay time, or if the ejection charge doesn't work, and it's disaster time. John is musing upon a low energy notion in which a parachute section separates like a fairing, letting the parachute fall outside the rocket. This parachute is then allowed to open by a Chute Release-style device. He produced a 3D printed concept to illustrate his point. Mr. Beans is quite an innovator - his Chute Release has transformed mid power and lower impulse classes of high power - and I really look forward to see what comes out of his shop in the next year or two. Exciting times are ahead!

Folks start to gather for Gary Rosenfeld's "History of High Power Rocketry" talk (Click to enlarge).

3) Gary Rosenfeld of Aerotech regaled us with the history of high power rocketry, taking us from its beginnings with Irv Wait of Rocket Development Corporation (which was bought by Centuri and became their Enerjet line) up to the Aerotech fire, which gave room for Cesaroni to grab a share of the composite motor market. There were lots of pictures, and a lot of good ancedotes (I was amused by the name of an early HPR rocket - the "E-legal", and stunned by the images of the Aerotech facility after the fire). Gary was quite the rocketeer in his younger days - it takes a wild man to cluster composite motors - and was present for many of the events that shaped the development and adoption of high power. The hour went by all too quickly.

Part of the vendor room (Click to enlarge).

There were no big announcements at the vendor forum Friday night, other than Aerotech announcing its 18 mm composite A and B motors, which will be available in 2 packs this month. As usual, I could not keep my money in my wallet - I ended up purchasing a Mercury Redstone skin from John Pursley and an Odd'l Rockets F-104 kit from JonRocket. Only the lack of luggage space prevented me from buying more.

The traditional banquet was held on Saturday night, with the speaker being astronaut Scott Parazynski, veteran of 5 Space Shuttle missions with 47 hours spent outside on EVAs. He recounted his experiences of orbiting Earth, and of climbing Mt. Everest (twice - he succeeded on the 2nd attempt). It was a fascinating presentation, and each of us was lucky enough to receive a free copy of his book "The Sky Below"; Scott kindly took the time to autograph the books people brought to him -   a true gentlemen.

Sunday morning I packed my things and headed off to the airport - it had been a very good NARCON. Good talks, cool rocket stuff to buy and poke at, and lots of people to talk to. The latter is perhaps the best aspect of NARCON; it gives us a chance to link up with old friends from distant parts of the country and to meet new entrants in the hobby, filled with enthusiasm and excitement. A hobby for just one is not very exciting, and I was reminded that there is more to rocketry than TARC launches at Pegasus field. I came back with new goals and with new energy.

My signed 1970 Estes catalog (click to enlarge).

I also got Vern and Gleda Estes to sign my 1970 Estes catalog. It is now right up there with my Centuri catalog signed by Lee Piester. One of my bucket list items is now checked - I have the autographs of the Rocket Royalty.

More scale modeling...

Hawk missiles on their launcher (Click to enlarge).

At the December HARA meeting, Elliot Laramie gave a tech talk on the Hawk anti-aircraft missile, which was first deployed way back in 1959. Despite being nearly 60 years old, the missile system has undergone improvements over the years - the last major one being in 1995 - and is still in use by several countries, including Egypt, Israel, Saudi Arabia, Turkey, South Korea, and Greece. The U.S. phased out the Hawk in favor of the much more capable - and way more expensive - Patriot in 2002. However, the cost of the Patriot is such that many countries with Hawks are opting to go with far cheaper upgrades to their existing missiles rather than buy Patriots. This is for good reason - the Hawk has proven itself to be a decent anti-aircraft weapon over the years, with over 80 kills. Jane's gives a single shot kill probability of 85% for the 1995 Hawk. However, as any missile man could tell you, Hawks are launched in pairs or more, adding to the lethality of the system.

There have only been a few flying scale kits of the Hawk produced - Estes produced a not-even-close-looking Army Hawk from 1990 to 1992, and The Launch Pad put out a much better kit of the MIM-23A version, but this one is very hard to find with the company now out of business. However, scratch builders have better luck, as Peter Alway gives plans and tips in his "Scale Bash" booklet for constructing a Hawk in one of 4 different sizes (BT-5, BT-20, BT-50, and BT-55). This Hawk information is shown in the scanned image below.

Hawk page from Peter Alway's "Scale Bash" (Click to enlarge).

Acting on a whim, I took the dimensions from Peter's BT-55 version and entered them into Open Rocket. I found that the rocket required a lot of nose weight - 70 grams - to get a decent margin of stability with a D12 motor. This amount of weight jives pretty well with the Alway notes, which state that the CG needs to be 9.4" from the nose; my CG is 9.0" from the nose with a D12-5. I went with a 24 mm motor mount because you can always adapt down to smaller sizes, and because I liked the performance (900 feet) on the D12.

Open Rocket sim of a BT-55 based Hawk (Click to enlarge).

Open Rocket rendering of Hawk in flight (Click to enlarge).

I suppose that I must add this rocket to the build list, given that I went to the trouble of simming it in Open Rocket and also because it would make a nice addition to my collection of scale models. If you happen to be interested in building one for yourself, my Open Rocket file can be downloaded from here.

Warm December launch at Pegasus...

(Warning - this is a long post with many pictures)

Some of the activity at Pegasus field this Saturday; the Jurassic TARC team is in the foreground (Click to enlarge).

Huntsville has been experiencing a period of warm weather with little wind of late - a phase that is projected to end this Tuesday. Yesterday's conditions were so good, they positively demanded that rockets be put in the air, forcing me and Duane to join some of our colleagues and a couple of TARC teams at the field for some flying fun. We arrived at noon, and the next couple of hours saw a lot of activity, so much so that I had a hard time keeping up with it. What follows is a brief summary of that launch, which I figured I'd better post before I forget many of the details.

Duane giving advice to some members of Jurassic TARC (Click to enlarge).

"Velocity Raptor" on the pad (Click to enlarge).

"America" is launched (Click to enlarge).

The Falcons setting up their range table (Click to enlarge).

The two TARC teams turning out onto Pegasus yesterday were both familiar, being Duane's Falcon Rocketeers and Jurassic TARC from Pope John Paul II High School. Saturday saw their first practice flights, with each team getting one - and only one - flight in. Jurassic TARC launched first, and their "Velocity Raptor" rocket struggled to reach 691 feet on an Aerotech F42. Might have had something to do with the too-short 4 second delay, which also caused the shock cord to pull loose from its mount, sending Velocity Raptor back to the Mesozoic Era. The Falcon Rocketeers had their rocket - the red, white, and blue "America" - ready a few minutes later. The F42-8 motor powered it to an altitude very close to the goal - 808 feet - but the payload section pulled free of the sustainer, causing the payload to tumble to Earth. It too was out of action; hopefully these teams can overcome the bad mojo next time out.

"Fat Chance" starts its first successful flight (Click to enlarge).

Speaking of Duane, he only flew one bird. Determined to show that he could actually have a rocket land safely, he brought out "Fat Chance", equipped with a new shock cord mounting system. Fat Chance put in a beautifully straight flight to 862 feet on a F32-6, and held together at ejection. Both parts of the rocket made a gentle landing under parachute, and we congratulated Duane on breaking his curse (which seems to have transferred to his TARC teams).

Allen launches his Estes Patriot (Click to enlarge).

The Mammoth leaves the rod with camera and
telemetry active (Click to enlarge).

Frame from the Mammoth's onboard HD camera (Click to enlarge).

Another frame from the Mammoth camera (Click to enlarge).

The Inductor leaves the rod, its Aerotech D10 leaving a trail of black smoke (Click to enlarge).

Allen brought a slew of rockets to fly, and their performances did not disappoint. His A10 powered Sonoma (an Estes Sequoia with adapted decals) led off, followed by his venerable semi-scale Patriot on a B6-4.  He then launched his Mammoth on an F42, which was fitted out with a Missile Works T3  tracker system and an HD video camera. The T3 worked well, and the camera recorded some decent onboard video, which you can view here. A flight of his finless induction rocket (the "Inductor") was next; the Aerotech D10 motor chuffed quite a bit before building enough thrust to loft the model, which started doing a big loop in the sky at motor burnout. 

The QCC Explorer rides the fire of a D12
(Click to enlarge).

The Fletcher clears the rod on an A10
(Click to enlarge).

Allen's Skydart II on the pad (Click to enlarge).

Liftoff on the B6-4 (Click to enlarge).

The Skydart glides in for a landing (Click to enlarge).

Frame from the HD camera on the QCC Explorer (Click to enlarge).

The video camera got another workout taped to the side of Allen's Estes QCC Explorer, which went surprisingly high on a D12. However, I would have to say his most impressive flight was made by his Estes Skydart II; powered by a B6-4, the model flew arrow straight, with ejection of the power pod occurring right at apogee. The Skydart settled into shallow glide, moving in a wide lazy circle over the field for about 30 seconds. It was the best flight I have ever witnessed a Skydart make, and I congratulated Allen on his trimming skills. He also flew his Estes Fletcher (painted camouflage) on an A10, and risked his QCC Explorer on an Estes E9, but those flights - good as they were - were nowhere near as awesome as that of the Skydart.

"Purple Pink" is launched (Click to enlarge).

"Starshooter" clears the pad on a B6 (Click to enlarge).

Razor's A3 motor ignites (Click to enlarge).

"Shockwave" heads up into the sky (Click to enlarge).

Marc's Screaming Mimi on a D12 (Click to enlarge).

Big Bertha rises majestically on a C6 (Click to enlarge).

Marc had his son and daughter along for this launch, and it was his daughter's "Purple Pink" which started his series, flying on a B6-4. That same motor pushed his son's "Starshooter" (an Estes Make It Take It) off the pad, after which Marc's scratch built orange "Razor" took to the blue on an A3-4T. Also scratch built, "Shockwave" was up next, grabbing some decent air on a B6. His last two flights were those of an Estes Screaming Mimi on a D12 (despite 4 noise makers, that rocket almost never makes any audible screams on the way up), and a very nice black Big Bertha powered by a C6-5. There were no recovery failures -  Marc and family retired from the field happy, with all models intact.

James' School Rocket lifts off (Click to enlarge).

Yellow Walmart scratch build gets going
(Click to enlarge).

James' Orange Walmart rocket starts its voyage on an E9 (Click to enlarge). 

One of the JPII team members, James, had three rockets to fly. The first was that of a Balsa Machining 3" School Rocket, which lifted off on an E9-4. Unfortunately, the orange model caught the Duane curse, and the upper motor tube centering ring pulled out of the model. The nose cone drifted gently to the ground, whereas the sustainer plummeted straight down, being saved from total obliteration only due to impacting into a pile of soft mulch by the road - very lucky. James also flew a couple of similar rockets (one orange and one yellow) made from body tubes and materials bought at a local Walmart. These put in very good performances on Estes E9's, deploying their homemade parachutes for safe landings. I must say that I was surprise by the fact that there was not a single CATO out of all those E motors flown on Saturday.

Vince's Quest Starfire on a B6 (Click to enlarge).

The A10 in the MoonGo ignites (Click to enlarge).

Vince gets the Enterprise ready for launch (Click to enlarge).

The Enterprise leaves the pad, with a clothespin caught in its tractor beam (Click to enlarge).

The Enterprise descends under its parachutes as Vince starts his recovery trek (Click to enlarge).

Vince also brought along a few rockets; a venerable Quest Starfire was first off the pad, riding the thrust of the popular B6-4 motor.  Next was the much anticipated flight of his Estes Starship Enterprise, the long "atmospheric probe" being painted to look like phaser beams. The Enterprise did not quite achieve warp on the C6-3, but the flight was very nice, with the parachute deploying to ensure a safe landing on the grass. An inspection of the pad showed that the motor's thrust had destroyed the clothespin used as a standoff, which I captured flying behind the model in a cell phone pic. His last flight was that of a Semroc MoonGo, which was up and gone on an A10-3T.

Methinks the rod needs adjusting, else the Airwalker
may stray far away  (Click to enlarge).

My Stellar Photon Probe looking all pretty before
launch (Click to enlarge).

Duane launches my Estes Asteroid Hunter (Click to enlarge).

And what did I fly, you may ask? I put three in the air, all powered by B6-4's - my Estes Airwalker,  my orange and white clone of the Centuri Stellar Photon Probe, and an obscure out-of-production Estes RTF - the Asteroid Hunter. Everything went well, including parachute deployments. I probably should have brought along a few more models, considering that the weather is going to be sucky cold for the foreseeable future, but I suppose that's why we have winter coats.

And that, good readers, is the short story of the two hour December 2nd launch at Pegasus field. Sunny, warm, and windless, we shall probably not see its like until Spring.