Thursday, February 24, 2011

More Rocket Shenanigans

Just by luck, we stumbled on to a Estes model rocket "starter kit" at Bi-Mart for $26. It had a large enough diameter to fit my electronics package, so it was purchased, built, and pressed in to service. I hadn't built or flown a traditional model rocket in fifteen years or so, and was pleasantly surprised how easy the kit was to put together. We went down to Alpine on President's Day and got off six flights (a $3 engine a pop) off. The first and last had the improved telemetry package (lighter battery and a high-range accelerometer), and the remainder with a cheapo spy camera.

In all, not too bad. I've got plans for a lighter, more powerful rocket so I can fly both the electronics and the video camera at the same time (we were over the recommended limit by a ways). Here are the two instrumented flights. Flight 6 was a bit lighter due to removing the helicopter recovery system, and thus went higher. It also descended slower on the parachute. Look at the "zero G" coasting after the engines shut off!

The original water rocket shenanigans are here . . . 

Saturday, February 12, 2011

Model Rocket Telemetry

We completed another round of flights to test my JeeNode Arduino-compatible model rocket telemetry system. The first couple of minutes of the video show the highlights. The remainder explains the details, so feel free to leave the theater at any time.

I published instructions on how to build it on so anyone that wants to try something similar can see how I did it. Unlike my LED bug project, this one didn't need a custom PCB or special chip programming, so it would be far easier to duplicate.

Now I've got some flight data, but is it accurate? From the read outs in the field (final flight #4 shown above), I saw a quick (off-scale) acceleration followed by an arcing altitude graph. The results are a bit higher than I would have guessed, but within reason.

One approach to validating the numbers is theoretical. Dean Wheeler has put a water-rocket simulator on-line that accounts for everything down to air temperature. To put it to use, you need to know some key parameters. My total rocket (minus water) weighed 185 grams. I used 0.7 L of water pressurized to 60 p.s.i. (there I go mixing units again). Using Dean's suggested drag coefficient, which is the primary unknown in all this, the number fall short of measured (268 feet verses 176 feet). The calculated flight time and time to apogee are also shorter. So, what is a good scientist to do? Right, massage the data. Since the drag coeeficent is the primary unknown, and all the other data seem scaled low, I adjusted it until the maximum altitude matched. The time to apogee and the total flight time are pretty close! (see the dotted lines in the graph)

There was a lucky accident here. Did you catch it? The theoretical model assumes the rocket has no parachute on its return to earth. My parachute failed to open, allowing us to directly compare the total flight time.

Of the four flights we made, only flights 1 and 4 provided data. The crash from the first flight fractured the battery holder so the micro-controller would loose power during the acceleration, leaving an eerie end to data transmission. The final flight, the parachute, making up for past transgressions, opened to early, leaving a flight in my estimate of half as high as the first.

Some final observations on the data. The calculated maximum Gs are ~60! That would kill a human, and explains why I have a hard time keeping fins attached to the rocket body. It also looks like the initial acceleration causes havock with the pressure transducer. Mounting it in a different orrientation (parrallel to g-force) might help. Also of interest is that the bottle empties of water in under 0.2 seconds, while the rocket is still just 10 feet off the ground! Some things to think about.

But wait, there is more . . . 

Friday, February 11, 2011

They have trains in France?

Just kidding.  They also have blogs, one of which recently featured Dawson Station.

As near as I can tell, they said they found it through Carl Ardent's website and to visit if you are not too put off by English.

Wait, a blog about a blog, about a blog? That kind of recursion could bring down the inner-webs!

Sunday, February 6, 2011


The WetWesties crew has an annual tradition of camping out on Superbowl weekend. This was our first time attending.
They descend on Nehalem State Park near Manzanita.  We stopped for some squeaky cheese in Tillamook on the way.
It was wet and cold, but that is the Oregon beach in February. We had a heater in the van which made it cozy.
They had a potluck dinner. It was amazing what good food people living in vans can come up with. There were so many people we had to eat in shifts!
There were at least 50 vans there of all vintages. It was fun to see the different customizations. My favorite was a paint job using ferns and blackberry leaves. You can see more pictures here.

Chief cook Annie working on breakfast.
We are now home, warm and dry, and still don't know who won the point-ball thingie.

Wednesday, February 2, 2011

Real-time model rocket telemetry

My latest project involves wireless transmission of data from an Arduino-compatible micro-controller called a JeeNode. My first application was transmitting acceleration and pressure data from a water-powered rocket so that I can determine altitude and speed.

The first launch, done at the minimal pressure of 40 psi went 150 feet and reached 140 mph! The second launch also transmitted flawlessly, right up until it hit the roof of the house, knocking the batteries out.

The batteries were bent, and the holder destroyed, but remarkably the electronics still work. In addition to transmitting data, I'm saving it on board using non-volatile memory so it isn't lost in such an even, but it still didn't capture the final deceleration!
Someone did all the math, which is quite a bit more involved than for a regular model rocket, and put a simulator on line. This is very similar to what I do at work, analyze simulation data! I'm amazed at the initial acceleration and speed. There is a lot of energy being transferred rather quickly (~0.3 seconds)! Now I can optimize the flight parameters (up the pressure), and see what altitudes we can reach. Well, maybe I should work on the landings first.