Last Updated: February 3, 2001
Project by Sean Breheny and Anish Trivedi
Now with photos, video, code, and schematic!
Table of Contents
I've been interested in model rocketry for a long time, but as my interest in electronics became stronger and more developed, rockets fell by the wayside. Recently, though, I thought of a way that I could combine these two interests: try to put electronic devices onboard rockets! That is what I have been trying to do recently with my BTSAM project, and just this month, with the Rocket Altimeter Project.
Back in September of 2000, Anish Trivedi and I decided to build and test a very small rocket altimeter. I thought a great deal about the problems involved, and as usual, discussed it with several people on the PICLIST (Microcontroller discussion list), and decided that we should use an integrating accelerometer type altimeter, rather than a barometric altimeter, because of the ease of calibration, and because I have prior experience with accelerometers. Anish agreed, and so design began.
The final design used a Parallax Basic Stamp II (because of its internal EEPROM which could be used for datalogging, and because we wanted to use Basic as the programming langage), an AD7706 16-bit Sigma-Delta analog to digital converter, a Zetex ZRC250 2.5V reference, a National Semi. LP2950 low-dropout 5V regulator, and an ADXL190 +/- 80 g MEMS accelerometer. Analog Devices was good enough to give us two each of the ADC and accelerometer free as samples.
After laying out the board, the whole thing fit onto a board about 2.5 inch by 0.9 inch and weighed about 10 grams with components. This was small enough to fit into the payload tube of an Estes Hercules model rocket. Power was supplied by a 12V 23A battery in an N-cell holder in the nose of the rocket. This added only about 6 more grams.
The onboard software operated in several modes. If no data were previously recorded, it would go into a mode where it would continuously sample the accelerometer at 60 samples/sec and place the samples in a 720-sample (1440 byte) long circular buffer in the BSII's EEPROM. If an acceleration in excess of 2 g is detected, a pointer is set to the current location in the buffer and sampling is stopped when 660 additional samples have been taken (this leaves 1 second of data from before launch, and 11 seconds after). An LED comes on to indicate that data has been taken.
If data has been recorded before, the software goes into a command mode in which it will accept commands over the BSII's serial interface. It accepts three commands "r","c",and "l". An "r" command causes it to dump the entire contents of its circular buffer as human readable 16-bit numbers (5-character ASCII strings for each sample, eg 32566, followed by a line feed character). The unit then goes back to where it waits for another command.
A "c" command clears the EEPROM flag which indicates that data has been recorded. When you execute a "c" command, the next time you reset the BSII (by removing power and appyling it again), the unit will go into the launch-waiting mode described above (as if no data were previously recorded). After the "c" command, the program ends and the unit must be reset.
When you give an "l" command, the unit waits 1 minute, then takes 3 samples, 1 second apart. It then blinks the LED to indicate that this has happened. It averages the three samples, and stores the result in a dedicated EEPROM location. This routine is designed to find the accelerometer output level when the unit is standing still on the launch pad. This level is required for the launch detect routine to know what output is equal to 2 g (because it must measure the 2 g change relative to the stationary level). It is also required in order to interpret the final data. The 1 minute pause is to allow the operator to put the unit into the rocket and put the rocket onto the launch rail (in the launch position) before the samples are taken. After the "l" command, the BSII goes back to where it waits another command.
Once the data have been taken, they can be downloaded and analyzed. We used a Palm M100 to store the data from our tests, and then used MATLAB to analyze the data. By holding the unit vertical and taking some data and then flipping it over 180 deg so that the other end is down, and taking data again, the accelerometer's scale factor (e.g. how many units equals 1 g or 9.8 m/s^2) can be found easily. MATLAB can then be used to obtain the rocket's acceleration, velocity, and altitude versus time curves from the data. Because the AD7706 is an integrating ADC, it doesn't matter if the sample rate is too slow, and no interpolation is necessary. Integrating gives the correct final velocity and altitude (as long as you measured acceleration properly).
Although we used no other way to measure altitude and, therefore, have no way of determining exactly how accurate our accelerometer is, our test went very well and the resultant data are, for the most part, within the expected range. The data also show very clearly the stages of rocket flight, such as ignition, burnout, coast, and parachute ejection.
Although there were two test launches, only one resulted in useful data because of a catastrophic engine failure on the second launch. In fact, it was the most spectacular model rocket engine explosion I have ever seen, and not only destroyed the rocket, but even tore one of the supports off the launch pad and nearly removed a second one. I'm just glad we were far enough away when it went off! It scared us as well as some cows a few hundred feet away, and we have it all on tape :-) One good thing is that the altimeter seems to have survived it unharmed.
Below are the graphs from the good launch. We hope to conduct another test at a future date so we can obtain more data to verify that it is working properly. As you can see, the rocket's maximum altitude was recorded as 693 feet and it's maximum velocity as 155 mph. The theoretically predicted maximum altitude was about 920 feet with a maximum velocity around 206 mph. The primary reason for the discrepancy seems to be that the engine (an Estes C6-5) produced about 30% less thrust than expected (4.09N instead of 5.8N). The measured drag force coefficient was within 1.6% of the theoretical value, so at least the accelerometer calibration seems to be correct.
There are many possible accelerometer errors, but it seems as though the engine really did produce much less thrust than expected because the only way our measurement could be wrong would be if the accelerometer calibration were off, and it couldn't be off by 30%. Wynn Rostek, from the PICLIST, told me that he had measured Estes C6 class engines to be about 30% low in thrust, too, so perhaps they are being manufactured that way. Bill Westfield suggested that the cold temperature on the launch day might have reduced the thrust, and that it might have also cracked the propellent. Bill and Dave VanHorn explained to us how cracked propellent can lead to explosions like the one in the video (see below)
Thank you to all those on the PICLIST who helped with this project, including (in no particular order)
Dave VanHorn,Bill Westfield,Herbert Graf,Barry Gershenfeld,Lee Jones,Phil Eisermann,Roman Black,JB (Sorry, don't know your full name),Jinx (same, not sure of your real name),Ricardo Seixas,and I appologize to any others, please let me know if I left your name out
The Good Launch - Video of the launch where we obtained good data (the data for the above graphs) - 5.6MB .MPG
The CATO - Video of the launch where the rocket disintegrated in a loud blast which destroyed the launch stand! - 3.9MB .MPG
In the video of the explosion, look carefully at the group of cows in the background, just to the left of the center. About a second after the explosion, when the sound reaches them, there is a commotion among them and one comes out into the open to have a look at what happened!
The Altimeter Board Photos of the top and bottom of the altimeter board
The battery for the altimeter (a 12V 23A-type lighter battery, in an N cell holder) was placed in the nose of the rocket, packed in with wadding, and then tape placed over the hole in the base of the nose. The battery holder was oriented so that the non-springy contact was down, so that acceleration would not tend to compress the spring, and cause the battery to lose contact (Thanks to Dave VanHorn for the warning).
Normal Liftoff Two photos (frames from video) of the Estes Hercules lifting off during the "good" launch. Note prominent flaming exhaust jet!
Explosion! Two photos (frames from video) of the Estes Hercules being ripped apart during the CATO.
Altimeter Schematic - Here is the schematic of the altimeter. If you want to build it, don't hesitate to email me with any questions, but please be advised that I am sometimes late in answering due to being busy.
Altimeter BSII Code - Here is the Basic Stamp II code for the altimeter.
You can contact me at shb7@cornell.edu
You are visitor number to this page
