This blog covers the day to day progress of water rocket development by the Air Command Water Rockets team. It is also a facility for people to provide feedback and ask questions.

Saturday, April 26, 2008

Flight Computer Q & A

Over the last year we’ve had quite a few questions about our water rocket flight computers so I’ll try to answer some of the more common ones here:
  1. Why do you need a flight computer, wouldn’t a Tomy Timer do the same thing?

    When you only need a one-shot timer, we always recommend a Tomy Timer as they are inexpensive, you can get them anywhere, there is no need for batteries, very simple operation and light-weight. They are also a proven design.

    We wanted to be able to do more than just deploy a single parachute. A PIC-based micro-controller is an easy way to gain more functionality. Electronic timing is typically more accurate and repeatable compared to mechanical systems. You can also set very short timing delays for things like staging, something difficult to do with a Tomy Timer. There are elaborate mechanical systems that have been built that require no electronics that achieve the same thing.

    A flight computer can have: data logging capability, can control multiple actuators and have in-flight data processing and signal conditioning capability. If you wanted to experiment with active stability, or gather engineering data, it is difficult to do with a Tomy timer.

  2. Isn’t it much heavier than a Tomy Timer?

    With our current design when you add the battery, servo and the flight computer it is perhaps 4 or 5 times heavier than a Tomy Timer. However, we use the battery for both the flight computer and the altimeter, so we save weight that way. The same battery could power the camera as well so you could save weight even further. We haven’t shared the power between the cameras and the flight computer yet.

    If you were to use a small lightweight battery such as the 4LR44, a 4.5g micro servo and surface mount components on a small PCB it could weight about double that of a Tomy imer.

    This weight difference translates to perhaps 10-20 feet altitude loss on small rockets and negligible on larger rockets.

  1. Why do you call it a “flight computer” and not a “timer”?

    The terminology distinction is purely internal to our team. That way we differentiate between our simple electronic “timers” usually based around a 555 or 556 timer and the PIC based ones that have software running on them. Once the flight computer starts processing real-time flight data, the distinction will be more obvious.

  2. Does it do more than timing?

    The published versions of the flight computers mostly do just timing. Although through software they also drive the LED display, do switch de-bouncing and generate the correct PWM signals for the RC servo motors.

  3. Are you looking at adding more functionality?

    We have plans on our roadmap to add more functionality, but we are taking it one step at a time, experimenting with what actually works in the field what doesn’t. Take for example the various G-switch designs we’ve been testing. This involves multiple flights which takes time.

  4. Does your current flight computer control your camera and altimeter?

    The published ones and flown to date have not. The altimeter is powered from the same power source as the flight computer. The Z-log altimeter can be set up to start recording 10 seconds after power-on which means when we turn on the computer, power is also supplied to the altimeter and it starts recording. But there is no direct control between the altimeter and flight computer.

    The V1.5 design has a free port left open to allow the altimeter to be connected to the flight computer through a serial connection. The Z-log altimeter outputs altimeter data continuously over its serial port. However, even V1.5 will not initially have it connected.

    The plan is to feed this altimeter data to the flight computer and it will be able to monitor the altitude and deploy parachutes at preset altitudes or when altitude starts decreasing after apogee. The flight computer will always use the timer capability for backup should something go wrong with the altimeter. At the moment we are working to make the timing as reliable and usable as possible before adding more complex functionality.

    It was always our intention to wire the old cameras to the flight computer so that they could be turned on by the computer just before launch since they only had 30 seconds of record time. However, ever since we bought the new FlyCamOne 2 video cameras with their 30 minute record time, the flight computer/camera integration took lower priority. We start the camera separately before we pressurize the rocket.

  5. Do you have designs that you are keeping secret?

    No. We have no reason to. We only publish the designs once we have flown them a number of times. We like to verify the designs for ourselves before making them public, as it is much easier to fix things before publishing than having to make retractions or corrections later. We find it very useful in making the designs public as other rocketeers help suggested ways of improving them.

    We have already been contacted by 2 rocketeers that have built the flight computers based on our published designs, so we want to make sure we have confidence in the design before they are made public.

  6. Isn’t it expensive?

    Not really. The PIC controller costs AUD$2.84, the handful of discreet components around $10, the batteries are about $3 and the cheap 9g RC servos we get for around $6 each. This means with a PCB the whole electronics ends up costing in the order of ~$25. That is about 1/4 of the price of the camera and about 1/5th the cost of the altimeter.

    Of the ones we have crashed we have been able to reuse most of the parts. Really the only things that do brake are the PCBs, the old G-switches and servos. We have now learned to protect the servos better and have had 2 survive direct impacts since the change.

  7. Will they be available for sale?

    There are currently no plans to sell them in any great numbers as there really isn’t a market for them. Most water rocketeers prefer to build rockets out of inexpensive components. Personally I’d rather be flying rockets than handling order paperwork, chasing payments, etc. etc. We will likely offer 5 of the V1.5 for sale privately at cost price. (Contact us if you are interested - see contact page on our main site) The others we will continue to use for our experiments.

  8. How reliable are they?

    So far we are having relatively good success with deploying parachutes and staging 2-stage rockets with them. All together there have been 68 flights with on-board flight computers, of which 5 failed to deploy and 2 successful deploys but tangled parachutes. This means as part of an integrated recovery system they are about 90% reliable.
  1. What will be in the next version?

    V1.5 of the flight computer is the next iteration we are working on. This version has dual servo capability like V1.4, a loud buzzer for status feedback and helping to locate the rocket lost in tall grass or bushes. One of the new capabilities is that all the timing parameters are configurable in the field and stored in the on-board EEPROM to retain them after power is turned off. There are 15 parameters that are configurable from parachute/staging delays, to multiple servo positions, to the lost rocket sound alarm delays. We are having 9 more PCBs manufactured for this particular design as it makes it more compact and lighter.

  1. Future plans?

Eventually we would like to miniaturize it and use all surface mount components and a much smaller PCB. The final weight and size should be similar to the altimeter (~10grams), although realistically this is at least a year or two away.

Adding logging capability will also be a priority in the upcoming months. We have ideas for air speed sensors that could be used to detect apogee, but have no idea how well they will work or what the data will look like. The idea is to use the normal timing for recovery, and the logging capability to capture data over multiple flights. We will do this for each type of sensor so that we can see what processing will be needed before it can be used effectively for apogee detection.

None of these plans for the flight computer are set in stone and are likely to change along the way. We only work on these during spare time and as a result the development is drawn out.

There have been many people who have flown flight computers on water rockets over the years, many of them a lot more advanced and using accelerometers, logging capability, running science experiments etc. The oldest documented reference I have found is back from March 2000.


The following quote reproduced here in full is taken from a long exchange from the WRA2 forum and is included here because apparently we did not credit Bill with the invention of a water rocket flight computer and that we "stole" the idea from him. (See previous paragraph) In his own words:

Team Seneca Post subject: Posted: Tue Apr 15, 2008 12:13 pm

WRA2 Member
Joined: Sun Dec 31, 2006 4:40 pm
Posts: 97
Location: Seneca, N.Y.

It's not hostility. It's just that I never knew you guys would be so impressed with an electronic timer with a fancy name. I've put real computers on my rockets since the summer of 2005. A computer that does something too, not just a timer. I use an accelerometer to measure the flight and deploy. Back then I also used it to send signals to a small camera to take a snapshot at apogee. I'm the first one to put a computer on a water rocket and it was a real computer, not a tomy timer made from silicon.

Bill W.
Team Seneca


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