Electronics selection and/or Flight Computer (First rocket attempt)

[musketman]

Hi,

I'm currently participating in a G-class hybrid rocket competition for senior design at my university. My team is building the majority of our rocket from scratch except for the electronics. I was wondering if anyone could offer some guidance for flight computers? I am looking for an integrated system that has a simple interface and includes at least an altimeter and accelerometer (more sensors the better). I will be writing a lengthy report that will analyze the design of the rocket as well as the data received from test flights, so the more data I can obtain the better. We are not too worried about budget because we have about $2,000 sponsored for the project. I am considering getting the G-Wiz HCX flight computer for my integrated electronic system. I also had a friend recommend getting a AIM XTRA GPS flight computer but I think it may be overkill. I do not have much experience with electronics so any help would be much appreciated.

The competition goal is to get as close as 2,000 feet as possible so we are also trying to implement some sort of drag control system. Our current idea is to have servo controlled mechanism (small wings maybe?) come out at some critical point in the flight to ensure the rocket reaches 2,000 feet. The rocket is being design to overshoot the goal and then the drag control system will be used to hit the 2,000 ft mark.

One question I have is trying to understand what a "Pyro Output" is exactly. I'm familiar with circuits giving a voltage output such a 5V to trigger an event, but I'm not sure what a pyro output is. Could anyone clarify that for me please? I am guessing it is a high current output, but I am having trouble finding a definition anywhere.

That being said, I was also wondering if flight computers are usually easily programmable. For example I saw that the pyro output 4 on the G-Wiz HCX is user programmable. Does that mean I could use that output to initiate the drag control system? I was thinking of programming a function of two critical variables (height and speed) and having the drag system deployed once a point close to the function, f(v,h) is reached.


Thanks,

-Alex

 

[cerving]

The Eggtimer will do what you want, it has native servo support which is what you're looking for. It's a kit, so you'll have to put it together, but it sounds like that won't be an issue. It's $40 including the USB data cable... www.eggtimerrocketry.com

 

[bdureau]

What we call pyro output is an output that can deliever enough current to fire an ematch that will then fire a small quantity of black powder to eject the noise cone for exemple.
So if you look at the schema of my altimeter
https://rocket.payload.free.fr/FlightLoggerImages/altimulti-schema.pdf
you have 3 pyro output with 3 fet transistors that provide enough current for the ematches. If you were trying to light an ematch directly from your micro controller you would probably damage the output and the ematch would not fire....
The outputs are also able to drive servos with minor modifications of the code
Unfortunatly none of my altimeters have accelerometers yet but I am sure that you could easily modify them
It really depend where you want to concentrate in your project. Just one advice, whatever altimeter you choose make sure that you have 2 from 2 different brand. If one goes wrong the other one could save your rocket. So you could have a complex expensive altimeter that does all the measurements plus the ejection(s) and a cheaper one (on a different battery) that is a backup.
All info is an my site if you want to build some simple and efficient altimeters with Arduino board
https://rocket.payload.free.fr/
Prices for kits with all components starts at 15 euros + shipping.
Regards
Boris

 

[musketman]

Thanks for the information guys.

Cerving - I made a design so that I just need 2 pyro outputs. One for the parachute deployment and the other will initiate my drag control system (via pyrotechnic charge as well). Would it be possible to program the Eggtimer pyro outputs to deploy based on certain velocity and altitude conditions? For example use a series of if then statements to check the velocity at different altitudes and if it falls within a certain range it deploys the drag control system.

I see that many flight computers are "programmable" but I'm not sure what that exactly means or to what extent they are programmable. Thanks again for the help.


Regards,

-Alex

 

[Adrian A]

Alex,

You should also check out the Raven altimeter. This web page shows an example of the user interface programming screen that controls all 4 Raven outputs. You can set one channel to deploy on the way up (AGL > AGL2) and still have channels for apogee, main deployment on the way down, and one other channel.

 

[cerving]

You might be able to use the airstart function which can trigger on a minimum velocity after the expiration of a motor burnout timer. If you set the minimum to what you think your lowest threshold will be then it will almost certainly trigger after the timer; the "on-time" is programmable, so you can have it turn off after a few seconds. You'll need to play around with OpenRocket or Rocsim a bit to figure out what the values might be, but I think you're going to be doing that anyway.

The Eggtimer's current firmware doesn't have velocity-triggered events, other than the airstart logic. Barometric altimeters' derived-velocity figures have several sources of inherent error, they're OK for flight profiling afterwards, and certainly for deployments, but they aren't accurate enough to be a base for any kind of control device. You'll need an accelerometer-based device for that, which of course costs more.

 

[JohnCoker]

I see that many flight computers are "programmable" but I'm not sure what that exactly means or to what extent they are programmable.

What programmable means in this context is that you have a set of 2-4 outputs, and you can assign them to trigger at certain points in the flight:

• launch +n seconds (outboards)
• burnout +n seconds (staging)
• apogee +n seconds (drogue)
• descent to x ft (main)

The "outputs" typically just connect the battery across the output for a short period of time (typically 1s), which is plenty to fire an initiator (electric match) to trigger a larger charge (black powder).

In short, I think "computer", "programmable" and "output" are misleading you here.