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Author Topic: Arduino Rocket Ejection at Certain Altitude  (Read 6435 times)
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I mean its going mach .31, not too fast and i dont think i will have a problem

Where abouts on the vehicle do you propose to install it that will not be affected by dynamic pressure effects? The pressure changes you're going to be looking for will be tiny compared to the dynamic pressure at those speeds.
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Except for the part about the proprietary binary format with block acknowledge.

Who said anything about USB? Re-read my post, then RTFM on the MAWD manual, which has a telemetry mode.  I said  "If it's like the MAWD", i.e. if it also has a telemetry mode.

The USB part of the Alt15k interface is a separate board with an FTDI chip.  The actual interface on the altimeter is 5V TTL RS232.  No USB sniffing required.  If it comes down to reverse engineering the perfectflite protocol, rolling your own altimeter could be easier.

If the Alt15k doesn't have a telemetry mode (and there is no indication that it does), you may be better off getting a Pnut altimeter (the manual says it has a telemetry mode).

-j
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Where abouts on the vehicle do you propose to install it that will not be affected by dynamic pressure effects? The pressure changes you're going to be looking for will be tiny compared to the dynamic pressure at those speeds.

Static ports are perpendicular to the direction of flight, i.e. on the side of the rocket.

-j
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OK, I just dug out my Alt15k, and it does not respond to MAWD telemetry commands.  Doesn't look like you will be able to tap the Alt15k for altitude data.

Even if you could access the binary data, that's done post-flight, not in flight, so you're still out of luck.

I'd say either check out the Pnut (I don't have one of those to look at) or think about rolling your own altimeter.

One problem with the Pnut is that it requires 3.3V signals, and a standard arduino is 5V.  You'll have to take care of that problem (maybe by using a 3.3V arduino pro mini from sparkfun?)


If you decide to make your own altimeter, the older perfectflites (alt15k, MAWD) use the Freescale MPXH6115 pressure sensor.  Freescale offers free samples (use your school .edu email if you have one, or ask your teacher/mentor to order samples if you don't).

I have some code here that may contain useful info (it also contains a bunch of irrelevant stuff).  It reads pressure from an MPXH6115 and converts to altitude in feet.  This is code that flies on the tracker in my 4" Patriot, and the altitude reported by this code is noisy and off by quite a bit as compared to the perfectflite and R-DAS units.

-j

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Static ports are perpendicular to the direction of flight, i.e. on the side of the rocket.

There will be some suction there - will it be significant?

Does the rocket ever generate any angle of attack?
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There will be some suction there

And where will it come from?

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Does the rocket ever generate any angle of attack?

Nothing significant.  CP and CG geometry on a well behaved rocket correct to zero AOA.

You seem to be questioning the concept.  There are thousands of flights a year that use altimeters based on barometric pressure sensors connected to 8 bit microcontrollers.  There are several commercial products available.  There are well established techniques for mounting the altimeters and sizing and locating the static ports.  If I'm guessing right, the original poster is participating in a (US) national competition with several hundred other teams (thousands of students, thousands of flights between now and April) where such an altimeter is the official altitude recording device.

-j
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You seem to be questioning the concept.  There are thousands of flights a year that use altimeters based on barometric pressure sensors connected to 8 bit microcontrollers.  There are several commercial products available.  There are well established techniques for mounting the altimeters and sizing and locating the static ports.

I'm not a rocketeer, as you've certainly guessed. I'm coming at this from a theoretical perspective. There will be very large suction areas around the nose, and some slight suction down the side due to Bernoulli's principle (the body is displacing air and acting like one side of a venturi).  Maybe boundary layer effects would mask that if you get far enough back. If the rocket is prone to any sort of wobble, there could be quite significant pressure variations. Those could probably be compensated by ports on opposite sides feeding an internal plenum. So theory suggests that there are some effects which might need to be addressed.

Since this approach is evidently in regular use, I guess that either these effects are not significant at this scale in practice, or there are established ways to avoid them, or perhaps that we're simply not too fussed about the accuracy. Is anyone actually going to care if it triggers at 740' or 760'?
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Since this approach is evidently in regular use, I guess that either these effects are not significant at this scale in practice, or there are established ways to avoid them, or perhaps that we're simply not too fussed about the accuracy. Is anyone actually going to care if it triggers at 740' or 760'?

Most likely not. However failure to deploy is a pretty common failure in the hobby no matter what method is used, and can happen for a varity of reasons. Most don't consider it a successful launch until they do indeed see the 'cute deploy properly.

Lefty
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I can be accurate to +- 10 feet so 740 or 760 is ok. I ordered a MP085 barometric pressure sensor and my main concerns are, where i should mount the sensor, and the coding scares the crap out of me  smiley-sweat
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The coding is fairly straight forward, and if you do get stuck there's a lot of people on here who will help you. However, as to where you mount the sensor.... there's far fewer!
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I think the trick would be to have multiple pressure ports arranged around the perimeter to help maintain even pressure inside the test chamber.

Another option is a GPS sensor. The Fastrax unit that Adafruit sells comes out to $45 or less, place it in a plastic nose cone, allow it to get a fix prior to launch. You'll get 10Hz updates will likely have run the arduino on 3.3V @ 8MHz but that's plenty fast for such a simple task. 

Then have the Arduino log it's flight path in 3D. EEPROM?
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I also expect that you'll have trigger well below an Indicated 750 ft to get the parachute to deploy at 750 ft.
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Another option is a GPS sensor.

Consumer GPS is crap for altitude determination when the altitude is changing rapidly (e.g., in a rocket in flight).

GPS altitude measurements are not that accurate, and much more time is needed for an altitude fix to stabilize as compared to an X,Y position.

-j
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Interesting. I figured that at a 10Hz refresh rate while tracking 20+ satellites that z-coordinate determination would not be an issue. I still think its worth a shot considering that this module is available for $30 at digikey or mouser, IIRC.

I realize that civilian gear is intentionally not able to operate near cruise missle velocities for rather obvious reasons but didn't think this rocket would come close to those speeds.
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The problem isn't the speed (or not just the speed), it's the amount of signal processing done, and the assumptions in the algorithms.  The GPS units are more or less designed with ground travel and auto speeds in mind.

The satellites are LEO, so the elevation angles are low, which results in more error in the Z measurements.

There are GPS units more suited to rocket use, but they're expensive and hard to get.

-j
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