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Raven- accelerometer only deployment

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bobdog

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Would like to use a Raven in a stubby two stage rocket to airstart the second stage and execute the apogee chute deployment in the second stage. Due to space limitations, the location of the Raven would have to be in the nosecone. As such, the barometer-based deployment function would probably be unreliable. Do you see any problem with relying on the accelerometer function detection of apogee for chute deployment? Could use the timer function also I suppose.
 

djs

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Why not put some holes in the nose cone to give it access to air sampling?
 

mikec

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Do you see any problem with relying on the accelerometer function detection of apogee for chute deployment?
I've done this fairly often and it's worked well. Some Ravens have accelerometer scale errors that cause apogee detection to be off -- the degree of error is related to the time and duration of thrust. You might try a single-stage test flight on your Raven first to see how closely it detects apogee.
 

bobdog

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Why not put some holes in the nose cone to give it access to air sampling?
Am afraid that the nose cone curvature would affect the pressure readings.
 

djs

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Depends on the nose cone shape. I've used the av bays from APE-RC with good results.
 

bobdog

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I've done this fairly often and it's worked well. Some Ravens have accelerometer scale errors that cause apogee detection to be off -- the degree of error is related to the time and duration of thrust. You might try a single-stage test flight on your Raven first to see how closely it detects apogee.
Good to know that it has worked well in the past. Will try the single stage test.
 

Steve Shannon

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Am afraid that the nose cone curvature would affect the pressure readings.
Only at high velocities. As the rocket reaches apogee and approaches its lowest vertical velocity the pressure readings should be nearly accurate, especially if there's little horizontal velocity.
Be sure Mach inhibit is on.
 

cerving

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If you drill the holes just ahead of the break, or in the switch band if you're going HED, it shouldn't be an issue any more than it would be one for any other location. Modern altimeters filter out bad pressure readings and/or have algorithms to delay deployment until the rocket is near apogee, so the pressure hole location doesn't matter as much as it used to back in the day.
 

OverTheTop

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What about venting the NC through the base, and putting the static ports in the airframe lower down?
 

Buckeye

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...Do you see any problem with relying on the accelerometer function detection of apogee for chute deployment?....
Yes, I do! I had very unexpected deployment events using accel deployment in a Raven (and a MARSA, for that matter). I now only use baro for ejection events.

http://www.rocketryforum.com/showthread.php?140400-Featherweight-Raven3-accel-data

Your baro altimeter, even in a nose cone, will handle apogee detection more reliably. (assuming you are operating in the proper range for baros, ~40K ft MSL, I think)
 

bobdog

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Yes, I do! I had very unexpected deployment events using accel deployment in a Raven (and a MARSA, for that matter). I now only use baro for ejection events.

http://www.rocketryforum.com/showthread.php?140400-Featherweight-Raven3-accel-data

Your baro altimeter, even in a nose cone, will handle apogee detection more reliably. (assuming you are operating in the proper range for baros, ~40K ft MSL, I think)
Thank you for the thread link - very interesting discussion. Now leaning toward a single stage test with the barometer for apogee detection.
 

GRIFFIN

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I fly Many Ravens in head end deployment rocket. My sampling holes are located on the switch band the is just aft of the cone. No issues what so ever.

I wouldn't rely on accelerometer for apogee detection. I've looked at some of my data and the accelerometer detection of apogee isn't very accurate.
 

JimJarvis50

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Only at high velocities. As the rocket reaches apogee and approaches its lowest vertical velocity the pressure readings should be nearly accurate, especially if there's little horizontal velocity.
Be sure Mach inhibit is on.
There are Raven's where the accelerometer isn't accurate. You can't fix this by calibrating the unit. In most of the cases I've seen, apogee is detected too early (because the thrust it measures is low compared to the decelleration after burnout). If you do a test flight, you can see how the predicted apogee (calculated zero velocity) compares against the actual apogee. It's usually pretty easy to tell if you have a good or bad accelerometer. Roughly, you might see a predicted apogee that is about three-quarters of the way to actual apogee.

One thing to watch out for, though, is that if the accelerometer is bad, then calculations based on it have to be reviewed carefully. An example is the use of "velocity lower than" as the mack lockout, which is the normal way that Raven's do mach lockout. With a bad accelerometer, the calculated velocity value will be lower than the actual velocity at any given time, so you might be below the set mach lockout velocity based on the calculated velocity, but actually still be above mach. This is mainly an issue for long or higher velocity flights. But, it's always good to know if the calculated velocity is accurate so that you're not asking the Raven to do something that it can't.

Jim
 

cerving

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What about venting the NC through the base, and putting the static ports in the airframe lower down?
The problem with that scenario is that when the ejection charge blows it's going to pressurize the AV bay too. Generally, you don't want to do that.
 

OverTheTop

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The problem with that scenario is that when the ejection charge blows it's going to pressurize the AV bay too. Generally, you don't want to do that.
True. Not insurmountable though. I have heard of it being done, but I don't remember the rest of the details of how everything went together.
 

Rocketjunkie

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The problem with that scenario is that when the ejection charge blows it's going to pressurize the AV bay too. Generally, you don't want to do that.
My Goblin is built this way. I've used Missileworks and Perfectflite barometric altimeters with no problems. They don't react to a pressure spike lasting less than the sampling interval. A plot of altitude shows the pressure reaching zero or lower altitude.
 

SpaceManMat

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You can put a vent on the NC shoulder (near the bottom) with a matching hole in the body. The only issue with this is that the NC could spin blocking the hole, so you need to add an alignment pin to prevent rotation or use shear pins (or do both). You can also oversize the vent on the NC so alignment does not need to be perfect. If not using shear pins make sure you have ample friction to prevent the NC sliding forward after burnout.
 

Adrian A

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Only at high velocities. As the rocket reaches apogee and approaches its lowest vertical velocity the pressure readings should be nearly accurate, especially if there's little horizontal velocity.
Be sure Mach inhibit is on.
Exactly. The Raven's accelerometer is good enough to make sure the rocket is going slower than transonic speeds, but it won't detect apogee as accurately as the baro will, even for a non-optimal vent hole placement.
 

bobdog

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Just an update to bring closure to this thread.

Placed the altimeter vent holes just in front of the nose cone shoulder and programmed the Raven for barometric apogee detection. Finally flight tested it yesterday. Apogee detected accurately and apogee chute deployed as programmed.

Thanks for all the advice that lead to a successful result.
 

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