Who makes accelerometers with no baro sensor?

If you are looking for an independent method to delpoy a parachute at apogee, try a magnetic apogee detector.

- Robert Galejs

Brian Elfert wrote:

We've considered a MAD for apogee deployment. The problem with a MAD is the single event.

It has not been decided yet if this will be dual deployment or single.

The rocket is almost 20 feet tall and 12" diameter, but flying it at BALLS in the desert won't make dual deployment a necessity. Single deployment will make things less complicated.

Brian Elfert

I cannot imagine that being a problem if the goal is truly as stated:

backup apogee deploy.

But if you want dual deployment, then you need a baro sensor, which you said you didn't want to use. An accelerometer is also a single event device.

- Robert

I guess finding an acceleromoeter only device is not possible.

I did a little research and found both the Gwiz MC and the AltAcc2 have baro sensors and probably require venting. I didn't think the MC or the AltAcc2 had baro sensors.

The problem is my rocket has an 16" long 11.5" diameter altimeter bay and requires a ton of holes for venting.

I suppose I could rebuild the altimeter bay with smaller volume.

Brian Elfert

As an alternative, you could vent the altimeter differently.

I think at least one altimeter (don't ask me which one as I can't remember) uses a pressure sensor that has a package designed to be attached to tubing. I also think that these sensors are designed for use with 1/8" ID tubing that you can find at your local RC shop for use with fuel systems.

Pick up some tubing and some "T" connectors. Use the tubing and "T" connectors to connect to three external vents. You could probably make decent vents using brass tubing.

Now your vented volume is almost non-existant. But this might cause a problem. The usual vented altimeter bay acts like a low pass filter and decreases any variation in internal pressure caused by wind or other things. The tubing will not provide this filtering. Depending on the particular software in the altimeter, this may or may not be a problem.

This could possibly be extended to venting a small internal compartment but it gets tricky. The recommended vent sizes are based on the depth of the vent hole being small enough to ignore the flow effects. If you run a long tube to vent an internal compartment, this assumption is no longer valid.

Brian Elfert wrote: > Robert Galejs writes: >

Why not put the altimiter in a "box" inside the mammoth compartment.

Or fly smaller rockets.

Naaaaah

Jerry

Anything that has an altitude-based second event is going to require a barometer. Sure, you could theoretically do it with a 3-axis accelerometer and a tilt / rotation sensor, but I've never seen that done correctly in a hobby altimeter.

I bet that 3 or 4 1/4" holes would vent your alt bay just fine, as long as you have a mach delay on your baro-based altimeter. It might lag a bit, but while your rocket is coasting to apogee (which takes several seconds), the pressure will equalize with the outside.

According to the formula posted on ROL by Bob Krech, 4 1/4" vent holes in a bay of this size will have a delay af about 2 seconds between a pressure change on the outside and the inside of the bay.

But if the intent is to control the apogee event using an accelerometer algorithm and the main with a barometric, this much venting is just fine. A delay of two seconds should not make that big a difference in the main deployment. So long as the deployment altitude is high enough and the drogue descent rate isn't too high.

An AltAcc2C should work fine. I fly my AltAcc routinely in rockets from 2" to 6" diameter and the only holes are those required for arming and seeing the status LED.

David wrote:

How much of a change in pressure? Surely the time delay would depend on how quickly the pressure was changing - i.e. how fast the rocket is changing altitude.

If you use an accelerometer-based altimeter (which doesn't 'care' about the baro sensor until after apogee) with a barometric sensor for main deployment the rocket isn't likely (all being well) to be falling especially fast at the altitude of main deployment, so I can't see pressure equalisation time making any huge difference.

One thing to bear in mind about the G-Wiz altimeters is that the LC and MC do deployment at 400 or 800 feet for the main. Depending on the size of your rocket and how much margin you want, this may be a little low. Given that you're launching at Black Rock, could you perhaps use one altimeter to deploy the main at say 2000ft and then something like a G-Wiz for backup at

I'm not familiar with the G-wiz, but the Blacksky AltAcc uses an accelerator for apogee detection and a barometric sensor for deploying the main chute. You can set it up for main deployment at apogee, but if you are going to use it for dual deployment you will need to vent it.

Brian, As long as the main event is accelerometer based then you do not have to worry about venting of the bay.

My 12in dia 24" long bay had 4 1in dia holes. And that was more than effective with both my barometric only altimeters.

Consider this. You need more venting to detect apogee because if not properly vented it will deploy late as the pressure in the e-bay will take time to equalize to outside pressure. But under drogue there is plenty of time for the inside/outside pressure to equalize as the rocket descends much slower than it ascends.

So it will be f>