Barometer or Accelerometer

i was wondering what is the difference to these two types of altimeters: Barometer or Accelerometer? what are the pros and cons to both sides? which in your opinion is better and how have they resulted in past or memorable flights individually.
p.s still on my research phase of my rocket and knowledge from veterans at this will be extremely appreciated.

You need a baro sensor to deploy a main chute at a low altitude on the say down. A baro sensor also is usually better at firing at apogee accurately. An accelerometer complements the baro sensor well, because it's not susceptible to pressure transients during transitions in and out of supersonic flight, from shifting of rocket parts during liftoff, or ejection charges. It also peovides a wealth of interesting recorded information like the forces and accelerations from the deployment charges and chute openings. If you study the data, you can calculate how close you came to breaking parts during those events. You can also measure the drag after the motor burns out to calculate the rocket's Cd. It's a more reliable measurement of the rocket top speed during fast flights. I have been flying accelerometers for several years, and I still almost always find something surprising or interesting in the accel data. An accelerometer works well with the baro sensor to check combinations of flight conditions for igniting an air-started motor. So for me, a baro sensor is mandatory, an accelerometer is an important supplement, and the two together are worth more than the sum of their parts.

Alby said:

Adrian:

Are you still selling the 250G single-axis Ravens? Not that I need one, but
I don't see them on the website anymore. Was nobody buying them?

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They're still available and on the site. I may need to re-organize the web store a bit so they show up easier.

Adrian:


Why would somebody choose a Single Axis over a Dual Axis accelerometer?

Adrian A said:

They're still available and on the site. I may need to re-organize the web store a bit so they show up easier.

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You are too quick. I eventually found the link and deleted the msg..

https://www.shop.featherweightaltimeters.com/category.sc?categoryId=3

Alby said:

Adrian:


Why would somebody choose a Single Axis over a Dual Axis accelerometer?

Click to expand...

The 250G range accelerometer is only available in a single-axis version.

Alby said:

Adrian:


Why would somebody choose a Single Axis over a Dual Axis accelerometer?

Click to expand...

The second axis adds little or nothing in the way of quantitative data, because you don't know where the off-axis instrument is pointing. It can give you a qualitative idea of off-axis motion at particular moments.

[Actually, the only way we know where the axial instrument is pointing is the assumtion of stability. If the rocket goes unstable, even axial data are meaningless. That's why accelerometers don't work right when a rocket goes unstable.]

I'm personally preoccupied with accelerometers, and the second axis (by itself) doesn't influence my decisions at all.

Larry Curcio said:

The second axis adds little or nothing in the way of quantitative data, because you don't know where the off-axis instrument is pointing. It can give you a qualitative idea of off-axis motion at particular moments.

[Actually, the only way we know where the axial instrument is pointing is the assumtion of stability. If the rocket goes unstable, even axial data are meaningless. That's why accelerometers don't work right when a rocket goes unstable.]

I'm personally preoccupied with accelerometers, and the second axis (by itself) doesn't influence my decisions at all.

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Is it possible to use (2) 2-axis accelerometers to get 3-4 axis of information
to guard against unstable rockets? Or would you really just need an
accelerometer designed to see X, Y, and Z axis?

Alby said:

Is it possible to use (2) 2-axis accelerometers to get 3-4 axis of information
to guard against unstable rockets? Or would you really just need an
accelerometer designed to see X, Y, and Z axis?

Click to expand...

Interesting idea!

Unfortunately, a 3-axis accelerometer has the same problem. Ideally, you need 3 gyrocopes in addition to the 3-axis accelerometer. The trajectory problem has 6 degrees of frredom, rather than 3.

Still, I like the way you think!

Alby said:

Is it possible to use (2) 2-axis accelerometers to get 3-4 axis of information
to guard against unstable rockets? Or would you really just need an
accelerometer designed to see X, Y, and Z axis?

Click to expand...

In theory, yes, if the sets of accels are at two ends of the rocket, then you can deduce 2 axes of angular rate as well as acceleration by differencing the measurements to get the centripetal acceleration. In practice, the available accels that have a high enough range for rocket motions also have too much sensor drift to do this reliably, especially since you wouldn't know the roll orientation without a gyro or another set of accels that are even closer to the first two sets.

Larry Curcio said:

Interesting idea!

Unfortunately, a 3-axis accelerometer has the same problem. Ideally, you need 3 gyrocopes in addition to the 3-axis accelerometer. The trajectory problem has 6 degrees of frredom, rather than 3.

Still, I like the way you think!

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I was kinda thinking like the movie Stargate. Where you need 6 points to
know where you are and 1 point for a destination. Instead, what if the
1-destination point became the launch pad and the 6-points are the
various axis of the rocket? If you had such a device, you should, in
theory, be able to guard against an unstable rocket.

Adrian A said:

In theory, yes, if the sets of accels are at two ends of the rocket, then you can deduce 2 axes of angular rate as well as acceleration by differencing the measurements to get the centripetal acceleration. In practice, the available accels that have a high enough range for rocket motions also have too much sensor drift to do this reliably, especially since you wouldn't know the roll orientation without a gyro or another set of accels that are even closer to the first two sets.

Click to expand...

Hmmmm...... That sounds like a new product line for Featherweight Altimeters
Once you have a Raven, you could get add-ons that would work with the
Raven, like a gyroscope and other gadgets.

Adrian A said:

In theory, yes, if the sets of accels are at two ends of the rocket, then you can deduce 2 axes of angular rate as well as acceleration by differencing the measurements to get the centripetal acceleration. In practice, the available accels that have a high enough range for rocket motions also have too much sensor drift to do this reliably, especially since you wouldn't know the roll orientation without a gyro or another set of accels that are even closer to the first two sets.

Click to expand...

In the WGX algorithm, I use the barometer as a reference.

1) Compute inertial velocity and altitude assuming a vertical trajectory

2) Convert inertial altitude into inferred potential energy, and barometric altitude into observed potential energy

3) Add the discrepancy back to the inferred kinetic energy, derived from inertial velocity

4) Derive a Barometrically-Adjusted Speed Estimate (BASE) from the correced kinetic energy

5) VY = BASE*SineTheta

6) VY = VY(Last) + .5*DeltaT*(AccReadingSineTheta +LastAccReading*LastSineTheta) - g*DeltaT

7) Equate RHS from 5 and 6

8) Solve for SineTheta

9) Get VY from expression 6

Devil's in the details, but sometimes it works.