Altimeters vs. GPS?

[cerving]

...FWIW, This may be evidence that barometric altimeters are affected, not only by aerodynamic processes at the static ports, but also by acceleration. (Or maybe everyone knew that. I always suspected it...)

Baro sensors are actually cousins of accelerometers, so yes, they can be affected by shock. Since they have an open port to the atmosphere, they're also light-sensitive too... you don't want one pointing at one of the vent ports.

 

[Spacedog49Krell]

Baro sensors are actually cousins of accelerometers, so yes, they can be affected by shock. Since they have an open port to the atmosphere, they're also light-sensitive too... you don't want one pointing at one of the vent ports.

I have subjected baro sensors to 124 G's of launch acceleration without any detectable altitude or pressure variation in the data.
They are very sensitive to light. I paint the interior of my avionics bays flat black to prevent any light reflection reaching the baro sensor.

 

[kjhambrick]

Tangentially related... I think...

Noticed something interesting in Adrian’s F10 data. Here is a graph of the Blue Raven’s rendering of vertical velocity (BR VY), and also, the numerically-differentiated barometric altitude/time curve (Baro VY). Notice that the barometric curve is above the inertial curve until cutoff, at which point it falls precipitously.

View attachment 644316
Next, I took the sines of the tilt angles (BR Sine), and also the ratio of the BaroVY values (above) and inertial total speeds (ratio labeled BaroSine). Both should estimate trajectory sine. Here’s what I got:

View attachment 644317

Things come together near apogee; however, before that, the barometric sine seems to have an acceleration curve imprinted in it.

FWIW, This may be evidence that barometric altimeters are affected, not only by aerodynamic processes at the static ports, but also by acceleration. (Or maybe everyone knew that. I always suspected it...)

Larry --

I've been pondering your method for deriving tilt angles from inertial -vs- barometric altitude since you shared it with me last year.

I believe you are on to something there.

But in the case of Adrian's F10 flight, could the divergence be due to transonic static port pressure sampling issues ?

These are all the Altitudes I could muster from Adrian's Data Files along with Vertical Velocity from the low_rate file for Adrian's F10 flight:


Legend:

PAlt(u) - uncorrected Barometric Pressure Altitude
DAlt(c) - PAlt corrected for ISA Temperature Lapse Rate and Site Altitude via Richard Nakka's method.
IAlt(c) - Inertial Altitude from the Blue Raven Low Rate file
GPS - GPS Altitude from the Featherweight GPS
VVel - Vertical Velocity from the Blue Raven Low Rate file.

Note that the slopes of the PAlt and DAlt curves begin to diverge from the slopes of GPS Alt and IAlt when the rocket is moving about 800 ft/sec or so.

Also note that DAlt is really just PAlt corrected for Site Altitude, Site Temperature and Temperature Lapse Rate so they SHOULD have the same shape

Now look at the same plot for his G12 flight:

Note that the slopes for the G12 flight also start to diverge at around 800 ft/sec.

Also note that the shoulders on the PAlt and DAlt are shaped like 'classic' transonic static port sampling errors.

I believe in this case, we're seeing an issue with the static port position or geometry.

I do have one Q about your second plot ... how do you calculate a BaroSine value greater than 1 ?

-- kjh

p.s. I found and fixed a bug in my implementation of Nakka's DAlt calculation after my posts #32 and #39 so the DAlts will be different here.

And after pondering Adrian's post #35 about calculated IAlt in the Blue Raven Low Rate file, I've decided to 'trust Adrian' and use IAlt from the Low Rate File so the IAlts are also different

EDIT: Never mind ...

 

[kjhambrick]

I had 3 flights over the weekend of my own hybrids which I intentionally run crazy unstable. I'm still developing my own flight electronics, so there was a bug in the code of the backup timer and they all deployed prematurely (just as well for the L flight) from that backup timer. Nevertheless, I was impressed with the correlation between the Ublox M9N and the MS5611 Baro sensor.

TP

That's beautiful data @rocket_troy !

Do you have pics or better yet, videos of your flights ?

One correction: the MM/DD in your dates are reversed

-- kjh

 

[rocket_troy]

That's beautiful data @rocket_troy !

Do you have pics or better yet, videos of your flights ?

One correction: the MM/DD in your dates are reversed

-- kjh

Unfortunately, not handy. I'll try and track something down from one of our members. Yeah, aussie dates

TP

 

[Adrian A]

I have subjected baro sensors to 124 G's of launch acceleration without any detectable altitude or pressure variation in the data.
They are very sensitive to light. I paint the interior of my avionics bays flat black to prevent any light reflection reaching the baro sensor.

They can be affected by changes in pressure at the bottom of the interior of the rocket when under high Gs. If you have a 2' length of airframe and a 100 G boost, you'll get pressure differential equivalent to 200' between the top and the bottom of the av-bay just from the mass of air inside the av-bay. If you haven't seen any effect of a 124 G launch on pressure, then either you have your pressure sensor very close to the middle of your av-bay or it's unusually well-vented.

Depending on the configuration and how well sealed the av-bay is from the rest of the rocket, It's also not uncommon for chutes and pistons to move under boost, causing changes in pressure.

 

[ihbarddx]

Larry --

I've been pondering your method for deriving tilt angles from inertial -vs- barometric altitude since you shared it with me last year.

I believe you are on to something there.

But in the case of Adrian's F10 flight, could the divergence be due to transonic static port pressure sampling issues ?

These are all the Altitudes I could muster from Adrian's Data Files along with Vertical Velocity from the low_rate file for Adrian's F10 flight:
View attachment 644359

Legend:

PAlt(u) - uncorrected Barometric Pressure Altitude
DAlt(c) - PAlt corrected for ISA Temperature Lapse Rate and Site Altitude via Richard Nakka's method.
IAlt(c) - Inertial Altitude from the Blue Raven Low Rate file
GPS - GPS Altitude from the Featherweight GPS
VVel - Vertical Velocity from the Blue Raven Low Rate file.

Note that the slopes of the PAlt and DAlt curves begin to diverge from the slopes of GPS Alt and IAlt when the rocket is moving about 800 ft/sec or so.

Also note that DAlt is really just PAlt corrected for Site Altitude, Site Temperature and Temperature Lapse Rate so they SHOULD have the same shape

Now look at the same plot for his G12 flight:
View attachment 644360
Note that the slopes for the G12 flight also start to diverge at around 800 ft/sec.

Also note that the shoulders on the PAlt and DAlt are shaped like 'classic' transonic static port sampling errors.

I believe in this case, we're seeing an issue with the static port position or geometry.

I do have one Q about your second plot ... how do you calculate a BaroSine value greater than 1 ?

-- kjh

p.s. I found and fixed a bug in my implementation of Nakka's DAlt calculation after my posts #32 and #39 so the DAlts will be different here.

And after pondering Adrian's post #35 about calculated IAlt in the Blue Raven Low Rate file, I've decided to 'trust Adrian' and use IAlt from the Low Rate File so the IAlts are also different

EDIT: Never mind ...

Hi, Konrad
It's quite possible that the anomalies are caused by aerodynamic effects. Indeed, that's normally what we assume when we see them - and, of course, we see them all the time. (Transonic effects are not necessary for divergence, BTW, but they sure help) I'm merely speculating that this particular curve comes from acceleration because

1) The anomaly looks (qualitatively) like an acceleration curve from an F10. (This F10 burned for about 8.5 seconds, which is long.)
2) The curve turns down at the end immediately at cutoff, rather than somewhat after cutoff, as one might expect if it's speed-related.

BTW, I did not use the sine estimation method that I shared with you, although I got virtually identical results that way. I used the cosines of the Blue Raven tilt angles (which yields sines of the rocket's attitude) for the reference curve. For the altimeter sines, I divided the numerically-differentiated altitude-time curve by the total velocity from B.R. data. The differentiated curve is an estimate of vertical velocity. That divided by total velocity should estimate the trajectory sine - assuming a more-or-less planar trajectory.

As you point out, sines cannot exceed unity. Estimates of sines can exceed unity - particularly when there is noise in the data. Barometric vertical velocities are notoriously noisy. Numerically-differentiated data are inherently noisy, and then there are the aero and possibly acceleration effects. The sine estimates are affected.

Anyway, it will be interesting top do similar analysis on other datasets to see if acceleration-like anomalies pop up. The speculation could be dead wrong. OTOH, barometers have diaphragms, which can distort under setback forces. Ya never know.

 

[Spacedog49Krell]

They can be affected by changes in pressure at the bottom of the interior of the rocket when under high Gs. If you have a 2' length of airframe and a 100 G boost, you'll get pressure differential equivalent to 200' between the top and the bottom of the av-bay just from the mass of air inside the av-bay. If you haven't seen any effect of a 124 G launch on pressure, then either you have your pressure sensor very close to the middle of your av-bay or it's unusually well-vented.

Depending on the configuration and how well sealed the av-bay is from the rest of the rocket, It's also not uncommon for chutes and pistons to move under boost, causing changes in pressure.

You are correct on both accounts. The baro sensor(s) are located in the middle of the av-bay with two vent ports located 180° apart. The top port is a minimum of 2.5 calibers from the nose cone transition and the second port is a minimum of 1 caliber lower. The vent ports are also sized for the required equilibrium rate.

Where were your vents located? The air flow over them lowered your av-bay pressure below ambient.

120 G's will disconnect a 9 volt battery snap connection.

 

[ihbarddx]

Depending on the configuration and how well sealed the av-bay is from the rest of the rocket, It's also not uncommon for chutes and pistons to move under boost, causing changes in pressure.

Interesting possibility!