Example of mach transition?

[Worsaer]

I flew my 4" Nike Smoke this weekend on a K1440. Looking at the data this evening I noticed the anomaly in altitude that coincided with peak velocity. This was the most pronounced example of this that I've seen on one of my flights. Does this illustrate why mach immunity in altimeters matters?

For what it's worth, the flight was controlled by an RRC3, and it worked great. The data is from a Raven 3 that was just along for the ride.

 

[CORZERO]

I don't see anything funny on your graph in the mach 1 region. If you're in Cali, and launching at sea level, then the area of 1125FPS would be mach 1. Nice smooth lines on your graph in that area. Tell me why I'm retarded...

 

[cerving]

Yup, that's pretty typical of a mach transition. Also typical is the little jag around your drogue deployment, it tends to be more pronounced on altimeters that aren't actually doing the deployment (probably because the deployment altimeter fires the charge after it takes the reading, then it waits a little bit until the next reading).

 

[ksaves2]

Nice spike. I've seen smaller ones with an MAWD at about the time it was expected from a simulation. Kurt

 

[Worsaer]

I don't see anything funny on your graph in the mach 1 region. If you're in Cali, and launching at sea level, then the area of 1125FPS would be mach 1. Nice smooth lines on your graph in that area. Tell me why I'm retarded...

Here are the numbers that make it more pronounced...

At 1.54 seconds the altitude is 1,673 feet (AGL), and the speed is 1,342 fps.
At 2.06 seconds the altitude is 3,182 feet, and the speed is 1,121 fps
At 2.70 seconds the altitude suddenly drops to 2,464 feet (baro pressure rises), then altitude goes back up again to eventual altitude of 7,655 feet.

I can see why altimeters that didn't filter that out would have deployed at 2.7 seconds, instead of at apogee at 19.4 seconds. That would have been quite a mess to clean up.

 

[CORZERO]

The graph reads ft/sec, yet acceleration is calculated at ft/2^2, correct? Is this wherein my confusion lies? I see the spikes, but I seem to have misinterpreted the graph.

 

[Worsaer]

The graph reads ft/sec, yet acceleration is calculated at ft/2^2, correct? Is this wherein my confusion lies? I see the spikes, but I seem to have misinterpreted the graph.

I think I see your point. I think the graph is showing velocity, not acceleration - yet the label doesn't make that clear. My attention was drawn to the momentary drop in altitude ( rise in barometric pressure) that would confuse an altimeter that wasn't designed to filter it out.

 

[CORZERO]

Thank you. The graph description is unclear. To quantify velocity and acceleration with one value creates a conflict. Either I need to step up my graph reading skills, or some code work is necessary. Again, I advertise an open offer to criticize my intellect.

Edit: spelling of "value"

 

[Stable1]

Thank you. The graph description is unclear. To quantify velocity and acceleration with one value creates a conflict. Either I need to step up my graph reading skills, or some code work is necessary. Again, I advertise an open offer to criticize my intellect.

Edit: spelling of "value"

I believe the word "Accel" on the vertical velocity axis indicates that the velocity was calculated from the accelerometer data. I believe there is also a Velocity - Baro variable

 

[CORZERO]

I believe the word "Accel" on the vertical velocity axis indicates that the velocity was calculated from the accelerometer data. I believe there is also a Velocity - Baro variable

Well there it is, it is "believed" by someone. Any definitive experts out there care to chime in?

All due respect, Stable1

 

[kevinkal]

Well there it is, it is "believed" by someone. Any definitive experts out there care to chime in?

All due respect, Stable1

It's been my experience that true experts know better than to make absolute or definitive statements.

Bill,
Thanks for posting the question and flight data. Good stuff. I haven't yet pushed though Mach1 with my 4" Fiberglass Airframe, so its nice to see your data. I flew a K1103X Saturday, but my rocket tips the scales at 12.5 lbs.. so I'll be lucky to see more than 1100 ft/sec. I'll take a closer look at my baro alt data for this flight to see if anything interesting might have happened.
Aside, that was a hot day out there Saturday... I think I was experiencing a few effects of heat exhaustion after helping to break down, carry and load the trailer.
Kevin

 

[Worsaer]

It's been my experience that true experts know better than to make absolute or definitive statements.

Bill,
Thanks for posting the question and flight data. Good stuff. I haven't yet pushed though Mach1 with my 4" Fiberglass Airframe, so its nice to see your data. I flew a K1103X Saturday, but my rocket tips the scales at 12.5 lbs.. so I'll be lucky to see more than 1100 ft/sec. I'll take a closer look at my baro alt data for this flight to see if anything interesting might have happened.
Aside, that was a hot day out there Saturday... I think I was experiencing a few effects of heat exhaustion after helping to break down, carry and load the trailer.
Kevin

Thanks Kevin - it would be interesting to see your data when you can pull it down. I agree, it was brutally hot when we started tearing down the pads. After a couple of trips carrying pads & rails back to trailer I was feeling it too. Dehydration and heat exhaustion can sneak up on you.

 

[OverTheTop]

The graph shows that the velocity, calculated as a function of the acceleration input, is increasing monotonically (no negative dips) during the boost/coast phase of flight. The altitude, calculated as a function of barometric pressure, has increased non-monotonically (it has a negative slope region in the positive slope) likely (not definitively :wink due to the Mach transition during boost. We know from the velocity graph that the rocket is still ascending, so the feature on the altitude graph is an artifact of the measurement system, not reality. This is a good example of why Mach lockout (or other ways of dealing with this phenomena) is important in altimeters.