Keith: I think I have it now. I knew that gravity was a constant acceleration throughout the flight. My thinking was more wishful than understanding. I thought there was a magic algorithm. Since acceleration plots on normal flights display chaotic acceleration data after separation, I found a data set for a rocket that had a failed apogee charge and a ballistic return. Unfortunately, it was not a TeleMega so I only have only one axis of acceleration that I can plot. Not surprising the acceleration plot did not change at apogee. It was a flat line until the main opened and zippered the rocket. Hence, the acceleration due to gravity is the same going up as it is going down (minus some drag). Somehow, I thought with three axes you might be able to track the direction of acceleration looking for a vector change of acceleration. If lucky, you could see Z-axis acceleration change from negative 1 to positive 1. Even still, you are only working on a range of 0-1 if the zero on x and y don't include gravity. With rotation and tumbling possible, these axes would not be reliable either. Either way, I learned a ton from this exchange. I understand how you approximate apogee by calculating velocity and and projecting the zero cross point for apogee. The algorithm for detecting apogee was really the crux of my questions. Thanks for helping me get a deeper understanding. I hope to see you again in Argonia. Craig
Craig Klimczak <[email protected]> writes: > Your're right. :-) I really don't understand what the > accelerometers are reporting. I assumed that the accel would detect > the de-acceleration as the rocket reached apogee, reach a point of > zero acceleration and then start accelerating back toward Earth. Uh. You're still missing something. After motor burnout, save for the effect of air resistance, the rocket spends the entire flight accelerating at 1g towards earth. You may want to consider a bit of a refresher on newtonian physics :-) > how do you estimate apogee by acceleration alone? We assume the rocket is pointing upwards during ascent and integrate the z-axis acceleration to compute change in speed. If we assume the rocket starts at rest, then when the total change in speed gets us back to zero, then we've stopped moving in that axis. This is slightly complicated by gravity -- when the accelerometer measures zero acceleration, we're actually accelerating towards the ground at 1g, so we add 1g of acceleration to whatever the accelerometer measures. Don't expect TeleMega to work on the moon. -- -keith -----Original Message----- From: altusmetrum-request <[email protected]> To: altusmetrum <[email protected]> Sent: Tue, Jun 25, 2019 5:04 pm Subject: altusmetrum Digest, Vol 94, Issue 7 Send altusmetrum mailing list submissions to [email protected] To subscribe or unsubscribe via the World Wide Web, visit http://lists.gag.com/mailman/listinfo/altusmetrum or, via email, send a message with subject or body 'help' to [email protected] You can reach the person managing the list at [email protected] When replying, please edit your Subject line so it is more specific than "Re: Contents of altusmetrum digest..." Today's Topics: 1. Re: BALLS Project (Keith Packard) 2. Re: BALLS Project (Clay and Carly Dunsworth) 3. Re: BALLS Project (Clay and Carly Dunsworth) 4. Re: BALLS Project (Clay and Carly Dunsworth) ---------------------------------------------------------------------- Message: 1 Date: Tue, 25 Jun 2019 12:13:39 -0700 From: "Keith Packard" <[email protected]> To: Craig Klimczak <[email protected]>, [email protected] Subject: Re: [altusmetrum] BALLS Project Message-ID: <[email protected]> Content-Type: text/plain; charset="utf-8" Craig Klimczak <[email protected]> writes: > Your're right. :-) I really don't understand what the > accelerometers are reporting. I assumed that the accel would detect > the de-acceleration as the rocket reached apogee, reach a point of > zero acceleration and then start accelerating back toward Earth. Uh. You're still missing something. After motor burnout, save for the effect of air resistance, the rocket spends the entire flight accelerating at 1g towards earth. You may want to consider a bit of a refresher on newtonian physics :-) > how do you estimate apogee by acceleration alone? We assume the rocket is pointing upwards during ascent and integrate the z-axis acceleration to compute change in speed. If we assume the rocket starts at rest, then when the total change in speed gets us back to zero, then we've stopped moving in that axis. This is slightly complicated by gravity -- when the accelerometer measures zero acceleration, we're actually accelerating towards the ground at 1g, so we add 1g of acceleration to whatever the accelerometer measures. Don't expect TeleMega to work on the moon. -- -keith -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 832 bytes Desc: not available URL: <http://lists.gag.com/pipermail/altusmetrum/attachments/20190625/b375486b/attachment-0001.sig> ------------------------------ Message: 2 Date: Tue, 25 Jun 2019 15:30:28 -0500 From: Clay and Carly Dunsworth <[email protected]> To: Altus Metrum <[email protected]> Subject: Re: [altusmetrum] BALLS Project Message-ID: <[email protected]> Content-Type: text/plain; charset=utf-8 Not really In parabolic flight, gravity; a constant force perpendicular to the object has less effect on the absolute velocities, when horizontal velocity is higher. So it’s closer to 0g of axial acceleration through apogee. Thus, your center of gravity and center of pressure will use most of that 1g as an arm to rotate (laterally accelerate) the rocket rather than accelerate it on axis. > On Jun 25, 2019, at 2:13 PM, Keith Packard <[email protected]> wrote: > > Craig Klimczak <[email protected]> writes: > >> Your're right. :-) I really don't understand what the >> accelerometers are reporting. I assumed that the accel would detect >> the de-acceleration as the rocket reached apogee, reach a point of >> zero acceleration and then start accelerating back toward Earth. > > Uh. You're still missing something. After motor burnout, save for the > effect of air resistance, the rocket spends the entire flight > accelerating at 1g towards earth. You may want to consider a bit of a > refresher on newtonian physics :-) > >> how do you estimate apogee by acceleration alone? > > We assume the rocket is pointing upwards during ascent and integrate the > z-axis acceleration to compute change in speed. If we assume the rocket > starts at rest, then when the total change in speed gets us back to > zero, then we've stopped moving in that axis. > > This is slightly complicated by gravity -- when the accelerometer > measures zero acceleration, we're actually accelerating towards the > ground at 1g, so we add 1g of acceleration to whatever the accelerometer > measures. > > Don't expect TeleMega to work on the moon. > > -- > -keith > _______________________________________________ > altusmetrum mailing list > [email protected] > http://lists.gag.com/mailman/listinfo/altusmetrum ------------------------------ Message: 3 Date: Tue, 25 Jun 2019 15:35:08 -0500 From: Clay and Carly Dunsworth <[email protected]> To: Altus Metrum <[email protected]> Subject: Re: [altusmetrum] BALLS Project Message-ID: <[email protected]> Content-Type: text/plain; charset=us-ascii Sorry for my response, I guess we said the same thing, except gravity is why the mass is important for thrust to weight ratios during the burn. So if you update the code for the moon, we could launch from there to mars cheaper. > On Jun 25, 2019, at 2:13 PM, Keith Packard <[email protected]> wrote: > > Craig Klimczak <[email protected]> writes: > >> Your're right. :-) I really don't understand what the >> accelerometers are reporting. I assumed that the accel would detect >> the de-acceleration as the rocket reached apogee, reach a point of >> zero acceleration and then start accelerating back toward Earth. > > Uh. You're still missing something. After motor burnout, save for the > effect of air resistance, the rocket spends the entire flight > accelerating at 1g towards earth. You may want to consider a bit of a > refresher on newtonian physics :-) > >> how do you estimate apogee by acceleration alone? > > We assume the rocket is pointing upwards during ascent and integrate the > z-axis acceleration to compute change in speed. If we assume the rocket > starts at rest, then when the total change in speed gets us back to > zero, then we've stopped moving in that axis. > > This is slightly complicated by gravity -- when the accelerometer > measures zero acceleration, we're actually accelerating towards the > ground at 1g, so we add 1g of acceleration to whatever the accelerometer > measures. > > Don't expect TeleMega to work on the moon. > > -- > -keith > _______________________________________________ > altusmetrum mailing list > [email protected] > http://lists.gag.com/mailman/listinfo/altusmetrum ------------------------------ Message: 4 Date: Tue, 25 Jun 2019 16:04:07 -0500 From: Clay and Carly Dunsworth <[email protected]> To: Altus Metrum <[email protected]> Subject: Re: [altusmetrum] BALLS Project Message-ID: <[email protected]> Content-Type: text/plain; charset="utf-8" > “Accelerometers cannot measure gravity “ Accelerometers cannot measure Acceleration. If you knew the exact drag coef., mass, rotational forces and trajectory, your accelerometer would be useful to measure exact forces of gravity. “In axis” It’s why you get different values depending on orientation . Put a rocket on a launch pad angled from horizontal to vertical and note the reading of the accelerometer. > On Jun 25, 2019, at 12:22 PM, David W. Schultz <[email protected]> > wrote: > >> On 6/25/19 11:33 AM, Keith Packard wrote: >> That's not what we're waiting for -- we're just compensating for the >> inaccuracy in computing apogee based on acceleration alone. We don't >> have a lot of precision in measuring acceleration because we need such a >> large range, that means there's a significant error in each measurement, >> leading to (potentially) large errors in speed computation and missing >> apogee by many seconds. > > I have seen this concern before and I find it puzzling as it isn't > really a problem. The now ancient BlackSly AltAcc used an 8 bit ADC and > worked quite well. Digging out an handy data set I see it reporting > 3.1387 GHarrys / G. (GHarry being an ADC count.) > >> >>> What about Drew's concern over the lack of integration over the three >>> axis of accel? >> >> Yeah, having a full 6dof state computation would be 'better' in some >> way, but we still can't get much precision in the z-axis acceleration >> value, so I don't know how much more accurate apogee detection would be. >> > > If there is considerable error in alignment between the sensor axis and > the vehicle axis then you might want to do this. Or just align them. > (Made harder by sensor to package and package to PCB alignment errors.) > >>> Can we assume that as the rocket arc's over the change in acceleration >>> will become detectable and thus trigger apogee detection? Is there a >>> set threshold for change in acceleration for the algorithm to detect >>> apogee? >> >> Uh, I think you're missing something here -- there's no change in any >> acceleration values across apogee; the rocket trajectory is ballistic >> precisely because the only large force acting on it is gravity, which is >> constant. >> > > Accelerometers cannot measure gravity because it works on all parts of > the rocket/sensor equally. On the pad you see the acceleration from the > ground holding the rocket up. Near apogee there are only a couple of > things it could measure. One is aerodynamic forces and the other matters > only if the rocket is rotating. > > > > -- > https://web.archive.org/web/20190214181851/http://home.earthlink.net/~david.schultz/ > (Web pages available only at the Wayback Machine because Earthlink > terminated that service.) > _______________________________________________ > altusmetrum mailing list > [email protected] > http://lists.gag.com/mailman/listinfo/altusmetrum -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.gag.com/pipermail/altusmetrum/attachments/20190625/05bd9c0d/attachment.html> ------------------------------ Subject: Digest Footer _______________________________________________ altusmetrum mailing list [email protected] http://lists.gag.com/mailman/listinfo/altusmetrum ------------------------------ End of altusmetrum Digest, Vol 94, Issue 7 ******************************************
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