Jolly Logic Altimeter TWO versus Perfectflite Alt15K

[Zeus-cat]

I flew my Launch Pad Bolo on a D12-5 and a C6-3 with my Perfectflite Alt15K and my new Jolly Logic Altimeter TWO. The rocket was modified by adding an avionics bay for the Perfectflite altimeter. The D12-5 flight was very good, but the C6-3 was very underpowered.

I also flew both altimeters in a rocket of my own design - RFD IV. I flew that rocket only on a D12-5 as it does not fly well on a C6-3. I felt the breeze coupled with an overcast sky made an E9 flight very risky.

In both rockets the Perfectflite altimeter was in a dedicated avionics bay. The Altimeter TWO was attached underneath the avionics bay in the parachute/streamer compartment. Both rockets had two 1/16th inch holes drilled into the compartment for the Altimeter TWO. I tried to make sure that the holes were clear before launch.

I have attached screenshots of the graph of the Perfectflite altimeter data along with the data from the Altimeter TWO.

The descent speed of the Alt15K data is calculated from the data and is not a function of the altimeter.

 

[jms]

So the graphs look fairly close! Obviously since the 15K has been around awhile, do you have a good feel on how accurate it is vs. the AltimeterTwo? I am waiting for my A2 to arrive this week and then I'm going to do a review on my site, but this is great!

 

[Zeus-cat]

On the single RFD flight they are about 4% apart. This was actually the first flight I did.

There is a considerable difference on the first Bolo flight (#18). The Perfectflite shows 578 and the Altimeter Two is 650. I can't explain the difference. That is over 12% variation.

On the second Bolo flight they are less than 1% apart, but the rocket didn't go very high.

 

[jms]

Right on. Well I'll take it for a spin on some smaller rockets and see how it does. I'd love to see some pictures of your bolo with the additional bay. Drop one if you get a chance. Thanks!

 

[BEC]

It would be good to see some comparisons where the AltimeterTwo and the Alt15k/WD were in the same compartment and therefore much more exposed to the same pressure transients.

I have done this with some beta AltimeterTwos and generally have seen lesser variation that way. I have seen similar things doing it the same way as Z-c - ATwo in the body, Alt15k/WD in a dedicated payload section.

I'm working on a much broader comparison of various relatively inexpensive altimeters...though I'm not sure right now where those data will go besides to the NAR Contest committee that approves altimeters for NAR contest use (putting them in Appendix G of the Pink Book).

 

[Zeus-cat]

It would be good to see some comparisons where the AltimeterTwo and the Alt15k/WD were in the same compartment and therefore much more exposed to the same pressure transients.

I agree that a test as you described would provide a better environment for the Altimeter TWO. However, one of the main selling points of both the Altimeter ONE and TWO is that they do not special payload bays. They can be flown by placing them inside the rocket above the wadding/Nomex and with vent hole(s) in the body tube. A person would assume that such a method would work and produce accurate results. I wanted to test that method against the Alt15K with its required avioncs bay.

 

[BEC]

Understood. I love the AltimeterOne exactly for that reason. And I've done a number of tests exactly as you have - with generally less distance between the two devices' readings (but not always). However one explanation for the difference is that despite our best intentions, the two can't be exposed to the same pressure profile throughout the flight when we do this. There would be different perturbations in the airflow over the two sets of static ports depending on what's upstream of each set and how far. So they will read differently.

If apogee occurs after ejection, then the difference in environments is even greater.

Therefore we can't really know which one is "right" unless we have an independent (not pressure-based) way to determine the altitude. I've been wanting to do some of this sort of testing while having simultaneous two-station optical tracking, but haven't been able to arrange this yet.

As an aside, I've read several sets of altimeter instructions (not Jolly Logic's) which say to use one, or three or four, but never two static ports. I'm not sure why this is....it's just an observation.

 

[jms]

As an aside, I've read several sets of altimeter instructions (not Jolly Logic's) which say to use one, or three or four, but never two static ports. I'm not sure why this is....it's just an observation.

I wonder if this is the same effect as driving with two car windows open. You no longer get that "vacuum"/Venturi type of effect as you do with one window, but continuous buffeting. Who knows, but I'd be interested to test out the theory by doing three launches with the AltimeterTwo, with one, two and three holes. Has anyone seen an experiment like this done?

 

[Larry Curcio]

Although I can see how the A2 can be placed in a rocket in any orientation, I still do not see, without the addition of gyros, how it is any better than a 1-axis accelerometer at determining the rocket's orientation WRT gravity.

Could someone please explain the point to me?

Thank you.

-LarryC

 

[Zeus-cat]

I don't think it "cares" what orientation it is in. It simply combines the acceleration of all three axes mathematically and reports that. That should work just fine if done correctly.

 

[John Beans]

Although I can see how the A2 can be placed in a rocket in any orientation, I still do not see, without the addition of gyros, how it is any better than a 1-axis accelerometer at determining the rocket's orientation WRT gravity.

Could someone please explain the point to me?

Thank you.

-LarryC

I like to design products that "simply work," which is much harder than it sounds. It would have been much cheaper and simpler to make an altimeter that just monitored one axis. But then I would have had to tell people that they *had* to mount it rigidly aligned with the rocket's primary axis of flight. Personally, I don't want to have to baby an altimeter that much as I move it from rocket to rocket; I'd probably just rather not use it in that case.

With all three axes monitored, the user can just clip it to a parachute line and go, or just wrap it in foam and toss in into a payload bay. No muss, no fuss. It took a good deal of design work and testing to get it work reliably, but the result turned out to be worth it (IMHO).

By the way, it's surprisingly hard to find production quantities of 3-axis accelerometers that can sense this many Gs. They must be hard to manufacture (I guess). I'd love to up the G-limit, but 24G is the state of the art right now it seems.

--John Beans, Jolly Logic

 

[G2Rockets]

I wonder if this is the same effect as driving with two car windows open. You no longer get that "vacuum"/Venturi type of effect as you do with one window, but continuous buffeting. Who knows, but I'd be interested to test out the theory by doing three launches with the AltimeterTwo, with one, two and three holes. Has anyone seen an experiment like this done?

Don't take this as gospel, but the way I understand it is... if you use 2 holes the wind rushing by can inadvertantly pull the air out causing a false vacuum and if you use charges a false apogee and boom. If you use 3 or 4 the wind my pull the air out but it is pulled back in through the other holes. eliminating the false vacuum. I have seen people have a main go off, even though the rocket is sitting on the pad waiting for launch because they used 2 holes.

My kids use the Altimeter one for 4h rocketry and want a altimeter 2. they are perfect for 4h rocketry. probably one of the best products in a long time.

John

 

[Larry Curcio]

I like to design products that "simply work," which is much harder than it sounds. It would have been much cheaper and simpler to make an altimeter that just monitored one axis. But then I would have had to tell people that they *had* to mount it rigidly aligned with the rocket's primary axis of flight. Personally, I don't want to have to baby an altimeter that much as I move it from rocket to rocket; I'd probably just rather not use it in that case.

With all three axes monitored, the user can just clip it to a parachute line and go, or just wrap it in foam and toss in into a payload bay. No muss, no fuss. It took a good deal of design work and testing to get it work reliably, but the result turned out to be worth it (IMHO).

By the way, it's surprisingly hard to find production quantities of 3-axis accelerometers that can sense this many Gs. They must be hard to manufacture (I guess). I'd love to up the G-limit, but 24G is the state of the art right now it seems.

--John Beans, Jolly Logic

I can see how the 3-axis accelerometer can get total acceleration *except for gravity* when it's placed in *any particular* orientation. This is a great advance in ease of use, I agree. Indeed I have one of your units and I'm hoping to try it out this weekend or next.

I'm not trying to take anything away from you. It looks like a very convenient instrument to use, and a very sophisticated one. I just think some people on the list may have unrealistic expectations of a 3-axis accelerometer that has no gyros. I believe they have false impressions - unless I am operating under one myself.

As far as I know (please correct me if I'm wrong), you cannot relate the three axis acceleration readings to the three-dimensional space of a ground observer. In particular, you cannot, as far as I know, infer the rocket's orientation with respect to gravity from the three axis accelerometer alone. (i.e.; if the rocket turns about its long axis, you cannot tell where in N-S-E-W-Up-Down space the off-axis instruments are pointing when they are measuring accelerations.)

As far as I know, the underlying problem is 6-DOF and you are measuring only 3 DOF. Without gyros or *coordinated* use of the altimeter, or the use of significant (familiar) assumptions, you cannot infer the rocket's direction or its orientation with respect to gravity. Without that orientation, you cannot infer the trajectory in a totally empirical fashion.

I also do not see how a dangling accelerometer can represent the motion of a rocket.

Once again, I have the instrument, and I am eager to use it, and I hope/expect to find good things to say when I do. This is in no way a condemnation of the product. It's a question/clarification.

Best Regards,
-LarryC

 

[Zeus-cat]

LarryC,

I am not sure what you think the altimeter does. For acceleration you get single data points for Peak G and Average G (under thrust) as well as other data points realted to speed and time. You don't need to know the orientation to get Peak G or Average G.

 

[cjl]

I wonder if this is the same effect as driving with two car windows open. You no longer get that "vacuum"/Venturi type of effect as you do with one window, but continuous buffeting. Who knows, but I'd be interested to test out the theory by doing three launches with the AltimeterTwo, with one, two and three holes. Has anyone seen an experiment like this done?

I haven't tried any actual experiments, but I did run several cfd simulations, and I think this is somewhat of an old wives tale. The simulations I ran indicated that 3 or more holes was ideal, but 1 hole was noticeably worse than 2, and there wasn't any particular problem with two. There were cases where two holes gave the wrong pressure measurement, but in every case, one hole was even worse than two. Actual flight data to back this up would be interesting though.

 

[Larry Curcio]

LarryC,

I am not sure what you think the altimeter does. For acceleration you get single data points for Peak G and Average G (under thrust) as well as other data points realted to speed and time. You don't need to know the orientation to get Peak G or Average G.

But... you do need to know the orientation. The accelerometer reading is not the rocket's acceleration. The accelerometer reading doesn't include gravitational acceleration. In order to know the actual acceleration, you have to correct for gravity. To do that, you have to know the angle between the gravitational vector and the vector that is the resultant of the accelerations included in the accelerometer reading.

For example, consider a rocket pointed straight up, sitting on a table. It registers 1g. It isn't accelerating at 1g; that is the component of its acceleration from the force of the table underneath it. That force is equal to the earthly weight of the rocket. If you imposed that force in free space, the rocket would accelerate at 1g.

On earth, there is also gravitational acceleration which, in this case, is pointed opposite to this "table thrust" component. Gravity canels out the table thrust, and the rocket has a net acceleration of zero - not 1g, as the accelerometer reads.

If you replace the table with a thrusting motor in an airless environment, the accelerometer will register the same thing in any direction. The rocket will be accelerating differently in different directions.

(BTW Note that the reading of the instrument in the table-bound rocket isn't -1g, which is what it would be if it were registering gravity directly.)

So my question is indeed relevant, and I do realize it isn't a recording accelerometer.

But thank you. I appreciate your response.

-LarryC

 

[Zeus-cat]

LarryC,

As you said, the Altimeter Two is going to register 1G when sitting on the table pointing straight up. Let us call that the Z axis and the Z axis accelerometer is providing the 1G data. Now your cat comes along and knocks the rocket over onto its side so the X axis acceleromter is pointing up. The altimeter output is still 1G. Your cat thinks this is a fun toy and flips the rocket over 90 degrees so that the Y axis is now pointing up and down. Still a 1G output on the altimeter. Now your cat knocks the rocket off the table and the rocket lands on its nose, but is leaning against hat which the cat knocked off the table earlier. The rocket is leaning at a 45 degree angle. Now at least two of the accelerometers are registering something, possibly all three. The combined output is still 1G.

 

[G2Rockets]

But... you do need to know the orientation. The accelerometer reading is not the rocket's acceleration. The accelerometer reading doesn't include gravitational acceleration. In order to know the actual acceleration, you have to correct for gravity. To do that, you have to know the angle between the gravitational vector and the vector that is the resultant of the accelerations included in the accelerometer reading.

For example, consider a rocket pointed straight up, sitting on a table. It registers 1g. It isn't accelerating at 1g; that is the component of its acceleration from the force of the table underneath it. That force is equal to the earthly weight of the rocket. If you imposed that force in free space, the rocket would accelerate at 1g.

On earth, there is also gravitational acceleration which, in this case, is pointed opposite to this "table thrust" component. Gravity canels out the table thrust, and the rocket has a net acceleration of zero - not 1g, as the accelerometer reads.

If you replace the table with a thrusting motor in an airless environment, the accelerometer will register the same thing in any direction. The rocket will be accelerating differently in different directions.

(BTW Note that the reading of the instrument in the table-bound rocket isn't -1g, which is what it would be if it were registering gravity directly.)

So my question is indeed relevant, and I do realize it isn't a recording accelerometer.

But thank you. I appreciate your response.

-LarryC

Hi Larry,
I think you are bringing up 2 different question. Am I correct? One is concerning the accurate axial readings of the accelerometer, and the other has to do with the accelerometer sensing the correct g rating due to earths gravity. Both are tough questions because you are bringing some physics into this that some of us haven't touched since college, but on the other hand it is fun to try and remember how these things work. It makes all the money we spent on school seem useful.

I am not certain, but I think many components that use accelerometers use MEMS Gyros. If I remember right these are considered vibrating gyros and can sense multiple axis. There is a good chance the Altimeter two uses this inexpensive technology, but John at Jolly Logic would wisely not tell us how he makes it work, rather just tell us that it DOES work. I hope this helps answer your question. Then again I may be completely wrong. It is just a thought.

Best
John

 

[John Beans]

...but John at Jolly Logic would wisely not tell us how he makes it work, rather just tell us that it DOES work...

All,
It's not that I mind telling how it works, I just never know how interested anyone is in the gory details. Here's a little more I can add to the conversation.

Yes, as the rocket sits on the pad, the AltimeterTwo is sensing local conditions (this part is in the manual, which you can download). A manufacturer of an altimeter never wants to have to calibrate each individual accelerometer (actually, each axis of each accelerometer), so every unit is instead taught to calibrate itself constantly as to what "1G" feels like. So it's sensing the magnitude of rest acceleration, and the direction. We do not assume it's lying straight. In fact, it works better (can sense more Gs) if it's as tilted as possible away from "straight."

Then, suddenly, it senses a sustained acceleration that dwarfs the ambient 1G. That's a liftoff. It's usually (but not required to be) in the same direction (up) as the "apparent" acceleration due to gravity. Geometry ensues, and for the next 0.2 to 8.0-ish (love those long-burns) seconds, it does its best to "not count" the ambient 1G as it does its best to assess the relative motion of the rocket to the ground.

Larry C is correct that we cannot be perfect at isolating relative motion if the rocket rotates. You can't use a 3 degree of freedom sensor to completely track a rocket moving in 6 (three linear directions, three rotations). For the dollars spent, we have to be satisfied with being very, very close for rockets which (by and large, knock on wood) travel very straight at first.

Well, at least for the boost phase. After that, the accelerometer is only used to detect the timing of the explosive ejection.

You might think that someone who has devoted as many hours as I have to programming accelerometers and interpreting their data would want to use them for everything (like sensing apogee), but the opposite is actually true. In my humble opinion, accelerometers can be "trusted" for short, strong movements, but not long, slow ones. They just accumulate too much error when used that way.

Any of that help? (Zzzz....)

--John Beans, Jolly Logic

 

[Larry Curcio]

Hi Larry,
I think you are bringing up 2 different question. Am I correct? One is concerning the accurate axial readings of the accelerometer, and the other has to do with the accelerometer sensing the correct g rating due to earths gravity. Both are tough questions because you are bringing some physics into this that some of us haven't touched since college, but on the other hand it is fun to try and remember how these things work. It makes all the money we spent on school seem useful.

I am not certain, but I think many components that use accelerometers use MEMS Gyros. If I remember right these are considered vibrating gyros and can sense multiple axis. There is a good chance the Altimeter two uses this inexpensive technology, but John at Jolly Logic would wisely not tell us how he makes it work, rather just tell us that it DOES work. I hope this helps answer your question. Then again I may be completely wrong. It is just a thought.

Best
John

OK. I see some misplaced evasion, here. I'm attacking no one, but the public forum may be obscuring that fact.

Long and the short of it:

3-axis accelerometers can indeed be oriented in any direction WRT the long axis of the rocket and still work.

Accelerometers cannot work when they are dangling

3-axis accelerometers cannot solve the problem of orientation WRT to gravity

That said:

3-axis accelerometers, when analyzed in a recording instrument (which this is not), in conjunction with barometric data CAN SOLVE THE PROBLEM OF ORIENTATION WRT GRAVITY

3-axis accelerometers, when analyzed in a recording instrument (which this is not), in conjunction with barometric data CAN SOLVE THE ANGLE OF ATTACK PROBLEM as long as AOA is in the range of +/- 90 degrees.

Those last two items are considerably cheaper and simpler than adding gyros, and they accomplish quite a lot. I'd really like to talk to someone over there about them.

Regards,
-LarryC

 

[cjl]

Accelerometers cannot work when they are dangling

Sure they can. If you have a 3 axis accelerometer, you can simply sum all three axes in quadrature, assume the acceleration is axial, and subtract 1 G. That will give a reasonably accurate measurement of the rocket's acceleration during flight, even if the accelerometer's orientation is changing throughout the flight.

 

[G2Rockets]

OK. I see some misplaced evasion, here. I'm attacking no one, but the public forum may be obscuring that fact.


-LarryC

Larry,
on the contrary. I think you pose an excellent question and i really enjoy thinking about this stuff because as I said I spent money on classes for this stuff and It seems like I never use it at my job. I don't feel you were attacking anyone.

I know that if i had a product and i was being quizzed on how I made it work, i would be very elusive and just say "It works".

Best
John

 

[Adrian A]

Sure they can. If you have a 3 axis accelerometer, you can simply sum all three axes in quadrature, assume the acceleration is axial, and subtract 1 G. That will give a reasonably accurate measurement of the rocket's acceleration during flight, even if the accelerometer's orientation is changing throughout the flight.

I think you're both mostly right. Chris is right that a 3-axis accelerometer can be in any orientation and you can figure out the total acceleration, but that's only true as long as the rocket ascends vertically. The accelerometer doesn't have to be vertical (or even constant with respect to the rocket), but the path of the rocket does. So Larry's part about not being able to determine the path of the rocket with respect to gravity is true. Without gyros (or perhaps heavy post-processing of the data, but I'm skeptical) you have to assume that the rocket's ascent is vertical, which is a useful assumption as long as it's true.

 

[Larry Curcio]

Sure they can. If you have a 3 axis accelerometer, you can simply sum all three axes in quadrature, assume the acceleration is axial, and subtract 1 G. That will give a reasonably accurate measurement of the rocket's acceleration during flight, even if the accelerometer's orientation is changing throughout the flight.

OK. Consider a big mass smacking into a little mass. The little mass recoils much faster than the closing speed. Let the big mass be a rocket. Let the little mass be an accelerometer dangling on a shroud line within the rocket's compartment.

After the accelerometer knocks against the wall,

1) The accelerometer gives you a reading that is the resultant acceleration;

2) The reading tells you about the acceleration of the accelerometer banging around inside the rocket, and not of the rocket itself; and

3) The reading does not include the effect of gravity. (Subtracting 1G implicitly assumes the rocket is traveling vertically.)

Once again, I'm not knocking this product. I have this product; Ilove this product.


Best Regards
-LarryC

 

[Larry Curcio]

LarryC,

As you said, the Altimeter Two is going to register 1G when sitting on the table pointing straight up. Let us call that the Z axis and the Z axis accelerometer is providing the 1G data. Now your cat comes along and knocks the rocket over onto its side so the X axis acceleromter is pointing up. The altimeter output is still 1G. Your cat thinks this is a fun toy and flips the rocket over 90 degrees so that the Y axis is now pointing up and down. Still a 1G output on the altimeter. Now your cat knocks the rocket off the table and the rocket lands on its nose, but is leaning against hat which the cat knocked off the table earlier. The rocket is leaning at a 45 degree angle. Now at least two of the accelerometers are registering something, possibly all three. The combined output is still 1G.

OK! Deep breath!

Consider a rocket sitting on a table. The rocket has a three-axis accelerometer inside it. We get the resultant acceleration and the instrument reads 1g. This is an absolute value of acceleration, and tells us nothing about the orientation of the rocket. We can knock this rocket over and it still registers 1g. The instrument is obviously registering gravity, and the world is obviously flat. (Actually, I have many rocket-oriented computer programs based on the flat world model. :blush In fact, the accelerometer is not registering gravity *directly*.

The true total acceleration of the rocket, in all instances, is 0. This total acceleration of the rocket is the resultant acceleration of gravity and the force of the table underneath the rocket. The instrument is not registering 0, though; it is registering 1g. It must be failing to measure either the component of acceleration from gravity or from the table.

(Aside: Note that in the case of a single-axis accelerometer, one can tell right off that gravity is not sensed, because the reading there is 1g, the component from the table, and not -1g, the component from gravity. The three-axes obscure things, because we compute the absolute magnitude of the acceleration, which isn’t signed.)

Suppose we substitute a rocket motor for the table, and we arrange the motor to always thrust vertically with a force equal to the weight of the assembly. The rocket hangs in space with thrust equal to weight. We have an equivalent situation to the table scenario. The reading stays the same and the rocket remains (at least theoretically) motionless. I claim the accelerometer is not registering gravity; it is registering the thrust of the motor. Here’s the proof:

Suppose we cut the all thrust. The rocket is now in freefall and the accelerometer (before drag kicks in) registers 0g. Gravitational acceleration still pertains – You can see it! The rocket is actually accelerating at -1g before your very eyes! If the accelerometer registers gravity, it should be registering that acceleration, -1g (or 1g in absolute value). It’s registering 0g!

Still not convinced? Suppose we turn the thrust back on but adjust it to 1/4 the earthly weight of the assembly. The rocket now falls at the rate of 3/4g, and the accelerometer registers 1/4g! Gravity is still pulling at -1g, but the accelerometer doesn’t register it; it registers (before drag kicks in) the amount of acceleration from the thrust of the motor (1/4g) and not the acceleration from gravity (1g in absolute value).

If you take the same rocket motor and orient it in any direction, the accelerometer reading remains the same. The acceleration of the rocket differs according to is orientation, though.

Conclusion: When the rocket was on the table, it was registering the component of acceleration from the force of the table. That component was 1g. That reading registered gravity because of a property of tables (they push back as hard as you push down on them), and not because of a property of accelerometers.

If you apply the same force (the earthly weight of the rocket) in free space, the rocket actually accelerates at 1g there. The accelerometer in free space measures that acceleration and reads exactly what it read on earth under the same conditions with the same force applied. If you cut the motor in free space, the rocket stops accelerating (it still has velocity) immediately, and the accelerometer registers zero – just as it did in free fall (no table, no thrust) on earth.

The accelerometer reading is unaffected by gravity, but the rockets motion is affected.

In a rocket flight, the accelerometer registers the resultant of thrust, drag, lift, wind, and other aerodynamic forces. It doesn’t register gravity. That is why, in a vertical flight, you subtract 1g from the resultant – you are, in effect SIMULATING the effect of gravity, which is missing from the data. That simulation is based on the assumption of verticality.

If the flight is off-vertical, you have to subtract 1g *from the vertical component* of the resultant – and not from the entire resultant to get the correct acceleration. But since the flight is off-vertical, you don’t know the orientation of the acceleration vector without gyros.

An accelerometer analysis program, whether it be for a 1 or 3-axis instrument, is not entirely empirical. It's only a partially informed simulation – combining empirical accelerations with the simulated effect of gravity. If you have an independent reference for the orientation of gravity like gyros or an altimeter, you can get more empirical estimates. You need independent references, though. A 3-axis instrument by itself doesn’t do it.

I sure that helps .

For further discussion, please see:

https://www.lunar.org/docs/LUNARclips/v5/v5n1/Accelerometers.html


https://www.rocketryplanet.com/images/pdf/Analysis-of-Flight-Computer-Data.pdf

and

https://www.rocketryplanet.com/images/pdf/Barometric-Adjustment-of-Inertial-Flight-Data.pdf

 

[Larry Curcio]

Regarding 3-axis accelerometers and orientation WRT gravity:

In my attempt to avoid vector arithmetic, I set forth only vertical examples. Whereas these show that 3-axis accelerometers don't register gravity, all the examples are rectified by simply subtracting 1g from the resultant acceleration. That result is an artifact of the verticality.

Here is a very small PowerPoint presentation with a counter-example.

View attachment 3-Axis Tests.zip

 

[Zeus-cat]

Larry,

Great analysis; it shows that you understand the physics involved.

However, I think you are overanalyzing this. You are looking at this from the aspect of a mathematician (of which I am one). However, I think you need to pull this into the realm of engineers (I happen to be one of those too).

The Altimeter Two is measuring the peak force on the rocket. In most normal flights the rocket will be travelling straight up or reasonably close to that. Even if it is off vertical by say 10 degrees the gravity component of the measured acceleration is still over 98% of 1g which contributes only a tiny error to the measurement. Obviously there is an angle (X degrees) that if it were exceeded during the powered boost phase of the flight would introduce significant error to the measurement. I am assuming peak g forces are recorded during the powered boost phase which should be a valid assumption if the rocket does not have a hard landing.

The engineer in me says that even without gyros to confirm the orientation of the rocket it is likely that the rocket will be traveling in the proper direction for the altimeter reading to be considered valid. Since the g resolution is 0.1 on the display, the rocket's orientation with respect to vertical would have to be off by more than 25.8 degrees for the Altimeter Two's resolution to show it. At 25.8 degrees off vertical the force of gravity through the vertical (z) axis of the rocket is still 0.9g.

On the C6 flights I have conducted the peak g is around 6, while the D12 flights have shown peak g in the 10-11 range. If those numbers are correct park then whatever error introduced by not measuring gravity correctly due to the lack of gyros is darn close to the noise level in the resolution and measurement capabilities of the altimeter.

In conclusion, I agree with your analysis on the gyros. However, the sensors we are using will report results that are good enough without them as their accuracy and resolution isn't good enough to show the errors unless the flight is way off vertical.

 

[Larry Curcio]

Larry,

Great analysis; it shows that you understand the physics involved.

However, I think you are overanalyzing this. You are looking at this from the aspect of a mathematician (of which I am one). However, I think you need to pull this into the realm of engineers (I happen to be one of those too).

The Altimeter Two is measuring the peak force on the rocket. In most normal flights the rocket will be travelling straight up or reasonably close to that. Even if it is off vertical by say 10 degrees the gravity component of the measured acceleration is still over 98% of 1g which contributes only a tiny error to the measurement. Obviously there is an angle (X degrees) that if it were exceeded during the powered boost phase of the flight would introduce significant error to the measurement. I am assuming peak g forces are recorded during the powered boost phase which should be a valid assumption if the rocket does not have a hard landing.

The engineer in me says that even without gyros to confirm the orientation of the rocket it is likely that the rocket will be traveling in the proper direction for the altimeter reading to be considered valid. Since the g resolution is 0.1 on the display, the rocket's orientation with respect to vertical would have to be off by more than 25.8 degrees for the Altimeter Two's resolution to show it. At 25.8 degrees off vertical the force of gravity through the vertical (z) axis of the rocket is still 0.9g.

On the C6 flights I have conducted the peak g is around 6, while the D12 flights have shown peak g in the 10-11 range. If those numbers are correct park then whatever error introduced by not measuring gravity correctly due to the lack of gyros is darn close to the noise level in the resolution and measurement capabilities of the altimeter.

In conclusion, I agree with your analysis on the gyros. However, the sensors we are using will report results that are good enough without them as their accuracy and resolution isn't good enough to show the errors unless the flight is way off vertical.

Agreed. No argument. I use single-axis instruments in that context all the time, and I'm sure this product functions perfectly well that way. As I have said, I own and like the product. That's why - in near vertical launches it functions very well.

There was a lot of misguided discussion about the Altimeter II. There are many incorrect beliefs about three-axis instruments that were aired in that discussion. I just wanted to clear the air, but succeeded mostly in fogging it. Not knocking the product.

Standing down now.

Best Regards,
-Larry

 

[John Beans]

I agree with all of the above. I designed the AltimeterTwo with an expensive, three axis accelerometer for convenience, not extra accuracy. Unless you mount your one or two axis model improperly. Then, it's both.

From the manufacturer's perspective (mine), the most expensive thing about a product is user confusion. So, I was aiming to reduce that. "Put it in your rocket and it will work."

That's what I was going for.

Glad you like the AltimeterTwo, Larry, and I appreciate all of your comments.

--John Beans, Jolly Logic
Designer of the AltimeterOne and AltimeterTwo

 

[mIcahel]

I've studied this thread, done some additional research, and then used Rocksim to model my own accuracy increasing solution.