I could use just a little guidance

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Very nice. Thanks for the links Brian. Hope you get the video posted as it would be neat to see the Canards in action along with the video with no spin. These modern systems are getting pretty sophisticated as compared to the modified
R/C mechanical Gyros in the days of yore. I recall the Dr. Rocket fellow had a bulky mechanical system he flew with stabilization on and off videos probably close to 10 years ago. I believe there are others that used this older technology.
Kurt
 
I'm glad Brian's flight went well. I hadn't heard how it went. Brian is doing real well.

Me, not so much. I tried a flight last weekend at Las Cruces, but the flight uncovered a control program bug that cause a rapid spin starting just after booster separation. I guess that's what test flights are for, although it's an expensive method for debugging software. I did get two good videos of a bad flight though.

In short, the flight was with my "test rocket" on an N5600. Key events were supposed to be:

- Roll control from the start of the flight
- Booster separation at 2.4 seconds
- Yaw/pitch control starting about 6 seconds
- Stabilization spool separation at 15 seconds

Launched the rocket at about a 5 degree angle. Control looked OK up to about 2.4 seconds. There was some roll, but at a speed of Mach 1.2 with multiple cameras hanging off the airframe, that isn't surprising. Control interactions with the booster fins might also be a factor, and this may be evident in the spin of the booster as it separated from the sustainer. This may have caused the spin rate to exceed the range of the gyro (500 degrees/sec), and then a software bug at this maximum spin rate cause the spin control to reverse (something related to an integer overflow in the subtraction of the offset values, which I'm sure some of you know what that is). So, the canard action was reversed and just spun up the rocket. Very exciting! When yaw/pitch control kicked in, the gain for roll control was lower, so the spin rate decreased, and roll control was finally regained at 14 seconds, just before the stabilization spool separated.

I think we'll learn a few things from the flight, but the results don't say anything about yaw/pitch control, which is what I was really looking for in this flight.

Jim

[video]https://youtu.be/lhAnzNH5Zi0[/video]
 
I'm glad Brian's flight went well. I hadn't heard how it went. Brian is doing real well.

Me, not so much. I tried a flight last weekend at Las Cruces, but the flight uncovered a control program bug that cause a rapid spin starting just after booster separation. I guess that's what test flights are for, although it's an expensive method for debugging software. I did get two good videos of a bad flight though.

In short, the flight was with my "test rocket" on an N5600. Key events were supposed to be:

- Roll control from the start of the flight
- Booster separation at 2.4 seconds
- Yaw/pitch control starting about 6 seconds
- Stabilization spool separation at 15 seconds

Launched the rocket at about a 5 degree angle. Control looked OK up to about 2.4 seconds. There was some roll, but at a speed of Mach 1.2 with multiple cameras hanging off the airframe, that isn't surprising. Control interactions with the booster fins might also be a factor, and this may be evident in the spin of the booster as it separated from the sustainer. This may have caused the spin rate to exceed the range of the gyro (500 degrees/sec), and then a software bug at this maximum spin rate cause the spin control to reverse (something related to an integer overflow in the subtraction of the offset values, which I'm sure some of you know what that is). So, the canard action was reversed and just spun up the rocket. Very exciting! When yaw/pitch control kicked in, the gain for roll control was lower, so the spin rate decreased, and roll control was finally regained at 14 seconds, just before the stabilization spool separated.

I think we'll learn a few things from the flight, but the results don't say anything about yaw/pitch control, which is what I was really looking for in this flight.

Jim

[video]https://youtu.be/lhAnzNH5Zi0[/video]

By the looks of it when the first stage separated it did like a twisting corkscrew that threw off the gyros.

You might want to look into shear pins to hold everything together instead of friction fitting.
 
By the looks of it when the first stage separated it did like a twisting corkscrew that threw off the gyros.

You might want to look into shear pins to hold everything together instead of friction fitting.

The booster was pinned to the stabilization spool. It didn't start turning until it started to separate, which released the pin. I think that turning might have been induced by the canards, which were turned at that point. It's hard to say though. As it turns out, the stabilization spool was friction fit to the sustainer. I thought the fit was very tight, but you can see the canards move a little. I don't think that affected anything, but I'll do a key for that joint too.

Jim
 
Jim,

What are your current theories of failure? If your controller can damp your gain with respect to the square of the forward velocity you may want to implement it. Looks like the loop got in to a sustained underdamped oscillation. Where are your sensors located in the airframe and what kind are they? If it's a basic MEMS gyro/accel mounted off center and longitudinally displaced from the center of control it may be very difficult to stay out of roll coupled failures.
 
Jim,

What are your current theories of failure? If your controller can damp your gain with respect to the square of the forward velocity you may want to implement it. Looks like the loop got in to a sustained underdamped oscillation. Where are your sensors located in the airframe and what kind are they? If it's a basic MEMS gyro/accel mounted off center and longitudinally displaced from the center of control it may be very difficult to stay out of roll coupled failures.

The problem was literally a calculation error. I'm speculating a bit here, but when the booster separated, there was a short period of clockwise rotation (a few tenths of a second, which is hard to spot in the video unless you are looking specifically at the direction of turn versus time). The clockwise turn rate exceeded the range of the gyro. The canards responded correctly to this. However, at that point, the difference between the sensor value and the sensor offset exceeded a value that could be computed by the processor. I believe if everything had been reversed, the problem would not have occurred. That is, there is a problem if the rocket spins on one direction but not the other. I don't understand the details of the program, but I'm told this is a relatively simple error. It was found and fixed in the matrixpilot code, but the simple fix didn't make it to the rocket code. So, between about 2.6 seconds and 14 seconds, the canard response is reversed for roll control, and they are trying to increase the spin in the direction of the spin. I think useful information on roll control exists prior to 2.6 seconds, but not thereafter (unless the maximum roll rate is of interest, and it was around 540 rpm I believe). Now, what do you see? The board contains an MPU-6000 MEMS with a dsPIC33FJ256 CPU. It's about 1/2" off the axis of the rocket.

Jim
 
It was found and fixed in the matrixpilot code, but the simple fix didn't make it to the rocket code.

This seems to me to be a case of treating the symptom and not the cause: why, if the roll control was active, did the rate exceed 500 dps?

The problem is probably a simple case of exceeding the range of a 16 bit integer. The gyro measurements from the MPU-6000 are 16bit two's complement. If you had a rate near the maximum of 500dps and a negative offset, when you subtracted that offset from the measurement you would get a negative result somewhere near -500dps. Note that the same thing would happen with a positive offset and large negative rate.
 
This seems to me to be a case of treating the symptom and not the cause: why, if the roll control was active, did the rate exceed 500 dps?

The problem is probably a simple case of exceeding the range of a 16 bit integer. The gyro measurements from the MPU-6000 are 16bit two's complement. If you had a rate near the maximum of 500dps and a negative offset, when you subtracted that offset from the measurement you would get a negative result somewhere near -500dps. Note that the same thing would happen with a positive offset and large negative rate.

Yes, it was a signed integer overflow. If that could make it through to the code Jim flew, it may be worth reviewing all the code.

Regardless of SW implementation, there are still fundamental issues. It is well-known that canard vortexes will impinge on the fins and can cause roll reversal. I passed along some related papers to Jim. Then there is the 20ms update rate to the RC servos, and the overall system delay, which will limit gain&stability at higher roll/pitch rates. There's also the shape of the canards and its airfoil cross section, limiting the deflection angle at which it has non-separated flow.

Before he can get to worrying about gain vs velocity, for example, these fundamental issues need to be looked at more closely.

"If it was easy, everybody would be doing it". ;-)
 
I'm glad Brian's flight went well. I hadn't heard how it went. Brian is doing real well.

Thanks Jim, this is quite the compliment coming from you. I will get you guys flight data and videos as soon as some family medical issues are over.
 
I plotted up the y-axis gyro reading and also estimated the spin rate based on the video. The results are in the attached graph. There are a couple of things of interest. First, prior to "the problem", the spin rates were below 300 degrees/sec. That's not great, but remember that the software is designed to reduce the roll rate and not to stop it. If there is some natural spin in the rocket, or if the canards are not perfectly aligned or have a little wiggle, then there will be some spin "offset" that the proportional control can't correct. Also, this is a thin fiberglass rocket that bends (the wildman tubing), there are two cameras hanging off the rocket, along with four rail guides. This rocket really wouldn't be expected to fly that straight. Is 300 degrees/sec at Mach 1.2 good enough? I don't know, but it's what I got with this rocket. We've also been talking about PI control for roll.

Also, "the problem" clearly occurred between 2.4 and 2.5 seconds according to the video data (at 0.34 second frames). I think the 0.1 second data logging rate for the gyro missed this. I think we'll go ahead and increase the logging rate.

Jim

Spin.jpg
 
So why did "the problem" occur? I still don't know, but the video data below says that the problem occurred just after 2.4 seconds. Attached below are five screen shots around the time. The four differential spin rates from these five pictures are -75, -550, -500 and 362 degrees per second, and those points are on the graph in my previous post.

In the first picture, the spin of the rocket has been CCS (positive degrees/sec). The canard is therefore pointed to the right to try and stop this.

Going from the first to the second picture is the first point where the booster fins start to spin relative to the sustainer. This is the moment where the separation charge goes off, which frees the pin and allows the two parts to turn relative to each other. The spin continues in the third and forth pic, and the booster is free of the coupler in the fifth pic. Note that in the first and second pics, the angle of the canards could be inducing the booster spin. So, the booster is spinning CCW and the sustainer is spinning CW. It's almost as though there was some torque between the two pieces, and when they separated, it allowed the two pieces to spin in opposite directions (equal and opposite reaction?). That torque might have been from interaction between the canards and the booster fins, or just a difference in the natural spin rate between the two parts. I don't know.

In the third and fourth pics, the canard is moving left to counteract the positive rotation, but the gyro value reached the saturation point, probably in the fourth pic. The rocket is moving CW in the fifth pic. The canard is not moving to the right in response, and never does (it's output has flipped due to the software problem), and the fast spin starts right at the fifth pic. I think in the absence of the bug, the gyro would have simply saturated for a moment, and then roll control would have resumed. It would have been interesting to see how it recovered from that disturbance.

Jim

1.jpg

2.jpg

3.jpg

4.jpg

5.jpg
 
I'm not really sure if it would be better to locate the canards under the sustainer fins or out in clear air. It is possible that locating them under the sustainer fins would help with stall angle, but with a separation of 12 inches, I don't know if this would really help. But having them aligned might not be a problem either.

Regarding interaction with the booster fins, my belief is that with the long booster, that this would not be an issue. I am concerned about this for the test rocket, though, because the booster is much shorter and has 3 fins rather than 4. For the Balls rocket, there is a separate composite fin can. It would sit on a fiberglass ring that would be held in place using the bolts for the nozzle holder. It might be possible to configure this to allow the fin can to spin freely, although I suspect that as the motor heats, that the fin can would get pinned at some point.

Jim
I think you've done a great job, but you still have problems after booster separation.

I've mentioned this before, I believe the best mounting location of your canard guidance unit is at the top of the sustainer. The longer you can keep the pointy end vertical, the higher your rocket with go. Once the rocket begins a gravity turn, without active countermeasures you can't correct the trajectory and minimize the loss. Canards generate torque to turn the rocket, so the more forward of the CG you can locate the canards, the smaller you can make the canard area (and lower the drag) while generating the desired toque.

YMMV

Bob
 
I think you've done a great job, but you still have problems after booster separation.

I've mentioned this before, I believe the best mounting location of your canard guidance unit is at the top of the sustainer. The longer you can keep the pointy end vertical, the higher your rocket with go. Once the rocket begins a gravity turn, without active countermeasures you can't correct the trajectory and minimize the loss. Canards generate torque to turn the rocket, so the more forward of the CG you can locate the canards, the smaller you can make the canard area (and lower the drag) while generating the desired toque.

YMMV

Bob

For roll-control only, which is what Jim is looking for until booster sep, it is best to have the canards closer to the CG and not in the nose of the sustainer. Minor misalignments in the nose will couple roll corrections into unwanted pitch errors. After separation of the booster, during coast, the combined pitch/roll control with the guidance in the back has almost as much control authority as the guidance in the nose. The cost of carrying the guidance system with the sustainer outweighs the benefit of correcting for gravity turn during second stage coast. (All these points were made back when Jim starting thinking about this project).

The other factor is that canards generate a vortex that tends to impinge on the fins behind them, imparting an opposite roll. Carrying the guidance package in the back during pre-ignition of the 2nd stage removes this problem. It is still there during the 1st-stage boost and before the booster separates, which is most likely what caused the opposite torque on the booster as it separated. It's somewhat non-intuitive, but this vortex roll-induction on the fins is common and it's tricky to optimize. Smaller span on the canards compared to the booster fin span makes it worse because the vortex is hitting more of the fin surface. Long tail fins with a short span, and larger canard span is better for the impinging, but difficult to do.
 
We're starting to think that the actual gyro reading pathway followed the blue line in the attached pic. If this is correct, then the separation of the booster released the torque between the pieces at that point, and caused the rapid CW turn of the sustainer (the dip to negative roll rates). The canards responded to this and drove the roll rate positive to the saturation point, and this is where the gyro value flipped. What we don't know is whether the oscillation would have settled down or not. Would have been a fine test.

We're considering options including a helicopter-type heading hold, PI, or just increasing the saturation values for the gyros.

Jim

Spin2.jpg
 
Well, I'm still working on the stabilization project. I did a 7th test flight this weekend at Argonia, with mixed results as usual. This time, everything worked from a mechanical perspective, but the things that went wrong will probably provide the most useful data. They sure do have nice weather in Kansas in February!

So this flight was the third flight of my "two-stage" test rocket. It flies with the stabilization section between the two stages, but with no motor in the sustainer (there is a weight to simulate the weight of the motor that would normally be present). This time, it flew on an N3180. For the first 8 seconds, the stages were together with roll control only. The rocket accelerated to around 1200 ft/s and then slowed to around 800 ft/s at the point where the booster separated. I've been expecting to see the infamous "control reversal", where the vortex from the canards interacts with the booster fins. This can cause the roll to be in the opposite direction of the desired control action, and that appears to have happened in this flight. It would appear not to be a good idea to use roll control with the booster attached unless there is a free-spinning fin can. I might look into that. It is interesting that the roll was stopped as soon as the booster separated.

The second part of the flight, from 8 to 18 seconds, is where the control system was active for both roll and vertical stabilization. Due to the roll during the boost, the control system didn't really know where vertical was. This is the Achilles heal of the MEMS gyros I think. If there is roll, and if the accelerometers are turned off (which is necessary), then there is enough drift in the gyros in the MPU-6000 chip to confuse the control system. In this case, there was enough apparent tilt such that the canards were turned to their maximum of 7.5 degrees for both yaw and pitch. Based on the video (my gps decided to malfunction), this canard deflection moved the rocket about 15 degrees over the 10 second control period. I realize that the canards aren't very large, and that the deflection isn't that large, and that the lever arm to the CG is only about 2 feet, but I still would have expected more movement in the rocket than what occurred. But by accident, this is pretty much the data I need to figure out how things need to be sized.

Anyway, here's the video of the flight. Enjoy!

https://youtu.be/Abod_JBxnM8

There have been a lot of improvements made to the control system (someday, I might even demonstrate them). There is now separate enabling of the roll and yaw/pitch, improvements to minimize gyro drift and to zero out the offsets just before launch. We also increased the gyro range from 500 to 1000 degrees per second, based on prior flights, and this capability was needed in this flight. The cross-drift of the MEMS gyros is an issue though.

Jim

IMG_1056.jpg
 
That's amazing...the fins stabilizer fins redirected just enough air right at the booster fins to roll it the other way?

Thats some great data.
 
That's amazing...the fins stabilizer fins redirected just enough air right at the booster fins to roll it the other way?

If so, that is a hell of a bizarre aerodynamic phenomenon.

Jim, have you checked the booster to confirm the fins are dead straight? Or has that one been flown with no roll issues before?

If it is a bizarre phenomenon..... then has this been lurking for the other 6 flights?

If it was something that was repeatable, strange as it may seem, one possible fix would be to program for opposite roll when the booster is attached, then go to normal roll control after booster sep. I have done that sort of thing with some R/C RBG's like my Space Shuttle where I use rudder for steering, not ailerons. During boost, left rudder causes right roll, so I have the rudder reversed for boost so when I move the stick left, the rudder moves right, which causes left roll (because the rudder is acting like a single aileron on boost when attached to an ST/SRB stack or piggyback carrier rocket, causing opposite roll on boost rather than yaw as it does during glide).

I am a bit leery of suggesting fighting reversal with reversal, in case the phenomenon is not repeatable or only happens in certain situations.
 
If so, that is a hell of a bizarre aerodynamic phenomenon.

Jim, have you checked the booster to confirm the fins are dead straight? Or has that one been flown with no roll issues before?

If it is a bizarre phenomenon..... then has this been lurking for the other 6 flights?

If it was something that was repeatable, strange as it may seem, one possible fix would be to program for opposite roll when the booster is attached, then go to normal roll control after booster sep. I have done that sort of thing with some R/C RBG's like my Space Shuttle where I use rudder for steering, not ailerons. During boost, left rudder causes right roll, so I have the rudder reversed for boost so when I move the stick left, the rudder moves right, which causes left roll (because the rudder is acting like a single aileron on boost when attached to an ST/SRB stack or piggyback carrier rocket, causing opposite roll on boost rather than yaw as it does during glide).

I am a bit leery of suggesting fighting reversal with reversal, in case the phenomenon is not repeatable or only happens in certain situations.

Yes, a bit bizarre, but something NASA likely knew about before I was born. From the flights I have so far, I think the flights with the forward canards didn't show this because the flight speeds weren't that high and because the canards were small. I think that if the rocket isn't rolling, the effect might not be seen until something causes some roll to begin. The flight at White Sands didn't have much spin to get things started. I think there is evidence of the effect in my Las Cruces flight right at the point where the booster separated (the booster turns as it separates due to the vortex from the canards):

https://youtu.be/lhAnzNH5Zi0

If the booster hadn't separated, I suspect this one might have spun up.

My three stage flight didn't show the effect. The booster fins are further from the canards than for the test rocket, so maybe that's a factor.

Jim
 
Very interesting. Despite the unexpected roll reversal, it appears that your stabilizer system between the two stages is operating well. It immediately solved a significant roll issue after booster separation. (Of course, it also caused the roll, but that's the fun of experimentation.)

I'm sure you're not too interested in redesigning everything after a 7 test flights of this system, but what about losing the fins on the booster all together? I've never simulated anything like this before, but there are plenty of guided military rockets that don't have fins on their aft end. For example the Roland SAM or the Exocet MBDA.

Keep posting about this project. I'm living vicariously through it!
 
Very interesting. Despite the unexpected roll reversal, it appears that your stabilizer system between the two stages is operating well. It immediately solved a significant roll issue after booster separation. (Of course, it also caused the roll, but that's the fun of experimentation.)

I'm sure you're not too interested in redesigning everything after a 7 test flights of this system, but what about losing the fins on the booster all together? I've never simulated anything like this before, but there are plenty of guided military rockets that don't have fins on their aft end. For example the Roland SAM or the Exocet MBDA.

Keep posting about this project. I'm living vicariously through it!

My first thought was "make the aft fins active-stabilizers" but these are min-diameter rockets right?

Probably wouldn't be trivial to add teeny winglets to those canards like they do on the big planes to reduce vortex formation.

The thought experiments abound!
 
If so, that is a hell of a bizarre aerodynamic phenomenon.
It is a well-known effect. That's why some rockets have the fore and aft fins interdigitiated rather than in line.

Congrats on success of the flight. Lots of data to paw over and figure it out!

Any thoughts of using a different gyro? Some of the commercial ones peg out at about 2000deg/s which might give you some more wriggle room in the design.
 
I should have said in my other reply how neat it was to see the guidance system almost instantly kill the roll after booster sep, no sign of over-correction. Also so good that you have video cameras. Pretty for entertainment, but also so critical for technical review of guidance effectiveness.

I have a couple of ideas. One, pretty simple, but may not work. The other...fuzzy.

The simple one would be to orient the guidance section so its control fins would be in-line with the booster fins, or possibly right in-between. IIRC, you may have to remove and reposition a stage coupler alignment guide to be able to do that. Reason why it might make a difference would be how the airflow may be interacting downstream of the control fins. As it is right now, deflected airflow from an angled control fin seems to be impinging on the booster fin to cause slightly higher pressure on it to cause a roll.

I take note that you do not have the guidance control fin alignment exactly between the booster fins. Rather than 60 degrees apart, it looks more like 30 (fin seen on left) and 90 (fin to the right). So this aerodynamic effect probably is way more on the fin on the left and possibly not at all on the fin on the right. I take note than on flight #6's video, at first the rocket started to roll right (camera view) just a bit, then the guidance system stopped that right roll, stopped for a moment, then began a left roll that got faster and faster. So that may be a clue that the aerodynamic interaction is limited b how far away (in roll orientation) the lower fins are. So if the fins had been exactly in-between, 60 and 60, then this effect likely would be less effective on the fin at left, but then would affect the fin at the right equally.

What I am not sure of is whether the effect would be solved by having the control fins directly inline with the booster fins, or not. It may still have an effect, possibly make it even worse. If not, then at least the effect could be reduced by having the control fins exactly mid-way, at 60-60, not the current orientation that looks to be about 30-90.

The other idea is "fuzzy logic", in case you do not think this is a repeatable effect. If it is repeatable, then the simplest programming fix would be to have the control surfaces move the opposite way for first stage boost, fighting reversal with reversal to get the desired result.

But if it was not repeatable, but a random effect, the programming would need to be able to try to figure out if it is in a reversal situation or not, and correct accordingly. So if the rocket begins to roll one way, the guidance acts to correct the roll, but the roll increases almost directly in response to the control output trying to STOP the roll, the programming can consider this may be a control reversal situation. It could do a VERY brief test for that, by going to neutral for say 1/4 to 1/2 second (possibly even half-throw the opposite way), and detect whether the roll gets worse, or if the roll rate begins to reduce. If it begins to reduce, then the programming can assume a roll reversal situation and then give reverse roll control commands to the servos. Which should solve that for the rest of the first stage portion of the flight, then go to normal after booster sep. Though that probably should not be a one-shot programming tree branch, but some sort of routine that keeps monitoring whether actual rocket response seems to be as a result of control surface deflection, or not. Could even have a "When in doubt, go to neutral" routine in it if it had more than one roll reversal anomaly during boost and after the first changeover to roll control reversal to solve it, it started to roll too fast the OTHER way.

Also, at those speeds, and since aerodynamic forces increase with the square of the velocity, maybe you have a lot more control surface deflection in roll than you really need. For example the aerodynamic forces at 400 mph are 16 times greater than the forces at 100 mph. And the aerodynamic forces at 600 mph are 36 times greater than the forces at 100 mph. I mean, the ideal
guidance system would take into account velocity if possible, to have a lot more angular control surface deflection at lower speeds, and as the rocket gets faster, to have less control surface deflection for the same desired level of corrective guidance response (When you fly on an airliner at 600 mph, the ailerons are NOT as responsive to pilot commands as they are for landing!). But that's not at the core of this particular problem, not the reversal itself. But I think it may be a contributing factor. You could simply just try it with 1/3 the control surface deflection when the booster is attached (if not the entire guided flight), and see if it is sufficient to maintain roll control. And if nothing else was done to address the roll reversal problem, at least that should greatly reduce how fast it would roll

The one good thing is that the rolling tends to keep the rocket mostly straight, it's not like it is veering off course but is ballistic. Then after booster sep for the coasting vertical guidance phase it kills the roll fast and that allows for good control in pitch and yaw. More a matter of higher stresses on the rocket and some increase drag reducing the potential altitude.
 
My first thought was "make the aft fins active-stabilizers" but these are min-diameter rockets right?

Probably wouldn't be trivial to add teeny winglets to those canards like they do on the big planes to reduce vortex formation.

The thought experiments abound!

I think the vortex in question is the flow of air around the air frame (heading towards the booster fins) rather than the tip vortex. I could be wrong. I saw an interesting paper on this that included wind tunnel results. The same effect can mess with yaw/pitch control in the case of forward canards on a single stage rocket.

Jim
 
It is a well-known effect. That's why some rockets have the fore and aft fins interdigitiated rather than in line.

Congrats on success of the flight. Lots of data to paw over and figure it out!

Any thoughts of using a different gyro? Some of the commercial ones peg out at about 2000deg/s which might give you some more wriggle room in the design.

The MPU-6000 gyros can be adjusted from 500 to 2000 degree range. We bumped up the range from 500 to 1000 prior to this flight. This choice was based on previous flights, which showed that short-term transient rates were typically in the 500 to 1000 range (i.e., 1000 would be sufficient). Increasing this value decreases the accuracy of the gyros at lower rotation rates and also increases the effect of drift, so there is a trade-off.

Jim
 
I should have said in my other reply how neat it was to see the guidance system almost instantly kill the roll after booster sep, no sign of over-correction. Also so good that you have video cameras. Pretty for entertainment, but also so critical for technical review of guidance effectiveness.

Many things happen on my flights that could not otherwise be discovered except by video.

I take note that you do not have the guidance control fin alignment exactly between the booster fins. Rather than 60 degrees apart, it looks more like 30 (fin seen on left) and 90 (fin to the right).

It's worse then that. There are 4 canards and 3 booster fins.

What I am not sure of is whether the effect would be solved by having the control fins directly inline with the booster fins, or not. It may still have an effect, possibly make it even worse. If not, then at least the effect could be reduced by having the control fins exactly mid-way, at 60-60, not the current orientation that looks to be about 30-90.

What I have read is that it is difficult to design for this. My next planned use of the stabilization system will be for this year's three-stage attempt. With an N5800 booster, I think the right strategy will be just to turn off roll control during the boost, or reduce the gain as you suggest below, and then just separate the booster as quickly as possible. I haven't seen really bad spin during the initial boost, so one of these two approaches might be OK. Here's my three stage rocket with no stabilization (second stage didn't light). The booster could be separated at 4 seconds into the flight. I think the gyros might be marginally OK with the amount of roll that occurred up to that point.

https://youtu.be/4UtaQAj3Yug

Also, at those speeds, and since aerodynamic forces increase with the square of the velocity, maybe you have a lot more control surface deflection in roll than you really need. For example the aerodynamic forces at 400 mph are 16 times greater than the forces at 100 mph. And the aerodynamic forces at 600 mph are 36 times greater than the forces at 100 mph. I mean, the ideal
guidance system would take into account velocity if possible, to have a lot more angular control surface deflection at lower speeds, and as the rocket gets faster, to have less control surface deflection for the same desired level of corrective guidance response (When you fly on an airliner at 600 mph, the ailerons are NOT as responsive to pilot commands as they are for landing!). But that's not at the core of this particular problem, not the reversal itself. But I think it may be a contributing factor. You could simply just try it with 1/3 the control surface deflection when the booster is attached (if not the entire guided flight), and see if it is sufficient to maintain roll control. And if nothing else was done to address the roll reversal problem, at least that should greatly reduce how fast it would roll

At some point, I'll post the actual gyro data. At first blush, it looks like the induced roll rate is linearly proportional to the velocity - or pretty close anyway. It seems to me that the roll control might be more than necessary with yaw/pitch perhaps too low. I'd like more review of the data, but that proportion can be easily changed. Varying the flight profile or varying the gain with velocity (i.e., time) can also be done. For my application, however, I just need to be able to control within a narrow speed range (say, 800 down to 500 ft/s).

The one good thing is that the rolling tends to keep the rocket mostly straight, it's not like it is veering off course but is ballistic. Then after booster sep for the coasting vertical guidance phase it kills the roll fast and that allows for good control in pitch and yaw.

I've done a fair amount of turntable testing looking at how rotation in the vertical axis affects the gyros that would control yaw/pitch. It isn't good. It's pretty clear that with the tools that I have to work with, rotation has to be prevented or vertical stabilization won't work.

Jim
 
I've done a fair amount of turntable testing looking at how rotation in the vertical axis affects the gyros that would control yaw/pitch. It isn't good. It's pretty clear that with the tools that I have to work with, rotation has to be prevented or vertical stabilization won't work.

Jim

Sounds like you need a lockout condition on the yaw/pitch control. Ie when enabled the control is only on roll until the roll rate goes below a set threshold. Only once roll authority is established then yaw and pitch control are automatically enabled. This would of course need to be coupled with dropping the booster as soon as is safe and not using any control with the booster attached.
 
Sounds like you need a lockout condition on the yaw/pitch control. Ie when enabled the control is only on roll until the roll rate goes below a set threshold. Only once roll authority is established then yaw and pitch control are automatically enabled. This would of course need to be coupled with dropping the booster as soon as is safe and not using any control with the booster attached.

The roll and yaw/pitch controls can be enabled separately, but that doesn't help with the problem I was describing. Basically, you can turn the unit around a vertical axis such that the gyros that control yaw and pitch should read only zero. However, due to the way the gyros work, which I only understand at a basic level, those gyros will still accumulate an apparent rotation even though this isn't any in the yaw and pitch axes. So, the unit calculates an increasing tilt even though there is none. I have not found a way to avoid this other than not to have rotation. I think this is an issue with MEMS gyros in general, but I could easily be wrong about that.

Jim
 
I've done a fair amount of turntable testing looking at how rotation in the vertical axis affects the gyros that would control yaw/pitch. It isn't good. It's pretty clear that with the tools that I have to work with, rotation has to be prevented or vertical stabilization won't work.

Yep. Need to stop the roll first. Otherwise it ends up like the rotor dynamics of a helicopter. The rotation of the main rotor causes a shift in the phase of the applied moment from the fins. Helicopters can be almost 90deg out of phase with the position of the blades relative to the required force moment. For example, you need to apply the required AoA to the blade when it is almost across the flight path to provide fore and aft pitching moments (I seem to remember 86deg phase lead being required in one piece of reading). Likewise, the roll axis is similarly out of phase on the rotorcraft.

So priority one is to stop the roll, otherwise you can induce dutch roll (pitch-roll coupling) or maybe even the equivalent of PIO (pilot induced oscillations).


PS There are some seriously weird aerodynamics on helicopters. There is even one part of the blade cycle where the air flows from the trailing edge to the leading edge of the blade! Seriously peculiar :facepalm:
 
Last edited:
... Basically, you can turn the unit around a vertical axis such that the gyros that control yaw and pitch should read only zero. However, due to the way the gyros work, which I only understand at a basic level, those gyros will still accumulate an apparent rotation even though this isn't any in the yaw and pitch axes. So, the unit calculates an increasing tilt even though there is none. I have not found a way to avoid this other than not to have rotation. I think this is an issue with MEMS gyros in general, but I could easily be wrong about that.
Jim

Is your gyro chip mounted such that the center of rotation for the 3-axis sensors is centered on the long axis (roll axis) of the rocket? I think that would minimize the pitch and yaw sensitivity to roll. However, there will always be a little bit of cross axis sensitivity. From what I have seen, typical MEMS gyros are in the 1-2% range for sensitivity to off axis motion.
 
Is your gyro chip mounted such that the center of rotation for the 3-axis sensors is centered on the long axis (roll axis) of the rocket? I think that would minimize the pitch and yaw sensitivity to roll. However, there will always be a little bit of cross axis sensitivity. From what I have seen, typical MEMS gyros are in the 1-2% range for sensitivity to off axis motion.

My board is mounted vertically (a horizontal mount would be easier). The board is probably an inch off the axis. Most of what I know about the drift, though, is from turntable testing, but I didn't look at distance from the axis as a variable. It's not obvious to me how the gyros would know if they're on the vertical axis or not. If they're actually accelerometers and not "real" gyros, maybe it matters.

Jim
 
Back
Top