I could use just a little guidance

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Very nice. I noticed no T2T on these fins is that because it is sole purposed for relatively low velocity flights. On the debate as to when to engage the roll control, what do you see as the downside to having it engaged out of the gate?

So, tomorrow is the first flight of the new, stabilized rocket. Woohoo! The weather is supposed to be ideal. We will have fun.

I decided on one layer of reinforcement. No artistic awards on this job, but the fins are quite a bit stiffer. I also got the second #16 camera in time, mounted opposing per Nytrunner.

The plan is to launch at a 5° angle, or maybe a little more, with roll control engaged from the start. Vertical stabilization will kick in 1 second after liftoff, before the rocket is going too fast. Flight will be on a Gorilla L-1112 Blue to around 8K or so.

Question: With the rail at an angle, should I mount the rocket above or below the rail? I have three 1010 buttons with the highest one about 5 feet above the base of the rocket. Rocket weight without the motor is right at 20 pounds.

Jim

IMG_1416.jpg

IMG_1417.jpg
 
Well those are nice little mounts. Where do you get those?

I'd think under is better. Three 1010's will hold a 35# rocket I know from experience. If you mount it on top of the rail it'll likely lean to the side putting a bend on the buttons (they don't like the bends. I know That from experience too....)
 
Well those are nice little mounts. Where do you get those?

I'd think under is better. Three 1010's will hold a 35# rocket I know from experience. If you mount it on top of the rail it'll likely lean to the side putting a bend on the buttons (they don't like the bends. I know That from experience too....)

The mounts are just little square plates that I cut from the air frame of the rocket that did the Mach 3.7 shred at Balls 2016. I recycle! The mounts are sanded to fit the OD of 4" air frame, which lets me move the cameras around from one rocket to another.

I think you are right about the mounting on the rail.

Jim
 
I usually try to hang from the backside of the rail in a case like this, but my motivation is to avoid rail-rash. I am looking forward to reviewing the data and video.

Good luck.
 
OK, I did the flight on Sunday. Some good news and some not good news. Here's some video, and the gyro data are attached.

https://youtu.be/PrS-DufSDGE

The first three seconds of the flight were pretty good. The rocket was launched at an angle of about 5 degrees, and it weathercocked to the left to about 10 degrees. You can see the rocket turn to vertical after the vertical stabilization was engaged one second into the flight. There were a few oscillations from 1.5 to 3.0 seconds, but the rocket was staying vertical. Also, during that three-second period, the rocket only rolled about 45 degrees in total. At that point, however, the rocket started to roll at a rate greater than the 1,000 degree/second maximum for the gyros. From the sunlight on the air frame, it appears to be a series of 180 degree rolls. That caused the rocket to lose track of vertical, and things didn't go well after that.

I think what happened during the flight is that the carards were providing too much control for the speed of the flight. For one thing, it sort of appears to me like the rate of the oscillations is about equal to the full-sweep speed of the servos. It also appears like there is quite a bit of torque on the air frame. Since the rocket quickly went vertical 1 second into the flight, even thought the speed was relatively low, there is plently of yaw/pitch control. I think I just need to use less control action and/or smaller canards for this particular rocket and speed combination.

I was also dissapointed to loose some of the flight data. My down-looking camera, which would have been very handy, collected some videos on the morning of the flight for some reason. By the time it got to the pad, the battery was dead. I must have it set wrong. I'll check into it if I can get the dirt cleaned out of it. I also lost the gps file for some reason. It appears that the power cycled when the rocket landed, and the file was over-written. Crap. Hopefully, I can get the dirt cleaned out of all of the bearings and fly it again.

Jim

Gyro data.jpg
 
It does look like too much control authority on roll. Bummer. What is the slew limit (degrees off-center) for your canards? Do you have a mechanical stop?

It looks like it reduced the tilt nicely till it went oscillatory.
 
Congrats on a great test flight. That looks like plenty data to get your roll gains in the right ballpark. I'm looking forward to the next flight!
 
Looks like more data to go over, big plots on your Y axis. Seeing the rocket dragged through the field looks like Dave McCann's video at URRG a few years back. But his went on for at least 10 min and I think the farmer thanked him for tilling the field.
 
It does look like too much control authority on roll. Bummer. What is the slew limit (degrees off-center) for your canards? Do you have a mechanical stop?

It looks like it reduced the tilt nicely till it went oscillatory.

For yaw/pitch, the gains were set such that there was up to 10° of canard movement for up to 7.5° of tilt. Beyond 7.5° of tilt, the maximum canard movement is 10°.

For roll, there is up to 10° of canard movement at a roll rate of up to 250°/s. However, if the servo movement for yaw/pitch is not being used, then the roll response could continue to increase at higher roll rates, up to 20° of canard movement at roll rate of up to 500°/s.

From the gyro plot, the tilt was very low 3 seconds into the flight. Thus, there would have been unused yaw/pitch response (i.e., a tilt less than 7.5°, evaluated separately in the x and z directions), and when the roll rate increased to, and above, 500°/s, the canard angle would have exceeded 10°. Right at 3 seconds, where the tilt was about 2°, the canard deflection would have been on the order of 17° for just an instant. The rationale for this enhanced roll is that we know that cross sensitivity will foul up vertical control and we want to do everything possible to reduce the roll rate. But, I think we went a bit too far on this flight.

Jim
 
Interesting that it got (mostly) stable between 6-7 seconds. Any thoughts what went on there?

Thanks for sharing with us, Jim- this stuff is very interesting to me!
 
Interesting that it got (mostly) stable between 6-7 seconds. Any thoughts what went on there?

I haven't looked that much at what happened later in the flight. I think that roll stabilized again and that it was trying to control to a "new" vertical. I wish I had the gps file and the down-looking video. Those would make the flight much easier to interpret.

Jim
 
Sounds like a control systems problem.

If you were really lucky all it would require is limiting the gain but I suspect that you also need some loop compensation. For an example see this I wanted to look up reference [28] but the paper failed to mention a page number and I own the third edition rather than the tenth. Which gives you a hint of how long ago I studied control systems. :)

But when I say gain I have a particular meaning in mind. The output of your control system is not the angle of the control fin but the amount of force generated by that fin. Since that force is a function of both the fin angle, air density, and vehicle velocity (squared), the relation between error and fin angle will vary during flight. A lot.

Even worse is that this depends on the dynamic characteristics of the vehicle it is attached to. So the control system needs to be customized for each vehicle. You might be able to craft a control system that works reasonably well over a range of vehicle parameters but best performance will require tailoring the control system to the vehicle.
 
Sounds like a control systems problem.

If you were really lucky ...

I hope that's not a prerequisite.

At least on this flight, there is some data on control response to look at. The yaw/pitch data from 1 to 3 seconds is good, and it is interesting how well it stabilized from 5 to 7 seconds (just not to the actual vertical). It's been difficult to get much usable data.

I'd love to put in a function that adjusted the gains as a function of time (i.e., velocity).

Jim
 
I hope that's not a prerequisite.

At least on this flight, there is some data on control response to look at. The yaw/pitch data from 1 to 3 seconds is good, and it is interesting how well it stabilized from 5 to 7 seconds (just not to the actual vertical). It's been difficult to get much usable data.

I'd love to put in a function that adjusted the gains as a function of time (i.e., velocity).

Jim

I agree with Dave. It's a control system problem and a non-linear dynamics problem. Best you can do is to enable active control during short periods where the system model is simpler.

A major dilemma in control system design is to have enough control authority (force/gain & min delay), but then be able to tune the system to have stable feedback control.
 
I'd love to put in a function that adjusted the gains as a function of time (i.e., velocity).
Ahhhh...If your gains aren’t changing with flight condition, then I recommend adding that. If that’s not possible (cost/money/time/etc), then adjust the gains to be right for your worst case. That should keep it from going unstable (or at least stop over over controlling), but will make the response much more sluggish at slow speeds. Since you’re aim is to go straight up, slower response off-speed shouldn’t be a problem.

(So, divide the roll gains by 10 and fly again?)
 
One IREC team I'm involved with is using active control grid fins to get to a certain altitude. What I've been impressed with, even when the grid fins were static and not controlled, is the corrective force of the grid fins. We launched in 15mph winds last year and the rocket didn't weathercock nearly as much as their planar fin rocket. They flew both to see the differences. They launched them within a minute or two of each other and the conditions were as close as you could get. The grid fin rocket more translated with the wind instead of steering into it. I don't know if this would be an option for your base fins - to put grid fins on a spincan.

Edward
 
I'd love to put in a function that adjusted the gains as a function of time (i.e., velocity).

Gain scheduling would be a great way to keep the control system more responsive throughout a larger portion of the flight envelope. On the system I am readying for flight this year I am using a commercial autopilot for a fixed-wing RC aircraft so I don't have that luxury. If I feel sufficiently inclined (low probability) I might try customising something like the Naze32 flight controller for the controller. That would give me access to the firmware (open source) so gain scheduling could be added. I suspect I will keep the gain dialed down and tolerate the leisurely performance.
 
I agree with Dave. It's a control system problem and a non-linear dynamics problem. Best you can do is to enable active control during short periods where the system model is simpler.

A major dilemma in control system design is to have enough control authority (force/gain & min delay), but then be able to tune the system to have stable feedback control.

John & Dave ...

As I'm sure you're aware, I am completely fluent when it comes to discussion of topics like loop compensation, non-linear dynamics, simple system models, and stable feedback control. However, for the benefit of others here on TRF who may know less than we do, perhaps you could expound a little on these topics, particularly in relation to my flight (and possibly in English?). Thanks in advance!

Jim
 
Since you’re flight goes vertical under control quickly, what would take it off vertical in flight? (Wind, Motor thrust, separation) Maybe turning control off after 3 sec. allowing fins to do there job for a little bit.

Have you thought of pulsing control on and off after the first few seconds. , 1 sec on 1 sec off repeat. This probably is a bandaid of what you really want to accomplish. If your goal is perfecting control this probably not going to get you there, however if your goal is vertical flight then maybe something to consider.

I’ve been watching/ reading your posts with interest since I found TRF - but honestly don’t know your main goal (no spin and vertical for altitude is what I think).


Sent from my iPhone using Rocketry Forum
 
I’ve been watching/ reading your posts with interest since I found TRF - but honestly don’t know your main goal (no spin and vertical for altitude is what I think).


Sent from my iPhone using Rocketry Forum

The main goal is to get "just a little guidance" to straighten up the sustainer on multi stage flights so as not to land in the mountains at Balls. Unfortunately, the frequency of launches there, and the "things" that happen during those flights, don't allow much development to happen. I do have a two stage test rocket that I fly without a sustainer, but even that takes an N motor and a trip to Kansas. I like Kansas, but it's a bit of a haul.

So, I revived the forward canard rocket that I stated with to let me do more testing of the system. I can fly it locally and get a lot more data over time. Everything from the most recent flight survived its cross country trip, so we'll try again next month. The goal of no spin is because the control system that is available at reasonable cost does not function properly if the rocket spins more than just a little bit (this is true of other commercial products you might recognize that have the same gyros). And, acheiving no spin appears to require a spin can for fins below the canards once the velocity gets up a bit. The goal of vertical is not for altitude, but rather, for staying in the same county that the rocket is launched in. If you have a rocket that goes to 150K, and it heads off at a 10 degree angle for some reason, you land 30 miles away, plus or minus. I just want a little guidance to avoid that. For the test rocket, reasonable spin control with the ability to slowly correct to vertical would be just fine.

Jim
 
This probably is a bandaid of what you really want to accomplish.

What he likely needs is a transfer function S to t domain differential equation driven control system with a washout depending on S domain to adapt to situation at hand. I'm in a process dynamics and controls lecture and then I find yaw damper code for 747 by Boeing published by Matlab as tutorial and I see it gets even "shittier" than my simplistic textbook models without answers in back of book. Basically you have an input into a system, you have outputs say voltage versus time or pitch angle versus time and these oscillate, you have another plot trying to converge on it in real time. They have open loop and closed loop systems. You can ultimately fine tune servo movements by velocity or time by a given oscillating system in real time and have outputs progressively decrease.

The problem is with Boeing they give you inputs in matrices for a 747 aileron bank angle etc and a bunch of proprietary numbers. The numbers they found somehow Anyone's guess knows how for that object. The rocket wouldn't behave like the 747. It will have a lot more rapid inputs with progressively less pitch angles on fins as the rocket velocity increases. It's just a complex thing to model where y=mx+b linear algebra won't solve it. You can size capacitors and inductors in labs to a resistor for a desired voltage response through differential equations or guessing if you hate diff. And a secondary response follows a similar amplitude and frequency but less amplitude until it's neutral again and stabilizes on an oscilloscope. But I'm just a dumb undergrad mechanical student trying to graduate. Only time I used complex numbers were phasers in circuits long ago. This goes from real to imaginary or vice versa and it's infuriating with differentials. I didn't take a mechanical vibrations course but I'm certain some of the frequency aspects could get pulled off the airframe somehow. That other guy Dave was mentioning how the inputs were specific to airframe and the scary part is I haven't even learned how one would get those numbers. They were experimentally found somehow. Heck NASA had like 400 pages of the X-31 test program where they developed some dial a gain system a control system. And it crashed anyways.

I think he wants his gain to vary. Boeing multiplied their gain by H(s)=s/(s+a) but Matlab was doing the operations from proprietary data. How they found what rad/s went where beats me!
Perhaps they had a slight idea then iterated by tests. Currently doing beam deflection labs with vibrations math and there's this LVDT method with strain gauges by omega to get natural frequency but that's child's play compared to a hobby rocket airframe in flight. Supposedly you can similulink an arduino from Matlab. Dumb mechanical so they didn't teach that bit. Guess that's electrical engineer or on the job. Maybe someone smarter could get an actual actuator to response dampened to an input using the method. Tweak boeing's tutorial code is my opinion.

Not trying to snark. I feel f---ing stupid myself looking at it. The easy stuff starts with partial fraction decomposition and Laplace transforms if Laplace even applies! And the prof is check marking answers we know we missed. There's not answers in book back but I'm trying.

So with airplanes like Cessnas the control input by cable/pulley wasn't linear with velocity. You had regions near stall velocity where full control authority with maximized control deflection described as mushy feeling was required while near velocity never exceed you might need only a very slight touch rather than tens of pounds of effort to deflect control surfaces maybe a few degrees. The response was increasing with velocity despite decreasing forces applied. And that was a manual cable trainer no fly by wire gizmos or yaw dampers at a puny fifty to hundred knots. The controls system sorta changes the input from manual into a automated precise varying output reaction of what is sensed through coding and electronics/sensors. And you can adapt linkages, servos, motors, and all kinds of mechanical systems to it. I don't fault anyone for trying bandaids. It would be nice to average a linear model equation but if the control input changes proportional to output needed in flight you need a system like a control system using differentials that can adapt. Basically your control surface on an airplane has force components of lift when it's angled that generate the rolling or pitching. The drag on that surface increases as it deflects and it varies in force. And it causes oscillations on airframe elsewhere. And it's not linear. Putting flaps to 20 degrees on prolonged approaches caused the tail to bobble I remember. A damper via a control system could counter that by having elevator oscillate via math model but Cessna was pure mechanical linkages. And if on rocket the fin vibrates then god help you because it could chase the vibration with the control system picking up false inputs.

And the the theoretical is just as scary as how the heck did manufacturer X or Y actually develop that control system for Z problem. Nobody says hey start with math function this one. Is it suppose to be that complicated or is it just hard? I feel stupid. There's math tutors running away from control textbooks. Also seen control systems on VW based car factory robots using PLC server logic making cars every eight seconds. Door gates and part placement was inputs for when robot arms would move with relation to other robots. They had teams of programmers and electrical engineers. Teams. Beyond my understanding and pay grade...
 
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