Actively controlled rocket not flying straight

[Nv7]

We are a TARC team working on a rocket with actively controlled fins. Before moving the fins, we just set them to 10 degrees in opposite ways (hopefully creating a roll) and flew it.

In the first launch (where its more sunny) we had relatively weak servos so we thought that was why it wasn't flying vertical. For the next one we bought Bluebird BMS-115WV+ servos which we are confident are strong enough, but its still doing something similar.

Our OpenRocket sim says it should fly 826ft and it has a good stability.


However, here are the actual stats we got:
- Flight 1: 459ft
- Flight 2: 547ft
Both are not vertical like OpenRocket predicted.

Here is an image of our payload:

On the launches we put masking around the slit that the fins slide in through.

We are flying on a F51-10NT from Aerotech.

 

[Titan II]

We are a TARC team working on a rocket with actively controlled fins. Before moving the fins, we just set them to 10 degrees in opposite ways (hopefully creating a roll) and flew it.

What exactly are you trying to communicate?

Thanks for editing the post and adding the rest.

 

[neil_w]

Hmm, active controlled fins seem like an unnecessary and possibly counterproductive complication for TARC. An interesting technical challenge? Yes. Helpful for achieving TARC objectives? I'm not so sure. What's the motivation?

A braking system seems much more useful as a way to nail the target altitude.

 

[Nv7]

Active controlled fins seems like an unnecessary and possibly counterproductive complication for TARC. An interesting technical challenge? Yes. Helpful for achieving TARC objectives? I'm not so sure.

A braking system seems much more useful as a way to nail the target altitude.

We believe it can act as a braking system but also provide spin stability in windy conditions

 

[Nv7]

What exactly are you trying to communicate?

Thanks for editing the post and adding the rest.

Sorry, I didn't realize pressing Enter would post it. I finished adding all the information now

 

[Titan II]

What are you using to control the fins? RC...gyro control...? What is the purpose of active control in this situation?

I also agree with neil_w in post 3.

 

[neil_w]

Am I hallucinating or is there some sort of weird taper at the top of the body tube?


I have to say, I am concerned by the very messy construction there. Normally this wouldn't be much of an issue but if you're trying to implement something as technically challenging as active controls, it would be good to get your build technique under control first.

 

[Nv7]

What are you using to control the fins? RC...gyro control...? What is the purpose of active control in this situation?

I also agree with neil_w in post 3.

We made a flight computer that will control the fins using data from the barometer, accelerometer, etc. However right now they're just set to a constant 10deg in opposite directions because we want to make sure it flies correctly.

 

[Nv7]

Am I hallucinating or is there some sort of weird taper at the top of the body tube?
View attachment 600546

I have to say, I am concerned by the very messy construction there. Normally this wouldn't be much of an issue but if you're trying to implement something as technically challenging as active controls, it would be good to get your build technique under control first.

In that image the fins are tilted, I think that could be causing some perspective? Also in this image I didn't put it down the tube all the way, put masking tape over the top, etc. Normally it feels like its about as solid as our regular fins. The payload tube is just a regular BT-70 body tube.

For our final rocket I plan on CADding and printing out a stencil which we would use to cut some slits to slide in the fins. However, for this prototype we just want to make sure it will fly correctly and we thought that cutting this and putting masking tape over would be good enough

 

[Titan II]

Have you flown it without the canards to see what it does?

 

[Rschub]

I would guess that the moment imparted by the deflected fins is overpowering the main fins. Open rocket will not be accurate due to the increased drag of deflected fins.
Depending on the orientation of the upper fins the slipstream could be impacting one of the lower fins as well.

 

[Nv7]

Have you flown it without the canards to see what it does?

Yeah, this is an old rocket that we flew like 20 times successfully before we put the fins on.

 

[Tractionengines]

10 degrees is a LOT for the apparent length of the fins. Your fin shape is causing most of your control surface to be in the turbulence of the leading part of the fin.

I would guess you have much more drag from those fins than your SIM is accounting for. Between that and the rough surface finish (drops and runs in paint, non-uniform tube, etc.) it's robbing your missing altitude.

What do you have your surface finishes set to in the SIM for the nose, tubes, and fins?

 

[Nv7]

I would guess that the moment imparted by the deflected fins is overpowering the main fins. Open rocket will not be accurate due to the increased drag of deflected fins.
Depending on the orientation of the upper fins the slipstream could be impacting one of the lower fins as well.

Do you recommend we try flying it with a 0deg tilt? Any other suggestions to improve reliability?

 

[Nv7]

10 degrees is a LOT for the apparent length of the fins. Your fin shape is causing most of your control surface to be in the turbulence of the leading part of the fin.

I would guess you have much more drag from those fins than your SIM is accounting for. Between that and the rough surface finish (drops and runs in paint, non-uniform tube, etc.) it's robbing your missing altitude.

What do you have your surface finishes set to in the SIM for the nose, tubes, and fins?

This rocket had been flown without the fins multiple times and flew pretty close to the sims (like within 10 feet). I tried switching the surface finish to "Rough" but it only affected altitude by a couple meters. I think the main reason it isn't going the correct height is because its going in crazy paths that you can see in the video, nowhere close to vertical. Any ideas why it could be doing this?

 

[waltr]

I'm also thinking fin shape, length, not being out from the BT enough to also be in clean airflow.

Have you searched the forum for threads on active control???
There are a few with systems that work very well. A study of these would help.
You can also PM the designer for some help.

 

[Tractionengines]

I think you also likely have issues from having 2 control canards ahead of 3 main fins.... especially with your 10deg angle. You'll have all kinds of asymmetric forces.

 

[Nv7]

I think you also likely have issues from having 2 control canards ahead of 3 main fins.... especially with your 10deg angle. You'll have all kinds of asymmetric forces.

How can we make these forces symmetric? Do you think lower angles would have enough control authority? Also, do you think a small weight imbalance (like 10g) within the rocket could be causing it to go sideways?

 

[Nv7]

I'm also thinking fin shape, length, not being out from the BT enough to also be in clean airflow.

Have you searched the forum for threads on active control???
There are a few with systems that work very well. A study of these would help.
You can also PM the designer for some help.

We could try deploying airbrakes instead of having actively controlled fins. The reason we went with this design is because we thought it would provide the benefit of spin stability in addition to the altitude control. Do you think it would work or should we switch to an airbrake design?

 

[neil_w]

We could try deploying airbrakes instead of having actively controlled fins. The reason we went with this design is because we thought it would provide the benefit of spin stability in addition to the altitude control. Do you think it would work or should we switch to an airbrake design?

To me, for a rocket that's going to fly ~800', you do not need active stabilization. As I said before, that's a complex challenge that doesn't get you where you want to go, which is to accurately control apogee.

Do you know of any TARC teams that have achieved success with an active stabilization system?

IMHO airbrakes are a much more fruitful direction. Still plenty of effort required, but much better return on your investment of effort.

 

[Titan II]

Also, do you think a small weight imbalance (like 10g) within the rocket could be causing it to go sideways?

No

Do you think it would work or should we switch to an airbrake design?

Ditch the canards. You keep saying active control, but you say they are fixed at 10 degrees. I have not seen any indication of any active control. Was the intent to flip them 90 degrees at a specific altitude for braking action?

neil.w has good advice.

 

[neil_w]

It is worthwhile to go back to square one and practice thinking like an engineer and ask some questions, such as:

• What exactly is the problem you are trying to solve?
• Are there any proven techniques to solve the problem?
• What is the most efficient way to solve the problem?
• What resources and skills are needed to implement the solution?

 

[Nv7]

No


Ditch the canards. You keep saying active control, but you say they are fixed at 10 degrees. I have not seen any indication of any active control.


Than why bother with the canards at all?

neil.w has good advice.

Here is our plan:
1. Fly it at various fixed angles to get an idea of how it flies (we are here, its not flying straight)
2. Derive a model of how different fin angles make the rocket fly using Simulink
3. Tune a PID controller that moves the fins to bring us to a target flight path
Does this seem ok? Right now we're just trying to get it to fly reliably with a fixed angle before getting into the more complicated things.

 

[Nv7]

It is worthwhile to go back to square one and practice thinking like an engineer and ask some questions, such as:

• What exactly is the problem you are trying to solve?
• Are there any proven techniques to solve the problem?
• What is the most efficient way to solve the problem?
• What resources and skills are needed to implement the solution?

Our problem is that our rocket doesn't always hit the correct height and also doesn't always go straight up if its windy.
Proven techniques: correct stability, angling launch rod, correct mass, spin stability, airbrakes
Most efficient way - It depends on how accurate you want to be, we have a rocket that can get under 10 pts reliably in good conditions but we think active control could be more accurate and handle worse conditions
Our solution was to use the spin that occurs with canted fins to stabilize the rocket and then tilt them to control the altitude
Resources and skills - Engineering, simulation, control systems, probably more

 

[Alan15578]

So, if I understand your approach, you intend to fly a rocket that will nominally exceed your target altitude, use an onboard computer to predict apogee and then command canards to bend the trajectory over to achieve your target altitude, or pitch the rocket over 90 Degrees, increasing drag to achieve your target altitude? To do that you will need some years of aerospace engineering controls and performance courses, or a LOT of trial and error, and probably both.

I think you will have a better chance of success using a split rudder drag modulation system similar to what the Shuttle used on landing approach.

If you continue to use canard control, I would limit the size to 10% of the fins size.

I do applaud your attempts to use autonomous control. You can learn a lot from your failures.

 

[Nv7]

So, if I understand your approach, you intend to fly a rocket that will nominally exceed your target altitude, use an onboard computer to predict apogee and then command canards to bend the trajectory over to achieve your target altitude, or pitch the rocket over 90 Degrees, increasing drag to achieve your target altitude? To do that you will need some years of aerospace engineering controls and performance courses, or a LOT of trial and error, and probably both.

I think you will have a better chance of success using a split rudder drag modulation system similar to what the Shuttle used on landing approach.

If you continue to use canard control, I would limit the size to 10% of the fins size.

I do applaud your attempts to use autonomous control. You can learn a lot from your failures.

No, its not supposed to bend it over. They will tilt in opposite directions which will make the rocket spin, taking energy away from its vertical movement and increasing the drag (and hopefully increasing stability too). In the video it spins a little but not nearly as much as the spins expected. It also seems to be spinning around a point that is outside the rocket.

 

[techrat]

Have you tried flying it WITHOUT the actively controlled fins? I would say to start there and verify the rocket flies straight without all the extra junk. THEN, add back the the extra stuff to determine your problem area. If rocket is good without the Actively Controlled fins, then you know where the problem lies and may be related to software OR how you are reading the data from the sensors.

 

[techrat]

Ok, re-reading the thread, it seems that the rocket flew OK without the canards.
My thinking is that the canards are not acting as control vanes at all where they are located right next to the nosecone. They will not induce spin, because the airflow from the blunt nosecone is creating a bit of a shockwave of sorts, which leaves the canards in a dead-zone of airflow. If you have a spare piece of body tube, try lengthening the body of the rocket about 4 to 6 inches, then add the nosecone. This will more centrally locate the canards, and possibly allow more airflow to be affected by them... But that's just my mindsim...

 

[neil_w]

Our problem is that our rocket doesn't always hit the correct height and also doesn't always go straight up if its windy.

That's two problems.

What are the consequences of the rocket not going straight up? Why is that a problem? Exactly how "not straight" are we talking about?

Proven techniques: correct stability, angling launch rod, correct mass, spin stability, airbrakes

Again, you're mixing two problems here.

Our solution was to use the spin that occurs with canted fins to stabilize the rocket and then tilt them to control the altitude
Resources and skills - Engineering, simulation, control systems, probably more

How do you imagine your active-controlled fins might behave while the rocket is spinning?

I don't want to walk you through the entire process here, just trying to get you to think the whole thing through before you jump at a solution that sounds cool. If you conclude that active stability control is really what you want to do, you've got a tough road ahead of you.

 

[Nv7]

That's two problems.

What are the consequences of the rocket not going straight up? Why is that a problem? Exactly how "not straight" are we talking about?

Again, you're mixing two problems here.


How do you imagine your active-controlled fins might behave while the rocket is spinning?

I don't want to walk you through the entire process here, just trying to get you to think the whole thing through before you jump at a solution that sounds cool. If you conclude that active stability control is really what you want to do, you've got a tough road ahead of you.

We're hoping the gyroscopic forces created by a spinning rocket would provide stability (spin stability). If our rocket flies in heavy winds (like D.C) it would tilt into the wind and lose lots of altitude. To combat this we want to use spin stability. They would also be controlled to do a fixed altitude, which would solve both problems (this is what we were thinking when we made this)