Nv7
Well-Known Member
I was thinking that since the rocket is trying to slow itself down, it should tilt the fins in opposite directions so as to create drag. Do you think changing the direction would be more effective?
Actively controlled rocket not flying straight
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Many solid rocket missiles fly near ballistic trajectories. At max range they fly fine. To fly to shorter ranges, you can fly the high trajectory or the low trajectory, which can create problems. So they often fly an energy management maneuver instead, like an S curve during boost. So yes, more effective, but not for amateurs and sport flyers.
Nv7
Well-Known Member
Increased the size of the fins and expanded the range to 90 degrees! Weirdly however, the effectiveness seems to decrease after 60 degrees. To solve the oscillation issue I plan on enabling the control systems after 4 seconds. Excited to launch on January 15th!
Nv7
Well-Known Member
Doesn't stall increase drag?
Thanks for making me look, @Nv7 !
You are correct.
CD increases monatonically from AoA 0 to 90 and then decreases for AoA angles from 90 to 180.
I was thinking of the Lift whiich decreases at the stall angle.
This is a graph of the CL -vs- AoA for a symmetric NACA 0012 airfoil:
This is the CD for the same NACA 0012 airfoil:
Note the blip in CD at about 12 degrees where the NACA 0012 airfoil stalls.
Thanks for making me look, @Nv7 !
-- kjh
You are correct.
CD increases monatonically from AoA 0 to 90 and then decreases for AoA angles from 90 to 180.
I was thinking of the Lift whiich decreases at the stall angle.
This is a graph of the CL -vs- AoA for a symmetric NACA 0012 airfoil:
This is the CD for the same NACA 0012 airfoil:
Note the blip in CD at about 12 degrees where the NACA 0012 airfoil stalls.
Thanks for making me look, @Nv7 !
-- kjh
Nv7
Well-Known Member
Today I did 3 flights and things didn't work how I expected. This is the graph for altitude for one of the flights
The fins got tilted after 3.5 seconds, and because of this, I think a high pressure zone got created in front which led to a lower altitude reading, also causing the rocket to think it reached apogee (this wasn't a problem since at that point it just locks the fins at full deflection, which is where it should've been anyways). I'll adjust the position of the vent holes to fix this.
However, the fins were not effective. They tilted the same amount as in the sims but they were not nearly as effective in real life.
Here is the simulated acceleration (green) vs real acceleration (black):
You can see there is significantly less drag in real life. When I multiply the drag by 0.15, it lines up: (I think the spike is caused by the actual physical movement of the fins)
I'm not sure what I can do because increasing the size of the canards leads to a decrease in the stability. For example, here is the current design:
And here it is with canards that add up to the diameter of the tube - stability is a lot lower:
I have already increased the size of the fins a lot and I am up against the weight limit and I don't think much larger fins could survive a landing, so I can't increase the size of the canards much more.
What do I do to increase the effectiveness of the canards?
The fins got tilted after 3.5 seconds, and because of this, I think a high pressure zone got created in front which led to a lower altitude reading, also causing the rocket to think it reached apogee (this wasn't a problem since at that point it just locks the fins at full deflection, which is where it should've been anyways). I'll adjust the position of the vent holes to fix this.
However, the fins were not effective. They tilted the same amount as in the sims but they were not nearly as effective in real life.
Here is the simulated acceleration (green) vs real acceleration (black):
You can see there is significantly less drag in real life. When I multiply the drag by 0.15, it lines up: (I think the spike is caused by the actual physical movement of the fins)
I'm not sure what I can do because increasing the size of the canards leads to a decrease in the stability. For example, here is the current design:
And here it is with canards that add up to the diameter of the tube - stability is a lot lower:
I have already increased the size of the fins a lot and I am up against the weight limit and I don't think much larger fins could survive a landing, so I can't increase the size of the canards much more.
What do I do to increase the effectiveness of the canards?
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Nv7
Well-Known Member
Do you think the fins could be stuck in a turbulent boundary layer? I’ll try making them taller and less wide
I've been wondering about that for some time. Last week I searched for 'spin rockets' and 'spin control' rocket designs. I found several projects with recorded spin data for fin angles from 3° to 30°. All of them produced 2-22 RPS rates of spin. Your vehicles spin between 22-27 RPM. My latest flight with a gyro IMU system recorded a 1.8 RPS roll rate during the motor burn with the roll rate decaying to near zero just before parachute deployment. I need to correct this issue.
There are a number of questions to consider as to why you are not attaining a higher rate of spin.
Insufficient servo torque.
3-D printing material flex.
Aerodynamic flow interference between two canards and three fins.
Aerodynamic flow over the canards.
Wrong shape canards. Delta over rectangle.
Wrong location for canards.
I recommend that you test your vehicle for air flow spin dynamics.
Read “The Effects of Spin Stabilization in Amateur Rocketry” by Evan M. Gates
fetch.php (acemu.org)
There are a number of questions to consider as to why you are not attaining a higher rate of spin.
Insufficient servo torque.
3-D printing material flex.
Aerodynamic flow interference between two canards and three fins.
Aerodynamic flow over the canards.
Wrong shape canards. Delta over rectangle.
Wrong location for canards.
I recommend that you test your vehicle for air flow spin dynamics.
Read “The Effects of Spin Stabilization in Amateur Rocketry” by Evan M. Gates
fetch.php (acemu.org)
Wow! That is a very impressive and professional report for being written by an 8th grader. Even if it is Evan Gates.
waltr
Well-Known Member
Have you read this thread?
https://www.rocketryforum.com/threads/i-could-use-just-a-little-guidance.122042/
https://www.rocketryforum.com/threads/i-could-use-just-a-little-guidance.122042/
Nv7
Well-Known Member
- I am using Blue Bird BMS-115WV+ servos which at my voltage have a torque of 5.5 kg-cm, which is very high for a servo - I have seen a lot of TVC mounts and fin controlled systems with weaker servos do better.
- The 3d-printed fins did not flex under what was probably at least a pound of force from my hands on the ground, it would probably break before it flexed and it didn't break
- How would aerodynamic flow interference between two canards and the fins not create enough drag?
- What is the proper shape for the canards? A lot of missiles have trapezoids but I wonder if they don't use rectangles just for strength reasons or aerodynamic reasons? I'm going to try an inverted trapezoid to see if the surface area being further away from the body helps:
Here is the old fin design:
What do you mean by delta over rectangle?
- Where should the canards be? I just put them in the center where I could make them largest without affecting the Cp wile still being in the payload.
waltr
Well-Known Member
Did you go to the links posted and see what others have done???
Your original canards seem to be too long (nose to fine direction) and too narrow (across BT direction) to be effective. Also maybe to fat.
I'll guess they end up inside a turbulent layer (next to BT) thus not acting the same at different speeds.
The inverted trapezoid looks like a bad idea as it seems it would flex.
Flipping them around with long root next to BT seems better.
Your original canards seem to be too long (nose to fine direction) and too narrow (across BT direction) to be effective. Also maybe to fat.
I'll guess they end up inside a turbulent layer (next to BT) thus not acting the same at different speeds.
The inverted trapezoid looks like a bad idea as it seems it would flex.
Flipping them around with long root next to BT seems better.
Nv7
Well-Known Member
To keep the stability ok the surface area is very limited with just a rectangle going the other way though
Nv7
Well-Known Member
Did a launch today on our new F35-8W motors, but unfortunately it ended up going a lot lower (not sure if it went vertical, hard to tell from POV) and because of that the delay was way too late so parachutes ejected at a very high velocity. Because of this the ring that attaches the parachutes to the payload tube broke. Then, the payload tumbled and the egg mount broke upon impact. Finally, the parachutes got tangled and failed to open so the fin can broke. Here is an image of every part that is broken (which is every exterior component):
View attachment IMG_6916.jpg
Luckily the electronics, egg, and motor were completely unharmed (all the delicate and expensive components)!
In the video you can see there is a line coming off the fin of the rocket. This follows the rocket throughout its flight and starts when the motor ignites. What is it? Did it cause the rocket to go lower? Here is the video so you can take a closer look:
View attachment IMG_6910.MOV
I have also attached the flight data but its not too useful because the fins didn't tilt since it went so low. However, I can see the rocket's oscillations until it tilts over as it approaches apogee. It seems like it flew straight though, until it tilted as the effectiveness of the fins reduced.
View attachment IMG_6916.jpg
Luckily the electronics, egg, and motor were completely unharmed (all the delicate and expensive components)!
In the video you can see there is a line coming off the fin of the rocket. This follows the rocket throughout its flight and starts when the motor ignites. What is it? Did it cause the rocket to go lower? Here is the video so you can take a closer look:
View attachment IMG_6910.MOV
I have also attached the flight data but its not too useful because the fins didn't tilt since it went so low. However, I can see the rocket's oscillations until it tilts over as it approaches apogee. It seems like it flew straight though, until it tilted as the effectiveness of the fins reduced.
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It almost looks like the safety cover + flag from a launch rod. Or like a remove-before-flight pin + flag?
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NTP2
Well-Known Member
no! It’s a UFO coming to get us!!!!
Nv7
Well-Known Member
We did 3 launches today and I saw it in 2 of the 3 launches. I know its not something physical because you can see that it only appears once the rocket starts moving, and I can confirm the launch rod is clean. However it doesn't seem to be causing any problems so I guess I'll just ignore it?
Ansys workbench decided to stop working so once I fix that I will do some more analysis on the flight today and post about it.
Nv7
Well-Known Member
Got the simulations working, and the data makes very little sense...
The mass is down to 552g, which was a lot lower than my expected mass of 590 (I was trying to reduce the mass for this new iteration). I checked everything several times and had other people check and I guess I just saved a lot of weight.
The graph still only lines up when I multiply drag by 15%. However, a lot of things don't make sense.
- Why is the thrust measured a lot longer? I am using the same thrust curve
- The drag created by the fins is very rough instead of a nice smooth curve like simulated
- It takes 0.1 seconds between the command sent to tilt the servo and when it is detected on the accelerometers
This chart is even more confusing. The green demonstrates simulated altitude, black shows measured. I closed up some vent holes and tried offsetting them and the dip in altitude from the fins tilting is lesser but it is still there. However, I am confident of the accuracy of the final apogee value because airspeed at apogee is 0, meaning the canards can't be creating any drag there so the air pressure should be accurate.
What I don't understand is why the real rocket went so much lower than what was simulated. To get this to line up I have to greatly decrease the mass of the simulated rocket while increasing the Cd a lot, which doesn't make sense, I tested the mass with a variety of scales and I am confident in its accuracy. In addition, the final apogee of the real rocket was much lower than the simulated rocket.
I think I will try doing a flight with no fin tilt to see how well it matches up with the simulations at the next launch in addition to a fin tilt test (assuming it doesn't crash next time)
The mass is down to 552g, which was a lot lower than my expected mass of 590 (I was trying to reduce the mass for this new iteration). I checked everything several times and had other people check and I guess I just saved a lot of weight.
The graph still only lines up when I multiply drag by 15%. However, a lot of things don't make sense.
- Why is the thrust measured a lot longer? I am using the same thrust curve
- The drag created by the fins is very rough instead of a nice smooth curve like simulated
- It takes 0.1 seconds between the command sent to tilt the servo and when it is detected on the accelerometers
This chart is even more confusing. The green demonstrates simulated altitude, black shows measured. I closed up some vent holes and tried offsetting them and the dip in altitude from the fins tilting is lesser but it is still there. However, I am confident of the accuracy of the final apogee value because airspeed at apogee is 0, meaning the canards can't be creating any drag there so the air pressure should be accurate.
What I don't understand is why the real rocket went so much lower than what was simulated. To get this to line up I have to greatly decrease the mass of the simulated rocket while increasing the Cd a lot, which doesn't make sense, I tested the mass with a variety of scales and I am confident in its accuracy. In addition, the final apogee of the real rocket was much lower than the simulated rocket.
I think I will try doing a flight with no fin tilt to see how well it matches up with the simulations at the next launch in addition to a fin tilt test (assuming it doesn't crash next time)
@Nv7 --
It kinda-sorta looks like your accelerometer might have pegged out from 0.0 to 1.3 sec ?
What are the specs on your accelerometer ?
Was this flight also on an F35-7 ?
And what are the units on the graphs ?
According to Thrustcurve.org > AT F35W, the motor mass is 85 g with 30 g of propellant.
Suggestions:
1 - record the motor and the batch
2 - weigh you motor before the flight
3 - prep your rocket for flight
4 - weigh your ready-to-fly rocket
5 - fly and recover your rocket
6 - remove the motor and weigh it after the flight
You'll be able to calculate the propellant mass ( initial motor mass) - (final motor mass) ) and then the 'coast mass' of your rocket.
Acceleration is simply an effect of Force / Mass and you'll need to calculate the Forces at play to make sense of the measured acceleration.
It kinda-sorta looks like your accelerometer might have pegged out from 0.0 to 1.3 sec ?
What are the specs on your accelerometer ?
Was this flight also on an F35-7 ?
And what are the units on the graphs ?
According to Thrustcurve.org > AT F35W, the motor mass is 85 g with 30 g of propellant.
Suggestions:
1 - record the motor and the batch
2 - weigh you motor before the flight
3 - prep your rocket for flight
4 - weigh your ready-to-fly rocket
5 - fly and recover your rocket
6 - remove the motor and weigh it after the flight
You'll be able to calculate the propellant mass ( initial motor mass) - (final motor mass) ) and then the 'coast mass' of your rocket.
Acceleration is simply an effect of Force / Mass and you'll need to calculate the Forces at play to make sense of the measured acceleration.
is it possible the rotation of the canards opens a hole where the axle goes into the body of the rocket and servos? I wonder if there is a sudden opening that causes a pressure spike, and is exacerbated by the pressure building up on the canards in their aero-braking mode. Do you only have one static pressure hole? I wonder if adding a second will help so it doesn't go over-pressure if the first one is in the direction of the wind instead perpendicular to the angle of attack...
Try just pointing a fan at your rocket's nose and rotating the fins while on the ground; that way you have a static altitude, but you can experiment with what might be causing these spikes. If the fan isn't enough, try compressed air across the nose.
E: another thought, try graphing the difference between the calculated and actual altitude. If it is pressure from increased drag, it will decrease exponentially; if it is a mechanism blocking the altimeter, it will be a straight line. Drag is a function of velocity squared, so we should see it correlate with the speed of the rocket; whereas if the altimeter is just blocked by something, the altitude difference would be a relatively consistent.
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Nv7
Well-Known Member
Yes the accelerometer is limited to 4G so that portion of the thrust curve is out of its range
This flight was on an F35W-8, I got the thrust curve in the sims from thrustcurve.org.
On altitude its meters, on acceleration its m/s^2 (SI units)
I weighed it ready-to-fly and got 552g. The propellant mass is only 30g, so I don't think it could have explained the discrepancy (although propellant mass loss calculations are missing from the sim). Earlier the flight data used to match up a lot better with the sims. However, I am confused why the thrust tapers off much later in the accelerometer data rather than the sim.
Yes, I implemented the sim myself in Scilab Xcos so that I could simulate the PID controller:
Nv7
Well-Known Member
I moved the vent holes since the last launch to be further away and perpendicular to the fins, however, there are some small holes near the fins so that I can screw in the servos. I think this is where the high pressure is coming from. I noticed that the dip is significantly less than it was last time when the vent holes were in front of the fins.
I graphed the difference but it doesn't look like any type of function, its sort of like a small hill (because it goes down when the fins start tilting). However, this estimate should get accurate as it approaches apogee because there are lots of vent holes near the altimeter and the fins would be creating very little pressure when the rocket is not moving at apogee. Yet, there is still a large discrepancy at apogee.
@Nv7 --
You won't be able to integrate accelerometer data to speed and distance traveled if the sensor cannot read real-world acceleration accurately.
As for the motor burn time ...
What is the percent difference between the F35 thrust curve and the same inflection point in the measured acceleration ?
Real world motors are allowed to vary 20% from the certified thrust curves.
What were the launch site temperature and barometric pressure when you flew your rocket ?
A cold motor may not burn the same as a motor tested on a warm day.
Next to last, the 'pressure -to- altitude' conversions assume that the atmosphere is at STP.
Look around here on TRF for posts by @ihbarddx -- Larry has done some good work on converting pressure altitude to 'density altitude' where launch site temperature is factored in.
Along the same lines, Richard Nakka's Experimental Rocketry Web Site has empirical data and he provides plug-n-play formulae for temperature correction for pressure -to- altitude.
There are a blips in pressure but they are also evident in your raw accelerometer data between 3.5 and 4 secs.
Could your canard control motor be causing a voltage drop when it deploys ?
HTH ...
-- kjh
p.s. I think your're doing a great job !
After all, this *_IS_* real rocket science your doing from scratch here
p.p.s. As has been discussed before ... if you want useful acceleration data you'll need an accelerometer that can measure higher values.
4-G acceleration is just barely on the edge of 'legal' for an HPR
p.p.s. here is some real data from a Blue Raven in two different rockets showing the difference among inertial altitude, pressure altitude and density altitude
This was a flight on Nov 4 where the site temperature was 76 F and the site pressure was 997 mb:
This was a flight on Dec 30 where the site temperature was 68F and the pressure was 997 mb:
Note how the green lines ( density altitudes ) differ from the red lines ( raw pressure altitude )
Also note that the Inertial Altitude ( blue line ) is meaningless after the drogue is deployed at apogee
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Is there a reference you could point me to that has more graphs like this ? Very informative.This is a graph of the CL -vs- AoA for a symmetric NACA 0012 airfoil:
Here you go, @Finicky
Airfoil database search (NACA 4 digit)
-- kjh
If you like history, this Order of Magnitude: A History of the NACA and NASA, 1915–1990 was kinda-sorta semi-related ...
EDIT: it bugged me to not find those two graphs.
I still can't find that specific NACA paper but this is close: NACA Technical Note 3361, Jan 1955
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Sorry for the response delay, I've been fighting a virus.
- Simulations are just that, a close approximation of what is reality. You use your real time data to make improves in the simulation calculations, not the reverse action.
- Solid propellant batch to batch burn rates vary. A ±100 ms variation in burn time is not unusual.
- Your 4G accelerometer rail severely limits knowing what is occurring above that value for burn time and velocity calculations. You need to record the actual acceleration values and not a simulated acceleration from a thrust curve.
- Check your servo response speed to understand the servo response delay. This value is given as a time/degree movement, usually 0.1/60° or 0.1/90°. My cheap SG90 test servos are rated at 0.1/60°, but my 90°movement test was 0.2/90°.
- Your canards are producing drag, just not the drag values I would expect.
@Nv7 what accelerometer are you using? I know some of the stock LSMs I've used default to a lower magnitude of measurement to get more accuracy, but I was able to reconfigure mine to do up to 12G (which is still mostly insufficient for HPR, but has done well for MPR and less). Before you go buy new hardware, check if you can modify the configuration of the accelerometer to get larger magnitudes.
Nv7 is using the BNO055 9DoF IMU. Past data had it in M4G Fusion mode. The data graph from the Feb 4 post appears to be in IMU Fusion mode.
I use the LSM9DS1 equivalent 9Dof IMU, but it does not have a fusion mode for absolute orientation. I do post flight quaternion calculations.
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