I'm a bit confused? If the rocket is vertical with some wind, is there not an AOA in that condition (along with a horizontal component to the flight path)? A vertical flight path would be a higher AOA?
Your question got me thinking about my three-stage strategy of putting stabilization under the second stage motor. I had thought that having the stabilization would allow a lower speed at second stage ignition. But, when stabilization stops and the motor lights, the rocket will weathercock into whatever wind there is, depending on the relative velocities, defeating the purpose of stabilization. I don't think I can let it slow down all that much.
Jim
Just because the rocket is pointed straight up does not mean that its velocity is in that direction.
In any case, vertical might not be the direction you want to go. Based on preflight simulations using the measured wind profile you would set a launcher azimuth and elevation to achieve the desired landing location. The guidance system would then maintain the angle set on the launcher.
Umm, hold on. If the guidance is pre-set on the launcher, at an angle (let's say 10 degrees from vertical, pointing East), then if the rocket rolls 180 degrees, it is going to fly to the WEST at 10 degrees from vertical.
I know Jim is talking abut adding roll control. I am concerned it will over-control in roll, there is a lot of forgiveness in the system for pitch/yaw guidance that is a bit too responsive. But if roll is too responsive, it will over-control, and can easily lead to rolling so far so fast that it locks up and can never stop the roll. I had that happen on my first test of roll control with a sunguidance rocket. It turned out that the vanes were too big, and there was a lot of deflection angle. But then that rocket did not boost very fast either, and aerodynamic forces increase with the square of the velocity.
So, until the behaviour of the roll control system is proven, one cannot assume the rocket would not roll 180 degrees.
Jim, for your roll control, you might consider using only two of the four canards to control roll. I think the rocket is not going to need much deflection angle for roll, compared to pitch/yaw at the velocity your bird will be flying. Let's say for example, one degree plus or minus for al four canards. How well can the servos and mechanical "slop" in the control system handle angular resolution that low? THere's going to be some amount of error or slop. But if you used two canards, and not four, then you could make those rotate twice as much (2 degrees in this theoretical example), so that the extra degree is free from the resolution and slop contained to some extent, in that first degree. If you get what I mean (and i'm sure you do).
However, I do not know if it is practical for the controller's software to control only two for roll and not four for roll. Probably is possible, but may be more complex to find out (though with the designer helping to progrsm it, that should not be as much of an issue.)
Now a wild card in this is how well the Flight Controller handles it. There is term called PID which involves three factors for the feedback and control loop:
https://en.wikipedia.org/wiki/PID_controller
I have barely touched on that myself. But if tuned well it may handle high speeds better without over-control. But there will be some speed beyond which it would over control. It needs far less control for roll than it does for pitch/yaw
Oh, the roll guidance probably should be active from liftoff. I forget if that is planned or not. If the roll guidance was not active until pitch/yaw was active, the rocket would probably already be rolling a LOT. It would take several precious seconds to stop the roll. Now, Pitch/Yaw guidance does work when the rocket is rolling, but there is a certain amount of roll (degrees per second) beyond which the system (mostly servo speed) can not keep up.
BTW - what UhClem said abut angling the rocket for launch, I hep he has misunderstood something. The procedure for a guided rocket like this should be something like:
Rocket on pad, everything prepared for flight except for guidance (and probably not arm ejection or staging system yet). Angle rocket vertically. Turn on guidance system so it can initialize and know where vertical and horizontal are. THEN angle the launcher however many degrees, say 10, in the desired direction for an impact zone if it goes ballistic. And do any final arming of anything else, get way, and hopefully the bird will not wait long time wasting batteries before it is launched (Generic safety issue, no rocket depending on electronics for a safe flight should be forced to wait a long time, but also the battery capacity onboard should be overkill and freshly charged or new).
As for angle of attack, AOA, a vertical rocket that is pointing vertically and is thrusting vertically, should have effectively zero angle of attack. Now if it took off in a 10 mph wind and was trying to steer vertical from liftoff, sure, at first it would weathercock some until the velocity was enough for the controls to be effective. It would begin to pitch up to vertical. By the time it would then become vertical, it is NOT flying a vertical LINE up at 90 degrees (unless there was no wind).
It is pointed straight up, flying vertical through the AIR...... so the vertical model and the air mass are moving together downwind at 10 mph (OK technically the winds tend to be higher at higher altitude, but this is for example). Think of people riding in a hot air balloon. The wind is 10 mph. But shortly after the balloon is in the air, drifting at 10 mph, the people in the gondola do not feel any wind at all. To them, there is no wind. This is ignoring wind gusts and the balloon moving up or down. Another classic example would be an aircraft carrier sailing downwind at 10 mph, to the sailors on deck.... it feels calm (again an ideal 10 mph wind, ignoring gusts and such)
So the path of motion is sort of a steep diagonal line, whose angle is relative to the windspeed versus rocket speed. I say sort of since the faster the rocket gets relative to the wind, the more towards vertical the line would become.
I have seen some vertical guidance flights in wind, models with long burn (8 sec) low thrust engines flying at probably 50-80 mph, and that is exactly what they look like, model vertical, with model and smoke under it vertical.... drifting downwind on a flight path that to a ground observer looks steeply diagonal, but to a passenger in a nearby (not too close) hot air balloon it would look vertical.
So, Jim, that approximately 8 degree off-vertical path. Was it tilted towards the wind drift direction? I am SO hoping it was.
- George Gassaway