mardlamock

This is what I did. http://imgur.com/4I3cFUB. Im using Newton drag, an exponential model for atmospheric density, thin airfoil theory to estimate the coefficient of lift as a function of the angle of attack. I do account for area difference, the area being an orthogonal projection. http://www./download/led0x2ubszo7avv/10740687_10203439585645884_287536767_n.xlsx This is the older excel spreadsheet I used, I know changed it for component drag and on the side im doing Runge kutta, and yes you can do 3rd order, 4th is more common though. As this particular equation is a stiff equation I might need to use a higher order Rk, but It doesnt hurt to try with 3rd first. As you may see on the side, the whole idea of this is to then detach the fin orientation from the rocket orientation and test dead reckoning and active stabilization systems.

Exactly, I figured that determining the moment of inertia would be better done with real life measurements as opposed to doing it analytically. The behaviour I am not seeing is the ¨Path-Bending¨. I understand why it would happen if the lift force exerted translational acceleration on the body, but that is not the case with a rocket, therefore, the oscillations are not the result of that. Having changed the drag and lift to be dependant on the total velocity and component velocity, the oscillations are still there. Im doing switching to a 3rd order Runge Kutta but its taking been taking me some time. I still believe it is a problem inherent to the use of euler´s method to approximate harmonic motion.

I never said that, I just arbitrarily added a 1/12 moment of inertia constant and then used that. No matter what the moment of inertia is the rocket will tend to stablity if the model is correct, only the time it takes will change. - - - Updated - - - Im not seeing any of the behaviour you described, and im sorry if this sounds rude, but even though the drag breakdown makes sense, the oscillations you talked about dont (at least tome=. I sent you a message to see if you want to talk about this in a much more dynamic fashion and maybe see if I can help you with your model and you with mine.

Of course I dont need an arduino, but it would add to the fun. And imagine the possibilities, if you ve got a working dead reckoning unit, you can strap on some servos on the sides and stabilize the rocket, or even better, guide it! You can have ascent profiles that keep it within certain boundaries, return to launchsite, or you can just plat around with it. I do it mainly because I one day want to work in aerospace, and for the time being, this is the closest i ve got. Besides, it may (though im not too sure) help me get admitted i to college. I have the permission of the owner of the field i am launchig it from, its in the middle of nowhere, and the airspace regulations are sort of greyish regarding rockets. Putting an arduino with dead reckoning and stabilization might violate some international arms regulations, but i havent found any conclusive evidence of that and i am a long way from actually doing that. - - - Updated - - - Thanks for the links, ill be sure to check them out. I too have been watching a lot of yt videos of kalman filters and whatnot, its reallt hard, but i sort of get it. Problem is, i might have some intuition on how and why it works and the formulae, but i cant even get my l3g4200d to print raw data. Thanks a lot man!

Its 10cm in diameter, 120 cm long, propulsion section goes from the bottom up to 55cm. It uses a J/K (I have to do a few static tests to get some numbers on the propellant specifications and grain design) class PVC R-Candy engine, BATES grain, with a total propellant mass of 1kg. The first rocket will go up to around 6100 meters, (not exact because I still need to do the full static test I mentioned) and 340m/s (though Im not taking into account drag coefficient variations, so it may be less), and use a delay charge to release a parachute, though I wouldnt put much faith in it and personally believe it will spectacularly lawn dart. As the motor diameter is smaller than the airframe, in case of a CATO it will absorb the pressure. The mission is to test the airframe and propulsion, so that i dont waste an arduino in a spectacular explosion.

Yeah, I believe that for my rocket's flight time it would be ok-ish, not that much drift from the gyro and the errors in the accelerometer wouldnt have that much of an effect MECO would be somewhere near the 3 second mark, apogee at 40. I will just have to test and see how it works. Im using an l3g4200d gyro with I2c interfacing and an adxl345 which can sort of make it up to the gees im going to be running into (10 gees). Any good textbooks or resources you might recommend to me when learning about this sort of stuff?

Crap, I had no idea you couldnt separate the components like I did, it just seemed like a reasonable thing to do. Could you draw something for me to illustrate what you mean by saying the drag force is applying a torque on the craft? What I understood in your comment is that when there is a horizontal drag force being applied to the craft, it will always generate a torque and no translation due to a missplacement from the center of mass, therefore doing what I did and breaking the translational motion taking into account sideways drag is wrong, because the drag wont generate any translational movement. What I fail to understand though is how, without me having done it, that torque is being taken into account in the equations I formulated. http://i.imgur.com/4I3cFUB.png Here is a pic with a bit of explaining of what I thought. I will change the model and see if it removes the oscillation, but I still dont see quite how. Oh and btw, the units arent broken, its Force/(initialmass-massflowrate*t)-g. If you dont mind me asking, what where you building a drag force model for? Anyways, thanks a lot man, you ve been extremely helpful!

What I was saying was just a wild guess, I'll try to build something and see if by reducing the friction and adding more mass I could get it to work, I doubt it though. Without using multiple accelerometers it would be impossible to distinguish angular acceleration and regular translational acceleration, and even during the coast phase of the ascent, you'd still get problems if you used a constant approximation for gravity, because the rocket would also be experiencing drag. I guess that you could say that during the time the engine is active the orientation would remain pretty much the same as during the time it is in the pad, which would in turn (so long as the engine cuts off very rapidly) mean that that accelerometer data could be used for dead reckoning. You could then use the information you got during the ascent phase (integrated to get velocity and position), to estimate the drag, subtract it from the current readings, and then estimate orientation. That would be tricky though, as soon as I get the model im using working with RK I will tell you if it could be used or not. What are your thoughts on that?

Yeah, thats exactly what made me think of saying bye bye to the gyros and just using gimbals. If it doesnt work with translation then I probably cant use it at all, as I am building it for a rocket. My electronics knowledge is poor to say the least, I understand what I need to do with the data from a gyro or an accelerometer, I just dont freaking know how to get it. What would be a good resource for a complete newbie wanting to learn how to decently work with an arduino for the project above? I know its probably way above my level (turning on a light with a button is already above my freaking level), but im still interested in how it could work. I ve been trying to develop a model for the rocket's flight and then experiment with that by simulating an arduino using dead reckoning to estimate position and actively stabilize the rocket. I'd get the data from the model and feed it to a "fake" program with added errors, the program would then execute commands which would reflect upon the model, testing how useful the program is or isnt. I dont know how feasible it is with my little understanding of math and electronics, I am still in high school, know a decent amount of calculus and just a little bit of differential equations. I understand the different numerical methods, but I have no idea on how to use them say on Matlab, or how to data I need from my arduino board. Anyways, thanks a lot man, tell me what you think! - - - Updated - - - Not if you used 3+ gimbals inside one another, only problem would be the gimbal lock. The question is, can you get the friction between the gimbals to be so low that you dont need gyros to keep it pointed in the same direction? And I have no idea, hence the thread.

What I posted above, a free floating platform resting ontop of properly lubricated gimbals with an arduino+accelerometer in the middle, for the time being I dont need gyros.

They are cool, but I cant get them to work properly with my arduino. Not talking about the mechanical gyros, but electronic ones. I could use 3 mechanical gyros to stabilize the platform, but I was just wondering if without them, and using proper lubrication, you could get acceptable results.

Yeah, but I thought it would be easier to use a gimbaled platform and just an accelerometer if i want simple dead reckoning, that way i dont have to deal with freaking gyros (I hate them, or better said, I dont understand solid state gyros interfacing with arduino)

Hello, just a random question i had today, is it possible to build a gimbaled platform such as those used for Inertial measurement units without using gyroscopes to keep the platform stabilized? It would have to be lubricated pretty well, but i dont see any reason why it couldnt work, what do you think? This is what i am talking about https://www.hq.nasa.gov/alsj/lm_imu.gif Thanks!

Cool, less work to do if I only need 2nd order RK. I havent tried linearizing it but i will have to learn what the hell that is and try it out (still in junior hs, any good resources you might recommend). The problem seems to become apparent when the velocity reaches speeds below 50m-s (that is it is launched somewhere close to a 90 degree angle), the rocket then has to do a pretty large turn to start facing the velocity vector and it just fails and flops all over the place. Changing the moments of inertia to something realistic could help, but I dont have any flight data to compare it with. Tomorrow I´ll try doing the 2nd order RK and see if it is any better, thanks a lot man!

Thats what I did, http://i.imgur.com/bXkdEyH.jpg I used thin airfoil theory to estimate the lift coefficient. By making the change in time smaller I was able to get almost rid of the osicllation but it is still there (+- 1 degreee isnt that bad IMO). https://cdn.fbsbx.com/hphotos-xpa1/v/t59.2708-21/10740687_10203439585645884_287536767_n.xlsx?oh=223ccd3ae4dae86deba3711280b4af32&oe=54A5B90E This is the excel file, sorry for the weird colours and ....ty colour coding, im terrible at making things look pretty (or work at all). Getting a 3rd or 4th degree runge kutta will take me a lot of time, but it may be worth it.

http://latex.codecogs.com/gif.latex?%5Chuge%20x%27%27_%7Bj%7D%28t%29%3D%5Cfrac%7B-%5Cfrac%7B1%7D%7B2%7D*A*C_%7Bd%7D*1%2C225*e%5E%7B-x_%7Bj%7D*10%5E%7B-4%7D%7D*x%27_%7Bj%7D%5E2+F*sin%28%5Ctheta%20%29-g*%28M_%7B0%7D-M_%7Br%7D*t%29%7D%7BM_%7B0%7D-M_%7Br%7D*t%7D http://latex.codecogs.com/gif.latex?%5Chuge%20x%27%27_%7Bi%7D%28t%29%3D%5Cfrac%7B-%5Cfrac%7B1%7D%7B2%7D*A*C_%7Bd%7D*1%2C225*e%5E%7B-x_%7Bj%7D*10%5E%7B-4%7D%7D*x%27_%7Bi%7D%5E2+F*cos%28%5Ctheta%20%29%7D%7BM_%7B0%7D-M_%7Br%7D*t%7D - - - Updated - - - I ll be posting a the rest in a few minutes

Soo, I ve been trying to add orientation to my model of the flight dynamics of a rocket but I ve been running into a lot of problems. I didn't bother actually doing the math for the moments of inertia and everything because I guess it really doesnt have that much of an effect on the general behaviour of the rocket,but rather simple specifics of its movement. The problem is the following, when the rocket is offset from a perfect 90 degree angle at launch, it gains horizontal velocity faster than vertical (due to gravity) and therefore acquires another offset form the launch angle (say it was 89 degrees). The fins of the rocket should stabilize it by using the lift gained from the small angle of attack and point it back towards the velocity vector, this is what produces the gravity turn we all know and love (I think, I might be wrong though). In my model though, the rocket fails to stabilize properly and starts rotating uncontrollably after a certain amount of time. At first it oscillates as a pendulum would,but deviating just a bit more with each period and gaining more and more angular velocity. Is this something that naturally arises when using euler's method to analyse this type of motion or would it also happen if I were to use Runge-Kutta's? I was thinking it may be something that naturally happens when using discrete time to analyse something that would be continous. By reducing the time step I was able to increase the time it would take for the deadly rotation to take place, but im running out of memory on excel (Its waay easier than using matlab or anything else, at least for me). Anyways, please tell me what you think, tell me if you want the sheet to see if there's anything wrong, and if you need i ll draw some pictures on paint to better illustrate the problem. Thank you very much!

Thanks man!

Who? Me? Impossible, i would never try such a thing! Ehhmm it seems the problem has solved itself, no need to get violent or anything. Now, would you mind if i asked you how you calculate the moments of inertia? Im building a model for a rocket's movement with atmospheric drag lift and everything but i cant get anything done with orientation without the moments, and for the time being i have no idea how to calculate them. Would you mind referring me to some book or anything i could use?

It doesnt seem to be that IMO, ive been playing with FAR for a long long time but this is the first time this ever happens to me. http://imgur.com/ImGLC39,iQ4uXYV,dqgCrvU,wniQcX7,asBQ6qk,fIOXU4C,6GyDrsj#1

Am i the only one getting some ridiculous aerodynamic effects when crossing the 200m/s mark? Every plane i ve flown just flips upwards and enters into an uncontrollable spin for no apparent reason

http://www.nbcnews.com/watch/live-video/watch-live-officials-discuss-virgin-galactic-s-spaceshiptwo-crash-322485827772 Virgin is doing an awful job at explaining what happened, i guess they dont want to say too much about it.

Ok, ill try and to that. Thanks for the info, i hadnt really understood what paul had originally said. And for the time being i am not goint to get into a 3d analysis, that would be way too much for my small brain. Im hoping to use this model to try and test how self stabilization software would work and see how accurate dead reckoning could be. Thanks!

Hello everyone, I've been trying to build a decent model for the flight of a rocket but recently I started having a lot of problems calculating the angular acceleration. The pivot point for the rotation will be the center of mass,which changes as the fuel burns out, I found a decent way to calculate where the center of mass will be, but the other thing needed to calculate the angular acceleration is the moment of inertia. How would one go about calculating said moment if the center of mass changes as a function of time? Would an approximation using the regular 1/2*m*r^2 formula be decent enough? Anyways, sorry if its a dumb question, its just that I really need to know if my rocket will be stable. Thanks!

I live in a large city as well and I do my tests on my building´s rooftop, if you keep your designs simple and not too ridiculous then the chances of a CATO are very low. I ve had my motors fail 6 feet from me and nothing happen, so long as you know the estimated pressure is not going to burst the pvc you are alright.