The discussion revolves around the possibility of determining the terminal velocity of a model rocket based on data collected from a launch to 20,000 feet. Participants explore methods to analyze velocity data, calculate drag coefficients, and the implications of different phases of the rocket's flight (ascent vs. descent).
Participants generally agree on the need to analyze the data carefully, but there is no consensus on the methods to determine terminal velocity or the implications of the rocket's orientation during descent. Multiple competing views remain regarding the feasibility of calculating terminal velocity during different flight phases.
Participants note that the rocket's drag coefficient may vary between ascent and descent, and the limited data available from the coasting phase may restrict the analysis. There is also uncertainty regarding the accuracy of the reported launch height and the implications for the calculations.
You are going to make mistakes, and you'll learn. Get ready to swallow your pride. Thats probably universal for any young engineer.LT72884 said:
This is why scale is really important. How large are these fluctuations when you step back and examine it. You are taking measurements with some sensor, that has some error. You aren't going to get perfection. How much does the average ##\beta## change from 50 -100 , vs 100- 150, vs 150-200? How much is the ##C_d## based on the average for each range?LT72884 said:
I agree about the pride. That is why im REALLY grateful that you showed me ALL the steps. I do not expect that from everyone or anyone, and when i have no single clue what i am doing, its so nice to see an example ALL the way through. That is how i learn. step by step and examining each section.erobz said:
beta changes a lot from 50 to 100m/s but then the system settles around 200m/s. The Drag Coef seems to be an average of 0.6 from 150 to 200m/s.erobz said:
That looks like what I'd expect. In my experience (having done similar Cd estimation on some of my rockets), you get pretty noisy data but the higher speed average value ends up being pretty reasonable (and the 0.6 that you found does appear reasonable to me).LT72884 said:
so what is happening at 200m/s with this graph? thats where the system settles too. Is this terminal velocity?cjl said:
The graph finds ##\beta##. Now, assuming that it is a constant, to find the terminal velocity you have solve the eq in post #40.LT72884 said:
"To do that: What is the acceleration of the rocket at terminal velocity?" Well, my thinking is either 0 or 9.81 since terminal velocity is where Fd and Fg equalize. So probably 0 thenerobz said:
Actually, I was being a bit hasty. You need to write the DE for the falling rocket first, and then do what I said.LT72884 said:
ok, cool. I have noticed i do not have 0 in my data for acceleration so im guessing its when acceleration is a different value.erobz said:
Its similar. But you have to think about the directions of the forces when its falling.LT72884 said:
You are not going to find terminal velocity in your data. Why do you think you would?LT72884 said:
somewhere along the line we were discussing finding the terminal velocity. We discussed how its exponential and if i integrate the equation we have, i do indeed get the terminal velocity equation inside. Somehow i thought we were finding both the Cd and terminal velocity thats why haha. but maybe somewhere we changed our minds because hte math gets to crazy. You also said something about how you think Vt is around 100m/s or so...erobz said:
Firstly, that is a different equation altogether. If you want to solve for the velocity as a function of time you will have to solve the new differential equation (that you haven't yet derived like I asked)LT72884 said:
there are other Vt equations as well i could probably use.erobz said:
Correct. It wold be in there if you dropped you rocket from 20k ft, but you launched it to 20k ft , and ejected a parachute.LT72884 said:
Also correct, the β values fluctuate around because of the instrument you used. This is the difference between Accuracy and precision. If you want both in an instrument you usually have to payLT72884 said:
. If you want accuracy,precision, and range .The hope in using the average values was that the “truth” was somewhere in the middle.ok, thats what i thought. I told that to my professor and he didnt have time to think about my response during the team meeting and he looked confused haha, but i re-assured him that a $80 sensor may not be the most accurate and precise.erobz said:Also correct, the β values fluctuate around because of the instrument you used. This is the difference between Accuracy and precision. If you want both in an instrument you usually have to pay
Correct.LT72884 said:
i think so haha. I just got home from my 3 hour train ride from school.erobz said:
LT72884 said:
The biggest problem with that is that 600m/s is well supersonic, and thus your real terminal velocity would be lower thanks to the drag rise that occurs at mach (as seen in my sample data above). I'd also note that your very high sectional density is actually making your data a lot muddier - since you're limited by accelerometer resolution, you'd get much better data with a rocket with higher drag relative to weight, but obviously that goes against the general design goals here.LT72884 said:
Newton’s 2nd Law for the rocket in free fall.LT72884 said:
My friend (someone from the rocket forums) used an L1030 motor.cjl said:
well, no time like the present. Lets give it a go.erobz said:
where did you get ##kv##? Do you not remember the whole thing was about quadratic drag?LT72884 said:
just from a basic FBD. so technically it should be kv^2 or the Bv^2.. i did forget about the quadratic drag hahaerobz said: