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OK then; sunlight it is!
OK then; sunlight it is!
You are making this much too difficult. So far as Rocksim is concerned a rod/rail is an all or nothing thing. If you tell it the rail is 8' then after the rocket moves a simulated 8' Rocksim assumes that the rocket has left its rigid guidance. So use an appropriate distance.I also wondered about the position of the rail buttons on the rail, but there's no data entries for this with Rocksim and I don't know what is used in the program.
OK, I appreciate the help on this.ou are making this much too difficult. So far as Rocksim is concerned a rod/rail is an all or nothing thing. If you tell it the rail is 8' then after the rocket moves a simulated 8' Rocksim assumes that the rocket has left its rigid guidance. So use an appropriate distance.
OK, I appreciate the help on this.
I re-ran the sim at 100 F and 88.5" for the launch rail. This is how far the bottom button traveled from start to the top of the rail and the sensor.
The sim shows 107.4 ft/sec for launch guide departure.
If I use the mid-point between the two rail buttons at 78.5" the launch guide departure is 101.4 ft/sec.
I guess the point is, the simulation is about twice the velocity as the sensor reading indicated.
I would like everyone to help with this and appreciate the input so we can determine a standard of testing and continue to understand why it would be different.
RockSim simulation details states that user specified minimum velocity for stable flight is 44 ft/sec and this should have been achieved at 17.2". This didn't happen.Rocksim can tell you the distance the rocket travels to meet your 50 f/s condition - so does that help you with your calculations?
A very useful test would be 3 flights in a row using the same rocket and motor and comparing that data, rather than using a different rocket and motor.
Easiest and best way (IMO) put two switches about 3-6" apart at the end of the rail and measure time it takes between switch actuations. The switches could be activated by the rail button (microswitch) or if you want get fancy put a magnet on the airframe and use 2 hall effect switches on the rail.
See graph in previous post. Teensy 3.6I wonder what it would take to put a kilosample/sec acceleratometer on the rocket? That really should do the trick.
Rocksim takes the thrust curve and applies that. F=ma. Where F is the sum of all forces. You can plot both thrust and acceleration where (depending on the thrust curve of course) you can see where there is zero acceleration but non-zero thrust.But, does RockSim take into account that as soon as the rocket goes over 1:1 weight to thrust that rocket is going to start to move?
A very useful test would be 3 flights in a row using the same rocket and motor and comparing that data, rather than using a different rocket and motor.
What other information should I be noting during the test? For the simulation, what should I enter for the rail length? Are we good with the midpoint between the buttons to the top of the rail?
Until then, I suggest starting to consider the possible reasons why the sim and actual would be different.
That, sir, sounds like an amazingly good point.I think the biggest source of difference will be between how your actual motor is starting up vs. what the thrust curve says it does...
OK, the Teensy line looks nice, and certainly faster than Arduinos. However, an Arduino is fast enough for 1 k sample/sec on an analog input. The trick is finding an accelerometer with an analog output (since that can be sampled at an arbitrary rate, unlike modules with serial outputs) and capable of the the peak acceleration that would be needed.See graph in previous post. Teensy 3.6I wonder what it would take to put a kilosample/sec acceleratometer on the rocket? That really should do the trick.
Dave likes to be clever. His answer is a link to the correct graph, which is post #51:OK, to which "graph in previous post" are you referring?
Thanks. UhClem, would you be so kind as to share the raw data?Dave likes to be clever. His answer is a link to the correct graph, which is post #51:https://www.rocketryforum.com/threads/launch-rail-velocity-simulated-vs-actual.160656/post-2032952
Not rail friction, but see below.That said, it doesn't surprise me that the real world number is slower than the sim. I would expect the sim to be a best case number, since it likely doesn't account for rail friction, slower ignition, etc. But, it does raise a lot of questions if you are consistently off by orders of magnitude.
Predicted, yes that makes sense. But the published curves are from tests. While ignition timing down to milliseconds could surely introduce variability, the published won't be consistently better than any other test run (including test runs where the test bed with fins and stuff moves) as sims might be.In my experience with static testing motors (load and pressure), the real-life curves never look as good as the published or predicted curves, especially in motors with 2+ grains.
It's big. Over 30MB zipped so if there is something in particular you want maybe I can cut that down a bit.Thanks. UhClem, would you be so kind as to share the raw data?
If you're saying that the thrust curve published by a manufacturer won't be consistently better than an 'in the field' test run, I might disagree. I have no doubt that for a qualification test, the manufacturer ensures that a fresh motor is used, from a known good lot, and the best possible igniter is used. These might consistently provide better results.the published won't be consistently better than any other test run
The first quarter second after T₀ should be plenty. Do you know why these happen?It's big. Over 30MB zipped so if there is something in particular you want maybe I can cut that down a bit.
I may be mistaken but I think the certification thrust curve is taken at the 5% and 95% levels of the burn. Any anomalies before the 5% point is ignored an not shown.If you're saying that the thrust curve published by a manufacturer won't be consistently better than an 'in the field' test run, I might disagree. I have no doubt that for a qualification test, the manufacturer ensures that a fresh motor is used, from a known good lot, and the best possible igniter is used. These might consistently provide better results.
If you want to compare your video data then the stripes should be at the same location as the buttons. If you want to watch for roll, you need a roll pattern.
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