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Important data missing from .eng thrust files?

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OZ1SEJ

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I'm writing a model rocket flight simulator, and I've just finished a module that reads and parses .eng thrust files from e.g. thrustcurve.org.

It suddenly occurred to me that the .eng file format (http://www.thrustcurve.org/raspformat.shtml) only contains "total weight" (i.e. before firing) and "propellant weight", which, on the page I just linked to, is defined as "the weight of all consumables in the motor".

This is horrible in itself, because it makes it impossible to know how much propellant is in the engine, thus rendering it equally impossible to calculate the specific impulse.

The problem doesn't stop there, though. If for instance I look at the Estes A8-3 engine, the data sheet (http://nar.org/SandT/pdf/Estes/A8.pdf) reads

Initial mass 16.7 g
Mass after firing 10.2 g

-which should give a value of 6.5 g for the data file (http://www.thrustcurve.org/simfilesearch.jsp?id=1251). However, in the data file, it says 3.3 g, which the data sheet states as the mass of propellant!

Conclusion:

1. The data definition for .eng files is insufficient for simulating accurately the flight of a rocket, since is it not possible to extract propellant mass from total mass,

2. This definition isn't adhered to in at least one case. How many others might there be?

What is one to do in this situation?
 

Buckeye

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Welcome to the world of model rocket engine files. They can be maddening but are perfectly good enough to do hobby model simulation.

As to your question #1, I am pretty sure you don't need specific impulse to simulate the flight of the rocket. All you need is thrust, time (ie, total impulse), initial and final masses. Many people have written perfectly good flight simulators with eng files, myself included.

90% of the consumables are probably propellant anyway, so the associated mass errors in your analysis are likely small.

Edit: Try the .rse file format. They have more detailed mass info.
 
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markkoelsch

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You are wrong. From a simulation standpoint you want to know the weight of the consumables and the total weight. These are the numbers you need. You want more data for your sim you will need to recreate the wheel. Enjoy yourself.

Total propellant mass is not the only thing consumed.
 
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GregGleason

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research and rewrite one's own database, fitting to their own needs?
This is the way to go. It's likely that whatever you are designing you want to test and run with a smaller pool of motors anyway.

Plus, will your program account for statistical variances found in each motor?

Greg
 

Peter Olivola

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A classic case of over engineering. The variation in motors exceeds to accuracy of any/all simulations.
 
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