Our team is thinking about next year's TARC competition, and we're wondering which motors are the most consistent or suitable for TARC (ex. reloads or single-use/ Aerotech or CTI?). This year we used F42 motors, they can vary up to around plus or minus 0.5ish grams. Although we never got the accuracy to actually figure out if it contributed a significant variation in our flights, we want to make sure next year's motors can get really good scores. We just placed 43rd in nationals, barely missed the 2nd round (42 teams) by 0.72 points... luck wasn't on our side today.
TARC motor consistency
- Thread starter Brian2007
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Hey Brian!
Great question, unfortunately I don’t have an answer. 0.5 grams seems pretty small to me, perhaps even less than weather condition changes between launches or variation in thrust curve. Then again, I have not competed myself and I’ve only scored once.
I’ll reach out to my contact at Vocational STEAM Works Rocketry and see if he has any input from mentoring his own teams.
Great question, unfortunately I don’t have an answer. 0.5 grams seems pretty small to me, perhaps even less than weather condition changes between launches or variation in thrust curve. Then again, I have not competed myself and I’ve only scored once.
I’ll reach out to my contact at Vocational STEAM Works Rocketry and see if he has any input from mentoring his own teams.
mhanna
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I worked with a TARC team that once fired a case of motors (20 I think) on a test stand to see what the variations in thrust were. The results surprised us all - actual thrust varied very minimally but the delay on the motors varied by +/- 2 seconds from what it was supposed to be.
What motor did you fire?
mhanna
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It was a long time ago but I'm pretty sure it was a Cesaroni F59-12A white thunder.
TheAviator
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So let's break down the question a little bit here. There is variability induced from a variety of factors, but I think the one you're implicitly asking for is production variability. A list of things that can vary within a design tolerance are as follows:
In addition to these factors, there are also environmental factors that determine the performance of the motor. Most notable is the there is a direct correlation between the temperature of the motor in a pre-fired condition and the burning rate of the propellant. Consistently igniting the propellant in the same manner is the other (Cessaroni motors really shine over AT motors in this aspect).
Add all of this together with the variability in atmospheric conditions and the condition of your rocket and you'll find that they really aren't as consistent as you'd maybe like. The funny thing about it is that if you were to make the motors yourself, there's a good chance they'd be even worse! There is something to be said for commercial mass production.
One of the biggest things that you can do to reduce motor variability is to buy your motors in large batches. If you get motors from the same lot, there's a good chance they were all cast from the same batch of propellant, which means better consistency within that batch. Just make sure you order enough for the whole season!
If you really want to quantify the effect that all of this has on your performance, you need to set up a Monte Carlo analysis and do thousands upon thousands of simulations. Setting up a good Monte Carlo is a combination of good research, good experimental design, attention to detail, and patience. I think what you'll find is that the true variability of a typical TARC rocket is probably in the neighborhood of 20-30 points or more, and that reducing this variability is a possible if somewhat arduous task.
- Propellant mass
- Propellant burn rate
- Propellant density
- Geometric dimensions of propellant features
- Diameter of the nozzle throat
- Rate of erosion of the nozzle throat (Yes, the propellant can start to erode the nozzle enough to impact the performance. It's hard to measure on these small motors.)
In addition to these factors, there are also environmental factors that determine the performance of the motor. Most notable is the there is a direct correlation between the temperature of the motor in a pre-fired condition and the burning rate of the propellant. Consistently igniting the propellant in the same manner is the other (Cessaroni motors really shine over AT motors in this aspect).
Add all of this together with the variability in atmospheric conditions and the condition of your rocket and you'll find that they really aren't as consistent as you'd maybe like. The funny thing about it is that if you were to make the motors yourself, there's a good chance they'd be even worse! There is something to be said for commercial mass production.
One of the biggest things that you can do to reduce motor variability is to buy your motors in large batches. If you get motors from the same lot, there's a good chance they were all cast from the same batch of propellant, which means better consistency within that batch. Just make sure you order enough for the whole season!
If you really want to quantify the effect that all of this has on your performance, you need to set up a Monte Carlo analysis and do thousands upon thousands of simulations. Setting up a good Monte Carlo is a combination of good research, good experimental design, attention to detail, and patience. I think what you'll find is that the true variability of a typical TARC rocket is probably in the neighborhood of 20-30 points or more, and that reducing this variability is a possible if somewhat arduous task.
This is a good question. TARC is all about hitting the target value head-on. I remember one team even accounting for "atmospherics", i.e., air drag, air density, etc., by adjusting the amount of B-B's for weight.
boatgeek
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If you do decide to go CTI, you might look at the 2G 24mm motors. They're a hair more impulse than the 1G 29mm, but having two grains makes it easier to mix and match to get equal propellant weight. They also tend toward a bit higher thrust, so the rocket is a bit faster off the pad and a bit less likely to weathercock.
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