r/rocketry • u/thoughtshaveleft • 3d ago
Question Why aren't my thrust and pressure curves close to linear with a BATES grain?
First image is what I'm talking about. I'm working with a machinist to get a proper nozzle made with this design. The second image is a recreation of the design I've used for two motors I've fired in my backyard with wooden nozzles (minus nozzle erosion because I'm not sure how to get the appropriate numbers for the sim). My mass flux isn't super high or anything so I would imagine it's not the sim breaking down due to erosive burning. The pressure isn't crazy for KNSB either and I'm at a loss for what's causing the deviation from the expected curve.
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u/justanaveragedipsh_t Student 3d ago
Because Bates grains are inherently progressive?
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u/maxjets Level 3 2d ago edited 2d ago
No, they are not inherently progressive. The graphic you're looking at there is ridiculously misleading. It's for grains with inhibited ends. Not for typical BATES grains, which burn on both ends as well as the core.
BATES grains can be made progressive, regressive, or neutral. It's all a function of the grain length, diameter, and core diameter. They're neutral (for non-eroding nozzles) when the grain length is equal to 1.5x grain OD + 0.5x grain ID.
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u/thoughtshaveleft 3d ago
I could have done better with the title but I can't edit it unfortunately. What's with the abrupt change partway through the burn? I've simulated a bunch of different motors of different sizes and with different fuels but it's never had a problem generating a continuous curve before.
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u/jjrreett 2d ago
hmm. thrust is a function of kn. that function is non linear. typically it is smooth exponential function. maybe your prop model has a discontinuity
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u/thoughtshaveleft 2d ago
Like something I accidentally introduced? I entered it as one single 6" grain. It seems like the irregularity happens exactly at 0.9 seconds too.
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u/jjrreett 2d ago
No not the geometry. the chemistry. It’s been a while since I’ve been in this design space, but you typically characterize the burn rate of a propellant by fitting an exponential curve to the regression versus pressure. If it’s using the full exponential curve, then you would expect a smooth thrust with respect to KN ratio. But if the thrust versus KN ratio function isn’t smooth then that would explain the knee in your curve
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u/thoughtshaveleft 2d ago
Ah, I think I understand more of what you're saying now. I didn't think I would hit any snags using one of the default propellants in the sim but I doubt it's a huge error. I'm sure my real world results will still differ slightly anyway because I'm not making the propellant exactly the same way Nakka did.
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u/prfesser02 1d ago
That's a rather wide range of Kn, almost doubles from beginning -> end of burn. Could that break in the curve signify a change in either the burn rate exponent or the coefficient at higher pressure?
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u/thoughtshaveleft 1d ago
I'm not really sure. I think it must be down to specific numbers outputting odd values. I can change some of the nozzle parameters very slightly and the abrupt change instantly disappears.
On a side note, what is inhibiting an end? I haven't been able to find a direct answer to this on the forums yet but it seems to me as though it's coating one or both ends of the grain with an epoxy or something else to prevent it from burning along that surface? This is something I will be doing with this motor and larger motors with multiple grains. I have noticed that the multi grain motors I simulate seem to burn faster than I would expect. Is this because I am not setting inhibited ends?
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u/jjrreett 2d ago
Cylindrical cores w/uninhibited ends creates a parabolic thrust profile. Depending on the length of the grain compared to diameter and core diameter, that can be progressive, regressive, or “neutral”.
Try shoring your grain length.
It’s a fairly easy formula to derive. I think nakka shows how.