rickster75
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What is a "Grain"? When looking to purchase engines, it refers to them in quints of "Grains".
What is a "grain"
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jimzcatz
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Propellant segments.
A propellant grain is a "slug" or unit of propellant. BP motors have the propellant pressed as a single solid grain into a paper casing. Most APCP motors have several grains, each having a hollow core. The grains are loaded into a casing. The grains below are the tubes with green stuff in them and a hole through them.
One of the difficult things for people new to the hobby to figure out is the relationship between vendor hardware and reloads and the terms we use. For example, a person may say they used a 3-grain 38 mm Aerotech motor. However, Aerotech designates the cases by impulse as 38-240, 360, 480, etc.
Similarly, the reloads area not intuitively matched to cases. For example, J275W, J90W, J460T reloads all fit the 54-852 case. Without experience, or a secret decoder ring, it can be difficult to figure out. The Aerotech price list (or the old RouseTech matrix) can be used to match reloads to cases, but none of them say how many ‘grains’ are in a particular reload.
Similarly, the reloads area not intuitively matched to cases. For example, J275W, J90W, J460T reloads all fit the 54-852 case. Without experience, or a secret decoder ring, it can be difficult to figure out. The Aerotech price list (or the old RouseTech matrix) can be used to match reloads to cases, but none of them say how many ‘grains’ are in a particular reload.
gldknght
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Anyone know why they are called "grains"? Why not just call them slugs or segments?
This is a guess, open to correction.
The grain is the smallest component of a propellant mixture. Its geometry plays a key role in how the motor burns and how must thrust is produced. IOW, in a motor of a particular diameter, a grain is a cylinder of that diameter. Its length can vary, but its diameter is a constant. You can stack them together to make a multi-grain motor, but you cannot have a motor with less than one grain.
even if you cut them in half; you now have two grains instead of one grain in two halves.
I have no idea, but the term has been used by the pros for nearly 70 years.
Edit: I'd like to know why that term was selected, too.
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The Internet's ultimate authority 
, Wikipedia:
Solid propellants are used in forms called grains. A grain is any individual particle of propellant regardless of the size or shape. The shape and size of a propellant grain determines the burn time, amount of gas, and rate produced from the burning propellant and, as a consequence, thrust vs time profile.
And from lexico.com:
2.4 A piece of solid propellant for use in a rocket engine.
, Wikipedia:Solid propellants are used in forms called grains. A grain is any individual particle of propellant regardless of the size or shape. The shape and size of a propellant grain determines the burn time, amount of gas, and rate produced from the burning propellant and, as a consequence, thrust vs time profile.
https://en.wikipedia.org/wiki/Propellant
https://en.wikipedia.org/wiki/Solid-propellant_rocket#Grain_geometry
https://en.wikipedia.org/wiki/Solid-propellant_rocket#Grain_geometry
And from lexico.com:
2.4 A piece of solid propellant for use in a rocket engine.
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Edit...sorry..I read the OP question and went out to work on my truck..came in for a break and looked up the below ...
borrowed from wikipedia....
"Propellant - Wikipedia
Grain. Solid propellants are used in forms called grains. A grain is any individual particle of propellant regardless of the size or shape. The shape and size of a propellant grain determines the burn time, amount of gas, and rate produced from the burning propellant and, as a consequence, thrust vs time profile."
Tony
borrowed from wikipedia....
"Propellant - Wikipedia
Grain. Solid propellants are used in forms called grains. A grain is any individual particle of propellant regardless of the size or shape. The shape and size of a propellant grain determines the burn time, amount of gas, and rate produced from the burning propellant and, as a consequence, thrust vs time profile."
Tony
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They are called grains because before they invented APCP they used to make rocket motors out of oats and quinoa.
gldknght
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From
https://joepastry.com/2011/whats-the-difference-between-a-grain-and-vegetable/
————————————-
Grains are the seeds of grasses. ... A vegetable can be any edible part of a plant that's not a) the seed or b) the ripened ovary. The ripened ovaries of seed-bearing plants are what are known in scientific terms as “fruits”
—————————————
Thank goodness someone picked “grain.” I just wouldn’t feel good about putting “ripened ovaries” in the tail of my rocket.
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Exactly. Then, Aerotech introduced "grain" spacers to fit in "impulse" cases with little to no explanation of how this works. Horrible nomenclature and documentation by Aerotech. Cesaroni is far better in this regard.
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Makes sense to me. Didn't CTI have the advantage of looking at how several other vendors, including AT, had already done it?
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I believe CTI came out with the spacers first.
Quick quiz for you: Will a J401FJ reload fit into a 54/2560 case, and if so, how many spacers, and of what length, are required?
5...4...3...2...1...time's up.
Hint: I already bitched about this in the Propulsion forum.
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I was speaking of course to CTIs update to the multi-vendor "customary" OD-Ns nomenclature, not your bug report on AT 54mm spacer documentation ( which I believe they've already amended and published ).
Yes x-grains can be more intuitive especially in a spacer context -- still doesn't tell us what the OAL of that case is and thus whether it will fit in our MMT. Further since the length of "grains" varies by load, it's worse in other contexts.
And if we're to be making propulsion selections in five seconds or less, I'm not sure any amount of simplification will help us.
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Nope, AT amended somebody else's bug report, not mine. Answer to the quiz: No, that load will not fit in that case, even with all possible spacers. I didn't figure that out until I purchased the load, tore it open, and tried to assemble it.
A simple table, like this, would provide a 5 second answer. However, this table is incorrect in some instances and led me astray on the J401 purchase:
mikec
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AFAIK the only error on that table is that for the 54/2800 case you need the short spacer and two regular spacers to use a 54/1706 reload, and for the 54/2800->54/2560 you need only the short spacer.
It would be a lot cheaper than AP!
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No, that table is far from complete as it doesn't even distinguish between "regular" and "short" spacers. There is at least one more major error: my J401FJ experience.
The J401FJ fits the 54/1280 case. Per the table, it should fit a 54/2560 case with two spacers. Nope, not even close, even with 2 black spacers plus one silver spacer. The stackup of the motor is still an inch or two shy of fitting in the case. Another member, who apparently has the super-secret decoder ring, chimed in to my other thread that the J401 is a "3 grain" motor. Well of course, silly me!
I don't think it is too much to ask of Aerotech to make a complete matrix of loads, cases, and spacers.
mikec
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Oops, OK, you're right. The 54/2560 line should say that with 2 regular spacers you can use 54/1706 and there are no options for 1 spacer. I tried to explain the situation in your thread but I guess I didn't do a good enough job. To compute the number of grains in an AT 54mm case, divide by 426. So 54/426 is 1G, 54/852 is 2G, 54/1280 is 3G, 54/1706 is 4G, 54/2560 is 6G, and 54/2880 is a weird special case, it uses 6 grains that are longer than the standard grain length, hence the need for the short spacer. (Of course, not all AT reloads even have multiple grains -- the long-burns are single monolithic grains, for example.)
Same basic idea applies across the whole Aerotech product line: a grain in 38mm is 120 N-s, in 75mm it's 1280 N-s, etc.
Hope that helps!
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Nope, AT amended somebody else's bug report, not mine. Answer to the quiz: No, that load will not fit in that case, even with all possible spacers. I didn't figure that out until I purchased the load, tore it open, and tried to assemble it.
A simple table, like this, would provide a 5 second answer. However, this table is incorrect in some instances and led me astray on the J401 purchase:
This chart is a bit misleading. It doesn't spell out the differences between the regular spacers and the short spacer, which as others have already mentioned makes only sense in the 54/2800 casing. It also ignores that there is no 5G casing. Dropping a single spacer in a 54/2560 (6G) makes therefore no sense, except for research use maybe.
@mikec already mentioned the NS/grain nomenclature that AT uses. With the exception of a few odd ducks that diverge from the regular full grain sizes (29/100, 54/2800 and 98/18000) and somewhat imprecise rounding (54mm), AT sticks to that scheme. So understanding that is quite helpful. This is also useful to spot gaps in the lineup (neither 29/5G, 38/7G, 38/8G, 38/10G, 54/5G or 98/5G does exist) to see where spacers make no sense. Those gaps have existed long before CTI and later AT introduced grain spacers. Before that, ATs diameter/impulse naming convention made more sense for most fliers and other companies like AMW and Loki are doing the same.
CTI, by the way, has it's own idiosyncrasies. 29/6GXL and 75/6GXL are actually 7G casings requiring no special spacers. The other casings are either longer or shorter than a 7G casing would be. They amount to 38/7.6G, 54/6.9G and 98/7.5G casings. The latter casings where introduced before CTI offered spacers and where often designed with specific motors in mind.
Reinhard
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First commercial motor line to have grain spacers was CTI.
I started using them for my own motors around 2001. I found that standard PVC pipe is the right OD for both 38mm and 54mm snaping cases. I hadn't seen anyone doing this before that time, but it's possible others came up with the same idea. I've made many posts over the years explaining how to use grain spacer for EX/Research motors, long before CTI had them.
For example (2006): https://www.tapatalk.com/groups/tqc/case-question-t1229-s10.html#p13205
Due to the way Aerotech's casings compress an o-ring on the case of the closure face, the length of the spacer is more critical. On snapring cases, the radial oring still seals if there's a gap.
The largest grain spacer I've made (or anyone has made?) is to make my 8" hardware reduce from an R to a Q. Turned down from 6" schedule 80 drain pipe couplers (very thick):
https://photos.app.goo.gl/U4txfxAQX3Hf7UJq8
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