Grain spacers

I have been meaning to mention this about Loki Research motors for some time now. I came across a thread showing a photo of the assorted parts of an AT J-510 motor. One comment asked what all the small o-rings were for. If this were a CTI motor, it might have been more obvious since they are much more common with that product line.

Well at Loki Research, I started to go down that path at one point and I even bagged a few 54mm reloads at one time with them. In the end, Loki Research does not use o-ring grain spacers on any reloads. I do something that I feel is better.

Grain spacer o-rings are very thin, and at least on snap ring sizes in 54mm, I've had more than a few that wanted to roll up between the grain and the liner which results in a stuck grain. They also take away 1/16" of propellant space for every o-ring used. Call me stingy, nit-picky, whatever, but I'd rather put propellant in a motor rather than empty space. Sure, the performance loss isn't great, but high performance and low cost was always one of the things Loki Research set out to deliver back in 2004-2005.

So instead of using o-rings, here's what I do. Every grain at Loki Research from the 38/1200 and up is cast and cut by me, by hand. The only slightly automated process is done in the mixing of the propellant and the cutting of the 38/120 thru 740Ns grains. Since these are all "hand made" reloads, it is quite easy for me to take a little more time and cut a chamfer into the face of each propellant grain. The loss is about 1 gram of propellant for this sliver thin slice, but honestly I'd have to go back and look if that was in 54mm or 76mm. I'm pretty certain that was the weight taken from these 54mm grains. 1/16" of propellant is about 4 grams. So 4 grains, 8 faces for this reload would equal 32g of propellant loss (~64Ns) for o-rings, or 8g of propellant loss (16Ns) for cutting in chamfers.

Pictured below are 4 grains from the 54/2800 red reload that is awaiting testing certification. There is a1/4" wide phenolic flat edge running across the center of all 4 grains with a bright light held behind them so you can somewhat see the chamfer in the grain faces. It was rather hard to get the camera to capture this as good as the eye can, and the 1/4" wide flat edge hides more of the light at the OD of the grains. Only a tiny gap is needed for the flame front to reach the surface of the grain faces, and this is only showing one of two sides. Mate two of these together and you have plenty of a gap for fast, complete ignition, but with about 1/4 the loss of propellant and Ns over using o-rings. On this size motor with around 3,300Ns and 1,684g of propellant, that's a 2% loss loss compared to a .5% loss. To me, every little bit counts.

Not having grains spacer mean there are several less o-rings for you to worry about doing the right/wrong thing with. It means a slight performance edge, and in the case of Loki Research, it means you have a hand made, custom reload that will be a joy to assemble and awesome to watch fly.



Thank you,

Scott,
That is quite interesting that you cut the grain ends to provide for a flame space. I too have been casting grain for over 10 years and I found it comforting that you post this information. I cut my grain by hand too, and often thought that having some slight imperfections in the ends would actually be a benefit as the grain don't stack together closely, leaving room for the flame to propagate easily across end grain end moments after ignition. Having them all perfectly cylindrical is a fool-hearty effort that would certainly impede the flame progression. Also, adding spacer o-rings seems like a dangerous method since one might foul and temporary plug the nozzle as it was ejected, causing a pressure spike.

I'll aim toward the shape you show as I will adapt this method as well.

Thanks for sharing,

Tom

Thanks for the response Tom. I wanted to add that this feature will not be found on older Loki Research reloads. Only in the last year or two with the advent and testing of new reloads has this feature been actively added to production propellant grains. 76mm grains were probably the first, followed by the 54mm White. I believe most everything in the 38/1200 and up has been this way for the entire year now.

Scott -

Thanks for sharing some very useful information.

I have only been casting and mixing for a few years and I sand the ends of my grains trying to get that little convex space for the flame front.

I have been wanting to make a conical sanding block with a 1 or 2 degree slope that would work for 54mm - 98mm motors or maybe thru the 6" grains but have not done that yet.

rdbones said:

Scott -

Thanks for sharing some very useful information.

I have only been casting and mixing for a few years and I sand the ends of my grains trying to get that little convex space for the flame front.

I have been wanting to make a conical sanding block with a 1 or 2 degree slope that would work for 54mm - 98mm motors or maybe thru the 6" grains but have not done that yet.

Click to expand...

Maybe we can add that project to our next mixing session in-between additions.

Hardline said:

Maybe we can add that project to our next mixing session in-between additions.

Click to expand...

I was thinking about seeing if Jay could make something out of wood on this lathe and the put a small T handle on it. just glue a piece of sandpaper to it and put it to work.

Loki Research said:

Every grain at Loki Research from the 38/1200 and up is cast and cut by me, by hand.

Click to expand...

After making a total of 1 motor I'm most impressed by what Scott said above. It's hard work and takes alot of time and then to take the extra time to cut a chamfer into the face of each grain is outstanding workmanship.

Good reading.


TA

I made this up about 10 years ago. 3/8 on one side and 3/4 on the other (necked down for a 3/8 drill). It works well even if the core sizes are bigger then 3/8 or 3/4. Not sure what the angles are..I just eye balled them. I use rubber cement to glue on 60 gritt. For those Isp pinching types only one end of the grain needs to be beveled.



Tony