understanding BATES propellant grains

[High Desert Rocketry]

I was recently teaching a class on rockets to an 'unnamed group' and got on the topic of assembling an Aerotech motor.
When asked what the white things were I talked about the propellant and how it burns and why the different grain geometries produce different burn profiles and why you might choose to use one over another. When discussing the BATES (Balistic Test And Evaluation Systems) grains, I mentioned that by having the core get larger in diameter as it burns and the burning ends making the length of the core get shorter, the amount of burning surface stays about the same thus keeping the motor chamber pressure the same as would be the thrust and produce a 'neutral' burn profile.

I also mentioned the length of BATES grains is a bit over 1.5 times the diameter of the grain. Someone listening in said it was 2,2 times the diameter. I replied is was really more like 2.3 times the diameter. Not wanting to get into a protracted 'debate' I ended it by saying the geometry and placement of the core also has an effect on the burn profile.



I found this old paper on BATES grains and thought it might be of interest to people wondering why BATES grains look as they do. The originals were quite large in size.
Also interesting as what a Super BATES is/was.

chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://apps.dtic.mil/sti/pdfs/ADA405742.pdf

 

[prfesser]

I think the characteristics of the two motors got swapped somehow by the publisher or the author of the article. A 70-lb motor should be bigger than a 15 lb...

Anyway, yes, the optimum length/diameter depends on the core diameter relative to the grain diameter. In Figure 2 above the core is not much smaller than the grain diameter. I did a quick-n-dirty run and the chamber pressure and thrust in such a motor varied by less than 3% over the burn.

In our hobby motors we don't need a perfectly neutral burn, a reasonably neutral burn is good enough. The smaller cores (vs. grain diameter) allow more propellant in each grain and a more powerful motor.

 

[bad_idea]

Good reading, thanks! I cleaned up the link to the paper:

The History of the BATES Motors at the Air Force Rocket Propulsion Laboratory

 

[dhbarr]

Good reading, thanks! I cleaned up the link to the paper:

The History of the BATES Motors at the Air Force Rocket Propulsion Laboratory

8ms is not enough dwell time for aluminum combustion, but 17ms is enough to burn most of the aluminum in most propellants. That's the first time I've ever seen that range bracketed, so.... nifty!

EDIT: PS, I'll further guess that ~1in-per-ms of motor length is a pretty okay rule of thumb for hangtime.

 

[cls]

that's a very interesting paper, thanks for posting! I learned a few things.

 

[AeroTech]

I first learned about BATES grains from Dr. Claude Merrill, while employed at the Bermite Div. of Whittaker Corp. from 1980-1982. Dr. Merrill had previously been the head of the solid propellant research facility at AFRPL. As far as I know, AeroTech was the first company to use the design for commercial hobby rocket applications.

 

[jsdemar]

Here's a clip from something I wrote in 2006 when I was giving free propellant classes in the Houston area.