Question about end grains and additional pressure.

Hello all,

How much additional pressure do end grains really produce, and how?

A vendor was cleaning out some old misc. stock and I was given 3. 75mm redline grains, 2 of which are end grains with a larger core. The grains were leftovers from a couple M1550R motors and I also have the nozzle kit with o-rings etc. from the M1550 as well. I was told that a L1150 uses the same grains (3 of 5) and I could effectively make that motor. However since two of the three that I was given are end grains with the larger core I should use a larger nozzle. I was told I could simply drill out the nozzle for the M1550 a little larger to compensate for the added pressure the second end grain would cause. If this is true, how much larger?

I looked all over on Aerotech's site and of course my luck, there is nothing on the L1150 with info on if it even uses an end grain. No instructions posted for that motor that I could find but there are for the M1550.

Thanks!

...Fudd

Fuddrucker said:

I looked all over on Aerotech's site and of course my luck, there is nothing on the L1150 with info on if it even uses an end grain.

Click to expand...

https://www.aerotech-rocketry.com/c...75mm_hp_rms/75-3840/75-3840_l1150r-p_assy.pdf
https://aerotech-rocketry.com/uploads/4cbb2bed-d22a-47e8-80fc-f40fc95f4222_75-6400_m1550r-p_assy.pdf

Looks like the M1550 does use a larger core end grain and the L1150 doesn't.

This is, of course, all research/EX if you do it. At a minimum someone would need to run Burnsim or the like and figure out the right nozzle diameter, get an estimate of max thrust and total impulse, etc.

Thanks Mike. Yes, I would only launch this at an EX Launch. Looks like the L1150 in those specs uses a medusa, and I read somewhere that it was changed over to a single throat but could not verify. I will ask a vendor if that can tell me what the current nozzle used is.

...Fudd

A 'stock' L1150R would use a single throat nozzle with a diameter of .594 https://www.rocketmotorparts.com/75mm_Medusa_Nozzle/p1577809_7763077.aspx

The following is a guess on my part.......
That gives a stock L1150R a starting Kn of 249. changing the bottom grain core from .875 to 1" That brings the Kn up to 256.

I misread your question a tad..

If the top grain remains at .875 and the other two grains have 1" cores your Kn will be 262

As for how much more pressure do "end grains" produce...very little. In your case maybe 50 psi more. What they actually do is..On longer motors: they create more space/room for all the 'stuff' above that grain, to get by easier on it's way out the nozzle. Otherwise it starts eroding (eating itself up) and it gets ugly quickly.
Tony

Tony, I think you mean how much additional pressure do larger throat end grains produce...

Note, if too much of your core uses a different core size, then you'll have a road flare for the end of the burn, and quite possibly a late chuff. Rapid depressurization is one way to put out an APCP motor. When playing with dual thrust EX motors I had combustion nearly extinguished on one test due to too rapid pressure decrease. Learned a lesson so it won't happen again... But, that's getting into EX territory so not a subject for this subforum.

Gerald