Hybrids 2018

[jderimig]

Has anyone studied if volume used for a post combustion chamber yields more impulse than using that same volume for more N2O?

 

[drewnickel]

That’s what we’re currently trying to figure out. On larger motors it definitely helps, but at this scale may be unnecessary. We’ve been able to shorten fuel grains significantly due to the CNVA, so we now get ~200 cc of nitrous in a 16 inch motor and still burn a roughly optimal OF ratio. As for the bell nozzle, it can be 75-85% the length of a conical nozzle and get identical performance, so less weight and more space for nitrous.

 

[AlphaHybrids]

Has anyone studied if volume used for a post combustion chamber yields more impulse than using that same volume for more N2O?

That would be very interesting. Also very hard to test. You'd have to have either a pre-filled tank setup, or test two motors at the same time, filling at the same time so that filling conditions and nitrous temps are as close as possible. One with, one without a post combustion chamber.

I've found that there are so many variables in hobby hybrids that getting reliable data to compare is very hard. I had a 4" HDPE grain and on two firings the same day, everything the same (tank fill, initial pressure, etc) I had a difference of 15% in total impulse. The change was the preheater on the firing with more impulse burned about 5 seconds longer before burning through the U/C valve. I had HDPE dripping out of the nozzle before it fired. This meant that the surface of the grain was liquid, and when nitrous was introduced it readily reacted and was up to full thrust. The one that opened the U/C valve faster had about a 2 second period ramping up to full thrust.

75% the length of a nozzle- on a typical 98mm nozzle that is an extra 138cc, or 94 grams. That could represent between 150 and 200 Ns total impulse.

Edward