Saturday, January 16, 2010

Hybrid Tips

At 8 bars chamber pressure and 3.5 nozzle area ratio, a nozzle is horrifically over expanded. An over expanded nozzle will contribute significantly to poor ISP performance

c* is a better measure of combustion efficiency, being a function of mass flow rate, chamber pressure, and nozzle throat area. At low chamber pressures, use frozen chemistry rather than equilibrium in PROPEP.

It is common practice in hybrid motor design to include a "pre-chamber" upstream of the fuel grain - it will help improve performance and stability.

Back when I was doing N2O/HTPB hybrids, I measured c* efficiencies from 50% to 84% all with the same hardware - changing mainly fuel grains.


Ben Brockert said...

(Thank you for publishing this in your official industry publication medium.)

But do hybrids require drastically higher L*? In a liquid I'd consider 2 meters to be really long, so an L* of 7.5 to 14 meters is crazy huge. In a well operating engine, that kind of L* should hurt, not help.

If the combustion is relatively stable, making the L* longer isn't going to help if the issue is fuel droplet size. Vaguely related to rockets, in (I think) Reynst's book on pulsejets he suggests that pulsejets are particularly suited to burning coal dust because the severe oscillation of the gas inside the engine does a great job of scouring off the gas around the particles so they can be completely burnt. Similarlay, a hooting hybrid might actually run better than a pretty one.

Rocket Dude said...

(You are welcome.)

I think with hybrids and L*, the complication comes with what you count as "chamber". If you count the volume of the grain's port and even the pre-chamber, L* can be quite large.

As far as combustion efficiency with LOX, imagine a LOX/HC engine where the fuel is injected axially at the walls.... I think Paul B. tried something like this and it didn't work too well.

Ben Brockert said...

Yeah, in a liquid engine you call that "film cooling" and you're hoping the fuel doesn't burn. Getting regen working i a bit difficult, but beyond that, it seems like getting a liquid near theoretical (and ours is over 95%) is way easier than a hybrid.

I'm still going to do a wood hybrid just for fun, though.

tredude said...

Hello Rocket dude, I was wondering if you could e-mail me at, I am very keen to buy one of those servo-valves you made a while ago?


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