You can keep shortening L* until combustion performance is so bad that
you lose sonic choke and it stops being a rocket engine. In any
well-functioning injector that would be well under 12". The
appropriate L* isn't a fixed number for a given propellant; it varies
with injector and can certainly go up or down depending on packaging
requirements of the engine. Though in most cases making the engine a
bit bigger to get higher performance is favorable in a system
performance trade.
The engine in the classic "A High-performance 250-pound-thrust Rocket
Engine Utilizing Coaxial-flow Injection of JP-4 Fuel and Liquid
Oxygen" had an L* around 14 inches and achieved an C* efficiency of
95%. I don't know what your definition of "small" is, but if it's plus
or minus 2x of 250lbf I'd start with that engine as very strong
inspiration.
On Wed, Jan 27, 2021 at 11:51 AM Jonathan Adams <jonadams2002@xxxxxxxxx> wrote:
I was wondering about something I thought the insight of this group could
help me with.
For a small liquid propellant rocket engine running Kerosene and LOX, what
would be the consequences of a characteristic length of around only 12 inches?
Could small attitude control thrusters run with an L* as low as that under
reasonable operating conditions (i.e. combustion completion issues, etc)?
I'm wondering since someone here mentioned the Rocketdyne LR-101 engine uses
an L* of 20", yet the texts I've found have RP-1 and LOX L* values much
higher than that, and that made me wonder whether the characteristic length
could be taken even smaller for amateur engines.