Bob Truax once told me they did an L* test with a variable length CC and c*
didn't drop significantly until into the single digits. But it's really
about how efficiently the injector atomizes and mixes the propellants,
usually easier to use a higher L* than spend the time/money designing an
efficient injector.
What's your priorities?
K
On Wed, Jan 27, 2021 at 10:29 AM Wyatt Rehder <wyatt.rehder@xxxxxxxxx>
wrote:
Something to consider on the LR-101 was its intended application. Which
was as a vernier control thruster for the Altas. So they had other driving
design constraints than just performance. I'd hazard a guess that they were
happy to take a hit on performance if it made the packaging smaller. So I
wouldn't be surprised if the LR-101 was optimized for minimum size with
still an acceptable margin on it functioning properly.
So as Ben said there isn't really anything to stop you shortening L* until
the engine just stops working. You will just be trading performance for
that shorter engine. A lot of small liquids do end up on the shorter side
for L*, as if you went with the upper ranges on the suggested L* table your
engine can end up having the L:D of a pringles can. Which can be hard to
keep cool. How bad of hit you take from that short L* will depend on how
good your injector is.
On Wed, Jan 27, 2021 at 6:12 AM Ben Brockert <wikkit@xxxxxxxxx> wrote:
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:
could help me with.
I was wondering about something I thought the insight of this group
what would be the consequences of a characteristic length of around only 12
For a small liquid propellant rocket engine running Kerosene and LOX,
inches?
under reasonable operating conditions (i.e. combustion completion issues,
Could small attitude control thrusters run with an L* as low as that
etc)?
uses an L* of 20", yet the texts I've found have RP-1 and LOX L* values
I'm wondering since someone here mentioned the Rocketdyne LR-101 engine
much higher than that, and that made me wonder whether the characteristic
length could be taken even smaller for amateur engines.