Another potential problem with measuring the exhaust composition for closed
loop control is chemical equilibrium (particularly for deep throttling). For a
given typical hydrocarbon:oxygen mix ratio, the chamber composition should stay
pretty constant for varying chamber pressures over a pretty broad range,
however, the exhaust composition (optimally expanded) – particularly in
relation to CO vs CO2 can vary considerably from significantly different
chamber pressures which could potentially provide you with some subtly
misleading data if you were measuring CO or CO2 concentration at the exit over
varying chamber pressures and didn’t take such into account.
Troy.
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On
Behalf Of Wyatt Rehder
Sent: Thursday, 10 December 2015 2:13 PM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: Closing the loop on rocket engines
Most of the advantage for O2 sensors in car engines stems from your inputs are
not well known. Gasoline is a combination of chemicals that can vary pretty
significantly between gas stations on the same street, not to mention between
different states or countries. Because of this and other reasons your mixture
ratio tends to vary quite a bit. So an O2 sensor on your exhaust tends to be
handy since you can fine-tune the mixture ratio in your car, by the end results
vs. trying to analyze your fuels for composition.
A rocket is a much more controlled system than a car. Your propellants are very
well defined, So you have a pretty good idea of what will be going into your
rocket. A conventional O2 sensor also would not work on a rocket since they are
designed for use in the exhaust stream, which might see temps up to 1200F, at
pretty low pressures (a few ATM). If you tapped of your combustion chamber, or
your nozzle you could see temperatures in excess of 4000 degrees, and at much
high pressures. It would be more equivalent to measuring the combustion right
after top dead center right after ignition. So an optical measure would be a
likely way, but even then the properties of the plume can vary significantly
between edges and the interior. So your plume could look rich on the outside
but be very lean in the center. This gets worse if you use film cooling.
So to measure combustion products on a rocket in the same way as the car you
would have to measure up to 100 ft+ or so behind the rocket. Quite a few engine
controllers control off of chamber pressure, and sometimes injector pressure.
As long as you know the performance of your injector that is going to give you
a pretty accurate idea of mixture ratio. Just going off of pressures is a
pretty solid way to go since it is very reliable and if you need any finer
control than that, you would do it on the test stand. You can characterize your
mixture ratio pretty accurately on the ground, and have a pretty solid
expectation that it is going to be the same in flight. Main reason this works
is again you have a high level of control over your propellants.
The SSME has a much more sophisticated suite of sensors than that, I cant
remember the exact number of sensors but it is quite a few.
So liquid engines have been doing closed loop control for quite awhile.