Is there a chance spectral emissions could me monitored and used for useful
feedback ?
I guess Troy's comments may rule this out.
The sensors exist but are a little pricey.
On Thu, Dec 10, 2015 at 2:02 PM, Troy Prideaux <GEORDI@xxxxxxxxxxxxxxxx>
wrote:
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.