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.
On Wed, Dec 9, 2015 at 4:29 PM, Monroe L. King Jr. <
monroe@xxxxxxxxxxxxxxxxxx> wrote:
Thanks for the PDF Lloyd, well yeah I look at it this way before we had
fuel injection we had carburetors. Engine management has made automotive
engines more efficient.
It seems to reason it can do the same for rocket engines.
Deep throttling is one area that it should help. The other aspect I'm
interested in is how much it could help the design process. Data
collection and analysis.
I'm working on a test stand for the turbopump and there are quite a few
data points and as I see it adding a few more, it just looks like an
engine management system. So why not just go all the way with it?
Turbocharger maps look like engine maps to me not much different than
timing maps or pressure temperature matrices. Pretty much all the same
to me. The combustion sensor is the only one I don't see in the current
array of sensors for a complete system.
I'm a pretty good tuner of engine management systems for race cars and I
understand them pretty well. Makes sense to me to apply it to rockets if
I can.
I am sure there are engine management systems for modern rockets there
just is not much talk about it. I could be wrong but I doubt it. It's
being done I'm pretty sure. Is it closed loop? That I'm not so sure of
but it would not surprise me.
-------- Original Message --------nor
Subject: [AR] Re: Closing the loop on rocket engines
From: Lloyd Droppers <ldroppers@xxxxxxxxx>
Date: Wed, December 09, 2015 4:34 pm
To: arocket@xxxxxxxxxxxxx
I have not heard of anyone using combustion sensors in a flight engine,
do I think that it would provide much value as most designs are eithernot
throttlable or already have an accurate measurement of propellant flowrate
in the feed system. What actionable information would an O2 sensorgather,
am I missing something?a
In the lab there are some extremely cool methods of measuring the
compounds,
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980201356.pdf is
bit old but it covers most of the basics of an H2/O2 systems in sectionchamber.
3.2, including direct measurement of gO2 using raman spectroscophy. But
these are primarily used for injector element experiments and design and
CFD code validation.
Lloyd
On Thu, Dec 10, 2015 at 3:33 AM, Monroe L. King Jr. <
monroe@xxxxxxxxxxxxxxxxxx> wrote:
I have not heard of using modern techniques such as those in the
automotive industry to control rocket engine combustion.
It occurs to me that if we want to have complete control over our
engines performance we need this same control over the combustion
process.
This would be especially useful for a preburnner system that
reintroduces the turbine gasses into the main rocket combustion
I'm wondering if anyone on arocket has heard of a combustion sensor
much like the O2 sensor in automotive use used to detect oxidizer/fuel
ratio in the exhaust gases?