Granted a car has a much more complex engine than a rocket. Engine
management for your average car is designed to extract the most
efficient use of a confined explosion and to reduce emissions.
It is an air breathing engine that has to operate at varying densities
of propellant and oxidizer. It has to have VERY deep throttling
capabilities and operate under a large variation of load.
The effort that has gone into the development of such an engine is
staggering. Several orders of magnitude more than any rocket engine.
A race engine is a different story depending on the flavor which there
are MANY.
The rocket is closest to a dragster engine designed to extract the most
power from the propellant and oxidizer possible.
There are not a lot of secrets that can be kept. The shear number of
brilliant people working on just this one type of racing is staggering.
There are plenty of lessons rockets can learn from automotive engines.
It surly is not the other way around.
There is no doubt in my mind engine management systems for rockets are
no different and they are behind the automotive curve not in front of
it.
That said all that is left is to find out how rocket engines can be
better managed.
Yes, under optimal conditions you can calculate how things will work.
The real world is usually less than optimal and better engine management
will give higher performance. Allow more data to be collected and
INCREASE SAFETY.
I am sure at the top of the rocket food chain engine management is being
done. It's not exactly the same as automotive engine management but the
principals are very similar. Forgive me if I dumb it down too much as I
progress on this journey I will be more specific.
The specific thing I'm looking for right now is how to monitor
combustion effectively.
-------- Original Message --------
Subject: [AR] Re: Closing the loop on rocket engines
From: "Troy Prideaux" <GEORDI@xxxxxxxxxxxxxxxx>
Date: Wed, December 09, 2015 11:02 pm
To: <arocket@xxxxxxxxxxxxx>
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.