Fuel/oxidiser ratios

Why are engines usually run fuel-rich? Is there any advantage to running
them oxidiser-rich?

Can anyone confirm whether my understanding of this is correct?

Normally engines are run fuel-rich, to decrease chamber temperature and
provide a reducing environment. Lox/lh2 engines are run fuel rich also
because it increases the Isp, but this isn't that significant for
Lox/hydrocarbon engines.

This has the disadvantage of producing nitrogen oxides as atmospheric
pollutants when the exhaust burns in air.

Assuming a reducing environment is not required, for lox/hydrocarbon engines
might it not be better to run oxidiser-rich (in order to keep the chamber
temperature down). The oxidiser is cheaper than the fuel, and it should be
less polluting?


--
Peter Fairbrother

Peter Fairbrother wrote:
Why are engines usually run fuel-rich? Is there any advantage to running
them oxidiser-rich?

ISP, and reducing atmosphere.

Assuming a reducing environment is not required, for lox/hydrocarbon engines
might it not be better to run oxidiser-rich (in order to keep the chamber
temperature down). The oxidiser is cheaper than the fuel, and it should be
less polluting?

Right.
Now, work out what percentage of the cost of a typical launch is fuel.
This is not a large number.

Peter Fairbrother wrote in message ...
Why are engines usually run fuel-rich? Is there any advantage to running
them oxidiser-rich?

Can anyone confirm whether my understanding of this is correct?

Normally engines are run fuel-rich, to decrease chamber temperature and
provide a reducing environment. Lox/lh2 engines are run fuel rich also
because it increases the Isp, but this isn't that significant for
Lox/hydrocarbon engines.

It is significant for LOX-hydrocarbon engines for two reasons. First,
it keeps the average molecular weight of the exhaust species down (CO
is lighter than CO2 for example) which raises exhaust velocity (AKA
Isp), and second, because LOX is denser than hydrocarbon fuels and
therefore the vehicle tanks are lighter.

This has the disadvantage of producing nitrogen oxides as atmospheric
pollutants when the exhaust burns in air.

It's not that simple. Diesel engines always run lean but create more
NOx than Otto cycle (spark ignited) engines because the peak
combustion temperatures are higher. The *temperature* of the rocket
plume, in addition to the chemistry of the plume, has a big effect on
creation of NOx.

Assuming a reducing environment is not required, for lox/hydrocarbon engines
might it not be better to run oxidiser-rich (in order to keep the chamber
temperature down). The oxidiser is cheaper than the fuel, and it should be
less polluting?

Most long life reusable rocket engine combustion chamber and nozzle
parts are made of metals. Metals generally burn, and oxidizing flames
tend to burn the metals. Therefore, another reason rockets are run
rich is to minimize component oxidation. For the same reason, airplane
pilots are trained to manually lean their piston engines to the rich
side of peak exhaust gas temperature (stoichiometric mixture) for
longer exhaust valve life even though lower temperatures can also be
achieved on the lean side.

Dan DeLong

In article ,
Peter Fairbrother wrote:
Why are engines usually run fuel-rich? Is there any advantage to running
them oxidiser-rich?

They're run fuel-rich because with typical propellant combinations, you
get the best Isp that way. The penalty in energy release per unit mass
from including some non-reacting fuel is made up for by better gas
properties in the exhaust, giving more efficient conversion of thermal
energy to kinetic energy.

Running oxidizer-rich would increase thrust some, particularly for LOX/LH2
engines, but would have no other particular advantages.

(Many rocket stages run *less* fuel-rich than maximum Isp would seem to
dictate, because the oxidizer is denser than the fuel, so a leaner mixture
means smaller, lighter tanks, and so maximum *stage* performance comes at
less than maximum Isp. This effect is small except for LOX/LH2, where
max Isp would be at perhaps 4:1 but real stages typically run at 6:1.)

Normally engines are run fuel-rich, to decrease chamber temperature and
provide a reducing environment.

Decreasing chamber temperature is a virtue only if performance is not a
high priority. The one place in typical rocket engines where mixture
ratios are deliberately skewed to reduce temperatures is in gas generators
for driving turbopumps.

As for the reducing environment, the importance of this is overrated.
Keeping the walls cool is vital even in a reducing environment, and a
*cool* oxidizing environment is not that big a materials problem. The
reducing environment does *help* -- maximum service temperatures, and
useful lifetimes, of materials are lower in an oxidizing environment --
but it's a secondary issue. The flame temperature in most any liquid
rocket engine far exceeds the maximum service temperature of the wall
material, reducing environment notwithstanding.

The RL10 has been run oxidizer-rich, experimentally, without damage.

Lox/lh2 engines are run fuel rich also
because it increases the Isp, but this isn't that significant for
Lox/hydrocarbon engines.

No, it's significant even for hydrocarbon fuels. The improvement in Isp
is small, yes, but because high-performance LOX/hydrocarbon stages are
typically in quite a steep part of the mass-ratio curve, even a small
improvement in Isp can make a big difference. Rocket engineers will go
to great lengths to add even a few seconds of Isp.

This has the disadvantage of producing nitrogen oxides as atmospheric
pollutants when the exhaust burns in air.

You'll get nitrogen oxides when any hot exhaust jet hits atmosphere.
Whether it has some unburned fuel in it is much less significant.

Assuming a reducing environment is not required, for lox/hydrocarbon engines
might it not be better to run oxidiser-rich (in order to keep the chamber
temperature down).

As noted above, reducing temperature is generally a priority only in
turbopump gas generators. Those are sometimes run oxidizer-rich, notably
by the Russians. For the engine proper, you generally want the highest
possible chamber temperature.

The oxidiser is cheaper than the fuel, and it should be less polluting?

LOX/hydrocarbon propellant costs are not generally significant, and even a
typical fuel-rich mix is mostly LOX by mass. There's little difference in
pollution, which is in any case a very minor issue at even very optimistic
flight rates.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

On Wed, 10 Nov 2004 16:45:46 GMT, in a place far, far away,
(Henry Spencer) made the phosphor on my monitor
glow in such a way as to indicate that:

Running oxidizer-rich would increase thrust some, particularly for LOX/LH2
engines, but would have no other particular advantages.

It might make for more efficient usage of propellants if they're being
produced by electrolyzing water. In that event, you might want to
have an average ratio close to stoichiometric.

Henry Spencer wrote:

Peter Fairbrother wrote:

Lox/lh2 engines are run fuel rich also
because it increases the Isp, but this isn't that significant for
Lox/hydrocarbon engines.

No, it's significant even for hydrocarbon fuels. The improvement in Isp
is small, yes, but because high-performance LOX/hydrocarbon stages are
typically in quite a steep part of the mass-ratio curve, even a small
improvement in Isp can make a big difference. Rocket engineers will go
to great lengths to add even a few seconds of Isp.

???

A 1% difference in Isp is going to make a 1% difference in delta-vee, no
matter where on the mass ratio curve it occurs, unless I have seriously
misunderstood the rocket equation.

Now I'll grant that 1% of a lot is more than 1% of something smaller, but if
you're multiplying your 1% by some factor relating to the mass ratio and the
payload, that's just bad accounting. It's the wrong way to look at it.



--
Peter Fairbrother

Peter Fairbrother :

Henry Spencer wrote:

Peter Fairbrother wrote:

Lox/lh2 engines are run fuel rich also
because it increases the Isp, but this isn't that significant for
Lox/hydrocarbon engines.

No, it's significant even for hydrocarbon fuels. The improvement in Isp
is small, yes, but because high-performance LOX/hydrocarbon stages are
typically in quite a steep part of the mass-ratio curve, even a small
improvement in Isp can make a big difference. Rocket engineers will go
to great lengths to add even a few seconds of Isp.

???

A 1% difference in Isp is going to make a 1% difference in delta-vee, no
matter where on the mass ratio curve it occurs, unless I have seriously
misunderstood the rocket equation.

Now I'll grant that 1% of a lot is more than 1% of something smaller, but
if
you're multiplying your 1% by some factor relating to the mass ratio and
the
payload, that's just bad accounting. It's the wrong way to look at it.

But it makes more that a 1% diffirence in the mass ratio if the delta-v is
fixed. And mutliple stages shows this more.

Earl Colby Pottinger



--
I make public email sent to me! Hydrogen Peroxide Rockets, OpenBeos,
SerialTransfer 3.0, RAMDISK, BoatBuilding, DIY TabletPC. What happened to
the time? http://webhome.idirect.com/~earlcp

In article ,
Rand Simberg wrote:
Running oxidizer-rich would increase thrust some, particularly for LOX/LH2
engines, but would have no other particular advantages.

It might make for more efficient usage of propellants if they're being
produced by electrolyzing water. In that event, you might want to
have an average ratio close to stoichiometric.

Yes, if you add external constraints that force you to take extra mass
along -- so that the question is not whether you have it, but how to make
best use of it -- that changes the picture. Carrying it as dead weight
during the burn is an obvious non-starter, which leaves you with the
choices of dumping it overboard (at roughly zero relative velocity) or
injecting it into the chamber. If memory serves, in general it turns out
to be better to inject it, despite getting a suboptimal mixture ratio as a
result.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

In article ,
Peter Fairbrother wrote:
No, it's significant even for hydrocarbon fuels. The improvement in Isp
is small, yes, but because high-performance LOX/hydrocarbon stages are
typically in quite a steep part of the mass-ratio curve, even a small
improvement in Isp can make a big difference. Rocket engineers will go
to great lengths to add even a few seconds of Isp.

???
A 1% difference in Isp is going to make a 1% difference in delta-vee, no
matter where on the mass ratio curve it occurs, unless I have seriously
misunderstood the rocket equation.

However, more commonly the problem is to meet a fixed delta-V requirement
without committing too many unspeakable acts :-) in the design. In the
steep part of the curve, a small increase in Isp takes the required mass
ratio down quite substantially.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

On Thu, 11 Nov 2004 18:54:35 GMT, in a place far, far away,
(Henry Spencer) made the phosphor on my monitor
glow in such a way as to indicate that:

Running oxidizer-rich would increase thrust some, particularly for LOX/LH2
engines, but would have no other particular advantages.

It might make for more efficient usage of propellants if they're being
produced by electrolyzing water. In that event, you might want to
have an average ratio close to stoichiometric.

Yes, if you add external constraints that force you to take extra mass
along -- so that the question is not whether you have it, but how to make
best use of it -- that changes the picture. Carrying it as dead weight
during the burn is an obvious non-starter, which leaves you with the
choices of dumping it overboard (at roughly zero relative velocity) or
injecting it into the chamber. If memory serves, in general it turns out
to be better to inject it, despite getting a suboptimal mixture ratio as a
result.

It seems like a no brainer to me. Hard to imagine that it could
degrade Isp enough to make it completely worthless as reaction mass.