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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 |
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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. |
#3
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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 |
#4
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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 | |
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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 |
#7
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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 |
#8
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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 | |
#9
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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 | |
#10
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