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I'm working through Sutton's "Rocket Propulsion Elements," (8th Ed.) and it is mostly gibberish. However, I understand a little bit. Anyways, the question is: When using RP-1 as a fuel, your oxidizer shipment contains 15% nitrogen by mistake. What effects would this have on performance, combustion gasses, etc? What would the optimum mix ratio be?

So, for performance, I can only assume that specific impulse would lessen due to unburnt nitrogen conveying heat to the supersonic gas flow in the nozzle. Combustion gasses would ultimately have a concentration of nitrogen, monatomic nitrogen, nitrous oxide and nitric acid, and possibly some nitrogen tetroxide, as well as the typical combustion gasses (I'm bad at chemistry...). For ratios, I am guessing that the optimum of 2.3 or 2.5 would need to be adjusted to be more oxidizer rich due to nitrogen being inert.

Thoughts? Suggestions? Anything?

EDIT: For the mix ratio, I did a simple calculation accounting for the decrease in oxidation efficiency and came up with a new ratio of 2:1 (for frozen equilibrium).

I also should have pointed out that these ratios are: 2.3:1 for frozen equilibrium expansion and 2.5:1 for shifting equilibrium expansion with gas expansion to sea-level pressure.

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  • $\begingroup$ Besides the efficiency hit, there may be combustion stability problems. $\endgroup$ Feb 26, 2016 at 16:49
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    $\begingroup$ 15-20% contaminants (nitrogen, carbon dioxide) would be quite expectable if you're into amateur rocketry and making own oxygen (not LOX, just compressed gas) using zeolite-based generator. $\endgroup$
    – SF.
    Feb 26, 2016 at 19:28

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I'm guessing, you are familiar with CEA, right? It's a nifty tool that you can use to calculate equilibrium chemistry and its effect on rocket combustion. By fiddling with the inputs, you should be able to answer all your questions.

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    $\begingroup$ I wasn't familiar with CEA until now. That is possibly the best tool for the job. Using that, I can see there would be hydrogen cyanide, ammonia, and inert nitrogen in the exhaust. I don't know if it is account for dissociation, which would have N and O in the exhaust as well, but they would recombine in the plume. Essentially, the engine would have lower Isp and thrust, higher temperature, and generally suck more than normal. $\endgroup$
    – Tawooh
    Feb 26, 2016 at 16:24
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Did you specify LOX for the oxidizer? I will assume that you are using LOX.

I certainly do not know all of those answers, but if you are already fuel rich, then your engine is burning 15% less propellants than expected. It seems that the performance hit would be proportional to the inert nitrogen concentration, although I do not know if it would be exactly 15%.

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Your performance also decreases. Either you burn 15% less propellant, or if you really want to burn all your propellant you need to put 15% more oxidizer in the tank to have the amount of oxygen you need, so you start out with more mass.

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It is demonstrably bad to introduce nitrogen into your liquid oxygen, at least for a hydrogen-fueled engine.

In the early 2000s the Space Shuttle Program struggled with performance predictions - the Space Shuttle Main Engines as tested on the stand in Mississippi routinely showed higher performance in flight.

This was eventually tracked down to be the result of the test stands pressurizing their run tank with nitrogen versus the flight vehicle using helium. The buzzword for this problem in the program was "LOX purity".

It may sound OK to get better performance in flight, but the whole reason for the tests was to determine the actual engine performance, and any difference between test and flight was bad.

A test with low LOX purity can result in flying an engine with a lower oxidizer-to-fuel mixture ratio as the engine will not require as much oxidizer to maintain chamber pressure (due to higher LOX purity seen on launch day). That condition results in flying with more LOX than is needed for a given mission and that difference in margin could be the difference between achieving the planned orbit or having to abort (e.g. low fuel level cutoff) due to unplanned ascent performance dispersions.

The problem was because

Two Dimensional Kinetic (TDK) and One-Dimensional Equilibrium (ODE) modeling illustrated that an impurity in LOX results in lower engine specific impulse (Isp). Argon is the main impurity for delivered LOX while a nitrogen impurity may be introduced, during hotfire, as gaseous nitrogen is used to pressurize the facility LOX tank.

This graph shows the insidious effect of nitrogen versus argon.

enter image description here

Quotes are from Summary of Results from Space Shuttle Main Engine Off-Nominal Testing The relevant portion of the paper documents attempts to quantify the LOX purity issue by test stand runs, but for various reasons, the results were inconclusive.

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"When using RP-1 as a fuel, your oxidizer shipment contains 15% nitrogen by mistake". Usual industrial standards are 99.7%. Medical oxygen is the same.

It may do no harm for your rocket to have 15% nitrogen. Most rockets burn fuel rich to protect engines from the high temperature of ideal stochiometric O2/fuel mixes, so you will need to adjust your fuel mix. N2 as a contaminant acts as a benign reaction mass. It has a fairly low molecular weight so it will form a high Isp component in the exhaust.

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