4
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ Besides the efficiency hit, there may be combustion stability problems. $\endgroup$ – Russell Borogove Feb 26 '16 at 16:49
  • 2
    $\begingroup$ 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 '16 at 19:28
4
$\begingroup$

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.

$\endgroup$
  • 3
    $\begingroup$ 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 '16 at 16:24
1
$\begingroup$

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%.

$\endgroup$
1
$\begingroup$

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.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.