5 ms·
You are looking for maximum impulse, not maximum energy. If you run fuel-rich, the extra fuel converts heat into impulse, it acts as reaction mass, so to speak.
by HarryHirsch 4y ago
You are looking for maximum impulse, not maximum energy. If you run fuel-rich, the extra fuel converts heat into impulse, it acts as reaction mass, so to speak.
Water injection in jet engines works along the same principles, convert heat into volume of vapour.
- AnimalMuppet 4y agoOK, but couldn't you do the exact same thing by running lean? Then the extra oxygen would convert heat into impulse. Or, as you said, by steam injection? (But then you'd need an extra tank for the steam, so that's probably less than optimal...) (I'm not saying they're wrong. I'm asking for an explanation.)
- HarryHirsch 4y agoThe real-life application for an oxygen-rich mixture is a cutting torch, you really don't want that. Also, maximum impulse means high exhaust velocity, that means exhaust gasses with low molecular mass. Oxygen wouldn't qualify.
- ncmncm 4y agoYes, burning away the interior surface of your combustion chamber runs into a resource limit pretty quickly, shortly followed by rapid uncontrolled dissociation. The exhaust colors leading up to that are often pretty, and finally dramatic. There have been rockets that ran lean during the first few seconds, deliberately eroding away a layer of the combustion chamber, to get higher-mass particles into the exhaust while trying to clear the launch tower. The higher-mass exhaust particles gives you less efficiency but more thrust, which is most important just then.
- nickff 4y agoRunning lean makes the combustion hotter, and melts the chambers on a rocket.
- avmich 4y ago> Running lean makes the combustion hotter Not necessarily, unburned oxygen will reduce average temperature.
- QuadmasterXLII 4y agoYou ask a good question! When you dig into it, the goal of almost all modern rocket engine optimization is to create a mixture rich in hot hydrogen. For thermodynamic reasons, smaller molecules move faster at a given temperature, hydrogen is spectacularly smaller than any other molecule, and the most important quality of a rocket engine is its exhaust velocity.
- ncmncm 4y agoAmong the most important qualities of a rocket engine is its exhaust velocity, which determines its efficiency, delta-V per unit mass of fuel/oxidizer burned. Not exploding is probably the actual most important quality. You don't really start tuning for efficiency until you have got that one nailed. Sometimes absolute thrust is more important than efficiency. Perhaps surprisingly, efficiency can be traded off for thrust by getting heavier molecules into the exhaust. They are coming out slower than the naked protons, but not as many times slower as the ratio of their mass to the protons'.
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