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I am a member of a university rocketry club and we are in the process of designing a liquid rocket engine. One of the previous propulsion leads did the primary conceptual design of this engine in RPA. Since he graduated, we have switched propellants from N2O/Jet-A to LOX/Jet-A. Because of this, we obviously need to modify some of our design parameters. I am asking here about the characteristic length, L*. This previous lead used an L* value of 2 m. According to this page, the L* for a LOX/RP-1 engine should be 102-127 cm. What would be the consequence of continuing to use the 2 m value?

In my rough understanding of what L* is, I would guess that no major problems will occur, but we may be designing the engine larger than is needed.

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From my (probably) similarily rough understanding of L*, it's a minimum size required for propellants to stay long enough in the chamber to mix properly. If the distance is shorter than that, you will experience problems with combustion instability.

As such, having a length that's too large is certainly not as bad as too short, although the engine is then needlessly bulky.

But more importantly, why would you want to keep the L* at 2m? A change in propellant necessarily means a redesign of the engine. Why not pick a known optimal value instead?

If the hardware is already built and can't be repurposed for any other use, consider running it with the new propellant and see if it explodes (at a proper safety distance).

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    $\begingroup$ Thanks for the answer, everything you said makes sense. We certainly will be changing L*, probably to the upper end of the range I mentioned. I was more so just curious what the effect would be if we didn't change it. We're still in the design phase, so no hardware has been built. $\endgroup$
    – landobean
    Commented Oct 21, 2020 at 16:17

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