Engine Cycle Doesn't Affect Exhaust Velocity

I'm really glad you found this issue before the release! Thank you so much for your help with beta testing!

Yay! @AndrewGarrison

@Kell ah, you are absolutely correct! This is due to the efficiency parameter which we multiply the thrust by at the last step, but perhaps we should apply it to exhaust velocity instead. Good catch!

@AndrewGarrison but according to Wikipedia:

In rocketry, where the only reaction mass is the propellant, an equivalent way of calculating the specific impulse in seconds is also frequently used. In this sense, specific impulse is defined as the thrust integrated over time per unit weight-on-Earth of the propellant:

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Isp = Ve/g0

where
Isp is the specific impulse measured in seconds,
Ve is the average exhaust speed along the axis of the engine (in ft/s or m/s),
g0 is the standard gravity (in ft/s2 or m/s2).

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In rockets, due to atmospheric effects, the specific impulse varies with altitude, reaching a maximum in a vacuum. This is because the exhaust velocity isn't simply a function of the chamber pressure, but is a function of the difference between the interior and exterior of the combustion chamber.

To summarize: exhaust velocity and Isp are directly proportional at all times.

Exhaust velocity should not change between the two. Exhaust gases are choked at the nozzle throat and are accelerated from there based on the ratio of chamber pressure to exit pressure, which is determined by nozzle area ratios. Notice here in equation 1.22 that it's not the chamber pressure, but rather the ratio of exit pressure to chamber pressure that sets the exhaust velocity. In both cases above, your ratio of Pe to Pc is 2631.578.

I do agree, it does seem very counterintuitive!