Summary of the changes made. They include control rod changes so that inserting the rods does not produce a reverse effect, fixed neutron absorbers to inhibit unstable low-power operation, and extra protection systems. The void coefficient is reduced, although it is still positive, which is an undesirable characteristic. A side-effect of the changes is the need for higher enrichment of the fuel.

https://www.world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-power-reactors/appendices/rbmk-reactors.aspx

The ideal rocket equation relates the delta-V (change in velocity) a rocketship is capable of to the specific impulse (propellant efficiency) of the engine and the mass ratio (mass including propellant divide by mass of everything that's not propellant).

https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

The equation takes the logarithm of the mass ratio, so adding more fuel brings diminishing returns - this is the "tyranny of the rocket equation". Chemical rockets launching to Earth orbit have only a few percent of their launchpad mass as useful payload, with some more mass in the rocket itself, but around 90% of the launchpad mass is the fuel.

The equation doesn't depend on how quickly the fuel is burned. Thanks to orbital mechanics you are better off burning it all as fast as possible then coasting rather than throttling back and burning slowly, all else being equal.