It would make the rocket too heavy. To add another engine, they would have to make the rocket larger in diameter, increasing the weight by a fair amount (they currently have the 9th engine in the middle, so it would have to get moved to the ring with the rest). Reducing the power of the center engine would also reduce the amount of payload to orbit.<p>Rockets that can reach orbit have very bad weight to payload ratios (Falcon 9 is ~2.5% takeoff weight to LEO) so there isn&#x27;t much room for dead weight.