A 25% more efficient engine sounds like a lot, but doesn’t necessarily mean squat for increasing total system efficiency. The RS-25 for the Shuttle and SLS has

A 25% more efficient engine sounds like a lot, but doesn’t necessarily mean squat for increasing total system efficiency. The RS-25 for the Shuttle and SLS has an ISP of 366 at sea level and 451 in Space. The first stage F1 for the Saturn V had an ISP of 261. SpaceX Merlin has an ISP of 282 at sea level, the vacuum Merlin achieves 311 in space, but require a different bell. That makes the RS-25 more than 30% more efficient than Merlin or F1. Yet this is the result. Payload fractions: SLS: 3.4%, Saturn V: 4.3%, Falcon 9: 4.1%, Falcon Heavy: 4.5% Thrust to weight ratio: RS-25: 54 F-1: 82 Merlin: 194 The big difference is the SLS choice of hydrogen as fuel. That provides the higher ISP, but also requires significantly larger and heavier tanks. It’s efficiency requires more dead weight in engines and structure that more than negates the ISP advantage. So much weight that the SLS and the Shuttle required the help of massively expensive solid rocket boosters just to reach orbit. If a detonation engine is heavier, that undercuts its efficiency. If it cokes up, it’s not going to be usable in deep space, if not easily reusable it useless. It requires hydrogen, which means it needs a sea level ISP of 450 to be worth the heavier structure in a launch system. But if it can do 550 in vacuum, it will make a great deep space engine.