The main reason SpaceX chose a methane-fueled engine wasn't really about efficiency. Rather, it comes from a more practical consideration: methane is plentiful on Mars, which is the final destination of Starship. //

Like the BE-4, the Raptor uses a staged combustion scheme to achieve better fuel efficiency. However, the Raptor has a full-flow, twin-shaft configuration. This means the Raptor has double the number of pre-burners, each driving a single propellant pump. What SpaceX got out of this design is greater reliability and safety.

The downside is that the engine is also far more complicated to build. Thankfully, over the years, SpaceX has simplified and iterated on the engine with each hot-fire test. Today, the latest version of the Raptor (Raptor 3), isn't just cheaper than the first one, it also has nearly 50% the nominal thrust — increasing from 408,000 pound-force at sea level on the Raptor 1, to almost 593,000 pound-force on the Raptor 3. //

However, while the design of the BE-4 has remained largely unchanged since its conception point in 2011, the Raptor series has gotten to its third iteration. The Raptor 3 — the latest version — was a complete redesign of the engine to make it smaller, more streamlined, reliable, and most importantly, offering more thrust. The Raptor 3 can deliver up to 593,000 pound-force of thrust, making it by far the most powerful Methalox engine ever. With 33 Raptor cores powering the Super Heavy first stage of Starship, SpaceX's massive launch system can bring up to 330,000 pounds to orbit, dwarfing Vulcan's best by nearly five times. //

Efficiency is everything in rocketry, which is why specific impulse (Isp) is such a big deal in this field. Simply put, this number represents the amount of thrust an engine can generate when burning a certain amount of propellant over a set period — typically in seconds. The higher the Isp of a particular rocket engine, the more efficient it is since it can produce more thrust, while consuming less fuel.

According to measurements made by Everyday Astronaut, the Isp for the BE-4 sits at around 310 seconds, while the initial version of the Raptor goes up to 330 seconds. The 20-second difference may not sound like much for most of us, it's actually very important for a rocket engine.

Basically, when you attach the two engines to the same rocket, each carrying an equal amount of propellant, the Raptor will be able to burn for 20 extra seconds compared to the BE-4. //

There's also the matter of availability. SpaceX is churning out one Raptor engine core per day at their in-house manufacturing plant in McGregor, Texas. Meanwhile, the BE-4 has (infamously) encountered troubles during development and production, resulting in a four-year delay in the engine's launch. In fact, it was because of all the issues surrounding the BE-4 that ULA had to postpone the launch of their flagship Vulcan rocket several times. And since the Vulcan's maiden flight in January, it has been the only time the BE-4 has been used in a realistic launch mission so far.

As for the Raptor? It performed brilliantly in the last few tests of the Starship system.