SpaceX continues to impress with their technological developments. Unlike most ‘new’ rocket engines, which are mainly evolutions of old designs (such as the J-2X), SpaceX’s Merlin is a clean sheet design. And it has evolved quite a bit since it was first introduced.
The original Merlin 1A had 77,000 pounds of thrust. It was used on the first couple of Falcon 1 flights. The Merlin 1B was an improved 1A with 85,000 pounds of thrust, but it was never flown. Instead the 1C replaced the 1A on the Falcon 1 and is used on the Falcon 9. The 1C initially had 78,000 pounds of thrust, but this was increased to 125,000 pounds. And now the Merlin 1D has achieved 147,000 pounds of thrust. This means the 1D has a thrust to weight ratio of 150:1, the highest ever achieved for a rocket engine. Nice work, SpaceX. The Merlin 1D will be used on Falcon 9 flights going forward.