NASA is fine-tuning its design for the new Space Launch System (SLS) that will send astronauts to the moon, asteroids and possibly Mars. Basically there will be three main types of rocket, the Block 1, Block 1A and Block 2. Block 1A and Block 2 will both have a crewed and a cargo variant, so that's a total of 5 models. The most important difference with what was announced last September is that the main core stage will now have four or five RS-25D/E Space Shuttle Main Engines (SSME) instead of three.
