NASA is testing the use of the RS-25 engine after modifications. The previous engine was reusable and came back with the shuttle after the launch. However, the modified engine is disposable. The main reason for this is to ensure that the engine is always in top condition every time.
"The next time rocket engine No. 2059 fires for that length of time, it will be carrying humans on their first deep-space mission in more than 45 years," stated NASA. "We feel that we are equipped enough with both information and technology to make this a success."
The RS-25 engine is viewed as the most successful Rocket engine to date, and that is why it is being considered for this important step. It has a very powerful boost while maintaining a stable state all through the journey.
However, suffice its high performance, it will be fitted with several rocket boosts to ensure that it maintains momentum and voyage the human cargo without any problems. The need to make a bigger shuttle for the humans also poses a problem that may require further modifications to the rocket itself.