The SLS is a Space Shuttle rocket stack without the orbiter. It will have taken over $30 billion and 33 years to make an expendable Space Shuttle rocket stack without an orbiter.

We will spend another $6 to 8 billion over the next two to three years to get to the first unmanned test flight. This mission could be done with less than $1 billion and two SpaceX Heavy launches. The SpaceX Heavy has already flown three times.

Two SpaceX Falcon Heavies could launch the Orion capsule in 2020 and complete the mission for $5 billion lower cost and probably one year sooner.

Exploration Mission 1 is going to use the SLS to launch the Orion, its Service Module, and the Interim Cryogenic Propulsion Stage (ICPS) into orbit around the Earth. SLS separates from the components at 157 kilometers. Orion operates independently, and the ICPS propels the craft towards the Moon. The SpaceX Falcon Heavy could put the Orion and its Service Module into GTO. This would be closer to the moon. The Orion Service Module would be able to complete the trans-lunar injection maneuver with enough propellant in reserve to perform any necessary course corrections. A second Falcon Heavy would be needed to bring the Interim Cryogenic Propulsion Stage to join it to the Orion in order to have the fuel needed to slow down and enter lunar orbit.