NASA will contract with Boeing for an interim cryogenic propulsion stage to power at least the first two flights of its planned heavy-lift Space Launch System (SLS), based on the company’s Delta Cryogenic Second Stage used on the Delta IV.

After reviewing other upper stages, the U.S. space agency picked the Delta unit because it meets most of the requirements for lofting Orion multipurpose crew vehicles in 2017 and 2021 atop a NASA-developed SLS main stage with strap-on boosters.

The two requirements it doesn’t meet – human rating and the ability to achieve a velocity change (delta v) of 3,050 meters/sec. with three engine burns – can be fulfilled with “relatively minor modifications,” NASA says in a notification of the selection posted on the agency’s procurement website by Marshall Space Flight Center.