Congress and Trump may compromise on the SLS rocket by axing its costly upper stage

For this reason, the House Appropriations Bill calls for the space agency to "evaluate alternatives" to the Exploration Upper Stage.

One of these, as Ars reported last year, is the Centaur V upper stage built by United Launch Alliance, which is already flight-proven. Another option is a "short" version of the upper stage Blue Origin currently flies on its New Glenn rocket. Sources indicated that Blue has already begun work on a modified version of the stage that could fit within the shroud of the SLS rocket. This smaller version of the stage, like the Centaur V, would allow NASA to continue launching the SLS rocket using the existing launch tower in Florida.