Common Questions

What was SLS Block 1B?

An upgraded SLS variant with the Exploration Upper Stage, allowing simultaneous crew and cargo missions to lunar orbit.Source: Lowdown briefing analysis

Why was SLS Block 1B cancelled?

NASA Administrator Isaacman cancelled it in 2026 citing cost and schedule overruns, with commercial alternatives available.Source: Lowdown briefing analysis

What SLS versions are still flying?

Only Block 1, with the Interim Cryogenic Propulsion Stage; both Block 1B and Block 2 were cancelled.Source: Lowdown briefing analysis

Background

SLS Block 1B was the first planned upgrade to the Space Launch System, replacing the Interim Cryogenic Propulsion Stage with the more powerful Exploration Upper Stage (EUS). The variant would have enabled simultaneous crew and cargo delivery to lunar orbit, a capability regarded as essential for sustained surface operations.

Development was cancelled by NASA Administrator Jared Isaacman in early 2026, writing off years of EUS development spending. Block 1B was already years behind its original schedule and over budget before cancellation.

The decision leaves the Block 1 variant — with its more limited payload capacity — as the only version of SLS still flying, and means simultaneous crew-and-cargo missions cannot be achieved without commercial alternatives or a future architecture redesign.

How the World Sees Them

United States

Cancellation saves near-term development cost but permanently removes the only domestic path to combined crew-cargo lunar launches.

Commercial sector

SpaceX positions Starship as the natural successor for heavy combined-payload missions that Block 1B was meant to handle.

International partners

ESA and Canada had mission architectures premised on Block 1B capability; cancellation forces replanning of contributions.