Numerical investigations of high-speed rarefied gas outflow into a vacuum through channels with a forward- or backward-facing step have been conducted using the direct simulation Monte Carlo method. Calculations have been performed for various free-stream Mach numbers, covering transonic, supersonic, and hypersonic flow regimes, and over a wide range of gas rarefaction from free molecular to near hydrodynamic conditions. Mass flow rates through the channel and the gas flow field have been accurately calculated both inside the channel and in the regions upstream and downstream. It has been established that channel geometry, the free-stream velocity, and gas rarefaction strongly influence the gas flow. In the flow field, in front of the channel, a phenomenon known as a detached shock occurs, while inside the channel, a gas recirculation zone may form. © The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2024.