CENTENNIAL, Colo. , August 04, 2022 – The VORTEX upper stage engine (VR35K-A) successfully completed a Critical Design Review (CDR) marking a significant milestone in the maturation of the 35,000 lbf thrust upper stage liquid oxygen (LOX)/liquid hydrogen (LH2) engine.  

Sierra Nevada Corporation (SNC), through its commercial space subsidiary Sierra Space, is co-investing with the United States Air Force to increase launch performance at a lower cost through development of an upper stage engine alternative. The VR35K-A, developed in conjunction with the Air Force Research Laboratory (AFRL), is a commercialized version of the VORTEX engine system. The team will provide a fully integrated engine, leveraging technologies initially developed under Small Business Innovation Research programs.

The VR35K-A team had been progressing toward CDR since the engine Preliminary Design Review (PDR) in December 2020. This review assessed Sierra Space’s capability to confidently manufacture and safely operate the engine on the test stand. The team presented the technical readiness of the engine and its critical components, the concept of operations and risk assessment and mitigations plans. AFRL representatives participated in the review held at Sierra Space’s Madison, Wisconsin, office.

Compared to existing upper stage engines, the VR35K-A provides both higher thrust and a higher specific impulse for increased capability for a given launch. Additionally, the per-engine cost is lower than existing options, enabled by the following key VR35K-A system attributes: 

  • Advanced single-shaft turbo-pump assembly for simplified operation and mechanically coupled mixture ratio control.
  • Low-cost, stable combustion chamber.
  • Fuel-rich staged combustion cycle for increased Isp.
  • VORTEX combustion for increased chamber pressure within small engine volume.
  • Lightweight assembly with fixed position (non-deployable) nozzle.
  • Additive manufacturing, enabling reduced production costs of complex components.

Moving forward, the Sierra Space team will continue to advance the VR35K-A engine by producing a flight-weight prototype engine to demonstrate capability and performance at Sierra Space’s Badger Propulsion Test Facility location outside of Baraboo, Wisconsin.  

For more information, visit

About SNC
SNC is a trusted leader in innovative, advanced technology solutions and open architecture integrations in aerospace and national security. Best known for its unique mission applications, SNC creates customized solutions for the world’s most pressing technology challenges in the fields of aviation, national security space, electronic warfare, command and control, mission systems and inline cybersecurity. SNC is owned by Chairwoman and President Eren Ozmen and CEO Fatih Ozmen, and is once again recognized as a US Best Managed Company by Deloitte Private.