Sierra Space, a commercial space company and defense tech prime that is Building a Platform in Space to Benefit Life on Earth, announced the successful completion of the test campaign for its fully integrated VR35K-A upper stage engine, under contract with Air Force Research Laboratory’s (AFRL), Rocket Propulsion Division, Edwards, Air Force Base in California, marking a major step forward in Sierra Space’s development of advanced propulsion systems.

In addition, the company has been awarded a $16 million firm-fixed-price contract from AFRL to continue upper stage engine maturation to fulfill future National Security Space Launch (NSSL) missions. This is a follow-on to a $22M+ contract awarded to Sierra Space in July 2023 by the Air Force Test Center at Edwards Air Force Base in California.

The VR35K-A is a high-performance liquid oxygen (LOX)/liquid hydrogen (LH2) engine that employs a fuel-rich staged combustion cycle, an innovative turbopump architecture and a VORTEX combustion chamber to produce 35,000 lbf of thrust at a higher performance than any engine currently on the market. Sierra Space’s patented VORTEX technology enables efficient and stable combustion while providing active cooling for the combustion chamber, ensuring high performance and reliability.

“Our powerful new upper stage engine, the VR35K-A, represents a groundbreaking advancement in propulsion technology, leveraging Sierra Space’s patented VORTEX engine design to achieve unprecedented efficiency, reliability and affordability,” said Dan Polis, Vice President of Engineering Solutions and Propulsion Systems at Sierra Space. “We are delivering high performance, reduced complexity and significant cost savings with this revolutionary new product, allowing our customers and partners to deliver up to 30% more payload mass to orbit. This engine is a testament to our company’s commitment to pushing the boundaries of space commercialization with cutting-edge, scalable solutions.”

Additional key attributes of the VR35K-A include:

• Simplified Operation: A single-shaft turbopump assembly for streamlined functionality, including start-up and shut-down.
• Mechanically-Coupled Mixture Ratio Control: Ensuring precise fuel and oxidizer balance.
• Additive Manufacturing: Reducing production costs and lead times for complex components.

The engine underwent a rigorous test campaign in 2024 at Sierra Space’s Badger Propulsion Test Facility in North Freedom, Wisconsin, and achieved all objectives, including: stable, efficient thrust chamber operation, robust and efficient turbomachinery operation, mixture ratio control, throttle demonstration and high overall engine efficiency.

Sierra Space partnered with Florida Turbine Technologies, a subsidiary of Kratos Defense & Security Solutions, Inc., to develop and test the turbomachinery used in the VR35K-A engine. The single-shaft main turbopump assembly provides for simplified and reliable operation including start-up and shutdown, and mechanically-coupled mixture ratio control. Prior to integrating the turbomachinery into the engine, Sierra Space completed a comprehensive turbomachinery standalone test campaign to validate operating points and predicted performance.

“Kratos is a leader in propulsion technology, dedicated to the rapid development, demonstration and deployment of innovative solutions,” said Stacey Rock, President of Kratos’ Turbine Technologies Division.  “Our philosophy is to collaborate with industry leaders who share our mission of delivering cutting-edge technologies and capabilities to market quickly and cost-effectively.  This partnership with Sierra Space has been highly successful and represents a cornerstone of our long-term strategic initiatives.”

Sierra Space’s VR35K-A engine will serve a critical industry need in supporting larger payload delivery to Low Earth Orbit and beyond. Sierra Space is also applying its VORTEX combustion chamber technology to engines and thrusters to support in-space propulsion for spacecraft orbital maneuvering and extraterrestrial landers. Sierra Space’s Dream Chaser spacecraft uses VORTEX in its reaction control system thrusters, for example.

In 2023, Sierra Space announced the VR35K-A had completed engine component testing, which allowed the company to advance to the full engine hot fire testing campaign. Next, Sierra Space will continue to mature its upper stage engine technology as it moves into a more extensive qualification phase.

Click here to learn more about Sierra Space's Propulsion Engines