WASHINGTON – Stoke Space has tested a highly efficient engine it is developing for the first stage of its fully reusable launch vehicle.
The company, based in Kent, Washington, announced June 11 that it conducted a brief engine burn at a test site in Moses Lake, Washington, on June 5. The engine, designed to produce up to 100,000 pounds of thrust, rose to 50% of its rated thrust in the two-second test.
The goal of the test was to see how the engine started and stopped, Andy Lapsa, Stoke's chief executive, said in an interview. “All the complexity and much of the risk is in that transitional start-up and transitional closure,” he said. “The test duration was short because the goal was to demonstrate the transient and then backtrack.”
The engine uses a design called full-flow staged combustion, where both the fuel and engine oxidizer (liquefied natural gas and liquid oxygen, respectively) pass through separate preburners before entering the main combustion chamber. That approach offers greater efficiency and longer engine life, but is more complex to develop. It is currently used only in SpaceX's Raptor engines that power its Starship vehicle.
Lapsa said Stoke chose this approach because it was necessary for rapidly reusable launch vehicles. “In a world of rapid reuse, high performance is needed,” she said. “Full flow staged combustion gives you the highest possible performance under the least stressful conditions.”
Currently, engine testing focuses on transient start and stop conditions. The company is building a larger testbed that will be completed later this summer that will allow for longer duration testing, including a full qualification test campaign.
Stoke plans to use seven of the engines in the first stage of Nova, the fully reusable medium-lift launch vehicle it is developing. The upper stage uses a very different engine technology, with liquid oxygen and liquid hydrogen fueling an engine integrated into an actively cooled heat shield to allow the upper stage to return for a powered landing. The company tested that system. with the flight of a “hopper” prototype last September.
Lapsa said there are some commonalities between booster and upper-stage engines in areas such as the technology used in the engines' turbomachinery, as well as analysis tools. “Other than that, it's very much a completely new and different system.”
Stoke is making progress on other aspects of the vehicle. Engine testing used flight software and avionics, he said, and the company is doing a “design iteration” on the upper stage. That work is being funded by a $100 million Series B round that the company raised last October.
“In many ways, it's all systems go, and the last big question mark I felt hanging over my shoulders was the first stage engine, and specifically how to get the engine through the transients and back successfully,” he said.
Lapsa said after the hopper test last September that the company had an internal goal of beginning orbital flight testing in 2025, with a desire to accelerate that timeline wherever possible. He declined to give an updated timeline after this engine test, saying it will depend on when the company can begin work on the vehicle's launch site at Cape Canaveral's Launch Complex 14. that the Space Force assigned to Stoke last year.
He noted that the test was carried out just 18 months after the company began designing the engine. “I think over time we will find that, just as reusable rockets will quickly make all the others obsolete, I think these high-performance engines that make that mission possible will eventually make the lower-performance variants obsolete as well,” said. . “I think it's an essential technological mountain to climb and I'm very excited to be on that mountain.”
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