With this initial funding and an engineering design in hand, the company hit the gas on development. In the 20 months since its initial seed round of funding, Stoke has built a second-stage engine, a prototype for the second stage, turbopumps, and manufacturing facilities. It also increased its headcount to 72 people and finalized the overall design for the rocket, which has a lift capacity of 1.65 metric tons to low-Earth orbit, in fully reusable mode.
Last month, the company started to test-fire its upper-stage engines at a facility in Moses Lake, Washington. The images and video show an intriguing-looking ring with 15 discrete thrusters firing for several seconds. The circular structure is 13 feet in diameter, and this novel-looking design is Stoke's answer to one of the biggest challenges of getting a second stage back from orbit.
Most commonly, a traditional rocket has an upper stage with a single engine. This second-stage rocket engine has a larger nozzle—often bell-shaped—to optimize the flow of engine exhaust in a vacuum. Because all parts of a rocket are designed to be as light as possible, such extended nozzles are often fairly fragile because they're only exposed above Earth's atmosphere. So one problem with getting an upper stage back from Earth, especially if you want to use the engine to control and slow its descent, is protecting this large nozzle.
One way to do that is to bury the engine nozzle in a large heat shield, but that would require more structure and mass, and it may not be dynamically stable. Stoke's answer was using a ring of 30 smaller thrusters. (The tests last month only employed 15 of the 30 thrusters). In a vacuum, the plumes from these nozzles are designed to merge and act as one. And during reentry, with a smaller number of smaller thrusters firing, it's easier to protect the nozzles.
"What you’re seeing in the photos of the test is a high-performance upper-stage engine that can operate within atmosphere at deep throttle to support vertical landing but then also perform at a higher ISP than some variants of the RL 10 engine in space," Lapsa said.
Stoke Space aims to build rapidly reusable rocket with a completely novel design
"We took a leap of faith and jumped off a cliff."
arstechnica.com