Stoke Space Aims To Build Rapidly Reusable Rocket With a Completely Novel Design

Andy Lapsa and Tom Feldman, former Blue Origin engineers and the founders of Stoke Space, are working to develop the first fully recyclable space rocket -- one that features a reusable first and second stage. Here's an excerpt from Ars Technica's exclusive report, written by Eric Berger: 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. Another significant second-stage problem is protecting the whole vehicle from the super-heated atmosphere during reentry. NASA's Space Shuttle accomplished this with brittle thermal tiles, but these required 30,000 employee hours to inspect, test, and refurbish between flights. SpaceX is using a different type of ceramic tile, designed to be more reusable, for Starship. Given Stoke's background in rocket engines, Lapsa said it made the most sense to try a regeneratively cooled heat shield. The vehicle's ductile metallic outer layer will be lined with small cavities to flow propellant through the material to keep it cool during reentry. The second stage, therefore, will return to Earth somewhat like a space capsule -- base first, with the regeneratively cooled heat shield. Stoke Space has a very long road ahead of it to reach space. Engine tests are an important step, but they're only the first step of many. Next up for the company is "hop" tests with a full-scale version of the second stage at the Moses Lake facility in central Washington. This prototype won't have a fairing as it would during launch, but it will still stand 19 feet tall. Initially, the tests will be low-altitude, probably measured in hundreds of feet. If there's an engineering need to go higher, the company will consider that, Lapsa said. But for now, the goal is to prove the capability to control the rocket during ascent and descent and make a soft landing. This is a shockingly difficult guidance, navigation, and control problem, especially with a novel system of distributed thrusters. "This is kind of a final proof point of this architecture," Lapsa said. "It is new. It's different. It's weird. It's original. There were a lot of questions that we had about how this thing is going to work. But we've already mitigated a lot of risk." If Stoke can manage to land the upper stage, it can move ahead with the first stage and start to turn the yet-unnamed rocket into an orbital vehicle. It sounds easy, but it's not...

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