¶Stoke’s unique upper stage. Stoke Space, a Washington State-based medium lift startup aiming at the fully reusable launch market from day one, has continued to pull back the curtains around its design for a uniquely multipurpose upper stage (Dodd also just released an in-depth walkthrough video). While the first-stage has its own challenges—the plan is to use an in-house full-flow, staged-combustion engine, only the fourth engine of this type to be developed—the upper stage is where Stoke is swinging for the fences. This stage features an actively cooled heat shield. Liquid hydrogen flows through tiny channels in its surface, absorbing heat during re-entry, expanding, and then, as a hot gas, powering the turbo pumps to feed yet more fuel through the heat shield to the upper stage’s 30 combustion chambers. (Notably, this system self-regulates its cooling by using heat from re-entry to drive the feed pumps. It is also theoretically fault tolerant: micro cracks in cooling channels will lead to over cooling instead of a loss of cooling.) The combustion chambers, laid out around the circumference of the heatshield, technically form a single engine since they share turbopump machinery. They utilize the shield to produce high expansion ratios and an aerospike effect for increased efficiency from high altitude, to full vacuum, and, finally, back to sea level. Expansion gas from this actively-cooled, open-cycle engine is exhausted from the center of the heat shield as “base bleed” for the aerospike effect, taking the place of a traditional long tapered spike shape and allowing the rounded, “truncated spike” heat shield dome to achieve high thrust (paper). Re-entry will be shield (and engine) first, and employs its skewed dome geometry to allow precise trajectory control by rolling with RCS thrusters during the un-powered re-entry phase. Close to landing, the upper stage will re-ignite its 30 thrust chambers and use phased shut-off pintle injectors for deep throttling, thrust differential, and precise shutdown to propulsively land. Together, the fully-reusable, 30 m tall, two-stage rocket will deliver ~1.65 tons to LEO. The upper stage is where the majority of Stoke’s technical risk lies, and so the startup—well funded by Breakthrough Energy Ventures and Alexis Ohanian’s 776—is tackling this challenge first. With initial hot fires complete, it is now moving quickly towards upper stage hop tests later this year at its Moses Lake, WA testing facility; meanwhile, the booster is much earlier in development, with component design still in progress. | 
