Successful Stoke Space Hopper 2 test:
Source: The Launch Pad via Stoke Space
Official update: Update on Hopper2: The Hopper Has Landed
During this test, known as Hopper2, we were able to successfully launch the Hopper test vehicle to an altitude of 30 feet and land at its planned landing zone following 15 seconds of flight. The test successfully demonstrated our novel hydrogen/oxygen engine, regeneratively cooled heat shield, and differential throttle thrust vector control system, as well as our avionics, software, and ground systems.
Stoke’s 100% Rapidly Reusable Rocket
Stoke Space is only four years old. Their goal from the start was to build a fully and rapidly (24 hour turnaround) reusable rocket. The goal wasn’t to use an aerospike engine, the final product just ended up looking like a pseudo-aerospike or a plug nozzle.
SpaceX has shown that re-flying first stages is possible. Seventeen times is currently the most times they have reused boosters. Their shortest turnaround time for a booster is down to 21 days. Even SpaceX has not achieved a 100% recycling rate because the second stage will burn up on re-entry.
Stoke Space is tackling the still unsolved problem: reusing the second stage.
Their idea was to build 30 independent small thrusters that form an effect that is similar to an aerospike engine with an actively cooled heatshield, which doesn’t require heat tiles.
Manny small thrusters offer benefits of scaled production, finely controlled thrust during landing, it enables differential thrust control, thereby eliminating the hardware and control to steer a nozzle.
Scott Manley Rocket Scientist Combine A Rocket Engine With A Heat Shield - Can They Revolutionize Space Launch?
A Falcon 9 booster has only limited power throttling capabilities. It lands with only one engine running, and the shut-off must be timed perfectly (power slam), or the booster would start climbing again.
The traditional bell-shaped rocket engine creates a few issues for engineers. The most prominent is flow-separation. Surprisingly, rocket exhaust is low-pressure. That means the atmosphere at sea level is pushing hard against it and can cause instabilities that will result in the destruction of the bell. When the rocket climbs, ambient pressure lowers and the exhaust overexpands, which is increasingly less efficient. That’s why bell-shaped engines have a sea-level and a vacuum version of the nozzle. An aerospike overcomes this limitation because there is no nozzle where flow can separate.
Tim Dodd, who is a big fan of aerospike engines, had the opportunity to tour the facilities in February of 2023:
How Stoke Space's Unique Rocket Works // Exclusive Tour & Interview
The re-entry of the second stage will work like any other capsule (e.g. SpaceX’ Dragon, Sojus Capsule). Whereby the heat shield has a slight asymmetry that works in tandem with an offset center of mass which can be used to ascend, descend, and left and right steering.
Great explanation by Scott Manley: Ballistic Reentry vs Aerodynamic Reentry
Their next effort is going towards developing a booster stage. They intend to use a full flow combustion engine engine, like SpaceX is developing for Starship.
Tim Dodd: Is SpaceX's Raptor engine the king of rocket engines?
The reason why they see the need for their own booster rocket is that the interface between first- and second stage is fairly unique and not compatible with currently flying rockets.
Company website: stokespace.com
Wikipedia: Stoke Space