Stoke Space Just Tested Its Hopper2 System
Stoke Space just completed one of the most significant tests in the company’s relatively short history. After a lot of testing and preparation, the Hopper test vehicle known as Hopper2, lifted off for around 15 seconds before landing back on the ground. Despite how short the test was it demonstrated some crucial systems including the engine design along with the heat shield and other core components.
Stoke Space is taking a very unique approach to spaceflight and this specific hopper test article is the upper stage of a bigger vehicle. Similar to how SpaceX began with hops of Starhopper before moving on to 10km flights and eventually full launches. With this test now complete, teams at Stoke will no doubt be looking at all the data gathered in order to improve and continue innovating the system.
While this test was successful, the rocket’s design is ambitious and will take a lot more work and testing before we see full launches and especially payloads in orbit. Here I will go more in-depth into the recent hop, why this is so significant for Stoke, what the company is planning next, and more.
Successful Hop Test
Starting a few months ago Stoke shipped its reusable second-stage prototype, the Hopper Test Vehicle, to their test site in Washington. By September 5th, they had completed a wet dress rehearsal with the vehicle. A week later on the 12th, they completed a static fire which produced promising results. This static fire was also special as it practically simulated the entire hop. Specifically, the test included everything from flight avionics, power systems, computers, GNC, RCS, tank pressurization, and, of course, the engine and heat shield. Stoke Space commented that “The only thing we simulated was the position data, which was derived in real time from engine data. Simulating the position gave us the opportunity to inject dispersions, such as a persistent roll” they said.
With the success of this test, we knew a hop was right around the corner. Yesterday the 17th Stoke tweeted both videos and images of the test. Here they successfully conducted a vertical takeoff and vertical landing (VTVL) developmental test flight of their reusable second-stage rocket. In a statement, the company said, “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.”
With quite a bit of past testing, yesterday’s test was the last test in their Hopper technology demonstration program. They pointed out that “We’ve also proven that our novel approach to robust and rapidly reusable space vehicles is technically sound, and we’ve obtained an incredible amount of data that will enable us to confidently evolve the vehicle design from a technology demonstrator to a reliable reusable space vehicle.”
Taking a closer look at the flight itself, the engines begin to ignite and around 5 seconds later the vehicle is in the air. It then rises another 15 feet or so before falling back toward the ground, making contact with a slight jump, and then stopping. You can also see that the vehicle ended up completing around a 180-degree spin counterclockwise. This likely wasn’t intentional but provided crucial data for the teams at Stoke. The company stressed that multiple milestones beyond just the use of the engines were completed in this test. The regen cooled heat shield for example was a large area of focus. They were quoted saying, “although this vehicle didn’t directly experience the heat from hypersonic atmospheric re-entry, it has successfully operated at 100% of the expected heat load in a simulated environment.”
This test was just one of the most recent of many. Back in June for example, the company tested differential thrust vector control. In these “hula hoop” tests, they rotated the thrust vector in a circular fashion at progressively faster speed. The “hula hoop” engine test verifies the dynamic response and the torque produced by our Differential Throttle Thrust Vector Control (DT-TVC). A traditional engine uses a gimbal to move the physical engine nozzle. This approach throttles individual thrusters to re-point the thrust vector. As the upper stage of Stoke’s larger launch vehicle, it will need both control and power for certain orbital maneuvers and deployments.
Stoke’s Plan
Stoke Space’s main goal is to create a 100% reusable launch system. The upper stage, which we just saw in early testing, will feature an actively (regeneratively) cooled metallic re-entry heat shield with integrated modular LH2/LOX rocket engines. Stoke believes this combination is robust, resilient to damage, and operates with passive failure modes. Designed for minimal refurbishment between flights, unlocking rapid turnaround: Refit, Refuel, Refly. It will feature direct access to GTO, TLI, and other high-energy orbits, return from orbit to the launch site with a precision, powered vertical landing, and also feature downmass capability.
The payload fairing supports a 180-deg by 360-deg deployment hemisphere and axial loads during ascent, descent, and landing. The first stage on the other hand, which has very little development progress when compared to the upper stage, is planned to use 7x LNG/LOX engines and return to the launch site (RTLS) or complete a downrange landing.
Founded only years ago in 2019, what’s arguably most impressive is the progress the company has made in such a short period of time. This has been partially thanks to pretty consistent funding. Starting in May 2020 the company won a $225,000 SBIR Phase I grant from the National Science Foundation to work on an integrated propulsion solution for reusable rocket upper stages. Just a year later February 2021 the company raised $9.1 million in seed funding in a round led by venture funds NFX and MaC Venture Capital. Later that same year in December, the company raised $65 million in a Series A round, funding development and testing of the upper stage of a reusable launch vehicle.
All of this funding was used to create test articles and establish various important infrastructure. For example, earlier this year in March, announced they had been allocated Launch Complex 14 at Cape Canaveral Space Force Station in Florida by the Space Launch Delta 45. “We are over the Moon excited by this opportunity,” said Julia Black, Director of Range Operations at Stoke Space. “To be trusted with the reactivation of the historic Launch Complex 14 is an honor, and we look forward to adding to its well-distinguished accomplishments for America’s space program” she said.
By now, Stokes prototype has had at least 22 static fires in total. This constant testing has allowed the company to iterate and improve the design rapidly. Following yesterday’s hop test, we can expect even more developments in the near future. In a statement, the company said, “we became the fastest company to go from initial seed funding to demonstrating an orbital-class vertical takeoff and vertical landing rocket, the second company in the world to fly a prototype of a fully reusable upper stage rocket, and just the third US company to develop a liquid hydrogen rocket engine” they said.
As far as what to expect in the future, Stoke is attempting to do something that has never been done before: design and build a rocket that is 100 percent reusable with a 24-hour turnaround. To reach that goal, they plan to continue moving through the development program by increasing focus on their reusable first stage.
Stoke’s approach to testing involves trying new things and accepting failure. Before the hop test they said “While we’re hopeful that Hopper2 will successfully complete this full series of tests, including a flight test with the vehicle upright and intact upon landing, there’s a good chance that won’t be the outcome. And that’s okay. With developmental tests of this nature, we’re pushing the hardware to its limits so we can obtain data that will help us refine the prototype’s engineering and design” they said. Earlier this spring, they took a similar approach with their Hopper1 tests. In that developmental campaign, they learned a lot about fluid systems, propellant conditioning, operational procedures, and terminal count.
In April of this year, the company even released a tool to let hardware companies track the design, testing and integration of parts. It’s a solution born out of “ubiquitous pain in the industry,” Stoke CEO Andy Lapsa said in a recent interview. Many of the existing tools are not optimized “for boots on the ground,” but for finance or procurement teams, or even the C-suite, he explained. “If you think about aerospace in general, there’s a need and a desire to be able to understand the part pedigree of every single part number and serial number that’s in an assembly,” he said. “So not only do you understand the configuration, you understand the history of all of those parts dating back to forever.” Tools such as these will hopefully only help Stoke as they continue to develop and innovate within the industry. While yesterday’s test was a big milestone, they have a lot of work ahead of them.
Conclusion
Stoke Space has been continuing to test its reusable upper stage. The most recent test was a 15 foot hop test which was very successful. The data gathered should help teams at Stoke improve the design and try again in the future. We will have to wait and see how it progresses and the impact it has on the space industry.