Stoke Space continues to test reusable second stage, looks ahead to full rocket

by John Sharp
Static fire

Stoke Space recently carried out the first test of the full-size 30-thruster version of the innovative engine that the company is producing for its in-development second stage. This will be an integral part of its future Nova rocket, which aims to be a fully reusable medium lifter.

The engine test took place on Feb. 26 and follows the engine’s first test flight on its prototype vehicle, Hopper 2, in September 2023. Although fitted with only 15 chambers for that flight, Hopper 2 flew for 15 seconds, achieved a maximum altitude of 30 feet, traversed to a landing site, and touched down softly.

NSF recently spoke with Stoke Space to discuss the Hopper campaign and the path ahead. According to Stoke, “The Hopper vehicle was/is a full-size second stage prototype, equipped with a fully functioning actively cooled heat shield. The test flight provided clear evidence of this functionality, as evidenced by the icy condensation visible on the heat shield during flight in released photos and videos.”

Hopper 2 takes to the skies in a mix of fire and ice. Note the ice accumplating on the heat shield, evidence that the active cooling was operating during the flight. (Credit: Stoke Space)

As to how comprehensive the testing campaign was, the company added, “The Hopper campaign demonstrated our engine’s full landing burn capabilities, including deep throttle control and precise steering through differential throttle thrust vector control, even under conditions of minimal propellant and no payload.” Furthermore, “In our tests so far, we’ve successfully achieved burn durations that deplete the entire run tanks.”

Stoke Space also clarified, Our heat shield is engineered for active cooling at multiple critical mission stages, ensuring optimal protection, performance, and mobility to, through, and from space.”

When asked whether there were any specific engineering issues with pumping the hydrogen through the longer-than-usual channels, the company responded, We’ve encountered no significant challenges in this regard. Engine test and Hopper flight data validated analytical predictions made by our in-house tools, confirming the ability to manage these requirements.”

A test firing of a 15-chamber second stage engine. Note the frost on the heat shield. (Credit: Stoke Space)

Interestingly, although there are multiple benefits, utilizing several small thrusters has some downsides: “The smaller size of our thrusters significantly simplifies manufacturing processes, presenting a clear advantage. However, this approach does introduce challenges in propellant distribution and net weight due to the cluster configuration compared to using a single large chamber. Yet, these challenges are offset by the benefits in terms of mass efficiency for flight, re-entry, and rapid reusability, ultimately justifying their presence in our design.”

Andy Lapsa – the Chief Executive Officer for Stoke Space – recently appeared on an episode of NSF Live with NSF’s John Galloway, expanding on many subjects around the philosophy and design of the company’s vehicles.

Stoke Space Technologies of Kent, WA, was formed in early 2021 by Andy Lapsa and Tom Feldman. Both had previously held senior positions working for Blue Origin on BE-4 engine development and managed to raise over $9 million in seed funding for their new venture.

Stoke Space was formed because Lapsa and Feldman had concluded that, as Lapsa put it during the NSF Live episode, “The economics of full rapid reusability are so powerful that there’s really no other future end state.  So, to me, that is inevitable. That will happen – full and rapid reusable rockets…. When I was thinking about what I wanted to do next, I looked out [at] the landscape, and it was really surprising to me that nobody else was pursuing it. I think that there absolutely needs to be more than one player to have a healthy economy for space mobility.  So, there’s got to be somebody else other than SpaceX doing this, and it wasn’t out there yet, so here we are.” 

“You get into this kind of virtuous cycle,” Lapsa further explained. “The lower cost you have, the more demand you have, the more frequently you’re flying, the better your reliability, and the better your availability. That is what Falcon has started to show us as an industry.”

The next step for Stoke was to set specific goals and establish how to achieve them. “What’s the part that the industry doesn’t have a playbook for yet?” Lapsa asked. “That part is the reusable second stage, the upper stage. So that’s when we started to think it was important for us.”  

Stoke Rocket

Render of Stoke’s in-development Nova rocket Credit: Stoke Space

Stoke is now aiming to build a rocket capable of a 24-hour turnaround, and it soon realized the constraints on its designs. “You have to think at least as hard about what you cannot do, as you think about what you can do, in those 24 hours,” Lapsa said. “I guarantee you, if it’s going to turn around in 24 hours, you’re going to be pretty damn busy – restacking the vehicle, reintegrating payload, getting it back on the pad, refueling, and flying. So, there’s absolutely no time for inspections between flights. There’s no time for refurbishment, for sure.”

Lapsa elaborated, “It’s my belief that you really have to design [reusability] from day one into the vehicle. You can’t back your way into it later.”  Expanding on the efficacy of this design approach, he continued, “We can do so many powerful things with modern computing and analysis tools. So, you can pull this stuff off with a very small team on a relative basis.”

Referring to images of the Hopper 2 vehicle, during the September 2023 testing, Lapsa was keen to point out a paradox: “You see there’s white stuff on the bottom of the heat shield. That’s frost, but there is a raging fire happening right under that heat shield, including where the ice is, and that’s kind of cool.”  The frost Lapsa refers to is caused by the regenerative cooling of the heat shield, which is achieved by passing cryogenic liquid hydrogen through channels in the heat shield. The hydrogen absorbs heat generated by re-entry, which is converted into mechanical energy by utilizing it to further drive the turbomachinery in the engine, which then pumps in more cold hydrogen into this cyclic system. The simplicity of this regenerative cooling appraoch is that it works throughout re-entry, even when the thrusters are not firing.

Hopper 2 on the test stand for pre-hop testing. (Credit: Stoke Space)

During engine firing, the heat shield and the 3D-printed thruster chambers and nozzles are all regeneratively cooled using this cycle.

Continuing to discuss the cryogenic hydrogen-fueled second-stage engine, Lapsa said, ”It’s a set of discretized thrusters, fed by a single set of turbomachinery, arranged outside the perimeter of the vehicle, and makes use, to some extent, of the aerospike effect. But certainly, it’s not like we set out and said we’re going to design the aerospike. It’s very different from a traditional aerospike…It’s a single set of turbomachinery, so there’s one turbo pump for the fuel, and one turbo pump for the oxygen, and it feeds a common distribution circuit, which is actually the heat shield. From there, it feeds all of the discretized thrusters around the perimeter. “

“All of those thrusters have individual throttle control ability,” Lapsa said. “They actually have throttle control in two different places. One is with the pumps, so the bulk engine thrust can be throttled. Then the relative thrust from one side to the other can be managed by the individual thrusters.”

Returning to the heat shield design, Galloway asked whether Stoke Space had considered water as a cooling medium: “Yes, water is an extremely efficient coolant,” Andy replied. “If you use water as a coolant circuit, you need an entirely separate system. You need water tanks, you need the water itself, you need water pumps, you need all of those things… they are dead weight.”

Lapsa noted that hydrogen is the best propellant available that also cools efficiently, allowing a margin to pay the reuse penalty. Its high specific impulse also provides access to higher-energy orbits not otherwise accessible by a smaller medium-class rocket.

In October 2023, Stoke Space announced a further funding round had raised $100 million and that its first rocket would be called Nova. Lapsa also confirmed the booster stage would be built from stainless steel: “I think one of the things that the industry is learning as a whole, is that there’s a huge amount of value in being able to manufacture at high speed, especially during R&D. So, you want to be able to build fast, you want to be able to iterate fast, you want each increment to cost as little as possible. I think manufacturing kind of dictated this trade as much as anything. In this case, we chose stainless steel. First of all, there’s a commoditized alloy that we buy off the shelf from any number of suppliers in just raw sheet metal form. So, that’s number one. Number two, it’s pretty easy to work with.”

At approximately 30.5 meters tall when fully stacked, Nova is being designed to launch with a wide variety of potential payloads and functions. These include not only deploying satellites but also performing manufacturing and science experiments in the vacuum of space and microgravity before returning to Earth. Furthermore, Nova could even be used for collecting and returning satellites or removing space debris. While these are lofty ambitions, Lapsa pointed out, “I think the first thing is you have got to do all those things, but it still has to perform its function, which is to deliver payload to orbit. And, because of that, it has to be very mass efficient.” 

Lapsa discussed the viability of continuing to discard and burn up material in the Earth’s atmosphere: “You have millions of pounds of heavy or rare metals coming back. And, if the way that we bring those things back is to burn them up in the atmosphere, at some point that’s going to have an effect. I think that’s only just beginning to be studied now. But is it viable? Is that the right way, to just burn stuff up in the atmosphere and let it crash? I guess my hunch is probably not. That’s got to be studied, but I don’t know. I think we’ll probably want to collect things and bring them back. “

Looking ahead, Lapsa gave an overview of what we can expect to see next: “We’re going to focus on first-stage development, first-stage engine. Hopefully, we’ll start to roll out some of those milestones that we’re working on. The first stage tank is coming right along. … We’ll [also] fire the engine with all 30 thrusters in the second-stage engine.”

In December 2023, a shortened first-stage test tank, built to the full diameter of 3.7 meters, was successfully proof-tested.

Although Nova is still deep in development and testing, Lapsa ended on an optimistic note: “It’s a development path and the next step, in a real sense, is orbit!”

(Lead image: Thirty chamber engine test. Credit: Stoke Space)

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