After a series of DARPA-supported tests of an all-composite cryogenic propulsion tank, Boeing has succeeded in advancing the new reduced-mass tank technology to Technology Readiness Level 6 (TRL 6).

The all-composite design contrasts to the welded tanks commonly used throughout the industry for rockets’ stages — although Rocket Lab’s Electron is an outstanding example of composites, albeit on a much smaller scale.

For Boeing’s new tank, the all-composite structure is 4.3 meters in diameter and “similar in size to the propellant tanks intended for use in the upper stage of NASA’s Space Launch System (SLS) rocket.”

And that is the part of the advancement that pushes composite technology forward: large, lightweight tanks that can help increase a rocket’s overall performance by reducing vehicle mass — which translates into a greater payload-to-destination mass capability for the overall system.

For the Boeing all-composite tank, testing and modeling so far indicate the potential to increase the SLS Block 1B’s performance by up to 30% if the Exploration Upper Stage’s design with welded tanks were switched to the new all-composite structure.

“That’s a near-term system that could benefit pretty greatly from this sort of technology … should NASA see this as a beneficial upgrade in performance for that part of the mission,” said Jim May, Technology Integration Specialist, Space, and Launch Division, Boeing, in an interview with NASASpaceflight.

Boeing’s all-composite cryogenic fuel tank undergoing pressure testing at NASA’s Marshall Space Flight Center. (Credit: Boeing)

“And, as a lot of us in the space industry know, mass is very important to space systems. And so, as we’ve kind of matured this composite technology, where composites are a good place to reduce the structural mass on things like tanks, we saw an ability or a way to reduce, pretty significantly, the mass in propulsion systems in the tank area — paired with a material that can support and hold in cryogenic fuels.”

But getting to the point of being able to test this technology, bring it to TRL 6, and be ready to offer this new, all-composite structure for future rockets and deep space missions took time and effort.

“So this kind of validated the scale, the ability to build this at scale and meet the same requirements [as metallic tanks]. And the hard part of this technology is doing it at large scale,” related May.

In particular, having a facility large enough to build the tank was a challenge. According to Boeing, “You need a fiber placement facility that can handle a structure of this size to layup in one piece within the material out-time and an autoclave that it will fit in. Not many organizations have composite fabrication facilities that can handle a part of this size.”

After finding the location, the tank underwent final assembly between 2020 and 2021 having originally been intended for DARPA’s Experimental Spaceplane Program.

“DARPA’s interest is in advancing the technology of these propulsion systems,” noted May. “And we’re building the tank in one large composite piece without an incremental metallic liner. But building it to the same requirements you would build metallic tanks to avoid losing your propellant to permissibility.”

Boeing’s reusable Phantom Express spaceplane would have used all-composite cryogenic tanks to deploy an expendable upper stage for orbital launches. (Credit: Mack Crawford for NSF/L2)

Following its construction, the tank was taken to NASA’s Marshall Space Flight Center in Huntsville, AL where it underwent a series of pressure tests and inspections.

“We did a number of pressure cycles and stopped partway through to thoroughly inspect the tank for any damage,” Boeing explained. “That requires draining the tank, moving it out of the test facility, and removing parts for interior inspection. Then putting it all back together, moving back into the test area, and connecting and testing all the instrumentation again. This was not just one simple test sequence.”

In a final test designed to over-pressurize the tank and burst it – known as a test to failure to validate computer modeling – the tank reached a maximum pressure of 3.75 times its design requirement and still did not burst — with Boeing noting that no major structural failures were identified.

In addition to pressure tests, leak and permeation tests were also conducted to verify tank performance and that the tank was not leaking cryogenics.

On top of the size challenge of the composite tank, “the cryogenic part is one of the harder parts of getting composites to work,” related Mr. May.

Per Boeing, “We ran the tank test with liquid nitrogen, but we have done coupon and element tests at both liquid oxygen and liquid hydrogen temperatures. At cold temperatures, some of the properties are slightly lower but coupon and element tests provide the appropriate data that allows us to design a robust composite tank that can meet requirements even at these extremely low temperatures.”

Our team built this cryogenic fuel tank that withstood 3.75 times its intended operational pressures in recent testing. Check out the video of the tank being manufactured. pic.twitter.com/5pq9OGyx42

— Boeing Space (@BoeingSpace) February 3, 2022

Liquid nitrogen boils at -196°C (-320°F). For comparison, liquid oxygen has a maximum stable temperature before evaporation of -183°C (-297°F) while liquid hydrogen has a maximum temperature of -253°C (-423°F).

As Mr. May said, “What our goal was, and what we have shown in our test here, is that we’ve raised the Technology Readiness Level of this technology to about a TRL 6. So what we see is this basically being ready for implementation into a production vehicle design.”

“And that’s the big step forward in this technology in that we think it’s ready for primetime, for real space applications now. And that’s what’s been missing historically in these larger composite tanks is that while there’s been some testing, no one’s really taken it to a point to be ready to push it on to production.”

So where does this fit in terms of future applications?

According to Mr. May, SLS’s pending Exploration Upper Stage — while an obvious candidate — is not the only space application. Boeing sees any deep space mission or rocket that uses cryogenics as potentially benefiting from the technology.

In addition, space is not the only realm where Boeing sees applications for this new tank technology. Aircraft represent another potential market.

Orion extracts a payload from the Exploration Upper Stage after a successful trans-Lunar injection. (Credit: Mack Crawford for NSF)

“Boeing is also looking at ways to have more sustainable aviation technologies. And one way we’re looking at are huge hydrogen-powered aircraft,” said Mr. May. “So this technology that works well for cryogenic storage in space also applies to hydrogen storage in atmospheric conditions as well.”

“And so these tank designs are kind of the first generation of things we’re gonna be looking at that can fit inside of an aircraft and burn hydrogen, which essentially just makes water vapor as its byproduct.”

But will companies and agencies be interested?

“I think with any new technology there can be some reluctance to go after the risk of something new,” admitted May. “The upside here is that we have some evidence of running these tests that these systems perform at least as reliably as historical metallic tanks with the performance gains we get mass-wise from having a composite system.”

“And so we’re ready to propose these into future programs and also be able to show that they’re going to meet the same safety, reliability, and performance statistics of historical different material systems.”

(Lead image: The Exploration Upper Stage prepares for a trans-Lunar injection burn to send Orion on its way to the Lunar Gateway. Credit: Boeing)