MIT Team Researching Reusable Rocket Lifespans
Researchers at the Massachusetts Institute of Technology have reportedly been developing new technology and coatings materials to extend the lifespan of reusable rockets while also reducing their risk of failure.
According to a release from MIT, researchers at the Cordero Lab, based in the MIT Aerospace Materials and Structures Laboratory, have begun multiple projects to solve the reliability problem that reusable rockets face.
About the Rockets
The release states that a new generation of reusable rocket engines and vehicles have the potential for larger payloads and greater reuse.
“The new class of reusable launch vehicles is likely to transform the space industry by lowering launch costs and improving space accessibility,” said Zack Cordero, the Esther and Harold E. Edgerton Career Development Assistant Professor of Aeronautics and Astronautics at MIT.
“This will enable applications such as mega constellations for space-based internet and space-based sensing for things like persistent, real-time CO2 emissions monitoring.”
However, the release mentions recent launch failures like the April 2023 explosion of SpaceX’s Starship prototype, suggesting that the new designs could still have reliability issues.
Now, working with partners including the National Aeronautics and Space Administration, which plans to use Starship for its crewed Artemis missions to the moon, Cordero is using expertise in additive manufacturing (AM), processing science, materials engineering and structural design.
The goal, Cordero stated, is to reduce the maintenance costs and extend the lifespan for reusable rockets while decreasing the chance of catastrophic failure.
The release states that unlike expendable rockets, the "reusable launch vehicles" need to use components and design elements to let them automatically maneuver for soft landings. The new rockets also reportedly need more thermal protection to survive extreme heating during reentry.
“Propulsion devices need to be designed differently for reusable rockets,” stated Cordero. “With reusable liquid propellant rocket engines, you must ensure safe operation over multiple flight cycles and ease off on performance to reduce stress.”
With larger and powerful reusable rockets, the design additions reportedly became even more challenging. “SpaceX’s Raptor and Blue Origin’s BE4 engines operate on different power cycles compared to the Merlin engine,” said Cordero.
“The new staged combustion power cycles are more amenable to reusability because they lower turbine inlet temperatures to extend the life of turbine hardware. Yet, the new power cycles pose a greater risk of catastrophic failure. Oxygen compatibility and metal fires represent critical challenges.”
Cordero stated that he is working to strengthen the components that cap the life of a reusable rocket engine, beginning with the turbopump that pressurizes the liquid propellant.
Other vulnerable components reportedly include the thrust chamber where propellants are burned to create a hot gas, and the nozzle through which the gas is exhausted.
The release adds that extended wear on turbopumps, chambers and nozzles does not always result in a large explosion. However, these do reportedly add to the maintenance and repair costs factored into overall launch payload costs.
“There is a wide spectrum of failure behaviors,” Cordero said. “Thrust chambers can start to crack but continue to function. Yet, turbopumps can have more serious issues. There could be a blisk [a type of rotor disc] failure or in the case of oxygen-rich turbopumps, a rub between the rotor and casing.
“The new engines are also vulnerable to particle impact ignition in which FOD [foreign objects and debris] are accelerated into a surface, igniting the hardware. In a turbopump, these ignition modes can lead to a metal fire and a catastrophic, single-point failure mode that results in the vehicle exploding.”
About the Developments
Cordero stated that additive manufacturing (AM) is now reportedly popular in the space industry, including printing parts for launch vehicles with laser power bed fusion printers. AM is reportedly often used to print metal propulsion devices like the small pumps in gas generator engines, though it is only selectively used in larger boost stage engines and their turbo pumps.
“Space is probably the heaviest user of metal AM and is basically dictating technological developments,” said Cordero.
“We are developing material advances that should enable greater use of AM for larger turbopumps. Our technology enables novel designs with improved thermal efficiency or resilience against high temperatures or rapid thermal transients.”
One major challenge for full-flow staged combustion (Raptor) and oxygen-rich stage combustion (BE-4) engines is the issue of oxidizer compatibility.
“In the turbine and downstream hardware, you often see high-temperature, high-pressure oxygen gas, which can drive metal fires and rapid energetic failure modes,” Cordero explained.
One option for the team is reportedly to design a pump with larger clearances in the rotating hardware, though this plan could degrade performance, causing Cordero to choose another path.
Now, the team is reportedly using metal AM to create more oxygen-compatible materials. “Building oxygen-rich turbopumps with metal AM makes it easier to integrate exotic materials that are more compatible with high-pressure, high-temperature oxygen environments,” he explained.
Cordero Lab is working towards this idea with two projects. The first is reportedly an oxygen-compatible ceramic coating to protect against particle impact ignition. The second is an ignition-resistant AM materials, printed into complex net shapes to avoid friction ignition.
For the coating project, stationary and rotating components in oxygen-rich turbopumps are reportedly covered in an inner ceramic coating to prevent heat transfer to the substrate and protect the metal from high pressure oxygen.
“The advantage of coatings is that you can apply them to almost any kind of hardware whether printed, cast, or forged,” said Cordero.
“Conventional aero coatings tend to delaminate and break apart under the rapid thermal transients that are typical in rockets,” he explained.
“In an aero engine, the engine starts up in over a minute, then idles a few minutes before taking off. By contrast, a rocket engine goes to full throttle in a split second. The rapid change from very low to very high temperatures generates incredible stresses that cause conventional coatings to pop off.”
To solve this problem Cordero stated that his team is using toughened ceramic coatings with embedded metallic ductile phases to keep down delamination through crack bridging. Cordero added that if cracks form in the coating, they could then be bridged and “held in place by metallic inclusions that help it to withstand the thermal transients.”
The release adds that the Cordero Lab has successfully tested the coatings with typical thermal transients seen in rockets and are currently looking into how to use them in real-world flight hardware.
The team, Cordero stated, also wants to “optimize their composition and design for higher turbine inlet temperatures.”
Researchers are now reportedly working with NASA to study the particle impact ignition resistance of the coatings using different thicknesses, particle sizes and operating conditions.
“Our research into fundamental principles of ductile phase toughened environmental barrier coatings should allow us to develop new coatings with chemistries and properties specifically tuned to different applications,” said Cordero, adding that one potential use is to “cover acreage aero-surfaces on hypersonic vehicles.”
Additionally, MIT stated that Cordero Lab’s research into ignition-resistant alloys is a collaboration with Aerospace Corp., a federally funded nonprofit R&D center. The lab is reportedly looking into the what causes frictional ignition, another ignition mode that can lead to metal fires.
Frictional ignition, which “is like striking a match when the match is traveling at 300 meters per second,” is sometimes caused by friction between the rotor and casing, Cordero explained.
To reduce the risk, Cordero is working to build a new printable superalloy material that incorporates oxide nanoparticles for dispersion-strengthening. Named the TGT100, the material reportedly has the ability to be “printed into complex net shapes and offers best-in-class frictional ignition resistance.”
The burn-resistant material is first expected to be used to print casing and stationary hardware. Cordero has reportedly begun a startup called Top Grain Technologies, that is planning to commercialize the material, as well as the ceramic coatings.
“The academics have more time to explore these more fundamental challenges,” he says. “The vision is to bring reliability and reusability of reusable rocket engines up to the standards of aero engines, which would transform the industry.”