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The race to space is being run at full speed by governments and private companies, fueled by a dramatic drop in cost thanks to SpaceX and reusable rockets. Advancing the maturity of reusable rocket technologies in Europe is the aim of the EU Horizon project (2022-2026). Within this project, (MTA, Augsburg, Germany) is developing carbon fiber-reinforced polymer (CFRP) composite landing legs for the Themis3 (T3) launcher.

is an ongoing (ESA, Paris, France) program led by (Paris) that is developing prototype reusable rockets toward the first fully reusable European launcher, dubbed ArianeNext, targeted to fly in the 2030s. These prototypes will advance multiple systems and technologies.

Based on previous knowledge gained from a sub-scale composite landing leg developed in the project (2019-2022), MTA has designed, manufactured and tested a full-scale, 7-meter-long CFRP landing leg demonstrator, successfully made using automated fiber placement (AFP), foam core and 3D printed tooling.

“The work package to build the landing leg demonstrator had a very limited budget and timeline,” explains Peter Ortmann, head of AFP and senior engineer in composite materials and process technologies at MTA. “Our objective was to deliver the full-scale landing leg demonstrator for testing on-time and on-cost. We received boundary conditions from the sub-scale demonstrator in the RETALT project, which had used five elements riveted together. But our approach was to produce the aerodynamic cover and load-carrying structure with interface points as a single, integrated part.”

Testing in 2024 validated MTA’s design, manufacturing approach and structural performance as well as successful deployment, locking and unlocking.

Advancing composites in space

MTA is part of (Bremen, Germany), one of the largest European space companies with more than 3,000 employees at 15 locations. Its SPACE SYSTEMS business includes development, launch and operations for satellites, manned spaceflight, space exploration and security/reconnaissance technologies as well as developing and implementing payloads, scientific equipment and devices for aerospace, research institutes and industry. Its DIGITAL business segment leverages its space capabilities and enables technology transfer, while MTA is part of its AEROSPACE business, which manufactures parts and structures for aviation and space travel.

MTA was established in 1969 as MAN New Technologies. With 500 employees, it builds components for aviation and space including for launchers, satellites and other spacecraft. It also builds critical structures and tanks and provides development services. The company has a ~10% workshare in the EU’s current launcher, Ariane 6, including metallic and composite structures. Composite structures include tanks and structures for the Prototype of a Highly OptimizEd Black Upper Stage (PHOEBUS) aimed for a future Ariane 6 evolution.

Landing leg requirements

To ensure reusability, the T3 landing legs must be robust, lightweight and serviceable. During flight, the legs are folded up to rest against the reusable first stage rocket. Upon return, they deploy under gravity without external systems. To enable this, MTA developed a new kinematic system which includes a mechanism to lock the legs in place once deployed and unlock them after landing for reattachment to the launcher during ground handling.

Each T3 landing leg comprises two assemblies. The shock absorber assembly (SAAY) includes three CFRP tubes, the locking/unlocking mechanism and a damper system which helps to dissipate kinetic energy during touchdown, while the landing leg assembly (LLAY) integrates CFRP load-carrying structures and aerodynamic cover/aeroshell.

Requirements for the landing leg are manifold. Its structure must be optimized for aerothermal and aerodynamic loads to reduce drag during ascent with a geometry that guarantees stable landing with no tipping. The system must also reliably provide correct deployment to ensure ground clearance for the rocket nozzles and withstand the estimated T3 launcher mass of 15,200 kilograms upon landing as well as static, dynamic, acoustic and buffeting/vibration loads during ascent and descent.

Developing the T3 landing leg design

Design development included a static loop using finite element modeling (FEM) and quasi static load (QSL) analysis as well as a dynamic loop using multi-body dynamics (MBD). The FEM and QSL analysis was done in Nastran 2017.1. The model was meshed with bar elements for the SAAY and 2D shell elements for the LLAY using material properties developed by MTA.

The initial LLAY design comprised a CFRP bay and struts that sustain tension, bending and torsion loads combined with a CFRP crossbeam that increases stiffness against buckling and bending effects. It must also withstand tension and compression loads during deployment and touchdown. The crossbeam was later replaced with local reinforcements in the aeroshell.

The MBD analysis was performed in the MSC Adams 2021 software (Hexagon AB, Stockholm, Sweden). This elasto-kinematic analysis was used to determine the reaction forces during deployment. The mathematical model includes:

• Rigid components for application using contact formulation, for example, on complex components.
• Flexible bodies for SAAY and LLAY stiffness representation via the super element technique (SET).
• Structural mass points to represent mass of inertia of components not represented in geometrical detail.
• Spring elements that represent the interface stiffness of the test rig.

Deployment simulation assumed gravity loading without friction — which has only a minor effect on reaction forces — and represents the complete kinematics of unfolding from vertical position to latching of the locking mechanism at the deployed position.

CFRP structures using AFP, foam core

The LLAY must withstand dynamic tension and compression loads during deployment and touchdown. It is mostly CFRP but does use titanium fittings at the top where it attaches to the launcher and also at the bottom for the landing pad. The CFRP structure includes bay and struts that sustain tension, bending and torsion loads integrated with the aeroshell, which uses local reinforcements to increase stiffness against buckling and bending effects.

Ortmann’s team actually did a small trade study comparing CFRP to a metallic approach for the leg. “But we could only meet the reduced mass requirements for the whole system with CFRP,” he explains. “We were also convinced that our integrated design would produce benefits for both reduced mass and cost. Assembly is an important cost driver, and when you have only one part you have a lot of cost savings.”

Ortmann’s team would manufacture the CFRP parts using its Coriolis Composites (Quéven, France) C1 AFP machine. This enables layup to be fully automated with high accuracy, reproducibility and quality assurance. Using the AFP CAM software CATFIBER2023 for Dassault Systems CATIA V5R28, the team quickly ran simulations to complete a design feasibility study for the integrated LLAY, evaluating the geometry parameters and AFP head accessibility on female versus male layup tooling.

“We have a large engineering and analysis department,” says Ortmann, “and we worked with them to further define the specifications for this approach. We calculated part thicknesses and fiber distribution in areas, and then my team optimized this for the AFP process. The design/analysis team built an FE model and performed the structural analysis, and we made a few iterations with them.”

Part of this work entailed modeling the localized fiber orientation AFP enables. A global fiber orientation in FEM would not accurately represent the actual fiber architecture. To deal with this, MTA established an interface between the FEM in Hypermesh and the AFP manufacturing environment from CATFIBER2023. The CATIA V5R28 Composite Link (CCL) plugin was used to map the actual AFP ply contours and fiber directions onto a mesh that was previously extracted from Hypermesh. After the reimport of the mapped file exported with CCL, this resulted in a complex 2D shell element property distribution in Hypermesh. The figure at right shows these results for the inner facesheet of the LLAY as well as the corresponding thickness of the laminate.

“We then finalized a design and also for the layup in our AFP software,” says Ortmann. “We used a lot of 0° fibers in the load-carrying structure and moved more toward a quasi-isotropic layup in the aerodynamic covering part.”

The AFP layup used standard 1/4-inch-wide slit carbon fiber/epoxy prepreg tape. “It was one of the three materials we have qualified, which we have used for satellite central structures produced over the last 2 years,” he explains. “We produce almost everything in our aerospace projects with intermediate modulus carbon fiber because it is often the best compromise between stiffness and strength.”

The part also used foam core. “The CFRP struts are the main load-carrying structures that must support the weight of the launcher,” says Ortmann. “This integrated design transitions from monolithic laminate to cored laminate at the ends that can be hollowed for the interface with the titanium hinge fittings at top and for the telescoping system and landing pad at bottom. We don’t need the foam for structure, but they create these hollow areas that must be rigid and handle high loads.”

This design, he notes, is a result of the optimization performed by his AFP team. “The whole structure is integral, including the monolithic and foam cored layups. So, there are some areas which were very complicated for the AFP programming and also for compaction feasibility. These pushed the limits of what the AFP head can do.” In the end, Ortmann’s team used structural foam only because other foams were not compatible with the layup’s autoclave cure at 7 bar and 180°C.

The CFRP tubes for the SAAY were also produced with AFP. The advantage of this approach over filament winding included the ability to produce 0° dominated laminates with high outer surface quality within tight geometry tolerances, says Ortmann, which was mandatory to meet the requirements of the telescope mechanism.

3D printed tooling

The conventional choice for the layup and cure tool would have been to use the same or similar CFRP prepreg as in the part. This ensures that the coefficient of thermal expansion (CTE) and resulting dimensional changes during cure are the same in the part and tool. But instead, Ortmann’s team chose to use 3D printed tooling to meet the demonstrator’s tight timeline and budget.

The tooling tapered from 2.6 meters to between 600 and 700 millimeters wide and was roughly 7 meters long. “For 90% of all the projects I've made for the last 10 years, we’ve used traditional CFRP prepreg tooling,” says Ortmann. “But when we talked to our conventional tooling supplier, the lead time was too long.”

This is when the team turned to Ingersoll Machine Tools (Rockford, Ill., U.S.), which produced the layup and cure tool using its MasterPrint large-format, thermoplastic composite 3D printer. “They were able to complete the tooling in less than 1 month, where the lead time for conventional tooling was three to four times that,” says Ortmann. “This was our first project to use 3D printed tooling. Ingersoll was able to produce it as a single print, and it enabled us to meet the project objectives.”

There were issues, however, mainly with CTE mismatch. Ortmann notes that the 3D printed tooling used polyetherimide (PEI) reinforced with chopped carbon fiber which had an anisotropic behavior. “The fibers inside the thermoplastic resin have an orientation in the printing direction,” he explains, “so you have a factor of 10 difference between the CTE in the print direction [in plane in each printed layer] versus through the print. Ingersoll made a lot of calculations and measurements. The CTE compensation was very complex. We learned a lot and not everything was perfect, especially the interfaces between the foam core areas and the monolithic laminate. But our plan is to work with them [Ingersoll] in the future because I’m convinced that, especially for prototyping, this is an extremely good technology.”

However, for serial production, Ortmann believes that classical CFRP tooling may still be the better option for 180°C cure of complex parts. “That’s only because it ensures there is no CTE mismatch,” he says. “But perhaps this will change in the future, as Ingersoll develops this technology further, and is able to reduce the CTE difference in the two directions. Because the lead time and cost for this 3D printed tooling are so much less, the final price was approximately half that of conventional autoclave-cured prepreg tooling.”

Manufacturing, testing, next steps

AFP layup of the CFRP demonstrator took approximately 2 weeks, says Ortmann, “but in serial production I think you could do this in 2-3 days. That’s one reason why I think this is a good approach. We used conventional vacuum bagging on the tooling and a conventional, standard autoclave cure cycle.” After demolding, the team performed manual ultrasound on some local areas. “But for serial production,” says Ortmann, “we’ll need to explore how to inspect this structure in an effective way using automation.”

In July 2024, MTA tested deployment of the 7-meter CFRP landing leg together with the telescoping system and integrated locking mechanism. The test campaign began with small drop distances and the alignment of myriad measuring points. The demonstrator was finally brought into an upright position for the final deployment test, simulating a realistic rocket landing. Both vertical deployment under gravity and locking were successful.

The dots shown in the images above were used as part of the (Zeiss, Oberkochen, Germany), a noncontact 3D digital image correlation (DIC) and metrology technology that enables high-speed measurements during static and dynamic load tests. High-speed cameras and software track displacements over time via numbered reference points and provide visualization and post-test analysis. MTA used this for correlating simulation and physical test results. Accelerometers and strain gauges were also used.

“This has been a big success for us,” says Ortmann. “The leg has been tested at full scale, proving its operation and mechanics including deployment and locking. The results also correlated well with our simulation.”

When asked what was the largest achievement, he notes the design for manufacturing. “We did not have a large team, but we worked in a very pragmatic way that functioned well. We were able to produce an integrated, manufacturing-optimized design using automation in a very short time with an equally limited budget. Normally, a system with these dimensions and requirements would require at least three times more money. And many said it would be impossible to produce the whole demonstrator. But we were very focused from the beginning and were able to deliver. We have also collected a lot of data to continue product development and will now further optimize this landing leg system toward future launches.”