Sealing/coatings for cryogenic CFRP surfaces

Objectives
In general tanks requires sealed interfaces (e.g. line IF or removable covers). For both type of interfaces high performance surfaces are needed for leakage tightness, especially in combination with liquid hydrogen and oxygen. The need of further maturation of the surface quality, was identified for the sealed tank (Boss, Cover, LineIF).

The main requirement for the coating are:

• Good bonding to the CFRP base surface (RT, 77K 20K)
• Good coating surface (flatness, roughness, hardness…)
• LOX compatibility
• LH2 tightness
• Processability at 120°C multicuring

A process for surface improvement based on the existing CFRP materials at MTA shall be developed and assessed.

Besides the required application processes of the coating, the assessment of the surface quality is fundamental. Therefore, an inspection method is required to assess the sealed/coated surface with respect to the surface evenness, hardness and layer thickness.

Solution approach
The methods used for surface enhancement, etching, coating, sealing, etc. must be evaluated for their suitability in a cryogenic environment and for the desired surface quality that can be achieved. This includes evaluation of surface roughness, thermo mechanical properties. The process temperature must be below 120°C. For operational use, in addition to a qualified surface inspection method, the scalability of the method for industrial application for components up to 2m in diameter must be clearly recognizable.

Input
MTA will provide a set of geometric and material characteristics of a typical sealed interface of a cryogenic CFRP tank. In addition, a set of surface quality requirements will be provided. Finally, MTA will provide coupon samples of CFRP for manufacturing and inspection trials.

Activities
Based on the specified requirements the contractor/partner shall provide a candidate list of suitable surface improvement methods e.g. etching, coating etc. or laminating and a matching inspection method. A trade shall be performed for the most promising candidates from technical and economical point of view. The most innovative technique shall be selected and further matured proven on dedicated samples provided by MTA. The final surface quality must be assessed at cryogenic temperatures.

Benefits
High performance CFRP surfaces will enable new functional surfaces for cryogenic tanks to be offered within the FLPP Space Transportation Ecosystem Bavaria, reducing cost and improving the quality of launcher and aerospace structures.