Space Sector

Application

The European Space Agency (ESA) and a UK Space-backed startup specialising in electric propulsion systems for satellites needed a supplier for a fuel tank diaphragm that had been previously produced using a Teflon FEP material. This custom rubber diaphragm is intended for a tank with an internal diameter of approximately 90 mm, which is part of the propulsion system they were developing. Within the tank, the diaphragm separates two fluids: the propellant (H₂O, Kr, Xe, NH₃, and others) and the cooling fluid (R134A).

Problem

To maximise the volume available, the diaphragm needed to follow the internal form of the chamber. This created a significant undercut on the part, making moulding difficult. Careful consideration of material selection was also needed to ensure chemical compatibility, processability and in application performance.

Solution

TRP had previously developed a material for satellite propulsion fuel tanks. A white EPDM (TRP Ref E334), this compound was specifically formulated for use with A.D.N (Ammonium Dinitramide) monopropellant. It also provides excellent resistance to the fuels and coolant on this new project.

Design Concept

Following the geometry of the chamber, the diaphragm was redesigned to include a sealing bead to seal the two halves when assembled.

• The square sealing bead will locate into the current groove. The radiused profile should allow the membrane to invert without causing excess stress to the elastomer.

• The diaphragm was designed with ribs on the outside of the membrane. This has several functions:
  • It reduces stiction of the elastomer to the walls of the vessel. This allows more control in application.
  • Secondly, it gives the diaphragm structure that should allow deformation to be more predictable.
  • Thirdly, it should give the diaphragm support in low-gravity environments to prevent excessive distortion.

Tool design

The three-plate compression mould tool design concept demonstrated the feasibility of removing the diaphragm from the core, overcoming concerns with the undercut. 

The proposed material EPDM E334 with 225% Elongation at Break will stretch over the core during removal without tearing.

Outcome

With hardware already produced, the customer was keen to finalise the design and produce samples for the engineering build phase. The initial design was the first of three sizes, which TRP will continue to develop to support this rapidly growing company with the manufacture of their complete range of propulsion fuel tank diaphragms.