While pinching copper tubes is a standard practice for many applications on Earth, it presents challenges for spaceflight applications.
Pinching copper tubes is not ideal for critical spaceflight applications due to uncertainty in the verification process and the potential for a seemingly “good” pinch to separate later if a cold weld is not correctly achieved. The Mass Spectrometer observing lunar operations (MSolo) team performed a copper tube pinch quality verification analysis that included visual inspections followed by computerized tomography (CT) scans and inverted metallography on several copper tube pinches.
While CT scans and metallography were not useful for definitively deeming a pinch to be acceptable or not, they provided insight into the shape of the pinch, which was useful in conjunction with how the operator “felt” the pinch went.
The pinch point is delicate and can potentially leak if impacted. The likelihood of damaging the pinch point significantly decreases when a physical protective barrier — often an epoxy — is used to protect the pinch point.
NASA Kennedy Space Center Subsystem Lead and Systems Engineer Prital Johnson on the importance of this lesson learned:
Spaceflight can take the most standard practices and turn them into worrisome operations. While pinching copper tubes is a routine, widely used procedure here on Earth, it’s a risky design choice for space applications. A microscopic gap due to a pinch that isn’t properly cold-welded during the pinching procedure can deplete the gas supply during a mission in the vacuum of space. A gap from lack of proper cold-welding may not show up during ground processing even when viewed under a microscope — only to open up during the harsh environment of launch vibrations or thermal fluctuations during the mission. Unlike circumstances on Earth, there’s often no opportunity to re-pinch a tube once the mission launches, even if pressure transducers hint that there may be a leak. Moreover, there are currently no standards to reference how to reliably perform and evaluate copper tube pinches for spaceflight applications.
This lesson learned reviews one copper tube pinch failure instance that was luckily caught before launch. It also highlights best practices that were learned from this failure — practices that may help decrease the risk of a failed copper tube pinch during a mission if the pinch can’t be designed out altogether.