Postmortems

Helium buoyancy troubles in rocketry

When there’s an object in a liquid, held down to counteract buoyancy, the force needed to hold it increases when the liquid is accelerated upward. So, the high pressure vessels holding helium for pressurisation or other purposes need to be firmly attached inside the tanks they are placed in, when those tanks are part of a rocket. The hold-downs failed in the SpaceX launch CRS-7, and now a similar problem has afflicted South Korea’s Nuri launch:

South Korea has figured out why its Nuri rocket failed

After weeks of investigation, the committee concluded that the helium tank fell off of its anchoring device inside the oxidizer tank of the rocket due to increased buoyancy during the flight.

The detached tank then caused cracks in the oxidizer tank and damage to the tank pipes as it flew around unfastened, causing leakage of helium and oxidizer.

The lack of oxidizer flowing into the third-stage engine eventually caused the engine to shut off prematurely, officials at the Ministry of Science and ICT said during a press briefing.