A dish best served cold
SpaceX uses supercooled, “deep cryogenic” liquid oxygen, or LOX, to propel its Falcon 9 rockets. This is colder than more commonly used liquid oxygen which is chilled only to its condensation temperature. Principle 35 – Parameter Change
The advantage is that the oxidizer is more dense, allowing SpaceX to cram more onto their rocket.
Here’s a handy guide to LOX temperatures.
Deep Cryogenic LOX: minus 340.6 degrees Fahrenheit
Conventional LOX: minus 297.3 degrees Fahrenheit
Bagels and lox: 72 degrees Fahrenheit
Supercooled LOX has not found widespread use. Conventional LOX is held at its boiling point and as it warms it turns to a gas which can vent harmlessly while the rocket sits on the pad. The tank can be topped up just by adding more LOX.
By contrast, the Falcon rocket can’t sit on the pad indefinitely as its LOX will warm and lose density. SpaceX missions have been aborted because a minor problem couldn’t be resolved in time and the fuel had to be purged.
The explosion
Even worse, through a complex series of interactions, supercooled LOX contributed to the destruction of a SpaceX rocket and its payload in Florida on September 1, 2016.
The explosion was caused by a breach in the cryogenic helium system in the rocket’s second-stage liquid oxygen tank, which in turn caused the LOX tank to rupture and the entire vehicle to be lost in an explosion.
Each SpaceX Merlin engine has a turbo pump bring fuel and oxygen to the rocket engine. This pump has to work very fast and if there were no pressure in the oxygen and fuel tanks, the liquids would cavitate (basically bubbles would form and impact the efficiency of the pump).
To prevent this SpaceX uses pressure vessels nestled within the fuel and oxygen tanks that contain pressurized helium. This helium is released into the tanks to keep the pressure up. Not only does this improve fueling, but it also keeps the tanks rigid. Principle 29 – Pneumatics and Hydraulics
To resist the high pressures inside these helium tanks, COPVs, or composite over wrapped pressure vessels are used. In the Falcon 9, these vessels use a thin tank of aluminum wrapped in carbon composite material to resist the high internal pressures. Principle 40 – Composite Materials
In order to hasten fueling and avoid loss of propellant capacity due to the oxygen warming, expanding and being vented, SpaceX simultaneously filled the COPV tanks with even colder helium. This stressed the COPVs and opened cracks in the composite over wrap. Supercooled LOX found its way into the cracks, then froze against the even colder helium, which caused it to expand (like an asphalt road cracking when ice is formed within its structure).
SpaceX concluded that the carbon fibres shifted and broke as a result, friction “or other mechanisms” ignited oxygen in the upper stage. This caused sudden failure of the COPV tank and the rest of the failures cascaded from that point.
Mitigation
In the short term SpaceX resolved to fill the helium tanks in advance of LOX loading, to prevent this mode of failure. These changes have proven to be successful, with SpaceX resuming launches four months later. 2017 proved to be its most successful year so far with 18 successful launches.
SpaceX has also committed to redesign the Falcon’s COPV tanks with the hopes of winning Nasa’s approval for launches with humans aboard.
