The quote:
"We sub-cool the oxygen and methane to densify it, so compared to... propellants normally used close to their boiling point in most rockets, in our case we actually load the propellants close to their freezing point, and that can result in a density improvement of up to around 10 to 12 percent, which makes an enormous difference in the actual results of the rocket."
"It also makes the... it gets rid of any cavitation risk for the turbo pumps, and it makes it easier to feed a high pressure turbo pump if you have very cold propellant."
edit: This answer to the question "What fundamentally distinguishes cavitation and boiling as different phenomena?" by a self-described mechanical engineer and former US Navy nuke is particularly helpful!
Background:
Some examples of process which include some material undergoing a phase changing from liquid to gas are evaporation, boiling, and cavitation.
As we usually see it, evaporation takes place at a pre-existing and often stable boundary between a liquid and a gas. Even if the temperatures are low and pressure is high, evaporation will continue slowly until the partial pressure of the material that is the liquid is reached in the gas. So if you have room-temperature water sitting in air and they are both at atmospheric pressure, the water will continue to evaporate until the partial pressure of water in the air reaches the vapor pressure of water at those conditions.
But boiling and cavitation generally happen inside a liquid, not outside or above it, and so they depend on both temperature and absolute pressure on the liquid, not just partial pressure as in the case of evaporation.
In both cases, a bubble of vapor is produced within the liquid. When the vapor pressure has been increased above the local hydrostatic pressure in the liquid by the application of heat, we usually call this boiling, and when the hydrostatic pressure has been lowered below the vapor pressure in a transient or highly localized manner, we usually call it cavitation.
But let's just consider the effects of reduced pressure, and not temperature.
If the hydrostatic pressure of a volume of a liquid is lowered below its vapor pressure in bulk, as in the case of a bubble chamber or even a vacuum flask of water attached to a vacuum pump pulling almost an atmosphere of pressure, the liquid will boil. The bubbles may reach the surface and release the vapor, or over time they may re-condense within the liquid, especially if the pressure is restored.
The actual nucleation and growth of such bubbles a complicated process - liquids have a tensile strength just as solids do, and usually nucleation sites will dominate in the actually formation of the bubble - tearing a hole in the liquid so to speak.
In engineering, the use of the word cavitation is generally restricted to situations where the bulk pressure of the liquid is not by itself responsible for the bubble formation. Instead, the term cavitation generally refers to a situation where there is a highly localized and transient drop in pressure below the vapor pressure. Almost immediately after the bubble is formed, the conditions change and the pressure is above the vapor pressure again. While the process is complex, an important feature is the rapidity with which the bubble collapses under the influence of the higher pressure of the liquid.
The terms boiling and cavitation refer to complex phenomena which are related but different. The distinction is between the evolution of the pressure immediately after the bubble is formed. Conceptually there could be a conitnuum of situations between these two extremes, but that does not mean they are the same thing.
This video shows an example of water being boiled using an immersion heater. The temperature of the heating element is much higher than the water. Between about 01:00 and 02:00 the sounds gets louder and louder, and while a few bubbles can be seen, the process making the sounds is the production and collapse of mostly invisible vapor bubbles. While the temperature at certain points on the heater is higher than the boiling point, as soon as a bubble begins to form, part of the bubble expands into cooler areas of the liquid causing the vapor to quickly condense.
This results in a phenomenon similar to cavitation, but on a somewhat longer timescale so that it will not damage the heating element. Boiling water will not cause cavitation damage.
Cavitation is bad because the rapid and violent collapse of the bubble is a result of the very rapid change in pressure over time and or distance. It is this rapidity and localization of bubble collapse that generally determines if a phenomenon is called boiling or cavitation.
Subcooling:
Subcooling of liquid propellant as introduced into a turbo pump on a rocket engine can reduce the possibility of cavitation. At lower temperatures the vapor pressure is lower, and thus it is less likely that a given turpbo pump speed can produce a pressure below this point. However, subcooling also raises the density significantly - one of the main reasons that it is being adopted. This means that the turbo pump can rotate more slowly to deliver the same mass per unit time. Slower rotation means less cavitation even at a fixed temperature.
