Partial answer to
What is "anti-geysering"...
tl;dr Anti-geysering systems are intended to stop geysering, which is a phenomenon that can occur in long vertical pipes of cryogenic fluid that results in fluid erupting out of the pipe into the tank ullage with potentially bad results on the vehicle.
- What is geysering in the first place?
The geyser phenomenon is the rapid expulsion of a boiling liquid and it’s vapor from a vertical
tube. For example, in the case of a long vertical column of fluid connected to a reservoir, such as
the LO2 downcomer to the LO2 tank of the Space Shuttle, the line is subject to ambient heating
from top to bottom. As this heating begins, the fluid adjacent to the wall is warmed and becomes
less dense than the fluid in the center of the line. Because of this density difference, a convection
pattern is established, and warm fluid rises along the tube wall while cooler fluid from the
reservoir descends down the center of the line, keeping the system in equilibrium.
As heating continues, a boundary layer is established along the wall and grows in thickness with
time. This layer also grows in thickness from the bottom of the tube toward the top of the tube.
If the heating rate is sufficient to cause the boundary layer to grow and fill the tube, the cool fluid
flow from the reservoir is stopped, thereby halting the convection pattern. With the cessation of
convection, additional heating causes the temperature to rise in the fluid. Eventually, a portion of
the fluid becomes saturated and boiling begins. Bubbles form on the wall of the feedline and
then detach and start to rise in the liquid. As they rise, they coalesce and form a large "Taylor"
bubble. The formation of the large bubble results in reduced pressure below it, and consequently,
more bubbles are formed in the saturated liquid. This self-sustaining reaction occurs rapidly and
forms vapors faster than they can escape the feedline. As a result, the vapor is rapidly and
violently expelled from the feedline as a geyser. The geyser and subsequent water hammer
effect, caused by rapid tube refill from the reservoir, can cause severe damage to the system.
The expulsion of cryogenic liquid from the feedline into the vehicle tank by the rapidly rising
bubble, generally does not result in damage to the tank or feedline. However, if this expulsion
occurs when the tank contains a large warm ullage volume, implosion of the propellant tank can
occur. Large quantities of cold liquid droplets are expelled into the ullage volume. The expelled
droplets cause rapid cooling and contraction of the warm ullage gas. As a result, the tank
pressure may fall below atmospheric pressure as shown in Figure 6.4-I. If the tank pressure falls
significantly, tank implosion is possible.
This condition is prevented by the installation of a simple deflector over the tank outlet. The
deflector will disperse the expelled liquid and prevent rapid ullage gas cooling. Once the tank is
filled with liquid, and the ullage is small and cold, this geyser effect presents no problem.
The main geysering effect that can cause vehicle damage does not occur during the geyser but
during feedline refill immediately following the geyser. When the geyser occurs, the feedline is
emptied of essentially everything except cold propellant vapors. As the line refills with cold
liquid, the vapors condense at the interface of the liquid. As the vapors condense, they provide
no cushioning for the falling liquid.
The impact pressure at the bottom of the line, resulting from the uncushioned refill can be
extremely high and is unpredictable. This water hammer effect is shown in Figure 6.4-II. High
pressures resulting from the refill can result in catastrophic failure of low pressure, lightweight,
feedlines.
- "anti-geysering" is the provision of systems in the launch vehicle to prevent geysering.
To prevent geysering, the liquid in the feedline must be maintained in a subcooled condition.
- examples of anti-geysering systems
- systems that circulate cool fluid through the feedline
- cooling by helium injection which
...consists instead of the injection of helium directly
into the aft portion of the main feedline. Local saturation is prevented by the evaporation of LO2
into the rising helium bubbles. When the bubbles of helium rise, their volume is increased as the
local pressure in the line is decreased. Evaporation of LO2 into the bubble continues, keeping the
bubble saturated with LO2 vapor.
Source: Space Shuttle Booster Systems Briefs section 6.4
How this cooling by helium injection works physically is well explained in the answer here Why would sub-cooled LOX tanks need to "topped-off" until the last minute or so?
- without any knowledge of specifics on the Electron, I cannot comment on what system they use, or why they disable it at a particular point in the countdown. Shuttle turned off its anti-geyser system about 250 seconds before launch but for reasons that may or may not apply to Electron. This good answer Role of ground-supplied helium in S-1C stage explains that Saturn V turned off their anti-geyser system prior to ignition in order to provide bubble-free fluid to the engines.
