PROPELLANT SYSTEM

The propellant system is composed of seven sub­systems : purge, fill and replenish, venting, pres­surization, propellant feed, recirculation, and pro­pellant management..

Purge Subsystem

The purge subsystem uses helium gas to clear the propellant tanks of contaminants before they are loaded. The important contaminants art* oxygen in the liquid hydrogen tank (liquid hydrogen will freeze oxygen which is impact-sensitive) and moisture in the liquid oxygen tank.

The tanks are purged with helium gas from ground storage tanks. The tanks are alternately pressur­ized and vented to dilute the concentration of con­taminants. The operation is repeated until samples of the helium gas emptied from the tanks show that contaminants have been removed or reduced to a safe level.

Fill and Replenish Subsystem

Filling of the propellant tanks on the second stage is a complex and precise task because of the nature of thd cryogenic liquids and the construction of the stage.

Because the metal of the stage is at normal outside temperature, it must be chilled gradually before pumping the ultra-cold propellants into the tanks. The filling operation thus starts with the introduc­tion of cold gas into the tanks, lines, valves, and other components that will come into contact with the cryogenic fluids. The cold gas is circulated until

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Channel Installed—Feed line from IH2 tank to one of the five engines is installed.

the metal has become chilled enough to begin pump­ing in the propellants. The filling and replenishing subsystem operation consists of five phases:

Chilldown – Propellants are first pumped into the tank at the rate of 500 gallons per minute for LQX and 1,000 gallons per minute for LHa. Despite the preliminary chilling by cold gas, the tanks are still so much warmer than the propellants that much of the latter boils off (converts to gaseous form) when it first goes into the tank. Filling con­tinues at this rate until enough of the propellants remain liquid so that the tanks are full to the five per cent level.

Fast Fill —As soon as tank sensors report that the liquid has reached the five per cent level, the fill­ing rate is increased to 5,000 gallons per minute for LOX and 10,000 gallons per minute for LH2. This rate continues until the liquid level in the tank reaches the 98 per cent level.

Slow-Fill—Propellant tanks are filled at the rate of 1,000 gallons per minute for both LOX and LH2 until the 100 per cent level is reached.

Replenishment—Because filling begins many hours before a scheduled liftoff and the cryogenic liquids are constantly boiling off, filling continues almost up to liftoff (160 seconds before liftoff for LOX and 70 seconds before liftoff for LH2). Tanks

are filled at the rate of up to 200 gallons per min­ute for LOX and up to 500 gallons per minute for LH,, depending on signals from sensors in the tanks on the liquid level.

101 Per Cent Shutdown—A sensor in each tank will send a signal to indicate that the 101 per cent level (over the proper fill level) has been reached; this signal causes immediate shutdown of filling operations.

Filling is accomplished through separate connec­tions, lines, and valves. The ground part of the con­nections is covered by special shrouds in which he­lium is circulated during filling operations. This provides an inert atmosphere around the coupling between the ground line and the tanks.

The coupling of the fill line and the tanks is engaged manually at the start of filling operations; it is nor­mally disengaged remotely by applying pneumatic pressure to the coupling lock and actuating a push – off mechanism. A backup method involves a remotely attached lanyard in which the vertical rise of the vehicle will unlock the coupling. The fill valves are designed so that loss of helium pressure or electrical power will automatically close them.

Liquid oxygen is the first propellant to be loaded onto the stage. It is pumped from ground storage tanks. Liquid hydrogen is transferred to the stage by pressurizing the ground storage tanks with gaseous hydrogen. The liquid hydrogen tank is chilled before the liquid oxygen is loaded to avoid structural stresses.

After filling is completed, the fill valves and the liquid oxygen debris valves in the coupling are closed, but the liquid hydrogen debris valve is left open. The liquid oxygen fill line is then drained and purged with helium. The liquid hydrogen line is purged up to the coupling. When a certain signal is received (first stage thrust-commit), the liquid hydrogen debris valve is closed and the coupling is separated from the stage.

The tanks can be drained by pressurizing them, opening the valves, and reversing the filling opera­tion.