Propellant Tanks

In liquid bipropellant rocket engine systems propellants are stored in one or more oxidizer tanks and one or more fuel tanks; monopropellant rocket engine systems have, of course, only one set of propellant tanks. There are also one or more high-pressure gas tanks, the gas being used to pressurize the propellant tanks. Tanks can be arranged in a variety of ways, and the tank design can be used to exercise some control over the change in the location of the vehicle's center of gravity. Typical arrangements are shown in Fig. 6-5. Because the propellant tank has to fly, its mass is at a premium and the tank material is therefore highly stressed. Common tank materials are aluminum, stainless steel, titanium, alloy steel, and fiber-reinforced plastics with an impervious thin inner liner of metal to prevent leakage through the pores of the fiber-reinforced walls.

The extra volume of gas above the propellant in sealed tanks is called ullage. It is necessary space that allows for thermal expansion of the propellant liquids, for the accumulation of gases that were originally dissolved in the propellant, or for gaseous products from slow reactions within the propellant during storage. Depending on the storage temperature range, the propellants' coefficient of thermal expansion, and the particular application, the ullage volume is usually between 3 and 10% of the tank volume. Once propellant is loaded into

3 Oxidizer tanks

6 Fuel tanks

3 Oxidizer tanks

6 Fuel tanks

Tandem tanks with common bulkhead, internal piping

Concentric tanks l. y l. y

Multi-tank

Spherical tanks Tandem tanks, external piping

Tandem tanks with common bulkhead, internal piping

Concentric tanks

Multi-tank

FIGURE 6-5. Typical tank arrangements for large turbopump-fed liquid propellant rocket engines.

a tank, the ullage volume (and, if it is sealed, also its pressure) will change as the bulk temperature of the propellant varies.

The expulsion efficiency of a tank and/or propellant piping system is the amount of propellant expelled or available divided by the total amount of propellant initially present. Typical values are 97 to 99.7%. The losses are unavailable propellants that are trapped in grooves or corners of pipes, fittings, and valves, are wetting the walls, retained by surface tension, or caught in instrument taps. This residual propellant is not available for combustion and must be treated as inert mass, causing the vehicle mass ratio to decrease slightly. In the design of tanks and piping systems, an effort is made to minimize the residual propellant.

The optimum shape of a propellant tank (and also a gas pressurizing tank) is spherical, because for a given volume it results in a tank with the least weight. Small spherical tanks are often used with reaction control engine systems, where they can be packaged with other vehicle equipment. Unfortunately, the larger spheres, which are needed for the principal propulsion systems, are not very efficient for using the space in a vehicle. These larger tanks are often made integral with the vehicle fuselage or wing. Most are cylindrical with half ellipses at the ends, but they can be irregular in shape. A more detailed discussion of tank pressurization is given in the next section.

Cryogenic propellants cool the tank wall temperature far below the ambient air temperature. This causes condensation of moisture on the outside of the tank and usually also formation of ice during the period prior to launch. The ice is undesirable, because it increases the vehicle inert mass and can cause valves to malfunction. Also, as pieces of ice are shaken off or break off during the initial flight, these pieces can damage the vehicle; for example, the ice from the Shuttle's cryogenic tank can hit the orbiter vehicle.

For an extended storage period, cryogenic tanks are usually thermally insulated; porous external insulation layers have to be sealed to prevent moisture from being condensed inside the insulation layer. With liquid hydrogen it is possible to liquify or solidify the ambient air on the outside of the fuel tank. Even with heavy insulation and low-conductivity structural tank supports, it is not possible to prevent the continuous evaporation of the cryogenic fluid. Even with good thermal insulation, all cryogenic propellants evaporate slowly during storage and therefore cannot be kept in a vehicle for more than perhaps a week without refilling of the tanks. For vehicles that need to be stored or to operate for longer periods, a storable propellant combination must be used.

Prior to loading very cold cryogenic propellant into a flight tank, it is necessary to remove or evacuate the air to avoid forming solid air particles or condensing any moisture as ice. These frozen particles would plug up injection holes, cause valves to freeze shut, or prevent valves from being fully closed. Tanks, piping, and valves need to be chilled or cooled down before they can contain cryogenic liquid without excessive bubbling. This is usually done by letting the initial amount of cryogenic liquid absorb the heat from the relatively warm hardware. This initial propellant is vaporized and vented through appropriate vent valves.

If the tank or any segment of piping containing low-temperature cryogenic liquid is sealed for an extended period of time, heat from ambient-temperature hardware will result in evaporation and this will greatly raise the pressure until it exceeds the strength of the container (see Ref. 6-7). This self-pressurization will cause a failure, usually a major leak or even an explosion. All cryogenic tanks and piping systems are therefore vented during storage on the launch pad, equipped with pressure safety devices (such as burst diaphragms or relief valves), and the evaporated propellant is allowed to escape from its container. For long-term storage of cryogenic propellants in space vacuum (or on the ground) some form of a powered refrigeration system is needed to recondense the vapors and minimize evaporation losses. The tanks are refilled or topped off just before launch to replace the evaporated vented propellant. When the tank is pressurized, just before launch, the boiling point is usually raised slightly and the cryogenic liquid can usually absorb the heat transferred to it during the several minutes of rocket firing.

There are several categories of tanks in liquid propellant propulsion systems:

1. For pressurized feed systems the propellant tanks typically operate at an average pressure between 1.3 and 9 MPa or about 200 to 1800 lbf/in.2. These tanks have thick walls and are heavy.

2. For high-pressure gas (used to expel the propellants) the tank pressures are much higher, typically between 6.9 and 69 MPa or 1000 to 10,000 lbf/ in.2. These tanks are usually spherical for minimum inert mass. Several small spherical tanks can be connected together and then they are relatively easy to place within the confined space of a vehicle.

3. For turbopump feed systems it is necessary to pressurize the propellant tanks slightly (to suppress pump cavitation as explained in Section 10.1) to average values of between 0.07 and 0.34 MPa or 10 to 50 lbf/in2. These low pressures allow thin tank walls, and therefore turbopump feed systems have relatively low tank weights.

Liquid propellant tanks can be difficult to empty under side accelerations, zero-g, or negative-^ conditions during flight. Special devices and special types of tanks are needed to operate under these conditions. Some of the effects that have to be overcome are described below.

The oscillations and side accelerations of vehicles in flight can cause sloshing of the liquid in the tank, very similar to a glass of water that is being jiggled. In an antiaircraft missile, for example, the side accelerations can be large and can initiate sloshing. Typical analysis of sloshing can be found in Refs. 6-8 and 6-9. When the tank is partly empty, sloshing can uncover the tank outlet and allow gas bubbles to enter into the propellant discharge line. These bubbles can cause major combustion problems in the thrust chambers; the aspirating of bubbles or the uncovering of tank outlets by liquids therefore needs to be avoided. Sloshing also causes shifts in the vehicle's center of gravity and makes flight control difficult.

Vortexing can also allow gas to enter the tank outlet pipe; this phenomenon is similar to the Coriolis force effects in bath tubs being emptied and can be augmented if the vehicle spins or rotates in fight. Typically, a series of internal baffles is often used to reduce the magnitude of sloshing and vortexing in tanks with modest side accelerations. A positive expulsion mechanism can prevent gas from entering the propellant piping under multidirectional major accelerations or spinning (centrifugal) acceleration. Both the vortexing and sloshing can greatly increase the unavailable or residual propellant, and thus cause a reduction in vehicle performance.

In the gravity-free environment of space, the stored liquid will float around in a partly emptied tank and may not always cover the tank outlet, thus allowing gas to enter the tank outlet or discharge pipe. Figure 6-6 shows that gas bubbles have no orientation. Various devices have been developed to solve this problem: namely, positive expulsion devices and surface tension devices. The positive expulsion tank design include movable pistons, inflatable

Liquid out

Outlet

Liquid out

Gas in

Screen-type surface tension propellant management device showing one particular liquid distribution during upward acceleration

Gas in

Screen-type surface tension propellant management device showing one particular liquid distribution during upward acceleration

Typical liquid gas interface shapes for wetting liquids in zero-g conditions with different ullage volumes

FIGURE 6-6. Ullage bubbles can float around in a zero-gravity environment; surface tension device can keep tank outlet covered with liquid.

flexible bladders, or thin movable, flexible metal diaphragms. Surface tension devices rely on surface tension forces to keep the outlet covered with liquid.

Several basic types of positive expulsion devices have been used successfully in propellant tanks of pressurized feed systems. They are compared in Table 6-5 and shown in Fig. 6-7 for simple tanks. These devices mechanically separate the pressurizing gas from the liquid propellant in the propellant tank. Separation is needed for these reasons:

1. It prevents pressurizing gas from dissolving in the propellant. Dissolved pressurizing gas dilutes the propellant, reduces its density as well as its specific impulse, and makes the pressurization inefficient.

2. It allows hot and reactive gases (generated by gas generators) to be used for pressurization, and this permits a reduction in pressurizing system mass and volume. The mechanical separation prevents a chemical reaction between the hot gas and the propellant, prevents gas from being dissolved in the propellant, and reduces the heat transfer to the liquid.

3. In some cases tanks containing toxic propellant must be vented without spilling any toxic liquid propellant or its vapor. For example, in servicing

TABLE 6-5. Comparison of Propellant Expulsion Methods for Spacecraft Hydrazine Tanks

Positive Expulsion Devices

TABLE 6-5. Comparison of Propellant Expulsion Methods for Spacecraft Hydrazine Tanks

Positive Expulsion Devices

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