Tank Pressurization

Subsystems for pressurizing tanks are needed for both of the two types of feed systems, namely pressure feed systems and pump feed systems. The tank pressures for the first type are usually between 200 and 1800 psi and for the second between 10 and 50 psig. Refs. 6-1, 6-3 to 6-5 give further descriptions. Inert gases such as helium or nitrogen are the most common method of pressuriza-tion. In pump feed systems a small positive pressure in the tank is needed to suppress pump cavitation. For cryogenic propellants this has been accomplished by heating and vaporizing a small portion of the propellant taken from the high-pressure discharge of the pump and feeding it into the propellant tank, as shown in Fig. 1-4. This is a type of low-pressure gas feed system.

The pressurizing gas must not condense, or be soluble in the liquid propellant, for this can greatly increase the mass of required pressurant and the inert mass of its pressurization system hardware. For example, nitrogen pressurizing gas will dissolve in nitrogen tetroxide or in liquid oxygen and reduce the concentration and density of the oxidizer. In general, about 2 5 times as much nitrogen mass is needed for pressurizing liquid oxygen if compared to the nitrogen needed for displacing an equivalent volume of water at the same pressure. Oxygen and nitrogen tetroxide are therefore usually pressurized with helium gas, which dissolves only slightly. The pressurizing gas must not react chemically with the liquid propellant. Also, the gas must be dry, since moisture can react with some propellants or dilute them.

The pressurizing gas above a cryogenic liquid is usually warmer than the liquid. The heat transfer to the liquid cools the gas and that increases the density; therefore a larger mass of gas is needed for pressurization even if none of the gas dissolves in the liquid propellant. If there is major sloshing and splashing in the tank during flight, the gas temperature can drop quickly, causing irregularities in the tank pressure.

Chemical pressurization permits the injection of a small amount of fuel or other suitable spontaneously ignitable chemical into the oxidizer tank (or vice versa) which creates the pressurizing gas by combustion inside the propellant tank. While ideally this type of pressurization system is very small and light, in practice it has not usually given reproducible tank pressures, because of irre gular combustion the sloshing of propellant in the tank during vehicle maneuvers has caused sudden cooling of the hot pressurizing gas and thus some erratic tank pressure changes. This problem can be avoided by physically separating the hot reactive gas from the liquid propellant by a piston or a flexible bladder. If hot gas from a solid propellant gas generator of from the decomposition of a monopropellant is used (instead of a high-pressure gas supply), a substantial reduction in the gas and inert mass of the pressurizing system can be achieved. For example, the pressurizing of hydrazine monopropellant by warm gas (from the catalytic decomposition of hydrazine) has been successful for moderate durations.

The prepackaged compact experimental liquid propellant rocket engine shown in Fig. 6-8 is unique. It uses a gelling agent to improve propellant safety and density (see Section 7.5 and Ref. 7-11), a solid propellant for pressuriza-tion of propellant tanks, two concentric annular pistons (positive expulsion), and a throttling and multiple restart capability. It allows missiles to lock on to targets before or after launch, slow down and search for targets, loiter, maneuver, or speed up to a high terminal velocity. This particular experimental engine, developed by TRW, has been launched from a regular Army mobile launcher.

Thrust chamber with ablative liner and face shutoff valve in injector

Graphite fiber overw rapped propellant tank

Electronic controls

Hydraulic pilot valve

FIGURE 6-8. Simplified diagram of a compact pre-loaded, pressure-fed, bipropellant experimental rocket engine aimed at propelling smart maneuvering ground-to-ground missiles. It uses gelled red fuming nitric acid and gelled monomethylhydrazine as pro-pellants. A solid propellant gas generator provides the gas for tank pressurization and the hot gases are isolated from the propellants by pistons. The concentric spray injector allows restart, throttling, and flow shut-off at the injector face. The rocket engine is 6 in. diameter and 23.5 in. long. (Courtesy of Space and Electronics Group, TRW, Inc.)

Gas generator

Cable/waveguide passth rough

Fuel tank

Oxidizer tank

Fill valves

Thrust chamber with ablative liner and face shutoff valve in injector

Hydraulic pilot valve

Electronic controls

Graphite fiber overw rapped propellant tank

FIGURE 6-8. Simplified diagram of a compact pre-loaded, pressure-fed, bipropellant experimental rocket engine aimed at propelling smart maneuvering ground-to-ground missiles. It uses gelled red fuming nitric acid and gelled monomethylhydrazine as pro-pellants. A solid propellant gas generator provides the gas for tank pressurization and the hot gases are isolated from the propellants by pistons. The concentric spray injector allows restart, throttling, and flow shut-off at the injector face. The rocket engine is 6 in. diameter and 23.5 in. long. (Courtesy of Space and Electronics Group, TRW, Inc.)

Estimating the Mass of the Pressurizing Gas

The major function of the pressurizing gas is to expel the propellants from their tanks. In some propulsion system installations, a small amount of the pressurized gas also performs other functions such as the operation of valves and controls. The first part of the gas leaving the high-pressure-gas storage tank is at ambient temperature. If the high-pressure gas expands rapidly, then the gas remaining in the tank undergoes essentially an isentropic expansion, causing the temperature of the gas to decrease steadily; the last portions of the pressurizing gas leaving the tank are very much colder than the ambient temperature and readily absorb heat from the piping and the tank walls. The loule-Thomson effect causes a further small temperature change.

A simplified analysis of the pressurization of a propellant tank can be made on the basis of the conservation of energy principle by assuming an adiabatic process (no heat transfer to or from the walls), an ideal gas, and a negligibly small initial mass of gas in the piping and the propellant tank. Let the initial condition in the gas tank be given by subscript 0 and the instantaneous conditions in the gas tank by subscript g and in the propellant tank by subscript p. The gas energy after and before propellant expulsion is mgcv Tg + mpcv Tp +PpVp = m0cv T0 (6-5)

The work done by the gas in displacing the propellants is given by pp Vp. Using Eqs. 3-3 to 3-5, the initial storage gas mass mQ may be found.

cvPg vo/R + cvpp Vp/R + ppVp = m0cv T0 m0 = (pgVQ+PpVpk)/(RT0)

This may be expressed as

Po RT0 RT0\ 1 Pg/PoJ

The first term in this equation expresses the mass of gas required to empty a completely filled propellant tank if the gas temperature is maintained at the initial storage temperature T0. The second term expresses the availability of the storage gas as a function of the pressure ratio through which the gas expands.

Heating of the pressurizing gas reduces the storage gas and tank mass requirements and can be accomplished by putting a heat exchanger into the gas line. Heat from the rocket thrust chamber, the exhaust gases, or from other devices can be used as the energy source. The reduction of storage gas mass depends largely on the type and design of the heat exchanger and the duration.

If the expansion of the high-pressure gas proceeds slowly (e.g., with an attitude control propulsion system with many short pulses over a long period of time), then the gas expansion comes close to an isothermal process; heat is absorbed from the vehicle and the gas temperature does not decrease appreciably. Here T0 = Tg = Tp. The actual process is between an adiabatic and an isothermal process and may vary from flight to flight.

The heating and cooling effects of the tank and pipe walls, the liquid pro-pellants, and the values on the pressurizing gas require an iterative analysis. The effects of heat transfer from sources in the vehicle, changes in the mission profile, vaporization of the propellant in the tanks, and heat losses from the tank to the atmosphere or space have to be included and the analyses can become quite complex. The design of storage tanks therefore allows a reasonable excess of pressurizing gas to account for these effects, for ambient temperature variations, and for the absorption of gas by the propellant. Equation 6-7 is therefore valid only under ideal conditions.

Example 6-2. What air tank volume is required to pressurize the propellant tanks of a 9000-N thrust rocket thrust chamber using 90% hydrogen peroxide as a monopropel-lant at a chamber pressure of 2.00 MPa for 30 sec in conjunction with a solid catalyst? The air tank pressure is 14 MPa and the propellant tank pressure is 3.0 MPa. Allow for 1.20% residual propellant.

SOLUTION. The exhaust velocity is 1300 m/sec and the required propellant flow can be found from Eq. 3^2 = 1.06):

The total propellant required is m = 7.34 kg/sec x 30 sec x 1.012 = 222.6 kg. The density of 90% hydrogen peroxide is 1388 kg/m3. The propellant volume is 222.6/1388 = 0.160 m3. With 5% allowed for ullage and excess propellants, Eq. 6-7 gives the required weight of air (R = 289 J/kg-K; T0 = 298 K; k = 1.40) for displacing the liquid.

_ppVp k _ 3.0 x 106 x 0.16 x 1.05 x 1.4 m° ~ RT0 [1 - (pg/p0)] ~ 289 x 298 x [1 -(3/14)] = 10.4 kg of compressed air

With an additional 5% allowed for excess gas, the high-pressure tank volume will be

VQ = m0RT0/p0 = 1.05 x 10.4 x 289 x 298/(14 x 106) = 0.067 m3.

Was this article helpful?

0 -1
Project Management Made Easy

Project Management Made Easy

What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.

Get My Free Ebook


Post a comment