Liquid-propellant systems carry the propellant in tanks external to the combustion chamber. Most of these engines use a liquid oxidizer and a liquid fuel, which are transferred from their respective tanks by pumps. The pumps raise the pressure above the operating pressure of the engine, and the propellants are then injected into the engine in a manner that assures atomization and rapid mixing. Liquid-propellant engines have certain features that make them preferable to solid systems in many applications. These features include (1) higher attainable effective exhaust velocities (v_e), (2) higher mass fractions (propellant mass divided by mass of inert components), and (3) control of operating level in flight (throttleability), sometimes including stop-and-restart capability and emergency shutdown. Also, in some applications it is an advantage that propellant loading is delayed until shortly before launch time, a measure that the use of a liquid propellant allows. These features tend to promote the use of liquid systems in many upper-stage applications where high v_e and high propellant mass fraction are particularly important. Liquid systems also have been used extensively as first-stage launch vehicles for space missions, as, for example, in the Saturn (U.S.), Ariane (European), and Energia (Soviet) launch systems. The relative merits of solid and liquid propellants in large launch vehicles are still under debate and involve not only propulsion performance but also issues related to logistics, capital and operating costs of launch sites, recovery and reuse of flight hardware, and so forth.

The typical components of a liquid-rocket propulsion system are the engine, fuel tanks, and vehicle structure with which to hold these parts in place and connect to payload and launch pad (or vehicle). The fuel and oxidizer tanks are usually of very lightweight construction, as they operate at low pressure. In some applications, the propellants are cryogenic (i.e., they are substances like oxygen and hydrogen that are gaseous at ambient conditions and must be tanked at extremely low temperature to be in the liquid state).

The liquid-propellant engine itself consists of a main chamber for mixing and burning the fuel and oxidizer, with the fore end occupied by fuel and oxidizer manifolds and injectors and the aft end composed of the supersonic nozzle. Integral to the main chamber is a coolant jacket through which liquid propellant (usually fuel) is circulated at rates high enough to allow the engine to operate continuously without an excessive increase of temperature in the chamber. Engine operating pressures are usually in the range 1,000–10,000 kilopascals (10–100 atmospheres). The propellants are supplied to the injector manifold at a somewhat higher pressure, usually by high-capacity turbopumps (one for the fuel and another for the oxidizer). From the outside, a liquid-propellant engine often looks like a maze of plumbing, which connects the tanks to the pumps, carries the coolant flow to and from the cooling jackets, and conveys the pumped fluids to the injector. In addition, engines are generally mounted on gimbals so that they can be rotated a few degrees for thrust direction control, and appropriate actuators are connected between the engine (or engines) and the vehicle structure to constrain and rotate the engine.

Each of the main engines of the U.S. space shuttle employs liquid oxygen (LO₂) and liquid hydrogen (LH₂) propellants. These engines represent a very complex, high-performance variety of liquid-propellant rocket. Not only does each have a v_e value of 3,630 metres (11,909 feet) per second but is also capable of thrust-magnitude control over a significant range (2–1). Moreover, the shuttle engines are part of the winged orbiter, which is designed to carry both crew and payload for up to 20 missions.

At the opposite extreme of complexity and performance is a hydrazine thrustor used for attitude control of conventional flight vehicles and unmanned spacecraft. Such a system may employ a valved pressure vessel in place of a pump, and the single propellant flows through a catalyst bed that causes exothermic (heat-releasing) decomposition. The resulting gas is exhausted through a nozzle that is suitably oriented for the required attitude correction. Systems of this kind also are used as gas generators for turbopumps on larger rockets.

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Most liquid-propellant rockets use bipropellant systems—i.e., those in which an oxidizer and a fuel are tanked separately and mixed in the combustion chamber. Desirable properties for propellant combinations are low molecular mass and high temperature of reaction products (for high exhaust velocity), high density (to minimize tank weight), low hazard factor (e.g., corrosivity and toxicity), low environmental impact, and low cost. Choices are based on trade-offs according to the applications. For example, liquid oxygen is widely used because it is a good oxidizer for a number of fuels (giving high flame temperature and low molecular mass) and because it is reasonably dense and relatively inexpensive. It is liquid only below −183 °C (−297 °F), which somewhat limits its availability, but it can be loaded into insulated tanks shortly before launch (and replenished or drained in the event of launch delays). Liquid fluorine or ozone are better oxidizers in some respects but involve more hazard and higher cost. The low temperatures of all of these systems require special design of pumps and other components, and the corrosivity, toxicity, and hazardous characteristics of fluorine and ozone have prevented their use in operational systems. Other oxidizers that have seen operational use are nitric acid (HNO₃), hydrogen peroxide (H₂O₂), and nitrogen tetroxide (N₂O₄), which are liquids under ambient conditions. While some are somewhat noxious chemicals, they are useful in applications where the rocket must be in a near ready-to-fire condition over an extended period of time, as in the case of long-range ballistic missiles.

Liquid hydrogen is usually the best fuel from the standpoint of high exhaust velocity, and it might be used exclusively were it not for the cryogenic requirement and its very low density. Such hydrocarbon fuels as alcohol and kerosene are often preferred because they are liquid under ambient conditions and denser than liquid hydrogen in addition to being more “concentrated” fuels (i.e., they have more fuel atoms in each molecule). The values of exhaust velocity are determined by the relative effects of higher flame (combustion) temperatures and molecular masses of reaction products.

In practice, a variety of choices of propellant systems have been made in major systems, as shown in the table of liquid propellants. In flights where cryogenic propellants can be utilized (e.g., ground-to-Earth-orbit propulsion), liquid oxygen is most often used as the oxidizer. In first stages either a hydrocarbon or liquid hydrogen is employed, while the latter is usually adopted for second stages. In ICBMs and other similar guided missiles that must stand ready for launch on short notice, noncryogenic (or “storable”) propellant systems are used, as, for instance, an oxidizer–fuel mixture of nitrogen tetroxide and hydrazine–unsymmetrical dimethylhydrazine (also designated UDMH; [CH₃]₂ NNH₂). Systems of this sort also find application on longer duration flights such as those involving the space shuttle Orbital Maneuvering System and the Apollo Lunar Module. Solid motors have proved useful on long-duration flights, but liquid systems are often preferred because of the need for stop–start capability or thrust control.

Other systems

As suggested earlier, systems using energy sources independent of the propellant fluid have been studied, and they offer promise for several space missions. In certain systems the propellant is electrically heated at elevated pressure and then accelerated by exhaust through a nozzle. In others the propellant is accelerated without a nozzle (as in the ion and Hall thrusters) by electromagnetic means, in which case at least part of the fluid must be electrically charged first. In these systems the energy source may be nuclear, solar, or beamed energy from an independent source. The outlook for most current missions is that on-board energy sources of this kind would not be suitable for high-thrust missions. There are, however, missions such as flights to other planets where sustained low thrust from on-board energy sources would save on propellant. Such missions originate from an Earth orbit, with flight system and on-board materials being transported to Earth orbit by chemical rocket propulsion. Electrically heated fluids can be used in missions involving manned space stations, where low-thrust capability is needed to control orbit and station attitude. Consideration has been given to the use of human-waste products as propellants; these could be heated electrically from power systems already on board for station operational needs.