Liquid-propellant rocket

A liquid-propellant rocket or liquid rocket uses a rocket engine burning liquid propellants. Liquids are desirable propellants because they have reasonably high density and their combustion products have high specific impulse. This allows the volume of the propellant tanks to be relatively low.

Types

Liquid rockets can be monopropellant rockets using a single type of propellant, or bipropellant rockets using two types of propellant. Tripropellant rockets using three types of propellant are rare. Liquid oxidizer propellants are also used in hybrid rockets, with some of the advantages of a solid rocket. Bipropellant liquid rockets use a liquid fuel such as liquid hydrogen or RP-1, and a liquid oxidizer such as liquid oxygen. The engine may be a cryogenic rocket engine, where the fuel and oxidizer, such as hydrogen and oxygen, are gases which have been liquefied at very low temperatures.
Most designs of liquid rocket engines are throttleable for variable thrust operation. Some allow control of the propellant mixture ratio. Some can be shut down and, with a suitable ignition system or self-igniting propellant, restarted.
Hybrid rockets apply a liquid or gaseous oxidizer to a solid fuel.

Advantages and disadvantages

The use of liquid propellants has a number of advantages:

A liquid rocket engine can be tested prior to use, whereas for a solid rocket motor a rigorous quality management must be applied during manufacturing to ensure high reliability.
Liquid systems enable higher specific impulse than solids and hybrid rocket motors and can provide very high tankage efficiency.
A liquid rocket engine can also usually be reused for several flights, as in the Space Shuttle and Falcon 9 series rockets, although reuse of solid rocket motors was also effectively demonstrated during the Shuttle program.
The flow of propellant into the combustion chamber can be throttled, which allows for control over the magnitude of the thrust throughout the flight. This enables real-time error correction during the flight along with efficiency gains.
Shutdown and restart capabilities allow for multiple burn cycles throughout a flight.
In the case of an emergency, liquid propelled rockets can be shutdown in a controlled manner, which provides an extra level of safety and mission abort capability.

Use of liquid propellants can also be associated with a number of issues:

Because the propellant is a very large proportion of the mass of the vehicle, the center of mass shifts significantly rearward as the propellant is used; one will typically lose control of the vehicle if its center mass gets too close to the center of drag/pressure.
When operated within an atmosphere, pressurization of the typically very thin-walled propellant tanks must guarantee positive gauge pressure at all times to avoid catastrophic collapse of the tank.
Liquid propellants are subject to slosh, which has frequently led to loss of control of the vehicle. This can be controlled with slosh baffles in the tanks as well as judicious control laws in the guidance system.
They can suffer from pogo oscillation where the rocket suffers from uncommanded cycles of acceleration.
Liquid propellants often need ullage motors in zero-gravity or during staging to avoid sucking gas into engines at start up. They are also subject to vortexing within the tank, particularly towards the end of the burn, which can also result in gas being sucked into the engine or pump.
Liquid propellants can leak, especially hydrogen, possibly leading to the formation of an explosive mixture.
Turbopumps to pump liquid propellants are complex to design, and can suffer serious failure modes, such as overspeeding if they run dry or shedding fragments at high speed if metal particles from the manufacturing process enter the pump.
Cryogenic propellants, such as liquid oxygen, freeze atmospheric water vapor into ice. This can damage or block seals and valves and can cause leaks and other failures. Avoiding this problem often requires lengthy chilldown procedures which attempt to remove as much of the vapor from the system as possible. Ice can also form on the outside of the tank, and later fall and damage the vehicle. External foam insulation can cause issues as shown by the Space Shuttle Columbia disaster. Non-cryogenic propellants do not cause such problems.
Non-storable liquid rockets require considerable preparation immediately before launch. This makes them less practical than solid rockets for most weapon systems.
Principle of operation

Liquid rocket engines have tankage and pipes to store and transfer propellant, an injector system and one or more combustion chambers with associated nozzles.
Typical liquid propellants have densities roughly similar to water, approximately. An exception is liquid hydrogen which has a much lower density, while requiring only relatively modest pressure to prevent vaporization. The density and low pressure of liquid propellants permit lightweight tankage: approximately 1% of the contents for dense propellants and around 10% for liquid hydrogen. The increased tank mass is due to liquid hydrogen's low density and the mass of the required insulation.
For injection into the combustion chamber, the propellant pressure at the injectors needs to be greater than the chamber pressure. This is often achieved with a pump. Suitable pumps usually use centrifugal turbopumps due to their high power and light weight, although reciprocating pumps have been employed in the past. Turbopumps are usually lightweight and can give excellent performance; with an on-Earth weight well under 1% of the thrust. Indeed, overall thrust to weight ratios including a turbopump have been as high as 155:1 with the SpaceX Merlin 1D rocket engine and up to 180:1 with the vacuum version. Instead of a pump, some designs use a tank of a high-pressure inert gas such as helium to pressurize the propellants. These rockets often provide lower delta-v because the mass of the pressurant tankage reduces performance. In some designs for high altitude or vacuum use the tankage mass can be acceptable.
The major components of a rocket engine are therefore the combustion chamber, pyrotechnic igniter, propellant feed system, valves, regulators, propellant tanks and the rocket engine nozzle. For feeding propellants to the combustion chamber, liquid-propellant engines are either pressure-fed or pump-fed, with pump-fed engines working in a variety of engine cycles.

Pressurization

Liquid propellants are often pumped into the combustion chamber with a lightweight centrifugal turbopump. Recently, some aerospace companies have used electric pumps with batteries. In simpler, small engines, an inert gas stored in a tank at a high pressure is sometimes used instead of pumps to force propellants into the combustion chamber. These engines may have a higher mass ratio, but are usually more reliable, and are therefore used widely in satellites for orbit maintenance.

Propellants

Thousands of combinations of fuels and oxidizers have been tried over the years. Some of the more common and practical ones are:

Cryogenic

Liquid oxygen and liquid hydrogen – Space Shuttle main engines, Space Launch System core stage, Ariane 5 main stage and the Ariane 5 ECA second stage, the BE-3 of Blue Origin's New Shepard, the first and second stage of the Delta IV, the upper stages of the Ares I, Saturn V's second and third stages, Saturn IB, and Saturn I as well as Centaur rocket stage, the upper stages of the Long March 3, Long March 5, Long March 8, the first stage and second stage of the H-II, H-IIA, H-IIB, and the upper stage of the GSLV Mk-II and GSLV Mk-III. The main advantages of this mixture are a clean burn and high performance.
Liquid oxygen and liquid methane – the Raptor and BE-4 engines.

One of the most efficient mixtures, oxygen and hydrogen, suffers from the extremely low temperatures required for storing liquid hydrogen and very low fuel density, necessitating large tanks that must also be lightweight and insulating. Lightweight foam insulation on the Space Shuttle external tank led to the 's destruction, as a piece broke loose, damaged its wing and caused it to break up on atmospheric reentry.
Liquid methane/LNG has several advantages over LH. Its performance is lower than that of LH but higher than that of RP1 and solid propellants, and its higher density, similarly to other hydrocarbon fuels, provides higher thrust to volume ratios than LH, although its density is not as high as that of RP1. This makes it specially attractive for reusable launch systems because higher density allows for smaller motors, propellant tanks and associated systems. LNG also burns with less or no soot than RP1, which eases reusability when compared with it, and LNG and RP1 burn cooler than LH so LNG and RP1 do not deform the interior structures of the engine as much. This means that engines that burn LNG can be reused more than those that burn RP1 or LH. Unlike engines that burn LH, both RP1 and LNG engines can be designed with a shared shaft with a single turbine and two turbopumps, one each for LOX and LNG/RP1. In space, LNG does not need heaters to keep it liquid, unlike RP1. LNG is less expensive, being readily available in large quantities. It can be stored for more prolonged periods of time, and is less explosive than LH.

Semi-cryogenic

Liquid oxygen and RP-1 – Saturn V's first stage, Zenit rocket, R-7-derived vehicles including Soyuz, Delta, Saturn I, and Saturn IB first stages, Titan I and Atlas rockets, Falcon 1 and Falcon 9, Long March 5, Long March 6, Long March 7 and Long March 8 first stages.
Liquid oxygen and alcohol – early liquid rockets, like German A4, aka V-2, and Redstone
Liquid oxygen and gasoline – Robert Goddard's first liquid rocket
Liquid oxygen and carbon monoxide – proposed for a Mars hopper vehicle, principally because carbon monoxide and oxygen can be straightforwardly produced by Zirconia electrolysis from the Martian atmosphere without requiring use of any of the Martian water resources to obtain Hydrogen.