Most of the liquid propulsion rockets are used where long duration of operation is required. Here the oxidiser and fuel (both liquid) propellants are stored in separate tanks in the missile. There are basically two types of liquid propellants deployed: cryogenic (with boiling temperature below 120 K like liquid hydrogen, liquid oxygen, etc.) and noncryogenic or storable type (like kerosene, hydrazine, nitrogen tetraoxide, hydrogen peroxide, etc.). In space missions usually both propellants (oxidiser and fuel) used are cryogenic, whereas in missiles the propellants used are storable or non-cryogenic. Sometimes in space missions a combination is used where one propellant is cryogenic while the other is storable.
All liquid rocket engines have tankage and pipes to store and transfer propellant, an injector system, a combustion chamber which is very typically cylindrical, and one (sometimes two or more) rocket nozzles.
Liquid propellants, especially with cryogenic propellants, give better specific impulse and better stage mass ratio( mass ratio is a measure of the efficiency of a rocket. It describes how much more massive the vehicle is with propellant than without) for the same thrust force and duration of burn as compared to solid propellants.
A typical liquid propellant has a density similar to water, approximately 0.7–1.4g/cm³ (except liquid hydrogen which has a much lower density), while requiring only relatively modest pressure to prevent vapourisation. This combination of density and low pressure permits very lightweight tankage; approximately 1% of the contents for dense propellants and around 10% for liquid hydrogen (due to its low density and the mass of the required insulation).
In this system one of the propellants is solid while the other is liquid. Usually the oxidiser is in liquid state. This system is very rarely used though it has certain advantages. It has not found much favour with missile designers the world over.
In this case the advantage is taken of the atmospheric oxygen for burning the fuel thereby reducing the quantity of propellants to be carried by a missile. This lowers the weight of the rocket greatly 75 per cent of the total propellant’s weight is due to the oxidiser. This can be used either by using small turbojet engines to power the missile or ramjets(are a kind of propulsion it only works in high speeds commonly between mach 1 to 5 ,to accelerate ramjets to that speed we want to use another propulsion this system is called combined cycle ).
Unlike turbojets which have extensive rotary machinery (and are therefore costly), there is no such system in ramjets. Here the speed of incoming air is utilized, i.e., when we slow it down using the geometry to intake passage, its pressure rises. Then we add fuel to this and through a nozzle obtain the thrust force. Here a conventional rocket motor (normally solid type) called booster is used to provide the velocity initially at which a ramjet engine can start operating in a steady way. Ramjets cannot operate without atmosphere and also at extremely high speeds. They also have constraints of producing high thrust for a given size. They are highly suited for long range, low manoeuvre, steady and level flying missiles. For such missions they result in a lighter missile.
State-of-art propulsion systems use chemical combustion as energy source though nuclear, solar radiation, electrical, anti-matter, anti-gravity and the like are under varying stages of feasibility studies and research. It would not be surprising if superconductivity too is considered as a prospective candidate for missile propulsion systems in the coming decades.
Before a rocket engine can be put to use, it has to be tested. This is true whether it is in the case of the quality assurance of a rocket engine, R&D of a new or modified rocket engine or evaluation of the suitability of a new or a modified rocket motor to a specific application. Some of the tests are as follows.
Manufacturing, inspection and fabrication tests (pressure tests, bursts tests, leak tests, electro-mechanical checks).
Component tests (functional and operational tests on igniters, valves, injectors, structures, etc.)
Static rocket systems tests (with complete rocket engine on test stand): (a) simulated rocket operation (for proper function, calibration, ignition, operation-usually without establishing full combustion or nuclear reactivity); (b) complete engine tests (under rated conditions, off design conditions with intentional variations in environment or calibration).
Static vehicle tests (when rocket engine is installed in a restrained non-flying vehicle).
Flight tests: (a) on a specially instrumented flight test range with special flight test vehicle (b) with production vehicle.
Above all, flight testing of the integrated system is the ultimate in such tests. This is done in conjunction with tests of vehicles and other systems such as guidance, control, ground systems, and structures. These tests are usually conducted at missile or space launch ranges over the oceans. Data from most missile and space flight tests is telemetered to a ground receiving station as the test measurements are made. Some flight tests rely on salvaging some sections or pieces or data capsules. Some form a part of re-entry technology and recovery systems.