Space Rocket Engine

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Space rocket engine 

What is the difference between the jet engine and space rocket engine? They both produce thrust and, as explained by Newton’s Third Law of Motion, eject exhaust gases in an equal and opposite direction. The difference between them is that jet engines get the oxygen to burn fuel directly from the atmosphere and space rockets carry their own oxygen onboard, which allows them to operate in space. You can see main characters on following two Figures 1 and 2. 

Figure 1. Turbojet engine; source: Rolls-Royce plc (1996). The Jet Engine. Fifth Edition. Derby, England.


Figure 2. A liquid space rocket engine; source: Rolls-Royce plc (1996). The Jet Engine. Fifth Edition. Derby, England. 


There are four major components to space rocket: (1) the structural system, or frame, (2) the payload system, (3) the guidance system, and (4) the propulsion system. The propulsion system includes the tanks pumps, propellants, power head and rocket nozzle. The function of the propulsion system is to produce thrust.

In a space rocket engine, fuel and a source of oxygen, an oxidizer, are mixed and exploded in a combustion chamber. The combustion produces hot exhaust which is passed through a nozzle to accelerate the flow and produce thrust.

Figure 3. A Saturn V F-1 engine on display at NASA's Marshall Space Flight Center; source: NASA

A liquid-propellant rocket (see Figure 3) or liquid rocket utilizes a rocket engine burning liquid propellants. Liquids are desirable propellants because they have reasonably high density and their combustion products have high specific impulse (Isp). This allows the volume of the propellant tanks to be relatively low.

Liquid rockets can be either mono-propellant rockets using a single type of propellant, or bi-propellant rockets using two types of propellant, or tri-propellant rockets using three types of propellant, which is quite 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 (Refined Petroleum-1), and a liquid oxidizer such as liquid oxygen (LOX). The engine may be a cryogenic rocket engine, where the fuel and oxidizer, such as hydrogen and oxygen. The gases in that case have been liquefied at very low temperatures.

Most designs of liquid rocket engines are throttleable for variable thrust operation. Some designs allow control of the propellant mixture ratio (ratio at which oxidizer and fuel are mixed). Some can be shut down and, with a suitable ignition system or self-igniting propellant, restarted.

A simple solid rocket motor consists of a casing, nozzle, grain (propellant charge), and igniter.

The solid grain mass burns in a predictable way to produce exhaust gases. The nozzle dimensions are calculated to maintain a design chamber pressure, while producing thrust from the exhaust gases.

Once ignited, a simple solid rocket motor cannot be shut off, because it contains all the ingredients necessary for combustion within the chamber in which they are burned.  However more advanced solid rocket motors can be throttled, and also be extinguished, and then re-ignited by control of the nozzle geometry, or through the use of vent ports. Further, the pulsed rocket engines are another type that burn in segments and can be ignited upon command.

A solid rocket propellants are often tailored to and classified by specific applications, such as space launch booster propellants or tactical missile propellants; each has somewhat specific chemical ingredients, different burning rates, different physical properties, and different performance.

Figure 4. Schematic Diagram Of Solid Propellant Rocket; source: aerospacenotes.com

What Is Propellant Grain? 
The grain is the shaped mass of processed solid propellant inside the rocket motor, see Figure 4. The material and geometrical configuration of the grain controls motor performance characteristics. Propellant grains are cast, molded, or extruded bodies and their appearance is similar to that of hard rubber or plastic. Once ignited, the grain will burn on all its exposed surfaces forming hot gases that are then exhausted through a nozzle. Most rocket engines have a single grain. A few rocket engines have more than one grain inside a single case or chamber, and very few grains have segments made of different propellant composition, for example to allow different burning rates.

Solid Rocket Boosters
Blocks 1 and 1B of the SLS will use two five-segment solid rocket boosters. They use casing segments that were flown on Shuttle missions as parts of the four-segment Space Shuttle Solid Rocket Boosters.
The propellants for the solid rocket boosters are aluminum powder, which is very reactive, and ammonium perchlorate, a powerful oxidizer.
I will return to this topic later in more detail. 

Figure 5. SLS Block 1 with the Orion spacecraft launching from Pad 39B, source: Wiki



There are different types of liquid propellants. 

1. Cryogenic
  • Liquid oxygen (LOX, O2) and liquid hydrogen (LH2, H2) – Space Shuttle main engines, Space Launch System (SLS) 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 (water vapor is the only combustion product) and high performance.
  • Liquid oxygen (LOX) and liquid methane (CH4, liquefied natural gas, LNG) – the Raptor (SpaceX) and BE-4 (Blue Origin) engines. 
2. Semi-cryogenic
  • Liquid oxygen (LOX) and RP-1 (kerosene) – 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 (LOX) and alcohol (ethanol, C2H5OH) – early liquid rockets, like German (World War II) A4, aka V-2, and Redstone
  • Liquid oxygen (LOX) and gasoline – Robert Goddard's first liquid rocket
  • Liquid oxygen (LOX) and carbon monoxide (CO) – proposed for a Mars hopper vehicle (with a specific impulse of approximately 250 s), 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.

3. Non-cryogenic/storable/hypergolic

Many non-cryogenic bipropellants are hypergolic (self igniting).

  • T-Stoff (80% hydrogen peroxide, H2O2 as the oxidizer) and C-Stoff (methanol, CH3OH, and hydrazine hydrate, N2H4·n(H2O) as the fuel) – used for the Hellmuth-Walter-Werke HWK 109-509A, -B and -C engine family used on the Messerschmitt Me 163B Komet, an operational rocket fighter plane of World War II, and Ba 349 Natter crewed VTO interceptor prototypes.
  • Unsymmetric dimethylhydrazine (UDMH) and dinitrogen tetroxide (N2O4) – Proton, Rokot, Long March 2 (used to launch Shenzhou crew vehicles).
  • Aerozine 50 (50% UDMH, 50% hydrazine) and dinitrogen tetroxide (N2O4) – Titans 2–4, Apollo lunar module, Apollo service module, interplanetary probes (Such as Voyager 1 and Voyager 2)
  • Monomethylhydrazine (MMH, (CH3)HN2H2) and dinitrogen tetroxide (N2O4) – Space Shuttle orbiter's orbital maneuvering system (OMS) engines and Reaction control system (RCS) thrusters. SpaceX's Draco and SuperDraco engines for the Dragon spacecraft.
Engine cycles

Fuel and oxidizer must be pumped into the combustion chamber against the pressure of the hot gasses being burned. Engine power is limited by the rate at which propellant can be pumped into the combustion chamber. 
For atmospheric and launch phase, high pressure, and thus high power, engine cycles are desired to minimize gravity drag. For orbital phase, lower power cycles are usually acceptable.

  • Pressure-fed cycle:
    • The propellants are forced in from pressurized (relatively heavy) tanks. The heavy tanks mean that a relatively low pressure is optimal, limiting engine power, but all the fuel is burned, allowing high efficiency. The pressurant used is frequently helium due to its lack of reactivity and low density. Examples: AJ-10, used in the Space Shuttle OMS, Apollo SPS, and the second stage of the Delta II.
    • Figure 6. Pressure-fed rocket cycle. Propellant tanks are pressurized to directly supply fuel and oxidizer to the engine, eliminating the need for turbopumps. Source: Wiki


  • Electric pump-fed:
    • An electric motor, generally a brushless DC electric motor, drives the pumps. The electric motor is powered by a battery pack. It is relatively simple to implement and reduces the complexity of the turbomachinery design, but at the expense of the extra dry mass of the battery pack. Example engine is the Rutherford designed and used by Rocket Lab.
  • Gas-generator cycle:
    • A small percentage of the propellants are burnt in a pre-burner to power a turbopump and then exhausted through a separate nozzle, or low down on the main one. This results in a reduction in efficiency since the exhaust contributes little or no thrust, but the pump turbines can be very large, allowing for high power engines. Examples: Saturn V's F-1 and J-2, Delta IV's RS-68, Ariane 5's HM7B, Falcon 9's Merlin.
      • Merlin is a family of rocket engines developed by SpaceX for use on its Falcon 1, Falcon 9 and Falcon Heavy launch vehicles. Merlin engines use RP-1 and liquid oxygen as rocket propellants in a gas-generator power cycle. 
Figure 7. Gas-generator rocket cycle. Some of the fuel and oxidizer is burned separately to power the pumps and then discarded. Most gas-generator engines use the fuel for nozzle cooling. Source: Wiki


  • Tap-off cycle:
    • Takes hot gases from the main combustion chamber of the rocket engine and routes them through engine turbopump turbines to pump propellant, then is exhausted. Since not all propellant flows through the main combustion chamber, the tap-off cycle is considered an open-cycle engine. Examples include the J-2S and BE-3.
  • Expander cycle:
    • Cryogenic fuel (hydrogen, or methane) is used to cool the walls of the combustion chamber and nozzle. Absorbed heat vaporizes and expands the fuel which is then used to drive the turbopumps before it enters the combustion chamber, allowing for high efficiency, or is bled overboard, allowing for higher power turbopumps. The limited heat available to vaporize the fuel constrains engine power. Examples: RL10 for Atlas V and Delta IV second stages (closed cycle), H-II's LE-5 (bleed cycle).
  • Staged combustion cycle:
    • A fuel- or oxidizer-rich mixture is burned in a pre-burner and then drives turbopumps, and this high-pressure exhaust is fed directly into the main chamber where the remainder of the fuel or oxidizer undergoes combustion, permitting very high pressures and efficiency. Examples: SSME, RD-191, LE-7.
    • Figure 8. Fuel-rich staged combustion cycle. Here, all of the fuel and a portion of the oxidizer are fed through the preburner, generating fuel-rich gas. After being run through a turbine to power the pumps, the gas is injected into the combustion chamber and burned with the remaining oxidizer; source: Wiki

  • Full-flow staged combustion cycle:
    • Fuel- and oxidizer-rich mixtures are burned in separate pre-burners and driving the turbopumps, then both high-pressure exhausts, one oxygen rich and the other fuel rich, are fed directly into the main chamber where they combine and combust, permitting very high pressures and high efficiency. Example: SpaceX Raptor.
Figure 9. Full-flow staged combustion rocket cycle; Source: Wiki



Figure 10. SpaceX Raptor FFSC rocket engine, sample propellant flow schematic, 2019; source: Wiki





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