Space Shuttle orbiter

The Space Shuttle orbiter is the spaceplane component of the Space Shuttle, a partially reusable orbital spacecraft system that was part of the discontinued Space Shuttle program. Operated from 1977 to 2011 by NASA. Orbiter is the part of the Space Shuttle system that looks like an airplane. The size is very similar to that of standard medium size commercial passenger jet such as Boeing 747.  

Figure 1: Discovery approaches the International Space Station (ISS) on STS-121; source: wiki

Six orbiters were built for flight: Enterprise, Columbia, Challenger, Discovery, Atlantis, and Endeavour.

Space Shuttle Enterprise (Orbiter Vehicle Designation: OV-101) was the first orbiter of the Space Shuttle system. Rolled out on September 17, 1976, it was built for NASA as part of the Space Shuttle program to perform atmospheric test flights after being launched from a modified Boeing 747. It was constructed without engines or a functional heat shield. As a result, it was not capable of spaceflight.

Columbia was the first space-worthy orbiter; it made its inaugural flight in 1981. Challenger, Discovery, and Atlantis followed in 1983, 1984, and 1985 respectively. In 1986, Challenger was destroyed in a disaster shortly after its 10th launch, killing all seven crew members. Endeavour was built as Challenger's successor, and was first launched in 1992. In 2003, Columbia was destroyed during re-entry, leaving just three remaining orbiters. Discovery completed its final flight on March 9, 2011, and Endeavour completed its final flight on June 1, 2011. Atlantis completed the final Shuttle flight, STS-135, on July 21, 2011.

Figure 2: The nozzles of Space Shuttle Columbia's three RS-25s following the landing of STS-93; source: wiki

Space Shuttle Orbiter used RS-25 engine, which is also known as Space Shuttle Main Engine (SSME). RS-25 is liquid fuel cryogenic engine, which was used on Space Shuttle and is used on the Space Launch System (SLS), see SLS. 

The Space Shuttle used a cluster of three RS-25 engines mounted at the stern of the orbiter, with fuel drawn from the external tank, see Figure above. The engines were used for propulsion during the spacecraft ascent, with total thrust increased by two solid rocket boosters and the orbiter's two AJ10 orbital maneuvering system engines on the upper side, see the Figure above.

RS-25 burns cryogenic liquid hydrogen and liquid oxygen propellants, with each engine producing 1,859 kN thrust at a liftoff. Development began in the 1970s. First flight os STS-1 occurred on April 12, 1981. 

The engine produces a specific impulse (Isp) of 452 seconds (4.43 kN-sec/kg) in a vacuum, or 366 seconds (3.59 kN-sec/kg) at sea level, has a mass of approximately 3.5 tones, and is capable of throttling between 67% and 109% of its rated power level in one-percent increments. Components of the RS-25 operate at temperatures ranging from −253 to 3,300 °C.

The RS-25 engine consists of pumps, valves, and other components working in concert to produce thrust. Fuel (liquid hydrogen) and oxidizer (liquid oxygen LOX) from the Space Shuttle's external tank entered the orbiter at the umbilical disconnect valves and from there flowed through the orbiter's main propulsion system (MPS) feed lines; opposite to the Space Launch System (SLS), where fuel and oxidizer from the rocket's core stage flow directly into the MPS lines. Once in the MPS lines, the fuel and oxidizer each split into separate paths to each engine (three on the Space Shuttle, four on the SLS respectively). In each branch, pre-valves then allow the propellants to enter the engine.

Figure 3: A functional diagram showing the flow of propellant through an RS-25 engine. source: wiki

Figure 4: Simplified diagram of RS-25 rocket engine(SSME), source": wiki

Nozzle 

There are three bell-shaped rocket engine nozzles. The structure is designed triangularly, one nozzle on top and to nozzles below, as seen on the Figure 2. Two smaller nozzles are visible to the left and right of the top engine. The engine's nozzle is 3.1 m long with a diameter of 0.26 m at its throat and 2.30 m at its exit. 

The nozzle is a bell-shaped extension bolted to the main combustion chamber, referred to as a de Laval nozzle, see Rocket engine nozzle. The RS-25 nozzle has an unusually large expansion ratio (about 69:1) for the chamber pressure. At sea level, a nozzle of this ratio would normally undergo flow separation of the jet from the nozzle, which would cause control difficulties and could even mechanically damage the vehicle. However engineers varied the angle of the nozzle walls from the theoretical optimum for the thrust, reducing it near the exit. That raises the pressure just around the edge to an absolute pressure between 32 and 39 kPa, and so it prevents flow separation. The inner flow has much lower pressure, around 14 kPa or less. 

The inner surface of each nozzle is cooled by liquid hydrogen flowing through brazed coolant channels in the stainless steel tube walls. On the Space Shuttle, a support ring welded to the forward end of the nozzle is the engine's attachment point to the orbiter-supplied heat shield.

Gimbal

Each engine is install with a gimbal bearing, for more detail about gimbal see Thrust vectoring. It represents the thrust interface between the engine and the launch vehicle, supporting 3,390 kg of engine weight and withstanding over 2.2 MN of thrust. The gimbal bearing allows the engine to be gimballed around two axes of freedom with a range of ±10.5°. This motion allows the engine's thrust vector to be altered, thus steering the vehicle into the correct orientation. The comparatively large gimbal range is necessary to correct for the pitch momentum that occurs due to the constantly shifting center of mass as the vehicle burns fuel in flight and after booster separation. The bearing assembly is approximately 290 by 360 mm, has a mass of 48 kg, and is made of titanium alloy.

Engine variants:

During the Space Shuttle program, the RS-25 went through a series of upgrades, including combustion chamber changes, improved welds and turbopump changes in an effort to improve the engine's performance and reliability, which means to reduce the amount of maintenance required after use of the shuttle. As a result, several versions of the RS-25 were used during the program:

RS-25 RS-25A, RS-25C, RS-25D.

RS-25E: It will be used on the Space Launch System (SLS) for future Artemis program missions beginning with Artemis 5, as the RS-25D stock is on purpose being used up.

There is so much more what could be written but I will keep it for the future articles.