RocketEngines

Rocket Engine

The general principle may be simple, but there are numerous ways of achieving it. Different features and properties differ between existing rocket engines, and they all have consequences on complexity of manufacturing, complexity of operation, cost and weight for example.

We gather in this table the main properties of existing rocket engines.

Rocket engines features

Company	Rocketdyne	NPO Energomash	XCOR	XCOR	Armadillo
Model	SSME	RD-107 series (Soyuz)	XR-4A3 (EZ-rocket)	XR-5K18 (Lynx)	LOX/methane (no name)
Combustion
Propellants	LOX & LH2	LOX & Kerosene	LOX & Alcohol	LOX & Kerosene	LOX & LCH4
Tank pressurization	Yes, with O2 and H2 gases	Yes, with Nitrogen (same pump than propellants)	No	No	Yes, with Helium
Fuel pump	Turbopump	Turbopump driven by gaz generator using hydrogen peroxide decomposition (8300rpm)	Piston pump	Piston pump	No
Cooling	Regenerative w/ LH2 in three stages	Regenerative w/ kerosene (5 mm deep channels milled in the inner wall) and film of kerosene	Regenerative (w/ Alcohol?)	Regenerative w/ Kerosene	?
Injector	?	337 swirling/mixing injectors, ring of kerosene only for film cooling - view cut	?	?	?
Chamber metal	Copper or iron?	6 mm thick chromium bronze alloy inner wall, steel outer wall	Copper	Copper	?
Ignition system	?	Pyrotechnic, soon hypergolic	?	?	?
Actuators
Energy	Hydraulic				Electric
Provided by	Engine's turbopumps				?
Actuator	Six hydraulic servoactuators	Static engine, control by vernier engines	None	None	Servo-motor
Others
Valves	Hydraulically or pneumatically (helium) actuated	?	?	?	?

Pumps and tank pressurization

In order to get fuel from the tanks into the combustion chamber, the tanks must be either pressurized or the fuels pumped. In some cases, both techniques are used. The choice for this concern has a large impact on the design of the engine's hardware, and the complexity of manufacturing and operations.

Traditionnaly, only turbo pumps have been able to feed the engine at a large enough rate. Innovative solutions appeared in research projects or private space projects, like the use of piston pumps for LOX or simple pressurization using liquid helium.

Several possibilities exist for tank pressurization:

• vaporization of liquid propellants back into their own tanks
• external vaporization of inert gas like Helium (can Nitrogen be used for that?)
• smoke generator, that basically react fuel and oxidizer and use the resulting smoke for pressurization.

Cooling

There are four known ways to cool a rocket engine:

• Film cooling (aka the cooling curtain) takes place inside the chamber, generally using a ring fuel injector at the periphery of the injector plate, and acts both by cooling the chamber walls by contact and by isolating the walls from the combustion
• Regenerative cooling is most widely used in rocket engines, since it is the most efficient way to have the chamber not being destroyed by heat. The general principle is to use the fuel, or sometimes the oxidant, to cool the chamber walls before injecting those propellants into the chamber. The cooler flows into a series of pipes or crafting in the external or intermediate wall of the engine, around the nozzle, around the chamber, or around both of them.
• Ablative cooling is based on materials that provide cooling by being gently destroyed, like the heat-shield of spaceships, or the carbon fiber composite nozzle of SpaceX Merlin 1A engine.
• Radiative cooling uses the natural capacity of materials to radiate (in infrared light for example) when they are hot. Doing this, they lose energy, and thus cool. This is efficient in the void of space, and is used as the nozzle cooling method for the SpaceX's Merlin Vacuum nozzle (with regenerative cooling for the chamber).