Rocket engine nozzle

It was first used in an early rocket engine developed by Robert Goddard, one of the fathers of modern rocketry.

If ambient pressure is lower, while the force balance indicates that the thrust will increase, the isentropic Mach relations show that the area ratio of the nozzle could have been greater, which would result in a higher exit velocity of the propellant, increasing thrust.

In some cases, it is desirable for reliability and safety reasons to ignite a rocket engine on the ground that will be used all the way to orbit.

However, a nozzle designed for sea-level operation will quickly lose efficiency at higher altitudes.

This was the technique employed on the Space Shuttle's overexpanded (at sea level) main engines (SSMEs), which spent most of their powered trajectory in near-vacuum, while the shuttle's two sea-level efficient solid rocket boosters provided the majority of the initial liftoff thrust.

In the vacuum of space virtually all nozzles are underexpanded because to fully expand the gas's the nozzle would have to be infinitely long, as a result engineers have to choose a design which will take advantage of the extra expansion (thrust and efficiency) whilst also not adding excessive weight and compromising the vehicle's performance.

Additionally, as the temperature of the gas in the nozzle decreases, some components of the exhaust gases (such as water vapour from the combustion process) may condense or even freeze.

Magnetic nozzles have been proposed for some types of propulsion (for example, Variable Specific Impulse Magnetoplasma Rocket, VASIMR), in which the flow of plasma or ions are directed by magnetic fields instead of walls made of solid materials.

These can be advantageous, since a magnetic field itself cannot melt, and the plasma temperatures can reach millions of kelvins.

From the throat the cross-sectional area then increases, the gas expands and the linear velocity becomes progressively more supersonic.

Thrust is generated by the propulsion system of the rocket through the application of Newton's third law of motion: "For every action there is an equal and opposite reaction".

The thrust of a rocket engine nozzle can be defined as:[2][3][5][6] the term in brackets is known as equivalent velocity, The specific impulse

In English Engineering units it can be obtained as[7] where: For a perfectly expanded nozzle case, where

for any given engine thus: and hence: which is simply the vacuum thrust minus the force of the ambient atmospheric pressure acting over the exit plane.

This is often unstable, and the jet will generally cause large off-axis thrusts and may mechanically damage the nozzle.

[8] In addition, as the rocket engine starts up or throttles, the chamber pressure varies, and this generates different levels of efficiency.

The ratio of the area of the narrowest part of the nozzle to the exit plane area is mainly what determines how efficiently the expansion of the exhaust gases is converted into linear velocity, the exhaust velocity, and therefore the thrust of the rocket engine.

The shape of the nozzle also modestly affects how efficiently the expansion of the exhaust gases is converted into linear motion.

These designs require additional complexity, but an advantage of having two thrust chambers is that they can be configured to burn different propellants or different fuel mixture ratios.

Some ICBMs and boosters, such as the Titan IIIC and Minuteman II, use similar designs.

Figure 1: A de Laval nozzle, showing approximate flow velocity increasing from green to red in the direction of flow
Density flow in a nozzle
Diagram of a de Laval nozzle, showing flow velocity (v) increasing in the direction of flow, with decreases in temperature (t) and pressure (p). The Mach number (M) increases from subsonic, to sonic at the throat, to supersonic.
Nozzles can be (top to bottom):
  • underexpanded
  • ambient
  • overexpanded
  • grossly overexpanded.
If a nozzle is under- or overexpanded, then loss of efficiency occurs relative to an ideal nozzle. Grossly overexpanded nozzles have improved efficiency relative to an underexpanded nozzle (though are still less efficient than a nozzle with the ideal expansion ratio), however the exhaust jet is unstable. [ 1 ]