Combinations of Ducted Jet Engines and Rocket Engines

The Tomahawk surface-to-surface missile uses two stages of propulsion in sequence. The solid propellant rocket booster lifts the missile away from its launch platform and is discarded after its operation. A small turbojet engine sustains the low level flight at nearly constant speed toward the target.

A ducted rocket, sometimes called an air-augmented rocket, combines the principles of rocket and ramjet engines; it gives higher performance (specific impulse) than a chemical rocket engine, while operating within the earth's atmosphere. Usually the term air-augmented rocket denotes mixing of air with the rocket exhaust (fuel-rich for afterburning) in proportions that enable the propulsion device to retain the characteristics typifying a rocket engine, for example, high static thrust and high thrust-to-weight ratio. In contrast, the ducted rocket often is like a ramjet in that it must be boosted to operating speed and uses the rocket components more as a fuel-rich gas generator (liquid, solid, or hybrid), igniter, and air ejecter pump.

The principles of the rocket and ramjet can be combined so that the two propulsion systems operate in sequence and in tandem and yet utilize a common combustion chamber volume as shown in Fig. 1-7. The low-volume configuration, known as an integral rocket-ramjet, can be attractive in air-launched missiles using ramjet propulsion (see Ref. 1-8). The transition from the rocket to the ramjet requires enlarging the exhaust nozzle throat (usually by ejecting rocket nozzle parts), opening the ramjet air inlet-combustion chamber interface, and following these two events with the normal ramjet starting sequence.

Rocket Engine Fuel Tank
FIGURE 1-6. Simplified schematic diagram of a typical hybrid rocket engine. The relative positions of the oxidizer tank, high pressure gas tank, and the fuel chamber with its nozzle depend on the particular vehicle design.

A solid fuel ramjet uses a grain of solid fuel that gasifies or ablates and reacts with air. Good combustion efficiencies have been achieved with a patented boron-containing solid fuel fabricated into a grain similar to a solid propellant and burning in a manner similar to a hybrid rocket propulsion system.

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