The Air Turbo Rocket: An Introduction

The Air Turbo Rocket, also known as the Air Turbo Ramjet or by its acronym, the ATR, is an airbreathing propulsion engine which combines elements from both turbojets and rocket engines.  The ATR belongs to a general class of propulsion engines known as Turbine-Based-Combined-Cycle, or TBCC, engines. 

The basic operation of the ATR is as follows.  A source of hot, high energy, fuel-rich gas is used to drive a turbine, which in turn drives a compressor. The compressor compresses incoming atmospheric air, like a turbojet, which then flows into a combustion chamber, which is much like an afterburner (or reheater) on a turbojet.  The fuel-rich gas which was used to drive the turbine also flows into the combustion chamber.  Within this combustor chamber, the compressed air from the compressor and the fuel-rich turbine drive gas are mixed, burned, and then expanded through a nozzle to produce thrust. 

The source of the hot, high energy fuel-rich gas is generically known as the gas generator, and it can be configured in a number of different ways.  The hot gas can be produced by mixing and burning a liquid or gaseous fuel and oxidizer combination, by the use of a mono-propellant gas generator, or by the burning of a solid propellant.  These configurations are known as gas-generator, or gg-cycle ATRs.  Figure 1 below shows schematically the basic gg-cycle ATR configurations.

figure 1  Gas Generator-Cycle ATR Configurations

Alternatively, the hot, high energy gas can be produced by running a fuel through a heat exchanger located in the combustion chamber of the ATR. This last configuration of the ATR is known as an expander-cycle ATR.  In figure 2 below the expander-cycle ATR configuration is shown.

figure 2 Expander-Cycle ATR Configuration

The essential feature that characterizes the ATR is the fact that, unlike a turbojet, the gases which pass over the turbine contain no atmospheric air.  All of the constituents of the turbine drive gas come from some on-board propellant supply, and it is this characteristic which is the defining characteristic of the ATR, and differentiates it from the turbojet.  In most other respects, the ATR behaves much like an afterburning turbojet.

ATR Operating Characteristics

Although functionally very similar to the turbojet, the ATR possesses a number of operating characteristics that are significantly different from that of the turbojet.  The fundamental difference between the ATR and the turbojet lies in the relationship between the compressor and turbine.  In a turbojet, the turbine and the compressor are coupled thermodynamically, i.e. power output of the turbine is the power input into the compressor, and aerodynamically, i.e. air flow through the compressor equals airflow through the turbine.  In the ATR, the turbine and compressor are coupled thermodynamically, but are not coupled aerodynamically.   This coupling difference is the source of the major performance differences between the turbojet and the ATR.

It allows the ATR to fly much faster and higher than the turbojet, and to have the highest thrust-per-unit frontal area of any airbreathing engine over its operational speed range.

In the next posting, we’ll be discussing performance attributes of the ATR which emerge as a consequence of its operating characteristics.  We’ll then give some consideration to propulsion applications which might be able to best take advantage of these attributes.