The Air Turbo Rocket: Performance Attributes (part 1)

As mentioned in the previous ATR post, 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.

For a turbojet, the limitation on its achievable flight speed is determined by its maximum allowable turbine inlet temperature.  As you fly at higher and higher flight speeds, the stagnation temperature of the incoming air increases roughly as the square of the flight Mach number.  As the air passes through the compressor, compression work on the air raises its stagnation temperature further.  Adding fuel in the combustor section, the combustion process raises the gas temperature even higher, nearing the allowable temperature limitations on the turbine.  As you fly at ever higher flight speeds, the amount of fuel you can add before you reach the turbine temperature limit decreases continuously, until it reaches the point where you can’t add any more fuel.  This becomes effectively the upper flight speed of a turbojet.  Advances in materials and cooling techniques have continuously pushed the turbine temperature limitation upwards, but it still remains the flight speed limiting characteristic of the turbojet.  With current materials and cooling techniques, turbojets can now reach into high Mach-two range.  Advanced engines now in the development phase should push this number well above Mach-three.

For the the ATR, the situation is different.  Because no atmospheric air ever sees an ATR’s turbine, the flight speed limitation of the turbojet is by-passed. Instead, the flight speed limitation of the ATR is passed on the material limits of the compressor.  With current materials and cooling techniques, the ATR should be able to operate in the high Mach-five range.

Further advantages accrue by aerodynamically decoupling the turbine and compressor.  In the ATR, as well as an afterburning turbojet, the majority of the net forward thrust produced by these engines comes from that produced by the combustor (for an ATR) or afterburner (for a turbojet).  Because there is no pressure drop through a combustor or across a turbine, the pressure within the combustor of the ATR is higher than that of the turbojet, for a given compressor pressure ratio.  This feature allows the ATR to produce considerably more thrust than a comparably-sized turbojet.  In addition, it allows the ATR to fly at a much higher altitude than that of the turbojet.

It is this higher combustor pressure that allows the ATR to have the highest thrust-per-unit-frontal area (sometimes called the “thrust density”) of any airbreathing engine.  In addition to the high combustor pressure, the relatively simple geometry of the ATR allows this engine cycle to be relatively light, and gives the ATR a very high thrust-to-weight ratio.

Further discussion of ATR performance attributes will continue in subsequent posts.  For further reading of ATR attributes, I recommend this paper (which I wrote in 2000).

     

Comments

[john hare]

Never work, the noodlin pin just won't stay in the wobblin shaft. :-)

I have a few ideas on the cycle when I have time to flesh them out. Regen cooling of the turbine and preccoling some air to boost gg mass among others.

[plasma wind]

John,
It seems I've always had problems with my noddlin pin, just won't stay put. you never cease to amaze me with the abundance of your ideas! Many of them good!
I hope we can get together over a few cool ones, and discuss airbreathing propulsion as an enabling technology for low cost space access.