# What is the thrust balance between compressor and exhaust nozzle for a turbojet engine?

I had this question asked in university and the teacher was unable to provide more than a theoretical estimate. The ratio I was given was 15/85.

Obviously in the case of a turbofan this ratio increases.

Is there any professional pilot or engineer able to tell even on a real example, what is the ratio of thrust distribution between compressor and jet for a known reactive jet engine, turboprop or turbofan?

I believe that helicopter turboprops produce no measurable thrust in the jet. Anyone?

• What do you mean developed by suction? Do you mean the force applied to the compressor (basically all of it, actually; there is otherwise more forward-facing than aft-facing surfaces, so the force on the compressor is more than the total thrust), the force due to decrease of pressure ahead of the compressor as opposed to increase behind it (there is overpressure ahead of it once the aircraft gains ~M0.5, the “pressure recovery”, and it is important part of the reason jet engines remain efficient to high speeds)…? – Jan Hudec Dec 31 '19 at 15:51
• The compressor acts as a tubed multi blade propeller. It compresses the airflow and the reactive effect is thrust on each rotor blade. What is the ratio between the thrust developed in compressor and the thrust developed in the exhaust nozzle? – WindSoul Dec 31 '19 at 18:16
• For engines with subsonic exhaust, which is what most airliners use, the nozzle is purely convergent, so it does not have any aft-facing surface on which positive thrust could be generated. The compressor is the only major aft-facing surface, so the forward force on it is most of thrust, plus force to compensate the backward force on the turbine and the convergent part of the nozzle. It wouldn't be producing nearly as much thrust without the nozzle though, and it would not produce any without the combustor and turbine. – Jan Hudec Dec 31 '19 at 20:15
• Are you looking for a breakdown of the forces on each section, like in the answers here? On which point(s) in a jet engine does the reaction force act? – fooot Jan 3 at 5:49

The answer depends on two major factors: the engine design, and the flight speed.

1. Engine design. As mentioned in this answer, there are four major types of turbine engine design: turbojet, turbofan, turboprop, turboshaft. The turbojet has all air flow through the engine itself and derives all thrust from the combustion gas jet exhaust. At the other end of the spectrum, the turboshaft has negligible thrust from the hot exhaust and transfers all useable power to the output shaft.

1. Flight speed. At high speed (case a in above picture) air streams into the inlet at high velocity and is decelerated before it hits the compressor, which sees a static pressure at the front that is higher than ambient pressure. No suction at the compressor from end at high speed! Only at the lower speed of case b is the static pressure at the front of the compressor lower than ambient.

I believe that helicopter turboprops produce no measurable thrust in the jet. Anyone?

Helicopter engines are turboshafts, which transfer all useful power onto the output shaft. No thrust from the exhaust stream here, ratio 100/0

Then the title question, which is a bit confusing because of the use of the word suction. IF we take the question as posted in a comment, it seems to focus on turbofans, and on the thrust ratio between hot stream and cold stream:

The compressor acts as a tubed multi blade propeller. It compresses the airflow and the reactive effect is thrust on each rotor blade. What is the ratio between the thrust developed in compressor and the thrust developed in the exhaust nozzle?

1. From The Jet Engine 5th edition by Rolls Royce, page 94:

A compressor is a device that raises the pressure of the working fluid passing through it - in this case, air. A fan is a large, low-pressure, compressor found at the front of most modern aero engines.

For a modern large civil engine:the fan passes over one tonne of airflow per second; this flow produces around 75 per cent of the engine thrust

2. Gas Turbine Theory by Saravanamuttoo a.o. example 3.2, finds a ratio of 73.5% at sea level:

   Thrust of a twin-spool bypass engine with the following data:
* Overall pressure ratio           25.0
* Fan pressure ratio                1.65
* Bypass ratio                      5.0
* Total air mass flow             215 kg/s
* Fan thrust (cold stream)     52,532 N
* Exhaust thrust (hot stream)  18,931 N


These above percentages are from the engine itself - but in cruise flight, a large part of thrust is developed by different structural parts of the engine, notably the engine intake.

A supersonic turbojet engine with central intake cone, from an old paper format uni lecture book. The inlet starts developing thrust at below M = 1. A subsonic engine has its inlet optimised for lower velocities and contributes more to the thrust in cruise.

• Thank you for the answer. I understand pretty much of it. With some parts I’m not comfortable as to agree. I’ll be very brief: the four jet categories are not what I’ve learned so far. Turbo shaft is the driver and turboprop is the assembly turbo shaft plus propeller assembly. Turbofan is derived from turbojet. But these are minor differences and I guess different sources could lead to variations. You have focused on turbofan, I thank you for that. I would really appreciate if you- or anyone could provide any relevant data for a turbojet engine. – WindSoul Jan 2 at 5:08
• All thrust of a turbojet is delivered via the hot stream exhaust. I cannot make sense of your question when related to a turbojet, do you mean what portion of power does the compressor absorb? – Koyovis Jan 2 at 5:47
• The thrust developed in the turbojet is split between thrust developed in the axial compressor and the thrust developed in the exhaust nozzle. The axial compressor develops thrust because is a tubed propeller. No turbojet develops thrust in the exhaust nozzle alone. It is possible for the ramjet to develop thrust in exhaust nozzle alone, but that is a different engine flying in supersonic mode only. I am interested to find what is the balance of thrust between compressor and exhaust nozzle in a turbojet. – WindSoul Jan 2 at 15:49
• The compressor in a turbojet develops rear facing force all right, but it is an internal force. It pushes back onto the rest of the engine aft of it, like a person sitting in a sailboat and directing a fan at the sail.: no net thrust. Internal compression develops no thrust, only compression released into the environment does. The net gain in exhaust energy comes from the fuel combustion, behind the compressor. – Koyovis Jan 4 at 6:46
• The compressor does not develop rear facing force. It develops a forward force because each compressor blade forces the air through and in exchange the blade is forced forward, pulling the entire compressor forward. A person operating a fan on a boat induces thrust because the fan moves the air which in turn acts on each fan blade. The net gain in energy... what exactly are you contending in your answer already syndicated by the upvoting machine? – WindSoul Jan 4 at 22:51