# Is it possible to determine the mass flow rate through the engine of MiG-21 during its various cruise Mach no. operations?

Is it possible to obtain the mass flow through the Tumansky R-25 turbojet engine mounted on a MiG-21 during its various standard operating (cruise) Mach numbers?
Note: standard as in older planes where there were just 3 positions of the inlet cone - upto Mach 1.5, between 1.5-1.9 and above 1.9.

Also, what should be the cruise condition for this plane, given that it is capable of flying at various Mach numbers and thus speeds?

• Just out of curiosity, what is this project you are working on? Sep 30 '15 at 19:03
• oh it is a homework project :P I suppose assignment would be more appropriate term. Sep 30 '15 at 19:08
• I wish my homework had been like that...would have probably helped if I'd been doing aeronautical engineering of course... Sep 30 '15 at 19:19
• Tell me what is given and I can start to answer. Otherwise, just google it ;-) Dec 14 '15 at 14:04

## 2 Answers

The mass flow is approximately $20 \ldots 100~kg/s$.
You can try to calculate it yourself using thermodynamic equations (see for example NASA Resources, MIT Resources or dedicated books).

Although the (idealized) calculations are not very difficult, please note that you will have to make a lot of assumptions concerning the efficiency of the turbojet components (particularly intake, compressor, combustion chamber, turbine and nozzle), since this data is not published by engine/airplane manufacturers. Some variables that are usually available are the thrust, the total compressor pressure ratio and maybe the (approximate) geometry of the intake and the nozzle.
Also, you are asking for supersonic calculations, which are more complex than the subsonic ones -- including the assumptions.

I don't have any data on the Tumanski engine, only on comparable western fighter jets of the same generation.

• General Electric J79 mounted in the F-104 Starfighter: mass flow = 77 kg/s.
• Snecma ATAR9K50 mounted in the Mirage III: mass flow = 73.5 kg/s.

These are the max. mass flow rates, at maximum thrust. They don't scale much with the thrust at operating RPM, usually the inlet nozzle is choked.