How to calculate thrust ??
01-31-2007, 06:06 AM
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How to calculate thrust ??
Hello there, My name is joe.
Currently i am a student embarking on a project for jet propulsion test rig.
I really need to clarify certain doubts that had been bugging me.
If i am not wrong Thrust equation at take off follows as Ae(Pe-Pa) + M(Ve) ........ (1)
Where Ae = Area at exit, Pe = Pressure at exit, Pa = Ambient pressure, M= Mass flow rate and Ve = air velocity at exit.
I got muddled in the following, when throat is chocked or critical condition, Mach no = 1. So when Mach no = 1, Ve= square root(YRTe) .............(2)
where Y and R are gas constants
It follows that Mass flow rate = density*Ae*Ve ....... (3)
It also follows that density = Pressure/(R*Te) .........(4)
subt (3) into (4) we have (Pe)(Ve)(Ae)/(R)(Te) = Mass flow rate
Now when i try to subt into my momentum thrust which is M(Ve) my temperature equation seems to be cancelled away.
from my theoretical calculation, it seems that temperature is not having significance in thrust which is totally wrong. Please could some one pin point my mistakes.
Thanks and i hope i am not being too draggy.
Currently i am a student embarking on a project for jet propulsion test rig.
I really need to clarify certain doubts that had been bugging me.
If i am not wrong Thrust equation at take off follows as Ae(Pe-Pa) + M(Ve) ........ (1)
Where Ae = Area at exit, Pe = Pressure at exit, Pa = Ambient pressure, M= Mass flow rate and Ve = air velocity at exit.
I got muddled in the following, when throat is chocked or critical condition, Mach no = 1. So when Mach no = 1, Ve= square root(YRTe) .............(2)
where Y and R are gas constants
It follows that Mass flow rate = density*Ae*Ve ....... (3)
It also follows that density = Pressure/(R*Te) .........(4)
subt (3) into (4) we have (Pe)(Ve)(Ae)/(R)(Te) = Mass flow rate
Now when i try to subt into my momentum thrust which is M(Ve) my temperature equation seems to be cancelled away.
from my theoretical calculation, it seems that temperature is not having significance in thrust which is totally wrong. Please could some one pin point my mistakes.
Thanks and i hope i am not being too draggy.
02-01-2007, 11:07 AM
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RE: How to calculate thrust ??
ORIGINAL: av8tor1977
NO, NO... No "Bungees"!! If you have a flame out, it will come flying back at you...
AV8TOR
NO, NO... No "Bungees"!! If you have a flame out, it will come flying back at you...
AV8TOR
02-02-2007, 12:37 PM
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RE: How to calculate thrust ??
Joe,
Consult your Thermodynamics text. I believe you are oversimplifying. While a jet engine can be modeled as a simple open Brayton cycle, you must remember to account for adiabatic losses (compressor and turbine efficiencies). Your Thermodynamics professor will be able to help you with the particulars.
Best regards,
Tom Johnson
Consult your Thermodynamics text. I believe you are oversimplifying. While a jet engine can be modeled as a simple open Brayton cycle, you must remember to account for adiabatic losses (compressor and turbine efficiencies). Your Thermodynamics professor will be able to help you with the particulars.
Best regards,
Tom Johnson
02-03-2007, 01:19 AM
#17
RE: How to calculate thrust ??
Yep, that's about the extent of my jet propulsion knowlege too. Except for this little video of the Mentos & Diet Coke thing....
http://youtube.com/watch?v=20WGMTWsyCw&search=mentos
AV8TOR
http://youtube.com/watch?v=20WGMTWsyCw&search=mentos
AV8TOR
03-02-2007, 06:24 PM
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RE: How to calculate thrust ??
Hey, I think this is a joke but not a cruel one. I'm not a student anymore but I can tell you that your formula is good. For rocket thrust I would use the following formula: F = m dot * Ve + (pe - p0) * Ae. This is an equation. What you stated was an expression. Also, temperture is not lost; it's a function of mass and velocity or E; you transformed it. Nasa has a great website and simulator; it can answer all your questions. Just thought I could poke some fun back at ya,
. Thanks for making me use my brain again!
. Thanks for making me use my brain again!
