Rocket Thrust

[grifter]

Well I have a feeling this is gunna be a headache to answer !!! any help would be sooooooooooooo apreciated:

 

I'm in the process of working out the thrust produced by a rocket, I'm using the following equation: (i apologise for not using latex)

 

Fg=Mdot x Ve + Ae(Pe-Pamb)

 

where:

Mdot = Exhaust gas mass flow

Ve = Jet velocity at nozzle exit plane

Ae = Flow area at nozzle exit plane

Pe = Static pressure at nozzle exit plane

Pamb = Atmospheric pressure (101.325 kPa)

 

I can get Mdot with:

(A star * pt/sqrt[Tt]) * sqrt(gam/R) * [(gam + 1)/2]^-[(gam + 1)/(gam -1)/2]

 

where:

 

Astar = the area of the throat.

pt = the total pressure in the combustion chamber,

Tt = the total temperature in the combustion chamber,

gam = the ratio of specific heats of the exhaust,

R = the gas constant. (8.314472(15) J • K-1 • mol-1

 

I can calculate the exit pressure (pe) and exit temperature (Te) from the isentropic relations at the nozzle exit:

 

pe / pt = [1 + Me^2 * (gam-1)/2]^-[gam/(gam-1)]

 

Te / Tt = [1 + Me^2 * (gam-1)/2]^-1

 

once i've got Te i can calculate the exhaust exit velocity (Ve) with

 

Ve = Me * sqrt (gam * R * Te)

 

and then its back to:

 

F = mdot * Ve + (pe - 101.325 kPa) * Ae

 

any aprox Values for the above would help sooo much !!!

and any actuall examples or work-through would also be a mahoosive help!!!

 

Thanks guys

[D H]

The force varies a lot depending on the size of the rocket. The specific impulse of a rocket is a lot less size-dependent:

[math]I_{sp} \equiv \frac F{\dot m g_0} = \frac 1{g_0} \left(v_e + (p_e-p_{\infty})\frac {A_e}{\dot m}\right)[/math]

 

Some numbers (BIG thrusters):

Shuttle main engine (three engines on the vehicle):

Vacuum force = 2170 kN per engine

Vacuum Isp=453 secs

Chamber pressure: 18.9 MPal

 

Soyuz 1st stage (four engines on the vehicle):

Vacuum force = 991 kN per engine

Vacuum Isp=310 secs

Chamber pressure: 5.85 MPal

[grifter]

thats great, I will use that

 

But does anyone have any, even rough, values for the following (as being a few thousand pounds of thrust out isn't a real issue, don't feel that the values need to be correct, so long as they are accurate so when they are put in the equation I will get an answer to the correct power of 10!!)

 

Mdot = Exhaust gas mass flow

Ve = Jet velocity at nozzle exit plane

Ae = Flow area at nozzle exit plane

Pe = Static pressure at nozzle exit plane

Pamb = Atmospheric pressure (101.325 kPa)

 

Thanks