Simple Examples¶
RocketCEA always begins with an import statement and an instance of a CEA_obj:
from rocketcea.cea_obj import CEA_Obj
C = CEA_Obj( oxName='LOX', fuelName='LH2')
If the above is done at a command prompt, we can query the CEA_Obj as shown below:
>>> from rocketcea.cea_obj import CEA_Obj
>>> C = CEA_Obj( oxName='LOX', fuelName='LH2')
>>> C.get_Isp(Pc=100.0, MR=1.0, eps=40.0)
374.3036176557629
>>> C.get_Isp(Pc=100.0, MR=6.0, eps=40.0)
448.190232998362
Note that the number of significant figures in the Isp above are much higher than in the standard CEA output.
While there is likely no physical significance to this, it can sometimes be useful numerically in computations that take derivatives of Isp with respect to a design variable. (for example optimizers.)
N2O4/MMH Performance¶
Successive queries of the CEA_Obj can be made to create tables of information. The script below will make a table of N2O4/MMH performance data.
from rocketcea.cea_obj import CEA_Obj
C = CEA_Obj( oxName='N2O4', fuelName='MMH')
def show_perf( Pc=100.0, eps=10.0, MR=1.0 ):
IspVac, Cstar, Tc, MW, gamma = C.get_IvacCstrTc_ChmMwGam(Pc=Pc, MR=MR, eps=eps)
print( '%8.1f %8.1f %8.1f %8.1f %8.1f %8.1f %8.2f %8.4f '%\
(Pc, eps, MR, IspVac, Cstar, Tc, MW, gamma))
print(' Pc(psia) AreaRatio MixtureRatio IspVac(sec) Cstar(ft/sec) Tc(degR) MolWt gamma')
Pc = 250.0
eps = 50.0
for MR in [1.0 + i*0.1 for i in range(20)]:
show_perf( Pc=Pc, eps=eps, MR=MR )
The resulting table is shown below:
Pc(psia) AreaRatio MixtureRatio IspVac(sec) Cstar(ft/sec) Tc(degR) MolWt gamma
250.0 50.0 1.0 306.1 5378.5 4243.4 16.73 1.2539
250.0 50.0 1.1 311.6 5476.1 4532.8 17.39 1.2377
250.0 50.0 1.2 316.7 5554.9 4791.1 18.03 1.2216
250.0 50.0 1.3 321.4 5617.1 5018.0 18.63 1.2062
250.0 50.0 1.4 325.6 5664.5 5214.0 19.21 1.1918
250.0 50.0 1.5 329.3 5698.4 5379.7 19.75 1.1786
250.0 50.0 1.6 332.4 5719.6 5515.7 20.26 1.1668
250.0 50.0 1.7 335.1 5728.7 5623.4 20.74 1.1569
250.0 50.0 1.8 337.4 5726.4 5704.9 21.19 1.1489
250.0 50.0 1.9 339.3 5713.9 5763.2 21.60 1.1428
250.0 50.0 2.0 340.8 5692.9 5801.9 21.99 1.1384
250.0 50.0 2.1 341.9 5665.4 5824.6 22.34 1.1353
250.0 50.0 2.2 342.7 5633.5 5834.8 22.67 1.1333
250.0 50.0 2.3 343.0 5598.6 5835.2 22.98 1.1319
250.0 50.0 2.4 342.8 5562.1 5828.0 23.27 1.1311
250.0 50.0 2.5 341.6 5524.8 5814.9 23.54 1.1307
250.0 50.0 2.6 338.7 5487.2 5797.4 23.79 1.1305
250.0 50.0 2.7 335.5 5449.7 5776.3 24.03 1.1306
250.0 50.0 2.8 332.4 5412.5 5752.5 24.25 1.1308
250.0 50.0 2.9 329.3 5375.8 5726.4 24.47 1.1311
LOX/LH2 Delta V¶
Conducting an analysis with RocketCEA is much easier than the standard approach to running CEA and reviewing the pages of CEA output (as we did in the LOX/LH2 example from Standard Examples)
We can query the CEA_Obj instance repeatedly for specific information, as opposed to simply printing a page of CEA output.
If we wanted to run some deltaV calculations on a LOX/LH2 stage to see what impact changing the engine's area ratio would have, we might do the following.
from math import log
from rocketcea.cea_obj import CEA_Obj
C = CEA_Obj( oxName='LOX', fuelName='LH2')
Wpayload = 8500.0 # lbm
Wstg = 5106.0 # lbm
Wpropellant = 45920.0 # lbm
Winit = Wstg + Wpropellant + Wpayload
Wfinal = Winit - Wpropellant
def show_deltaV( Pc=475.0, eps=84.0, MR=5.88 ):
IspVac = C.get_Isp(Pc=Pc, MR=MR, eps=eps)
IspDel = 0.969 * IspVac
deltaV = 32.174 * IspDel * log( Winit / Wfinal ) # ft/sec
print( '%8.1f %8.1f %8.2f %8.1f %8.1f %8.1f '%(Pc, eps, MR, IspVac, IspDel, deltaV))
print(' Pc(psia) AreaRatio MixtureRatio IspVac(sec) IspDel(sec) deltaV(ft/sec)')
show_deltaV( Pc=475.0, eps=84.0 )
for eps in range(100, 251, 50):
show_deltaV( Pc=475.0, eps=eps )
show_deltaV( Pc=475.0, eps=280.0 )
The script above calls RocketCEA for a number of area ratio values to get ideal vacuum Isp. An efficiency is applied to that ideal Isp to arrive at a delivered Isp. The delivered Isp is then used to calculate a stage deltaV.
The script gives the following output:
Pc(psia) AreaRatio MixtureRatio IspVac(sec) IspDel(sec) deltaV(ft/sec)
475.0 84.0 5.88 464.9 450.5 21392.5
475.0 100.0 5.88 467.7 453.2 21518.6
475.0 150.0 5.88 473.5 458.8 21785.7
475.0 200.0 5.88 477.1 462.3 21954.9
475.0 250.0 5.88 479.8 464.9 22075.5
475.0 280.0 5.88 481.0 466.1 22133.4