Hi,

Your system is incomplete. You will need to add one more equation for x.
All models in Modelica need to be balanced, that is, the number of variables needs to be equal with the number of equations.

Cheers,
Adrian Pop/

adrpo,
so,how to get a solution for it????????
y''  - (y'/(x-1)) = x*(x-1) with y'(2) = -1 and y(2) = 1;

@All : I am .... so i not good at  English

Modelica can't give you an analytical solution.
You need to specify what x is (add x = time for example).

Ok thanks

plz help me again

model rocket

    parameter Real G=6.67*(10^(-11))  "gravity constant";

    parameter Real M = 59736*(10^(24)) "mass of earth";

    parameter Real m = 7.347*(10^(22)) "mass of mooth";

    parameter Real D = 384400 "distance bettwen earth and mooth(mean)";

    parameter Real V = 100000.0 "velocity of rocket";

    Real x(start = 0.0) "distance bettwen rocket and earth";

    Real y(start = V) "der of x";

equation

           der(x) = y;

           der(y)*((D-x)^2)*x =  (-1)*G*M*((D-x)^2) + G*m*x ;                  

end rocket;

where i sinulate -> error:

simulate(rocket,stopTime = 5)

record SimulationResult

    resultFile = "Simulation failed.

"

end SimulationResult;

plz .I need it soon.I don 't know what is error

./rocket
DASSL--  AT T (=R1) SOME ELEMENT OF WT
      In above message,  R1 =   .2220446049250E-15
DASSL--  HAS BECOME .LE. 0.0
Error, simulation stopped at time: 2.22045e-16 with idid: -3
Result written to file.

Thanks for all!
But I can't fix it.plz help me.
I get a other solution by ODE :

model rocket

    parameter Real G = 6.67*(10^(-11))  "gravity constant";

    parameter Real M = 59736*(10^(20)) "mass of earth";

    parameter Real m = 7.347*(10^(22)) "mass of mooth";

    parameter Real D = 384400.0 "distance betwen earth and mooth(mean)";

    parameter Real V = 50.0 "velocity of rocket";

    Real x(start = 1.0);

    Real y(start = (V/7.92));

    Real temp1 ;

    Real temp2;

equation 

    temp1 = m/M;

    temp2  = D/6400;

    der(y)*((temp2-x)^2)*x*x =  -1*((temp2-x)^2) + temp1*x*x ; 

    der(x) = y;   

end rocket;

it excute good and can switch to

model rocket 

    parameter Real G=6.67*(10^(-11))  "gravity constant"; 

    parameter Real M = 59736*(10^(24)) "mass of earth"; 

    parameter Real m = 7.347*(10^(22)) "mass of mooth"; 

    parameter Real D = 384400 "distance bettwen earth and mooth(mean)"; 

    parameter Real V = 100000.0 "velocity of rocket"; 

    Real x(start = 0.0) "distance bettwen rocket and earth"; 

    Real y(start = V) "der of x"; 

equation 

           der(x) = y; 

           der(y)*((D-x)^2)*x =  (-1)*G*M*((D-x)^2) + G*m*x ;                  

end rocket;

(************)


so why (************) don't run when i simulate??????It can error of modelica??????


Thanks

./rocket
DASSL--  AT T (=R1) SOME ELEMENT OF WT
      In above message,  R1 =   .2220446049250E-15
DASSL--  HAS BECOME .LE. 0.0
Error, simulation stopped at time: 2.22045e-16 with idid: -3


It's simple. It can't solve the equation because you made it unsolvable. You set x(start = 0), which means for the first steps:
der(y)*((D-x)^2)*x =  (-1)*G*M*((D-x)^2) + G*m*x ;
der(y)*((D-0)^2)*0 =  (-1)*G*M*((D-0)^2) + G*m*0 ;
0 = -5.8874703126643202e+29

Clearly you can't solve this.

ok

I will write to clean my problem!!!!!!!11

The first i have a ODE :                   => x’’  = G*M* /(x²)  + G*m*/( (D-x)² )       , x(0) = R , x'(0) = V    

G = 6.67*(10^(-11))  "gravity constant";

M = 59736*(10^(20)) "mass of earth";

m = 7.347*(10^(22)) "mass of mooth";

D = 384400.0 "distance betwen earth and mooth(mean)";

V = 50.0 "velocity of rocket";

After i write this on modelica (**********)

model rocket

    parameter Real G = 6.67*(10^(-11))  "gravity constant";

    parameter Real M = 59736*(10^(20)) "mass of earth";

    parameter Real m = 7.347*(10^(22)) "mass of mooth";

    parameter Real D = 384400.0 "distance betwen earth and mooth(mean)";

    parameter Real V = 50.0 "velocity of rocket";

    Real x(start = 6400.0) "distance bettwen rocket and earth";

    Real y(start = V) "der of x";

    Real temp1;

    Real temp2;

equation

           temp1 =     G*M;

           temp2 = G*m;

           der(y)*(D-x)*(D-x)*x*x =  -1*temp1*(D-x)*(D-x) + temp2*x*x ;       

           der(x) = y;   

end rocket;

i simulator it -> error

But where i change it by :   y(t) = Rx(t) và u(a) = y(a*T) với t = a*T with T² = R/g
I have new ODE :                               u’’ = -1/(u2)  + 0.012/((60 - u)2);   u(0) = 1 ;u’(0) = V*T/R

I write on modelica

model rocket



    parameter Real G = 6.67*(10^(-11))  "gravity constant";



    parameter Real M = 59736*(10^(20)) "mass of earth";



    parameter Real m = 7.347*(10^(22)) "mass of mooth";



    parameter Real D = 384400.0 "distance betwen earth and mooth(mean)";



    parameter Real V = 50.0 "velocity of rocket";



    Real x(start = 1.0);



    Real y(start = (V/7.92));



    Real temp1 ;



    Real temp2;



equation 



    temp1 = m/M;



    temp2  = D/6400;



    der(y)*((temp2-x)^2)*x*x =  -1*((temp2-x)^2) + temp1*x*x ; 



    der(x) = y;   



end rocket;

-> it is ok

So , why the first(**********) way was error

For me, the first one succeeds and the second one fails.
If I make M,m and D constant, it succeeds. So something in initialization seems to go wrong. Note that using simulate(rocket, method = "euler") works fine regardless.

sjoelund.se wrote:

For me, the first one succeeds and the second one fails.
If I make M,m and D constant, it succeeds. So something in initialization seems to go wrong. Note that using simulate(rocket, method = "euler") works fine regardless.

But the first falied and the second true.I can't  explain,

So something in initialization seems to go wrong

??????

Please tell me!!!!!!!!

thanks for all!!!!!!!!!!!!!

adrpo,

Hello,

   Actually, I want to translate a Matlab code to openmodelica

Can you help me to translate the instructions below

options = odeset('RelTol',1e-4,'AbsTol',[1e-4 1e-4 1e-4 1e-4 1e-4 1e-4 1e-4 ]);

[t, s]=ode45('Modello3',tspan,T0,options);

thank you in advance for your aswer