Why am I getting this interp1 error?

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Shawn Alcorn
Shawn Alcorn on 3 Feb 2023
Answered: Bhanu Prakash on 13 Mar 2023 at 10:49
%% AEM 588 HW1 Question 2
clear all;
close all;
global A_const;
global B_const;
global C_const;
mu = 398600; %km^3/s^2
% % nu0 = 1e-3;
Isp = 100;
r0 = 2378 + 12800; %km
m0 = 8; %kg
T = 12/1000;
v0 = sqrt(mu/r0);
rho0 =1;
tau0 = 0;
nu0 = T/m0/9.81;
T0 = nu0*(mu*m0)/r0^2*1000.;
mdot = T0/Isp/9.81;
A_const = T0*r0^2/1000;
B_const = mu*m0;
C_const = mdot*sqrt(mu*r0^3);
tspan = [0 10000];
initialConditions = [1.;0.;1.];
%Solve ODE
[tau,rho] = ode45(@odefun,tspan,initialConditions);
t = tau;
r = rho(:,1);
rMoonNorm = ones(length(r),1).*(384400/r0);
b_line = polyfit(t,rMoonNorm,1);
y_line2 = polyval(b_line,t);
x_int = interp1((y_line2-r),t,0);
Error using matlab.internal.math.interp1
The sample points must be finite.

Error in interp1 (line 188)
VqLite = matlab.internal.math.interp1(X,V,method,method,Xqcol);
y_int = polyval(b_line,x_int);
figure(1); clf(1);
hold on
xlabel('Time, Non-Dimensional')
ylabel('Radius, Non-Dimensional')
t = tau./sqrt(mu/r0^3);
r = rho(:,1).*r0;
rMoon = ones(length(r),1).*384400;
time_intercept = x_int/sqrt(mu/r0^3);
hours_intercept = time_intercept/3600;
days_intercept = hours_intercept/24;
Prop_mass = mdot*time_intercept;
fun = @(x) T0./(m0-mdot*x)
dV = integral(fun,0,time_intercept);
figure(2); clf(2);
hold on
xlabel('Time (s)')
ylabel('Radius (km)')
function dydt = odefun(t,y)
global A_const;
global B_const;
global C_const;
dydt = zeros(3,1);
dydt(1) = y(2);
dydt(2) = ((y(3)^2)-y(1))/(y(1)^3);
dydt(3) = (A_const/(B_const - C_const*t))*y(1);
KSSV on 3 Feb 2023
Check your tspan llimits.....You may reduce it? Also see are all the parameters used are in the same unit system.

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Answers (1)

Bhanu Prakash
Bhanu Prakash on 13 Mar 2023 at 10:49
Hi Shawn,
As per my understanding, you are trying to interpolate a 1-D function using "interp1" function and are facing some errors in it.
If you look at the code, there is a matrix "r" which is passed as an argument to the "interp1" function. The matrix "r" contains integers values along with "NaN" and "Inf (infinity)" values. These values are the reason for the error.
One workaround for this error is to replace the "NaN" and "Inf" values with finite values. I have tried this at my end and the code worked fine.
Hope this answer helps you.
Bhanu Prakash.




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