MATLAB simulation runs to infinity, but not when run step by step in debugger - matlab

Question

My code simulates the shock loading of a rocket upon parachute deployment. When the script is run as a whole the value of the shock goes to infinity. However, when the script is run step by step in the debugger mode it works as intended. I am unsure of what is happening or how to fix it. My code is written as follows:
clear all; close all; clc; tic
h_init = 100; %initial altitude, ft
%INPUTS
E = .38e6; %elastic modulus of nylon, psi
A = (1/16)*(1); %cross sectional area of cord, in^2
CD = 2.20; %parachute coefficient of drag
D_p = 144; %parachute diameter, in
W_p = 2; %weight of parachute, lbm
W_r = 105; %dry weight of rocket, lbm
L = 17*3*12; %length of shock cord, in
v_init = -350; %initial upwards velocity of rocket/parachute, ft/s
dt = 0.00001; %simulation time step, s
t_sim = 4; %total simulation time, s
g = 32.2; %gravitational acceleration, ft/s^2
%CALCULATIONS
k = (3900)./L; %spring constant, lbf/in
S_p = pi*D_p.^2/4/144; %parachute cross sectional area, ft^2
m_p = W_p/g; %mass of parachute, slugs
m_r = W_r./g; %mass of rocket, slugs
N = ceil(t_sim/dt);
N_L = length(W_r);
time = dt:dt:t_sim + dt;
Y1 = zeros(N_L,N);
Y2 = zeros(N_L,N);
V1 = zeros(N_L,N);
V2 = zeros(N_L,N);
FS = zeros(N_L,N);
Dr = zeros(N_L,N);
Shock = zeros(N_L,1);
Exten = zeros(N_L,1);
VDiff = zeros(N_L,1);
DrMax = zeros(N_L,1);
TDrop = zeros(N_L,1);
for j = 1:N_L
v1 = v_init; %initial velocity of parachute after deployment, ft/s
y1 = 0; %initial parachute position relative to deployment altitude, ft
v2 = v_init; %initial velocity of rocket after deployment, ft/s
y2 = 0; %initial position of rocket relative to deployment altitude, ft
Y1(j,1) = y1;
Y2(j,1) = y2;
V1(j,1) = v1;
V2(j,1) = v2;
FS(j,1) = 0;
Dr(j,1) = 0;
slack = 1;
for i = 1:N
rho = findDensity(h_init+y1);%air density, slug/ft^3
Q = 144*0.5*rho*v1^2; %dynamic pressure, psi
D = -sign(v1)*(Q/144)*S_p*CD; %drag, lbf
if y1-y2 >= L/12
delta = 12*(y1-y2)-L; %shock cord extension, in
if slack ==1
TDrop(j) = dt*i;
slack = 0;
end
elseif y2-y1 >= L/12
delta = -12*(y1-y2)-L; %shock cord extension, in
else
delta = 0;
end
fs = k*delta; %spring force, lbf
F1 = D - W_p - fs; %net force on parachute, lbf
F2 = fs - W_r(j); %net force on rocket, lbf
a1 = F1/m_p; %acceleration of parachute, ft/s^2
a2 = F2/m_r(j); %acceleration of rocket, ft/s^2
y1 = y1 + v1*dt; %update parachute position, ft
y2 = y2 + v2*dt; %update rocket position, ft
v1 = v1 + a1*dt; %update parachute velocity, ft/s
v2 = v2 + a2*dt; %update rocket velocity, ft/s
Y1(j,i+1) = y1;
Y2(j,i+1) = y2;
V1(j,i+1) = v1;
V2(j,i+1) = v2;
FS(j,i+1) = fs;
Dr(j,i+1) = D;
end
Shock(j) = max(FS(j,:));
Exten(j) = 12*max(Y1(j,:)-Y2(j,:)-L/12);
VDiff(j) = max(abs(V1(j,:)-V2(j,:)));
DrMax(j) = max(Dr(j,:));
end
H1 = h_init+Y1;
H2 = h_init+Y2;
fprintf('Max Shock Load: %.2f lbf', Shock)
toc
function density = findDensity(h)
temp = findTemp(h);
pressure = findPressure(h);
density = pressure / (1718 * (temp + 459.7));
end
function pressure = findPressure(h)
temp = findTemp(h);
if(h > 82345)
pressure = 51.97 * (((temp + 459.7) / 389.98) ^ -11.388);
elseif(h > 36152)
pressure = 473.1 * exp(1.73 - (0.000048 * h));
else
pressure = 2116 * (((temp + 459.7) / 518.6) ^ 5.256);
end
end
function temp = findTemp(h)
if(h > 82345)
temp = (0.00164 * h) - 205.5;
elseif(h > 36152)
temp = -70;
else
temp = 59 - (0.00356 * h);
end
end