Indexing in a while loop

I need the results to be the length until mfuel is equal to or less than 0. Please help.

time(i)=0;
dt=0.01; % change in time
height(i)=1;% elevation
i=1;
BR=4.8; % burn rate
mfuel(i)=134; % mass fuel
mtot=150; % total mass
mcase=16; % mass of casing
fthrust(i)=1560; % force of thruse
athrust(i)=0; % thrust acceleration
ag=-9.81; % acceleration of gravity
atot(i)=athrust(i)-ag; % total acceleration
v(i)=0; % velocity
ke(i)=0; % kinetic energy
pe(i)=0; % potential energy
te(i)=0; % total energy
while height > 0
    time(i+1)=time(i)+dt;
    mfuel(i+1)=max((mfuel(i)-BR*dt),0); % add if statement to make sure mfuel isnt <0
    mtot(i+1)=mfuel(i+1)+mcase;
    fthrust(i+1)=1560*sign(mfuel(i+1)); % -1 if neg 0 if zero 1 is pos
    athrust(i+1)=fthrust(i+1)/mtot(i+1);
    v(i+1)=v(i)+((atot(i+1)+atot(i))/2)*dt;
    height(i+1)=height(i)+((v(i+1)+v(i))/2)*dt;
    ke(i+1)=1/2*mtot(i)*v(i)^2;
    pe(i+1)=mtot(i)*(-g)*height(i);
    te(i+1)=pe(i)+ke(i);
end

Answer 1

[Walter Roberson]

Your line

atot(i)=athrust(i)-ag; % total acceleration

does not define an equation. It is executed only once, with the current value of i, so atot becomes a scalar.

Then in your line

    v(i+1)=v(i)+((atot(i+1)+atot(i))/2)*dt;

on the first iteration you have the problem that atot(1+1) does not exist.

After your while loop,

    nofuel_pos = find(mfuel <= 0, 1);
    

and then you can index your arrays at nofuel_pos or nofuel_pos - 1 as appropriate.