Merge pull request #915 from martincmartin/main

Lunar: Add explanation of variables and code structure.

Author: coding-horror

59_Lunar_LEM_Rocket/README.md

@@ -35,3 +35,33 @@ http://www.vintage-basic.net/games.html
 #### Porting Notes
 
 (please note any difficulties or challenges in porting here)
+
+### LUNAR
+
+Variables:
+
+`A`: Altitude in miles.  Up is positive.
+`V`: Velocity in miles / sec.  Down is positive.
+
+`M`: Weight of capsule in pounds, both fuel and machine
+`N`: Empty weight of capsule in pounds.  So, weight of fuel is M - N.
+
+`G`: Gravity in miles / sec^2, down is positive.
+`Z`: Exhaust velocity in miles / sec
+
+`L`: time in seconds since start of simulation.
+`K`: Burn rate for this 10 second turn, pounds of fuel per sec
+`T`: Time left in this 10 second turn, in seconds.
+`S`: Burn time in this 10 second turn, input to subroutine 420.
+
+Subroutines:
+
+330, Apply updates from one call to subroutine 420.
+
+370, If you started descending and ended ascending, figure out whether you hit the surface in between.
+
+420, Compute new velocity and altitude using the Tsiolkovsky rocket equation for S seconds:
+
+`Q`: Fraction of initial mass that's burnt, i.e. 1 - mf / mo, exactly what we need for the Taylor series of `ln` in the rocket equation. Local to this subroutine.
+`J`: Final velocity after S seconds, down is positive.  Return value.
+`I`: Altitude after S seconds, up is positive.  Return value.

59_Lunar_LEM_Rocket/javascript/lunar.js

@@ -113,7 +113,7 @@ async function main()
         print("\n");
         a = 120;
         v = 1;
-        m = 32500;
+        m = 33000;
         n = 16500;
         g = 1e-3;
         z = 1.8;

59_Lunar_LEM_Rocket/lunar.bas

@@ -12,7 +12,7 @@
 110 PRINT: PRINT: PRINT: PRINT "GOOD LUCK"
 120 L=0
 130 PRINT: PRINT "SEC","MI + FT","MPH","LB FUEL","BURN RATE":PRINT
-140 A=120:V=1:M=32500:N=16500:G=1E-03:Z=1.8
+140 A=120:V=1:M=33000:N=16500:G=1E-03:Z=1.8
 150 PRINT L,INT(A);INT(5280*(A-INT(A))),3600*V,M-N,:INPUT K:T=10
 160 IF M-N<1E-03 THEN 240
 170 IF T<1E-03 THEN 150

59_Lunar_LEM_Rocket/python/lunar.py

@@ -127,7 +127,7 @@ def advance(self, delta_t: float) -> None:
 class Capsule:
     altitude: float = 120  # in miles above the surface
     velocity: float = 1  # downward
-    m: float = 32500  # mass_with_fuel
+    m: float = 33000  # mass_with_fuel
     n: float = 16500  # mass_without_fuel
     g: float = 1e-3
     z: float = 1.8