This program was first published in the HP41C Users' Library Solutions: Games by HewlettPackard and is used here by permission.
This program is supplied without representation or warranty of any kind. HewlettPackard Company and The Museum of HP Calculators therefore assume no responsibility and shall have no liability, consequential or otherwise, of any kind arising from the use of this program material or any part thereof.
The object here is to perform a vertical descent ending in soft landing on the planet of your choosing. You select the planet before you begin by specifying the acceleration of gravity in feet per second per second. Some values are given below:
Body g(f/s^{2}) Earth 32.2 Moon 5.32 Mars 12.3 Ganymede 5.25 Pluto 7.25 Icarus (asteroid) 0.394
For interest, zero and negative values of g are allowed. The fuel allocated, as calculated from g, is more than adequate for a minimum use landing. At least twice as much fuel as needed is given. Although it takes longer to calculate, 3 seconds is the time your burn is stretched over. You can only key a burn in during the zero of each three second count down.
Note that if zero or negative g is selected and you run out of fuel, you may not impact. In this case you will see "DEEP SPACE. . ." instead of the normal "VF=" for final velocity.
Step 
Instructions 
Input Data/Units 
Keys 
Output Data/Units 
1 
Enter program 

2 
Key in gravity. 
g(f/s^{2}) 
[XEQ] GRAVITY 
G= 
3 


FUEL= 




*V= 





A= 
THREE... 

TWO... 





ONE... 
ZERO... 

4 
You have one second during the "ZERO..." prompt to key in a fuel burn. 
burn 


5 
Go to 3 for next status. 


6 
For a new game with the same g go to step 3. 

[A] 
G= 
7 
For a new game with a different g go to step 2. 



* 
When you see "VF= ", this is your landing or crash velocity. 


Try a landing on the moon (g = 5.32 f/s^{2}).
Keystrokes: Display: [XEQ] [ALPHA] SIZE [ALPHA] 007 5.32 [XEQ] [ALPHA] GRAVITY [ALPHA] G=5.32 FUEL=5456 V=500 F/S A=5000 V THREE... TWO... ONE... ZERO... (to free fall, just do nothing) FUEL=5456 V=516 F/S A=3476 F THREE... TWO... ONE... ZERO... 20 FUEL=5436 V=512 F/S A=1 934 F THREE... TWO... ONE... . . .
LINE KEYS 01 LBL "GRAVITY" Initialize 02 SF 27 03 STO 01 04 ABS 05 800 06 * 07 1200 08 + 09 STO 05 10 LBL A 11 5000 Set up the initial conditions 12 STO 06 13 500 14 STO 02 15 RCL 05 16 STO 03 17 "G=" 18 FIX 2 19 CF 29 20 ARCL 01 21 AVIEW 22 PSE 23 FIX 0 24 LBL 09 25 "FUEL=" 26 ARCL 03 Display status 27 AVIEW 28 PSE 29 RCL 02 30 RND 31 RCL 06 32 .5 33 "V" 34 X>Y? 35 "F" 36 "=" 37 ARCL Z 38 " F/S" 39 AVIEW 40 PSE 41 X>Y? 42 RTN 43 "A=" 44 ARCL 06 45 " F" 46 AVIEW 47 PSE 48 RCL 03 49 X=0? 50 GTO 02 51 "THREE..." Countdown 52 AVIEW 53 PSE 54 "TWO..." 55 AVIEW 56 PSE 57 "ONE..." 58 AVIEW 59 PSE 60 CLX 61 "ZERO..." 62 AVIEW 63 PSE 64 CLD 65 STO 00 Calculate acceleration 66 ABS 67 RCL 03 68 X>Y? 69 RDN 70 ST 03 71 RCL 00 72 SIGN 73 * 74 3 75 / 76 RCL 01 77  78 STO 04 79 RCL 06 80 * Calculate time to impact 81 2 82 * 83 RCL 02 84 X^2 85 X<>Y 86  87 SF 00 88 X<0? 89 GTO 01 90 SQRT 91 RCL 02 92 + 93 CHS 94 RCL 04 95 X=0? 96 GTO 01 97 CF 00 If greater than 3 seconds then use 3 98 / 99 3 100 X<>Y 101 X>Y? 102 RDN 103 LBL 01 104 FS?C 00 105 3 106 STO 00 107 RCL 04 108 * Delta V 109 RCL 02 110 + 111 X<> 02 112 RCL 00 113 X^2 114 RCL 04 115 * 116 2 117 / 118 X<>Y 119 RCL 00 120 * Delta Altitude 121 + 122 ST+ 06 123 GTO 09 124 LBL 02 125 "DEEP SPACE..." Find velocity calculation 126 RCL 01 127 X!=0? 128 GTO 03 129 0 g=0 130 STO 06 131 RCL 02 132 X<0? 133 GTO 09 134 PROMPT 135 LBL 03 g!=0 136 RCL 01 137 RCL 06 138 * 139 2 140 * 141 RCL 02 142 X^2 143 + 144 X<0? 145 PROMPT 146 SQRT 147 RCL 02 148 + 149 RCL 01 150 / t 151 X<0? 152 PROMPT 153 RCL 01 154 * Delta V 155 ST 02 156 0 157 STO 06 158 GTO 09 159 .END.
R00 Used R01 g R02 Velocity R03 Fuel R04 Used R05 Initial Fuel R06 Altitude
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