Jesus Just Left Chicago

by James Classen

I am developing a “Lunar Lander” game for the Lake Afton Public Observatory computers. The one they have is DOS-based, entirely textual, and needs at least a visual upgrade if nothing else. It’s the most popular game they have. So, starting with zero information, I decide to see what it would be like to simulate the actual lunar landing. I’m guessing those 12-15 minutes were harrowing experiences for the LM pilots, though they’d probably done plenty of ground simulations to make it easier.

The problems begin right off the bat: the average altitude at which the 6 LMs began their descent was about 15188 m (8.2 nmi), traveling 1694 m/s over the lunar surface. Ignoring the immense horizontal speed (considering the vertical component only), they had enough fuel on board for just 100 s of full throttle. Obviously, if they just fell, they’d hit the ground at about 222 m/s and leave quite a crater, and it’d take just over 2 minutes to get there. But the records show that it took Apollo 11 a full 12:36.39 to touch down. I’m trying to figure out a) how they managed that and b) how I can keep the game fun and educational. For instance, I haven’t found any data on the throttle position during landing, or how hard each LM hit the ground, or what, given the suspension of the craft, would be survivable (while keeping the ascent stage sufficiently intact for the return trip). Most of this data I would expect NASA to have record of, and perhaps I need to write them and find out if it’s freely available somewhere (or if they’ll just send it to me).

My program does do some nice things that others may not: it adjusts the mass of the craft based on the amount of fuel remaining, gravitational acceleration is adjusted based on the distance the LM is from the ground based on Newton’s law of gravitation (using mean lunar radius and estimated lunar mass), and uses all the (averaged) stats I could gather from the actual Apollo missions (Isp of the main thruster, acceptable thrust values, mass). I suspect I’ll edit starting values for “easy”, “medium”, and “hard” modes, and I’d like to add a “scenario” mode for each mission that actually landed. Keep in mind, Apollos 15, 16, and 17 had the lunar roving vehicles on board. Even though the rovers themselves were each just 210 kg, each of the LM were roughly 1179 kg more massive (probably for mounting equipment and shielding), yet held roughly the same amount of fuel.

One thing I do not have is terrain on which to land. I could (and probably will) make an artificial 2D terrain generator when I start building the GUI, but I’ll have to ensure that it’s realistic (that the parameters for a random midpoint displacement aren’t too extreme for the lunar surface).

Many sample programs I’ve examined use ambiguous altitude and speed measurements, things like “fuel units”, in addition to lives. I’m sorry, but if anyone crashed on the moon, they’d be dead. They wouldn’t get another chance five seconds later to try again. And instead of making decisions about how much throttle to use (or fuel to burn, as in the case of the current LAPO program), the user gets arrow keys to control pitch and the down arrow to control thrust–so it winds up being on or off. One boasts about using “true gravity physics”, but even if you land successfully, your craft explodes. Another talks about using realistic mass, thrust, and fuel consumption, but starts you off 50 m above the surface with a zero-magnitude velocity vector.

I went to Lindy Bombers tonight. Haven’t danced in probably 3 ½ years (since just after I moved to this house, I believe). It turned out that knocking some of the rust off my moves was easier than I expected it would be. I can do the swing out and the 6-count pass. Lots more of the basics to recall, but I’ll get there. Just gotta stick with it. Perhaps I can convince Karyn to join me some evening, at least over at Care to Dance.

So Corvi has grown…a lot in a short while. He’s probably passed 30 lbs already. Certainly not fat, either, just growing like a weed. I have to work on his biting, though, and a lot of it’s my fault for not spending enough time with him. Things will become much easier next week, as we finally get that extra hour of daylight in the evening (even if I do have to sacrifice an hour of sleep for it). So, running, dancing, dog, raise, side-job for more pocket money, things are going much better now than they were a month ago when I had the flu! I just have to find the time to do things like put away the laundry (it’s all clean, most of it’s folded, there’s just no room in the closet right now).

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