Comments URL: https://news.ycombinator.com/item?id=48877356

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Comments URL: https://news.ycombinator.com/item?id=46784580

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Comments URL: https://news.ycombinator.com/item?id=44490740

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If you can rebuild the cQbit from just the bath, then there's no information in cQbit, right?

So it turns out, before discovering the bug, I actually wrote code to find the optimal sequence when your choices are restricted to integers. I thought, along the same lines as you, "maybe if you burn 165 or 170 or something in the first non-zero term, then you could burn less on the 14 turn and still land."

And this is how I know it's not possible, at least with integer burn rates. :) I checked all 201^9 combinations, with a few optimizations to cut down the search space.

That's different than what you said, of using floating point for the last burn. But it is in a similar spirit.

OP here. I see your point. But think of what's needed to create this game:

- From high school physics, you know to start with a free body diagram. There are two forces, gravity and thrust. So far, your average high school student with an A in physics should be able to do that.

- Gravity depends on the distance to the center, which of course is changing, that's the whole point of a lander. I mean, you start 120 miles up. You have to realize it doesn't change much, so can be approximated as a constant. But you've been exposed to that in physics class, so maybe you just assume it's a constant.

- How the hell does thrust work as a function of burn rate? Is the exhaust velocity higher when you burn more? In other words, considering 100 lbs/sec vs 200 lbs/sec, when you double the flow of fuel into the engine, and then you burn it, it turns into twice as much fuel in the same volume. Wouldn't it be forced out at twice the speed? Or at least a higher speed? Maybe you think of the universal gas law, PV=nRT. The volume is constant (the volume of the engine), n is doubled, R is a universal constant. So that means P or T changes, or both. Why is T, which is a function of the velocity of the molecules, constant why P is doubled? Why don't both change?

- So you talk to your Dad, who happens to be a physicist. Most high school students, even those getting an A in physics, don't have a physicist for a father who can look up the properties of rocket engines and find the Tsiolkovsky rocket equation. So a high school senior finding the rocket equation is impressive to me.

- To go from velocity to position, you need to integrate. I'm not sure your average A physics senior would think of replace the FLOG() call with a Taylor series and integrating it term by term.

- How many terms of the Taylor series do you need? Does it even converge for you? If he thought of these subtleties, that's impressive. But it's possible that young Jim didn't realize these issues and just uses 5 terms because that seemed like a lot of terms.

- So now you can simulate it in near the moon. Cool! But how do you detect when it hits the ground? You could try to solve for altitude equals zero, and see if there are zero or more solutions. But even if there are solutions, they might be in the past or the future. So instead you decide to look where the velocity is zero, since you know this happens exactly once during the turn. I think that shows some ingenuity there, although I don't know if that was 18 year old Jim's thought process.

- So you try to invert the rocket equation: given a desired delta-V, how much fuel do you need to burn to achieve it? If you try this with pencil and paper and high school math, including Calculus, you keep getting stuck. You don't have the tools to show that it's actually impossible and needs you to introduce a new function, the Lambert W.

- So maybe you give up, or maybe your physicist Dad helps you again. Using your Taylor series, you now have to solve a 5th degree polynomial. So you decide to scrap the 3rd, 4th and 5th term to get yourself a quadratic. Why is it ok to scrap these now, when it wasn't ok when computing the regular dynamics? I'm impressed that he realized he can use different levels of approximation in different circumstances, without it generating some inconsistencies or other problems.

- You somehow figure out how to use the alternate form of the quadratic equation, which means you didn't just look it up and type it in. Possibly impressive.

Even when the bug is fixed, the code is still only approximating the lowest point. Also, even when it detects that you've landed, now it needs to compute the time when you land, i.e. when the altitude is zero, not velocity. It also uses an approximation for that.

So the times will be a little off. If you're burning 200 or 199 during that last time step, you have a high acceleration so even a small amount of time turns into a large amount of velocity. Instead, if you're burning say 10 lbs/sec, then being off by say 0.08 sec won't change your velocity much.

Also, the IF syntax is a little limiting and hard to read, although I suppose programmers would get used to it.

In terms of physical accuracy, especially when you get near the surface, the mass does change significantly if you have a high rate of fuel burn. But in terms of how challenging/fun it would be, and what strategies you use as a player, I doubt it would make much difference. In fact, there are other lunar lander simulations in the BASIC computer games book, I think one of them does use the naive approach.

If 10 seconds is too long, you could still leave that as the turn in the UI, but internally break each turn into a bunch of smaller time steps, e.g. 10 time steps of 1 second each. Actually, the existing game does that in certain places, which is why his physics simulation takes in an arbitrary time S rather than always the full 10 seconds.

Comments URL: https://news.ycombinator.com/item?id=40680218

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[1] https://data.bls.gov/cgi-bin/cpicalc.pl?cost1=82098&year1=19...

https://www.economist.com/science-and-technology/2024/05/17/...

(Accompanying editorial: https://www.economist.com/leaders/2024/05/23/hacking-phones-...)

https://news.ycombinator.com/user?id=simonsarris

which says:

I make GoJS, a powerful canvas-based diagramming library:

http://gojs.net/

Which is not what I think of a graphing (time series, x-y points joined by lines), but otherwise seems relevant to their comment.

https://en.wikipedia.org/wiki/Orbital_period#Small_body_orbi...