yea, we want to keep physics simulation as limited as possible.
Its not at all suitable for an RTS. Imagine that the engine will have to render about 500 tanks in some cases, PLUS calculate paths and behaviour.
If we included physics the game would probably only run on high-end comps which many people don't have.
Yes, physics would need to be kept as simple as possible.
The most would probably be where Chandler said, a tiger is slower than a lynx but can negotiate steeper terrain.
The engine is already going to be pumping out polygons left and right as yes there could easily be 500 units on screen, but don't forget other stuff like the terrain itself, and any effects (smoke, environmental effects).
There's no real gain to using a full blown physics engine in an RTS anyway, just extra bloat (of both size and speed).
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As for plastics, where could they get the resources to make plastics? Unless they find underground oil reserves like on earth it probably isn't going to happen - the game makes no mention of oil reserves.
I could see them using plastics for the more basic units since they didn't need them to be shielded against weapons (that wasn't till later that weapons buildup started happening, and then they needed protection).
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As far as the amount of material you can load into a rocket, it's got to be size limited. You only have so much physical room in the rocket. One way I could see of doing this is kinda like the game Starcraft, where they have a grid to show what's inside a dropship. The grid is subdivided into 8 squares I believe. Small units take up 1 square, medium sized units take up 2 squares, large units take up 4 squares. This means you could fit 8 small units (like SCVs - again this is a starcraft example), 4 medium units (vultures, goliaths), or 2 large units (tanks), as well as any combination of these.
(Not that I'm saying it needs to be like starcraft) but that is how they space limited the cargo amount. Of course it's not that realistic since they assume a fixed size-to-weight ratio, so OP3 could choose to have it go by weight instead (or a combination of the two).
All I'm saying is, I'm sure that a tiger takes up more room than a scout. A limitation like this also prevents people from abusing the lander size to drop massive military force all over the map (for example they might be able to do research to drop the metals cost of the tiger, lowering its weight) -- but the tiger wouldn't actually get any smaller in size. Thus don't expect to load more tigers on after it gets made cheaper.