# Re: Space sim in the works

```Hello again, sorry for the delay!

```
I get your point. Let me explain that "realistic flight physics" a bit.
For now, the ships fly taking inertia into account, so you actually
don't use the thrusters all the time, only when maneuvering - so you give it a push, and it keeps on going indefinitely, until you steer the
ship and give it another push, so it changes direction accordingly.
There is, however, an option to "automagically" compensate the ship's
inertia so you can fly around to whatever direction the ship is pointing
to, *almost* like atmospheric flight.
Do you think that this system is too simplistic for us to call it
"realistic" ? There is always room for improvement .
```Sorry. My mistake, my fault. I've seen your demo movie and now I have a better idea about your flight physics. Beforehand made a mistake and didn't explain myself clearly enough.

So now let me explain my reasoning slowly and clearly.

I'll start with some basic physics (I know you know them, but I want to state them explicitely anyway), then I'll explain a basic spaceship turn. After that I'll explain some consequences to spaceship designs, I'll create a simple classification of the degree of how spaceflight physics are realistic in a game. I'll finish by classifying your engine and explaining my whole point.

SPACESHIP FLIGHT PHYSICS
------------------------
Let's consider the spaceship to be a rigid body. It has a point called a "centre of gravity." (COG from now on) The movement of the ship can be divided into:

(i) translation of the COG
(ii) rotation of the ship around the COG.

Let us consider a thruster (force) F situated at a point P. The force F can be divided into two forces: FT parallel to the line COG-P and FR perpendicular to the line COG-P.

The force FT causes translation, while the force FR causes rotation.

BASIC TURNING MANOEUVRE
-----------------------
If a ship wants to, say, constantly turn left, going around in a circle, it is actually doing two things:

1. altering the direction of it's momentum -> it's going around a circle
2. rotating around it's COG -> it's always heading in the direction of it's movement (<=> momentum)

As for 2, small (invisible) thrusters are used. Not important, let's just suppose, that the ship is always heading the direction of it's movement and study 1.

Let the ship be situated in point P, while the center of the circle is C. Let F be the translational force applied by the ship's engines. We can split this force into FD parallel to the line C-P, and to the force FV perpendicular to the line C-P.

The force FV alters the ship's velocity, while the force FD alters it's direction.
We want to change the direction, so we just need a nonzero FV.

TO REMEMBER:
Thus in order to go around in a circle, we must apply a constant force perpendicular to the ship's direction. (e.g. to the left)

SHIP DESIGN - ENGINES
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In many science-fiction worls, space fighters have engines on the rear side.
Example 1: Star Wars - giant (sublight) engines on the rear side ("dart design")
Example 2: Star Trek - e.g. USS Enterprise 1701A: sublight engines (impulse drive) only on the rear side (though small and invisible)

A spacefighter mainly exercises turns, it accelerates and brakes much less often. Thus it mainly needs engines on the sides (starboard side, portside, dorsal and ventral side.) Engines at the front and rear sides are also needed, but usually need the same, or most probably less power, than the "side" engines.

As we can see, these sci-fi worlds don't have realistic physics. But these worlds are designed mainly for the visuals, so it's not a great problem.

The only problem is, that one can't make a game with realistic physics, that would be set in these worlds. (We can however use a trick, I'll explain that later.)

If we want to design a realistic ship, the side engines must have at least the same size (-> power) as the rear engines. (And there must be front engines, too.) A ship with giant engines on all sides might look ugly, thus we might use smaller ("more efficient") engines (perhaps even virtually invisible ones).
Moreover, one might want to keep the "dart design." Solution: add huge superluminal engines as in Star Trek.

CLASSIFICATION OF SPACEFLIGHT ENGINES
-------------------------------------
From the point of view how realistic they are, both translation and rotation can be simulated in a realistic or simplified way, thus spawning four classes:

B. "Between" Realistic translation only.
C. "Completely real" Both realistic.
(D. Realistic rotation only.)

Here D doesn't make sense, so we'll be just left with A,B,C. It cannot be stated that "C is better than B is better than A."

Class A
-------
gives an atmospheric-like feeling

HIGHS: simple to implement, intuitive ship controls
LOWS: cannot achieve complex manoeuvres.
Example: X-Wing from LucasArts.

Class B
-------
Gives an almost completely realistic feeling.

HIGS: complex manoeuvres possible.
LOWS: more difficult to implement. Controlling the ship's engines directly is not a reasonable solution. There must be a "piloting computer." The controls are a compromise between being too complicated and falling almost back to class A. The more complicated controls are better usable for capital ships, where one player is solely a PILOT and can turn all his attention to controling the ship.
Realistic collisions are not possible.

Class C
-------
Almost reality.

LOWS: It is impossible for a human to control a ship directly using it's control thrusters. The only solution is to use a piloting computer and fall back almost to class B.
HIGHS: Realistic collisions possible.

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Your flight engine is probably class B, which is actually highly realistic. I was mistaken by the fact that your ships have the "dart" design, so the designs are actually "class A".
This is not critical, however it still is an inconsistency. You probably used a trick and added "virtual" engines on the craft's sides, in order to have a "class B" ship.

(When your ship accelerates, it's rear engines glow, so they're not superluminal or any other special-purpose engines).

I've actually seen a commercial game, which was exactly the same: class B engine, "dart" ships.

CONCLUSION
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The whole point of this awfully long text: Your game engine is realistic. Your ship designs are not. As a consequence, the game in whole is not realistic.

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Whew! I hope that explains what I meant. So the claim "realistic physics" on your webpage is right (even though the game as a whole is not realistic) and I apologise for calling it wrong.

And remember: it's not important, how realistic a game is. The important thing is, how playable it is.

Jiri Svoboda
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