ft:planets:gravity
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- | The following rules are designed to model the use of planets within a Full Thrust game, both as an obstacle | + | The following rules model gravity from the bottom up, so require a few extra measurements when a ship is manoeuvring close to a planet. |
- | {{../ | + | The following are values for some common planets and moons. |
- | All these rules assume //vectored movement// is being used. If you're using cinematic movement, then these rules won't make much sense. | + | ^ ^^^ Acceleration at distance from surface |
+ | ^ Planet | ||
+ | | Earth | 13" (12, | ||
+ | | The Moon | 3" (3,475 km) | ||
+ | | Mars | 7" (6,787 km) | ||
- | ===== Units of Measurement ===== | + | According to physics, gravity will cause a ship to accelerate towards the planet. For this reason, gravitational strength is treated as thrust, as if the ship were thrusting towards the planet. A ship within 2" |
- | Normally, Full Thrust doesn' | + | If you want to work out values for your own worlds, there there's a simple(ish) formulae for doing so, which will be given at the end of this page. |
- | measurements, | + | |
- | ' | + | |
- | often taken as an inch on the table top). If we | + | |
- | start using real world objects though (such as | + | |
- | planets), it becomes necessary to translate these | + | |
- | abstract terms into real world units. | + | |
- | A turn is taken to be fifteen minutes, and 1" is | + | ===== Gravitational Movement ===== |
- | assumed to represent one thousand kilometres. From | + | |
- | this, we can work out that a ship thrust factor of 1 | + | |
- | is equivalent to an acceleration of approximately | + | |
- | 1.125 ms-2. To keep things simple, I'll assume that | + | |
- | 1g is equal to a thrust factor of 8. | + | |
- | Unless | + | The rules here assume that you are using the vector movement system, and work best if you have a counter for each ship showing its vector. When a ship is moved, you apply the ships normal manoeuvring |
- | consider using 1" = 1cm on the table top. This provides | + | |
- | a lot more room for manouevre around the planets, and | + | |
- | also makes modelling planets easier. | + | |
- | ===== Modelling Planets ===== | + | - Work out which zone the ship is currently in, and look up the gravitational acceleration. |
+ | - Work out the direction from the centre of the ship to the centre of the planet. | ||
+ | - Apply the gravitational acceleration to the movement counter in the same direction as in step 2. | ||
+ | - Move the ship as normal. | ||
- | The easiest way to model a planet | + | As an example, consider the NAC ship // |
- | of a piece of cloth or paper, and simply place it on | + | |
- | the table top. This also makes it very easy to place | + | |
- | planetary installations on the planet, since they can be | + | |
- | drawn/ | + | |
- | Another easy way is to buy a polystyrene sphere from a | + | {{ : |
- | craft shop, cut it in half to form a hemisphere and | + | |
- | paint it to look planet-like. The largest spheres I've | + | |
- | found though are only 10cm in diameter, so this works | + | |
- | best for either dwarf planets or if you're using | + | |
- | centimetres. See the picture above for an example. | + | |
- | ====== The Effects | + | According to the table above, the ship is currently in the 2" |
- | Every large body, from planets to stars and black holes, | + | {{ : |
- | will exert gravitational attraction on nearby ships. This | + | |
- | attraction will act as an acceleration directly towards | + | |
- | the body, just as if the ship were thrusting towards it. | + | |
- | Since a thrust of 8 is considered to be equivalent to 1g, | + | |
- | so the force of gravity at the surface of an Earth-like | + | |
- | world will cause an acceleration of 8". | + | |
- | Each planet is considered to have gravity bands which reach | + | We find the vector |
- | from its surface out into space. The first band is out to 4", | + | |
- | the second to 8", the third to 16" - and so on, doubling in | + | |
- | distance each time. For an Earth-like world, the gravity in | + | |
- | the first 3 bands is 6"/turn, 2"/turn and 1"/turn. This means | + | |
- | that any ship that begins a turn within 4" of Earth will get | + | |
- | dragged towards the surface by 6". | + | |
- | + | ||
- | The steps to work out the effects of gravity are as follows. | + | |
- | We assume that vectored movement is being used, and that | + | |
- | a //vector marker// is used to show each ship's position | + | |
- | at the end of the next movement. | + | |
- | + | ||
- | At the beginning of movement, work out the distance | + | |
- | between the ship and the surface of the planet. If | + | |
- | the ship is outside any effective gravity bands for | + | |
- | that planet, then the planet has no effect (e.g., | + | |
- | outside 16" for an Earth-like world). | + | |
- | + | ||
- | Lookup the gravitational acceleration for that planet | + | |
- | for that range. There is a table below which lists | + | |
- | acceleration values for some common worlds. | + | |
- | + | ||
- | Work out the gravity vector from the ship towards the | + | |
- | centre of the planet for this acceleration. This vector | + | |
- | has a magnitude equal to the acceleration, | + | |
- | direction | + | |
- | + | ||
- | Apply the gravity vector to the ship's vector marker. | + | |
- | + | ||
- | Apply any acceleration due to the ship's thrust. | + | |
- | + | ||
- | Move the ship and its marker. | + | |
- | + | ||
- | It is the distance of the centre of the ship to the | + | |
- | surface of the planet | + | |
- | of the vector marker has no effect. | + | |
- | + | ||
- | ===== Acceleration due to Gravity ===== | + | |
- | + | ||
- | The acceleration due to gravity on a ship depends on the size of the planet and the distance the ship is from it. For a planet of Earth size and mass, the following zones of influence are suggested. | + | |
- | + | ||
- | + | ||
- | ^ | + | |
- | ^ Planet | + | |
- | | Earth | 13" (12, | + | |
- | | The Moon | 3" (3,475 km) | 0.5" | -- | -- | -- | -- | -- | -- | -- | | + | |
- | | Mercury | + | |
- | | Venus | 12" (12,104 km) | 4" | + | |
- | | Mars | 7" (6,787 km) | + | |
- | | Jupiter | + | |
- | | Saturn | + | |
- | | Uranus | + | |
- | | Neptune | + | |
- | | The Sun | + | |
- | | Model | 10" (10, | + | |
- | + | ||
- | + | ||
- | Remember that the distances given above are the distance from the //surface// of the body. The Sun will not fit | + | |
- | into any sensible gaming area, and is given merely to show how insignificant everything else is. Even Jupiter would require at least 2.5m (at centimetre scales) to allow for manoeuvring at the edge of its gravity well. | + | |
- | + | ||
- | Where acceleration less than 1" is given, it can be ignored. However, this does mean that small worlds such as Mercury would have no effect, so it's listed anyway in case you want to use it. | + | |
- | + | ||
- | ====== Examples ====== | + | |
- | + | ||
- | + | ||
- | ===== Fast Fly-by ===== | + | |
- | + | ||
- | + | ||
- | + | ||
- | The following example shows the Light Cruiser | + | |
- | //Colbert// approaching a planet. At no point | + | |
- | does the //Colbert// apply any thrust | + | |
- | own. The world is slightly smaller than the Earth, | + | |
- | with statistics as follows: | + | |
- | + | ||
- | ^ | + | |
- | ^ Planet | + | |
- | | Model | 10" (10,000km) | + | |
- | + | ||
- | We begin the example with the //Colbert// 10,000 km (10") from the planet' | + | |
- | + | ||
- | {{../ | + | |
- | + | ||
- | The gravitational acceleration at this point is 1"/ | + | |
- | + | ||
- | This moves the //Colbert// to its new location, somewhat closer to the planet but halfway around it. | + | |
- | + | ||
- | {{../ | + | |
- | + | ||
- | At the new distance | + | |
+ | {{ : | ||
+ | {{ : | ||
ft/planets/gravity.txt · Last modified: 2015/04/27 18:05 by sam