[FT] Space-to-surface combat

I seem to be in a rule-writing mood today. This is the FT end of orbital
bombardment, distilled from posts around the end of last year and with a
lot of my own invention...

Space-to-surface combat

It is assumed for the purpose of these rules that 1MU = 1000km. On this
scale, Terra is a 13MU diameter circle; a large asteroid (Ceres) is 1MU
in diameter. These rules deal only with solid planets with no
atmosphere, or atmosphere that is thin relative to the size of the
planet.

It is further assumed that the _More Thrust_ rules for orbital and
atmospheric operatioNs are in use (pages 13-14).

Gravity

If the planet has significant gravity (more than 1 m/s/s - normally
anything over 2MU diameter), all weapons other than pure energy weapons
(beams, stingers and pulsers) which are fired or launched from it have
their ranges halved. Multi-turn weapons, such as capital ship missiles
and fighters, halve their movement on the turn of launch only. (Fighters
expend a CEF as well.) Fighters entering this gravity well (to land or
to attack ground targets) must have half their movement available at the
point at which the miniature reaches the edge of the planet.

Atmosphere

If the planet has a significant atmosphere, this will act as a level-2
screen against all attacks which pass through it. (This supersedes any
screens on the targets.) A fighter group flying through atmosphere
expends a CEF each turn on which it does so. (Thus a fighter group being
launched from a world with significant gravity and atmosphere expends 2
CEF just to get into the fight.)

Landing thrust

A ship attempting to land on a planet must possess thrust equal to 2/3
(rounded down) of the surface gravity in m/s/s in order to do so safely.
If the planet has significant atmosphere, the ship must also be at least
partially streamlined; if it is fully streamlined, it can get by with
thrust of 1/2 (rounded down) of the surface gravity.

Planetary bases

Planetary weapon installations are constructed as space stations. They
pay normally for basic mass and hull boxes, since even on a world with
breathable atmosphere it's considered wise to have sealed systems.
However, they do not pay for armour; instead, they gain infinite free
armour by being buried within the planet. (That is, only damage that
penetrates armour will affect them.) They do not have core systems.
Races with multiple armour layer technology may buy additional armour
layers to be placed behind the infinite layer.

A base must be placed at a specific point on the planet template, and
may not have more than three, adjacent, arcs of fire.

Ortillery

An ortillery attack does d6 damage to a planetary base, automatically
penetrating the armour layer (and screens). Its range is 12MU, and it
can only be used to attack a base on a planetary surface - even the
manoeuvre thrusters on a space station are sufficient to allow it to
avoid such an attack.

Splash damage

Splash damage is the reason for ortillery to exist. All that damage
that's soaked by armour doesn't just vanish; it's going to disrupt the
surface of the planet. If there are civilians about, they'll die in
large numbers. Scenarios may specify penalties for excessive splash
damage.