Moon defense bases

[The_Caveman]

Guns fired in orbit aren't going to stay in orbit.

I had to test this in Universe Sandbox 2 just to satisfy my own curiosity. It was surprising, I expected the influence of the sun's gravity to have more of an effect and produce a long-term solar capture orbit. But even firing the "projectile" (in this case a Tesla roadster) on a near-miss trajectory at the sun itself wasn't enough to curve the trajectory more than 90 degrees. It just bent around the sun and kept going off into infinity.

Probably the worst-case scenario would be firing something like a NAC/40 on a trajectory retrograde to an inner planet's orbit. Then you could actually get the shell into solar orbit in the opposite direction of the planet.

[Alsadius]

I had to test this in Universe Sandbox 2 just to satisfy my own curiosity. It was surprising, I expected the influence of the sun's gravity to have more of an effect and produce a long-term solar capture orbit. But even firing the "projectile" (in this case a Tesla roadster) on a near-miss trajectory at the sun itself wasn't enough to curve the trajectory more than 90 degrees. It just bent around the sun and kept going off into infinity.

Probably the worst-case scenario would be firing something like a NAC/40 on a trajectory retrograde to an inner planet's orbit. Then you could actually get the shell into solar orbit in the opposite direction of the planet.

Escape velocity is a scalar, not a vector. As long as you don't hit anything, direction is totally irrelevant. (Surprising, and a bit counterintuitive, but it's actually true). And for a bit of quick math, escape velocity from altitude X is always exactly equal to [the orbital velocity for a circular orbit of height X] multiplied by [the square root of 2]. For something leaving Earth, the escape velocity from Earth's gravity well is 11.2 km/s (which will leave it in the solar system).

Escaping from a satellite is more complex, because you need to escape both gravity wells, but you have the satellite's orbital velocity to help you. That means it does depend on direction. If you fire it in the optimal direction, the total escape velocity to get you out of the Sun's gravity well is 16.6 km/s. So if you fire your Tesla at 15 km/s, it'll leave Earth orbit but stay within the solar system. But if you fire it at 20 km/s, it can go bye-bye. And if you go in the opposite direction, you could fire it as fast as [11.2 km/s to escape Earth] + [29.8 km/s to counter the Earth's orbital velocity] + [42.1 km/s to escape solar gravity] = 83.1 km/s. Or at least I think that's how the math works out. For comparison, it seems like capital weapons go at a minimum of [56 hexes max range] * [18 km/hex] / [60 seconds/round] = 16.8 km/s, since they hit the round that they're fired, and I suspect that most will go faster.

[cray]

This is an interesting thread. We haven't had a good, crunchy aerospace discussion in a while.

Technically, if you are on the dark side of Earth's moon

The dark side moves around the moon once a month. Do you mean the far side, which gets 2 weeks of light like the near side?

The ever-mobile dark side

, and a spaceship is in geostationary orbit above you, Earth's moon could be said to be 'shielding' the Earth from the spaceship.

Geostationary orbit is 40,000km from Earth. The moon is 400,000km. The moon would never get between the ship and Earth.

Airless moons may be a good place for damaged craft to recover, as it avoids problems with re-entry.

If they have working engines, then they could avoid high speed reentry on a planet with an atmosphere, too. With fusion rockets you don't have to hit atmosphere like a bat out of hell.

Still, an airless, low G moon is very convenient for WarShips, JumpShips, and vacuum-only DropShips, or fighters with barely more than maneuvering thrusters.

[skiltao]

I guess Battletech never intended to have Surface to Orbit weaponry or satellite defense systems due to the original setting for the Succession Wars Era was low tech.

I used to think that too, but I've come across enough old references to castles with (what seems to be) anti-ship weaponry that I've reconsidered the point.

[cray]

I used to think that too, but I've come across enough old references to castles with (what seems to be) anti-ship weaponry that I've reconsidered the point.

There are rule provisions for capital weapons firing into orbit, but they have the same problem as capital weapons bombarding from orbit: very short range.

[Alsadius]

There are rule provisions for capital weapons firing into orbit, but they have the same problem as capital weapons bombarding from orbit: very short range.

Though given it's the same range as weapons mounted on a WarShip, it does let you cover a given patch of land against orbital bombardment, or against most DropShip attacks. Mount some capital missiles and you can even do anti-fighter work pretty well too. A small-ish island(which, given BT capital weapons ranges, could easily be "small" like Cyprus or something) with a Castle Brian on it is a genuinely tough nut to crack.