How hard is orbital bombardment

[teviet]

Originally Posted by schmeelke

How fast would an object have to be moving for atmospheric deflection to be negligible? :P

To a first approximation speed is irrelevant: the strength of the deflecting force goes as v^2 while the time that it has to act goes as 1/v (so the amount of deflection goes as force/v^2). The important criteria are (i) size of projectile, (ii) density of projectile, (iii) any streamlining, (iv) terminal guidance, and (v) how accurate do you need to be anyway? Bombarding a city with 100-tonne rocks is easy and almost impossible to stop. Striking a particular building with 1kg tungsten rods is a whole lot trickier (I'd expect some terminal guidance would be necessary).

TeV

[Luke Bunyip]

To effectively answer this question, I will need to fish for some stuff I stumbled across on a discussion board run by Ken Burnside from Ad Astra Games, for his Ten World setting. The board was populated by a gang of hard science fiends.

The basics were that orbital bombardment was like hurling iron crowbars from space. It took a while for them to fall, and you could dodge them, either by foot or vehicle, if you had a radar net which could sense them. You could mount small pebble bed reactors on truck if you need to keep your power grid up, but bridges and static infrastructure would be difficult to protect.

For interdiction of transport networks, and destruction of factories and power generation, tis perfect. Not really suited for combat support purposes. However, orbital bombardment is cheap, and a viable harassment option even if there are existent ground to LEO defences.

[Anthony]

Originally Posted by Dangerious P. Cats

Orbital bombardment is pretty common in sci fi and yet I wonder how possible it is and how it would be done assuming a realitivly hard sci fi setting?

It's not particularly technically difficult to drop rocks (crowbars, whatever) from orbit. It's significantly more difficult to do in a way that's useful.

If you have a ship in a 500 km orbit, it takes around 400 m/s delta-V (a fast pistol round) to fire a projectile that will hit the ground. That's the easy part. There are three problems with this:

• The projectile will take 30-50 minutes to hit, depending on how you launch it, with launch windows coming every 90 minutes or so.
• The projectile is limited to hitting a fairly narrow band, maybe a thousand kilometers wide.
• The projectile will be blinded and unable to communicate for the last several minutes of approach, severely limiting its accuracy. Accuracy can be improved if the projectile slows down by enough for its plasma sheath to dissipate, but that also costs a large fraction of the power of kinetic weapons (it's not a problem for explosive warheads).

Basically, if you don't have a ship in the right place at the right time, you can't hit the target. Even if you do, it takes a good part of an hour for the projectile to even land on target, and you still probably can't hit a target that isn't huge and unmoving.

Energy weapons fire from orbit is a bit more promising, though you still need to deal with the problem of having a vehicle in the right place at the right time.

[sir_pudding]

Originally Posted by Anthony

Energy weapons fire from orbit is a bit more promising, though you still need to deal with the problem of having a vehicle in the right place at the right time.

That's no worse than having CAS on station or Naval Guns in range though. Except for mortars, indirect assets are always on a ad hoc basis. That's why you need Fire Support Coordinators.

[Anthony]

Originally Posted by sir_pudding

That's no worse than having CAS on station or Naval Guns in range though.

Well, once CAS or naval guns are in range, which may take a while, they can generally loiter for quite a bit. With the orbital bombardment you've got 2-3 minutes where they can fire, and then at least 90 minutes (and, for an orbital pattern that lets you cover the entire planet, about a day) before you can shoot again. In addition, your opponents will generally know exactly when that 2-3 minute will be.

This varies by quite a bit based on weapons range, though. If you have weapons with an effective range of 40,000 km (approximately range L/X for spaceships) that are capable of firing through atmosphere, you can just park in geostationary orbit. Spaceships reasonably lets you do this with SM+12 lasers spinal; I haven't done the math on how sensible that is.

[sir_pudding]

Originally Posted by Anthony

Well, once CAS or naval guns are in range, which may take a while, they can generally loiter for quite a bit.

CAS tends to have windows of a hour or so at most.

With the orbital bombardment you've got 2-3 minutes where they can fire, and then at least 90 minutes (and, for an orbital pattern that lets you cover the entire planet, about a day) before you can shoot again. In addition, your opponents will generally know exactly when that 2-3 minute will be.

That assumes that your orbital platform is in a single orbit and unable to maneuver, yes? If CAS, arty and warships are expected to move to provide support why can't spacecraft?

[Son of Dave]

Essentially, orbital weapons of this sort seem to be a strategic, rather than tactical asset. Their utility is found mostly in what you might do with them, rather than what your actually doing with them. They occupy a very similar strategic role as ICBMs; and it is easy to envisage ICBMs as one shot orbital weapon platforms (for certain degrees of 'orbit'), with the added bonus of a limited resistance to being shot down, as they arent in space/high atmosphere longer than needed to drop the nukes.

[Anthony]

Originally Posted by sir_pudding

If CAS, arty and warships are expected to move to provide support why can't spacecraft?

That type of orbital change tends to be fairly expensive in delta-V terms (up to around 5 mps, depending on what sort of change you want to make); if you don't have reactionless drives (in which case you can just hover, and ignore the whole concept of orbiting) it's just not something you want to do very often.

[Lord Carnifex]

Originally Posted by Anthony

Well, once CAS or naval guns are in range, which may take a while, they can generally loiter for quite a bit. With the orbital bombardment you've got 2-3 minutes where they can fire, and then at least 90 minutes (and, for an orbital pattern that lets you cover the entire planet, about a day) before you can shoot again. In addition, your opponents will generally know exactly when that 2-3 minute will be.

I use this as an excuse for why capital warships in my space opera setting have destroyer and cruiser sized escorts.

[sir_pudding]

Originally Posted by Anthony

That type of orbital change tends to be fairly expensive in delta-V terms (up to around 5 mps, depending on what sort of change you want to make)

More expensive than loitering fixed wings?