The target ship is assumed to have infinite fuel for the purpose of evading the missile (because the missile has much smaller fuel reserves and, as you pointed out, awful fuel efficiency). So if the ship never runs out of fuel the delta-v available for the evasive maneuvers will be :
delta-v = max. acceleration * duration of the maneuversIf we assume the maximal acceleration is 10 m/s2, then the available delta-v will be 100 m/s if you start maneuvering 10 s before impact, 600 m/s if you start maneuvering 1 mn before impact, and 36 km/s if you start maneuvering 1 hr before impact.
The missile, meanwhile, has a fixed available delta-v and will not be able to excess yours if you start maneuvering early enough.
edited 18th Jul '15 6:16:21 AM by Aetol
Worldbuilding is fun, writing is a chore
I believe that those are very shaky assumptions. The target ship should not be assumed to have infinite fuel because it has more mass to shove around. It's not the absolute amount of fuel that matters, it's the fuel / weight ratio. If the f/W ratio is the same in both cases (and why wouldnt it be?), then their delta-v budgets are identical. The ship might very well have greater fuel efficiency than the missile, but that also means it's accelerating more slowly than the missile, so it might not live long enough to enjoy it.
If the ship has the same total delta-v as a missile it's useless as a ship. Obviously a ship would have a much lower acceleration, but the flip side is that the missile can't thrust for very long. Given enough time, the ship can outrun it.
Worldbuilding is fun, writing is a chore
It was always going to be a bloodbath either way but in this case shrapnel minefields actually are more useful to the attacker. Any of the attacker's shrapnel that doesn't hit the picket line will sweep right into the missile ships regardless of whether or not the attackers can breach the front line.
Pretty well, it's tight beam laser comms over a couple kilometers. The enemy would need to be right between them to jam it.
edited 18th Jul '15 8:35:37 AM by Belisaurius
True, but this cuts both ways. If the defenders deploy their shrapnel field too soon it will disperse to ineffectiveness before the attackers get there. At the same time, if they deploy too late the field wouldn't saturate space enough to be effective. The Key here is that the attackers know when they'll arrive at the picket line. It's just a matter of calculations then.
...It's a LASER comm.
edited 18th Jul '15 3:28:39 PM by Belisaurius
"If the ship has the same total delta-v as a missile it's useless as a ship. Obviously a ship would have a much lower acceleration, but the flip side is that the missile can't thrust for very long. Given enough time, the ship can outrun it."
Why is that obvious? And why would a ship be useless if it isnt true? It still has it's point defenses, after all.
"Planetary bodies are moving predictably, enemy ships will not be in all probability."
They wont want to be, surely. But remember that every maneuver depletes the fuel tank. When to engage in evasion tactics and to what extent will be a difficult decision to make. In order to conserve fuel, space warships may have no choice to be delay evasive maneuvers until they can see the missiles approaching them, and can calculate the trajectories. And yes, by then it may be too late. It's a tactical dilemma with no perfect solution.
"Shrapnel Minefield"? Are you proposing to deploy such a thing against individual ships or entire fleets? In the latter case, you would have to spread a wall of "shrapnel" across thousands of kilometers of battle space, with sufficient density to seriously damage any ship that passes into it, which seems unrealistic to me.
Night Clerk of the Apacalypse.
How is laser guidance finicky again without the limitations of atmosphere and weather? Short of a wire it is then next best form of point to point direct connection. There is neither of the key limiting laser factors in space and dumping common ECM into space will do precisely nothing to it. Beam riding type guidance is one of the more difficult ones to interrupt especially for laser guidance.
edited 19th Jul '15 9:41:48 AM by TuefelHundenIV
Who watches the watchmen?
Not laser guidance, laser comms. To keep the signal constant you have to be aiming the beams very accurately, while the other ships are moving relative to you, so it's going to be more like a spotlight than a conventional laser if you want a constant link. For a tighter beam you'd need a way of constant checking the position of each and every ship in the formation, which sounds like radar, but that can be jammed.
edited 19th Jul '15 1:05:01 PM by MattII
IFF transponders? There's no stealth in space so you might as well announce you're location.
"No stealth in space" doesn't mean "everybody knows where you are at any time". They have to know where to look, or do a scan to find you. There's no need to make that task easier.
edited 19th Jul '15 3:20:17 PM by Aetol
Worldbuilding is fun, writing is a chore
If we were talking a small group or even individual combat, then yes announcing your position would be stupid. However, we're talking large fleet on fleet engagements where knowing where your allies are is as important as identifying enemies. In this case a close range tactical network isn't just useful, it's essential to coordinating so many ships.
Painting someone with a laser in space isnt that hard. Laser beams diverge as they travel, so a comm beam is likely to be several meters across, or larger, by the time it reaches the intended target.
On the other hand, there are ways to capture data from a laser beam without actually entering the beam. Any particles caught by the beam will scatter some if it, for example.
Night Clerk of the Apacalypse.
Beam riders are basically the same thing as laser comm. Beam feeds info to missile telling it where to go. There is nothing finicky about it. Simple stabilized turrets that can track automatically are already present. The only way to disrupt that is to put something in the path of the laser beam. Another upside is you don't need a weapons grade laser to do it.
Who watches the watchmen?
Minor engineering nitpick : how do you make it so the missile's receiver always see the beam while it's swiveling around changing course ?
Worldbuilding is fun, writing is a chore
A gyroscopic stabilizer will enable it to keep track of it's initial frame of reference at launch, including the direction of it's launch point.
Dreams of Revenge
Here's a thought for you all - there's a good chance that the media will become ever more prevalent in war zones. Therefore, I wonder if future militaries might factor that into how they conduct operations (or at least moreso than they do now)? What sort of strategies might they employ to manage such monitoring?
Locking you up on radar since '09
That's what "Embedded Journalists" was all about.
Night Clerk of the Apacalypse.
Also combat and warzone journos are a thing. Poor buggers get shot up on a regular basis either on purpose or because someone jumpy has an itchy trigger finger. One of the downsides of their big shoulder cameras is they do look a bit like the targeting optics for a missile system sometimes.
Who watches the watchmen?
On to a new topic, how big should spaceborne warships be?
As big as you can build it! Carve out the core of Mars and turn it into a planetship!
Depends on the power generation needed and the factors of Space travel. Keep in mind that you want to get from point A to point B relatively quickly to still be relevant on your mission. The fault of literal planet starships is that unless you have some sort of FTL they are going to be goddamn slow.
And that's just because mass in space is still affected despite there being (relatively) no friction. Stick a engine too small onto a massive object you are getting nowhere fast.
"Yes it matters, because the delta-v available to the target is limited by time. The missile's delta-v budget is fixed by the fuel it packs, but the target's delta-v budget increases as you launch your missile from further away."
I must be confused, because I cant think of any reason why this should be true. If the target detects the launch and can calculate the missile's trajectory given no further maneuvers, then their respective budgets should unaffected in any way by time or distance.
"There's no point putting a high efficiency engine or enough fuel for interplanetary travel."
You put one in there because you want your missile to hit. Otherwise there is no point in building them. BTW- it's not an efficient engine that you want, it's one that produces very high acceleration. The engine that produces the highest acceleration (at the worst fuel efficiency) is a chemical rocket. Missiles will almost surely use them.
"A nuclear propelled kinetic impactor as an attack stage."
That's an interesting trick. Essentially an Orion Drive for a missile.
"That's assuming you know the missile is coming. Also, how do you know that incoming object is a missile and not simply a decoy to let the enemy judge how well you can manoeuvre?"
Both are excellent points.
"He's talking strictly about railguns, so a range of 100 km isn't too bad a guess."
I'm nearly certain we could hit something with a kinetic round at much farther than 100 kilometers. After all, a space probe is essentially a guided munition, once the rockets get it out of Earth atmosphere. The question is how difficult it is to detect and track a small munition launched by a railgun or other means?
"So as I said, your missile needs to stay undetected until it's close enough. Basically, the missile's properties (fuel reserves, exhaust velocity, cruise speed...) and the target's maximal acceleration determine a range inside which the missile is all but guaranteed to hit."
The second sentence is true, but the first one isnt. If the missile is launched with more delta-v than the target ship has, it doesnt matter how soon the missile is detected, unless the target's weapons can intercept it.