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Old 02-13-2018, 06:31 PM   #61
Humabout
 
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Default Re: [Blog] n-Body Politics

Something else to consider:

Lasers aren't instant-speed-of-light weapons. They have to send some sort of initial ranging pulse so they can adjust their focal array so that the laser beam is at maximum focus on the target. This takes 2 * [Distance in Light-Seconds] / [1 Light-Second per Second] to range the target and fire the weaponized laser. The weaponized laser then takes another [Distance in Light-Seconds] / [1 Light-Second per Second] to reach the target. Adding these and simplifying units, we find that it takes 3 * [ Distance in Light Seconds ] seconds to from pressing the fire button to the target getting hit. That generally won't be much time, but space is big and ranges might be large. Let's see what sort of limits exist for ability to dodge.

The target can probably detect the ranging ping (or it's really in for a cruel surprise), so let's just assume it can. This gives the target 2 * [Distance in Light-Seconds] seconds between getting pinged and getting hit. That's how long the target ship has to react to the ping and dodge.

Next, let's take a look at two cases: (1) Human doing the dodging and (2) computer doing the dodging.

(1) Human Dodging.
An average human takes about 0.25 seconds for visual stimulus, 0.17 seconds for audial stimulus, and 0.15 seconds for tactile stimulus, as per random Google result I'm not taking the time to vet. It doesn't sound horribly off. That suggests that depending on the warning system, the pilot will have different lengths of time to react and dodge. This mean that any shot within 0.25 light-seconds for visual warning, 0.17 light-seconds for audial warning, or 0.15 light-seconds for tactile warning probably shouldn't allow any dodge that requires pilot input other than general evasive maneuvers.

(2) Computer Dodging.
This is very hardware and software dependent. Currently, (by similar unvetted Google search), computers are alleged to have comparable reaction times as humans, but you don't have TL 8 computers. You have high-TL sci-fi computers, so can probably set this arbitrarily fast, meaning a computer can always attempt to dodge a laser weapon.

Next step is factoring in the responsiveness of the ship itself. This is quite a bit harder to do, since it is so very dependent on the specific ship. Here, you probably want to just pick a number of seconds that sounds reasonable. - you mentioned humans dodge 3 * Height, so that's one measure. But really when you think about it, the ship never has to move more than half it's longest dimension get out of the way of a shot. How long does this take? That's pretty simple:

t = sqrt(2 * distance moved / acceleration)

or for our purposes

t = sqrt{[Ship Length in Meters] / (9.81 * [Ship Acceleration in Gs])}

We add this time to the reaction time of whoever is dodging to get the total time to dodge. If this is less than 2 * [Distance to Target in Light-Seconds] seconds, the target can attempt to dodge. If not, the target just gets hit.

It's also worth noting that a spaceship can simply rotate out of the way, as well, but this is very geometry-dependent. At best, it's nearly instantaneous, and at worst, it takes too long. That sounds silly to mention, but GURPS doesn't give enough details to narrow the domain any further. It does, however, provide you with a way to fudge numbers in the target's favor, if that's the flavor you want.

WORKED EXAMPLE
A SM+12 ship is 200 m long and has a maximum acceleration of 1 G. This means that it takes the ship

t = sqrt{[200] / (9.81 * [1])} = 4.52 seconds to clear a worst-case attack.

Let's assume it is piloted by a human receiving visual warnings of inbound attacks, resulting in a 0.25 second reaction time. This means the target needs at least 4.77 seconds of warning to have a chance of successfully dodging. We know the target has 2 * [Distance in Light-Seconds] seconds to dodge, so we can then set this equal to 4.77 seconds to find that any shot fired within 2.385 light-seconds or 715,005 km has a chance to prevent the target from dodging a shot aimed at the dead middle of the ship. At the other end of the spectrum, a shot fired at the very edge of the end of the ship would need to have a range of 0.125 light-seconds or 37,474 km to deny the pilot time to even react.

Note: A computer wouldn't have that reaction time to worry about and could always dodge, and could dodge a hit amidship fired from beyond 2.26 light-seconds (677,531 km) or any shot at its extreme edge. This suggests AI have the advantage at close ranges, but aren't necessarily superior at longer ranges.

Also, This assumes the attacker already knows of the target's existence. Add another [Distance in Light-Seconds] seconds to find the time from detection to hit, assuming the attacker fires as soon as the target is spotted. Add 2 * [Distance in Light-Seconds] seconds to the target's allowable reaction time if the target was detected via active sensors and detected the ping (probably a safe assumption for military craft).

[EDIT]
And having written all of that, I realize that it ignores the "Dodge includes moving erratically to avoid being hit" thing. It does, however, provide a hard line where Dodge no longer can represent an actual reaction to an attack. If you don't mind the added complication, you could apply Restricted Dodge Against Firearms from Tactical Shooting, p. 17 with awareness determined by range to target as shown above. And those range thresholds can be pre-calculated for ships, so you don't have to do them on the fly.
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Last edited by Humabout; 02-13-2018 at 06:38 PM.
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Old 02-13-2018, 08:53 PM   #62
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Default Re: [Blog] n-Body Politics

A Human being is 2 meters tall?! Where the National Basketball Association? It ignores the fact that each sex has different average heights. Different countries have different average heights too. I don't understand why somebody would say that without acknowledging the difference.
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Old 02-14-2018, 03:43 PM   #63
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Quote:
Originally Posted by doctorevilbrain View Post
A Human being is 2 meters tall?! Where the National Basketball Association? It ignores the fact that each sex has different average heights. Different countries have different average heights too. I don't understand why somebody would say that without acknowledging the difference.
Sexual dimorphism and astounding accuracy are pretty counter to the purpose of doing a Fermi Estimation (click the link). All humans are on the order of 2 meters tall in just as all SM +12 spaceships are on the order of 200 meters long. His Fermi estimation holds.
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Old 02-14-2018, 06:41 PM   #64
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I really don't care. I haven't changed my mind about what I said.
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Old 02-15-2018, 02:36 PM   #65
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Quote:
Originally Posted by Humabout View Post
[snip calculations]
Interesting point, although I am wondering whether you actually need ranging pings - you probably can get a fairly decent fix just by using passive sensors (or just ping several times per second; modern LIDARs like the high-end Velodynes already produce a few hundred thousand datapoints per second (granted, that's 360į). And, once you have the distance to the target determined to within a kilometre or so, do you actually lose much efficiency compared to a centimetre-exact distance?

Of course, the longest-range lasers I have have a 1/2d-range of 20,000 kilometres, meaning we have times measured on the order of 0.1 seconds.

Quote:
(2) Computer Dodging.
This is very hardware and software dependent. Currently, (by similar unvetted Google search), computers are alleged to have comparable reaction times as humans, but you don't have TL 8 computers. You have high-TL sci-fi computers, so can probably set this arbitrarily fast, meaning a computer can always attempt to dodge a laser weapon.
A computer, even today, should be able to get you some sort of dodging software within milliseconds. Assuming you have recognized a laser through some sort of heat detection, you should be able to propagate that information directly to the RCS and/or drive systems. I could probably write something like that today (if you give me the interface) that's limited by engine speed, not computation power.

Quote:
[EDIT]
And having written all of that, I realize that it ignores the "Dodge includes moving erratically to avoid being hit" thing. It does, however, provide a hard line where Dodge no longer can represent an actual reaction to an attack. If you don't mind the added complication, you could apply Restricted Dodge Against Firearms from Tactical Shooting, p. 17 with awareness determined by range to target as shown above. And those range thresholds can be pre-calculated for ships, so you don't have to do them on the fly.
That's a good idea. I'll have to see whether it adds anything to the game itself except for additional rolls, but it would be a start. Hm, I have the feeling I'm Tactical Shooting will soon become my second-most referred-to book...



Quote:
Originally Posted by doctorevilbrain View Post
A Human being is 2 meters tall?! Where the National Basketball Association? It ignores the fact that each sex has different average heights. Different countries have different average heights too. I don't understand why somebody would say that without acknowledging the difference.
As Humabout noted, it's a Fermi estimate, meaning I'm actually happy if I'm within 50% or so of the target. You'll note that the other values are average human speed (6m/s, which is higher than GURPS average of 5y/s = 4.5m/s), a human and a spacecraft having the same aspect ratio (I don't even know what the aspect ratio of a spacecraft is, maybe 5:1?), and even G at 10m/s.

But, looking at the relation between height and speed, we have 3s (original estimate), 2.7s (165cm for median female height from wikipedia plus 5y/s from GURPS average), and 2.57s (ditto for male height). Meaning I'm still within less than 20% and, in my opinion, perfectly fine.

Remember - everything's an abstraction, and in the end, what counts is getting rules that aren't too disbelief-inducing.
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Old 02-16-2018, 04:53 PM   #66
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Default Re: [Blog] n-Body Politics

Quote:
Originally Posted by kreios View Post
Interesting point, although I am wondering whether you actually need ranging pings - you probably can get a fairly decent fix just by using passive sensors (or just ping several times per second; modern LIDARs like the high-end Velodynes already produce a few hundred thousand datapoints per second (granted, that's 360į). And, once you have the distance to the target determined to within a kilometre or so, do you actually lose much efficiency compared to a centimetre-exact distance?
Well, a kilometer or two can be quite a lot. Lasers work because they are highly focused, and I've heard of cutting lasers not doing anything useful if their focal length is minute fractions of a percentage off. Really, though, Luke Campbell is the better person to speak to actual tolerances.

That said (and linked), remember that as soon as you use any active sensor to ping the target, they know about you and should be assuming you're going to pull the trigger as soon as the signal gets back to you. This basically starts the Dodge Clock ticking and sets a minimum time limit - and consequently a maximum undodgeable range - on speed-of-light weapons. Nothing says the pilot isn't asleep at the wheel or his reflexes kind of suck. But those sorts of things are determined through surprise rolls and active defense rolls.

It also tells you that the most weapons will attempt to extend their range to that point, but the number that try to exceed it will drop off due to reduced efficacy. I'd expect the typical range of space-based weapons to be something like a skewed Guassian distribution with the mean around the maximum undodgeable range and weighted toward the undodgeable side, since that's the more effective side of things. Of course, super-long-range lasers are still plenty effective against unsuspecting, unpowered, or particularly unmaneuverable craft.

Quote:
Originally Posted by kreios View Post
A computer, even today, should be able to get you some sort of dodging software within milliseconds. Assuming you have recognized a laser through some sort of heat detection, you should be able to propagate that information directly to the RCS and/or drive systems. I could probably write something like that today (if you give me the interface) that's limited by engine speed, not computation power.
I'm not really surprised. It looked pretty high, but I wasn't vetting anything when I grabbed the number, and we all know how reliable the internet is. Either way, even if it is that high, somehow, it wouldn't break my suspension of disbelief for futuristic computers to do this effectively instantly.


Quote:
Originally Posted by kreios View Post
That's a good idea. I'll have to see whether it adds anything to the game itself except for additional rolls, but it would be a start. Hm, I have the feeling I'm Tactical Shooting will soon become my second-most referred-to book...
That rule doesn't add any rolls to the attack resolution sequence. It just limits defenses in specific ways for certain weapons.
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Old 02-18-2018, 10:08 AM   #67
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Default Re: [Blog] n-Body Politics

Quote:
Originally Posted by Space Combat Iteration 2: Playtest Intercept
Letís playtest everything again! Weíll use the same scenario as beforehand, and simply use the new rules.
Continue reading...
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Old 02-25-2018, 11:24 AM   #68
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Quote:
Originally Posted by Space Combat Iteration 2: Playtest SDVs
Similar to last time - except now, weíll pit two SDVs against each other. Specifically, two modified Oberth-class SDVs from last time. Iíll remove the one multi-purpose system from the front and replace it with a full-sized weapon system: Either a railgun or a particle beam.
Continue reading...
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Old 03-04-2018, 12:15 PM   #69
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Default Re: [Blog] n-Body Politics

Quote:
Originally Posted by Space Combat Iteration 2: Ground Fire
Now that we have achieved orbital superiority, the planetís surface is no longer out of bounds - time for orbit-to-ground fire!
Continue reading...

A look at ortillery fire (using rules from High Tech), efficient orbital bombardment munitions, anti-orbital railgun tanks, and submerines.
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Old 03-18-2018, 12:24 PM   #70
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Default Re: [Blog] n-Body Politics

Since I forgot to post the link last week (though it got hit by GurpsDay), here's a double feature of Planetary Invasion posts:

Quote:
Originally Posted by Planetary Invasion
As weíve seen last time, thereís sort of a Mexican standoff in space: Spacecraft are vulnerable to ground fire, but planetary defences can be killed fairly easily by orbital bombardment. This, by the way, fits with the ďif you can see it, you can kill itĒ which seems to have been the military trend of the last few decades.

This implies that spacecraft can hang back (such that they can dodge incoming shots) and slowly search for any suspicious ground craft which they then bombard (using the fact that those railgun tanks cannot dodge as well as spacecraft). But this cannot kill any submarines or tanks lying low (a fuel cell or MHD turbine tank thatís idling will be almost invisible).

So - are the spacecraft screwed? I donít think so.
Continue reading...

Quote:
Originally Posted by Planetary Invasion: Logistics
An army marches on its stomach, as somebody (the quote is attributed to both Napoleon and Friedrich II) once said. Accordingly, letís take a very rough look at the logistics of a planetary invasion.

Iím saying a very rough look because, ultimately, we donít yet know what kind of technology is used, and this will change the number. But, for now, since we just want a rough estimate, Iíll be using roughly modern supply numbers.
Continue reading...
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