On 9 Jun 2020 at 16:44, xxxxxx@gmail.com wrote:

> On Tue, Jun 9, 2020 at 12:54 PM shadow at shadowgard.com (via tml
> list) <xxxxxx@simplelists.com> wrote:
>     On 8 Jun 2020 at 17:48,
>     xxxxxx@gmail.com wrote:
>
>     > Now, space is different.
>     >
>     > No hiding in space (your thermal gives you away and you can be
>     found > by visuals tracking transits). Not a lot of cover.
>
>     It's possible to reduce your IR emmission in several ways, but
>     most are *highly* directional. You may appear to be as cold as the
>     3K background from some direction, but in others your signature
>     will be *greatly* enhanced.
>
> Yes, but here's the rub: You also have an multi-spectral signature.
> Heat isn't your only way of being spotted.

Yes, but the others are "easily" covered by various EMCON measures.
That leaves IR and gravity.

> And more importantly: Against multiple spaced out sensors, your
> camouflage will fail. And it can't last for long anyway due to the
> problem of dispensing with or storing the heat.

Multiple, well scattered sensors yes. But if there are only a few
sensors, or they are just "near" the target, you can use directional
radiators to "beam" your IR emissions in the other direction.

> If you eject it, then the heated ejecta will be visible. If you sit on
> it, your ship will get hotter.

See above, no "ejecta" needed except (higher) IR emissiuons in the
direction you pick.

> It's not practical long-duration stealth and commuting around systems
> takes some time if you aren't jumping or having super drives...

It's doable indefinitely as long as there's a cone in which you can
direct your IR emmissions that (hopefully) doesn't have any enemy
sensors in it.

Jumping has the problem of "jump flash".

>     The two methods that don't have this flaw have major flaws of
>     their own.
>
> Yes.
>  
>     "Storing" the heat by dumping it into a "cold sink" is not
>     possible for an extended time (hours at most).
>
> We agree on that.
>
>  
>     Well, you *could* try hiding in an iceball/comet, but that too has
>     drawbacks. Pretty much the same drawbacks as the second method.
>
> Well, that can work, but there's also the fact you can't *manouver*
> without being noticed and to figure out a ballistic approach that
> could cover your foes moves and leave you in good position no matter
> what they do... that's a feat.
>
>  
>     The second method is a *huge shell (balloon) around the ship. This
>     spread the same amount of energy over a *much* larger area, thius
>     reducing the signature to something plausible for a natural body.
>
> It does, but it's own existence may be betrayed to other sensors.
>
> And again, likely if you manouver, you are detected.
>  
>     Trouble with that is that if it's a known system, all the bodies
>     that size will have been recorded and a "new" body, especiall well
>     inside the system is going to be *very* suspicious.
>
> Esp if heading on trajectories near major points of interest.
>
>     Worse, you are stuck moving at a plausible speed for a natural
>     body. Which means it'll take you months, if not years to get where
>     you want to be.
>
> Yes.
>
> Lol... one strategy is hide, power down, use low berths... see how
> everything comes back to low berths? ;)

Alas, this has the "Battle of New Orleans" problem. You may wind up
making an attack after the war is over.

>     > A big ship might be a big target, but at long distances, the
>     size of > targets may matter less (it's a fraction of a degree of
>     angle to > adjust) and for guided munitions, they aren't getting
>     small enough to > dodge those.
>
>     Dodging works differently in space, regardless of the weapon.
>
>     For guided munitions, it's a matter of max accel *and* total
>     available delta-V.
>
> Assuming the drive isn't reactionless and/or power isn't small,
> cheap, and available and easily to hand.
>
>  
>     If the ship has a higher accel, the odds of being
>     able to hit it are essentially nil unless the muntions are very
>     stealthy and lying in wait for the ship to come to them.
>
> I kind of assume a missie designer designs most of the time for
> higher accel than a ship. That's not 100% of the cases, but I'd
> imagine 80%+.

But in Traveller, especially the LBB period, missiles have the same
acceleration limits as ships. And being smaller (and likely not
having fusion reactors to power their drives) their endurance is
*sharply* limited.

Even in later versions, getting drives with realkly high accel *and*
significant delta-V is a major design hassle.

> And if you lie in wait, that's more mine than missile.

Yup.

>     If the munitions have a higher accel, they have a *small* chance.
>     It all depends on agility *and* delta-V. Doesn't matter if you
>     have a higher accel if you run out of fuel before you can hit the
>     target.
>
> In a non-reactionless thruster scenario or a situation where energy
> supply is limited by heavy fuel (vs. zero point energy or whatever).
>
>     > Aiming lasers beyond 10K km probably requires gravitic lensing
>     (hence > we need gravity control).
>
>     Actually the limit is not *aiming* (though that's a major problem)
>     but beam focus
>
> You are more explicit - that's what I consider part of aiming. If you
> can't get focus, your aim is meaningless.  

No, "focus" is actually the wrong word. It's a "loss of collimation"
issue.

>     The size of the beam aperture sets a distance at
>     which the beam will start spreading badly no matter *what* you do.
>
> Yes, and that was covered in some detail in Brilliant Lances for TNE.

I'd have to check, but I suspect it wasn't. It was more likely
handwaved around.

>     Well, at least until you get beam densities so high the the
>     photons start to auto-collimate due to their mutual gravitational
>     attraction. But a beam that strong would have no trouble slicing a
>     *planet* in half.
>
> Ah, Death Star! ;)

Nothing as trivial as that.

>     > I'm assuming that railguns and the like over 500 or 1000 km
>     probably > require amazing predictive gunnery software to hit with
>     because of > travel time.
>
>     This is where we get into the fundamental principles of evasion in
>     space.
>
>     First you've got "tracking lag". Tracking is either using direct
>     emission from the ship, or reflected radar/lidar pulses. Both
>     travel at lightspeed.
>
> Or a passive visual solution. Or a passive thermal solution. Or some
> energy detection.

Those all involve emissions from the ship. That is, they are
detecting something the ship is emitting on its own with no "help"
from outside.

> But yes, lightspeed is a boundary, but beyond that, the impulse of
> kinetics suggests more limited ranges that something like a laser or
> particle beam.  
>
>     So at 300,000 km, your tracking info is one second old. At 300 km.
>     it's one *millisecond* old. So that's "tracking lag".
>
> Yep. And you get a data point on your PPI (past-position-indicator)
> display and a probability cone of where the ship might be going based
> on the characteristics you expect from it.
>
>     In reality the lag will be longer due to things like signal
>     processing (ie how long does it take the sensor hardware &
>     software to turn the incoming signals to usable targetting info)j
>
> That might be nanoseconds depending on your gear at a high enough tech
> level.  
>
>
>     Second you've got "time of flight". That's how long it takes the
>     beam, missile or projectile to get from your ship to where you
>     expect the target to be.
>
> Yep, another lag. So your firing solution has to factor in:
>
> - Where the ship was last seen (with error bars)
> - The velocity the ship has last appeared to have (with error bars) -
> The predictive estimates of where it could go (given ballistic course
> or following existing acceleration, or if it did every sort of jink
> and manouver that it cold) - with error bars - Then a probabilistic
> estimate of where one should saturate with fire that incorporates all
> of that and tries to guess where to fire
>
>     Add the two together and you've got total lag. Which is also how
>     much time the target has to evade.
>
> Yes, of course it could fail to do that, but one has to expect
> manouver very regularly if there isn't a really tight delta-V issue.
> Without that, ships should almost always be jinking around in combat.
>
>     This is where a seemingly complex problem can be *drastically*
>     simplified. Rather than using *your* frame of reference, use the
>     frame of reference of the target.
>
>     Treat it as stationary in its frame of reference. In yours, it's
>     moving at a *constant* velocity on a particular heading. Yes, we
>     ignore acceleration for the moment.
>
>     By doing this, any evasion becomes acceleration from "at rest".
>     That's very easy to figure.
>
> Easy to figure, but if the vessel being targeted is highly
> manouverable, it can cover a lot of potential area and be perhaps a
> very small target in that envelope.
>
>  
>     For now, we'll assume the ship can apply its full accel in any
>     direction (ie we ignore how long it may take to rotate the ship to
>     get the main drive pointing in the right direction)
>
> Sure, that's the simple case.
>
> Real physics and inertia and momentum tend to significantly affect
> that simplification though.
>
>     So, for a laser, at 300,000 km, the ship has *two* seconds to
>     evade (one sec of tracking lag, plus 1 sec of flight time for the
>     laser pulses).
>
> Plus processing if one is being realistic.
> And if humans want in that loop, that's another 2 seconds minimum.
> Also you haven't mentioned errors in location, speed, vector, etc. for
> your target.  
>     We aren't counting detection time of the incoming pulses,
>     because the ship may be evading anyway, and they can detect the
>     pulses before they get there anyway. (for that matter, it's fairly
>     likely that they won't detect pulses that *miss* either)

Ooops that should say that they *can't* detect the pulses before they
get there.

>     so if the ship has 1 g of accel (we'll use 10 m/s^2 to make the
>     math simpler) in 2 seconds it can shift its center of mass by 20
>     meters (d=0.5*a*t^2)
>
>     If the ship has a radius greater than that, you are pretty much
>     guaranteed a hit. If it's max dimension is onl 10 meters, there's
>     a fairly good chance you'll miss.
>
> With a single shot, if your weapon is not pulsing across an area of
> space at a fast rate in some pattern.
>
>     Particle beams travel a *lot* slower than light speed. So there's
>     a lot more "time of flight" lag. and for railguns, any feasible
>     launch velocity pretty much guarantees a miss at long ranges.
>
> Yes, that's pretty much what I figure.
>
> Realistic space combat ranges in settings without magic power or
> reactionless drives and no inertia or momentum will tend to be short.

But given the sorts of velocities commonly seen in Traveller, "short"
combat ranges are very hard to attain except for second in a passing
engagement

> But given the dynamics of acceleration and vectors in meeting
> engagements, it can really make for low odds passes or it requires the
> two combatants to want to get close. If your reach (with accuracy) is
> low, then your odds of actually engaging effectively is a lot lower.
>  
>
>     At shorter ranges the evasion time gets shorter.
>
> Dramatically.
>  
>     also note that
>     "jinking" as it is normally understoood is pretty useless. To
>     actually move your center of mass enough from your predicted
>     location requires *seconds* of thrust on a bearing.
>
> Under real physics as we know them, yes.
>
> If, for instance, anti-grav cancels the properties of mass to create
> inertia, the manouvering might be dizzying.

But if that's the case, your acceleration is limited only be the
friction of the medium you are passing thru. You've essentially got
Doc Smith's "inertialess drive".

>     More seconds the bigger
>     the ship is (and it gets a lot worse when you include time to
>     change the ship's orientation)
>
> Depends on your setting for sure. Real physics as we understand them
> sure seems to agree with you.
>
>     For lasers, if the beam can stay focused that far out, you are in
>     trouble at a light second or more if your ship is at all large.
>
> Assuming no clouds of ablative gas, or ice armour, or reflective
> mirrors leading your ship in as a shield, etc.

Ice armor doesn't reduce *hits*, it reduces *damage*. Mirrors are
really a practical measure, especially if the ship is accelerating.

> There's lots of conditions that can fuzzy up this simple case.
>
>     For particle beams the "oh shit" distance is a *lot* shorter.
>     Missiles, shorter still. And for railguns, you might as well not
>     bother unless you are stupid close.
>
> I figure that's in the hundreds of kms. Maybe a thousand or two but
> not more.  

Max velocity of a railgun round depends on only two things: The
length of the "barrel" and the acceleration it can impart. Also, the
energy input to the round must be higher than the kinetic energy it
leaves the barrel with.

You can derive the relevant equations fairly easily, from some basic
physics.

d=0.5*a*t^2
V=a*t
E=0.5*m*v^2

You have to rearrange the first equation to get the time it takes the
round to traverse the barrel, and then substitute that into the
second equation. Then you can plug the velocity into the third one.

>     Now flip this around and consider *defensive* fire.
>
>     Nothing you can do about lasers except use sand.
>
> Water clouds can suck out energy and also cause dispersion. This was
> the basics of Knight Hawks' Masking Screen. Note: Ballistic, cannot
> manouver with you.

Water isn't going to stick around long enough to help.

Sand has to have several properties. Ideally it should have a high
heat capacity (amount of energy require to raise the temp 1 dgree C)
a high heat of liquifaction (amount of energy require to convert it
to a lquid). then the heat capacity of the liquid should be high as
well. Finally the heat of vaporization should also be high.

On the other hand you may want the gas to ionize easily as plasma is
apt to be more opaque than the gas and thus absorb more energy.

Oh yeah, it would be nice if the solid form was formed into crystals
of a shape that would reflect/refract the laser beam in directions
away from the ship.

The liquid form will automaticaly act as a retro-reflector (though
like not for long enough to matter).

You'll want to balance all these factor to ensure the maximum amount
of energy is prevented from reaching the ship. Of course, this is
going to vary with the wavelengtyh of the laser. So your "sand" will
be "tuned" for the most likely frequencies.

>     Ditto for particle beams.
>
> I thought the idea with C-PaWS was that a sufficient field could
> deflect the charged particle stream? (That sounds hard albeit in terms
> of getting the field up, strong enough and where and when you want
> it).

Problem is that requires *huge* field strengths and that the field be
aligned properly with regards to the incoming beam.

Also, CPAWS have a problem with beam dispersion as the charges make
the particles repel each other which spreads the beam.

>     Missiles you can try using your missiles, railguns and lasers on.
>
> Or sand. Or even kinetic ERA (reactive armour) if you are expecting
> contact hits.  

Reactive armore is useless because the projectile velocity far
exceeds the detonation velocity of the explosive. As noted below
hyper-velocity terminal ballistics don't much resemble terminal
ballistics for weaponry we are used to.

>     And
>     the closer they get the less effectively they can dodge. Unless
>     your point defense is overwhelmed, or you are stupid close they
>     aren't very likely to get thru.
>
> Flooding PDS is one thing, but also creating enough debris from
> destroyed muntions on the incoming trajectories has a likellihood of
> buggering up point-dense targeting for subsequent waves.  

And acting to intercept some of the incoming fire :-)

>     Railgun rounds are hard to spot (but give a very significant EM
>     signature when they are fired).
>
> Yes, but only at close ranges because of the cubic fall-off. But with
> railguns having short ranges, flight times will also be short so the
> spot might not do you a huge amount of good unless there is a really
> fast defensive system that can do something dramatic in short time
> frames.  

What "cubic fall-off"? The "signal" should be subject to the inverse
square law like anything else.

>     Worse, lasers and particle beams may
>     not be very effective. You have to *destroy* the rounds. Even if
>     you melt them they are gonna hit and do damage.
>
> Maybe. If you graze them, the sublimation of some of the round may
> alter the trajectory enough to miss. Similarly, some sloped armour
> panels may work well for directing a lot of the force away from the
> innards of the ship via deflection.  

Sublimation might help.

Sloping the armor wil *not* help with hypervelocity impacts.
Remember, at these speeds the projectile *and* some of the armor will
flash into plasma at rates that far exceed the sort of mechanical
effects that allow deflection at "normal" velocities.

>     Missiles are easier. If you knock out the electronics or
>     drive/fuel tanks, they can no longer manuever and can thus be
>     ignored unless they are still headed at you and are too close to
>     dodge.
>
> Assuming they are not in-line and packing a bomb-pumped laser or the
> like.

If the electronics are gone, the laser won't fire. Drive/fuel kills
are almost cetain to change the "facing" of the missile even if they
don't change the course.

>     > Smaller ships jink easier. Saturation of cones from rapid fire
>     guns > may be one way to generate hits, but individual damage will
>     be less > than a single heavy penetrator but they will hit more
>     readily.
>
>     At typical Traveller velocities, very few rounds are going to act
>     as "pentrators". At 3 km/sec relative velocity (5 minutes at 1 g)
>     a round will flash into plasma on impact releasing the enery of an
>     equal mass of TNT.
>
> Yes, but it is mass-limited. Penetration in this case may mean
> 'bigger explosion which vaporizes deeper into the ship'.

Still not a "penetrator". Those are *b y definition* rounds that
punch thru to the interior more or less intact and then do extra
damage there.

Hypervelocity rounds "explode" on impact. You may pentrate the hull,
but it'll be as a surface explosion. if the round is massive enough
you may still get a plasma jet of sorts, but it won't be as focused
as what you get with the tank and artillery rounds you are thinking
of.

>     Not only would that make an impressive crater in the armor if it
>     didn't blast a hole in it, but it'd also cause massive spalling on
>     the inner surface.
>
> That's somewhat dependent on armour design but sure. You and I see eye
> to eye on how the physics works in favour of more, lighter, faster
> projectiles.

>     If it does blow thru the armor, you'll have a multi thousand
>     degree jet of plama moving inwards at hundreds of km/sec.
>
> Which seems bad, but I'm sure having a laser shot through your hull is
> as bad. And a c-beam could be considerably worse for other reasons.

Actually, lasers will be doing the same sort of thing. The laser
pulse dumps energy into the hull/armor turning it into as high temp
plasma. Which, btw blocks the incoming laser until it has time to
disperse.

That's why a series of *very* short pulses (say nanosecond durations
over a period of a millisecond or so) work better than one long
pulse, much less a continuous beam.

>     As you can see the rules for hypervelocity projecftiles are *way*
>     different from anything we are used to currently.
>
> There's the issue though: Lasers just emit light. So they need to
> generate X numbers of joules that they can turn into photon streams.
> For railguns, the energy required to massively accelerate a vast
> stream of light projectiles is pretty high to get those high
> velocities. The range limits and the amount of power needed for a lot
> of railguns may limit their use, whereas lasers may be more useful for
> the range alone.

Remember, most of the energy in a ralgun hit doesn't come from the
velocity it left the barrel with, but from the velocity difference
between the launching ship and the target ship.

10 km/sec would be a high velocity for the railgun, but if the
difference in velocity between the ships is several *hundred* km/sec
the "muzzle velocity" is pretty much lost in the noise.

>
>     > If you do have a heavily armoured large ship, you may be able to
>     take > some of the small stuff without too much damage. Fighters
>     and ships > may have (at least near term) delta-V constraints
>     which will > drastically limit battle plans, manouvering, and the
>     ability of > missiles, fighter, and other small craft (drones,
>     etc) to speed > towards an enemy that is manouvering and have
>     enough fuel left to do > final attack runs. Most battles will tend
>     to look like jousts. > Adjusting to change and pass again or
>     pursue is a lot of delta V.
>
>     Keep in mind the sort of attack runs Babylon 5's fighters made.
>     Since orientaion has nother to do with velocity, when they could
>     make "close" passes on enemy ships, they'd just rotate the fighter
>     to keep the guns pointed at the enemy ship as they made a pass.
>
> Sure. But if they have a lot of V built up, they'll be out of
> effective range quickly for projectile or plasma weapons. And ship
> sized lasers may or may not fit on fighters in some settings.

In *most* settings anything mounted on a fighter will be pretty puny.

>     Then it was
>     decelerate to a stop (far passt the target and accelerate back to
>     the battle for another pass.
>
> Yes, passes happen, but the second one will likely be slower (or long
> delayed otherwise) - catching back up for another pass, esp if the
> other guy decides to not turn to come back, is going to be a long
> climb that burns a lot of delta V.

Well, in Traveller (at least the early versions) ships normal fuel
tankage is enough for *days* of acceleration. Which makes delta-V
pretty much irrelevant for them.

Fighters and missiles aren't as lucky.

>     Unless by some remarkable coincidence both sides have nearly the
>     same velocity (speed *and* direction) that's how most battles will
>     go.
>
> Yep.
>     Fire at the enemy as you aproach, keep firing as you pass by (or
>     *thru* the enemy formation) and keep firing until you are out of
>     range.
>
> Yep.
>
> Orbits, facilities, and so on or converging tracks (rare) could
> change that.
>
>     Going back for another pass pretty much requires either mutual
>     consent *or* that one side has vastly superior acceleration.
>
> Yep.
>
>     And in case you missed it above, "sneak" attacks are moderately
>     easy. With lasers you won't even know you are being fired upon
>     unless there's *very* near miss or you sensors are looking
>     *closely* at the firing ship at the right time.
>
> Or you've got a medium in the space around you that will fluoresce
> when a laser burns through. That'd be one way, but it has the same
> issues as Masking Screens or other methods that require you not to
> manouver.  
>
>     Particle beams are a lot "noiser" and likely to be detected unless
>     they miss by a lot.
>
>     Missiles, probably fairly easy to spot. 
>
> If a ship can stealth, I suppose ballistically tracking missiles could
> do that.
>
> Of course, if your enemy keeps steadily steaming on the same vector,
> they are pretty dense.

That's the whole *point* of a sneak attack. You don't look like a
threat until the target is getting hit.

>     Railgun or other dumb projectile weapon, you are unlikey to detect
>     the projectiles. Though there may be a firing signature (EM pulse
>     in the case of railgun, gas in case of conventional guns)
>
> And you may hear the bangs when they hit solidly or glance off.

As noted above, these are *hypervelocity* projectiles. They will not
"bounce off". And the "bangs when they hit will be *very* noticable.

But I'm talking about shots that *miss*.

>     So if done right your first clue you are being attacked might be
>     lasers or projectiles blowing holes in the hull.
>
> If you didn't see the other guy coming half a system away.

See above.

> One of my buddies is a space sensor scientists and he says the
> sensors can detect fractional Ks off the background radiation and
> future compute capacities will allow full sky scans with full
> resolution in multiple frequencies in real time. The tricks to slip
> past detection at great range will be few and limited.

Being *detected* is one thing. Knowing that what you've detected is
somebody that is planning to attack you is a very different thing.

ps. a tactic that doesn't work in Traveller, but would work very well
in settings where heat is dealt with realistically is aiming a
defocused laser at the targets radiators. This turns them into heat
*absorbers*, absorbing the full power of the laser(s).

This will overload things fairly soon, giving the target the choice
of surrendering or frying.

It also requires you to reduce power usage on board (cut the manuever
drive, etc) just to slow the heat build-up.

Thus, in those settings there's a quite practical method to force a
ship to surrender relatively intact.

If they don't surrender, there will still be a lot of salvageable
stuff after they all die.
--
Leonard Erickson (aka shadow)
shadow at shadowgard dot com