How Vulnerable Are 'mechs To Air Attack?

[Death by Lasers]

  You can certainly place detectors at the Zenith or Nadir points but the invader might not use the Zenith or Nadir points but instead some arbitrary point just outside the proximity zone.  If its a raider I would imagine they would choose to jump 15 AU out to avoid possible detection.  If its an invader they will probably jump as close as possible either at the Zenith or Nadir points or at some arbitrary point outside the proximity zone and have their Emergence Wave detected.

  Once you detect the Emergence Wave the enemy will be completely invisible until they hit Drive Plume detection range.  A clever invader will make sure his exact arrival time is unpredictable and manage his speed so that he can cut his engines just before hitting Drive Plume detection range and reactivate them as soon as he hits Radar range for the final approach.

  From the defenders perspective they will detect an unscheduled emergence wave somewhere about 10 AU out.  Then a little more than week later they will detect the enemy Dropships less than 100k km out approaching rapidly and decelerating at full thrust.  They might be able to double their detection range if they have enough radar bouys (26 I think) with Dropship grade radars but this can get pretty expensive.

 

 

[Weirdo]

...so while skimming over most of the details, all this is telling me that sending out long-range shuttles on patrols whenever you detect an emergence save is a good idea. Odds of them actually detecting an incoming force are far from 100%, but if they do catch a contact, the advance warning is invaluable.

[Charistoph]

  The details are in the Raiding thread but this model is for a "side intercept".  The attacker has the advantage in that all he has to do is decelerate because he is approaching the planet from space while the defender needs to accelerate and then decelerate in order to not overshoot his target.  It should be noted I am using the default suborbital hops data from Strat-Ops so tighter intercepts would be possible with faster fighters and heavier fuel usage.  Also, if you are content with just doing a high speed pass I imagine your intercept time would be cut significantly. 

I admit I'm not talking about the rules, but the perception that the stories the universe presents.  I'll go in to more detail later.

This model isn't for a suborbital hops from one part of planet to another as you are describing though.  Haven't thought through what the mechanics of that would be in that case but  the attacker would have the advantage in that the majority of enemies planes will be grounded and need 20 minutes to take off safely although fighters on CAP could respond immediately.  How close you can get without intercept depends heavily on where you started from in that case.

I was speaking more to intra-orbital, which is why I mentioned sub-lunar.

The whole trick was more in to causing someone to lose their targeted window for entering the atmosphere.  You burn a lot of fuel going through the atmosphere, so going by orbit saves those resources, especially if you are already in the middle of a breaking maneuver.

Now, if the attacker has allowed for themselves a wide window for dropping in to the atmosphere, they have to be approaching the planet at a rather slow rate which makes them easily intercepted by anyone on planet or on a lunar base.  So, that usually means the Attacker moves quickly in, burns harder on final approach, and makes for a much smaller window in order to reach your target.  If you slow down or alter your direction, it can often mean that you either have to burn more to travel in atmosphere or stop decelerating and accelerate and go for another orbit.  Sometimes it's faster and more efficient to run that option.

No, that's not what the rules say, which is what Tai Dai is saying. It's not easy to track an invading force. You can't even see them until they're inside the moon's orbit. It's easy to see that a jumpship has arrived in system, but you can tell where it arrived from and you can't see what it detached until they're almost on the planet.

Interesting.  The stories usually talk about being able to track those dropships as well as being able to have a reasonable profile as to mass and type.

Not to mention, any good telescope should be able to track the drive plumes if you have a rough idea where to find them.  Most of that latter part is just math that we could do in the 60s, much less with the computers in the 80s or today.

And that didn't seem to matter if it was from a traditional Jump Point or a Pirate Jump Point.

[Tai Dai Cultist]

Not to mention, any good telescope should be able to track the drive plumes if you have a rough idea where to find them.  Most of that latter part is just math that we could do in the 60s, much less with the computers in the 80s or today.

And that didn't seem to matter if it was from a traditional Jump Point or a Pirate Jump Point.

Well, prior to turnover (halfway thru the transit) it's theoretically possible to optically pick up a dropship hull against the black of space.  But picking up dropship sized objects optically, out in interplanetary space involves not just seeing it once, but sequentially over time.  If someone accidentally happened to pick up a dropship, odds are it'll only be in one time lapse frame and quite likely dismissed as a simple glitch in the image.  Frankly, it's not a realistic circumstance for dropships to be picked up (in transit) prior to turnover.

Now *after* turnover, yes with the drives pointing at the destination, they're "easier" to pick up since that energy now doesn't have the dropship hull blocking the view. Still the rules for optical detection in SO don't account for detection on the scale of half of an entire transit (they're shorter range than radars).  The rules (in SO) just don't appear to support the kinds of "days out" detection ranges that we sometimes get in lore.  What, if anything, to make of that apparent disparity may itself be another nugget of contention.  Picking up a dropship post turnover but still days/weeks away from hitting the planet involves noticing that there are "new" star(s) out in the night sky.  SO just doesn't have rules for that, and saying how likely that is... is well talking lots of conjecture.  Since we're not talking about the sort of routine astronomy that goes on today on earth, but on a world of the Inner Sphere during the 31st century.  Who's to say how common observatories even are, let alone how integrated they are into military C2.  Who's even to say how far away drive plumes even can be picked up via simple optical magnification.  Drive plumes could easily be said to just not be "bright" enough in the visible spectrum to be picked up by means of raw optical magnification until military grade search radars are already getting returns from the hulls anyway...

[Death by Lasers]

  The detection ranges may be a conceit of the universe akin to the Battletech weapons ranges.  I think Cray said as much in a post.   Still, drive plume detection distance is nothing to laugh at as you can see them from 35 million kilometers away.  I don't know how that compares with our capabilities today given how much radiation a drive plume is pumping out to slow 10,000+ tons at 1G.  IIRC that will give the defenders a day or two of warning. 

  The whole not seen until radar range trick is something I came up with to avoid early detection but it costs you a couple of days time and involves 45 minutes of serious high G burning.  The human body can handle it, but it won't be fun.  I would imagine if an attacker doesn't want to waste time on stealth and would rather not strain their men and equipment with a high G burn normal breaking would be involved and the defenders would have a couple of days to prepare.  In that case though if the attacker is not bringing superior aerospace they just made a huge mistake (which can totally happen by the way, unless their intelligence is spot on they are going to have no idea what is on the planet until it hits them).

[Charistoph]

Well, prior to turnover (halfway thru the transit) it's theoretically possible to optically pick up a dropship hull against the black of space.  But picking up dropship sized objects optically, out in interplanetary space involves not just seeing it once, but sequentially over time.  If someone accidentally happened to pick up a dropship, odds are it'll only be in one time lapse frame and quite likely dismissed as a simple glitch in the image.  Frankly, it's not a realistic circumstance for dropships to be picked up (in transit) prior to turnover.

Masking the drive plume with your hull would only qualify if you were coming straight on to the observer.

Now *after* turnover, yes with the drives pointing at the destination, they're "easier" to pick up since that energy now doesn't have the dropship hull blocking the view. Still the rules for optical detection in SO don't account for detection on the scale of half of an entire transit (they're shorter range than radars).  The rules (in SO) just don't appear to support the kinds of "days out" detection ranges that we sometimes get in lore.  What, if anything, to make of that apparent disparity may itself be another nugget of contention.  Picking up a dropship post turnover but still days/weeks away from hitting the planet involves noticing that there are "new" star(s) out in the night sky.  SO just doesn't have rules for that, and saying how likely that is... is well talking lots of conjecture.  Since we're not talking about the sort of routine astronomy that goes on today on earth, but on a world of the Inner Sphere during the 31st century.  Who's to say how common observatories even are, let alone how integrated they are into military C2.  Who's even to say how far away drive plumes even can be picked up via simple optical magnification.  Drive plumes could easily be said to just not be "bright" enough in the visible spectrum to be picked up by means of raw optical magnification until military grade search radars are already getting returns from the hulls anyway...

You are making an assumption that the incoming jump ship was not considered hostile and not arriving as part of a scheduled visit.  You are also assuming that the planet is not taking care to track any of the incoming drop ships for emergency purposes.  They would probably be half-arsed tracking, but being tracked nonetheless.

Of course, that latter part is making assumptions, too.  Such as the world having a population and industry sufficient to have a spaceport that does track those things for both security and safety reasons.  If they have such an industry, they are a target.  If they don't have such an industry, why are they a target (outside of piracy)?

Though, there are a lot of dirt ball worlds out there, and many on the borders.  They may have been something once, but the First War saw to most of that, leaving them as either subsistent, or barely subsistent worlds only good for a food/water stop on to worlds that actually matter now.

[Tai Dai Cultist]

Masking the drive plume with your hull would only qualify if you were coming straight on to the observer.

But that's how transits work.  Nose towards the destination for the first half of the transit, then flip over and point the thrusters towards the destination for the latter half of the voyage.

You are making an assumption that the incoming jump ship was not considered hostile and not arriving as part of a scheduled visit.

According to the detection rules in SO, Jumpship emergence waves can be fairly easily detected.  Proviso: so long as the jump was within 15 AU of the sensor.  Most proximity limits are a good bit shorter than that, so if you want to hide your emergence wave you have to jump in WAAAAAY outside the proximity limit and add all kinds of transit time.  Maybe it's a doable option for hostiles to add days/weeks to their transit if it helps make a surprise attack.  Maybe.

Besides.  Emergence waves are easy to detect, yes.  But not every emergence wave means dropships dropped off the JumpShip.  If you're raiding a world 6 jumps away, you left emergence waves (and no incoming raiding ships) in 4 systems that didn't have accompanying inbounds.  I'd wager that many detected emergence waves are no more significant than the dull rumble one hears when a heavy truck drives near one's house.  Doesn't mean it is coming to YOUR house; it's probably just passing thru the neighborhood.

You are also assuming that the planet is not taking care to track any of the incoming drop ships for emergency purposes.  They would probably be half-arsed tracking, but being tracked nonetheless...

But that's the whole point.  Tracking (basically) can't be done.  Not without a craft physically chasing the dropships. (SO says radars are only good out to a hundred thousand km or so. Earth to Moon, for reference, is ~400,000km. 4x as far as radars can see a dropship)

It's more like the Age of Sail, where you basically don't have forewarning of a ship pulling up to port.  None beyond what a guy up in a really tall lookout perch can give (the analogy for actually having a search radar scanning the skies for incoming dropships).  Arrival schedules were quite vague guesses rather than firm dates & times.  There's no way to keep railroad-esque (or airline-esque) schedules crossing an ocean under sail, and with all the things that can necessitate delays or detours in space travel... (oops! pirates.  Oops! got dragooned into supporting a military operation.  Oops! Had to replace a blown Helium seal.  Oops! Tore the jump sail, had to spend a week repairing it.  Etc)... I'm quite sure that in the BTU "regularly scheduled" dropship arrivals are certainly not expected on a firm day, much less a firm time of day.

[Charistoph]

But that's how transits work.  Nose towards the destination for the first half of the transit, then flip over and point the thrusters towards the destination for the latter half of the voyage.

The destination of one spot on a planet.  It may be a narrow angle from the perspective of the drop ship, but it is rarely ever straight on with every possible observer.  In addition, in the standard transition time, a drop ship is heading to where the planet will be, not where it is.  That leaves a sufficient angle that masking a plume with your hull is almost impossible.

According to the detection rules in SO, Jumpship emergence waves can be fairly easily detected.  Proviso: so long as the jump was within 15 AU of the sensor.  Most proximity limits are a good bit shorter than that, so if you want to hide your emergence wave you have to jump in WAAAAAY outside the proximity limit and add all kinds of transit time.  Maybe it's a doable option for hostiles to add days/weeks to their transit if it helps make a surprise attack.  Maybe.

Besides.  Emergence waves are easy to detect, yes.  But not every emergence wave means dropships dropped off the JumpShip.  If you're raiding a world 6 jumps away, you left emergence waves (and no incoming raiding ships) in 4 systems that didn't have accompanying inbounds.  I'd wager that many detected emergence waves are no more significant than the dull rumble one hears when a heavy truck drives near one's house.  Doesn't mean it is coming to YOUR house; it's probably just passing thru the neighborhood.

Too much based on assumptions. 

Any system that would have traffic like that would also likely have recharging stations in place which would send off reports of invaders to the main planets.

Any system that is used to being a hyperspace bypass without a recharging station would be less likely to be the target of a House-sponsored raid.

But that's the whole point.  Tracking (basically) can't be done.  Not without a craft physically chasing the dropships. (SO says radars are only good out to a hundred thousand km or so. Earth to Moon, for reference, is ~400,000km. 4x as far as radars can see a dropship)

It's more like the Age of Sail, where you basically don't have forewarning of a ship pulling up to port.  None beyond what a guy up in a really tall lookout perch can give (the analogy for actually having a search radar scanning the skies for incoming dropships).  Arrival schedules were quite vague guesses rather than firm dates & times.  There's no way to keep railroad-esque (or airline-esque) schedules crossing an ocean under sail, and with all the things that can necessitate delays or detours in space travel... (oops! pirates.  Oops! got dragooned into supporting a military operation.  Oops! Had to replace a blown Helium seal.  Oops! Tore the jump sail, had to spend a week repairing it.  Etc)... I'm quite sure that in the BTU "regularly scheduled" dropship arrivals are certainly not expected on a firm day, much less a firm time of day.

And here we see a divergence of rules, and the story, as well as with reality.  All it takes is an initial departure time with a rough idea of a target, and it's "basic" math from there.

[Tai Dai Cultist]

Let's do some math. 

Max detection range: 100,000km
1 Astronomical unit: 150 million km
Proximity Limit for Sol: 8.2 AU  (I'm assuming that's comparable to most BTU planets)
EDIT: Oops.  Terra is 10.2 AU.  Whatever.  the numbers are even worse for a bigger proximity limit than what I calculated :D

So a ground (or orbital; no difference at this scale) based sensor can detect the JumpShip's emergence wave fairly reliably (50%-60%ish odds, iirc from SO) in a Sol-like system.  Yet the rules don't allow for the dropships to be picked up until they're within the last 100,000km of their 1.23 billion km transit.  They're invisible to the planet for ~99.99992% of their transit.

I take your point that there might be sensors at points of view other than the planet.  But again, 100,000km detection range.  All you got to do is jump a fraction of an AU away from a recharge station and while they'll detect the emergence wave, the planet was probably going to do so anyway.  Plus, the recharge station won't see the dropships depart either.

Think about the sheer volume of interplanetary space.  A Sol-like system has a proximity limit of 8.2AU... that's a sphere of some 7.3 octillion cubic km. (that's 27 zeroes after the decimal). A military grade radar can cover a sphere of about 3.9 quadrillion cubic km.  That's a lot; 15 zeroes after the decimal.  But it's still a teeny tiny fraction of the entire volume of the space within the proximity limit.  If you wanted to guarantee that you can track dropship traffic in your system (that is, have complete radar coverage) you'll need.. 7.3x10^27/3.9x10^15 radar bouys floating around in interplanetary space.  (that's over 1.8 quadrillion sensor bouys, or put another way more than 1.8 trillion batches of 1 million detection satellites. 

Ain't nobody ever had that kind of infrastructure described in lore.

[Tai Dai Cultist]

But what about optical detection?

Raw image enhancement: SO says it caps out around 2,500km.  Way better off using radars than telescopes.

But what about drive plume detection?

First of all: given that people generally aren't killed by the transit drives it stands to reason that the energy expelled out the aft (or bottom, for aerodynes) doesn't pass through the hull itself.  Ergo, prior to turnover with the drives pointed away from the destination, there is logically about zero probability of detecting drive plume energy from that said destination.

Post turnover, lore and rules both state that all kinds of high energy radiation, to include vislight and xray, are being shot at the destination.  Turns out I missed rules for detecting that in the other thread, so here's my mea culpa:
SO does indeed cover that afterall.  (see page 119... honestly I'm not sure how I missed it.  It's literally right next to radar detection rules)

But the rules break it down into groups of 5 million KM.  Refresher: 1 AU = 150 million km.  Picking up dropships at turnover in a SOL like system is a hilarious TN35. Yes on 2d6.  By the time it's mathematically possible to detect a drive plume under the SO rules (TN12 and getting your boxcars) the DropShips are already within 35 million km.  (35 millionkm/18,000km per space hex = ~1944 spaces hexes out)

At 1g deceleration, they're hitting the planet in less than 45 minutes.

Even if some astronomer happens to be looking up in the night sky and sees some new lights that he realizes are hostile dropships, he better have a hotline to planetary Command & Control.  Even if he does, that's not a lot of time for defenders to prepare a welcome party. 

Again, that 45 minute forewarning is predicated on getting your nat 12 on a 2d6.  By the time an hour has passed and you're eligible for another check where you only need an 11+, they're already down.  (perhaps someone should alert Cray to recheck the rules.  Or my math.  I'd welcome either).

EDIT: See posts downthread for correct math.

[Death by Lasers]

  I don't think it diverts with the stories at all.  I can't think of a Battletech novel where someone tracked an invader from their jump point all the way to the planet.  I'm not saying it hasn't happened but I honestly don't recall it happening.  In a standard invasion you are A) Going to know when they jumped in system and B) when they are about day out (35 million kilometers at 1G) or slightly less if they pull a hard burn.

  As for detecting incoming Jumpships and predicting their trajectory I'm not sure how good detectors are at triangulating the incoming drive signature of an incoming Jumpship.  At the very least it is fluffed as being very hard to single out the emergence wave from background radiation.  For example to detect an incoming invader 10 AU out the target number is going to be 11 for an average skilled crew.  That means you will need at least 12 stations to pick up a lone Jumpship.  Considering how sensitive the equipment has to be I would also consider false positives to be a regular issue.

  A raider can jump 15+ AU out, it only adds a couple of days travel time for the dropship although the Jumpship will have to spend more than twice as long recharging at the jump point.  Not a big deal if they have to stick around to pick up the raiders anyway but in an invasion that's over a weeks time the where the Jumpship could have been ferrying more troops/supplies to the front.  In a raid they can also afford to do a last minute hard burn and therefore will only be detected 100km (ie. before they hit radar) or about 45 minutes out.

  For the canon narratives a certain level of stealth is absolutely necessary.  It's hard to imagine Birddog working if the Jags could detect and track them the instant they Jumped in system.

 

[Tai Dai Cultist]

Actually I see my math problem on the drive plume detection.  Space hexes are 18,000 meters, not 18,000 kilometers.  Recalculating!  Stand by
Actual space distance is ~1,944,444 space hexes.  That'll be significantly longer than 45 mins :D

Still, it's only mathemetically possible to detect (according to SO rules) at 35 million km, which is not only a fraction of an AU, it's a teeeeny fraction of an entire transit.  No way are you tracking dropships prior to their beginning "final approach", even by watching the drives.

That's a max detection (getting 12s) of about 24 hours out at 1g.  Hilariously, 23 hours and 59 minutes would cover 1,946,000 space hexes.  but we can round to a nice neat 24 hours. 1/36 = about a 3% chance of picking up the hostiles this early, however.

1 hour later, another check can be made.  At T minus 23 hours, the dropships are still >34 million km, so another nat 12 or 3% to detect.  Or, a cumulative ~5% percent chance to pick them up by this point.

22 hours from planetfall: TN to detect is now 11+, or ~8.33% chance to detect

21 hours from planetfall: TN to detect is now 10+, or 16.67%

20 hours from planetfall: TN to detect is now 10+, or 16.67%

19 hours from planetfall: TN to detect is now 9+, or 27.8%

18 hours from planetfall: TN to detect is now 9+, or 11.1%

17 hours from planetfall: TN to detect is now 8+, or ~41.67%

so on.

So, with roughly a 2/3rds probability to pick hostiles up 19 hours out at 1 g, that's probably about when they "usually" are detected.  Assuming there's anyone actually looking.  I'm not on board with that being a safe presumption.

BTW:  I've piqued my own curiosity with nonstandard times, such as 1.5g and 2g.  Since the lore does afterall support harder burns to catch defenders more off-guard.  I'll post what I find in a bit.

EDIT: Fixed math error with cumulative probability

[Death by Lasers]

  I just wanted to add for space travel calculations this is a very good resource.  Just a side note though it automatically assumes you want to arrive at your destination with a neutral velocity so if you want to calculate how fast you can get somewhere without slowing down (or in this case to calculate time traveled once you are already slowing down) just put in twice the distance and divide the time by 2.

http://nathangeffen.webfactional.com/spacetravel/spacetravel.php

[Tai Dai Cultist]

  I just wanted to add for space travel calculations this is a very good resource.  Just a side note though it automatically assumes you want to arrive at your destination with a neutral velocity so if you want to calculate how fast you can get somewhere without slowing down (or in this case to calculate time traveled once you are already slowing down) just put in twice the distance and divide the time by 2.

http://nathangeffen.webfactional.com/spacetravel/spacetravel.php

I've done all mine with the formulae given in SO on page 259.  However, that includes thrust/turnover/brake.  It doesn't do one way thrust/braking.  I muddled through figuring out a gaussian method of shortcutting extraordinarily tedious N! addition arrays.  My (admittedly ugly) sausage maker CAN however tell you how far you thrust in 24 hours, or how long it takes you to thrust a given distance.  Very handy for the terminal braking of approaching dropship....

I'm not sure how my math compares to yours, DbL.  Adding 2 thrust (1G) every space turn (60 seconds) for 24 hours works out to 24x60=1440. 1440(1441)=2,075,40 space hexes. or 37 million km and change.  Likewise: Starting with 1140 thrust at 2,075,400 hexes distance from planet and subtracting 2 thrust (1 G) every space minute brings you to a nice perfect zero at the planet.

Oh and spoiler alert: coming in faster (technically, braking harder) only helps by a few hours.  Assuming the skies are being monitored for incoming dropships, they're probably spotted on xray telescopes around 12 hours or so from planetfall. 

[Death by Lasers]

Oh and spoiler alert: coming in faster (technically, braking harder) only helps by a few hours.  Assuming the skies are being monitored for incoming dropships, they're probably spotted on xray telescopes around 12 hours or so from planetfall. 

  Not sure if this referring to my 45 minute detection run but that one works by timing your velocity such that you only start slowing down within 100k km/ radar range.  It adds a few more transit days because you need to coast in through drive plume detection range but allows you to be undetectable until you hit radar range.  It requires a high burn because if you did it at 1G the time to the planet from detection at 100k km is 90 minutes (ie enough time for the defenders to scramble and intercept).

  I started my calculations by hand you Strat-Ops or even working out by hexes but the formula on the website gives the same results, its just more convenient.