Grav Tanks

[AchanhiArusa]

This makes my second post on this forum.  I haven't been part of a Battletech online community since the old days of rec.games.frp.battletech (but I've been a member of EnWorld for a very long time) and I've been lurking, reading the forum posts and I saw someone post on Grav Tanks at the end of an Aliens in Battletech thread and using the designs from Traveller Book 5-6, GURPS:  Traveller, and Renegade Legion I was able to make a design for them that fits the specs of Third Imperium Level technology which seems inferior to Renegade Legion (except for Meson Artillery.  What has the range of a Long Tom, does 15/10/5 Area damage all to internal structure while ignoring armor, and has the following statistics:  Heat 45, 35 tons, and 14 spaces?).  The Third Imperium (except for Grav Tanks, Nuclear Dampers, and Meson Artillery) are on par with the Clans but only field Aerospace Fighters, Grav tanks, Battle Armor, and, of course, Warships when they go into battle.  So here are the construction rules:

Grav Tank
Vehicle Type:  Grav
Suspension Factor:  0
Terrain Restrictions:  None
Tonnage:  20–200 tons
Space:  5 + 1/5 tons (round down)
Cockpit & Controls:  5% Tonnage
TS&R System:  2 tons
Internal Structure:  10% Tonnage
Grav Drive:  10% Tonnage
Engine Rating:  Tonnage x [Desired Thrust + 1], XL Fusion required, no shielding required

Motive System Damage Roll (the Grav tank has more protection for its motive system than other units because it is basically a flying brick):

2d6
2-7  No Effect

8  Direction Vane hit (appropriate to side, if rear or front it is a 50/50% chance which one will be hit, cannot make turns in that direction for the rest of the game.

9  Minor Damage, +1 Modifier to all Driving Skill Rolls.

10  Moderate Damage, -1 Thrust, +2 modifier to all Driving Skill Rolls.

11  TS&R Hit (halve all speed limits, a second hit will disable this system rendering the unit unable to move and if in motion it must immediately ground), +2 modifier to all Driving Skill Rolls.

12+  Major Damage; no movement for the rest of the game.  If moving the vehicle must immediately ground.

Grav Tank Movement
Grav Tanks have a number of MP equal to previous round's MPs plus or minus Thrust applied.  They pay the normal amount of MPs to enter or leave a hex as any other unit.  If they don't have enough MPs to enter a hex they must immediately ground taking normal ramming damage to the front armor (as if they had run into themselves) unless a piloting check is made.
   Grav tanks are not very agile and must move a certain number of hexes before they can turn (much like an aerospace fighter) and they can only turn one hex face at a time (unless they are at 0 MPs).


Current    #Hexes moved before a turn is allowed
MPs
0              May spin in place
1-5           1
6-10         2
11-19       3
20-29       4
30+          5

These are the terrain speed limits, going above this require a Grav tank to immediately ground unless a piloting check is made.

Terrain                                   Speed Limit
Clear/Road/River/Lake             20
Buildings                                 10
Rubble/Lt. Woods                    8
Heavy Woods                          6
Under Water                           1
Up or down 1-3 elevations       20
Up or down 4 or more             16

A Grav Tank can enter TTF or LAF.  At TTF the Grav tank is 2 elevation levels high and can ignore any terrain that is 2 elevations or less and can travel as if on clear terrain.  At LAF the grav tank is actually at aerospace elevation and can travel the board at 75 MPs (or about 5 Aerotech low altitude MPs).  They are treated as envirosealed, but cannot leave the atmosphere.

[imperator]

I would say that they are more vulnerable than you think because of the way traveller does GRAV movement, which is by countering gravity and using thruster plates around the vehicle to move and control the GRAV tank.  I would keep it on par with the tracked hit location for movement loss.  Also, if you want to AU/cross genres BT with Traveller or Renegade Legion, you gotta control the toughness,mobility, and firepower of the Grav designs or you will overpower the BT universe.  For example, 3rd Imperium uses a lot of nukes in warfare.

[AchanhiArusa]

I know it is more of the Renegade Legion style tank which causes the tank to fall parallel to the ground which would probably be a bit higher tech than standard Traveller.  And yes, having nuclear dampeners and having nukes would be a bit unfair.  The Meson artillery was a bit of a joke because I wouldn't want to be in a scenario where my vehicles and 'mech exploded from the inside and out.  I stuck with the standard BT armor because of the similarities in fluff between the two.  This is just all part of my expanding BT into a general wargame because I believe it has that potential.

[Col Toda]

It is just a special effect of hover or WIGE or areospace .  It need no special rules as I
seem to recall that in the old days it was just a lost tech option for hover.  I have heard
of a unit that uses Grav hover tanks that trac heat like a mech but can have double HS .
Costing about 43+  Million C bills each .  I can buy more than a company of conventional
combat vehicles with fuel cell engines for about 36 Million C Bills . 

[AchanhiArusa]

The problem with that is WiGEs still use movement points and Aerospace fighters only use thrust to change their MPs in space.  In this case a grav tank is an atmospheric vehicle that moves as a fighter does in outer space. 

As for their existence in the Inner Sphere I quote page 234 of Tactical Operations:  "Don't fall prey to Succession War-era nostalgia.  The Star League did not have anti-gravity. ..."

[evilauthor]

As for their existence in the Inner Sphere I quote page 234 of Tactical Operations:  "Don't fall prey to Succession War-era nostalgia.  The Star League did not have anti-gravity. ..."

Although, some of the old illustrations of hovertanks LOOK like they have antigravity...  :D

[Daemion]

That and the Gs most BT fighters go through give me the impression that they have some sort of anti-grav built in. Especially if you still look at AT1 as canon.

[AchanhiArusa]

Evilauthor, I keep forgetting there isn't a "Like" button on these forums.  I wanted to press it because of your comment.

And yeah, AT1.  If we assume that the satellite is the same distance as the Earth is from the Moon, each hex is 24,025 km.  Then each round on that map should be 4.5 hours not 10 seconds.  And let's not talk about weapon ranges.  They are still ridiculous in the current version though they are more sensible than AT1 (I personally like the 1-4 hex range of fighter weapons from Interceptor).

[Daemion]

Evilauthor, I keep forgetting there isn't a "Like" button on these forums.  I wanted to press it because of your comment.

And yeah, AT1.  If we assume that the satellite is the same distance as the Earth is from the Moon, each hex is 24,025 km.  Then each round on that map should be 4.5 hours not 10 seconds.  And let's not talk about weapon ranges.  They are still ridiculous in the current version though they are more sensible than AT1 (I personally like the 1-4 hex range of fighter weapons from Interceptor).

Wow. Really? You missed the blurb at the opening of the AT1 rules which specifically state that a turn is 1 minute long in space and the hexes are 6500 km across?

[AchanhiArusa]

Since I haven't seen those rules in about two decades, apparently yes.

[Nebfer]

That and the Gs most BT fighters go through give me the impression that they have some sort of anti-grav built in. Especially if you still look at AT1 as canon.

I believe you can get 25 to 30 some odd Gs in some turns in AT2/TW rules for Fighters, so they have some impressive G suits it would seem.   :D

What was interesting IIRC was when I looked into it, it oddly enough while the unit was pulling 20+ Gs the time taken (in turns) actually corresponded with the real time it should take to do a turn at with the indicated perimeters (360 degree turn at 2000kph in under 3km).

http://www.csgnetwork.com/aircraftturninfocalc.html
Using the above calculator.
2,160kph (12 movement at low altitude, or apx 1,342 MPH)
AFAIK At that speed a ASF must move three hexes before it can do a facing turn, to do a 360 degree turn it would take 18 hexes of movement and 2 turns (one could argue 1.5 turns), a turn is 10 seconds and each hex is 500m across. The diameter of this circle is 6 hexes or 3km.

Converting the above info from metric, and imputing it into the calculator (with a bank of 87.7 degrees), one gets the following.
Turn Diameter 1.9 miles (3km)
360 degree turn time in seconds 15.8 sec.
G load 24.4 Gs

Who would of thought it was reasonably accurate...

[AchanhiArusa]

And now, let us make a different set of assumptions.  Assume the planetary objective is the mass of the Earth (5.972 x 10^24 kg).  With the 6500 km/hex scale the moon is 104,000 km away from its primary (center of mass to center of mass).

By using a simple Hill's Sphere calculation

H = a(M1/M2)^(1/3)

and seeing that the Hill's Sphere is actually 12.5 hexes or 81,250 km away from the primary by inspection (it is where the arrows change direction) we can figure out that the moon is 2.85 x 10^24 kg or 44.7% the mass of the Earth.

If we use those numbers in Newton's Law of Universal Gravitation

F = G M1 M2/r^2

then the two bodies would exert 1.05 x 10^23 N of force on each other.  This is comparable to the Earth-Moon system which has a net force of 1.98 x 10^20 N.  So this system would have 529.5 times the attractive force of our own system.  By calculating relative seismic moments

Mw = (2/3)(log [ (rigidity of rock)(average slip along the fault)(area of the fault) ] ) – 6.05

(I have to use relative moments because otherwise the numbers are too large to work with) this would lead to tidal forces that would register on the Richter scale at 3.7.

As for the orbital speed, using

T = 2 pi sqrt (R^3/(G*(M1 + M2)))

and

v = sqrt (G*(M1+M2)/R)

This moon would make a full orbit every 76.3 hours at a rate of 143 km/min or 0.02 hexes/minute.

All and all a very unusual system.

[Daemion]

And now, let us make a different set of assumptions.

[snip]

All and all a very unusual system.

Dude. I found this amusing. You just made my day.

[truetanker]

All I can say is...  :o

TT

[AchanhiArusa]

Okay, I promise after this I will stop showing off  O:-) (maybe) >:D:

With a density equal to the Moon (3.346 g/cm^3) this satellite would have a diameter of 11,758 km.

Using the small angle equation (d/D = alpha/206265) where d is the distance and D is the diameter this satellite would be 6.48 degrees of arc in the sky.  The Moon is 0.52 degrees of arc, so this will be 12.5 times larger.

It will move across the night sky at a rate of 4.72 degrees of arc per hour, so it will take 1.37 hours to move its own angular distance (the Moon takes 2.5 minutes).

If it had the same albedo as our Moon it would be 2,120 times brighter (square of the relative size times square of the relative distance).  Since our Moon is magnitude –12.4 when full this would be –21.1 when full (the Sun is –27.6).  That is 0.36% as bright as the Sun, but it would take staring full at it about 16.3 minutes to damage your eyes like the Sun can (using the Blackbody radiation equation and the fact it takes a 10 C change to damage your eyes).

So we have Mechwarriors on a geological unstable planet looking at an incredibly large, bright, and fast moving moon in the night sky.