[Spaceships] Homebrew - Alternate Contragrav

[DaltonS]

Another new design switch suggestion for GURPS Spaceships:
Force Screen Variant

Gravity Screen: Provides no DR (although it does affect Tractor Beams[1] and Graviton Guns[2]) but instead lessens the gravitational effect of outside masses on the ship. Read "dDR" as the divisor applied to a planet's gravity to determine its effect on the ship*. You can have both a gravity screen and another force screen.

* The ship's mass remains the same however, so the rate of ascent in a vertical take-off would be A-G/dDR where A is the spacecraft’s drive acceleration in G, G is the world’s gravity and dDR is the gravity divisor. To find the effective escape velocity of planet while using the screen, divide its natural escape velocity by the square root of the dDR. (FWIW: The solar escape velocity from a planet's orbit is that planet's mean orbital velocity times sqrt(2).)

Example: An SM+10 (10,000 ton) ship with a gravity screen installed has an effective weight of 66 2/3 tons on a 1G planet. If that planet is Earth, its effective Ve would be 6.96/sqrt(150)=0.568 mps.

While not a defensive system, the gravity screen does have an effect on gravity-based weapons:

1. Tractor Beams: The force applied to a target with a gravity screen is divided by the screen's dDR.
2. Graviton Guns: Gravity screens protect with 1/10 dDR.

Dalton "treating this with the appropriate gravity" Spence

[Proteus]

Should it be a high-powered component?

(Not with my books, so I don't remember whether that's already true of force screens.)

[Phantasm]

Originally Posted by Proteus

Should it be a high-powered component?

(Not with my books, so I don't remember whether that's already true of force screens.)

Yes, force screens are already high-powered components.

[Anthony]

Why is it a force screen variant? It's just a contragravity variant, and there's no reason the effect would scale with the size of the ship.

[ericthered]

Originally Posted by Anthony

Why is it a force screen variant? It's just a contragravity variant, and there's no reason the effect would scale with the size of the ship.

its a little of both: it protects against a class of weapons and counter-acts gravity. In some universes its a very effective shield.

As for why it scales in size, the generator seems to effect things not based on the mass of what it effects, but a large area determined by the size of the generator.

That said, In a setting with this, I'd probably get a 1/3 or 1/10 one for my ship, because there is very little punishment for going undersized unless graviton beams are a dime a dozen. Its very effective at any size.

[DaltonS]

FWIW, I worked out the effect of the Gravity screen on effective escape velocities (and thus orbital ones). Multiply the velocity by the modifier below.

Escape Velocity Modifier
 SM   TL11^    TL12^
 +4  0.25820  0.22361
 +5  0.22361  0.18257
 +6  0.18257  0.14142
 +7  0.14142  0.11952
 +8  0.11952  0.10000
 +9  0.10000  0.08165
+10  0.08165  0.07071
+11  0.07071  0.05774
+12  0.05774  0.04472
+13  0.04472  0.03780
+14  0.03780  0.03162
+15  0.03162  0.02582

For SM+16 and greater, divide the modifier for size-12 by 10. An important feature here is that this make geosynchronous orbits at a lower altitude possible. (I haven't worked out the table for that yet. )

Dalton “who didn't get his birthday card from the Illuminati” Spence

[David Johnston2]

Originally Posted by ericthered

its a little of both: it protects against a class of weapons and counter-acts gravity. In some universes its a very effective shield.
.

Against a weapon nobody would use considering that this would be a standard propulsion system if available. Actually I'd suggest making it effective against missiles as well, but not beam weapons.

[DaltonS]

Originally Posted by ericthered

That said, In a setting with this, I'd probably get a 1/3 or 1/10 one for my ship, because there is very little punishment for going undersized unless graviton beams are a dime a dozen. Its very effective at any size.

How about this then.

Commercial G-Screen: As above, but provides 1/10th the gravity shielding at 10% the cost for 1/3rd the power (use small energy points). Commercial vessels use this system in conjunction with underpowered reaction(less) drives (which would otherwise be unable to liftoff against local gravity) to save money. Provides no protection against Tractor Beams or Graviton Guns. Can be jammed by a G-Screen Damper (see below).

 System Size    SM+4 SM+5 SM+6 SM+7 SM+8 SM+9 SM+10 SM+11 SM+12 SM+13 SM+14 SM+15
Gravity Divisor
        TL11^    1.5  2    3    5    7    10    15   20    30    50     70   100
        TL12^     2   3    5    7    10   15    20   30    50    70    100   150
Cost ($)                                     
  Light screen   15k  50k 150k 500k 1.5M  5M   15M   50M  150M  500M  1.5B    5B
  Heavy screen   50k 150k 500k 1.5M  5M  15M   50M  150M  500M  1.5B   5B    15B

G-Screen Damper (TL11^) [Hull!]
Also known as a Landlock, this system is used by spaceports to prevent vessels using a commercial G-Screen and underpowered drives from leaving without paying their docking (and other) fees. It has no effect on standard (military) grade gravity screens. This system works by disrupting the G-Screen’s integrity and is typically located beneath the surface of the landing pad (or deck). Each system has a base radius of effect equal to range distance for its SM (see the table below) within which it can jam any number of G-Screens of equal or smaller size.

System Size SM+4 SM+5 SM+6 SM+7 SM+8 SM+9 SM+10 SM+11 SM+12 SM+13 SM+14 SM+15
Range (yds)  10   15   20   30   50   70   100   150   200   300   500   700
  Cost ($)  100k 300k  1M   3M  10M  30M  100M  300M    1B    3B   10B   30B

Or for something a little different.

Armor, Structural Field Variant

Gravity Armor: For double the price, the “unstable exotic matter” of Structural Field Armor (S7:p7) can also act as a commercial grade gravity screen. Use 1/10th the armor’s "dDR" as the divisor applied to a planet’s gravity to determine its effect on the ship. The ship’s mass remains the same however, so the rate of ascent in a vertical take-off would be A-10*G/dDR where A is the spacecraft’s drive acceleration in G, G is the world’s gravity and dDR is the armor’s dDR. To find the effective escape velocity of planet while using the armor, divide its natural escape velocity by the square root of 1/10th the dDR.

Example: An SM+10 (10,000 ton) streamlined ship with gravity armor installed has an effective weight of 666 2/3 tons on a 1G planet. If that planet is Earth, its effective Ve would be 6.96/sqrt(15)=1.797 mps or 6,469 mph.

Note: this assumes that one Grav Armor system is applied to each of the three hull sections.

Dalton “still looking for my Serenity” Spence