[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.

[Flyndaran]

Originally Posted by David Johnston2

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.

Just curious, but what would be your in game reasoning / techno-babble as light is just as effected by gravity as matter?

[Anthony]

Originally Posted by Flyndaran

Just curious, but what would be your in game reasoning / techno-babble as light is just as effected by gravity as matter?

Well, sort of. Gravity is generally more relevant to slow objects. A lot of 'gravity'-based drive systems are likely to have boundary conditions that would be problematic for low velocity projectiles, though I'm not sure why this particular system would.

[David Johnston2]

Originally Posted by Flyndaran

Just curious, but what would be your in game reasoning / techno-babble as light is just as effected by gravity as matter?

No, not just as. My explanation is that the higher the mass the greater the effect.