Ad Astra: Gravitic Propulsion

What a diametric drive looks like, according to Google

One of the things I've had in the works for a while was a post about gravity manipulation technology as a setting element, how it affects other technologies, daily life, etc. This was originally conceived as a single post but quickly grew too big and unwieldy to be digestible. One of the most mature elements of the post was a discussion of gravitics used for space propulsion.

The standard approach to using gravitic propulsion in GURPS is to just slap reactionless drives on your spacecraft and call it a day. But this misses out on some cool opportunities! So let's take a deeper look at how gravitic propulsion might work.

Reactionless Drives
Gravitics usually leads to reactionless drives. If you can create free-floating gravity fields that accelerate their contents, why not make one around your spaceship? Mechanically, gravitic reactionless drives should be represented with standard reactionless, super reactionless, or subwarp drives with a maximum velocity of the speed of light. You may want to give them the pseudo-velocity design switch (GURPS Spaceships, p. 33) but this is only half right. Everything travelling in a gravitic drive's field experiences exactly the same force, including the passengers, so there's no acceleration effects – the ship is in "free fall". But the kinetic energy the spaceship accrues is real, so it should retain it's velocity when the drive is turned off.

Hot reactionless drives may or may not be a good idea to represent gravitic drives, depending on your setting. As written, hot reactionless drives give off a heat signature "equivalent to a conventional drive exhaust". While this might not sound like a big deal at first, remember that the temperature of a fusion rocket's exhaust plume is measured in millions of degrees. This isn't a trivial amount of heat! That said, if your gravitic reactionless drives have exposed hot elements, like superheated metal pylons, plasma toruses, or haloes of nova-hot negative matter, hot reactionless drives might be appropriate!

If you want to give a veneer of realism or scientificness to your gravitic reactionless drives, some names to consider are: diametric drive, pitch drive, bias drive, and differential sail. All of these are theoretical forms of propulsion that NASA came up with and named. The jury is still out as to whether they're actually possible – many require either negative or imaginary mass, or ways to alter the universal gravitional constant.

Look and Feel
Gravitic reactionless drives will almost certainly not look like traditional rockets. The gravity generator may be safely secreted within the bowels of the ship, giving no outward signs of its presence or location. In which case, ships can be designed in very creative or free-form ways, ignoring symmetry, acceleration direction, fuel tanks, etc. and taking on any shape you want. In combat, disabling a ship's drive is impractical, especially since something as important as the engines are likely to be placed in core systems, making them harder to hit even in long skirmishes. Which direction a ship will travel in becomes harder to predict, since the ship doesn't have to rotate or bank to accelerate in a different direction.

Alternatively, gravitic drives may require pylons, toruses or other exposed components on the outside of the ship, sticking out into space to project or shape the field. These limitations create equally interesting caveats in ship design. The pylons need to be arranged to produce the field correctly, putting constraints on the way ships can be shaped. Weapons fire or errant space debris might damage or knock the structures out of alignment, either disrupting the field or making it react unpredictably to controls. Atmospheric re-entry risks ripping them off, unless they can be retracted somehow. In mechanical terms, treat this kind of gravitic reactionless drive as being an exposed system (GURPS Spaceships, p. 66).

Either way, there's no big glowing bits on the back of your ships. If this makes you sad, keep reading!

Reaction Drives
Just because gravitics exists doesn't mean reactionless drives have to be common in your setting. You can tweak how they work for the effect you want – perhaps the energy requirement increases exponentially with size and strength, so a car-sized vehicle lifting itself takes only a battery the size of a car engine, but a field large and strong enough to accelerate a thousand-ton spaceship needs an antimatter reactor too big to fit into the ship! This is a way you can have your cake and eat it too, with regards to reaction drives. You can have artificial gravity but spacecraft with rockets that need fuel. The downside is you also can't have cities floating in the cloud decks of gas giants. Science giveth and science taketh away!

But gravitics still has something to offer in the field of rocketry. A rocket is any kind of device that imparts energy to a working medium to throw it out the back, pushing itself forward from the recoil. How you impart energy to the medium and what that medium is is a matter of technicality. Gravitics provides us with pressor beams, which easily compress any kind of matter without touching it. Basic thermodynamics tells you that compressing something makes it heat up. Over more conventional methods, pressors give us the benefits of arbitrarily high compression and not needing to use a physical object to contain the (extremely hot) working medium. So here are some additional spaceship systems to use for gravitic-powered rockets:

Reaction Engine, Gravitic (TL^) [REAR!]
These are reaction drives using gravitic technology to produce thrust. All are high-energy systems that require one Power Point to operate.

  Gravitic Thermal Rocket (TL10^): A gravitic rocket using pressor beams to compress reaction mass, causing it to heat up and expand out the back, providing thrust. This is a "realistic" gravitic rocket! Each engine is a high-energy system that gives 0.1G acceleration. Each fuel tank of hydrogen gives a deltaV of 180 mps (TL10), or 600 mps (TL11+).
  Gravitic Torch (TL10^): A high-performance version of the gravitic rocket. Each engine is a high-energy system that gives 1G acceleration. Each fuel tank of hydrogen gives a deltaV of 45 mps (TL10), or 150 mps (TL11+).
  Gravitic Fusion Rocket (TL11^): A gravitic fusion rocket that compresses the reaction mass so much that it undergoes nuclear fusion, like in the core of a star, heating the reaction mass even more and producing better performance. Each engine is a high-energy system that gives 0.3G acceleration. Each fuel tank of hydrogen gives a deltaV of 270 mps (TL11), or 675 mps (TL12).
  Gravitic Fusion Torch (TL11^): A high-performance version of the gravitic fusion rocket. Each engine is a high-energy system that engine gives 3G acceleration. Each fuel tank of hydrogen gives a deltaV of 70 mps (TL11), or 210 mps (TL12).
  Super Gravitic Torch (TL11^): A cosmic variant of the gravitic torch drive. Each engine is a high-energy system that engine gives 100G acceleration. Each fuel tank of hydrogen gives a deltaV of 560 mps.
  Options: Any engine may be high thrust (double acceleration but halve deltaV per tank). Any engine can also use water, methane or ammonia as reaction mass, with the normal effects. All gravitic reaction engines may have the ram-rocket design feature (see GURPS Spaceships, p. 30).

Gravitic Engine Table

SM	+5	+6	+7	+8	+9	+10	+11	+12+	+13	+14	+15
Workspaces	0	0	0	0	0	1	30	10	30	100	300
Cost ($)	600K	2M	6M	20M	60M	200M	600M	2B	6B	20B	60B

 Repair Skill: Mechanic (High-Performance Spacecraft or Gravitic Engine)
 Multiply cost by 2 for gravitic torch and gravitic fusion torch, and 4 for super gravitic torch.

Jet Engine (TL7) [HULL!]
As far as GURPS Spaceships is concerned, a "jet engine" is any reaction drive that uses air as the reaction mass, which includes fission and fusion air-rams. Gravitics has a little more to offer here too, in the form of the grav air-ram.

  Grav Air-Ram (TL10^): This engine sucks in air, compresses and heats it with pressor beams, then expels it as reaction mass. Grav air-rams can also function underwater, at one-third thrust. Each is a high-energy system that provides 1G acceleration for calculating atmospheric speed.

Jet Engine Table

SM	+5	+6	+7	+8	+9	+10	+11	+12+	+13	+14	+15
Workspaces	0	0	0	0	0	1	30	10	30	100	300
Cost ($)	800K	2.4M	8M	24M	8M	240M	800M	2.4B	8B	24B	80B

 Repair Skill: Mechanic (either Aerospace or Gravitic Engine)