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An Explanation for M-Drives

A few weeks ago I did some calculations on how much kinetic energy a M drive ship has during a 1 g trips from a world’s jump limit or to a neighboring planet and it turned out to be a lot.


Ever since I’ve been toying around with different ways of reducing the energy consumption of M-Drives in a way that explains their cannon behavior as cleanly as possible and I think I finally got one I like.

Grav cars, AirRafts and shuttle craft use your classic grav drive system, where the craft pushes off the planet or star’s gravity well and goes hurtling about, their kinetic energy is all perfectly normal. This means that the power requirements of the drive increases linearly as the velocity of the craft increases. Due to power limitations basic grav craft can only accelerate at multiple g's if they are moving less than 1 km/s relative to the planet they are pushing off of. They can still maneuver at higher relative speeds, but their acceleration is reduced, 1/10 normal accel at 10 km/s, 1/100 normal accel at 100 km/s and so on.

M-Drives get around the kinetic energy problem of basic G-Drives by being slower than light warp drives. A ship forms a bubble of warped space about itself which can then be seemingly accelerated up to high speed, but the ship isn’t actually moving, its being carried along by the bubble of warped space, its actual “intrinsic” velocity is unchanged. If the warp bubble is destroyed in some manner the ship’s speed reverts to its true intrinsic speed. In effect the drive acts as a kind of teleporter, it transports a ship from point A to point B without changing its velocity.

When the ship becomes inertialess inside of the warp bubble the G-drives can’t accelerate it any faster than normal, they are limited in how quickly they can fold space time, however the advantage of the warp bubble is maneuvering the ship takes vastly less power because the kinetic energy of the ship isn’t increasing. Once the bubble is turned off the ship’s speed reverts to its original value. The only thing the ship has to pay for is changes in gravitational potential energy.

The main tweak I made to this somewhat standard idea is that the field can only safely be turned off if the warp bubble’s effective velocity matches the ship’s intrinsic velocity, if there is a mismatch of more than a km/s or so the ship’s drive suffers a critical hit and is disabled until it is repaired and refueled. This is why ships bother preforming flip and burn maneuvers, as they need to slow down at their destination to turn their drive off properly. The ships then spend a few minutes or hours adjusting the difference in orbital velocity between their origin and destination using their G-drives.

The other bigger tweak I made was to make it so objects can freely pass through the warp bubble without changing their velocity. The drive adds or subtracts kinetic energy from the objects to make collision happen like normal, up to a point. Eventually the drive runs out of energy, so this is a way of making high speed collisions happen like normal, but if the collisions start entering nuclear bomb level energies the drive runs out of power and the ship reverts to its intrinsic speed.

I decided that the maximum amount of energy a M-drive can handle is 500 terra joules per dton of drive. This allows a 100 dton ship massing 500 metric tons starting at Earth orbit to escape the solar system using its 1 dton M-Drive, with some wiggle room for efficiency losses. Around stars significantly more massive than the sun, or when starting out very close to a star, like going from Mercury to Earth, gravitational potential energy changes are more extreme so Travellers might need to stop and refuel their drive but that is a relatively rare occurrence.

Now 500 terajoules is still a lot of power, around 8 Hiroshima bombs worth, so I decided that while changing the gravitational potential energy of a ship is nearly 100% efficient, the process of adding or removing kinetic energy from an object passing through the ships warp bubble is only around 1% efficient, a lot of energy is wasted creating a pulse of relatively harmless gravitational waves. So if someone seized control of a 100 dton ship with a 1 dton M-Drive, the most damage they could deal in a single flyby is 5 terajoules, around 1.2 kilotons worth of tnt, significantly better than the 39 megatons of tnt a classic M-Drive ship travelling at 1000 km/s could impart.

At 10 km/s the maximum amount of mass you could dump overboard before you exceeded the 5 MJ limit is 100 tons. At 100 km/s it is 1 ton, and at 1000 km/s it is 10 kilograms. Most weapons in Traveller don’t involve a lot of mass being thrown out, the most massive is missiles and they get around the mass limit by having their own (slightly unstable) M-Drives, so their intrinsic velocity doesn’t have to be adjusted when they leave the ship. The really big rail guns and mass drivers typically require massive ships with much larger drives, so they have larger mass budgets to play with.

Now the other problem is that is still a lot of energy for the ships power plant to supply, and the power requirements of the drive would also vary massively depending on whether the ship was deep in a gravity well or not. However some of the technobabble on the Traveller Wiki gave me an idea, because they mentioned that Maneuver Drive fields require manipulating both gravitational and nuclear forces. So I decided that one of the techs in this setting was a way of gravity generators being powered directly by nuclear reactions such as fusion.

Most of the power of a G-drive or M-drive comes from a series of internal fusion cells where nuclear forces are directly converted into gravitational forces to move the ship. The drives require a bit of electrical power to control the direction and output of the fields, which is why the drives requires electricity when maneuvering, but the energy needed to actually execute those maneuvers is provided internally by the fusion reactions. This allows the electrical requirements of a maneuver to be handwaved as a flat X power points per g regardless of circumstances. This also allows me to set how much energy a power point is to pretty much whatever I want. I decided to make it equal to 1 MW, rather than the standard 250 MW because that makes waste heat management and the power point requirements of life support and cargo handling equipment slightly less absurd.

To explain the waste heat management of the M-Drive, I took inspiration from all the Traveller art which shows M-Drive ships with rocket nozzles and exhaust plumes. The nozzle structures are the field emitters which produce the warp drive field around the ship, typically they are mounted in line with the ship’s primary direction of travel. The ship can accelerate off axis, allowing it to jinx around in combat, but it is not as quick or efficient in those directions. While most of the space inside the warp bubble is in freefall (which is why Traveller ships don’t have to worry about aligning decks with the direction of travel), there is a tubular shaped region of strong tidal forces between each emitter and the rear of the warp bubble. This region is prone to picking up and confining bit of dust and gas, a feature which engineers used to their advantage by using it as a confinement system for a dusty plasma radiator for cooling the M-Drives. Hydrogen coolant is pumped over the star hot powerplant and M-Drive fusion cells and vented out the back of the ship where it circulates in the gravitational eddies for several minutes, gradually cooling off before being drawn back in to cool the drives again in a continuously loop. This creates the illusion that the reaction less M-Drive has an exhaust plume, but in reality it is the M-Drive and powerplant radiator system.

To explain why ships can coast even if their M drive is shot out I decided once turned on the M drive field is self sustaining, only requiring external power to control it. Even if an M-Drive is hit in combat and disabled the field itself remains up until it runs out of energy in its fuel supply. The ship continues to coast until the drives fuel is exhausted in some way, either by collisions requiring kinetic energy adjustments, or by rising up a gravity well, however by default a ships drive typically has enough fuel in it that these processes usually are trivial. Most of the energy loss of a broken drive comes from the drive leaking fuel if its fusion cells are breached, which causes the fields holding the fuel in place to gradually lose cohesion and let the fuel leak out over the course of several weeks or months. Once a drive finally runs out of energy its field collapses and the ship reverts to its intrinsic velocity, however given how long it takes for a broken drive field to decay this is not something that Traveller characters typically experience. A ship fleeing to the 100 D limit of a world can still coast upwards if its drive is shot out halfway there, a field collapses only happens after several weeks of coasting or if the ship is so thoroughly obliterated that the drive’s fusion cells are completely scattered.

So yeah, I’m pretty happy with this. The only major gameplay change this system imposes is that there is a limit to how much stuff a ship can dump overboard when traveling at high speeds, but that shouldn’t come up too often, especially if self-sealing hulls to minimize decompression are standard. Thoughts?