Propulsion, Sublight

Overview
From the earliest days of rocketry, civilizations have learned to harness the forces of chemical, and later nuclear energy, to propel vehicles and missiles alike through space. One of the earliest challenges in space travel was finding a means of propulsion that was more efficient in its use of fuel, so those early vessels could accelerate to greater speeds and cover distances in shorter durations. Every space faring civilization went through this stage at some point: mastering a speedy and efficient means of sublight travel to explore interplanetary space before making the leap to faster than light travel and exploring the stars.

Chemical Propulsion
The earliest form of propulsion in space, chemical propulsion involves using a chemical reaction within its fuel (usually by combining the fuel with an oxidizer) to release thermal energy from the reaction. That thermal energy in turn heats the spent fuel, creating pressure from the expanding vapors that is released from the nozzle of the rocket, providing thrust. It is simple in principal, and there is a great variety of fuels that can be used, ranging from solid to liquid. Another advantage to this means of propulsion is that the release of energy is fast and provides for a very strong thrust, thus a very quick acceleration.

An later use of chemical propulsion uses a nuclear reactor, usually fission powered, to heat up liquid propellant to a very high temperature and then using the pressurized gas to propel the spacecraft.

The major drawback of chemical propulsion is that the energy density of the fuel is always very low: the majority of the energy released from the fuel goes towards moving the fuel itself. This makes breaking out of the gravity well of a planet especially expensive. In the early days of interplanetary space flight the fastest a vessel could hope to go by this means was roughly around 19 km/s. This made interplanetary journeys a matter of months, years, even decades. Another problem was the volitility of the fuel itself...it was expensive to produce, difficult to store, and had an annoying tendency to explode at the slightest provocation.

Chemical propulsion is almost never used in modern spaceflight, having long ago been replaced by far more efficient technology. However it does still see some use in orbital bases, as well as colonies on low-gravity planetoids, moons, and asteroids, where chemical fuel is plentiful and its energy is better spent.

One place where chemical propulsion is still widely in use is in weaponry. Bullets, artillery shells, missiles, and torpedoes alike still make frequent use of chemical propulsion, taking advantage of a cheap supply of fast burning fuel.

Solar Sails
An early attempt to higher speeds within the solar system, a solar sail is a huge net of metal foil that measures many kilometers in diameter. The sail is intended to ride the solar wind, creating a gradual but steady acceleration that could, in months time, push a probe to near-relatavistic speeds. Some Terran probes were eventually sent out of the Sol system by this means in the late 21st Century; but before the probes could reach their destinations the Terrans made first contact with the Selvens and discovered FTL.

Solar sails still have a nostalgic appeal, evoking images of the Age of Sail from ancient Terran history, but they are too impractical to be of any real use today for spacecraft. However they do find some use as a means of diverting potentially dangerous asteroids and comets, by gradually pulling them into another orbital trajectory, many years before they could have a chance of a near miss with any inhabited worlds.

Orion Drive
Also sometimes called the Omega Drive, this proposed means of propulsion predated the development of the advanced Plasma Drive, and was an attempt to push spacecraft to near-relavitistic speeds. Ultimately it was never built, and advances in propulsion technology made the idea obselete. This drive would have worked by mounting a huge concave metal plate on the aft section of a spacecraft, and then dropping fission bombs through a hole in the center of the plate, and detonating them. The release of energy from the nuclear explosion pushes on the plate, mainly by vaporizing a small layer of matter off of the plate. When detonated in quick succession, the fission bombs propel the spacecraft forward with an impressive rate of acceleration.

The problem with the drive was extreme radiation hazard involved in using fission bombs as a means of propulsion. The crew compartments would have needed to be heavily shieled to protect them from the radiation. Also the bombs themselves would be expensive to produce, and provided a limited quantity of fuel that could not easily be replaced when far from civilized worlds.

Even though the drive was never put into any official use, a few private developers have built a small number of ships using this drive, just to prove that it could be done. It is still illegal to this day to use such a drive within inhabited systems.

Plasma Propulsion
The most common means of sublight propulsion in use today, plasma propulsion involves exciting a gas into plasma and then condensing and accelerating the plasma through magnetic coils to provide enormous thrust.

Early versions of plasma drives used very little power and provided a very slow rate of acceleration, but were capable of maintaining accelearation for weeks or even months on end, adding up to an impressive cumulative velocity. With the advent of nuclear powered spacecraft, the next generation of plasma drives operated at much greater levels of power, enabling them to provide acceleration that rivaled that of chemical propulsion, but with a vastly superior energy density. Spacecraft could now launch themselves from the surface of a planet and still have enough fuel to travel across interplanetary distances without having to refuel.

Nearly every spacecraft in service today uses some form of plasma propulsion, the variety of drives being great, but the underlying principals being the same. They are relatively inexpensive and easy to repair on any civilized world. The reactive propellant can range from atmospheric gasses, gathered by turbines when a ship is flying within an atmosphere, to liquified gas stored in tanks onboard the craft.

The only real negative to plasma drives is that they do operate under very high pressures, and the magnetic coils that contain the plasma streams can fly apart catastrophically if they are not properly maintained.

Gravitic Mass Reductor Drive
The Gravitic Mass Reductor Drive (GMR) is the result of early experiments with the strange realm of hyperspace. Physicists on various worlds, at various times, have discovered that it was possible to shunt a certain amount of a ship's mass into hyperspace, allowing the ship to gain a sudden boost of sublight speed; the energy that intially produced the momentum is the same, but the ship moves faster because its overall mass has effectively been decreased.

The Tacyon Ranger, the first Terran GMR propelled ship (in service from 2133 to 2145), had managed in 2145 to approach 99.9998% of the speed of light after an enormous expendature of energy. The pilots tried to push it to 100% and briefly touched hyperspace in the process, causing the ship to convert completely into energy, dissolving into an intense burst of gamma radiation. After that point safeties were installed in all GMR drives to prevent them from pushing a ship to those speeds.

The GMR drive is also known colloquially as a "Pulse Drive".

That boost in speed remains as long as the GMR is in effect; when the GMR is shut off the ship will instanty revert to its original speed, gaining back the mass that it had traded off.

It is possible to use a ship's thrusters to change its velocity while the GMR is engaged; the boost provided by the GMR will vastly increase the effective acceleration while the crew on board will only experience inertia according to the ship's true speed. For example, if a ship has a GMR with a multiplier of 60x and it can ordinarily accelerate at 10 g, then if it accelerates while the GMR is in effect it will be effectively accelerating at 600 g while the crew will only experience 10 g of inertia (or less, depending on inertial dampeners).

It is possible to use a GMR to cause a sudden deceleration, using the GMR effect to absorb all of a ship's momentum and bring it to a full stop (along its particular vector), but as soon as the GMR is deactivated all of that energy will return to realspace in the form of heat. In order to avoid having its engines vaporised by that heat, the ship needs to radiate it off into space in a hurry, usually by flooding the engines with coolant and then venting it into space. The expanding gas cloud from such a coolant flush can be visible from a considerable distance.

While a GMR drive is active a ship cannot make any sudden adjustments in its course as its momentum becomes effectively locked by the partial contact with hyperspace. Gradual course corrections still work, as well as changes in direction caused by nearby gravitational fields, but if a ship wants to make a major change to its course, or engage in complex maneuvers, it needs to first deactivate its GMR drive.

Any missiles or smaller spacecraft fired from the ship while engaged in GMR will immediately lose the boost in speed as soon as they leave the ship, dropping back down to the ship's original momentum (plus any momentum gained from whatever means of propulsion the missile has)

While transitioning between regular sublight speed and GMR-enhanced speed (usually called Pulse speed) there is no inertia, no sense of acceleration or deceleration.

Regardless of how fast a ship's Pulse Speed is, it gains no additional momentum because of the GMR; if it impacts another object while using the GMR drive then the force of impact will always be calculated according to the ship's real speed before engaging the drive.

The increase in velocity caused by a GMR drive is usually reflected as a multiplier; the original speed is multiplied by a certain number based upon the grade of the GMR drive. This works as follows:

 Grade E: 

5 - 10 x original speed.

 Grade D: 

10 - 25 x original speed.

Grade C:

25 - 40 x original speed.

Grade B:

40 - 60 x original speed.

Grade A:

60 - 100 x original speed.

The GMR drive is still restricted by the speed of light, and cannot boost the acceleration of a ship to the speed of light, let alone beyond. Physicists have found that the GMR's effectiveness drastically begins to drop as the ship's Pulse Speed goes past one quarter the speed of light, and so typically this is the upper speed limit to a ship traveling at sublight speed.

Talesian Pulse Drive
Talesian Pulse drives utilize a greatly advanced form of GMR technology, amplifying the mass displacement field through the warp drive, allowing them to achieve multipliers on the GMR drive of up to 200x. The best performing Talesian Pulse Drive can accelerate up to 3000 gs (28,000 m/s/s), allowing them to reach a velocity of 0.5 c in just under 90 minutes.

Comparative Travel Times
The chart below illustrates the time required to travel a series of given distances at different speeds, as well as the acceleration time required to achieve those speeds.

Most ships do not accelerate beyond 10 - 15 gees (for larger ships, upwards of 20 gees for starfighters), as it requires the assistance of intertial dampeners to prevent the high gees from causing harm to the crew, and that's not even taking into consideration the ships that don't have intertial dampening technology.

The GMR is instrumental in cutting down the time and energy needed to accelerate. For example, a freighter that has a Grade B GMR could take just over 15 minutes of hard acceleration to get up to 100 km/s, then activate the GMR at a 60x multiplier and boost its speed up to 6000 km/s, vastly cutting down on the time it needs to travel across interplanetary distances. A top of the line military ship with a Grade A GMR could take around 2 hours to accelerate to 750 km/s and then activate its GMR at a 100x multiplier to boost its speed up to 75,000 km/s...a quarter of the speed of light....allowing the ship to rapidly cover the distances within a solar system.

The GMR can act in tandem with a ship's sublight thrusters to effectively boost the acceleration rate by the GMR's multiplier; a cruiser that ordinarlily accelerates at 5 gees and has a Grade B GMR with a 60x multiplier can actually accelerate at 300 gees (2,880 m/s/s). This combination of a GMR drive and a regular plasma thruster is called a "Pulse Drive".