Ion vacuum drive

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The Ion Vacuum Drive is the most common type of starship engine throughout the Known Worlds. It is used on every class and scale of ship, from fighters to capital ships, and is widely regarded as the single most versitile engine ever devised.


There is no recorded date for the invention of the Ion-Vacuum drive. Very likely, numerous civilizations developed the technology independently of one another. Earliest recorded use dates back to the Mage Wars though it may have been used in Antiquity.

By the Golden Age it was the standard in starship engine. There are many different designs of Ion-vacuum drives, making the technology as diverse as the internal combustion engine. All operate around the same basic principles.

Operating Principle

The Ion-Vacuum drive employs a ramscoop to suck in fuel from the vacuum of space. Anything will do; trace gasses, dust, whatever matter is in front of the ship gets sucked into the engine and combined with "dry" plasma where it burns intensely. The plasma is then ionized in the primary drive chamber, adding energy to it and causing further expansion as it is expelled out the back of the ship, causing forward motion.

Dry Plasma

Dry Plasma is pure, highly-energetic plasma formed by a matter/anti-matter reaction. This plasma is generally used as a 'spark' to burn materials gathered by the ramscoop, forming "drive" plasma.

Drive Plasma

Drive Plasma is the plasma used to propel the ship before ionization. Drive Plasma is produced when fuel is mixed with dry plasma and burned. In a traditional ion-vacuum design, the fuel source is material gathered by the ramscoop, though anything can be used for fuel.


Technology similar to that of Bussard collectors are used in the ram-scoops. The traditional fuel for an ion-vacuum drive is whatever is gathered from these collectors. In larger ships, waste from the vessel is burned for fuel. Deuterium is also a popular fuel source as it is plentiful and highly effective. Once a fusion reaction is sparked in the deuterium, the matter/anti-matter reactor is no longer required. Most large ships will use a system like this as deuterium is plentiful and easy to store while anti-matter refinement and storage is not.

Power Requirements

The Ion Vacuum drive is curious in the high variability of its electric-to-fuel ratio. More available fuel = less electricity. Less fuel = more electricity. Its efficiency then varies according to fuel availability, with highlier-available fuel allowing for greater speeds while retaining greater economy. However, the equation is often more complicated than that.

Aside from energy used in plasma manufacture and containment, the engine requires electrical energy to ionize the plasma in the pre-fire chamber. When more fuel is available, less electricity is required. More fuel being burned results in less-energetic plasma, which produces less thrust. At lower speeds, such as the cruising velocities of large ships, this is no problem, as even with a very high fuel/dry plasma mix, 30-40 PSL can still be reached with little ionization.

However, smaller craft, such as fighters, lack the power generators needed to produce highly-ionized plasma. For thousands of years, the Bussard Ramjet was the preferred solution, as it required relatively low power and no fuel. However, it was slow at changing velocities and useless for maneuvering.

In the late Fifth Age, the technique of "sweetening the mix" allowed the Ion-vacuum drive to dominate as a main engine. This simply involved burning more antimatter fuel to achieve higher velocities, thus resulting in a higher dry plasma/fuel ratio. It significantly decreased fighter range, but allowed for 70PSL+fighters to become a reality.

Engine Stages

The sections of a standard engine are as follows:

  • Bussard Collector
  • Pre-Compression Chamber
  • Compression Chamber
  • Plasma Combiner
  • Re-Compression Chamber
  • Plasma Re-Combiner
  • Magneto Accelerator
  • Magnetic Nozzle
  • Bussard Collector
    • The Bussard Collector is not actually part of the engine itself, but it a large megnetic and artificial gravity field use to sweep up matter in front of the ship. Bussard Ramjets, commonly used on smaller spacecraft before the development of small-scale ion-vacuum drives, uses a similar technology.
  • Pre-Compression Chamber
    • After matter has been gathered by the collector, it goes into pre-compression. This section of the engine has little function beyond a place to store material scooped up by the collectors, but it is separated from the collectors and the compression chamber by force fields, and essentially forms a lock to keep material from flowing forward back out of the compression chamber.

Some larger engines may use multiple independent pre-compression chambers. This improved efficiency with one large engine over multiple smaller units, but adds redundancy.

  • Compression Chamber
    • Materials gathered by ram scoop are funneled through the pre-compression chamber into the compression chamber where a series of fans, force fields, and artificial gravity compress the material to a near-fusion state.
  • Plasma Combiner
    • After undergoing compression, dry plasma is added to the compressed fuel, at which point it undergoes a fusion reaction
  • Re-Compression Chamber
    • After fusing in the plasma combiner, the material heavily expands, and must undergo a second compression stage for maximum efficiency.
  • Plasma Re-Combiner
    • During initial combination, the highest-energy plasma is allowed to escape and channeled around the re-compression chamber. This plasma is then added back to the main plasma in the re-combiner. High-performance drives may also add additional materials to the plasma stream at this stage(See: Cindy's AfterBurner). Another common practice is to "burn" waste materials from the ship. While higher efficiency may be gained by adding the waste material to the initial compression chamber, behavior can be unpredictable, so the re-combiner is the safest opportunity.
  • Magneto Accelerator
    • After re-combination, the now highly-energetic plasma is channeled through a magnetodynamic system where it is energized and guided to the magnetic nozzle. In normal modes of operation, this section of engine only channels the plasma; however it can be used to increase acceleration if sufficient electrical power is available.
  • Magnetic Nozzle
    • Actually a combination of electromagnets and force fields, the magnetic nozzle is where the plasma leaves the ship.


Engine-output can be increased in three ways:

  • The first is by over-volting the magnetic constrictors in the Magneto Accelerator. Further speed can be obtained by doing the same in the compression stages or anywhere else magnetic constrictors are used; however, doing so produces significant stress on the engine, and the only "safe" place to carry out this procedure is in the accelerator. Over-volting the constrictors will also reduce constrictor-life.
  • The second method is by adding more reactant mass. This can be achieved simply by channeling extra material(preferably fluid or gas into the drive. Under normal operations, the engines function by drawing in the extremely scant amount of gas found in the vacuum of space. Therefore, adding a comparatively small amount of water can have a significant impact on engine performance. The water is converted into plasma by the engine and adds additional reactant mass. Some engine variants are even designed to carry a reactant mass. However, doing this in the long-term is not efficient, and is mainly an augment-mode.
  • The third method is to increase the power of the nuclear reaction happening in the engines by adding additional fusion material. While method two above does this, using an isotope compound such as deuterium or tritium produces considerably more power. This augment method is best used when sufficient electrical power is unavailable or in short supply, and is a way to increase the reactant rates when the compression chamber is not operating at full efficiency.

While all three methods can be used simultaneously, only any combination of two is likely to have a significant impact on engine output. All three together will result in plasma that is hotter, but not necessarily exiting the ship any faster.

Engine Power Descriptors

An ion vacuum drive is usually described as two numbers seperated by a slash, E.G. x/x. This number describes the drives output as a percentage of forward and backward, and is used regaurdless of whether or not the design is by-directional.

A 100/100 engine, then, is capable of producing 100% thrust in either direction, forward or back. A 0/100 is an engine that only goes one way. Occasionally you will see an engine described with the second number higher than 100.

Design Variants

Bi-Directional Designs

Because of their design, ion-vaccum engines can very easily be engineered to to work in either direction. Meaning, instead of needing one set of engines to accelerate, and a separate set to decelerate(or having to turn the entire ship around), the same engine can go both ways.

  • Magnetic Nozzle/Bussard Collector
    • On a hardware level, the magnetic nozzle and the buzzard collector are the same. Both use magnetic and force fields, one simply projects a far, weak field, while the other needs a strong, close field. With relatively minor adjustments to the design, the two can share completely common parts and are only differentiated by programming. This practice is common even in single-direction engines, since it simplifies maintenance.
  • Pre-Compression Chamber/Magneto Accelerator
    • In order to produce a frictionless surface, the pre-compression chamber is lined with magnetic constrictors. Additional components are required to allow it to function as a magnetodynamic accelerator. This also aids in engine efficinecy, as materials flow faster into the compression chamber.
  • Compression, Combination, and Re-compression Chambers
    • At this stage of the engine, the four chambers, with relatively few design changes, can function in either direction. The changes also add redundancy to the system, further increasing reliability. When the engine is being operated "in reverse" the Re-Combiner becomes the Compression Chamber, the Re-Compression Chamber becomes the Combiner, and so on. In some especially high-availability designs, every chamber is built to function as any part of the chain.

Self-Powering Variant

A self-powering variation of the Ion Vacuum drive was developed for the Harpy-class of long-range bomber built by the Gudersnipe Foundation. The design simply used the mechanical motion of the expanding dry and drive plasma in the combiner and re-compression stages as well as all through the engine to produce electrical energy, which is then used both to ionize the plasma and to power the ship. The problem with this variant is that it requires drastically more anti-matter. In the Harpy, it allows the ship not to have an internal power generator.

Though completely inefficient in a large starship, this design works well for fighters where speed is of higher value than endurance.

Reduced Complexity Engines

In some variants, the engines reduce complexity by using just a Compression and Combination chamber, without re-compression and recombination. This is common on commercial vessels where reduced maintenance costs outweigh increased efficiency.

In the simplest engine design, the collector and nozzle use the same components, no pre-compressor is used, and only a compression and combination chamber sit before the magnetodynamic system, which itself is typically much shorter. This type of engine scales well and is ideal for use on very small space craft. It also preforms well on very large spacecraft where speed is not requires and reduced maintenance costs are beneficial.

Very High Performance Variants

This design has no use outside military applications. Even top-end racing spacecraft cannot afford the complexities involved. The design also scales poorly, and is found mostly on some Cruisers, most Destroyers, Frigates, and Cutters where a very high delta-V and good Acceleration curve are vital.

In this variation of the drive, an additional compression chamber is added immediately between the Bussard collector and the pre-compression chamber. This would have to be included at both ends for a bi-directional engine, and at the exhaust point would be configured as a pass-through.

More powerful magnetic constrictors are used in the pre-compression stage and the vector is narrowed, so that gathered material is already compressed somewhat as it enters the compression chamber. The compression chamber then uses inertial confienment coupled with gravitational compression to speed the initiation of fusion. During quick-start routines, helium and hydrogen are injected in this stage along the edges and fusion is initiated using lasers. The same technique can be used to provide a speed-boost.

After the plasma re-combination stage, pure water is injected into the plasma stream, which immediately fuses and produces considerable additional thrust; though quite a bit of water is required to make a difference. The water can be substituted for deuterium oxide or tritium if sufficient quantities are available.

A second set of magnetodynamic accelerators is added offset from the first, able to apply more energy to accelerate the plasma stream. The power requirements are doubled.

Speed is still limited to 70 PSL, but the Acceleration curve is much higher, and the ship is capable of producing very high delta-V.

Plasma Duct

Not a design variant by itself, but in very high-performance versions the plasma flow can sometimes be split into a series of ducts. This is used chiefly when the intake and outlet are not in a direct line. Ducting is used additionally by wrapping the ducts hellically along the path of travel, which increases the effective length of the engine. This is done along the Magneto Accelerator to provide additional area to add acceleration. Further, the space in between the ducts can be used to fit a gravity field generator employing a spinning field design, which adds extra force to the plasma at a cost of greater electrical requirements.

Linear Drive

The Linear Ion Vacuum Drive is actually the closest to ion-vacuum technology. It works on identical principles, but with no moving parts. Collection, compression, expulsion, all happen very fluidly. Also unlike a typical ion vacuum drive, there is no spiral, the engine is a straight line.

Linear drives offer some advantages over a typical ion-vacuum, such as much faster start up times and higher delta-v. Actual thrust levels are lower, but it takes a linear drive less time to reach maximum output, giving a basic version of the drive an Acceleration curve matched only by the highest-performance ion-vacuum kits. For their output, the linear drive also requires less power.

However, the drives suffer from numerous drawbacks. They are much more costly to manufacture, requiring more exacting standards, exotic materials, and calling for a number of components that cannot be replaced once failed. This results in a high maintanace cost and lower operational life-span. Compared to ion-vacuum drives, which can sometimes last the lifetime of the ship, linear drives have a total use life measured in thousands of flight hours.


Due to their drawbacks, the Linear Ion Vacuum Drive sees very little actual usage.

  • They are primarily found on the racing circuit, where their high degree of performance and augment-ability provide a serious edge. Race ships, typically little more than a cockpit with engines, can afford to use a drive system that only lasts the life of the race.
  • The Reliant Robin class of scout ship was the most widely-built user, with millions constructed during the Kamian Succession Wars. Robins used an effectively hot-swap-able version of the engine and typically saw less than a thousand flight-hours per drive. These ships also had a conventional ion-vacuum engine as a backup.
  • The Sahar Jusenkyou-Class Carrier was the first large spacecraft and first capitol-class warship to employ the technology, though in a heavily modified form. Little information was publicly released about the engines, but they have been reffered to on official documentation as "MK IX Linear Ion Vacuum Drives". Since the Foundation would never field an engine with so many problems on a warship, its clear that they overcame any issues.

Notable Examples of Ion-Vacuum Technology

  • The Ion-Vacuum drive on the Saratoga is notable for being under-powered for the ship's size (one of several serious design short-comings of the Glorious Heritage-class). The crew, who had not read the ship's operation's manual, dealt with the issue by running the engine at much higher pressure levels than they were designed for. Every time a component failed, the crew replaced it with a custom-made, over-built version, until the engines could cope with the stresses they were placing on it.
  • A.S. Sundew was a privately-owned freighter that gained wide attention when it was found that the ship's ion-vacuum drive had been in continuous operation for one hundred and six years. To quote the captain: "Grand-Pappy said if he ever turned off the engine, he'd never get it started again... so he never did. If it worked then, it otta' work now.".