The impulse engines of starfleet vessels are fusion powered. They have two primary functions, both of which are shared with the warp engines: they propel the ship though space and supply power for ships systems.
Impulse engines propel the ships at sublight speed, during normal operations, full impulse is only one-quarter the speed of light; above this, problems tend to occur. Travel at half the speed of light will cause a fall in engine efficiency to 85 percent, travel at impulse above three quarters the speed of light may cause relativistic problems.
The impulse drive is normally used within a solar system or within regions of space such as black clusters and the Badlands, which are incompatible with the warp field. Control of the impulse engines is maintained by a combination of computer automation and crew command input.
Early versions of the impulse engine propelled ships at sublight speeds using conventional Newtonian physics. Following experiments on Ambassador class starships early in the 24th century, a driver coil assembly was introduced into the system; to give later, larger starships the proper acceleration, it is necessary to use a fusion-driven compact space-time driver coil in association with the impulse engine. A simple Newtonian reaction driver acting alone will not do the job. In emergency situations, a small amount of antimatter can be admitted to the impulse reaction chamber to further increase output.
On Constitution-class starships, the impulse engines are on the aft of the saucer section. On Galaxy-class starships, the main impulse engine is actually located on the aft of Deck 23 of the engineering hull; the saucer section is propelled by two engines, each forming a group at the aft of Deck 10. Each engine is made of four parts: the impulse reaction chamber, the accelerator/generator, the driver coil assembly and the vectored exhaust director.
On Galaxy-class starships, fuel for the main engine is kept in the primary deuterium tank in the engineering section of the ship. Antimatter storage for the main engine is on Decks 41 and 42. All fuel tanks are made of alternate layers of forced-matrix cortanium 2378 and stainless steel. Fuel for the saucer section impulse engine is supplied by 32 auxiliary cryogenic tanks; antimatter storage is on Deck 10.
Due to the nature of the energy released during the fusion process, the impulse propulsion system needs slightly more maintenance than the warp propulsion system, even though warp engines are a million times more energetic than impulse engines.
Uses of impulse engines
Impulse engines may be used to propel the ship and as a power source at the same time. Parts must be replaced periodically to maintain the engines maximum efficiency and safety.
Impulse engines can be used to manoeuvre a ship and keep it aloft in planetary atmospheres, even if it does not have an aerodynamic shape. Impulse engines are not to be confused with manoeuvring thrusters which are used when pinpoint accuracy is needed, such as inside a spacedock.
This is in many ways the "heart" of the ship. The principle function of any reaction chamber is to allow the matter and antimatter streams to come together and direct the resultant energy flow into the power transfer conduits. This apparently simple task is rendered highly complex by the need to allow the various sensor and other monitoring and control equipment to function within the chamber. The addition of dilithium to regulate and control the reaction, while allowing far higher efficiency and so increasing the power output, has also lead to ever more complex designs - most especially in more recent starships which are designed to allow continual recompositing of the dilithium whilst in use. Nevertheless, reaction chambers of today perform fundamentally the same task as those of a century ago or more.
The magnetic constrictors make up the bulk of the warp core. They provide physical support to the reaction chamber, pressure containment for the whole core and, most importantly, guide and align the fuel flow onto the desired location within the reaction chamber.
The matter constrictor is typically longer than the antimatter constrictor, as antimatter is easier to focus and so requires a shorter distance for the same accuracy. Typically, the magnetic constrictors are divided into segments; each segment will contain several sets of tension frame members, a toroidal pressure vessel wall, several sets of magnetic constrictor coils and related power and control hardware. Constrictor coils will have dozens of active elements, and on more advanced designs these will be configures to contain the magnetic field almost wholly within the constrictor, with minimum spillage into the exterior environment. Starfleet warp cores usually have the outermost layers of the constrictors constructed of a semi-transparent layer which allows harmless secondary photons to escape from the inner layers, creating a glow effect. This gives an immediate visual cue to the current activity rates within the warp core.
As the fuel is released from the injector nozzles, the constrictors compress it and increase the velocity considerably. This ensures the proper collision energy and alignment within the reaction chamber.
Dilithium is a key factor in the design of any efficient matter / antimatter reactor, and has been incorporated into Federation Starship designs since it replaced lithium crystals in 2265.
The key to the success of dilithium lies in the remarkable properties of this material. When subjected to a high frequency electromagnetic field in the megawatt range, dilithium - or 2<5>6 dilithium 2<:> diallosilicate 1:9:1 heptoferranide to give it the full scientific name - becomes completely porous to antimatter. The field dynamo effect created by the iron atoms within the crystalline structure allows antimatter atoms to pass through without actually touching it; it is thus the only known substance which does not react to the antimatter fuel commonly used in Starships. Dilithium can thus be used to mediate the reaction, boosting efficiency.
Eventually reliance on natural dilithium was reduced after breakthroughs in nuclear epitaxy and antieutectics made it possible to synthesize dilithium for Starship use through theta-matrix compositing techniques utilizing gamma radiation bombardment. However, refining dilithium ore is a procedure which is still viable for Starships which are unable to obtain synthetic dilithium from a Starbase or other manufacturing facility.
After its long journey from the fuel systems, the flow is finally directed down the warp coils. These devices are large split toroids which take up the bulk of the nacelle. In order to increase efficiency they are usually made from multiple layers of various materials; this complicates the manufacturing processes greatly and has - so far - kept the replication of warp coils beyond Federation science.
The warp coils generate a multi-layered set of fields around the craft, creating the propulsive forces that enable a Starship to travel beyond light speed. Manipulation of the shape and size of the field determines the velocity, acceleration and direction of the vessel.
Since its confirmed existence in the 1930s, the concept of a form of matter with the same mass but reversed charge and spin has intrigued scientists and engineers as a means to produce unprecedented amounts of energy, and to apply that energy to drive large space vehicles.
Cosmological theory maintains that all constituent parts of the universe were created in pairs; that is, one particle of matter and one particle of antimatter. Why there seems to be a propensity toward matter in our galactic neighbourhood is, to this day, a topic of lively discussion. All of the basic antiparticle have been synthesized, however, and are available for continued experimental and operational use
When, for example, an electron and an anti-electron (or positron) are in close proximity, they mutually annihilate, producing energetic gamma rays. Other particle-antiparticle pairs annihilate into different combinations of subatomic particles and energy. Of particular interest to spacecraft engineers were the theoretical results presented by deuterium, and isotope of hydrogen, and its antimatter equivalent. The problems encountered along the way to achieving a working M/A engine, however, were as daunting as the possible rewards were glorious. Antimatter, from the time of its creation, could neither be contained by nor touch any matter. Numerous schemes were proposed to contain antihydrogen by magnetic fields. This continues to be the accepted method. Appreciable amounts of antihydrogen, in the form of liquid or, better yet, slush, posed significant risks should any portion of the magnetic containment fail. Within the last fifty years, reliable superconducting field sustainers and other measures have afforded a greater degree of safety aboard operational Starfleet vessels.
As use aboard the USS Enterprise, antimatter is first generated at major Starfleet fueling facilities by combined solar-fusion charge reversal devices, which process proton and neutron beams into antideuterons, and are joined by a positron beam accelerator to produce antihydrogen (specifically antideuterium). Even with the added solar dynamo input, there is a net energy loss of 24% using this process, but this loss is deemed acceptable by Starfleet to conduct distant interstellar operations.
The antimatter is kept contained by magnetic conduits and compartmentalized tankage while aboard the fueling facility. Early starships were also constructed with compartmentalized tankage in place, though this method proved less desirable from a safety standpoint in a ship subjected to high stresses. During normal refuelling, antimatter is passed through the loading port, a 1.75 meter-wide circular probe-and-drogue device equipped with twelve physical hard-dock latches and magnetic irises. Surrounding the antimatter loading port on Deck 42 are thirty storage pods, each measuring 4x8 meters and constructed of polyduranium, with an inner magnetic field layer of ferric quonium. Each pod contains a maximum volume of 100 mE3 of antimatter, giving a 30-pod total starship supply of 3000 mE3, enough for a normal mission period of three years. Each is connected by shielded conduits to a series of distribution manifolds, flow controllers, and electro plasma system (EPS) power feed inputs. In rapid refuelling conditions, reserved for emergency situations, the entire antimatter storage pod assembly (ASPA) can be drawn down on jackscrews and replaced in less than one hour.
In the event of loss of magnetic containment, this very same assembly can be ejected by microfusion initiators at a velocity of 40 m/sec, pushing it clear of the ship before the field decy and the antimatter has a chance to react with the pod walls (see also: Emergency Shutdown Procedures. While small groups of pods can be replaced under normal conditions, the magnetic pump transfer method is preferred.
Antimatter, even contained within storage pods, cannot be moved by transporter without extensive modifications to the pattern buffer, transfer conduits, and transporter emitters for safety reasons due to the highly volatile nature of antimatter. (Specific exceptions apply for small quantities of antimatter stored in approved magnetic containment devices, normally used for specialized engineering and scientific applications).
Refueling while in interstellar space is possible through the use of Starfleet tanker craft. Tanker transfers run considerable risks, not so much for hardware problems but because refine antimatter is a valuable commodity, and vulnerable to Threat force capture or destruction while in transit. Starfleet cruiser escorts are standard procedure for all tanker movements.