The principal sublight propulsion of the ship and certain auxiliary power generating operations are handled by the impulse propulsion system (IPS). The total IPS consists of two sets of fusion-powered engines: the main impulse engine, and the Saucer Module impulse engines.
During normal docked operations the main impulse engine is the active device, providing the necessary thrust for interplanetary and sublight interstellar flight. High impulse operations, specifically velocities above 0.75c, may require added power from the Saucer Module engines. These operations, while acceptable options during some missions, are often avoided due to relativistic considerations and their inherent time-based difficulties.
During the early definition phase of the Ambassador class, it was determined that the combined vehicle mass of the prototype NX-10521 could reach at least 3.71 million metric tons. The propulsive force available from the highest specific-impulse (IĄ) fusion engines available or projected fell far short of being able to achieve the 10 km/sec® acceleration required. This necessitated the inclusion of a compact space-time driver coil, similar to those standard in warp engine nacelles, that would perform a low-level continuum distortion without driving the vehicle across the warp threshold. The driver coil was already into computer simulation trials during the Ambassador class engineering phase and it was determined that a fusion-driven engine could move a larger mass than would normally be possible by reaction thrust alone, even with exhaust products accelerated to near light speed.
Experimental results with exhaust products temporarily accelerated beyond lightspeed yielded disappointing results, due to the lack of return force coupling to the engine frame. The work in this area is continuing, however, in an effort to increase powerplant performance for future starship classes. In the time between the Ambassador and the Galaxy classes, improvements in the internal arrangement and construction of impulse engines proceeded, while continuing the practice of using a single impulse engine to perform both propulsion and power generation functions like its larger cousin, the warp engine. Magnetohydrodynamic (MHD) and electro plasma system (EPS) taps provide energy for all ship systems in a shared load arrangement with the warp reaction core.ENGINEERING OPERATIONS AND SAFETY
All main impulse engine (MIE) and saucer module impulse engine (SMIE) hardware is maintained according to standard Starfleet MTBF monitoring and changeout schedules. Those components in the system exposed to the most energetic duty cycles are, of course, subject to the highest changeout rate. For example, the gulium fluoride inner liner of the impulse reaction chamber (IRC) is regularly monitored for erosion and fracturing effects from the ongoing fusion reaction, and is normally changed out after 10,000 hours of use, or after 0.01 mm of material is ablated, or if 2 fractures/cm measuring 0.02 mm are formed, whichever occurs first.
The structural IRC sphere is replaced after 8,500 flight hours, as are all related subassemblies. Deuterium and antimatter injectors, standard initiators, and sensors can be replaced during flight or in orbit without the assistance of a starbase.
Downstream, the accelerator/generator (A/G) and driver coil assembly (DCA) are changed out after 6250 hours, or if accelerated wear or specific structural anomalies occur. In the A/G, the normal need for changeout is brittle metal phenomenon resulting from radiation effects.
During flight, only the accelerator assemblies may be demounted for nondestructive testing (NDT) analysis.
Similarly, the DCA is subject to changeout after 6,250 flight hours. Normal replacement is necessitated by EM and thermal effects created by the driver coils. None of the DCA assemblies may be replaced in flight and all repair operations must be handled at a dock-capable starbase. The vectored exhaust director (VED) is serviceable in flight, requiring the least attention to deteriorating energy effects. All directional vanes and actuators may be replicated and replaced without starbase assistance.
Operational safety is as vital to the running of the IPS as it is to the WPS. While hardware limits in power levels and running times at overloaded levels are easily reached and exceeded, the systems are protected through a combination of computer intervention and reasonable human commands. No individual IPS engine can be run at >115% energy-thrust output, and can be run between 101% and 115% only along a power/time slope of t=p/3.
The IPS requires approximately 1.6 times as many man-hours to maintain as the WPS, primarily due to the nature of the energy release in the fusion process. The thermal and acoustic stresses tend to be greater per unit area, a small penalty incurred to retain a small engine size. While warp engine reactions are on the order of one million times more energetic, that energy is created with less transmitted structural shock. The major design tradeoff made by Starfleet R&D is evident when one considers that efficient matter/antimatter power systems that can also provide rocket thrust cannot be reduced to IPS dimensions.
While the Frontier class starship is the most advanced space vehicle in Starfleet's inventory, it is perhaps ironic that one of its most sophisticated systems can actually cause a number of annoying problems with extended use.
As fledgling journeys were made by fusion starships late in the twenty-first century, theoretical calculations concerning the tau factor, or time dilation effect encountered at appreciable fractions of lightspeed, rapidly crossed over into reality. Time aboard a spacecraft at relativistic velocities slowed according to the twin paradox.During the last of the long voyages, many more years had passed back on Earth, and the time differences proved little more than curiosities as mission news was relayed back to Earth and global developments were broadcast to the distant travelers. Numerous other spacefaring cultures have echoed these experiences, leading to the present navigation and communicationstandards within the Federation.
Today, such time differences can interfere with the requirement for close synchronization with Starfleet Command as well as overall Federation timekeeping schemes. Any extended flight at high relativistic speeds can place mission objectives in jeopardy. At times when warp propulsion is not available, impulse flight may be unavoidable, but will require lengthy recalibration of onboard computer clock systems even if contact is maintained with Starfleet navigation beacons. It is for this reason that normal impulse operations are limited to a velocity of 0.25c.
Efficiency ratings for impulse and warp engines determine which flight modes will best accomplish mission objectives. Current impulse engine configurations achieve efficiencies approaching 85% when velocities are limited to 0.5c. Current warp engine efficiency, on the other hand, falls off dramatically when the engine is asked to maintain an asymmetrical peristaltic subspace field below lightspeed or an integral warp factor. It is generally accepted that careful mission planning of warp and impulse flight segments, in conjunction with computer recommendations, will minimize normal clock adjustments. In emergency and combat operations, major readjustments are dealt with according to the specifics of the situation, usually after action levels are reduced.
IMPULSE ENGINE CONTROL
The impulse propulsion system is commanded through operational software routines stored within the spacecraft main computers. As with the warp propulsion system command processors, genetic algorithms learn and adapt to ongoing experiences involving impulse engine usage and make appropriate modifications in handling both voluntary external commands and purely autonomic operations. Voice commands and keyboard inputs are confirmed and reconciled by the current active main computer, and then handed off to the IPS command coordinator for routing to the engines for execution.
The IPS command coordinator is cross-linked with its counterpart in the WPS for flight transitions involving warp entry and exit. Specific software routines react to prevent field energy fratricide (unwanted conflicts between warp fields and impulse engine fields). The command coordinator is also crosslinked with the reaction control system (RCS) for attitude and translational control at all speeds.
IMPULSE ENGINE CONFIGURATION
The main impulse engine (MIE) is located on Decks 2 & 3 and thrusts along the centerline of the docked spacecraft. During separated flight mode, the engine thrust vectors are adjusted slightly in the +Y direction; that is, pointed slightly up from center to allow for proper center-of-mass motions.
Each impulse engine consists of three basic components: impulse reaction chamber (IRC, three per impulse engine), accelerator/generator (A/G), driver coil assembly (DCA), and vectored exhaust director (VED). The IRC is an armored sphere six meters in diameter, designed to contain the energy released in a conventional proton-proton fusion reaction. It is constructed of eight layers of dispersion-strengthened hafnium excelinide with a total wall thickness of 674 cm. A replaceable inner liner of crystalline gulium fluoride 40 cm thick protects the structural sphere from reaction and radiation effects. Penetrations are made into the sphere for reaction exhaust, pellet injectors, standard fusion initiators, and sensors.
The Frontier class normally carries four additional IRC modules primarily as backup power generation devices, though these modules may be channeled through the main system exhaust paths to provide backup propulsion.
Slush deuterium from the main cryo tank is heated and fed to interim supply tanks on Decks 7 & 8, where the heat energy is removed, bringing the deuterium down to a frozen state as it is formed into pellets. Pellets can range in size from 0.5 cm to 5 cm, depending on the desired energy output per unit time. A standing pulsed fusion shock front is created by the standard initiators ranged about the forward inner surface of the sphere. The total instantaneous output of the IRC is throttleable from 10Ž to 10¹ megawatts.
High-energy plasma created during engine operation is exhausted through a central opening in the sphere to the accelerator/generator. This stage is generally cylindrical, 3.1 meters long and 5.8 meters in diameter, constructed of an integral single-crystal polyduranium frame and pyrovunide exhaust accelerator. During propulsion operations, the accelerator is active, raising the velocity of the plasma and passing it on to the third stage, the space-time driver coils. If the engine is commanded to generate power only, the accelerator is shut down and the energy is diverted by the EPS to the ship's overall power distribution net. Excess exhaust products can be vented nonpropulsively. The combined mode, power generation during propulsion, allows the exhaust plasma to pass through, and a portion of the energy is tapped by the MHD system to be sent to the power net.
The third stage of the engine is the driver coil assembly (DCA). The DCA is 6.5 meters long and 5.8 meters in diameter and consists of a series of six split toroids, each manufactured from cast verterium cortenide 934.
Energy from the accelerated plasma, when driven through the toroids, creates the necessary combined field effect that (1) reduces the apparent mass of the spacecraft at its inner surface, and (2) facilitates the slippage of the continuum past the spacecraft at its outer surface.
The final stage is the vectored exhaust director (VED). The VED consists of a series of moveable vanes and channels designed to expel exhaust products in a controlled manner. The VED is capable of steerable propulsive and nonpropulsive modes (simple venting).
IPS FUEL SUPPLY
The fuel supplies for the IPS are contained within the primary deuterium tank (PDT) in the Battle Section and a set of thirty-two auxiliary cryo tanks in the Saucer Module. Redundant cross-feeds within both spacecraft and fuel management routines in the main computers perform all fuel handling operations during flight and starbase resupply stopovers.
While the PDT, which also feeds the WPS, is normally loaded with slush deuterium at a temperature of 13.8K, the cryo reactants stored within the Saucer Module tanks are in liquid form. In the event that slush deuterium must be transferred from the main tank, it is passed through a set of heaters to raise the temperature sufficiently to allow proper fuel flow with minimal turbulence and vibration.
As with the PDT, the auxiliary tanks are constructed of forced-matrix cortanium 2378 and stainless steel, laid down in alternating parallel/biased layers and gamma-welded. Penetrations for supply vessels, vent lines, and sensors are made by standard precision phaser cutters. They are installed by Fleet Yard transporters and may be transporter-removed for servicing at Starfleet maintenance docks. The internal volume of each auxiliary tank is 113 cubic meters and each is capable of storing a total of 9.3 metric tonnes of liquid deuterium.
Emergency flight rules allow for the injection of minute amounts of antimatter into the impulse reaction chamber in the event that short periods of overthrust or increased power generation are required.