Star Trek's Enterprise: Main Impulse Engines

Galaxy Class Starship
U.S.S ENTERPRISE NCC-1701-D
Main Impulse Engines

Impulse Drive

The principal sublight propulsion of the ship and certain auxiliary power generating operations are handled by the impulse propulsion system (IPS). The total IPS consist of two sets of fusion-powered engines: the main impulse engine, and the Saucer Module impulse engines. During normal docked operation the main impulse engine is the active device, providing the necessary thrust for interplanetary and sublight instellar flight. High impulse operations, while acceptable option during some missions, are often avoided due to relativistic considerations and their inherent time-based difficulties.

Main Impulse Engine Located On Aft Dorsal

During the early definition phase of the Ambassador class, it was determined the 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 fusion engines available or projected fell far short of being able to achieve the 10km/sec² acceleration required. This necessitated the inclusion of a compact spacetime driver coil, similar to those standard in warp engine nacelles, that would perform a low-level continuum distortion without driving the vehicule 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 lightspeed.

Experimental results with exhaust products temporarily accelerated beyond lightspeed yielded disappointing results, due to the lack of return force to the engine frame. The work in this area is continuing, however, in an effort to increase powerplant performance for the 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 ingine 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.

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 performs all fuel handling operation during flight and starbase resupply stopovers. While the PDT, which also feeds the warp propulsion system (WPS), is normally loaded with slush deuterium at a temperature of 13.8K, the cryo reatancts stored within the Saucer Module tank 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.

Impulse Drive Systems

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 required. The main impulse engine is supplied by the Battle Section's antimatter storage facility on Decks 41 and 42. The Saucer Module impulse engines are supplied by two dedicated antimatter storage on Deck 10. There is no transfer capability of antimatter between the two vehicles (See: Antimatter Storage And Transfer).

Impulse Engine Configuration

The main impulse engine (MIE) is located on deck 23 and thrust along the centerline of the docked spacecraft. During separated flight mode, the engine thrust vectors are ajusted slightly in the Y+ direction; that is, pointed slightly up from the center to allow the proper center-of-mass motion (See: Engineering Operation and Safety). The Saucer Module impulse engines are located on Deck 10 on the vehicle XZ plane and thrust parallel to the vehicle centerline.

Four individual impulse engines are grouped together to form the MIE, and two engines form the Saucer Module impulse engines. Each impulse engines consist of three basic components: impulse reaction chamber (IRC, three per impulse engines), 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 on 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.

Typical Impulse Fusion Reactor

The Galaxy 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 tank on Deck 9, where the heat energy is removed, bringing the deuterium down to a frozen state as it is formed into pellets. Pellets can range 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 10E8 to 10E11 megawatts.

High-energy plasma created during engine operation is exhausted through a central opening in the sphere to the acceleratior/generator. This stage is generally cylindrical, 3.1 meters long and 5.8 meter 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.

Impulse Drive System

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 neccessary combined field effect that (1) reduces the apparent mass of the spacecraft at its inner surfaces, 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).

Impulse Engine Control

The impulse propulsion system is commanded through operational software routine 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 modification in handling both voluntary external commands and purely autonomic operations. Voice command 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 routine 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.

Relativistic Considerations

While the Galaxy class starship is the most advanced space vehicule 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 fraction 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 communication standards 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 lengthly 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 vilocity 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 intergral 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.

Engineering Operations And Safety

All main impulse engine (MIE) and saucer module impulse engine (SMIE) hardware is maintained according to standard Starfleet mean-time between failure (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 minitored for erosion and fracturing effects from the ongoing fusion reaction, and is normally changed out after 10,000 hours of use, or after 0.01mm of material is ablated, or if >= 2 fractures/cm³ measuring 0.02 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 starbase.

IPS Fusion Reactor Replacement

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 bristtle 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 be 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, requirering the least attention to deteriorating energy effects. All directional vanes and antuators may be replicated and replaced without starbase assistance.

Operational safety is as vital to the running of the IPS as it is to the warp propulsion systems (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 run at > 115% only along a power/time slope of t = p/3.

The IPS requires approximately 1.6 times as many manhours 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 on 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.

Emergency Shutdown Procedures

Hardware failures and override commands can place abnormal stress on the total impulse propulsion system (IPS), requiring various degrees of engine shutdown. System sensors, operational software, and human action work in concert to deactivate impulse propulsion system components under conditions such as excessive thermal loads, thrust imbalance between groups of individual engines, and a variaty of other problems.

The most common internal causes for low-level emergency shutdown in Starfleet experience include fuel flow constriction, out-of-phase initiator firings, exhaust vane misalignment, and plasme turbulence with the accelerator stage. Some external causes for shutdown include asteroidal material impacts, survivable combat phaser fire, stellar thermal energy effects, and crossing warp field interaction from other spacecraft.

Emergency shutdown computer routines generally involve a gradual valving off of the deuterium fuel flow and safing of the fusion initiator power regulators, simultaneously decoupling the accelerator by bleeding residual energy into space or into the ship's power network. As these procedures are completed, the driver coil assembly (DCA) coils are safed by interrupting the normal coil pulse order, effectively setting them to a neutral power condition, and allowing the field to collapse. If the shutdown is in an isolated engine, the power load distribution is reconfigured at the first indication of trouble.

Variation on these procedures are stored within the main computer and IPS command coordinators. Crew monitoring of a shutdown is a Starfleet requirement, although many scenarios have seen engines being safed before reliable human reactions could be incorporated. Voluntary shutdown procedures are dependable and accepted by the main computer in 42% of the recorded incidents.

Catastrophic Emergency Procedures

As with the warp propulsion systemm, the IPS may sustain various degrees of damaged requiring repair or deliberate release of the damaged hardware. Standard procedures for dealing with major vehicule damage apply to IPS destruction and include but are not limited to: safing any systems that could pose further danger to the ship, assessing IPS damage and collateral damage to the ship structures and systems, and sealing off hull breaches and other interior areas which are no longer habitable.

Deuterium and fusion-enhancement antimatter reactants are automatically valved off at points upstream from the affected systems, according to computer and crew damage control assessments. Where feasible, crews will enter affected areas in standard extravehicular work garments (SEWG) to assure that damaged systems are rendered totally inert, and perform repairs on related systems as necessary. Irreparably damaged IPS components, starting with the thrust vents and moving inboard to the drive coils and reaction chambers, can be taken off-line and released if their continued retention adversely affects the integrity of the rest of the starship.

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Copyright © 1997 Tan Ngo-Dang Contact: tangowebmedia@sympatico.ca URL: http://www.tht.net/~tan/ncc1701d/impulse.htm	Created on 08/03/97 Updated on 12/11/97 Page status: final