Impulse engines are complex deuterium fusion reactors, generating
energy by pulsed fusion reactions of accelerated deuterium pellets.
Fuel is stored safely stored in shielded containers adjacent to the
These reactors provide thrust by exhausting fusion plasma into an
accelerator, which provides delta-vees of up to 900 m/sec^2 (or 90
times standard gravity). This acceleration is normally compensated
for by the Inertial Dampening System. While the amount of power they
generate is minor compared to the warp drives, it is an important
backup since warp drives occaisionally must be shut down.
TM: (In the accelerator/generator) Magnetohydrodynamic and
electroplasma system taps provide energy for all ship systems in a
shared load arrangement with the warp reaction core.
TM: (In the driver coil assembly) 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 impulse engines on the Galaxy class starships are different in design
from the Constitution class vessels in TOS. The Galaxy class impulse
propulsion system (IPS) generates a subspace distortion field, as does the
warp propulsion system, but the IPS field is induced by an energy flux
several orders of magnitude smaller than that of the warp engines. Because
of this, the warp field produced by the IPS does not have the power to
propel the ship faster than light.
Starfleet operational guidelines restrict impulse operations to a small
fraction of the speed of light to reduce relativistic effects.
So impulse engines are true Newtonian (non-warp) engines. They are powered
by sequential fusion reactors that fuse the slush deuterium into helium,
and the high-energy exhaust is expeled out the back, utilizing a
vector-controllable nozzle. This was added to the TNG ships (the impulse
thrust alone was no longer sufficient to propel the larger ship). Thus,
fusion exhaust products from the impulse engines are used to energize a
small warp coil which produces a non-propulsive warp field. This field
effectively lessens the mass of the ship and means the impulse engines have
less work to do.
There actually is a warp field generated, but it is a non-propulsive field
which only reduces the effective mass of the ship and gives the impulse
engines less of a job to do.
Some sources say that impulse engines must be used near inhabited planets
and other systems, because the field created by the warp drives would tear
the area apart.
TM: "During normal
docking operations the main impulse engine is the active device, providing
the necessary thrust for interplanetary and sublight interstellar flight.
High impulse operations, specifically 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 (See: 6.2)"
And section 6.2 goes on to say: "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 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 velocity of 0.25c."
John H. Kim <jhkim-at-darkshire-dot-net>
Last modified: Mon Jan 22 18:28:07 CST 1996