This is a huge contradiction, based upon what's been overtly stated on-screen and in the "canon" tech manual, which states quite clearly that impulse drive is purely sublight.
However, as some of you are aware, I'm one of those who is convinced that it must be possible for "impulse" to be FTL as well, under certain circumstances.
We can't "remember" this because it's never been established on-screen or in any official or even pseudo-official fashion.
However... and if you're interested in reading more about my take on it, check out this thread:
http://www.trekbbs.com/threads/showflat....rt=all&vc=1
I'm convinced that all this fits together, and that in fact impulse drive CAN (in conjunction with other hardware) produce FTL travel.
Evidence? (All inconclusive, mind you...)
1) The Romulan War was fought over interstellar distances using ships that were limited to "simple impulse." The "Balance of Terror" ship also was "simple impulse" but clearly was not limited to sublight. Not to mention that it has what are very clearly intended to be subspace field generation nacelles (key point there in my choice of terms).
2) It's established, semicanonically, that TOS shuttlecraft were "impulse only," though again they DO have external nacelles which I believee are for subspace field generation. We've seen Kirk and Commodore Mendez in pursuit of a warping 1701 in a shuttlecraft, and we've seen a shuttlecraft moving in a measureable fashion relative to a quasar, neither of which makes ANY sense if you're talking about being limited to sublight only.
3) We know that in "Where No Man Has Gone Before" the Enterprise is damaged and is incapable of warp drive. Yet she is still able to travel across interstellar distances to get to the lithium cracking station on Delta Vega. (Note that there is no visible evidence of damage to the nacelles, just to the hardware CONTROLLING them. Lots of controller computers, mainly.)
4) We know that at some point within the prior 20 years or so before "The Cage" there was a breakthrough in FTL propulsion. According to Jose Tyler, "the time barrier's been broken" and the new ships with the new propulsion system can travel incredibly fast compared to the prior designs. Yet clearly the prior designs had SOME form of FTL propulsion. Note, also, that they were still using overtly technical terminology to discuss the propulsion system on Enterprise when Pike was in command... "time warp factor five" instead of just "warp factor five" or so forth.
5) In TNG we see that it's possible to maintain "FTL momentum" without warp engines... by handing off and maintaining a subspace field (first seen in "farpoint"). You simply need a "sustainer coil" to keep the handed-off field from decaying too much.
6) In TNG it's established that a subspace field can be used, even far away from the source of the field, to reduce the "apparent mass" of an object (allowing the Enterprise to move a massive asteroid, as I recall).
7) In DS9, we see the station moved at very high speeds in a very short time (speeds that seem obviously to exceed the speed of light, though it's not overtly stated as such). They do this by creating a stable subspace bubble around the ship and using the thrusters to move it within that field.
In TAS, we see the Bonaventure, the first ship fitted with warp drive. It's not as ancient as you'd expect and it does have a STARFLEET history and crew... not just a "testbed."
So... all this leads me to believe the following:
1) Early FTL travel was obtained by generating a subspace bubble around the vessel, which had the effect of vastly reducing the "apparent mass" of the vessel relative to the "normal universe" outside the bubble, and also of increasing, dramatically, the speed of light relative to the outside unverse. This bubble created NO PROPULSIVE EFFECT OF ITS OWN, note. But by creating this bubble, you can then use "simple impulse" and propel yourself at speeds well beyond the speed of light in the "real" universe.
2) There is an upper limit of speed within this method of propulsion, however... commonly called the "time barrier." Based upon some prior published work, as well as some common-sense analysis (not into TOO much theoretical detail mind you), I've always accepted that this is about 75C, or (on the TOS warp scale, no the TNG one) roughly the equivalent of "warp 4.2."
3) The idea behind TOS-style warp drive is that you create an assymmetrical subspace bubble. The fabric of spacetime "presses" on this bubble, trying to make it perfectly spherical, and that "pushes" the ship inside of the bubble through real space at FTL speeds in a totally non-newtonian manner.
This was probably discovered by accident, as most breakthroughs are. Some ship probably had an accident that resulted in it's subspace field generator accidentally creating an assymetrical field, and as a result the ship found itself "slipping downhill" in an uncontrolled FTL slide until they were able to disperse their field.
So... in order to go FTL, a ship would need to be able to create a stable subspace bubble (which is probably pretty easy, relatively speaking) and just use thrust-based engines inside there. But to go to WARP, you'd need to have a tremendously powerful control system to perform the subtle manipulations of the subspace field to allow it to controllably "warp" space/time locally and result in this form of propulsion.
As time went on, they played with adding more and more field generation elements... see the Tritium and the Federation class. These didn't provide the hoped-for improvements in propulsion efficiency, but they did yield information about the effects of interacting warp bubbles. Which led directly to the next major breakthrough in subspace-field-based travel:
4) Linear warp drive. Linear drive was first put into production use around the time of TMP. This was a totally different propulsion system, though based upon the same physics. In this case, the propulsion system was based upon a string of field generation coils creating NESTED, expanding bubbles of subspace, rather than having to create a stable bubble (which is probably much more power-intensive to do). The linear offset between one bubble and the next one inside of it gives a stronger field density at one end of the "composite bubble" than at the other, and by having multiple ROWS of these coils as well, you could not only have fwd and aft translation but also roll, pitch and yaw in a purely non-Newtonian fashion.
This is the same form of propulsion, without major change, in use during the TNG era. However, at some point between TMP and TNG it was determined that the energy usage of the new system was significantly different than that of the original "distorted bubble" warp drive technology, so eventually they recalibrated the scale. Around the time of TNG, however, they're seeing the folly of this recalibration, since most travel seems to occur in the asymptotic range where you increase speed vastly from WF9.95 to WF9.951, for instance. So eventually, they'll undo this politically-driven SNAFU and go back to a more reasonable scale, possibly the original TOS-type scale (which was based upon relative real-space velocity, not shipboard energy consumption as the TNG scale was).
ANYWAY... my point is that "Warp Drive" refers ONLY to the "subspace field warpage" form of FTL travel, but that does not refer to the only form of FTL travel, just the most effective and efficient one at the time of TOS/TNG/NEM.
"Simple impulse" based FTL travel is possible, and IS simple compared to warp drive, but still requires the generation of a stable subspace bubble around the ship.
That's what my "ringship" uses... a massive fusion impulse drive with a redundant pair of "simple subspace bubble" generators.
Sources like the Encyclopedia, though not perfect, at least have the benefit of being compiled by staff who actually worked on the show.
