Vents vs Spheres.

[MAGolding]

A different take on this:

In the TOS timeline space travel technology advanced faster than the TNG timeline. So in 1960's TOS had orbital nuclear weapons, first manned moon mission. By the 1990's they were already exploring and colonizing nearby star systems with atomic powered DY-series sleeper ships. Colonies are already in place in Alpha Centuri and the system that has the planet Canopius (at the very minimum). Aliens like the Vulcans, etc are already encountered. By 2018, faster propulsion (impulse power, IMHO) becomes available eliminating the need for sleeper ships.

One of these newer and faster ships, the SS Valiant, travels to the galactic edge and is swept up in a magnetic storm and carried half a light year past the barrier. Since we know that the ESP abilities manifest in days then in order to get back past the barrier from a distance of 0.5 light year in a short enough period of time before the ESPer was unstoppable by ship self-destruct the Valiant would have had to travel at FTL speeds (like 100c to cross .5LY in 3 days). We also know from "Mirror, Mirror" that "magnetic storms" are the same as "ion storms" and in "Court Martial" you can fly through an ion storm with warp drive so if the Valiant had warp drive then that would have been the go-to to get out of the storm instead of the weaker impulse engines of the day. That leaves just impulse engines or an unknown other type of FTL drive that isn't warp drive for FTL travel for the Valiant.

Sometime after SS Valiant goes exploring, Zefram Cochrane discovers the "space warp" in the Alpha Centuri system. The technology spreads across the "known galaxy".

In the TNG timeline space technology development is significantly slowed. There are no colonies offworld and other races already have space warp technology. Zefram Cochrane builds Earth's first warp drive in 2064? and the Vulcans make first contact. Most of the TOS episodes happen differently and at a much later date. There is no SS Valiant lost at the galactic barrier.

YMMV

There is one big problem with:

By the 1990's they were already exploring and colonizing nearby star systems with atomic powered DY-series sleeper ships. Colonies are already in place in Alpha Centuri and the system that has the planet Canopius (at the very minimum).

Kirk & Spock:

KIRK: Kirk out. Seventy two alive. A group of people dating back to the 1990s. A discovery of some importance, Mister Spock. There are a great many unanswered questions about those years.
SPOCK: A strange, violent period in your history. I find no record what so ever of an SS Botany Bay. Captain, the DY-100 class vessel was designed for interplanetary travel only. With simple nuclear-powered engines, star travel was considered impractical at that time. It was ten thousand to one against their making it to another star system. And why no record of the trip?

DY-100 class ships like the Botany Bay were not designed for interstellar travel. Possibly DY-200 or DY-300 class vessels were designed for interstellar travel and were already in use by the 1990s.

But when Kirk thinks the Botany Bay could be a DY-500 class ship:

IRK: An old Earth vessel, similar to the DY=500 class.
SPOCK: Much older. DY-100 class, to be exact. Captain, the last such vessel was built centuries ago, back in the 1990s.

So it is unlikely that more than one or two later classes of DY ships were available in the 1990s. Khan would take the most advanced ship he could get.

And:

MARLA: Captain, it's a sleeper ship.
KIRK: Suspended animation.
MARLA: I've seen old photographs of this. Necessary because of the time involved in space travel until about the year 2018. It took years just to travel from one planet to another.

So it took years to get just from one planet to another during the 1990s. Neptune orbits the Sun at a distance of 29.81 to 30.33 Astronomical Units (AU) while the former planet of Pluto orbits the Sun at a distance of 29.658 to 49.305 AU. A light year is 63,421 AU and Alpha Centauri, the nearest star, is about 4.3 light years from Earth, or about 5,531 to 9,195 times the distance to the farthest planets in our solar system. Thus it should have taken thousands of years to reach even the nearest stars with the ships available by 2018, and so no colonization of other stars should have been possible in the 1990s.

Of course, I did once have a theory that in Star Trek Earth discovered methods of faster than light (FTL) travel twice. The first time a secret group secretly built starships and left Earth to explore and to settle a colony in another star system from which they continued to explore the stars, leaving no record of this on Earth. Tarbolde who wrote Nightingale Woman on the Canopius Planet in 1996 was probably one of them. The Valiant was one of their ships.

Meanwhile on Earth the Eugenics Wars happened and interplanetary travel took years, and Khan left Earth in suspended animation hoping to reach another star.

In 2018 Earth based space travel got much faster, so Earth developed FTL star travel for the second time in 2018 or later, and began to explore the stars, and eventually the explorers from Earth and the explorers from the Earth colony met and united sometime before Kirk's era. If one group had "broken the time barrier" and the other hadn't, the reunion might have happened between the departure of the Columbia and "The Cage".

When I believed that all dates in Star Trek were given as Anno Domini dates, I calculated that the Valiant should have been voyaging sometime between AD 1896 and AD 1996, so I came with with the theory that Earth discovered FTL interstellar travel twice, and the first time it was kept a secret from the rest of Earth.

But now I believe that Earth dates in Star Trek are given in several different dating systems, so that the year 2100 in one story is not necessarily after the year 2000 in another story. Thus it is not no necessary to assume that Earth developed FTL star travel twice, though some Star Trek chronologists might find it useful.

 

[Henoch]

If there was another FTL technology, does Kirk's Enterprise has 2 FTL technologies signified by sphere tailpipes on the nacelles (newest tech warp drive) and vents on the saucer (older tech impulse drive). Kirk's Enterprise also has 2 power systems intertwined with each engine system: M/AM reactors (regenerative, high power) and Mr. Fusion reactors (fuel consuming, lower power). Impulse engines are primarily used for STL speeds around planets. Going FTL with impulse engines really burns the fuel reserves (WNM, Mudd's Women, BoT, Doomsday Machine, etc.)

 

[blssdwlf]

There is one big problem with:
Kirk & Spock:
DY-100 class ships like the Botany Bay were not designed for interstellar travel. Possibly DY-200 or DY-300 class vessels were designed for interstellar travel and were already in use by the 1990s.
But when Kirk thinks the Botany Bay could be a DY-500 class ship:
So it is unlikely that more than one or two later classes of DY ships were available in the 1990s. Khan would take the most advanced ship he could get.
...
So it took years to get just from one planet to another during the 1990s. Neptune orbits the Sun at a distance of 29.81 to 30.33 Astronomical Units (AU) while the former planet of Pluto orbits the Sun at a distance of 29.658 to 49.305 AU. A light year is 63,421 AU and Alpha Centauri, the nearest star, is about 4.3 light years from Earth, or about 5,531 to 9,195 times the distance to the farthest planets in our solar system. Thus it should have taken thousands of years to reach even the nearest stars with the ships available by 2018, and so no colonization of other stars should have been possible in the 1990s.

All very good points. Now looking more into this and have some revisions to my take on this...

Tarbolde was most likely written by an alien on the planet Canopius. The love sonnets are translated.

"Where No Man Has Gone Before":
MITCHELL: My love has wings. Slender, feathered things with grace in upswept curve and tapered tip. The Nightingale Woman, written by... by Tarbolde on the Canopius planet back in 1996. It's funny you picked that one, Doctor.
​

We have a rough idea that in TOS Earth history that they had launched orbital nuclear warhead platforms and went to the moon in the late 1960's. Colonel Christopher was probably in his 30's for the first successful Saturn probe putting that also in the 1990's to early 2000's. Sometime before the Saturn probe probably were missions to Mars. We can assume trips to the Moon and nearby Mars (1.52 AU from Sun) were the quickest (days to months) and trips to the outer planets took years. As dialogue indicated, "star travel was impractical in the 1990's." One must wonder how many DY-100s were built. And if this was their most advanced at the time there probably were lesser DY-series ships around.

Around 2018 is when a faster drive went mainstream. Since it doesn't seem to be as important as the discovering of the "space warp" it is most likely something else like impulse technology. It's so fast that the need for sleeper ships went away probably making travel to the nearest stars practical. (I'm in the camp that TOS Impulse was capable of FTL.)

The SS Valiant is one of these fast ships and made a run to the edge of the galaxy.

A little while later, Zefram Cochrane, (an an alien living in the Alpha Centuri system that met Earth and Vulcan people? Or an Earth person that was living in a colony in the Alpha Centuri system?) discovers the "space warp" and helps usher into the known galaxy a faster or perhaps more efficient form of FTL travel.

YMMV

As to Vents vs Spheres on the end, there probably isn't a pattern to when they rotate in and out so it's probably something in-universe that engineering is doing that we don't have a line-of-sight to...

"Assignment Earth":

Captain's log. ...We are monitoring Earth communications to find out how our planet survived desperate problems in the year 1968.
...
KIRK: Mister Spock, historical report.
SPOCK: Current Earth crises would fill a tape bank, Captain. There will be an important assassination today, an equally dangerous government coup in Asia, and, this could be highly critical, the launching of an orbital nuclear warhead platform by the United States countering a similar launch by other powers.
KIRK: Weren't orbital nuclear devices one of this era's greatest problems?
​

"Tomorrow is Yesterday":

KIRK: Manned Moon shot? That was in the late 1960s.
...
SPOCK: Poor choice of words on my part. I neglected, in my initial run-through, to correlate the possible contributions by offspring. I find, after running a crosscheck on that factor, that your son Colonel Shaun Geoffrey Christopher headed, or will head, the first successful Earth-Saturn probe, which is a rather significant
​

"Return to Tomorrow":

KIRK: They used to say if man could fly, he'd have wings. But he did fly. He discovered he had to. Do you wish that the first Apollo mission hadn't reached the moon, or that we hadn't gone on to Mars and then to the nearest star? ...
​

"Space Seed":

KIRK: An old Earth vessel, similar to the DY-500 class.
SPOCK: Much older. DY-100 class, to be exact. Captain, the last such vessel was built centuries ago, back in the 1990s.
...
SPOCK: Hull surface is pitted with meteor scars. However, scanners make out a name. SS Botany Bay.
KIRK: Then you can check the registry.
SPOCK: No such vessel listed. Records of that period are fragmentary, however. The mid-1990s was the era of your last so-called World War.
...
SCOTT: Definitely Earth-type mechanism, sir. Twentieth century vessel. Old type atomic power. Bulky, solid.
I think they used to call them transistor units. I'd love to tear this baby apart.
MARLA: Captain, it's a sleeper ship.
KIRK: Suspended animation.
MARLA: I've seen old photographs of this. Necessary because of the time involved in space travel until about the year 2018. It took years just to travel from one planet to another.
...
KIRK: Scotty, any records, log books of any kind?
SCOTT: Negative, Captain. It appears they were in suspended animation when the ship took off.
KIRK: How many alive?
SCOTT: Twelve units have malfunctioned, leaving seventy two still operating. Thirty of those are women.
KIRK: Kirk out. Seventy two alive. A group of people dating back to the 1990s. A discovery of some importance, Mister Spock. There are a great many unanswered questions about those years.
SPOCK: A strange, violent period in your history. I find no record what so ever of an SS Botany Bay. Captain, the DY-100 class vessel was designed for interplanetary travel only. With simple nuclear-powered engines,
star travel was considered impractical at that time. It was ten thousand to one against their making it to another star system. And why no record of the trip?
...
SPOCK: Your Earth was on the verge of a dark ages. Whole populations were being bombed out of existence.
A group of criminals could have been dealt with far more efficiently than wasting one of their most advanced spaceships.
...
KIRK: Name, Khan, as we know him today. Name, Khan Noonien Singh.
SPOCK: From 1992 through 1996, absolute ruler of more than a quarter of your world. From Asia through the Middle East.
MCCOY: The last of the tyrants to be overthrown.
​

"Metamorphosis":

COCHRANE: You speak English. Earth people?
...
COCHRANE: You're a Vulcan, aren't you?
...
KIRK: Zefram Cochrane of Alpha Centuri, the discoverer of the space warp?
COCHRANE: That's right, Captain.
MCCOY: But that's impossible. Zefram Cochrane died a hundred and fifty years ago.
SPOCK: The name of Zefram Cochrane is revered throughout the known galaxy. Planets were named after him. Great universities, cities.​

 

[Henoch]

MARLA: Captain, it's a sleeper ship.
KIRK: Suspended animation.
MARLA: I've seen old photographs of this. Necessary because of the time involved in space travel until about the year 2018. It took years just to travel from one planet to another.

The films 2001 and 2010 both used sleeper ships to get to Jupiter. The Discovery and Leonov were more advanced than the DY-100. The Americans and Soviets are on the brink of another nuclear war in 2010.

I prefer our altered timeline, where instead, the Soviet Union fell in late-1991 and eugenics were not pursued avoiding a nuclear war and tyranny in the mid-1990's. As a result, it will take us many more years to catch up with space technology as shown in TOS and 2001.

 

[uniderth]

My thoughts are that the 2018 innovation was applicable in the Solar system "getting from one planet to another". However, sleeper ships still would have been required for travel to other stars, IF any such attempts were ever made prior to warp drive. Also all the references to poems and books being written on other planets back in the 1990s doesn't necessarily mean contemporary humans used FTL drives to colonize other worlds and then wrote them. They could have been written by aliens or humans transplanted from our past. And then the poems gained interplanetary fame after we got the spacewarp from Zefram.

Also on the note about Alpha Centauri vs Alpha Centuri; I pronounce it Beytulgeez(hard G as in game) not Beatlejuice, so just becasue Kirk pronounces it differently doesn't mean its a different place.