Sorry, what I meant was all ships that bear that hooded engine design might be newer than the Connie engine. The issue is less clear with the Shepard-class, etc. But since that line indicates the Discovery is newer, perhaps the hooded bussard collector is closer to The Motion Picture. And as someone previously said, there may have been a refit throughout the fleet, even if classes like the Shepard are older.
As has been said, there's no reason they can't be contemporaries. The teeny domes on the DSC ships remind me a bit of some cutaway drawings that show
24th century ships like the Sovereign have multiple small TOS-style nacelle caps inside their bussard collectors. The DSC-style engines could be the first step to "enclosed" nacelle caps and the other machinery, but they still don't work quite as well, so more powerful but less advanced "exposed" engines are still preferred for some designs, like the Constitution, where speed and power are at a premium, even if they're less efficient and require constant maintenance. It'd be analogous to the development of the electric car, where even though higher performance is possible than using a gasoline engine, the first generations of the technology can't match the older, more refined version in practice quite yet, but it's still being rolled out in parallel because that's the direction the proverbial puck is going. By TMP, "enclosed" warp nacelles can match or exceed the performance of cylindrical ones, so they begin rolling them out on ships that need the performance, starting with the
Enterprise
As for the
Constitution itself, "Lethe" made it seem like those ships were prestige postings and officers who made it on board were considered to be on the fast-track to career advancement, and both Discovery novels (produced with closer-than-usual coordination of the TV production team) are very clear that the
Constitution-class is the Cadillac of space, brand-new and completely tricked-out and overpowered compared to anything else in the fleet (the
Shenzhou, specifically).
Discovery is said to be a new ship, but unless I'm forgetting a line (please tell me), that assessment is made by a non-Starfleet spree killer commenting on how clean the floors were. I take it that the base
Crossfield design is a bit older than the
Shenzhou and related classes, but
Discovery and
Glenn were gutted to the rafters
a la TMP to be fitted with spore-drive equipment. They don't need big, powerful, finicky cylindrical engines because it's a science ship, so it'll spend most of its time in one place or another and not racing between crises, and, of course, if the spore drive is a success, warp drive as a whole will be obsolete, anyway.
is using the Kelvin as his transport. Like the Federation's version of Air Force One.
