This is going back a little while, however this photo was brought to my attention just today, which was probably the basis for why I believed the 7m UVLS was capable of accommodating HHQ-9 back during this discussion.

In this image HQ-9 has a length of some 6.5m at most, which I believed would've been sufficient to accommodate a cold launch mechanism. The 6.8m count was likely from old brochures of FT-2000.

Two problems. The cold launch bottle of the S-300, in which the HQ-9 system maybe copied from, is at least a meter. The ship VLS might possibly have a higher clear requirement than the land based VLS in case of a launch failure but who knows? Partly because it is already a proven system, the naval VLS may still be using the exact part used for the land based HQ-9 which in turn may just be a direct copy of the system used on the S-300 which is also like the one you see here on the bottom. Look at the 9M96 missiles here and despite the missiles are much smaller, they still use the same bottle from the S-300 because well, its already available and proven with countless firings, with no need for reinvention.

The second is that the VLS must have substantial growth overhead for future missiles, and that includes SAMs. The above may just be the base version of the HQ-9, but what about HQ-9B and future variants? There is a size growth between the 5V55 and the 48N6E missiles for the S-300. The most common way for a missile to extend its range is length growth. See the various members of the 9M96 missile family from the shorter ranged 9M96 to the long range 9M96E2.

Even with hot launched missiles, you still need significant clearance. The Strike version of the MK. 41 was intended to launch the Tomahawk missile. The length of the VLS is 6.8 meters long. The length of the Tomahawk, including booster, is 6.25m. The Tactical version of the Mk. 41 was created to launch the SM-3 and SM-6 missiles. The length of the Tactical version is 7.7m. The SM-6 is 6.6 meters long and the SM-3 is 6.55 meters long. And these are for hot launched missiles. The SCALP naval missile for the A70 Sylver is 6.5 meters long, and the A70 is 7 meters. That's about right for a .5 meters clearance but this is also a hot launch system.

My theory for the supposed "length" of the U-VLS is that 3.3m, 7m, and 9m quoted is not the physical length of the VLS themselves, but the maximum length allowed for the missiles for each tier of U-VLS, with their actual physical length exceeding the specificied maximum length of the missiles. Note that 3.3 meters makes absolutely no sense of a VLS length, given that a PL-12, without the booster needed for a surface variant, would already be at least 3.7 meters in length. The only missile that I can think that could fit this is the CAMM at 3.2 meters, and that's British. If the VLS is 3.3 meters long, then even CAMM would not fit, as it is cold launched.