I think the reason they initially design CZ-9 with boosters was so they could adjust the payload from 50 ton to 100 ton and 140 ton to LEO depending on the number of booster. Since the appearance of triple cored CZ-5DY (70 ton LEO) the scalability option is unnecessary so they change it to single cored full power version (150 ton LEO). Also if they want to send sample return mission to Mars they could use CZ-5DY instead of CZ-9B.
I think that is very much a plausible explanation.
Looking at all of China's forthcoming rocket projects, I think the three most important ones are CZ-5DY (70 ton LEO) and the single first stage CZ-5DY variant (20+ ton LEO), and CZ-9(21) (150 ton LEO), for space goals into late this decade and going into the 2030s.
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I consider them important because CZ-5DY, CZ-5DY single and CZ-9(21) all have the commonalities of having the potential to be adaptable for reusability of their first stage, and also because they slot neatly into what I consider to be useful load categories.
Furthermore, I think that consolidating their rockets into these two broad types -- CZ-5DY (single and triple) and CZ-9(21) -- can produce economies of scale if other rockets (CZ-7, current CZ-5 family, and CZ-8 and of course older rockets like CZ-3 family) are abandoned once they've fulfilled their roles this decade.
Of course, "fast reaction" rockets will still be retained (like CZ-11), and encouragement of commercial endeavours like the KZ family and other companies would still be useful.
But for major state launchers, I think the single CZ-5DY with 20+ ton LEO should be the "smallest" launcher they should pursue, with middle being the triple CZ-5DY 70 tons to LEO, and large being CZ-9(21) with 150 tons to LEO
The scientific and space exploration options that these categories will offer, naturally is significant.
But in terms of pursuit of technologies and ISR for military applications, it is also very significant.
The US has already broadcasted its intent to deploy a large number of LEO satellites for ISR purposes and for their missile and hypersonic defense purposes. I expect equally ambitious goals for GEO satellites as well, to become further solidified.
The limitations of satellites for ISR and defense purposes in LEO and GEO can basically be separated into:
1. number of satellites -- where more satellites provide better coverage (whether it's as a sensor, as a communications relay or whatever), as well as redundancy in case one is incapacitated
2. mass of each individual satellite -- where a larger satellite at any given orbit will inevitably be more capable than a smaller satellite at the same given orbit (due to a combination of a more powerful sensor, transmission/receiving capabilities, more propellant, etc -- which of course are all dependent on the satellite's inherent mass)
1. and 2. are both limited by the throw weight and cost of the rocket transporting it.
Greater throw weight means you can launch a larger number of smaller satellites in the same launch, or you can launch a smaller number of larger rockets in the same launch.
The cost of the rocket (which can be lowered by reusability) in turn means you can do more launches overall.
The ability to regularly put up 20 tons, or 70 tons or 150 tons into LEO, massively broadens the scope of the capabilities that you can bring to the table both in terms of LEO satellite swarms and more capable individual LEO satellites.
The ability of those same rockets to put payloads into GEO is even more consequential due to their ability to constantly observe a region of earth, but are currently limited by resolution -- currently the most capable GEO satellite China arguably has is the 4.6 ton GF-4 with an EO sensor, launched by a CZ-3B.
Imagine what a CZ-5DY with 70 ton to LEO could launch for GEO, or what a CZ-9(21) with 150 ton to LEO could launch for GEO, and how much more capable their payloads could be compared to current CZ-3 and CZ-5 families -- how regularly they could launch, if their first stages were recoverable?
Then remember that the US is already closing in on that capability with Falcon 9, Falcon Heavy and Starship/Super Heavy, and that is why I see the likes of CZ-5DY family and CZ-9(21) as very important.
Furthermore, if the unthinkable happens and the US pursues full fledged weaponization of space to kill each other's space based assets or to even use space based platforms against targets on earth (rather than only using space as a means of ISR/networking/navigation), you can bet that the likes of Starship and Falcon Heavy would be involved in deploying those assets.
Now, obviously I am being somewhat pessimistic here in projecting a worst case scenario regarding the militarization of space. However, I think it would be irresponsible to talk about these new launch vehicles from the US/SpaceX without considering their vast potential for achieving superiority in space based systems (even if it is "only" ISR/networking/navigation purposes), and China's likely assessed need to at least match them.
