> armour underneath a tank is usually much lighter, because it's difficult to hit. Missiles have to do fancy armour-penetration stuff to get through the main armour on a tank. If you can get a shaped charge stuck to the underside of a tank, you don't need to do the fancy stuff.
I'm not even sure if the air-pressures under a tank would allow a drone to fly under there. Let alone the myriad of anti-air defenses a modern tank has.
The M1A1 Abrams tank has not only the main cannon, but also a .50 Caliber M2 Browning and two M240 machine guns. In addition, there's a general expectation that a tank would be surrounded by supporting troops and equipment (and vice versa: the tank supports the troops, as per the theory of Combined Arms)
So a tank, facing a swarm of drones, probably can just fire its two machine guns and take them out. Its not like drones have any armor of any kind. Similarly, anti-personnel rifles from the supporting infantry would probably be effective against those drones.
Once we start considering weapons like air-burst grenades, I'm finding it less-and-less likely that a "10,000 Swarm" makes any sense what-so-ever. The __reason__ we mix tanks-with-infantry is because a singular weapon (ex: air-burst grenade) works only on infantry (tank armor is too thick), while a Tandem-shaped charge RPG only really works on tanks (Personnel are relatively cheap and move in groups. If you kill one, the rest of their buddies gang up and kill you).
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Any scenario where you're just lobbing uniformly made drones into the same area just opens them up to air-burst or machine gun fire.
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EDIT: I had a lot of unnecessary words. Lets just point this out... I'm not sure if you understand how good machine guns are today.
https://en.wikipedia.org/wiki/Phalanx_CIWS
We have machine guns that can autonomously aim and shoot-down a group of missiles that travel at 1500 mph (Mach 2). And you think a swarm of 10,000 drones flying at lol 100 mph has a chance?
https://www.youtube.com/watch?v=IB8d3OaFEco
Depending on the armament: you could have up to 9000 "pellets" per shot that airburst at the expected distance to maximize the chance of hitting the target.
If a missile flying at 1500mph has no chance, why do you think a drone at 100 chance or 200 mph has a chance?
Israel's "Iron Dome" machine guns: https://www.youtube.com/watch?v=gGaqBWXM8Ko
These platforms are "portable", for a definition of portable.
https://www.youtube.com/watch?v=iAw82h-IhdQ