>Even e.g. a cargo 747 has ~750 m3 (=~200k gal h2o) of cargo space, but 112000 kg (=~30k gal h2o) of payload, so one could only fill ~1/6 of the cargo volume with water.
yep. Yet they still have to push through the air and have the mass and the cost of the plane for that unnecessary 6x volume. That is because the plane is retrofitted instead of having been built for the purpose of carrying high density payload. (also that 6x larger than needed volume makes those retrofitted planes much more susceptible to the high wind gusts, etc. which almost always an issue with those fires)
CA has C-130 water tankers with 20 ton payload (16 ton is the water itself). Total capacity of C-130 when used as fuel tanker - ie. when its own fuel and payload combined - is 30 ton. Empty it is 35 ton, max - 70 tons, and cost $60M+.
Taking a real cheap large drone as an example to have real base numbers to work with - German V1 (pulsejet doesn't matter, the piston engine of course would be the way to go here as we don't need jet speed) - 2.2 ton total weight with 0.5 ton fuel and 0.85 ton payload. The ratio of fuel plus payload to weight is already better than C-130's (at that ratio the C-130 would have to carry 35 ton payload instead of its current 20 and 20 ton fuel instead of its current 10), and with piston engine of the same weight as the pulsejet we'd get even larger thrust - i.e can get even higher payload ratio - while using much less fuel (the pulsejet had atrocious efficiency).
So, it would take 15 drones of that size for one C-130. These drones are much simpler and easier to build and thus cheaper than even small Cesna, thus 15 of them would be at least ~10x cheaper than one C-130.
Dropping 15 loads of 1+ ton water instead of one 16 ton load seems to be better in most cases as the fire line isn't a straight line, and the large plane like C-130 has to maneuver, etc. while flying low over the fire and being subjected to the fire draft, wind gusts, etc.
The 15 drones can drop those 15 loads of 1 ton using dive-bomber style - impossible for C-130 or any other air tanker currently in use - thus avoiding that prolonged horizontal low-flying over fire and thus having much less risk/danger form the wind gusts, etc.
I think I (and maybe others) understood 'drone' to mean something closer to quad-copters. Sounds like you're actually proposing something closer to water-filled cruise missiles? Which actually seems like a compelling idea. I'd guess there are some questions around how to make the vehicle recoverable/reusable, or else how to avoid causing more fire than you're putting out when the vehicle splatters against terrain, and even more questions around operating the things (autonomously vs. remotely, launch/recovery, ...).
But I do love the image of a satellite spotting a quarter-acre IR hotspot and firing a dozen USFS Tomahawks filled with with retardant to nip it in the bud.
>I think I (and maybe others) understood 'drone' to mean something closer to quad-copters. Sounds like you're actually proposing something closer to water-filled cruise missiles?
as i said in original post - "not small quadcopters, more like WWII size bombers". I use V1 as a close and real existing example of a drone wrt. size and simplicity/cheapness of construction. Though i think 2x-4x of that size would be more suitable and convenient.
>how to make the vehicle recoverable/reusable
just regular software for a plane type drone. The drone comes, drops the water and returns. Can do it GPS/inertial, AI targeted or remote controlled (like say put a Starlink on it how Ukrainians do it sometimes).
>else how to avoid causing more fire than you're putting out when the vehicle splatters against terrain
while normally the drone would return back, accidental crashes of course unavoidable. You don't have to carry much fuel as the drones can be spread around the state on say a 200km grid. The fuel tank can be made with automatic foam generator or something like this triggered if/when the drone goes down crashing.
it works fine for Ukraine sending drones to the targets 1000 km away (commercial large drones bought from China). And their task is more complex like for example they have to care about flying low to avoid air-defense for example, etc.
yep. Yet they still have to push through the air and have the mass and the cost of the plane for that unnecessary 6x volume. That is because the plane is retrofitted instead of having been built for the purpose of carrying high density payload. (also that 6x larger than needed volume makes those retrofitted planes much more susceptible to the high wind gusts, etc. which almost always an issue with those fires)
CA has C-130 water tankers with 20 ton payload (16 ton is the water itself). Total capacity of C-130 when used as fuel tanker - ie. when its own fuel and payload combined - is 30 ton. Empty it is 35 ton, max - 70 tons, and cost $60M+.
Taking a real cheap large drone as an example to have real base numbers to work with - German V1 (pulsejet doesn't matter, the piston engine of course would be the way to go here as we don't need jet speed) - 2.2 ton total weight with 0.5 ton fuel and 0.85 ton payload. The ratio of fuel plus payload to weight is already better than C-130's (at that ratio the C-130 would have to carry 35 ton payload instead of its current 20 and 20 ton fuel instead of its current 10), and with piston engine of the same weight as the pulsejet we'd get even larger thrust - i.e can get even higher payload ratio - while using much less fuel (the pulsejet had atrocious efficiency).
So, it would take 15 drones of that size for one C-130. These drones are much simpler and easier to build and thus cheaper than even small Cesna, thus 15 of them would be at least ~10x cheaper than one C-130.
Dropping 15 loads of 1+ ton water instead of one 16 ton load seems to be better in most cases as the fire line isn't a straight line, and the large plane like C-130 has to maneuver, etc. while flying low over the fire and being subjected to the fire draft, wind gusts, etc.
The 15 drones can drop those 15 loads of 1 ton using dive-bomber style - impossible for C-130 or any other air tanker currently in use - thus avoiding that prolonged horizontal low-flying over fire and thus having much less risk/danger form the wind gusts, etc.