Secks,
There are many different types of fires, types of fuels, rates of spread and burn, etc. There are also many ways to fight fire. Going direct, meaning putting the retardant directly on the fire, is often not the best way to do it, for a number of reasons.
To answer your question about effectiveness going direct, one must consider the fire. I've worked many fires in which flame length was 300', with high or extreme activity. Nothing will put out a fire like that, with temperatures exceeding 2,000 degrees, trees exploding, and fire whorls (tornado-type manifestations made of fire). Such fires are too hot; they're preheating other fuels, and even if we could apply something to make a dent, the radiation heat has already affected other fuels ahead of the flame front. It would be like trying to juggle water.
Much of the time we're modifying the burn. We may be pretreating a ridgeline, for example. We're going to direct where the fire will go, alter it's course or work it to a natural barrier and stop it there. We may be building safe zones for ground troops, or reinforcing handcrew handlines or dozer lines, to prevent the fire from spotting past them. Sometimes ground crews will be backfiring, or starting a fire to put out a fire, and we're supporting their firing operation.
The retardant is a thick, slimy concoction with salts, phosphates,and other chemicals in it. It coats a fuel, and does several things. It can serve to cool a fire, but that's not our primary use. Nothing as you see it actually burns. It vaporizes, and the vapor burns. It's part of the burning process, or pyrolosis. By placing a barrier against the fuel, we're reducing or stopping the vaporization, or gassing process. We're putting in a thermal barrier that helps prevent the fuel from igniting. The retardant doesn't evaporate easily, and when the moisture in the retardant is gone, the chemical is still attached to the fuel. If the fuel does ignite, it can slow the burn rate.
When it rains later, the chemical is partially washed off, and acts as a fertilizer to aid in regrowth. That's the purpose of the phosphates and other chemicals in the retardant.
The retardant also has fugitive color, or the red color you see on television. This serves to help us see where the retardant went, judge wind drift, look for gaps in the line, etc. It helps us build line, when one airplane tags on to another airplane's drop to build a protective line outside the burn, or on the burn.
When watching a drop in person or on television, you see a lot of mist, but it's different to be on the ground, or even looking straight down. A lot more of the chemical is on the drop line than you might think. Often when I drop and pull off, a shiny wet slick can be seen reflecting on the target. I've gone back on occasion and walked the drop line, and it's still sticky or slimy two days later. I've examined the line for consistency, coverage, etc, and it really is some good, uniform coverage.
We're trying to avoid shadowing when we drop. That occurs when the drop is too low; the retardant is still moving forward, and it covers one side of the fuel. It leaves the other side unprotected. To avoid that, we drop high enough let the retardant stop it's forward motion and begin to mist and fall straight down. The type of fuel, wind conditions, amount of chemical being dropped, coverage level, terrain conditions, and other factors determine how high we'll be doing that. We may need to be lower with a high coverage level, meaning that we're putting out a lot of chemical at once, to penetrate tree canopies or fuels to get to the fire.
Other times, we are going direct on the fire. I've seen retardant work very effectively. I've done initial attack on several fires this year in which the retardant stopped the burn cold. Ordinarily we don't attack the head of the fire, or the most active part of the fire, because there's a danger of splitting it and make it worse. But on some occasions that's appropriate. By splitting a load and running a line at angles across the head of the fire with several drops, it can be contained or stopped. On several occasions this year, that's happened dramatically.
That's probably a lot longer answer than you wanted, but retardant drops can be very effective. It's a matter of using them properly. They can be wasted, too...but that's part of what they pay us for...making sure it's used effectively. Flying the airplane is important, but part of the reason it takes so long to train someone (years) isn't just the flying; it's the firefighting experience. The idea is that we're not just there to dump on the fire, we're there to employ sound fire tactics as a tool to be used on the fire; just like a McLeod or Pulaski, chainsaw, or shovel. We're just another tool...but often a very effective one.
