One of my professors had told us in class that the planes were able to destroy the towers easily because they had just taken off and therefore still had a lot of kerosene.
According to this professor, if the hijacked planes had been planes that were about to land (with much less fuel), the attack on the WTC would have been less devastating and the towers might not have collapsed.
Unfortunately I don't know how true or realistic this scenario is.
It's definitely true that the planes did a lot more damage because of their full tanks of fuel, but that was the point - the hijackers deliberately chose planes taking off from eastern airports on transatlantic flights to maximize their fuel loads.
Your professor makes a good point. The 10,000 gallons of kerosene remaining on each aircraft burned at 1,500 - 1,800°F (800 - 1,000°C). Steel loses 50% of its structural strength at 600°C, and 90% at 1,000°C; more than enough to weaken the towers' frame to the point of collapse.
On the other hand, the fuel actually burnt itself out within 10 minutes; but the combustible interior fittings (rugs, curtains, furniture, paper, etc.) ignited by the initial fire kept burning at extreme temperatures, sustaining the heat transfer that eventually brought the towers down. A plane running on empty might have lead to a much smaller and less intense fire initially, but even that might have only delayed the towers' collapse for a few more hours - the structural damage from extreme impact velocity would remain, the fire would inevitably becomes self-sustaining, and there was simply no way for any firefighting effort to even reach (let alone effectively combat) a blaze between
77-95 floors above ground. And as
@Wolfram have said, the hijackers knew about a high fuel load's effectiveness anyway.
I believe that if the south tower’s attack failed, the north tower might not have collapsed. Avoiding the south tower’s attack is actually pretty easy, United 175 came within 300ft of crashing into Delta Flight 2315, which in aviation is nothing. Keep in mind that the south tower fell first and was attacked second, and it’s collapse caused a 2.1 magnitude earthquake on the ground. So this is not as impossible as it seems.
Huh,
there's actually a thread discussing this scenario posted four years ago. It's a more plausible outcome, but I still doubt the WTC complex would survive in the long run: the South Tower earthquake was, by all accounts, too weak to meaningfully destabilise the relatively untouched foundations of the North Tower. It was very much secondary to the weakened steel frame that eventually caused the sagging floors to "pancake" downwards.
United 175 crashing would indeed mean the South Tower’s survival, but the North Tower's collapse would have almost certainly damaged it well beyond repair. The facade would be pummeled by thousands of tons of debris, with secondary fires being ignited across the tower's length (as happened with 7 WTC).
Eventually, the ruined South Tower would have to be torn down; as I've said above, both the economic and psychological impact cannot be understated. Controlled demolitions are banned in NYC for obvious reasons; the only option would be to gradually dismantle the South Tower floor by floor. An unprecedented and immensely costly undertaking, but still less so than the equally unprecedented repair and reconstruction of the towers - which would also lead to far less economic gain, since
who the hell would want to return?
Billions would have saw the North Tower burning and collapsing on live TV, just like OTL; even if no expense was spared in restoring the Twin Towers, would any corporate tenant be willing to put their headquarters at risk of instant destruction? Would any of their employees be willing to live and work where if anything went wrong, they might have to choose between being buried alive and having to jump to their death? Note that a pure repair means that incorporating major design changes, such as the new 1 WTC's
improved security features, would be flat-out impossible. The restored Twin Towers would instantly become a financial black hole and public relations disaster.
The quoted scenario below would probably be the likeliest outcome.
My suspicion is that the South Tower would be damaged beyond economic repair by the collapse of the North Tower, the way many buildings around the site were badly damaged by the collapse IOTL. If anything, this probably delays the construction of a successor, because instead of carting away debris quickly, clearing the site means carefully and cautiously deconstructing one of the tallest skyscrapers on Earth, full of fun things like asbestos.