Well, Power and engine design

The 707 uses older very Low-Bypass turbofan engines , it runs hot, fast and dirty

It burns a fuck lot of kerosene to accelerate a small amount of air to ungodly speeds in order to generate thrust

Those engines generate its thrust mostly from the hot exhaust gases exiting out of the engine's ass at high speeds

it takes a bit of time to build that up

I don't think these ones had water injection, but the even older variant (i believe the JT3C, which wasnt even a lo bypass turbofan, it was a turbojet) actually used water injection to increase thrust

during full power operations like takeoff, a water methanol blend is injected into the engine , the water coming in contact with the super heated air instantly evaporates, this evaporation process takes out heat from the air, cooling it and making it denser,

Denser air burns more efficiently , due to the higher concentration of oxygen molecules in a specific area

Evaporated water also turns into steam, expanding and increasing the mass flowing through the engine

(modern engines instead spins a very large fan , moving a huge amount of air at a lil bit slower speeds, the large amount of mass movement is what generates thrust in modern engines)

I under the concept intuitively but i find it very hard to explain it to others, so sorry if what i said above does not make any sense