I’m not quite convinced Iron Man is entirely realistic.
“Proposterous!”, you say, “Hollywood makes its superhero films to near-documentary accuracy!” No, hear me out. Iron Man can fly, using rockets in his hands and feet. We know from the commercials that the suit can in fact fit in a briefcase and be carried around by hand, so as an estimate let’s say the Iron Man suit + Tony Stark weighs 200 pounds even. In more proper physics-style units, that’s 889.6 newtons. Just to hover, let alone fly into the air at high speed, his rockets are required to generate at minimum 889.6 newtons of force.
Force, as we know, is equal to the rate of change of momentum. Momentum is conserved, so the force required to keep Tony aloft is equal to the change of momentum of the rocket exhaust as it leaves his thrusters:
And the momentum is just mass times velocity:
But we’re interested in dp. the rate of change of momentum. The exhaust velocity v is constant, so dp = v dm, where dm is the amount of mass the rocket spits out in the time interval dt. In other words, we can rewrite the force equation with that substituted in, and dm/dt will be the rate at which the rocket spits out its fuel through the thrusters:
So what’s the exhaust velocity? Based on the fact that he doesn’t deafen everyone standing near him when he takes off, I’d say it’s subsonic, but let’s be generous and say v = 330 m/s, which is about the speed of sound in air under normal circumstances. Solving for dm/dt, I get:
Which is a pretty darn hefty burn rate. We know from the fact that the suit is portable that there aren’t all that many kilograms of fuel in the suit to begin with, and even if there were then the thrust (and the burn rate) would have to be higher to get the stored fuel off the ground as well. It’s diminishing returns, and one of the reasons rocketry is so expensive. It looks like in real life Tony Stark would probably take off dramatically before ignominiously sputtering out and crashing in a heap. This, incidentally, is one of the major reasons personal jet packs never became practical. (That and the whole “certain death” thing if anything goes wrong.)
Switching to rocket fuel with a higher v is a possibility. A good rocket fuel might have an exhaust velocity of 4000 m/s, with a corresponding burn rate of 0.22 kg/s, which is better. Maybe Iron Man then has a couple minutes of flight before having to call a refueling time out in the heat of battle. The noise factor would be tremendously antisocial to everyone in the vicinity, but that might be an acceptable price to a superhero saving the day. Or showing off at sporting events, whatever.
Alternately the suit might not use rockets at all. Instead it could be a jet engine, pulling in air from the environment, heating it and spitting it out the rocket thrusters. With an appropriate energy source, this could go indefinitely so long as there’s air around. The suit doesn’t seem to have visible air intakes though, and if the suit has been shown working in space in the comics (confirm/deny, Marvel fans?) then that explanation is right out.
But I have to admit if someone did manage to invent and market a personal rocket suit I’d be the first in line.