I was watching one of those slow motion crash tests where a car rams a solid wall. As the airbag deployed, I saw the crash dummy bounce off the airbag and not go through the windshield. But I also saw an imaginary airbag extend FORWARD from the bumper before impact so that the bag could absorb some of the energy prior to the bumper and the rest of the vehicle hitting the wall.
I'm not an engineer. I don't know if such a bag could absorb enough energy to make a difference in the forces that eventually make their way to the human body. But why not?
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The problem is that it's not so much the impact that causes damage, it's the deceleration, and the distance in which that deceleration has to happen. If you imagine an emergency stop, from a fairly high speed, and then (say) divide that stopping distance by two to get the minimum stopping distance in which the human passenger is not damaged by the deceleration forces, that's how far ahead of the car the bumper-bag would have to extend on inflation to stand a chance of extending the deceleration period for long enough to prevent injury. Depending on the speed of the car, that could easily be several meters.Now, assuming that we have such an airbag, we need some way of setting it off. So we have a radar system that reads everything in front of the car, and predicts where every object is going to be over the next half-second, so that impacts can be predicted in advance, with the knowledge that no matter how good the driver is, they will be incapable of avoiding the obstacle, and that the object can't get out of the way itself. And to be 100% reliable - no false positives or negatives can be allowed. Given the marginal profits on cars, that advanced a radar system just can't float with current technology, but I don't think it's completely unfeasible in (maybe) the next 20 years.
What I really like about this idea is the potential to even the "Agressiveness" between different vehicles. Currently, when a large heavy car collides with a small light car, physics dictates that the small light car undergoes more extreme accelerations. This encourages people to buy large heacy cars with the commensurate toll on gas mileage. By deploying active bumpers or bags in anticipation of collision these accelerations can be significantly modified. There is precedent for these types of systems, in the military they are called (I think) active armor and where pieces of a tank are thrown at incoming progectiles to encourage the explosion further away from the tank itself. The radar/sonar systems necessary for the prediction of iminent collisions are starting to become available. There has been discussion of these types of systems for pedestrian protection as well. Adding the systems for side and rear collision on small vehicles where there is little room for crumple zones would significantly improve injury outcomes of these types of impacts. A main improvement that these systems could provide is that the mere act of deploying the device begins the acceleration away from the oncoming vehicle. Timed properly, the kinetic energy to stop the deploying device would begin to slow the oncoming vehicle all before the device starts to crumple from the impact. Particularly in side impacts, any gain in crumple zone would be a significant gain.
BumperBags are an excellent idea. The point is that it does not have to be an airbag. This is the functionality that is required for head on collisions.
"Should decelerate both vehicles appropriately at an impact velocity of 240km/hr - this being the combined speed of both vehicles"
It may not be an airbag but any device, mechanism or system that accomplishesthis.