The only thing allowed in front of a jet engine is air
The suction generated in front of a jet engine may be explained mathematically, but other than that it is beyond human comprehension until you see an engine suck a guy off the deck of an aircraft carrier who was standing near (not in front of) the engine. He was behind and to one side and got sucked in like a bug. He survived because a harness he was wearing caught on an engine component forward of the turbine blades, which wreaked havoc with his hands and head. If you could somehow place a screen in front of the engine, which did not get sucked in at first, the passage of air would heat it up pretty quickly (this is why turbine blades are all hollow), and it would either melt or expand so much that it would break the engine cowling (cover) apart. The reason jet engines have to warm up for at least 5 minutes before the pilot can move the jet is so that he can "grow the engine" (have the cylindrical component of the engine expand) before the turbine blades expand lengthwise and slice the engine apart.
