Why do spacecraft have to reach escape velocity when leaving earth?
For a start, military aircraft are exactly that – AIRcraft, not SPACEcraft. They need the pressure of air-flows on their wings to stay aloft. Spacecraft rely on sheer forward speed to ‘beat’ the effects of gravity; they fall towards the Earth, but they fly forward so fast that the curvature of the Earth causes its surface to fall away from the spacecraft. That’s what we call being in orbit. Next point: Military aircraft do not attempt to leave the vicinity of the Earth, whereas some (though not all) spacecraft do this. The reason spacecraft need to reach escape velocity is related to the reason ‘escape velocity’ is so named. If you throw something upward, it will fall back down. The harder you throw it, the higher it will go before it starts to fall. If an object could be sent up at a great enough speed, it would never fall back down because the ever-decreasing effect of gravity would be insufficient to curb the (also ever-decreasing) outward speed of the object. If a spacecraft could
“Escape velocity” is the speed a spacecraft has to achieve in order to leave the earth’s gravity and orbit. Military aircraft can get to the edge of space, but they are either air-breathing aircraft or else they are lighter than air aircraft. Air-breathing aircraft can’t leave the atmosphere because their engines require air from outside to mix with fuel to produce power. Balloons filled with hydrogen or helium that rise because they are lighter than the air around them. Once they are floating on top of the atmosphere, they can’t go any higher. It’s like a block of wood floating in the water. It’s at the top, it won’t float any higher than that.
They don’t need to reach escape velocity to leave Earth, but if they don’t then some time after the engine is turned off they will fall back to Earth. Escape velocity is only the speed at which something has to be going straight up so that with NO ADDITIONAL POWER being used it will never fall back. In theory, if you have some kind of infinite power source, you could leave Earth at any speed you like, as long as that power was on all the time to counteract gravity. But real rockets can only carry enough fuel for a few minutes of power, so they must reach escape velocity quickly so that can travel the rest of the way without any power at all, just coasting, like a rock thrown up at very high speed.
Spacecraft have to reach orbital velocity to orbit Earth, or escape velocity to leave it and go to the Moon or Mars, because they switch off their engines once they get there and coast the rest of the time. An aircraft has to keep using its engines to maintain its altitude. Were it to shut down its engines, would fall to the ground in a neat parabolic arc. If an object can get up to 17,500mph, however, before shutting down the engines its forward motion is enough that, as it curves down to the Earth, the Earth’s surface curves away from it at the same rate, hence it never actually reaches the ground.
Without special propulsion engines for thrust in very, very thin atmosphere of high altitudes, most regular engines (Jet engines) starve of oxygen (required for the combustion of fuel) somewhere around 40 to 50 thousand feet. Also, aircraft flight depends upon an air foil within the wings to generate lift which permits the airplane to fly in the first place. As you go higher and higher in altitude, the air gets thinner and thinner, so the “lift” generated by your wings’ air foil drops lower and lower, until it is insufficient to sustain the weight of the aircraft, and it falls back to Earth. Propulsion systems at very high altitudes usually require some kind of rocket engine or at least one in which all components needed for combustion are carried within the fuel tanks. Example: Liquid oxygen and Aviation Gas. There are many other exotic fuels which are used depending upon duration of the flight and purpose of the flight. Escape velocity (around 17,000 MPH) is needed to overcome the pu