Forgive me for my ignorance, but...
If gravity is stronger on earth than in the stratosphere and if commercial airlines can reach 35k feet under normal power then why can’t a spaceship simply fly into space? The edge of space is only 65 or 70 miles up. Why not fly into space?
Someone on the thread, please educate me.
Gravity doesn’t drop off that fast as air does.
An Airplane at 35K feet isn’t hovering on engine power, It’s using differential air pressure on the wings to stay up... which takes a lot less energy than direct thrust.
Above 25K feet, there isn’t enough air to breathe. Above 60K ft there is barely enough air to generate lift... and you still have 50 more miles to get to lower “space”.
Airplanes need air to fly. Planes that fly above 80K feet need to go Mach 2 to hit enough air molecules to generate enough lift to stay up. To fly much above 100K ft in a steady state, the plane would likely need to go Mach 5 and its still a long way from space.
A rocket going straight up can go slower in the atmosphere (and not pay such a big drag penalty) and speed up outside of the atmosphere (where drag is much less).
This is the reason we don’t have “aerospace planes” yet. Rockets are much easier/cheaper.
Gravity is not very different between the ground and 35k feet. The term “Zero Gravity” in space is misleading. There is plenty of gravity outside the atmosphere. Being in “Free fall” is a more accurate description of space flight.
The reasons to launch by drop are 7 miles of altitude - potential energy gain - you don’t have to carry the fuel for and the much thinner air takes less energy to accelerate in.
In addition to the lift problems others mentioned jet engines would shut down if they go too high. Jets engines provide energy by combining fuel with an oxidizer, similarly to any conventional car. Gas for the car, aviation gas aka jet fuel for the jets and that most basic oxidizer, oxygen, for both. Both save A LOT of weight by carrying the fuel, but taking the oxygen from the air through which they’re traveling. If you get high enough there’s no oxygen to take and the engine shuts down. Rockets get around that by carrying both their fuel and oxidizer. They don’t need air to run, but that oxidizer they carry is difficult to transport, adds weight and adds explosion risk.