There are orbital air-launched rockets, but the amount of energy needed to get into orbit is still vast. Burt's SS1, which uses the concept for suborbital flight, can't be scaled up for orbital flight due to this issue. I used to know exactly why it was done...but don't anymore. Damn memory...
Pegasus
To cut on fuel costs, rockets are launched near the equator: Cape Canaveral, Kwajalein Atoll, French Guyana (sorry for any spelling errors). By using the rotation of the earth, rockets get a velocity boost. Supposedly Israel gets royally screwed, having to fly west against the rotation of the earth and then make a coarse correction for orbital flight due to airspace restrictions.
Ramjets have been theorized, but not capitalized on. Hypersonic aircraft (NASA's X-43 Scramjet) will probably be the future. Essentially, engineers are trying to reduce weight by substituting some stored oxidizer with atmospheric air.
Aerodynamics are of concern especially in regard to engine efficiency. Rocket nozzles are generally fixed in size and based on an average atmospheric pressure experienced during the flight. This means that on launch, where pressure is higher, the rocket plume detaches from the nozzle walls and has reduced efficiency (less pressure recovery?). At high altitude, where pressure is extremely low, the rocket plume exceeds the width of the nozzle and causes air drag.
To combat this issue, novel nozzle designs have been developed. The coolest is the Aerospike engine, where atmospheric pressure solely regulates plume size. Trident SLBM's use aerospike nozzles to increase range.
Aerospike
As far as aerodynamic loads, as others have said, it mainly lies in supersonic flow. Rockets only stay in the atmosphere for a few minutes at most, and max-q (max dynamic pressure, aka max aerodynamic load) usually happens seconds after launch (~30 I believe). Fairings are usually jettisoned well before the rockets quit, just to give an idea of how much tangible atmosphere the rocket experiences during its flight.
I used to know more about escape velocities and elementary orbital mechanics, but all that has gone the way of the buffalo...
. If I remember right, LEO velocity is 7km/s and escape velocity is 11km/s...just to get an idea of the amount of energy needed. Unfortunately, rocket scientists have a reputation for a reason. Trying to learn and master that stuff is about as much fun as putting you nuts in a vice.
- LostCause