The speeds and energy required to achieve earth orbit is almost beyond conventional understanding.  To maintain a low earth orbit, a satellite must travel at over 5 miles each second.  At even a fraction of those speeds in the "lower" atmosphere (below say, 80 miles high), air friction converts that vast kinetic energy into tremendous heat.  Thus meteors or re-entering space junk are vaporized in a flash.

It is not enough to get to orbital altitudes where there is negligible air friction, getting to speed is critical to establish an orbit.  To compare with commercial air travel may be helpful.  Typical airline travel is around 6 miles high (30,000 feet or higher).  Typical airline speed at cruise is around 500 miles per hour.  To be in a safe orbit, a satellite needs to be 20 times higher (120 miles is safe for a few weeks) and going about 40 times faster (18,000 mph).  But energy, the real measure of the difference, is directly related to height (altitude) but is the square of the speed.  So to achieve earth orbit requires 1,000 times the energy that an airliner has at cruise.  Do the math. 

This partly explains why war-surplus V-2/WAC Corporal rockets could reach orbital altitude in the late 1940's but it took another decade to develop rockets that could not only get that high but propel a payload to the extreme velocity required for earth orbit.

Satellite launchers seek the most efficient way to get to orbit -- they want to use the least "energy" to get the most payload to orbit.  Simplistically, one would want to get to altitude first, then accelerate, accelerate, accelerate.  So most expendible satellite launch vehicles go high early and then pitch over toward the horizontal for the largest part of the rocket burn. 

Unfortunately, this does not work well if you want to protect a crew from a failure of the rocket.  Because a steep, suborbital ballistic reentry leads to extreme heating and extreme g-loads.  This is not obvious, so lets examine this closely. 

In a typical planned re-entry, the capsule or shuttle enters at a fairly shallow angle so that it encounters thicker atmosphere gradually.  As the re-entry proceeds, the speed (kinetic energy) is bled off gradually limiting the maximum heating temperature and holding structural loads relatively low.  For a suborbital ballistic type re-entry, the trajectory is quite steep, encountering the denser parts of the atmosphere while the speed and energy is quite high leading to a high heat impulse and very high structural loads.

The trajectories for manned spacecraft try to avoid these steep re-entries even on an emergency case.  For complete loss of thrust this is not always possible.  The one real life case turned out moderately well.  On April 5, 1975 the crew of what would be known later as Soyuz 18A, Vasili Lazarev and Oleg Makarov, were more than half way to orbit - at altitude and about 10,000 mph - when their second stage refused to be jettisoned.  During a normal Soyuz entry, decelerations of 5 g are normal.  Due to the steep angle of the Soyuz 18A abort trajectory, the crew endured up to 21g.  Fortunately they survived, the capsule did not break up and they landed safely.  But the two crew members never flew in space again.

An expendible rocket sending a satellite on a one way trip to orbit optimizes its trajectory by lofting high early on.  If an engine fails, the mission would be lost no matter what the trajectory; abort modes and crew rescue are not a consideration.  There has been some speculation that if an EELV were to be used to power the Orion capsule into orbit, there would be large parts of the trajectory where early aborts would cause loss of the capsule and crew during re-entry:  the dreaded black zone.  By adjusting the launch trajectory lower, these black zones can probably be eliminated -- but at a cost.  The cost is performance:  mass to orbit is decreased by flying a safer, more depressed trajectory.

The shuttle flies a trajectory that is more depressed than expendible launch vehicles>  This allows for potentially graceful abort trajectories following a premature engine shutdown.  After the Challenger, the first shuttle flight followed an even safer "abort shaped" trajectory -- but the performance price was too high to pay for long and all subsequent flights have gone back to the standard shuttle launch trajectory.  Which itself is not nearly as steep as the expendible rockets fly. 

Recent computer analysis from the Apollo missions had lead many analysts to conclude that the moon launch trajectories did not avoid all black zones.  More on this tomorrow.