Since ground-assisted space launch has never been used, some readers have asked for references.   Rocket sled launch is viable today since the basics have been demonstrated hundreds of times on test tracks at China Lake, Sandia, and Holloman.  The photo below is a rocket sled at Mach 4.4 at Sandia National Labs, while the track at Holloman recently boosted a 192 lb. missile to Mach 8.5 on steel rails.  Steam launches like Navy aircraft carriers have been proven thousands of times, but scaling up to the size needed for space launch seems impractical.  Pneumatic launch is also feasible, but never tested.  Maglev is promising, but in its infancy and requires major scientific breakthroughs to move beyond the small model level.  A huge cannon can launch objects into space as Dr. Gerald Bull proved in the 1960s, but the G force is far, far too great for manned launches.  A downhill gravity assist or jet turbine powered train may be used to start a rail launch up to 200 mph.

 photographed by Alan Sehmer of Sandia National Laboratories

Here are the best studies:

Inclined Launch Facilities and New Mexico's Spaceport; 2015 Raymond S. Leonard PE and James Daniel PE

Mass Drivers for Space Construction and New Mexico's Spaceport; 2015 Raymond S. Leonard PE

"A Light Gas Gun Approach To Achieving 'First Stage Acceleration' for the Highly Reusable Space Transportation System"   1997  M. Frank Rose, R .M. Jenkins, M. R. Brown, Space Power Institute, Auburn University, AL, 36849 

Here are some links:"B" Bird launch at the Cape

Holloman Test Track Video

Studies on Ground-Based Accelerators - the Alna Space Program

Multiple G - NASA concluded that 8 Gs for two minutes is safe

Lockheed Proposal - for a sled launched fly-back ramjet powered booster

Mountains Made for Launchtracks - "Moon Miners" four best sites

Current Air  Force Holloman Sled Tests - good info and photos

MX ICBM missile (short tube launched) video

2012 Sky Ramp Technology