both Project HARP and the Quicklaunch project. The magnitude of such correction may be small; for instance, the StarTram Generation 1 reference design involves a total of of rocket burn to raise perigee well above the atmosphere when entering an low Earth orbit. In a three-body or larger system, a gravity assist trajectory might be available such that a carefully aimed escape velocity projectile would have its trajectory modified by the gravitational fields of other bodies in the system such that the projectile would eventually return to orbit the initial planet using only the launch delta-v. Isaac Newton avoided this objection in his thought experiment by placing his notional cannon atop a tall mountain and positing negligible air resistance. If in a stable orbit, the projectile would circle the planet and return to the altitude of launch after one orbit (see Newton's cannonball). Acceleration For a space gun with a gun barrel of length (), and the needed velocity (), the acceleration () is provided by the following formula: For instance, with a space gun with a vertical "gun barrel" through both the Earth's crust and the troposphere, totalling ~ of length (), and a velocity () enough to escape the Earth's gravity (escape velocity, which is on Earth), the acceleration () would theoretically be more than , which is more than 100 g-forces, which is about 3 times the human tolerance to g-forces of maximum 20 to 35 g during the ~10 seconds such a firing would take.This calculation does not take into account the decreasing escape velocity at higher altitudes. Practical attempts V3 Cannon (1944-45) The German V-3 cannon program, during World War II was an attempt to build something approaching a space gun. Based in the Pas-de-Calais area of France it was planned to be more devastating than the other Nazi 'Vengeance weapons'. The cannon was capable of launching , diameter shells over a distance of . It was destroyed by RAF bombing using Tallboy blockbuster bombs in July 1944. Super High Altitude Research Project (1985-95) The US Ballistic Missile Defense program sponsored the Super High Altitude Research Project (SHARP) in the 1980s. Developed at Lawrence Livermore Laboratory, it is a light-gas gun and has been used to test fire objects at Mach 9. Project Babylon (1988-90) The most prominent recent attempt to make a space gun was artillery engineer Gerald Bull's Project Babylon, which was also