longer time The second and third are the typical amounts of thrust and the typical burn times of the method. Outside a gravitational potential small amounts of thrust applied over a long period will give the same effect as large amounts of thrust over a short period. (This result does not apply when the object is significantly influenced by gravity.) The fourth is the maximum delta-v this technique can give (without staging). For rocket-like propulsion systems this is a function of mass fraction and exhaust velocity. Mass fraction for rocket-like systems is usually limited by propulsion system weight and tankage weight. For a system to achieve this limit, typically the payload may need to be a negligible percentage of the vehicle, and so the practical limit on some systems can be much lower. Table Notes Testing Spacecraft propulsion systems are often first statically tested on Earth's surface, within the atmosphere but many systems require a vacuum chamber to test fully. Rockets are usually tested at a rocket engine test facility well away from habitation and other buildings for safety reasons. Ion drives are far less dangerous and require much less stringent safety, usually only a large-ish vacuum chamber is needed. Famous static test locations can be found at Rocket Ground Test Facilities Some systems cannot be adequately tested on the ground and test launches may be employed at a Rocket Launch Site. Planetary and atmospheric propulsion Launch-assist mechanisms There have been many ideas proposed for launch-assist mechanisms that have the potential of drastically reducing the cost of getting into orbit. Proposed non-rocket spacelaunch launch-assist mechanisms include: Skyhook (requires reusable suborbital launch vehicle, not feasible using presently available materials) Space elevator (tether from Earth's surface to geostationary orbit, cannot be built with existing materials) Launch loop (a very fast enclosed rotating loop about 80 km tall) Space fountain (a very tall building held up by a stream of masses fired from its base) Orbital ring (a ring around Earth with spokes hanging down off bearings) Electromagnetic catapult (railgun, coilgun) (an electric gun) Rocket sled launch Space gun (Project HARP, ram accelerator) (a chemically powered gun) Beam-powered propulsion rockets and jets powered from the ground via a beam High-altitude platforms to assist initial stage Air-breathing engines Studies generally show that conventional air-breathing engines, such as ramjets or turbojets are basically too heavy (have too low a thrust/weight ratio) to give any significant