The highest specific impulse chemical rockets (liquid-propellant rockets) use liquid fuel propellants. Approximately 170 different liquid propellants have undergone lab testing. This estimate excludes minor changes to a specific propellant such as propellant additives, corrosion inhibitors, or stabilizers. In the U.S. alone at least 25 different propellant combinations have been flown. However, there has not been a completely new propellant used in flight for nearly 30 years. Many factors go into choosing a propellant for a liquid propellant rocket engine. The primary factors include ease of operation, cost, hazards/environment and performance.   Bipropellants can be either hypergolic propellant or nonhypergolic. A hypergolic combination of oxidizer and fuel will start to burn upon contact. A nonhypergolic needs an ignition source.