of the fuel and engines may negate the potential performance gain, and the added cost of the extra fuel may constrain the profitability of a commercial aircraft. On the other hand, replacing an older, less efficient engine with a newer engine of more advanced design can increase both power output and efficiency while sometimes even decreasing weight. In this situation, the only real disadvantage is the cost of the upgrade. Utilize assisted take off devices, such as rockets, to increase acceleration and rate of climb. Inject distilled water into the engine compressor or combustor. The primary purpose of water injection into jet engines is to increase the mass being accelerated, thereby increasing the force created by the engine. A secondary purpose is to lower the combustion temperature so that higher power settings may be used without causing engine temperatures to exceed limits. Jet or rocket assisted take off Auxiliary rockets and/or jet engines can help a fully loaded aircraft to take off within the length of the runway. The rockets are usually one-time units that are jettisoned after takeoff. This practice was common in the 1950s and 60s, when the lower levels of thrust from military turbojets was inadequate for takeoff from shorter runways or with very heavy payloads. It is now seldom used. Auxiliary jets and rockets have rarely been used on civil aircraft due to the risk of aircraft damage and loss of control if something were to go wrong during their use. Boeing did, however, produce a version of its popular Boeing 727 with JATO primarily for "hot and high" operations out of Mexico City Airport (MMMX) and La Paz, Bolivia. The boosters were located adjacent to the main landing gear at the wing root on each side of the aircraft. Specialized aircraft Several manufacturers of early jet airliners offered variants optimized for hot and high operations. Such aircraft generally offered the largest wings and/or the most powerful engines in the model lineup coupled with a small fuselage to reduce weight. Some such aircraft include: The BAC One-Eleven 475 combined the short body of the series 400 with the more powerful engines and improved wings of the series 500. This aircraft also featured stronger landing gear for rough field operations. The Boeing 707-220, which was a 707-120 airframe fitted with more powerful Pratt & Whitney JT4A engines, civilian versions of the military J75. The 707-220 had extremely