Main Tank Injection (MTI) Pressurization of Liquid Rocket Propellant Tanks

During the 1960s, a number of investigations into a wide variety of liquid propellant tank pressurization systems for both storable and cryogenic propellants were conducted at Martin Marietta Denver Aerospace. One area of interest was main tank injection (MTI) pressurization. This technique was applicable to hypergolic propellants that react spontaneously when brought into contact with one another. Numerous propellant combinations were studied. For each combination, the oxidizer was injected into the fuel tank, and the fuel was injected into the oxidizer tank to form a small combustion zone. The resulting hot combustion gases were used to pressurize the tank to operating pressure and then maintain this pressure during outflow. A pressure sensor controlled the injection rate. Applications envisioned were launch vehicles, upper stages, and air-to-ground missiles. Feasibility of using MTI pressurization was demonstrated for a number of propellant combinations. Testing was conducted in aluminum and stainless steel tanks having volumes of 5.33 cubic feet, 279 cubic feet, and 1,940 cubic feet. The MTI pressurization method was never incorporated into an operational launch vehicle or upper-stage system, because combustion of the propellant inside the tank was always perceived as risky when compared to the more conventional high-pressure stored gas or gas generator methods. However, it was used in the rocket sled at Alamogordo, New Mexico, where the effects of high acceleration levels were tested on primates and humans.