Tripropellant rocket

A tripropellant rocket is a rocket that uses three propellants, as opposed to the more common bipropellant rocket or monopropellant rocket designs, which use two or one fuels, respectively. Tripropellant rockets appear to offer fairly impressive gains for single stage to orbit designs, although to date no tripropellant rocket design has been developed to the point of testing that would prove the concept.

There are two principally different kinds of tripropellant rockets. One is a rocket engine which mixes three separate streams of propellants. In the 1960s, Rocketdyne fired an engine using a mixture of liquid lithium, gaseous hydrogen, and liquid fluorine to produce a specific impulse of 542 seconds, likely the highest measured such value for a chemical rocket motor.

The other kind of tripropellant rocket is one that uses one oxidizer but two fuels, switching between the two in mid-flight. In this way the motor can combine the high thrust of a dense fuel like kerosene early in flight with the high specific impulse of a lighter fuel like liquid hydrogen (LH2) later in flight. The result is a single engine providing some of the benefits of staging.

For example, injecting a small amount of liquid hydrogen into a Kerosene burning engine can yield significant specific impulse improvements without compromising propellant density. This was demonstrated by the RD-701 achieving a specific impulse of 415 seconds in vacuum (higher than the pure LH2/LOX RS-68), where a pure kerosene engine with a similar expansion ratio would achieve 330–340 seconds.

Although liquid hydrogen delivers the largest specific impulse of the plausible rocket fuels, it also requires huge structures to hold it due to its low density. These structures can weigh a lot, offsetting the light weight of the fuel itself to some degree, and also result in higher drag while in the atmosphere. While kerosene has lower specific impulse, its higher density results in smaller structures, which reduces stage mass, and furthermore reduces losses to atmospheric drag. In addition, kerosene-based engines generally provide higher thrust, which is important for takeoff, reducing gravity drag. So in general terms there is a "sweet spot" in altitude where one type of fuel becomes more practical than the other.

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