Tripropellant rocket
Encyclopedia
A tripropellant rocket is a rocket
Rocket
A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...

 that uses three propellants, as opposed to the more common bipropellant rocket or monopropellant rocket
Monopropellant rocket
A monopropellant rocket is a rocket that uses a single chemical as its propellant.-Chemical-reaction monopropellant rockets:...

 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. For example, a mixture of lithium, hydrogen, and fluorine produced a specific impulse
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...

 of 546 seconds; the highest ever of any chemical rocket motor. The other kind of tripropellant rocket is one that uses one oxidizer but two fuel
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...

s, switching between the two in mid-flight. In this way the motor can combine the high thrust-to-mass of a dense fuel like kerosene
Kerosene
Kerosene, sometimes spelled kerosine in scientific and industrial usage, also known as paraffin or paraffin oil in the United Kingdom, Hong Kong, Ireland and South Africa, is a combustible hydrocarbon liquid. The name is derived from Greek keros...

 early in flight with the high specific impulse of a lighter fuel like liquid hydrogen
Liquid hydrogen
Liquid hydrogen is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.To exist as a liquid, H2 must be pressurized above and cooled below hydrogen's Critical point. However, for hydrogen to be in a full liquid state without boiling off, it needs to be...

 (LH2) later in flight. The result is a single engine providing some of the benefits of staging.

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 implies less loss to atmospheric drag. In addition, kerosene-based engines generally provide higher thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....

, which is important for takeoff, reducing gravity drag
Gravity drag
In astrodynamics and rocketry, gravity drag is a measure of the loss in the net performance of a rocket while it is thrusting in a gravitational field...

. So in general terms there is a "sweet spot" in altitude where one type of fuel becomes more practical than the other.

Traditional rocket designs use this sweet spot to their advantage via staging. For instance the Saturn V
Saturn V
The Saturn V was an American human-rated expendable rocket used by NASA's Apollo and Skylab programs from 1967 until 1973. A multistage liquid-fueled launch vehicle, NASA launched 13 Saturn Vs from the Kennedy Space Center, Florida with no loss of crew or payload...

s used a lower stage powered by RP-1
RP-1
RP-1 is a highly refined form of kerosene outwardly similar to jet fuel, used as a rocket fuel. Although having a lower specific impulse than liquid hydrogen , RP-1 is cheaper, can be stored at room temperature, is far less of an explosive hazard and is far denser...

 (kerosene) and upper stages powered by LH2. Some of the early Space Shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

 design efforts used similar designs, with one stage using kerosene into the upper atmosphere, where an LH2 powered upper stage would light and go on from there. The existing Shuttle design is somewhat similar, although it uses solid rockets for its lower stages.

Almost all of the cost of operating the Shuttle is for the payroll for the army of workers needed to refurbish the Shuttle after it has landed. The fuel used is orders of magnitude cheaper, and, if a single stage to orbit design SSTO avoided some of this refurbishment, costs would drop, although this could require more repairs. But in this case the staging solution is not available, by definition, so it becomes harder to use both fuels.

SSTO rockets could simply carry two sets of engines, but this would mean the spacecraft would be carrying one or the other set "turned off" for most of the flight. With light enough engines this might be reasonable, but an SSTO design requires a very high mass fraction and so has razor-thin margins for extra weight.

And thus the tripropellant engine. The engine is basically two engines in one, with a common engine core with the engine bell, combustion chamber and oxidizer pump, but two fuel pumps and feed lines. The engine is somewhat heavier and more complex than a single-fuel engine, but the complexity is generally a little less than 50% more than a single engine, hence less than two engines would be. Of course there are numerous practical reasons why this would be more complex.

At liftoff the engine typically burns both fuels, gradually changing the mixture over altitude in order to keep the exhaust plume "tuned" (a strategy similar in concept to the plug nozzle
Plug nozzle
The plug nozzle is a type of nozzle which includes a centerbody or plug around which the working fluid flows. Plug nozzles have applications in aircraft, rockets, and numerous other fluid flows.-In Rockets:...

 but using a normal bell), eventually switching entirely to LH2 once the kerosene is burned off. At that point the engine is largely a straight LH2/LOX engine, with an extra fuel pump hanging onto it.

The concept was first explored in the US by Robert Salkeld, who published the first study on the concept in Mixed-Mode Propulsion for the Space Shuttle, Astronautics & Aeronautics August 1971. He studied a number of designs using such engines, both ground based and a number that were air-launched from large jet aircraft. He concluded that tripropellant engines would produce gains of over 100% in payload fraction
Payload fraction
In aerospace engineering, payload fraction is a common term used to characterize the efficiency of a particular design. Payload fraction is calculated by dividing the weight of the payload by the weight of the otherwise empty aircraft when fully fueled...

, reductions of over 65% in propellant volume and better than 20% in dry weight. A second design series studied the replacement of the Shuttles SRBs
Space Shuttle Solid Rocket Booster
The Space Shuttle Solid Rocket Boosters were the pair of large solid rockets used by the United States' NASA Space Shuttle during the first two minutes of powered flight. Together they provided about 83% of liftoff thrust for the Space Shuttle. They were located on either side of the rusty or...

 with tripropellant based boosters, in which case the engine almost halved the overall weight of the designs. His last full study was on the Orbital Rocket Airplane which used both tripropellant and (in some versions) a plug nozzle, resulting in a spaceship only slightly larger than a Lockheed SR-71, able to operate from traditional runways.

The only tripropellant engines built were in Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

. Kosberg and Glushko developed a number of experimental engines in the early 1990s for a SSTO spaceplane
Spaceplane
A spaceplane is a vehicle that operates as an aircraft in Earth's atmosphere, as well as a spacecraft when it is in space. It combines features of an aircraft and a spacecraft, which can be thought of as an aircraft that can endure and maneuver in the vacuum of space or likewise a spacecraft that...

 called MAKS
MAKS space plane
The MAKS is a cancelled Russian air-launched orbiter project that was proposed in 1988, but cancelled in 1991. The orbiter was supposed to reduce the cost of transporting materials to Earth orbit by a factor of ten...

, but both the engines and MAKS were later cancelled due to a lack of funding. Glushko's RD-701
RD-701
RD-701 - liquid-fuel rocket engine developed by Energomash, Russia. It was proposed to propel the reusable MAKS space plane before cancellation of this project. The RD-701 is a tripropellant engine that uses staged combustion cycle afterburning of oxidizer-rich hot turbine gas...

 was built and test fired, however, and although there were some problems, Energomash
Energomash
The Energomash Corporation is a Russian power and engineering company. Energomash manufactures small cogeneration plants as well as wide variety of components for the energy industry...

feels that the problems are entirely solvable and that the design does represent one way to reduce launch costs by about 10 times.

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