Kerosene - almost as common to American life as gasoline - is being considered as a fuel for two main engine candidates for a second generation reusable launch vehicle, now in development by the Space Launch Initiative. The Initiative is NASA's technology development program to design a complete space transportation system with increased safety and reliability at a lower cost.
Managed by the Marshall Space Flight Center in Huntsville, Ala., the Space Launch Initiative Propulsion Project Office is developing the kerosene-fueled RS-84 prototype engine with Boeing Rocketdyne of Canoga Park, Calif., and the TR107 prototype engine with TRW Space and Electronics of Redondo Beach, Calif. Two hydrogen-fueled main engine candidates are also in development.
Kerosene rocket engines are not a new idea. Kerosene was used as a propellant in the F-1 engines on the Saturn V rockets that took Apollo astronauts to the Moon back in the late 1960s.
What is new is the design of the engine.
Second generation engines need a more integrated design that is more reliable, easier to operate, offers fewer components and has a lower cost. The new, kerosene-fueled engine design will feature a staged combustion cycle that results in greater fuel efficiency than the F-1's gas generator cycle by reusing some of the fuel and oxidizer used in the preburner to power the main combustion chamber. Preburners heat the propellants to ready them for the engine's turbopumps before the propellants are injected into the main combustion chamber - where the fuels combust to create thrust.
Another difference is overall engine size. To achieve a higher performance level, the propellant is burned at higher pressure which reduces the size of the main combustion chamber. That, in turn, increases thrust.
Chamber pressure in the new kerosene engine will be approximately 2,600 pounds per square inch absolute (psia) compared to 965 psia chamber pressure on the F-1 engine. This increased pressure allows a smaller engine to provide nearly as much thrust as the larger F-1 engine: The new kerosene engine will generate a powerful 1.1 million pounds of force - only 400,000 pounds of force lower than the F-1 engine.
The most significant difference between the F-1 engine and a second generation kerosene engine is reusability.
The F-1 engines were expendable - only capable of use for one flight. Both the RS-84 and TR107 prototype engines could become the first-ever reusable engines using kerosene and oxygen-rich gases.
The new, reusable engines would dramatically lower maintenance costs and allow quicker turn-around time between missions. "A reliable, low-maintenance, reusable engine is key to affordable space launch," said Garry Lyles, Space Launch Initiative propulsion project manager at the Marshall Center. "Long engine life, whether 50 missions or 100 missions, will be selected to minimize operational cost."
Kerosene does have its challenges, however. It is not as efficient a coolant as hydrogen - a fuel more commonly used as a first- or second-stage propellant in the U.S. space program. Kerosene is a hydrocarbon fuel that becomes gummy and deposits a hard film on engine components as its temperature increases - a process known as coking. Coking makes it difficult for the rocket propellant to pass through the small coolant pipes that make up the engine combustion chamber. Kerosene combustion also deposits soot on the turbine blades, presenting a major challenge for a reusable rocket engine with a potential 100-mission lifespan.
To help offset these challenges, engineers are looking for ways to limit the kerosene temperature while cooling the thrust chamber, and limiting the sooty kerosene build-up in the turbine drive systems.
The RS-84 engine design includes an inventive arrangement of small sections of manifolds - pipes to funnel the flow of fuel - placed along the combustion chamber and engine nozzle. The manifolds weave the flow of kerosene fuel to the coolant tubes and collect the fuel after it has cooled the hot tube walls. The manifolds are spaced so the kerosene is not heated above the coking limit. Some RS-84 manifolds will also inject a small amount of kerosene directly into the thrust chamber to create a thin film of fuel along the chamber wall, helping lower the temperature of the wall. This arrangement reduces the temperature of the chamber walls and shortens the time the fuel is exposed to hot temperatures.
"Developing a safe, reliable and reusable kerosene engine has emerged as a top priority for the Space Launch Initiative Propulsion Office," said Danny Davis, RS-84 assistant manager at Marshall. "The Space Launch Initiative has determined that kerosene has significant advantages for reusable launch vehicle concepts. Our RS-84 design team is answering the reusability issues with the manifold arrangement to make this kerosene concept possible. In addition, our oxygen-rich staged combustion concept uses a cleaner, heated oxygen gas to drive the turbine to avoid past issues with kerosene-rich gasses."
Another approach being taken by the TR107 engine team is duct-cooling the main combustion chamber and using materials that do not catalyze, or interact, with the kerosene to form coking. Adding a duct or liner inside the chamber to separate the fuel from the chamber wall allows a controlled way to cool the chamber while keeping the temperature of the kerosene down.
"The potential benefits of duct cooling certainly address the coking issues," said Rick Ryan, TR107 assistant manager at Marshall. "In addition, duct cooling has the potential for significant simplification of the rocket engine. By eliminating the regenerative cooling circuit, you eliminate many large manifolds and associated plumbing. This, in turn, eliminates several potential failure modes that affect the engine reliability," Ryan noted.
While the duct cooling concept is very different than past large rocket engine nozzle concepts, early analysis shows significant potential. Like the RS-84 engine, the TR107 engine uses cleaner and cooler oxygen-rich propellants to help alleviate the sooty build-up on the turbine blades.
Kerosene is a relatively low maintenance fuel that allows for easier ground handling and decreased operational costs. In addition, because it is not a cryogenic, or extremely cold, fuel like hydrogen, the propulsion system does not require insulation for propulsion-related ducts, valves, lines and actuators - saving weight and cost.
One comparison between the new kerosene engines and the F-1 engine remains unchanged: As in the early Apollo days, NASA engineers and industry partners are writing a new chapter in space propulsion.
"It's an exciting time for NASA," Lyles said. "We are working on cutting-edge engine designs that include oxygen-rich, staged combustion options that have never been tried in the United States with the goal of making space launch safe, affordable and routine."
The Space Launch Initiative continues to further technologies needed to develop a future generation in space launch. The Marshall Center manages the Space Launch Initiative for NASA's Office of Aerospace Technology.