(U.S. Air Force) |
Launch Vehicle Propulsion
Jeff Emdee
Rocket engines have evolved over the course of several decades. Research at Aerospace has helped make valuable improvements in performance, cost, and reliability.
In 1920, The New York Times responded to a scientific paper in which Robert Goddard, the father of modern rocketry, discussed the possibility of sending a rocket to the moon. The Times editorial stated that Goddard's ideas were all wrong and that rockets could not reach the moon because there was "nothing for a rocket to push against in outer space." Of course, Goddard was correct, and in 1969, just after the launch of the Apollo 11 moon mission and 24 years after Goddard's death, the Times issued a belated retraction.
The technology underlying the propulsion systems that power today's rockets is being pushed to new limits. The analytical tools that Aerospace and contractors use to design and analyze engines have made significant improvements in speed and fidelity, but the hardware itself has evolved slowly compared with that of other high-tech industries. Characterized by extreme power density (enough pumping power to empty a swimming pool in 25 seconds) and severe temperature gradients (up to 3600 Kelvin), propulsion systems are understandably difficult to design with high reliability. Challenges have included reducing propulsion system mass to allow more room for payload, pushing propellant combustion performance closer to the theoretical maximum, and increasing reliability to make launch vehicles as dependable as aircraft. The future holds promise in these areas, but to appreciate the changes taking place, one must first be familiar with the basic physics of rocket propulsion.
Rocket Science
To understand what Goddard knew in 1920, one must go back to the 17th century and Isaac Newton's three laws of physics. The first law is simple enough: Objects at rest will stay at rest and objects in motion will stay in motion, in a straight line, unless acted upon by an unbalanced force. The second law describes the relationship between force, mass, and acceleration—that is, an object will accelerate when a force is applied to it. The third law—for every action there is an equal and opposite reaction—explains why rocket propulsion works in a vacuum. The simplest way to think of this is in terms of someone on a small boat jumping onto a nearby dock. When the sailor leaps for the dock, he moves forward. His action imparts a force, or reaction, to the boat, sending it in the opposite direction. Disregarding friction, the acceleration of the boat is proportional to the mass of the boat and the force imparted to it.
|
In a rocket, propellants are burned in a combustion chamber and the combustion products are exhausted through a nozzle. The individual exhaust molecules can be thought of as little sailors jumping from the rear of the rocket at very high velocity. Although each molecule may not weigh much, its individual action imparts a small reaction to the rocket and accelerates it forward, just like the small boat in the example. When one ton of combustion products exit the rear of a rocket at supersonic speeds—every second—they can generate enough force, or thrust, to push the rocket into space.
Typical launch vehicle propulsion systems generate thrust through the combustion of a fuel and an oxidizer. By definition, a rocket propulsion system does not rely on the oxygen in the atmosphere. Liquid-fueled engines use liquid propellants—such as kerosene and liquid oxygen—which must be rapidly pumped into the combustion chamber at a suitable mixture ratio (see sidebar, Liquid Propellents). Solid rocket motors, often used to supplement liquid-fueled engines, burn propellants that are held together in a solid rubber-like binder (see sidebar, Solid Propellents). Liquid-fueled engines typically provide more thrust per kilogram of propellant, but they're also more complex because of the turbomachinery involved. Solid rocket motors are generally lower performing but are self-contained propulsion devices, which makes them suitable for smaller rockets or strap-ons with minimal integration.
Liquid Engine Power Cycles
Liquid engines can be categorized according to their power cycles—that is, how power is derived to feed propellants to the main combustion chamber. The most common arrangements include the gas-generator, staged combustion, expander, and pressure-fed cycles (see sidebar, Power Cycles).
The selection of one power cycle over another must be made after careful design trades are considered. In design studies, Aerospace engineers use weight codes and power balance models developed in-house to make these trades. In-house design codes are used in many cases because Aerospace is in a unique position to employ a diverse set of contractor data to calibrate and correlate the models, making them more accurate than public codes.
The power balance models are used to simulate the engine pressures, temperatures, and pump speeds. The flow rates and pressure drops are balanced to produce a working design complete with dimensions of major components such as the pumps and chambers. The engine mass is then calculated using the pressures, temperatures, geometry, material strength properties, and appropriate factors of safety. Often, advanced lightweight materials are inserted into a concept design to judge the benefit of using these materials against the development risk of creating, testing, and certifying them. In the final analysis, the engine mass, performance, and cost are traded to best meet the program needs.
Propulsion Today: An Evolutionary Approach
The U.S. rockets flown today evolved from the ICBM fleet deployed around 1960. The Delta IV and the Atlas V—the two rocket families in the Air Force's Evolved Expendable Launch Vehicle program—trace their roots to the original Thor and Atlas missiles. The Delta IV's main engine, the RS-68, is based on a gas-generator cycle with lessons learned from the shuttle program. One of the reasons for this evolutionary approach is purely financial; the cost of developing and certifying a large booster engine can easily exceed $500 million, so risk must be managed carefully.
Liquid-Fueled Engines
Some of the more prominent liquid-fueled engines used in the United States today include the RS-68, the RL10, the RD-180, and the space shuttle main engine.
RS-68. The Delta IV RS-68 employs a gas-generator cycle using liquid hydrogen and liquid oxygen. It's the first new engine designed and built in the United States to fly since the 1970s. At 3310 kilonewtons vacuum thrust, it's also the most powerful hydrogen/oxygen system in the world. Still, the goal of this design was not to incorporate advanced technology. The commercially developed engine was designed with cost as an independent variable and as such used existing technologies to minimize risk.
In the past, engines for the ICBM fleet and the space shuttle had strict performance requirements; cost was often considered secondary. Propulsion technology was pushed to its limits to meet program goals. The Delta IV program chose high performing hydrogen for its fuel instead of the kerosene used in the Delta II and Delta III so that it could meet its payload performance requirements with a relatively inexpensive, low-technology engine. The RS-68 chamber pressure is only about half that of the space shuttle main engine. As a result, the engine is relatively large at 5.2 meters tall. This is a disadvantage in terms of mass, but an advantage in terms of manufacturing because large tolerances can be used in the design. The main pump housings use castings rather than machined and welded parts, a decision that increased mass but reduced cost. Also, the engine's ablative composite nozzle extension weighs more than a sheet-metal nozzle or cooled nozzle would. This nozzle was selected in a design trade that pitted manufacturing cost against performance. The result is a new engine with a cost-competitive design.
The Delta IV RS-68 main engine is the world's most powerful hydrogen/oxygen engine. At 100 percent power level, the engine produces 3.3 meganewtons of thrust. The turbopumps can pump more than 815 kilograms per second of propellant into the combustion chamber when operating at full power. The engine can also be throttled to 57 percent power to meet mission trajectory needs. Three RS-68 engines will power the Delta IV heavy launch vehicle. (NASA) |
The RL10 engine propels the Delta IV and Atlas V upper stages to their final orbit for payload delivery. Capable of generating 110 kilonewtons of thrust, the Delta IV RL10B-2 shown here has a large carbon-carbon nozzle extension with an exit-to-throat-area ratio of 285:1. The large nozzle increases the specific impulse, or fuel efficiency, of the engine, enabling higher vehicle performance. The nozzle extension can be seen glowing red during a qualification engine firing. (U.S. Air Force) |
RL10. The RL10 family of rocket engines has been around since the early 1960s. This expander-cycle engine was the first hydrogen-powered engine to fly in space and the first to be restarted in space. At 20 Kelvin, liquid hydrogen is a difficult fuel to handle but offers superior performance. Developed initially from a turbopump planned for a top-secret hydrogen jet program, the RL10 has gone through numerous design upgrades. Few people are aware that six RL10s were used to power the Saturn I second stage. The RL10 was also a critical part of the missions that launched the Voyager and Cassini interplanetary spacecraft. The RL10 now powers the second stage for both the Atlas and Delta family of vehicles. The RL10B-2 used in the Delta III and Delta IV produces 110 kilonewtons of thrust and has a large, lightweight carbon-carbon nozzle—the largest in the world. The large nozzle expands the supersonic exhaust gas to extremely high velocities, yielding the highest performing chemical engine ever built. The RL10A-4-2 found in the Atlas III and Atlas V, which generates 99 kilonewtons of thrust, employs a new redundant electronic ignition system that improves reliability for the critical start sequence. The engine's restart capability is used to propel payloads the final distance to parking orbit, insert payloads into geosynchronous transfer orbit, and circularize the final geosynchronous orbit.
Space Shuttle Main Engine. The hydrogen-powered space shuttle main engine is the only reusable passenger-rated engine in use today and was the first U.S. engine to use the staged combustion cycle. This cycle was chosen because of its ability to generate the high 206 bar chamber pressure needed to efficiently propel the shuttle orbiter. The development program was at times quite difficult. Many times, turbomachinery would explode during a test, or a valve would disintegrate after a few short seconds of testing as a result of catastrophic oxygen fires. Several of these problems arose because the program was pushing technology at the same time it was being implemented. For example, engineers were subjecting materials to high-pressure hydrogen for the first time and witnessing new problems such as hydrogen embrittlement. In addition, engineers struggled with pump cavitation phenomena that were never seen before. In the end, the space shuttle main engine saw more than 100,000 seconds of test time. As a result, the engine has been remarkably reliable in flight. In fact, the fleet of engines has been fired more than 300 times with only one engine shutdown in flight—and that was caused by a faulty sensor reading.
RD-180. The Russian RD-180 engine is also a staged-combustion engine using kerosene and liquid oxygen. This engine uses an oxygen-rich preburner, unlike the fuel-rich preburner used in the shuttle engine, to produce 255 bar chamber pressure. Kerosene is easier to handle but lower performing than hydrogen and produces soot and coking products that can clog chambers and turbines. The oxygen-rich preburner eliminates concerns over turbine soot and enables a higher chamber pressure to partly compensate for the performance shortfall; however, the hot oxygen-rich gas requires special coatings to keep the metal components from burning.
Solid Motors
Solid rocket motors have been in use for centuries as small rockets and fireworks. In the early 1960s, Aerospace helped pioneer the use of solid motors on large launch vehicles with the addition of strap-on motors for the Titan III rocket. An even larger version of this multisegment motor design concept is used on the space shuttle. These strap-ons provided the additional thrust-to-weight performance needed at liftoff. The solid motors on the Evolved Expendable Launch Vehicles are the latest in the solid motor design history.
The Atlas V solid rocket motor provides additional liftoff thrust for the Atlas V Evolved Expendable Launch Vehicle. The motor, shown here during one of the horizontal ground firings, makes use of Peacekeeper and Minuteman technologies. (International Launch Services) |
Atlas V. The solid rocket motor used on the Atlas V launch vehicle is the largest monolithic (single segment) solid motor in the world. It measures 1.5 meters in diameter by 19.5 meters long and produces 1130 kilonewtons average thrust. The Atlas V can accommodate up to five solid rocket motors, each weighing 46,500 kilograms fully loaded. Each motor has a fixed composite nozzle. Although it is a new motor, much of its heritage technology comes from the Peacekeeper and Minuteman missile programs, including the filament-wound graphite-epoxy case.
Delta IV. The Delta IV GEM-60 motor is an evolution of the GEM (Graphite Epoxy Motor) family. The 34,000-kilogram GEM-60 is a 1.5-meter-diameter motor more than 16 meters long, cast as a single segment. The motor case is filament wound by computer-controlled winding machines using high-strength graphite fiber and epoxy resin. The Delta IV GEMs are ignited on the ground to optimize performance. The average thrust of each motor is 850 kilonewtons. The Delta IV can employ up to four GEMs with movable or fixed composite nozzles.
Aerospace has developed new analytical tools to help evaluate solid rocket motor operation and performance. These tools include ignition transient models, ballistic models, and thermal-structural models. The ballistic model is used to predict motor pressure and the propellant-grain burn-back profile as a function of time. This tool is used to gain confidence that the performance specification can be met with the full range of operating temperatures and propellant properties. For the Atlas V solid rocket motors, new advances were required in the transient modeling to predict the three-dimensional flow patterns at ignition. Three-dimensional computational fluid dynamics can be rather intensive and time consuming. Aerospace developed new techniques that allow the 3-D flow to be represented by integration of multiple 2-D flow fields. The results of these models were used to predict the thermal-structural behavior of the Atlas V solid rocket motor.
In evaluating GEM motors, Aerospace developed unique inspection tools to gain confidence in motor designs and margins. These inspection tools include processing of ultrasonic signals to verify manufacturing integrity of the composite materials. Aerospace has also drawn upon the experience and expertise in motor manufacturing from multiple Air Force programs to improve reliability. In examples like these, additional confidence is gained through Aerospace's independent efforts.
What the Future Offers
The Air Force and NASA are funding several efforts to push launch vehicle propulsion technology to new levels. The Integrated High Payoff Rocket Technology program, for example, is using a phased approach to increase performance and reliability while reducing cost. To support this program, Aerospace is conducting trade studies, design evaluations, and source selection activities.
 Model of the Integrated Powerhead Demonstrator being developed by the Air Force Research Laboratory and NASA. The engine will provide higher power at more benign conditions than the space shuttle main engine by using both a fuel-rich preburner and an oxygen-rich preburner. (Air Force Research Laboratory) |
One important initiative within this program is the Integrated Powerhead Demonstrator. The goal of this effort is to demonstrate a highly reusable engine with less mass, more reliability, and higher performance than the space shuttle main engine at lower cost. To reach these challenging goals, the engine will use a new cycle, known as the full-flow staged combustion cycle.
As noted in the power cycles sidebar, the staged combustion cycle uses propellant efficiently and can generate high chamber pressures. Today, staged combustion cycles use either fuel-rich preburners (e.g., the space shuttle main engine) or oxidizer-rich preburners (e.g., RD-180) to generate the gas that drives the turbine. The Integrated Powerhead Demonstrator uses both types of preburners: A fuel-rich preburner drives the fuel pump and turbine, and an oxidizer-rich preburner drives the oxygen pump and turbine. There are several advantages to this arrangement.
First, all of the propellants are burned in the preburners, thus providing more mass flow for turbine drive power than the conventional staged combustion cycle. This additional power can be used to increase the chamber pressure and produce a smaller engine; alternatively, the preburner temperature can be reduced to provide the same power at lower temperatures. The lower turbine temperatures translate into longer turbine blade life—often the limiting factor on reusable engine life.
The second advantage is that the use of oxidizer-rich gas in the oxidizer turbine and fuel-rich gas in the fuel turbine eliminates the need for a complex propellant seal for the pumps. There is little risk with leaking liquid fuel into a fuel-rich gas or liquid oxygen into an oxidizer-rich gas. In contrast, the fuel-rich staged combustion cycle must use sophisticated purges and multiple seals in the oxidizer pump to prevent any liquid oxygen from leaking into the hot fuel-rich gas. A similar situation must be avoided in the oxidizer-rich cycle on the fuel pump side. The elimination of this failure mode increases system reliability.
Other reliability improvements are being pursued in design and manufacturing. For example, the pumps use hydrostatic bearings instead of ball bearings. Hydrostatic bearings allow the pump shaft to ride on a cushion of fluid instead of another hard material, thereby increasing the life of the pump. The Integrated Powerhead Demonstrator will be the first engine to successfully start and restart with these new bearings. In addition, modern design tools are being employed to gain a better understanding of the design margins and to ferret out potential failure modes during development.
Finally, the Integrated Powerhead Demonstrator program is developing new materials to be compatible with the oxidizer-rich hot gas. Steel alloys would burn in the hot gas generated by the oxidizer preburner, which can operate at pressures greater than 400 bar. Coatings and platings could be used to protect the steel, but these are not always amenable to long engine life. Therefore, nickel-based super alloys were created and tested until the right combination of compatibility and machinability was found.
NASA is now testing the pumps and preburners. The program has shown remarkable success, given the technical hurdles it had to overcome. Next year, the full engine system will be hot fired. Once the system is demonstrated, the technology risk will be reduced for future full-scale development programs. Aerospace has helped the Air Force with the early design evaluation, providing systems engineering expertise throughout the design process.
The Air Force is also working on technologies to improve upper-stage propulsion in general and the expander cycle in particular. Because the expander cycle uses heat from the combustion chamber to vaporize the liquid hydrogen that drives the turbine, the turbine power is dependent on the efficiency of the heat transfer. In the past, brazed steel tubes or slotted liners were used for the chamber cooling circuit. Both have drawbacks in manufacturing and heat transfer. Aerospace is supporting research geared toward improving the heat transfer while maintaining appropriate thermal margins. Chamber technologies under consideration include advanced copper alloys to enhance the heat transfer and new manufacturing techniques that reduce mass and production costs.
Also critical to the expander cycle is the hydrogen fuel pump. The fuel pump provides only 15 percent of the total propellant mass flow rate but can account for 80 percent of the horsepower requirements. Thus, inefficiencies can drive up size and weight. New high-speed turbopumps are being designed with a monolithic shaft, pump, and turbine rotor to decrease part count and increase reliability. Aerospace is a member of the government team pursuing this technology and is developing tools to assist in the design process. In addition, a systems engineering approach is being used to eliminate failure modes and produce a more reliable engine system.
In addition to launch verification, Aerospace develops concept designs for solid motors, liquid engines, and spacecraft propulsion systems. The next-generation ICBM, shown here, was modeled for the Air Force using sizing and performance codes for solid rocket motors. The codes are integrated into a single graphical user interface to facilitate rapid design turnaround. |
In the solid motor area, advancements are being made in new high-strength composite fibers for lightweight motor cases and improved low-erosion nozzle materials and energetic propellants. Recently, the Air Force demonstrated a test motor that used these new materials to reduce system inert mass by 15 percent and improve payload capacity by almost 30 percent. These performance improvements result in cost savings of more than 30 percent in dollars per kilogram to orbit.
Other solid motor efforts are focused on developing modeling and simulation tools to aid future design efforts. Improvements are being pursued in the area of two-phase-flow particle models, performance prediction tools, and motor mass fraction models. Aerospace has developed motor sizing and performance codes that permit trade-offs in the design of future missiles for the ICBM fleet and for the Missile Defense Agency. These codes are used with graphical interfaces to seamlessly integrate results from performance models and weight models to develop rapid concept designs for proposal evaluation.
Conclusion
Propulsion systems are quite literally the driving force behind any effort to get a payload into space. Advances in engine technology have helped the Evolved Expendable Launch Vehicle program realize significant gains in performance and cost. As the launch community looks forward to the next generation of systems, Aerospace tools and expertise will continue to play a central role in the development of more affordable and reliable launch technologies.
To Winter 2004 Table of Contents
