This article discusses the three federally funded projects that are underway to develop new rocket engines that can make it more affordable to send payloads into orbits. The new RS-68 propulsion system is Rocketdyne's entry in competition to power the US Air Force's new heavy-lift booster. The most ambitious of the new propulsion system designs is Rocketdyne's XRS-2200 linear aerospike engine, a seemingly nozzle-less oxygen/hydrogen powerplant that is designed to send the autonomously controlled NASA X-33 lifting body into orbit. The X-33 is being developed by Lockheed Martin Skunk Works, Palmdale, CA. The key for new launch vehicles, whether they're expendable or reusable, is to get the costs down. The article also highlights that the payload that can be lofted by a launch vehicle depends in large part on engine performance and the ratio of propellant to structural weight. Bell nozzles are designed to offer the best compromise of shape and length for a vehicle and flight path. Rocketdyne's R-68 engine is to be 17 feet tall and 8 feet wide at the base. The key to the R-68 engine design was the selection of hydrogen as the propellant rather than kerosene.
It has been nearly a quarter century since the 'American aerospace industry produced a new rocket engine that could propel a space vehicle of any size into orbit. Today, the U.S. government is funding three projects to develop new, less-costly booster rockets. Rather than focusing on pure performance, as propulsion engineers had in previous development efforts, substantially greater affordability is the key to the new, simplified launch engine designs.
The high price of traveling to orbit in today's "throwaway" or expendable launch vehicles is largely due to their need to jettison successive propulsive stages en route. In the case of the complex, reusable Space Shuttle, high maintenance requirements make orbital launch prohibitively costly for all but critical or lucrative missions. Success with the new trio of American rocket engines could lead to a new era of routine, reliable, and low-cost access to space.
The R-68 liquid oxygen/hydrogen-fueled rocket engine is a pragmatic new design effort by the Rocketdyne Propulsion & Power Business of the Boeing Corp. in Canoga Park, Calif. It is vying with the Lockheed Martin/NPO Energomash/Pratt & Whitney RD-180-a reengineered version of an existing liquid oxygen/kerosene rocket developed in Russia--to see which will power the first stage of the future Evolved Expendable Launch Vehicle, a part of the U.S. Air Force's $2-billion space-lift modernization program. The modular EEL V, which will be based either on Lockheed Martin's Atlas 3 booster or on Boeing's Delta 4, will be a flexible (building-block-type) system that the Air Force expects will cut medium- and heavy-lift launch costs by 25 percent or more. The R-68, which is to generate 650,000 lbs. of lift-off thrust (745,000 at altitude), will be the most powerful oxygen/hydrogen fueled engine in the world.
The National Aeronautics and Space Administration, meanwhile, is developing the oxygen/kerosene-fueled Fastrac engine, a bare-bones approach to a 60,000-lb.-thrust, single-use powerplant that is to propel the air-Iaw1Ched x-34 spaceplane into space and back to a wing-borne landing. The reusable X-34 is being built by an industry team led by Orbital Sciences Corp. of Dulles, Va. The single-stage-to-orbit (SSTO) vehicle will reach speeds of Mach 8 and greater, carrying small commercial and research payloads to sun-synchronous orbits at one-tenth the cost of current expendable launch systems.
The most ambitious of the new .propulsion system designs is Rocketdyne's XRS-2200 linear aerospike engine, a seemingly nozzle-less oxygen/hydrogen powerplant that is designed to send the autonomously controlled NASA X-33 lifting body into orbit. The X-33 is being developed by Lockheed Martin Skunk Works in Palmdale, Calif. The unique aerospike engines, each of which will produce 206,400 lbs. of thrust at sea level, allow engineers to design the smallest, lowest-cost reusable SSTO vehicle because they cut weight and aerodynamic drag. NASA is hoping this subscale, 76-foot-long spaceplane will lead eventually to a full-size Reusable Launch Vehicle that could slash launch costs for larger payloads early next century. The RLV is to take off vertically, gain orbit, deliver its package, and then fly back to Earth, landing horizontally.
Cutting Launch Costs
"The key for new launch vehicles, whether they're expendable or reusable, is to get the costs down," said Steve Bouley, director and program manager for RLV programs at Rocketdyne. Today, a typical satellite launch to geosynchronous transfer orbit using current expendable boosters-such as the American Atlas, Delta, and Titan; the European Ariane; the Russian Proton; or the Chinese Long March-can cost anywhere from $10,000 to $13,000 per pound, he said. Meanwhile, there is a lot of debate regarding the true cost-operational and otherwise--of the first and only reusable launch vehicle, the Space Shuttle, which NASA uses mainly for human missions to low Earth orbit. The belief tl1at lie complex Space Shuttle costs too much to maintain is widely held. NASA's "near-term" goal for launches using next-generation reusable vehicles is about $1,000 per pound of payload.
The U.S. Department of Defense has been the major customer for American expendable boosters since the early days of the space program, noted Art Weiss, director and program manager of Rocketdyne's R-68 program. Government-sponsored launches have always been relatively few in number, however, and low production volumes do not lend themselves to low cost'; he explained. Today there is a lot more interest in launching commercial satellites, particularly the new communications satellite constellations, Weiss said, but "the current launch infrastructure is 40 years old." Hence, the emphasis on new launch system technology.
Both Rocketdyne and NASA engineers cite modern computer-aided design and manufacturing technologies as the linchpin of their efforts. "Our Intergraph CAD system and the advanced engineering analysis packages we use allow us to confront design risks directly, as opposed to adding complexity to avoid design risk," said Danny Davis, project manager for low-cost technology at NASA Marshall. "We have more faith in the analysis and design, so we're more comfortable taking risks, which results in simpler, more elegant designs," he continued. "In the past, it was often easier to design around risks by adding parts. In our case, the components we develop are inexpensive enough to test to failure without killing the project," Davis said. "In fact, nearly all the equipment came out of the box and worked perfectly the first time."
The two Rocketdyne managers had similar positive comments about their company's. Pro / ENGINEER based-CAD and engineering analysis system. "The keys to the new designs are the 3-D solid-modeling tools we now have," Weiss said. "Our analysts now work with the same model and database as the designers." For instance, the first set of hardware for the R -68 fit together the first time. "That's never happened before," he reported.
High Launch Costs
The reasons why current launch costs are so high have to do with physics, according to T.K. Mattingly, former director of reusable launch vehicles for Lockheed Martin. To place a payload into orbit, a rocket must ascend above the atmosphere while giving its payload sufficient horizontal velocity so that when it falls back to earth, the planet's curved surface "falls" away at the same rate. This task takes a lot of power. Though the cost of the propellants containing the required energy is minor, the expense involved in building complex launch vehicles is not. The reason for " this seemingly profligate approach," he explained, relates to the basic principles of rocketry.
The payload that can be lofted by a launch vehicle depends in large part on engine performance and the ratio of propellant to structural weight, Mattingly stated. The designer thus must maximize propulsion efficiency and minimize the amount of mass to be accelerated. Unfortunately, even the best efforts to improve efficiency and reduce mass have so far fallen short of what is needed to reach orbital velocity with a single set of rocket engines. That feat requires something like 90 percent of me vehicle's weight to be allotted to propellant. Only by using two or more separate stages, each with its own engines and fuel, have designers been able to build practical boosters.
Such "staging" works because it lets segments of the booster be discarded along the way, he continued. That capability provides a great advantage. Quickly lifting a launch vehicle off the ground and out of the thickest part of the atmosphere requires high-thrust engines and voluminous propellant tanks to feed them. But such large engines are unnecessary in conditions up at altitude. By dropping these weighty components and using more suitably sized counterparts in the upper stages of the vehicle, the mass that must be accelerated to orbit can be minimized.
Using separate stages has other advantages, Mattingly noted. It turns out that a rocket engine is' most efficient when its exhaust gases exit the nozzle at the prevailing atmospheric pressure. At low altitudes, where air pressure is high, this effect favors a short nozzle. However, in the near vacuum of the upper atmosphere, a longer nozzle is more effective. Staging thus allows the use of nozzles that work reasonably well even as the craft climbs through progressively thinner air.
Though the introduction of multiple stages allowed practical rocket boosters, they require labor-intensive checking processes to ensure that their entire mechanism is working perfectly, Mattingly stressed. The cost of this stringent inspection accounts for about half that of the total system, he claimed.
Meanwhile, reusability has its own problems to overcome: For example, after withstanding a rough flight, everything must be brought back to Earth gently so it can fly again and again. The key challenge to the success of reusable launch vehicles, however, will be the development of an efficient propulsion system that is light weight yet powerful enough to allow for SS TO flights. The other major hurdle is a lightweight but strong and h e at-resistant structure.
The plan to meet the challenge of heavy-lift SSTO operations is to take a 30-year-old idea- the aero. spike engine-update it, and then match it to another vintage technology: a lifting-body design in which the entire airframe provides aerodynamic lift. In July 1996, NASA awarded a $1. 125-billion X-33 cooperative agreement to a Lockheed Martin-led development team. The federal space agency has budgeted some $941 million for the X -33 program through 1999.
Lockheed Martin plans to conduct 15 flight tests of the X-33, beginning in July 1999. Originating from Edwards Air Force Base in California, the completely autonomous and preprogrammed suborbital flights will traverse from 400 to 900 nautical miles. The lifting-body design, which generates a shock-wave shape that keeps extreme heat off all but certain surfaces, requires significantly less protective sheathing than the Space Shuttle.
The main difference between the linear aerospike and conventional rocket engines is the shape of the nozzle. A rocket nozzle's job is to expand the combustion products of the burning propellants, which decreases pressure and increases velocity. Unlike a standard rocket engine, which features a bell-shaped nozzle that constricts expanding exhaust gases to maximize thrust, the basic aerospike shape is that of a bell turned inside out (see the illustration below). Whereas the bell nozzle expands the hot exhaust gas on its inside surface, the aerospike nozzle expands the gas on its outside surface.
Bell nozzles are designed to offer the best compromise of shape and length for a particular vehicle and flight path. At higher altitudes, gases could expand further if there were more nozzle length, thereby improving performance. But since the nozzle cannot easily change shape--that is, grow longer-it gives up some performance. "For a bell nozzle, the designer selects the area ratio, which defines the length and the curvature of the bell," Bouley said. "The designer chooses a configuration that is optimized for a specific altitude. It therefore offers maximum efficiency only at that altitude." Because most of the flight time is spent at high altitude, designers prefer large area-ratio nozzles. But large area-ratio nozzles operating at sea level produce separated exhaust airflows, performance losses, and high nozzle structural loads. Variable bell nozzles are mechanically complex and not cost-effective for this application.
As stated , the linear aerospike nozzle h as a "V" shape, with the vertex pointing toward the rear. The aerospike has a series of small combustion chambers along the forward rim of what is called the ramp. Hot gases shoot along the ramp's curving outer surface, producing a spike -shaped plume, hence the name aerospike. The ramp serves as the inner wall of the bell nozzle, while ambient atmospheric pressure serves as the invisible outer wall. The combustion gases race along the ramp and the outer wall of air to generate the thrust force. In essence, Rocketdyne engineers removed half of the standard nozzles and canted them inward, thereby forming a central ramp or spike.
The high efficiency of the aerospike arises from the external expansion of the combustion gases. In a bell-nozzle engine, a particular nozzle shape and length will expand its combustion gases outward only as far as the nozzle allows. For maximum thrust, those gases should exit the nozzle after they have expanded enough so their pressure has dropped to match that of the surrounding• air. If they leave the nozzle sooner, energy is wasted expanding the gas far behind the vehicle.
As a linear aerospike shoots its exhaust gases across the central ramp, they are not enclosed by a nozzle, so the exhaust gases can expand to the ambient atmospheric pressure while reacting against the ramp. The aerospike can thus self-compensate for decreasing atmospheric pressure as the vehicle ascends, keeping the engine's performance very high throughout the entire trajectory. While the free side blooms outward to adjust the combustion gas pressure to the local atmospheric pressure, it is pushing against the nozzle ramp surface on the other side, producing thrust. This arrangement lets the engine operate near maximum effectiveness at all altitude.
Aerospike technology was pioneered by Rocketdyne engineers in the 1950s, first with Air Force and then NASA funding support, Bouley said. "So far, about $500 million (in equivalent 1996 dollars) has been spent on the technology from the 1950s to the 1970s," he noted. During that period, earlier full-size aerospikes have undergone some 73 tests, along the way accumulating more than 4,000 seconds of operation. Though the propulsion concept has been investigated thoroughly, an aerospike engine has not yet flown, he said.
Aerospike nozzles can be circular or linear. "The original design was a circular or annular aerospike, which features a plug-like ramp," said Bouley. "The linear-type aerospike is a two-dimensional version of the annular type." To save weight, the spike is typically truncated, which does entail some performance loss.
The novel engine's altitude-compensating feature allows a simple, low-risk gas generator cycle to be used as well, according to Bouley. "Since the free stream air acts as a variable area-ratio nozzle, the aerospike can capture performance levels close to that of [closed] staged combustion cycles," he said. " In the gas-generator system, we burn some propellant at low performance conditions to run the turbopump." (To save money, turbopumps recycled from Rocketdyne's J-2 engines for the Saturn 5 second and third stages will be used in the X-33.)
And rather than recirculating and burning the turbine exhaust gases, as does the staged combustion system of the Space Shuttle Main Engine, or merely dumping the exhaust, as do most other bell-nozzle engines, the aerospike is designed to exhaust gases through the truncated end of the spike, creating additional thrust. By dumping the exhaust into the truncated base region, Bouley explained, the pressure in that region is elevated, which gives you back some of the performance loss associated with base drag, which occurs when the slipstream behind a vehicle does not re converge immediately. Use of the gas generator also eliminates the extra weight of a staged combustion system whose high-pressure operations require heavier, more costly components.
Another benefit of the aerospike is that it makes a very good fit with the X-33's lifting body configuration, Bouley continued. The rectangular-shaped linear aerospike engine fits nicely into a vehicle with a rectangular aft end. And because the aerospike 's conformal nature lets it produce a hot gas plume that nearly fills the base area of the vehicle, it lowers base drag.
Despite being rigidly mounted to the airframe, the aerospike engine needs no gimbals to direct rocket thrust. The direction of the X-33's flight will be controlled by varying the thrust from side to side and engine to engine. "We'll use differential thrust to steer the X-33, which will be controlled by throttling the upper and lower banks of engines or some combination to get pitch, roil, and yaw control," Bouley explained. To generate the necessary moments, the X-33 will have four banks of thrusters (comprising two engines) mounted in a rectangular array. He noted that the lack of systems for thrust vectoring- gimbals, hydraulics, and flex lines-lessens weight and should make the aerospike easier to maintain than conventional engines. In the atmosphere, the X-33 lifting body will augment flight control with aerodynamic control surfaces; in space, it will use a thruster-based reaction-control system.
Like the X-33 vehicle itself, the linear aerospike is subscale_ Four engines, each weighing 7,500 lbs., will be built. Two will be installed on the vehicle, while the other two will be used for testing, with one rebuilt and used as a spare. Each of the engines will have a series of 20 combustion chambers-10 lining the forward end of each nozzle ramp. The propulsion systems will produce a specific impulse of 339.9 seconds at sea level and 429.8 seconds in a vacuum. The full-scale aerospike for the RL V is to make 431,000 lbs. of thrust with 14 combustion chambers (seven per side). Specific impulse levels will be in the 380- to 390-second range at sea level, and 450 to 455 seconds at altitude. T he X-33 engine will be able to throttle from 50 to 105 percent of its designed thrust level, while the RLV engine will throttle from 18 to 100 percent. Engine life is expected to be 100 missions.
Bouley reported that the design work for the X-33 aerospikes at Rocketdyne is complete. Fabrication, component testing, and engine assembly are in progress, and powerpack/ engine ground testing should be completed by mid-1999. The first flight is scheduled for 1999.
The decision of whether to proceed with the RLV (also called VentureStar) is to be made in the year 2000. Success with the new SSTO design will require a considerable improvement in performance, however. As with all SSTO vehicles, mass-fraction is the biggest technical challenge.
The widespread adoption of composites should help VentureStar to shave weight. Better volumetric efficiency will also be needed, because there is too much empty space on the X-33.
Another needed measure will be to lower the weight of the aerospike engine itself. "We plan to use ceramic, matrix composites in some rotating parts of the turbine assembly," Bouley said. "We may also take advantage of the high specific strength of metal-matrix composites to replace cast structures, as well as heat-resistant composites in the nozzle ramp." To obtain several additional seconds of specific impulse, higher turbopul11p turbine temperatures and greater chamber pressure may also be used.
Engineers at NASA's Marshall Space Flight Center are designing Fastrac, which may be the world's simplest turbopump rocket engine. Fastrac is to power the X-34 Technology Testbed Demonstrator, an air-launched spaceplane that is to take small payloads to orbit at one-tenth the cost of current booster systems. NASA, the U.S. Department of Defense, and an industry team led by Orbital Sciences Corp. are playing key roles in the development and eventual flight testing of the X-34. NASA awarded the $50-million X-34 contract to Orbital Sciences in August 1996.
Plans call for the autonomously operated, suborbital aerospace plane to be dropped from an L-1 011 airliner. At speeds up to Mach 8, the 58-foot-long vehicle is to fly to an altitude of approximately 50 miles. The single- stage X- 34 will also demonstrate the ability to conduct subsonic flights through rain or fog, as well as autonomous landings in crosswinds of up to 20 knots.
Fastrac itself is a gas generator-cycle engine, which will produce 60,000 lbs: of thrust by burning a mixture of liquid Oxygen and R P-1 kerosene. It is 7 feet long, 4 feet wide, and weighs almost 2,000 lbs. Specific impulse at altitude will be 310 seconds. The new engine design features a single turbopump as well as a single use combustion chamber liner and bell nozzle that will be replaced after each powered flight.
NASA Marshall's Danny Davis said that engineers have assembled and tested the first engine unit. Some 75 hot firings will be conducted at the NASA Stennis Space Center in Mississippi. Ground testing of the X- 34 is scheduled for March 1999, with the first flight test taking place in August of that year.
"NASA is interested in upgrading the nation's launch cap ability in the low end of the payload spectrum around 150 to 300 kilograms," Davis said. "Today, there's no dedicated launch service for this payload size, which is mostly research satellites placed in solar-synchronous orbits from 200 to 300 nautical miles up. In the future, we'll see more communications satellites in this size range," he predicted. " It's sort of a field of dreams: If you build a vehicle, the payloads will come.
"Though these payloads can cost a few million dollars to build," Davis noted, " the ride to space can cost $15 to $17 million. Since our goal was to get much of the cost out, we went after the single most expensive part of a launch vehicle-the engine." The simple, robust, and easy-to-build Fastrac engine is designed to achieve a dramatic reduction in cost, initially to about $1 million apiece-one-seventh the cost of current similar systems. In a departure from traditional engine-development processes, the Marshall team adapted off-the-shelf technologies and common manufacturing methods to develop the new engine in a faster than-usual design cycle. Fastrac's price should drop even further as future enhancements come on-line-eventually to $350,000 per engine.
While some concepts for the Fastrac engine have been around for decades, actual technology development and design on the new rocket engine began in March 1996. "Our goal was to eliminate labor- intensive manufacturing procedures;' Davis said. "We hope that eventually anybody could build it." Another focus of the program was "getting the parts count down."
A typical rocket launch produces a temperature in the range of 5,500° F, hot enough to melt most enginechamber and nozzle materials. The common solution is regenerative cooling, which circulates liquid fuel or oxidizer around the engine chamber and nozzle through hundreds of feet of tediously welded tubing. The Fastrac engine design avoids complex plumbing, opting instead to cool the rocket chamber by charring or scorching its inside surface as the engine heats, a process called ablative cooling. Fastrac features a thrust-chamber nozzle that uses a silica-phenolic liner strengthened with a graphite/epoxy overwrap, Davis said. "Thiokol Corp. [of Ogden, Utah] is building the combustion chamber and nozzle by wrapping silica-phenolic tape, followed by graphite/ epoxy tape, around a mandrel, at the same time embedding a steel flange for assembly purposes." The new nozzle should cost about $100,000, compared with $800,000 for previous similar systems.
The Fastrac engine uses a simplified gas-generator cycle, which burns a small amount of kerosene and oxygen to provide gas to drive the turbine and then exhausts the spent fuel, Davis explained. The new engine's simplified three-piece gas generator contains one-tenth the parts of previous designs. Summa Technologies Inc. of Huntsville, Ala ., is building the gas generator.
Chamber pressure is supplied by a single turbopump. The new engine design incorporates significant reductions in the cost of turbomachinery, which "is typically a big technical challenge," according to Davis. "We used system design to reduce the complexity of the turbopump." By using simplified casting techniques and by putting both fuel and oxidizer pumps on the same shaft, engineers at Marshall and Barber-Nichols Inc. in Arvada, Colo., believe they have developed a rocket pump that would cost only $320,000, compared with nearly $1 million each for the oxygen/ kerosene pumps on the current Atlas booster.
In addition, Fastrac 's injection system has only three parts: a face plate with a drilled hole pattern, a brazed injector body, and a gimbal block, Davis said. "The biggest problem was figuring out what the hole pattern should be so you don't cook the silica/ phenolic chamber," he noted. As a result of the simplified design, the injector system will cost about $50,000 a copy, as opposed to $200,000 to $300,000 each.
Evolved Expendable Launch Vehicle
Rocketdyne's R-68 engine is to be 17 feet tall and 8 feet wide at the base, according to Art Weiss, the program manager. As noted, the new rocket engine is to generate performance levels exceeding any previous oxygen / hydrogen rocket system. Company engineers conducted a full-scale main injector test in November 1996 and a key gas-generator test in February 1997.
Weiss said that "we're going for the lowest recurring cost we can get-if possible, an order of magnitude reduction. Since cost drives the design, we're using off-the shelf technology-only proven systems and processes."
The part count has been minimized as well. "The R-68 will have one-tenth the parts of the Space Shuttle Main Engine, even though it produces 50 percent more thrust," he said. For example, the engine's 2,000-lb. turbopumps have only 30 parts, compared with 200 components in the SSME turbopumps. The next-generation expendable engine will use a simplified gas-generator cycle, Weiss noted. And, as with Fastrac, a simple ablative nozzle made of silica- phenolic materials will be used rather than a regenerative nozzle with all its complex tubing.
Weiss stressed that the key to the R-68 engine design was the selection of (higher-energy-content) hydrogen as the propellant rather than (lower-energy-content) kerosene.
"This allowed us to fully implement cost as the independent variable in the design process," he said. "For example, we're expecting a mission average (for specific impulse) of 410 seconds, which might seem a little low for an oxygen/ hydrogen engine. But the use of hydrogen allows us to trade away some performance to get the costs down." The entire design exercise, he said, required a change in mindset from the propulsion engineer's traditional focus on high performance to a concentration on lower costs.