Hypersonic X-51 Scramjet to Launch Test Flight in May

Hypersonic X-51 Scramjet to Launch Test Flight in May
The Boeing Phantom Works X-51A vehicle with Pratt & Whitney Rocketdyne SJY61 scramjet hangs on B-52H wing mount under full moon.
(Image: © Pratt & Whitney Rocketdyne)

The first hypersonic X-51scramjet powered long-duration flights to give the Pentagon a new "PromptGlobal Strike" capability that ties atmospheric and space propulsion willbegin as early as May 25 at Edwards Air Force Base, Calif. The X-vehiclescramjet flight tests are also a key step for the use of air breathingpropulsion to launch into space.

As spectacular as space shuttleflights still are, they are also about the past. Scramjetpropulsion is about the future. Unlike rocket engines, scramjets(supersonic combustion ramjets) are air-breathing engines that inhale oxygenfrom the atmosphere to achieve near rocket engine velocities and altitudeswithout carrying tons of oxidizer supplies.

Scramjets should eventuallyenable flights to hypersonic near-space velocities and altitudes, just likehigh performance jet engines propelled the Anglo-French Concorde to supersonicspeeds in the upper atmosphere.

The tests will be flown by fourunmanned X-51"Waverider" vehicles developed by a team including the U.S. AirForce, the Defense Advanced Research Projects Agency, NASA, Boeing PhantomWorks and Pratt & Whitney.

This air-breathing scramjetcapability is expected by about 2025 to begin a revolution in U.S. space launchcapability especially for rapid, aircraft-like launch of small, but criticalU.S. military spacecraft.

Scramjet revolution

If the X-51 tests aresuccessful, President Barack Obama is also ready to approve a revolutionaryscramjet-powered conventionally-armed "Prompt Global Strike" missile.Within 60 min. of launch from Vandenberg Air Force Base, Calif., thisscramjet-powered hypersonic missile could strike anywhere on Earth withextraordinary kinetic impact energy. The Defense Dept./NASA X-51 program is themost critical element of this new weapons system concept. The Prompt Strikeprogram is directly under former astronaut Air Force Gen. Kevin Chilton, whoheads U.S. Strategic Command, the successor to the Strategic Air Command in thepost-Cold War world.

This new capability would givethe U.S. a new conventionally-armed fast strike weapon that could be readilyused on priority targets, as well as provide a deterrent without relyingtotally on destabilizing ballistic missiles, even if the ICBMs areconventionally-armed.

Flying at 4,000 mph, the PromptStrike scramjet could reach terrorists or threatening Iranian or North Koreanmissiles on their launch pads much faster than subsonic cruise missiles. Theneed is illustrated by an August 1998 Navy Tomahawk cruise missile strikeordered by President Bill Clinton that arrived on target just minutes too lateto kill Osama Bin Laden.

"Each X-51 will fly forseveral minutes and travel hundreds of miles in what will be the firstpractical demonstration of scramjettechnology," says Charlie Brink, the X-51A program manager at the AirForce Research Laboratory (AFRL), Wright-Patterson Air Force Base, Ohio.

Military applications comefirst

While the first priority isdevelopment of a Prompt Strike missile by about 2015, the scramjet is morebroadly "all about space lift," says Brink. "That is theapplication that AFRL is most excited about," he says.

After dropping from a B-52H at49,000 ft., the X-51 will be powered straight up by a solid rocket motor from theArmy Tactical Ballistic Missile Program. Rolling gently for stabilization, eachX-51 will climb to about 60,000 ft. and Mach 4, where the solid motor will beseparated.

The X-51's scramjet, fueled andcooled by conventional JP-7 based kerosene, will then complete the climb toabout 100,000 ft. firing for a planned 300 sec. while accelerating to Mach 6.5— more than 4,000 mph. Pratt & Whitney graphics show the vehicle inverted,with its inlet on top at ignition, prior to a final roll to complete the runwith its inlet to nadir.

As the space shuttle and its35-year-old technology is phased out, the Air Force, NASA and Obamaadministration see a bright future for a range of scramjet-powered vehiclesthat would cross the boundary between the atmosphere and space for moreaircraft-type operations to and from orbit.

In addition to air-breathingspace launch, scramjet/ramjet combinations would also enable air-breathingengine-powered landings instead of higher risk unpowered glides like the spaceshuttle is required to use.

Independent from commercialspaceflight

As a strategictechnology project, the scramjet development will proceed no matter whathappens in the near-term shift to commercial crew and cargo launch to theInternational Space Station.

If the four X-51 flights are evenpartially successful, they should hasten the development of advancedair-breathing engines for horizontal takeoff space launch vehicles. The NASAGlenn Research Center in Cleveland, Ohio, for example, has done extensiveresearch on an unmanned 130,000 lb. scramjet-powered winged reusable space planeconcept called "Trailblazer" that could carry 300 lb. payloads intoorbit.

The U.S. Defense Dept.'sHypersonic Roadmap is also heavily oriented to new U.S. space accesscapabilities as well as missile or reconnaissance air vehicle applications.

These scramjet applications wouldfulfill a major U.S. Air Force requirement for a Responsive Space Access (RSA)military launcher that would involve the seamless integration of"air-to-space" capability.

"It is to enable the abilityto get to space, return to Earth, then get back to space with an aircraft-likeoperations tempo," according to the Pentagon's Hypersonic Roadmap, aformal strategy governing the growing number of scramjet-related developmentsbeing pursued by the U.S. military and NASA.

Each X-51will be dropped from a B-52 bomber flying south over the Pacific followingtakeoff from Edwards. Each X-51 is 26 ft. long and weighs 2,000 lb.

Hypersonic Concorde

One U.S. Air Force ResearchLaboratory (AFRL) concept for future applications depicts a reusableConcorde-sized hypersonic transport equipped with four X-51 type scramjetengines. Carried on its back in the concept is a USAF X-37 type unmanned space planelike that currently undergoing tests in space after launch from Cape Canaveralon an Atlas 5.

The tests getting underway thismonth were to have begun in February or March but were delayed when initialB-52 captive carry tests found the coordination of chase aircraft and telemetryacquisition required greater definition, project sources said.

The X-51s will use a Pratt &Whitney SJY61 scramjet engine to accelerate to Mach 6.5-7 while"waveriding" atop their own shock wave to enhance both aerodynamicand engine performance.

An equally high scramjet goal isdevelopment by 2025 of an "operationally responsive space lift capabilitywith 'aircraft type' operations," according to Dr. William U. Borger,former head of the AFRL Propulsion Directorate, which developed the X-51s.

He cited the scramjet as one ofthe top four air-breathing propulsion developments in the history of aviation.Phase 1 was the Wright Brothers' 1903 Wright Flyer that conquered thepower-to-weight ratio necessary for human flight. Phase 2, begun in the 1950s,was the development of the jet engine that "revolutionized the shape andspeed of aerospace vehicles," Borger said.

In Phase 3 by 1970, large highbypass ratio turbofan engines enabled the development of jumbo jets like theBoeing 747, while Phase 4 is the present era of scramjet development that will"enable sustained hypersonic flight and routine access to space," hesaid prior to his recent retirement from AFRL.

In addition to Pratt &Whitney, the Utah-based Alliant Techsystems (ATK) that produces space shuttlesolid rocket motors has also been heavily involved in scramjet development andcould also someday be involved in scramjet engine production.

International scramjet race

The U.S. is also mindful aboutinternational competition in this important new propulsion sector. More than adozen countries have done some level of scramjet component development.

China has a large scramjettechnology effort underway, while Russia and France have also done scramjetwork, some of it cooperatively. Australia is also doing significant scramjetcomponent testing in cooperation with the U.S. But the X-51 is the firstscramjet flight test vehicle designed for long-duration flight.

Edwards Air Force Base has nothosted such high speed powered flight test since the end of the X-15 rocketplane program in 1968. John Armstrong, who heads the Hypersonic Joint TaskForce at Edwards, says that the X-51 has returned the excitement of the X-15program back to the legendary flight test center in the Mojave Desert east ofLos Angeles. There are major differences in a scramjet engine from any otherair-breathing powerplant.

The airflow inside a regular jetengine must be kept at subsonic velocities for it to maintain combustion as theair moves through dozens of rotating turbine blades. Even if an aircraft isflying at supersonicspeed, the airflow through traditional jet engines must be slowed tosubsonic speed by the inlet for the engine to function.

In a scramjet engine, theopposite is true. A scramjet has no moving parts and the airflow through itmust be kept at supersonic flow speeds. The scramjet's engine inlet is also aninherent part of the vehicle's overall shape.

How scramjets work

The overall inlet shape of theentire X-51 resembles what any larger scramjet-powered aerospace vehicle willalso look like. This is because scramjet-powered vehicles are essentially largeflying inlets that sculpture their aerodynamic shock waves for ingestion by theengine, and in the case of a waverider like the X-51, use the shockwave to pushthem upward as if generating lift.

Those conditions are achieved byunderstanding and harnessing extremely complex flow fields. This enablesscramjets to propel vehicles at up to about Mach 15 at altitudes where theupper atmosphere transitions to the vacuum of space. Future vehicles flyingthat fast in the upper atmosphere will need the same kind of thermalprotection pioneered by the space shuttle for re-entry starting at Mach 25.

This is why scramjets are anexample of the type of revolutionary propulsion that President Obama says willbe needed for future space operations. The Obama administration seeks more than$200 million in scramjet/Prompt Strike related development in the next budgetyear.

Substantially more federaldevelopment work is already being done on scramjets to get from Earth intospace, then even deep space propulsion for accelerating faster once in space toreach distant targets. More than a billion dollars in scramjet technology workhas been completed and the X-51 program has totaled about $200 million.

President Ronald Reagan in themid-1980s called for development of the X-30 National Aerospace Plane, ascramjet-powered vehicle that could demonstrate the capability to fly from NewYork to Tokyo in three hours. NASA, the Air Force and Defense Advanced ResearchProjects Agency never developed a flying model, but they pioneered technologythat is now making scramjet propulsion a reality.

Presidents George H. Bush, BillClinton and George W. Bush continued the technology work begun under Reagan.That, in turn, has put the Obama administration in position to approvedevelopment of a scramjet missile to broaden strategic strike options.

X-51 missile development wouldalso speed scramjet development for space launch. Future scramjet-powered spacelaunch vehicles could reduce launch costs and greatly increase launch flexibilitybecause they could make aircraft-type runway takeoffs accelerating to about 60percent orbital velocity without the need to carry heavy oxidizer and tankage.

In an earlier $240 million testprogram, the smallerNASA X-43A hydrogen gas-powered scramjet accelerated to Mach 9.6, or nearly7,000 mph, during a 10 sec. flight. It was dropped from the NASA Dryden FlightResearch Center's B-52, then boosted to its scramjet start speed by a Pegasusrocket motor.

Before complex computer modelingof internal air flow fields made scramjet propulsion possible, hypersonicvelocities could only be reached using rocket engine-powered vehicles with alarge percentage of mass devoted to oxidizer carried in heavy metal tanks.

At tests that will be separatedby several weeks, each X-51 will be dropped from a wing pylon on an operationalB-52H bomber flying at nearly 50,000 ft. over the Pacific Ocean off theCalifornia coast.

Future missions ahead

The X-51s will not be as fast asthe X-43 flown in2004, but it will use far more common JP-7 kerosene fuel and a uniquecooling design for a much longer flight of nearly 400 mi. With a larger fuel tank,the X-51's engine could run for as long as a mission dictates, says Brink.

For missions into space, futurescramjet-powered vehicles should achieve speeds approaching Mach 15 onair-breathing engines consuming oxygen from the atmosphere before a transitionto rocket power using oxidizer carried on board to accelerate to Mach 25velocities required to achieve orbit.

After the B-52H takeoff fromEdwards, each X-51 will be dropped over the Pacific Ocean above the highlyinstrumented Point Mugu Naval Air Warfare Sea Range, 50 mi. north of LosAngeles.

Years of Pratt & Whitneyengineering and internal engine flow modeling have come together to make theX-51 a true stepping stone to operational scramjet operations for eventualair-breathing missions to space. "We believe the X-51 will lead the pathto practical air breathing hypersonic flight," said George Thum, Pratt& Whitney Rocketdyne X-51A program manager.

As the engine ignites, it willinitially burn a mix of ethylene and JP-7 fuel before switching exclusively toJP-7. The use of a standard JP hydrocarbon fuel like an operational systemwould use is an important part of the test. There will be 270 lb. JP-7 on boardeach X-51.

Dean Andreadis, a specialist inflowpath aerothermal analysis and systems integration at Pratt & WhitneySpace Propulsion Hypersonics in West Palm Beach, Fla., summarized thescramjet's remarkable internal flow characteristics for the website"Industrial Physicist." It illustrates what the Pratt & Whitneyengineers have mastered to make the X-51 a reality.

"The scramjet's high-speedair-induction system consists of the vehicle forebody and internal inlet, whichcapture and compress air for processing by the engine's other components.Unlike jet engines, vehicles flying at high supersonic or hypersonic speeds canachieve adequate compression without a mechanical compressor," saysAndreadis.

In the scramjet, "theforebody provides the initial compression and the internal inlet provides thefinal compression. Although remaining supersonic, the air undergoes a reductionin Mach number and an increase in pressure and temperature as it passes throughshock waves at the forebody and internal inlet. The isolator, visible on theflow path diagram, is a critical component. It allows supersonic flow to adjustto a static back-pressure higher than the inlet static pressure," he says.

"The isolator enables thecombustor to achieve the induced rise in combustor pressure without creating acondition called an "inlet unstart," in which shock waves preventairflow from entering the isolator. The combustor accepts the airflow andprovides efficient fuel-air mixing at several points along its length, whichoptimizes engine thrust," said Andreadis in his presentation.

The scramjet will circulate the fuelbehind engine walls to cool the structures. Without such active cooling, thetemperatures in a scramjet could reach 5,000 deg. Fahrenheit, high enough tomelt virtually any metal on Earth. Solving the cooling challenge is a majorAFRL/Pratt & Whitney achievement.

In an operational design, theengine will be configured as a ramjet for acceleration to Mach 4 before itchanges geometry for scram operations. As the vehicle reaches Mach 4 ignitionconditions, air is compressed on the lower surface of the vehicle and entersthe engine inlet at high pressure.

In a conventional jet engine,turbine blades are used to compress the air after the inlet has slowed it tosubsonic speed. In a scramjetpowered vehicle already going Mach 4, the air entering the engine is alreadywell compressed, and is squeezed down further, but remaining at supersonicvelocity.

This supersonic compressed airthen is mixed with fuel in the combustion chamber where the air and fuel ignitewithin 0.001 sec. of meeting each other. The fuel must enter the combustor in avaporized condition or it will not ignite.

How Pratt & Whitney achievesthis vaporized fuel condition is why the X-51 engine is a revolutionary flightweight design, nearly ready for scale-up in an operational design for a PromptGlobal Strike missile.

The JP-7 fuel is first pumpedinto tiny passages behind engine walls at the front of the engine. It flowsbehind the walls of hot engine structure cooling the engine until it reachesthe rear of the scramjet. There is more going on than cooling, however.

The passages behind the enginewalls are coated with a catalyst material. As the JP-7 absorbs heat from thestructure, the fuel's temperature and pressure rises until the catalyst reactswith the JP-7 and "cracks" it, literally breaking it down intosmaller hydrocarbon components like hydrogen, ethylene and methane.

At the rear of the engine, thecracked fuel is collected and reverses course, now being pumped forward where,as a vapor, it is sprayed into the combustor and ignited. The combustionproducts then expand tremendously in the rear nozzle, where they fire out ofthe scramjet creating about 1,000 lb. of thrust. That does not sound like much,but in a test engine at 60,000-100,000 ft. with the vehicle already moving atMach 4 at ignition, it is enough thrust to continuously accelerate the vehiclepast Mach 6, if all goes well.

The Waverider aspect of X-51aerodynamics is also at work at this time. The shock waves generated by thevehicle moving through rarefied atmosphere at about 4,000 mph would normallycreate drag, but the X-51 design is shaped to turn shock waves into anadvantage. The shape of the vehicle vectors the shock waves so that they liftthe vehicle, pushing upward on the X-51 structure while also entering the inletat a proper angle.

Each X-51 is designed then to flyabout 350-400 mi. before fuel depletion. Telemetry on 300 elements of thevehicle will be transmitted to the Pt. Mugu control room manned by 35 scramjetproject personnel. A Navy P-3 Orion relay aircraft will also be positionedunder the ground track to relay data.

All four flights will share onegoal, "to light off and accelerate through as many Mach numbers as wecan," says Brink. As with any complex flight test program, he expectsthings will go wrong during the flights and he will be happy if only two offour tests are successful. The 300 sec. of flight will be followed by a 500sec. dive into the Pacific. The four X-51s will not be recovered, but futuretest vehicles may carry recovery parachutes.

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