Mission to Mach 5

Jan. 1, 1999

During the next few months, the Air Force will deliver to Congress a bomber roadmap, describing in detail how USAF plans to perform and equip for the long-range strike mission in the next century. The new plan likely will describe a successor to the B-2 stealth bomber, and it probably will represent a shift away from the tradition of building big aircraft.

The new program is expected to tilt toward heavy reliance upon smaller, hypersonic vehicles, both manned and unmanned, with air-breathing engines. If the US succeeds in perfecting the critical building-block technologies, these new kinds of aerospace systems could be in place around 2010.

The term “hypersonic flight” means traveling faster than five times the speed of sound. Working hard to make these Mach 5-plus vehicles a reality are the Air Force, NASA, and Defense Advanced Research Projects Agency. The three agencies are pursuing complementary projects to investigate separate elements of the air-breathing hypersonic flight problem.

NASA is focusing on characteristics of hypersonic flight, which will be tested and measured on a small demonstrator vehicle set to fly in 2000. DARPA is seeking an “affordable” hypersonic missile able to zoom more than 400 miles in under seven minutes. For its part, the Air Force is investigating the critical engine technologies that will be needed to make both types of vehicles work while, at the same time, studying slow-speed characteristics of a hypersonic airplane.

The missile project could reach prototype form in four years and be operational in 10 years. The larger manned vehicle is not likely to appear until around 2015 at the earliest.

The Payoff

Hypersonic flight offers obvious military utility for reconnaissance and strike. Such vehicles would allow US forces to operate farther than they do now from enemy lines, reducing their exposure to enemy fire, without paying a penalty in reaction time or effectiveness. The inherent kinetic energy of a hypersonic missile would magnify its penetrating power, particularly against deeply buried facilities, which are among the toughest targets to destroy. Reconnaissance aircraft would be able, within three hours, to provide imagery of any place in the world. Such speed would make aircraft reconnaissance competitive with satellites not already over the area of interest, because the spacecraft would have to change orbits.

The term “air breathing” is important in the context of these vehicles. The craft envisioned would use the oxygen in the upper atmosphere to carry out the combustion of their fuel. Rockets routinely fly at hypersonic speeds but must carry their oxygen with them, making them large, bulky, and expensive. The goal of the ongoing hypersonics programs is to sharply reduce the cost of extremely high-speed flight and make it routine and reliable.

Gen. Michael E. Ryan, the Air Force Chief of Staff, said that the bomber roadmap deliberations have focused mainly on near-term weapons and improvements for the existing fleet of aircraft. Congress, however, insisted that the roadmap specifically address what USAF has in mind for the long-range strike mission in the B-2’s twilight years, and that’s where the potential of hypersonics comes into play.

When “time is of the essence” and the platform–either for an attack or with a sensor–“positively has to be there overnight, I think we need to look at faster ways to do it,” than are now extant, Ryan said. The product could be a “high-Mach” craft or a spaceplane. In any event, he said, “I think we have to have something that does that mission, sometime in the future.”

The Air Force likely will state a requirement for a vehicle or system that can deliver “rapid response at intercontinental ranges,” Ryan added. Once such a requirement is formally stated, the Air Force would carry out “trade studies” as the first step toward building such a system. These analyses would consider the available–or imminent–technologies that could enter service in the “desired time frame,” though what that time frame may be is as yet undefined.

“We have to get started on it now,” warned Ryan, “because our acquisition system takes a long time to produce brand-new things.” He noted that only now is the B-2 beginning to offer a full combat capability, “and we started that back in 1981.”

How badly will the Air Force need replacements for its existing long-range systems? USAF has said it believes the B-52H fleet is “technically capable” of lasting beyond the 2020s, but if the Air Force could field a system that was faster to target, more effective when it got there, and cheaper to operate–which a senior USAF official said has risen to “paramount importance among the considerations”-the service would give a serious look at retiring the BUFFs much earlier.

The B-1B fleet starts running out of its planned life expectancy in the late 2010s, with the exact year depending on how heavily they are used in the 2000s.

The B-2’s service life has not been calculated, but the bomber conceivably could last into the 2040s, if the example of the B-52 is any indicator. Unlike the B-52, which is chiefly built of well-understood metal alloys, the B-2 is largely made of nonmetallic composite materials, the longevity of which has not yet been established.

New World Vistas

The idea of air-breathing hypersonic vehicles as the next step for USAF was prominently voiced in the Air Force Scientific Advisory Board’s “New World Vistas” technology forecast of three years ago. In it, SAB Chairman Gene H. McCall focused on the “striking increases in effectiveness” the Air Force would reap if it succeeded in developing hypersonic systems.

New World Vistas planners saw unpiloted, Mach 15 hypersonic missiles and airplanes attacking enemies a world away, possibly with lasers, maneuvering at 20g’s, and agile enough to elude most missiles.

The issue bubbled to the surface in a big way again last fall when Hans Mark, the Pentagon’s director of defense research and engineering (and a former Secretary of the Air Force) told reporters in Washington that “there are things on the horizon” in aerospace technology that could lead to “an air-breathing, high-altitude aircraft.” He predicted that the successor to B-2 would “probably … be hypersonic.” He cautioned, though, that this exotic new aircraft “probably … will be far in the future.”

Hypersonic vehicles typically have “a really marginal payload,” Mark explained, adding, “That’s [their] big problem.”

It is difficult to acquire a large payload in a hypersonic vehicle because of the fineness ratio required of most designs: Because they are typically long and skinny, hypersonic craft don’t have an obvious place to put supplies of fuel and weapons, and increasing the payload and/or range usually means making a larger vehicle. An informed decision about the military utility of hypersonic vehicles is a decade away, Mark speculated.

The National Aerospace Plane project, inaugurated in the mid-1980s, was to have developed a hypersonic, air-breathing vehicle by the late 1990s, but the decline and fall of the Soviet Union, coupled with greater-than-expected technical challenges, inherent difficulties in an interagency project, and an on-and-off funding commitment from Congress led to the project’s demise in 1994. According to NASA’s former NASP program manager Vincent L. Rausch, NASP died “when the threat went away.” Rausch, a retired USAF colonel, now serves as program manager for NASA’s Hyper-X, a follow-on project that will fly three small-scale hypersonic research vehicles.

Waning Interest

“Military interest waned” in NASP when the Soviet Union collapsed, and as the program progressed, it became “clear that it was quite a big technical challenge,” Rausch said. The X-30 vehicle, as NASP was known, would have required a “national effort” and “several billion dollars” to build. That kind of money became very scarce in the early 1990s.

After “13 separate reviews” by a host of government panels, it was decided that NASP was a “very laudable thing to do,” Rausch asserted, but the question arose whether the program envisioned “was the right way to do it.”

In 1995, NASA contemplated the technology and research data left over from NASP, looking for a way to move ahead. What it came up with was Hyper-X: a project to fly small-scale versions of a hypersonic craft to gather data and develop the basic knowledge needed to make a full-scale version fly.

The key task in making hypersonic craft a reality, Rausch said, is “flight validation of a scramjet engine.” Hyper-X, he said, is the “cheapest way to do it” and the logical first step before something as ambitious as NASP should be tried again.

In an ordinary jet engine, fan blades compress the incoming air, and, after combustion of fuel, the engine expels the air at greater pressure, producing thrust. In a ramjet, the air is compressed by the aircraft’s own forward speed, and combustion occurs inside the engine in a subsonic flow of air.

In a scramjet–short for supersonic–combustion ramjet-the airflow inside the engine is supersonic. A scramjet is necessary if a hypersonic vehicle is to be “air-breathing”; a ramjet or turbofan would not be able to take air in fast enough to travel at high-Mach speeds.

The X-15 series of test airplanes in the 1960s carried both fuel and oxygen, and achieved speeds of up to Mach 6.7, but offered little practical value as weapon systems, since they carried barely two minutes of fuel and had to be carried aloft by a B-52 mother ship. Having burned their fuel, the X-15s had to return to a dead-stick, unpowered landing. The data they generated, however, paved the way for the space shuttle’s own high-Mach re-entry and dead-stick landings.

The demonstration of a scramjet is the “top priority” of Hyper-X, Rausch said. The craft will use liquid hydrogen as its fuel.

Three single-use craft, each 12 feet long, are being built under Hyper-X. Each, bearing the designation X-43, will be mounted on the front of an Orbital Sciences Pegasus-type booster rocket, which in turn will be carried to launch altitude by a NASA B-52. In three successive tests, the booster will be released from the bomber and accelerate a Hyper-X vehicle to its test speed and altitudes of about 100,000 feet, at which point the test airplane will separate and fly on its own power for about seven seconds, followed by about six minutes of hypersonic glide. Though brief, these flights will generate “an eternity of data,” Rausch said.

“The Spatula”

The first two vehicles, to be flown in January and October 2000, will fly at Mach 7, while the third, slated to fly in September 2001, will fly at Mach 10. Each will resemble the last planned configuration of the NASP, called “the spatula” by Rausch, but each will have variations, particularly in the shape of the inlet, for the speed at which it will fly.

The three vehicles constitute Phase 1 of the program. If successful, Phase 2 would draw on the data obtained from Phase 1 to build a larger version, completely reusable. It would take off and land on a runway but operate on a preprogrammed course. How it will get from ground level to high altitude hasn’t been decided yet, Rausch noted, and considerations include rockets, a pop-out turbine engine for lower altitudes, and “something called pulse detonator engines.” The choice will depend on “what integrates best” with the rest of the vehicle.

After the scramjet, Rausch said, “thermal management”-resolving the problems of heat generated by friction at very high speeds–is the next-biggest challenge, followed by reliable fuel injection at high altitude.

“The rocket community was not very much in love with NASP,” Rausch noted. Many in the NASA launch vehicle departments saw the project as a competitor and a drain on resources when rockets could be pushed to operate more efficiently. Now, though, “there is a growing awareness that in order to make the improvements that [the government] wants to see” in the responsiveness and cost of both getting to orbit and going long distances, “they have to be open to something different. They’re looking for anything that will work.”

The Air Force Research Laboratory is working on power plants and flight control systems that will make air-breathing hypersonic craft a reality. Under the HyTech program, scramjets that would use “ordinary hydrocarbon fuels” are being explored, according to Robert A. Mercier, chief of the hypersonic technology program at the AFRL’s Propulsion Directorate.

The scramjets being designed “would work in the Mach 4 to 8 range,” and part of the effort will be to develop engines that are not merely testworthy but which would have the durability for operational applications, Mercier said.

The AFRL also conducted flight tests of a vehicle called LoFLYTE, for Low Observable Flight Test Experiment. The vehicle is an example of what is called a “waverider”–a craft designed to ride its own bow shock wave, much as a surfboard rides on top of an ocean wave. The 8.3-foot vehicle has only flown at very slow speeds and altitudes, to test the basic airfield suitability of its broad, arrowhead-like shape.

This and That

LoFLYTE is also a test platform for a flight control system with a neural network. Mercier explained that a neural network uses an adaptive logic that allows the program to “learn” how to control an unstable craft by “trying a little of this and that to see what works” to keep the vehicle stable. The neural network used in LoFLYTE will be transplanted into Hyper-X, and cooperation between the programs is strong, Rausch observed.

A 23-foot-long follow-on to the delta-shaped LoFLYTE would explore its performance at high subsonic speeds. Two different designs are being looked at, Mercier noted, but the task of his project is to provide basic technological data “to our product centers,” who then decide whether to pursue the technology.

“As lab people, we have to look far downstream,” he said. “Our brethren in the AFRL are looking very closely” at hypersonic applications in “unmanned aerial vehicles, uninhabited combat vehicles, and manned systems … both for strike and reconnaissance.”

He added, though, that “at this point, we are just looking at vehicle trade studies, looking to see where the gaps [in capability] are, and doing the groundwork” for future systems.

The HyTech project will produce a power plant by 2003 for demonstration with a “missile-size application,” Mercier said, and the missile to take advantage of it will likely be a DARPA project called the Affordable Rapid Response Missile Demonstrator.

Boeing is developing two different concepts for the ARRMD, which is envisioned as a Mach 6-cruising vehicle that would come in at under $200,000 a copy. Boeing is producing both vehicles because it acquired McDonnell Douglas, which was offering one of the two finalist concepts.

One of the vehicles is a waverider and the other is a spatula-type vehicle like Hyper-X and NASP, according to Boeing’s program manager, John Fox. The operational concept, he said, is to produce a missile that could be launched from a platform as small as a fighter and as large as a bomber, as well as from a canister aboard Navy ships and submarines. The missile would be used against time-critical targets such as newly discovered mobile missile launchers or surface-to-air missile sites. It would also be useful to attack deeply buried bunkers.

The missile would have to fit inside the bomb bays of USAF’s bomber fleet as well as in the Vertical Launching System canisters used by the Navy, meaning no more than 13 feet long. In order to be carried on the Navy’s F/A-18E/F, the missile must not exceed 2,320 pounds in weight. A disposable solid booster would propel each missile to a speed at which its hypersonic engines could kick in.

The ARRMD has only a 250-pound warhead, a size driven both by the advances being made in the yield of explosives, as well as the functional payload limit on a hypersonic vehicle. The waverider version will be powered by USAF’s HyTech scramjet engine, while the spatula type will be powered by a dual-combustion combination ramjet/scramjet built by the Johns Hopkins University Applied Physics Lab. Both versions would use an Inertial Navigation System/Global Positioning System guidance package, developed for Boeing’s Joint Direct Attack Munition, to achieve a precision hit within 30 feet of the target.

There is “no preferred concept” at this point, Fox said. “Both designs are viable … candidates.”

One of the two concepts will be picked to go ahead by the end of next year, after which an engineering and manufacturing development effort will begin to produce flight test vehicles. If they work, and if they can be built at the required cost, the program could put missiles into the hands of operators by 2010, Fox said.

“Hot Skins”

“The engines are the long pole in the tent,” Fox said. “They are extremely related to the airframe. This is not like airplanes used to be designed, where you built an airplane around an existing engine. The airframe and engine are integral.”

Keeping the vehicle from melting is the second biggest problem, given “the hot engine and hot skin” that will be encountered at high Mach numbers, he added.

The ARRMD is to fly at Mach 6.5 and fly at 90,000­100,000 feet. DARPA is giving Boeing leeway to “trade off anything we need to against the cost,” which must come in under the $200,000 target, which Fox believes is possible.

The same kinds of hydrocarbon fuels found in serving aircraft today will be used in the ARRMD, Fox said. The Navy insisted that hydrogen not be used because it would be too hazardous to store and protect on an aircraft carrier. The use of JP-7 for the waverider and JP-10 for the dual-combustion ramjet will also simplify handling of the systems under wartime conditions.

As many as 3,000 ARRMDs are envisioned for the Navy and Air Force. The services are involved in the effort but will not become official “sponsors” of the program until after it has cleared the demonstration phase, Fox said.

France and Russia are known to be pursuing hypersonic weapons, but Rausch and Mercier guessed that their systems are not as well along as the US effort. A Japanese program is aimed at creating a spaceplane capable of Single-Stage-To-Orbit flight.

Rausch said the US could build a manned, Mach 5 craft “today, if we decided to” for SSTO operations, but “it would require the kind of national effort and investment” that was made on the space shuttle program. Building a vehicle that will exploit the knowledge gained from Hyper-X and the other hypersonic research projects “is not going to be cheap” but will pay back the investment handsomely, he said.

The level of effort being expended on hypersonics is “probably about right,” Rausch asserted, given that the scramjet technology will make everything else possible and must, of necessity, “come first.”

When the Air Force decided to retire the SR-71–with no obvious successor in sight–speculation raged that some sort of secret hypersonic reconnaissance airplane must have been nearing deployment. Rausch said, “I wish we had it” but noted that “in the ’80s, when we were working on NASP, we pretty much knew everybody who was working on this technology.” None of them, he said, knew of any program that had magically leaped ahead of the state of the art.