Title: Page born: April 01 2002

The XST

"Jesus, if they can do that with a frigging pole, what can they do with their damned model?" Northrop XST program manager upon seeing the Skunk Work's designed pole for the XST "pole-off".

The history of the F-117A Stealth fighter dates back to 1974 when the Defense Advanced Research Projects Agency (DARPA) initiated, with U.S. Air Force participation, a program to study and possibly demonstrate the concept of a very low observable military aircraft.

The Air Force had used remotely piloted vehicles (RPV's) in Vietnam and was looking at much smaller, less complex versions to take their place. The RPV's were small and had proven difficult to see on radar. Meanwhile, the Pentagon's scientific consulting group, the Department of Science Board, had completed one of its annual studies for the Air Force; these periodic reports, usually secret, were conducted on the orders of Air Force leaders on topics of interest to them. Reviewing air battles over Vietnam and the Middle East, the board concluded that U.S. aircraft would soon 'have a real challenge getting through air defensives."

DARPA study

The study was initiated and led by Ken Perko, who had recently come to DARPA from the RPV System Program Office (SPO) at Wright-Patterson Air Force Base (WPAFB). Perko requested White Papers from five companies - Northrop, McDonnell Douglass, General Dynamics and Grumman. The study asked two questions:

• What were the signature thresholds that an air vehicle would have to achieve to be essentially undetectable at an operationally useful range?
• What were the capabilities of each company to design and build an aircraft with the necessary signatures?

Fairchild and Grumman did not express any interest in the study. The General Dynamics response emphasized countermeasures and had little substantive technical content regarding signature reduction. Northrop and McDonnell Douglass (MD) responded indicating a good understanding of the problem and some capability to develope a "reduced-signature" air vehicle. McDonnel Douglas was also the first to identify what appeared to be the appropiate RCS thresholds. In late 1974 DARPA awarded Northrop and McDonnell Douglass contects of aproximatly $100,000 each to conduct further studies. These initial studies were classified "Confidential", the lowest of three major levels of security classification: Confidential, Secret, and Top Secret. In the spring of 1975, DARPA used McDonnell Douglass's values (confirmed by Hughes radar experts) as the goals for the program, and challanged the two participants to find ways to achieve them.

Project Harvey?

Up until now, most sources have stated that this study was code named Harvey, a reference to the invisible rabbit that accompanied Jimmy Stewart in the movie of the same name. However, according to John K. "Jack" Twigg, former HAVE BLUE program manager (who was heavily involved with subsequent low observables programs), nothing related to HAVE BLUE or the F-117A was ever called "Project Harvey." "All activities that led directly to HAVE BLUE were never referred to as Project Harvey," said Twigg. "There was a gentleman in the Pentagon...who had some concepts that he entitled 'Harvey.' His focus was tactics and other means, not 'technical' low observables... The occasional mention of 'Harvey' did provide confusion to some, and may have served as an inadvertent cover story." Adding to the confusion is the fact that both "Harvey" and HAVE BLUE drew support from within the Office of the Director of Defense Research and Engineering at the Pentagon, making the distinction very unclear even to many program participants.

Lockheed Involvment

Lockheed was not invited to participate in the study because they were not considered to be active in tactical aircraft developement at the time. Lockhheed had not produced a fighter aircraft in over 10 years. The DARPA study was not highly classified, so it eventually came to the attention of Ben Rich, who took over control of Skunk Works in January 1975.

The following is from Rich's book "Skunk Works": "In the summer of 1975, Skunk Works in-house expert on Soviet weapons systems, Warren Gilmour, attending a meeting at Wright Field, in Ohio, and came back in a dark mood. He marched into my office and closed the door. "Ben, we are getting the shaft in spades," he declared. "One of my friends in the Tactical Air Command spilled the beans. The Defense Department's Advanced Reaserch Prodjects Agency has invited Northrup, McDonnell Douglas, and three other companies to compete on building a stealthy airplane. They're getting a million bucks each to come up with a proof of concept design, trying to acheive the lowest radar signatures across all the frequencies. If one works, the winner builds two demonstration airplanes. This is right up our alley and we are being locked out in the cold." Kelly Johnson received permission from the CIA to share the previously highly classified radar-cross-section test results on the SR-71/A-12, which was sent to Dr. George Heilmeier, the head of DARPA, together with a formal request to enter the stealth compitition. But Dr. Heilmeier called Ben Rich expressing regrets. "Ben, I only wish I had known about this sooner. You're way too late. We've given out all the money to the five competitors." The only possibility, he thought, would be to allow Skunk Works to enter if they would agree to a one dollarr pro forma government contract. Ben Rich was sitting on a major technological breakthrough, and if he took the government money the government would own the rights to all of the equations, shapes, composites-the works. Lockheed was taking major risks, and Ben Rich beleived that they deserved the future profits. (Background: Lockheed at the time was teetering on the edge of moral and financial bankruptcy in the wake of a bribery scandel and the failure of the L-1011. The losses ultimatley reached a staggering $2 billion, and in late 1974, Textron Corporation almost aquired all of Lockheed at a "fire sale" price of $85 million. The Skunk Works would have been sold off with the corporation's other assets and then tossed into limbo as a tax write-off. Needless to say, Ben Rich would be risking his new-found position as the head of Skunk Works and those who would support him would be risking their own jobs also.) After a lot of arguing the Skunk Works was allowed to enter the stealth compitition with no strings attached. Ben Rich states that "It was the only time I actually felt good about NOT receiving a government contract."

Hopeless Diamond

Lockheed built a model of the strange diamond-shaped airplane that resulted from applying Denys Overholser's principles that if you make an airplane entirely out of flat panels, each angled so that none is ever likely to be facing straight toward radar the energy will be reflected away from the radar. Aerodynamicist Dick Cantrell dubbed it the "Hopeless Diamond." (Other names by the skeptical included "The Flying Engagement Ring" and "Rich's Folly") When Overholser presented a sketch of the design to Ben Rich on May 5, 1975, Rich did not quite grasp what had been achieved. Rich kept asking how big the radar return of a full-size aircrfat would be-as large as a T-38, a Piper Cub, a condor, an eagle, ann owl? Overholser game him an unbelievible answer. "Ben, try as big as an eagle's eyball." The "Hopeless Diamond" also met a frosty reception from Kelly Johnson, who was still working as a consultant at Skunk Works. Having built gracefull aircraft such as the U-2 and SR-71, Johnson was not impressed with the odd design. Many senior engineers nd aerodynamicists at Skunk Works felt the same. They all prefered a disk shape design. However, a disk shaped design was not possible. Johnson thought the RCS of the "Hopeless Diamond" would be larger than that of the D-21. On September 14, 1975 a ten foot wooden model of the "Hopeless Diamond" was tested against the original D-21 mockup. The "Hopeless Diamond" had a radar return one-one thousandth of that of the D-21. "They decided that I wasn't the village idiot, so I became a genius instead," Overholser said.

Pole off

During the summer of 1975, Ken Perko of DARPA and Bill Elsner of the Air Force worked on a plan to fly one of the two competing aircraft. So in September Northrop and Lockheed's Skunk Works (McDonnell Douglas had fallen out of the competition.) were asked to design a small prototype aircraft and build a full scale model for a "pole-off" at the RCS range at Holloman Air Force Base in New Mexico, with the winner going on to flight testing. By now, the projects name had become the Experimental Survivable Testbed, or XST.

Lockheed XST Entry

Lockheed had designed a notched trailing edge for its XST to replace the original Hopeless diamond shape. This allowed the team to sharpen the sweep angles on the rear of the aircraft and meet the DARPA rear-quadrant requirement of 45 degrees on either side of the tail.

Overholser's design philosophy forbade curves, even in the airfoil, because their reflectivity could not be modeled or predicted. The XST's airfoil section, which was a series of six straight lines rather than a smooth surface, appalled aerodynamicists because it seemed clear that the airflow over the wing would separate at its first meeting with the junction of two flat surfaces, creating turbulent airflow and drag.

But Alan Brown (who joined the Lockheed team in August of 1975) had worked on Lockheed's supersonic transport design and knew that a delta wing as sharply swept as the XST's, with a sharp leading edge, "really flies on the vortex generated from the leading edge, and conventional two-dimensional aerodynamics don't apply at all."

Northrop XST Entry

Irv Waaland, a Northrop designer knew that Northrop had a problem. Northrop's analysts had concluded that it was most important to reduce its vehicle's RCS from the nose and tail and the nose-on RCS-the view an adversary had in the critical head-on engagment-was more important than the rear aspect. It's XST design was a diamond with more sweep on the leading edges than the trailing edges. From the rear, it had low RCS as long as the radar was no more than 35 degrees off the tail.

But the DARPA requirement treated RCS by quadrants: The rear quadrant extended to 45 degrees on either side of the tail, thereby including the parts of the airframe where the Northrop design's RCS spiked. Waaland could not solve the problem by increasing the sweep angle of the trailing edges, the aircraft would become uncontrollable.

And Northrop had an internal issue to deal with. "The level of security on the observables was higher than it was on the airplane," says Waaland, "and not too many of the airplane people were cleared into the [details of the low-RCS design theory]. It was a great source of frustration, because there was no ability to make compromises." This put Northrop at a disadvantage, because the program was all about compromise: to minimize RCS while attempting to preserve acceptable aerodynamics. the normally reserved Waaland recalls epic shouting matches in which he would question John Cashen (a Northrop electromagnetics expert) about some aspect of the mysterious electromagnetics. "You know just enough to be dangerous," was Cashen's usual retort.

By now a change had crept into the program. Alan Brown traces it to the first Lockheed and Northrop 1/3 scale tests at McDonnell Douglass' Gray Butte RCS range in California's Mojave Desert in the December of 1975. (Lockheed tested the D-21 vs. "Hopeless Diamond" here because it did not have a range of it's own at the time.) The RCS numbers were not merely half of those of a conventional aircraft, but a hundred or a thousand times smaller, enough to make most radars useless. "People realized we had a tiger by the tail" says Brown.

Throughout most of the testing, the competing contractor teams and their models were kept in isolation from each other, Temporary quarters were set up so that each team would have access to the range but could be kept apart from the other team. However, after most of the tesing was completed, each team was allowed to drive out on the range to view the other's model mounted on top of the 40 foot pylon.

Technical performance, risk, cost, and schedule of the two competitors were very close. Therefore, choosing the winning team for PHASE II was somewhat subjective (and since then has been argued with). Technically, Lockheed's entry had a slight edge (based on the required quadrants). Overall, Northrop's XST was possibly stealthier than the Lockheed entry, but this conclusion is based on factors that weren't part of the DARPA requirements. Although both companies had developed special materials and construction techniques, the perception was that the Skunk Works had more experience with the use of these technics on anctual aircraft. The Skunk Works also had a proven record of accomplishing advanced, high risk prjects quickly under high security. These factors provided confidence that the Skunk Works could execute the XST program successfully. In April 1976 Lockheed was announced the winner of the PHASE I compition.

Into the Black

The XST project was upgraded to top secret in early 1977, and when Lockheed won the contract for the construction of two 60-percent scale flyable test aircraft (April 1976), it became an unacknowledged Special Access program. Only those with a need to know would be told that the project existed. The designation XST was replaced with the code name Have Blue, the project was transferred over to Air Force System Command, and the project office moved from Washington to a secure "vault" at Wright-Patterson AFB. The name Have Blue seems to have no specific meaning, probably having been chosen at random from an approved list of secret project names. The airplane would fly from Area 51, the secret flight-test base at the edge of Groom Dry Lake, Nevada. Built for the U-2 and expanded for the A-12 program, Area 51 was home to a secret squadron of Soviet-built aircraft and officially did not even exist. No one knew that Have Blue existed.

The Northrop Team

DARPA recognized that the Northrop XST team was an important nucleus of expertise and urged the team to stay together. Shortly afterward, DARPA initiated design studies for a Battlefield Surveillance Aircraft, Experimental (BSAX). The BSAX study led to the recently acknoledged (1996) Tacit Blue program for a very low observable battlefiel surveilance aircraft. The 135 Tacit Blue flights from February 1982 to 1985 provided valuable data for Northrop's entry in another poleoff compition with Lockheed for the Advanced Technology Bomber (ATB). Northrop won that compitition by a slim margin and went on the produce the ATB, now known as the B-2 "Spirit" Stealth Bomber.

Sources:

"The Invisible Men"
Bill Sweetman
Air and Space Magazine/Smithsonian
May 1997, pp.18-27

"Technology in the Lives of an Aircraft Designer"
by Irv T Waaland, Vice President, Chief Designer
Northrop Corp. Advanced Technology & Design Center,
Pico Rivera, CA.
"1991 Wright Brothers Lecture
AIAA Aircraft Design & Operations Meeting"
23 September 1991, Baltimore, MD

HAVE BLUE And The F-117A: Evolution Of The "Stealth Fighter"
David C. Aronstein and Albert C. Piccirillo
AIAA/ANSER, Arlington, VA 1997

"Vehicle" Patent # US5250950 (Have Blue aircraft)
Inventor: Scherrer; Richard , La Canada, CA
Inventor: Overholser; Denys D. , Frazier Park, CA
Inventor: Watson; Kenneth E. , North Hollywood, CA
Applicant: Lockheed Corporation, Calabasas, CA

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