Republic XF-103 Thunderwarrior

The concept of dedicated interceptors had come to fruition during World War II.
The Messerschmitt Me163 Komet rocket plane being at early example of an aircraft intended solely to shoot down enemy bombers over its home territory.
After the war it seemed that all things became more specialized, furthering the concept of dedicated interceptors.
This was helped in the U.S. Air Force by the fact that there were two operating commands - the Air Defense Command (ADC) and the Tactical Air Command (TAC).
Each wanted its "own" aircraft, and so defense interceptors and tactical fighters began to
diverge in design and capabilities.
Even the weapons carried by each began to diverge:
the ADC pressed the development of the Falcon family of air-to-air guided missiles, while TAC fielded the Sparrow and Sidewinder, both ironically developed by the Navy.
In early 1949, the Air Force issued a Request for Proposals (RFP) for an advanced interceptor capable of attacking the new Soviet intercontinental bombers that were expected to enter service soon.
The interceptors being developed at the time, the North American F-86D Sabre, Northrop F-89 Scorpion, and Lockheed F-94 Starfire, were all subsonic aircraft, and the Air Force did not believe that they had sufficient growth potential to meet the anticipated threat.
This program came to be known as the "1954 Interceptor," after the year that the new aircraft was scheduled to enter operational service.

Always at the forefront of innovation, Republic chief engineer Alexander Kartveli had begun designing the Mach 3 AP-44A all- weather high altitude defense fighter in early 1948 - less than a year after the first super supersonic flight of Chuck Yeager in the XS-1.
When the Air Force first saw the preliminary data, they were greatly intrigued, at least partly explaining why Republic was selected to continue in the WS-201A competition.
But, like many visionaries, Kartveli wanted was “on the fringe."
During 1951, the Republic design evolved into the AP-57 that was estimated to be capable of Mach 4 at altitudes up to 80,000 feet.
By September 1951, when the Air Force made the WS-201A selection, it was obvious that the
Karveli design was far too advanced to become operational in the near term, but at the same time, it promised such a leap in performance that the Air Force felt the project had to continue.
Concurrently with the Convair design being selected for further development as MX-1554, Republic received a Phase I development contract for WS-204A, with the airframe designated XF-103.
It would use the same Hughes MX-1179 fire control system and MX-904 missiles intended for the F-102.
It was also scheduled to use a variation of the Wright J67 engine that was initially selected for the Convair design.

Although the entire XF-103 was extremely futuristic, perhaps its most notable feature was the dual-cycle propulsion system developed as Project MX-1787.
There was nothing particularly noteworthy about the Wright Aeronautical Corporation XJ67-W-1 turbojet engine itself, being a license-built version of the Bristol Olympus.
What was notable was its installation.
The engine is usually described as being equipped with an afterburner, but this is not truly the case for the XF-103, at least not in the conventional sense.
The afterburner was a separate unit (eventually designated XRJ55-W-1) located several feet behind the engine itself.
The XRJ55 could be used as a traditional afterburner - hot exhaust gas from the J67 was ducted into it, mixed with fresh fuel, and ignited.
It could also be used as a ramjet where the turbojet was shut down and all incoming air was ducted directly into the XRJ55, fuel was injected, and the mixture ignited.
This was a novel approach to a propulsion system.
The turbine inlet (compressor output) temperature of the J67 was limited to about 1,500 degrees Fahrenheit, a figure corresponding to just under Mach 3, because that was the limit of the materials used to manufacture the engine.
It would be difficult to maintain Mach 3 for any length of time, or to achieve higher speeds, without destroying the engine.
The dual-cycle approach eliminated this problem by removing the J67 from the propulsion system at high speeds.
It should be noted that the ramjet provided somewhat less thrust than the J67, and used more fuel while doing it.
This was not, however, considered a major liability.
The F-103 would take off and climb using the J67 with the XRJ55 functioning as an afterburner, and as the aircraft approached Mach 2.25, the combination was generating approximately 19,500 lbf.
At this point, the pilot opened the exhaust nozzle, disengaged the turbojet, and bypassed all incoming air to the XRJ55.
In less than one second the thrust fell off to about 14,000 lbf - the XRJ55 was now using bypass air, but it was doing so less efficiently than it had been using engine exhaust.
About 6 seconds later the airflow and temperature had stabilized, the fuel flow into the XRJ55 was increased, and thrust came up to about 16,000 lbf.
The entire transition took about 10 seconds and the aircraft continued to accelerate to its Mach 3 design speed.
Thrust reportedly increased to 18,800 lbf at 55,000 feet.

The concept was successfully demonstrated several times in test cells at the Air Force Arnold Engineering Development Center.
A Ferri-type two-dimensional air intake with a sharply forward- swept lip was located under the fuselage.
The intake divided into two separate ducts;
the lower one fed the J67 during turbojet operations and the upper bypass duct fed air directly into the remotely located XRJ55.
The two-dimensional exhaust nozzle had variable throat and exit surfaces moving within fixed side plates.
The nozzle employed a combination of film and convective cooling using bypass air from the
intake and discharge air from the aircraft cooling system.
The exhaust nozzle incorporated dive brakes at the sides and was designed to be the outer structure of the aft fuselage.
Fuel was located in five pressurized fuel cells, and a single drop tank could be carried under each wing.

Like the F-105 (also a Kartveli design) drop tanks, the pylon was an integral part of the tank and was jettisoned at the same time.
Despite the initial Mach 4 estimate, as the concept progressed Kartveli came to realize that the structural design of the aircraft – or more precisely the materials being used in its construction – would limit the XF-103 to just over Mach 3 due to aerothermal concerns.
An independent performance evaluation conducted by the Air Materiel Command (AMC) showed a sustained speed of just over Mach 2.5 (1,438 knots), still a fast airplane for the time.
The rate of climb at 45,000 feet was projected to be almost 70,000 feet per minute, equal to Mach 1.2 straight up, using the ramjet.
Even at lower altitude while using the normal afterburner, the initial rate of climb would be over 40,000 feet per minute.
Total time-to-climb to 60,000 feet was just over 7 minutes from brake release.
A service ceiling in excess of 75,000 feet was expected using the ramjet.
A full-scale metal mockup of the XF-103 was inspected on 2 March 1953 with encouraging results.
An 18-month extension of the Phase I contract was used for further studies of titanium fabrication, high-temperature hydraulics, escape capsules, and periscopic sights.
Although development money was hard to come by the Air Force decided that the Mach 3+ interceptor held so much promise that the program continued to be funded despite a variety of technical problems.

By July 1954, the design had advanced to the point that the Air Force awarded Republic a contract to manufacture three prototypes.
As was his style, Kartveli had designed an airplane that was certainly unique.
The main wing used a delta planform with a leading edge sweep of 55 degrees;
the horizontal stabilizer also used a delta planform, this time with a leading edge sweep of 60 degrees.
A rather odd-looking tricycle landing gear retracted into the fuselage – odd because the main gear were located very far back on the fuselage, retracting into wells on the sides of the fuselage between the trailing edge of the wing and the leading edge of the horizontal stabilizer.
This arrangement was dictated because the wheel wells were located between the J67 and XRJ55.
The J67 could be removed through large access doors located under the main wing and the XRJ55 was removed through the rear of the fuselage.
The cockpit of the XF-103 has always been the subject of discussion since it was very different from contemporary fighters.
At least one version of it was.

Kartveli had designed the XF-103 with a conventional windscreen and canopy and the mockup was constructed with this arrangement.
Concerns were raised at the mock-up review about the drag associated with the canopy, and over aerothermal problems with the plexiglass windscreen.
The Air Force ordered Republic to change the design, although Kartveli continued to refine the original concept as an alternative.
The mockup was revised with a cockpit that was completely flush with the fuselage, although a second full-scale forward fuselage was maintained with the original design.
In the new design, two large side windows provided a relatively good view for takeoff and landings, and a retractable periscope could be used for forward vision at high speeds
if necessary.
Late in the development effort, a fixed periscope was added under a fairing on top of the nose, a less than ideal solution but one that provided at least a little forward vision during all phases of flight.
Tracking the target during combat was accomplished using the MX-1179 radarscope positioned in front of the single pilot, although an optical sight was also provided.

To evaluate the visibility from the revised configuration, Republic built a full-scale cockpit section and mounted it on a truck in an attitude and elevation simulating the XF-103.
The truck then a taxied around the ramp and runways at Farmingdale, New York (where Republic was located), while an engineer took photographs through the periscope and windows for evaluation.
At the same time, photos were taken from an F-84G with an A-1 gunsight installed as a basis of comparison.
The periscope and large side windows of the XF-103 offered slightly better vision directly ahead (no windscreen frames interfered as on the F-84) and adequate vision to the front quarters.
Vision to the sides was excellent, but there was essentially no rearward vision.
In all, everybody agreed the concept appeared to be workable, if slightly less than ideal.

Another innovative feature of the XF-103 was its escape capsule.
Kartveli believed that in order to survive a Mach 3 escape, some sort of capsule was necessary - a view not universally shared in the aircraft industry, although the use of a capsule in all new aircraft was briefly codified by the Air Force during the late 1950s.
The use of a capsule also addressed another problem - how to get into the XF-103, which
sat fairly high off the ground.
The solution was to have the capsule lower on a rail to ground level where the pilot got in. Escape was also from the bottom, making it useless at low level or during takeoff and
landing (much like early F-104s).
The capsule itself had a forward door that was stowed in a down position.
When the pilot wanted to encapsulate, the door slid upward between the instrument panel and
the pilot, sealing along the top edge of the capsule.
The flight controls and throttle were located inside the capsule;
exactly how they would have been connected to the aircraft systems was never completely
explained.
Two large windows in the capsule allowed the pilot to see the instrumentation and fly the aircraft while encapsulated.
Six GAR-1 Falcon missiles were carried on retractable launchers inside the fuselage.
There was an upper missile bay and two lower missile bays on each side of the fuselage, just behind the cockpit, each with separate quick-action doors.
A retractable rocket pod was also located on each side of the fuselage, just above the lower missile bays.
Each pod contained eighteen 2.75-inch "Mighty Mouse" folding fin aerial rockets (FFAR).
The Air Force had some concerns about the lower missiles and all of the rockets since their motor exhaust would likely be ingested by the XF-103 engine air intake.'
Most of the fire control system electronics were located in a compartment adjacent to the radar equipment in the nose.

Maintenance access was deemed less than ideal since a ladder was required for access to all components.
This limited the use of test equipment and made minor adjustments difficult.
However, the entire nose compartment and its collection of rack-mounted components were designed to be removable for major servicing.
The XF-103 was one of the first programs be investigate the wide-spread use of titanium, paving the way for the Lockheed Blackbirds and North American XB-70 later in the decade. Actually, four different construction techniques were investigated by Republic:
all Ti-150B titanium, all 4130 steel, a combination that used Ti-1508 titanium alloy outer skins and a 4130 stainless steel inner structure, and an all-aluminum (245-T86) structure.
The aluminum aircraft was limited to Mach 2.8 (375 degrees Fahrenheit), and also turned out to be the heaviest at 10,250 pounds (empty airframe weight);
it was quickly dropped from consideration.
Not surprisingly, the all-titanium structure was deemed the lightest (8,750 pounds), with the combination structure being second (9,400 pounds).
The all-steel structure was 100 pounds lighter than the all-aluminum design.
Both the steel and titanium aircraft were capable of sustained Mach 3 operation.
A great deal was learned about the manufacture of high-strength titanium alloy and the
fabrication of aircraft parts out of it during the course of the XF-103 program, although neither Lockheed nor North American ever acknowledged the contribution to their later programs.
Despite a low priority and little funding, the program continued to advance, albeit slowly.
A few major subcontracts for the three prototypes were issued, primarily for the landing gear and various pieces of titanium tooling.

However, it was becoming obvious that the XF-103 was never going to fly.
The effort was experiencing serious difficulties by 1955, but the Air Force still felt a need for an interceptor with higher performance than the upcoming Ultimate Interceptor.
On 6 October 1955, the Air Defense Command released GOR-114 for the Mach 3 Long-Range Interceptor, Experimental (LRI-X) program.
In anticipation of the LRI-X program entering development, during early 1957, the XF-103 was cur back to a single prototype and two flight engines.
In this role, the Republic aircraft to be advanced research vehicle for the LRI-X program, but was no longer considered a possible operational aircraft.
The design evolved considerably during this period, with more than a little assistance from Hughes, which intended to use it as a demonstrator for its long-range radar and Super-Falcon missile.
External changes included a more slab-sided shape for the rear fuselage, a refined variable-geometry inlet, and a rearrangement of the armament.
Hughes proposed a weapons load of tour GAR-1 Falcons in individual bays (two on each side, one above the other) and two GAR-9 long-range missiles, located in bays behind
the upper GAR-1s.
Little progress had been made by 21 August 1957 when the XF-103 and Wright engine were cancelled entirely.
The program had cost $104 million over 9 years.