Subject: Number Ninety-One in the Series--Republic XF-91 Thunderceptor

From: [email-masked]
Date: 5 Oct 93 16:17:21 GMT

>From [email-masked]

The Republic XF-91 Thunderceptor was a mixed-power interceptor, being 
powered by both a jet engine and by a battery of rocket motors.  

The project began in 1946 when Alexander Kartveli and his team at 
Republic Aviation began to explore the possibility of using rocket 
engines to power aircraft.  The Messerschmitt Me 163 Komet of World 
War 2 had demonstrated that truly spectacular performance could be 
attained with rocket propulsion, although at the expense of an 
extremely short endurance and a very low combat radius.  Republic 
proposed to overcome this inherent disadvantage of rockets by using a 
conventional jet engine for cruising flight, with rocket engines being 
used only for fast takeoff and climb to combat altitude and for extra 
boost during combat emergencies.  

In 1948, Republic submitted its ideas to the USAF.  The Republic 
proposal was to be powered by a single 5200 lb.st.  General Electric 
J47-GE-3 turbojet engine, augmented by four Reaction Motors XLR11-RM-9 
rocket motors of 1500 lb.st. each.  Two rockets were located above 
the jet engine exhaust, two below.  

The fuselage had some resemblance to the F-84 Thunderjet, with some 
parts actually being in common.  However, an entirely new wing was to 
be fitted.  The cantilever, mid-mounted wing was swept back at an 
angle of 35 degrees and had a variable incidence.  This meant that the 
wing could be pivoted around its attachment point to the fuselage, 
allowing the angle of incidence to be adjusted by the pilot for the 
most effective angle during takeoff, cruise, and landing.  In contrast 
to the usual practice, the swept wing was thicker and wider at the tip 
than it was at the roots.  It was hoped that this unusual wing 
configuration would provide greater lift outboard and would reduce the 
tendency of the wingtips to stall at low speeds.  

The retractable landing gear consisted of a single nosewheel plus a 
pair of main landing gear members attached to the wings.  The 
nosewheel was fairly conventional, and retracted forward into the 
forward part of the nose.  The main landing gear was anything but 
conventional.  Each of the main landing gear members consisted of a 
pair of bogie-type wheels mounted in tandem.  Since the wing roots 
were too thin to accommodate the main landing gear wheels when 
retracted, the landing gear retracted outwards, the wheels being 
stowed inside the thickened wingtips.  

The design featured a ventral-mounted airbrake, and was to be equipped 
with a braking parachute to shorten the landing run.  A pressurized, 
air-conditioned cockpit was provided, and the cockpit canopy was 
similar to that of the F-84.  

The USAF was sufficiently impressed that they ordered two examples of 
this design under the designation XF-91.  Serials were 46-680 and 
46-681.  

The first prototype (46-680) took to the air for the first time at 
Edwards AFB on May 9, 1949, Carl Bellinger being the pilot.  The first 
flights were powered by the turbojet alone.  Later that year, flights 
were made with an afterburner and then later with the rocket engines 
installed.  In December 1951, the XF-91 became the first American 
combat aircraft to go supersonic in level flight.  When all five 
powerplants were running, the XF-91 could achieve a maximum speed of 
1126 mph (Mach 1.71).  Not bad for the early 1950s! 

The XF-91 showed signs of having the potential of developing into an 
excellent warplane.  However, the Korean War had demonstrated that 
Mach 1 could be exceeded by aircraft that were considerably less 
sophisticated than the XF-91, and this extremely advanced warplane 
never saw production.  Tests continued with the two prototypes.  One 
was flown experimentally with a V-type butterfly tail, and the first 
XF-91 was refitted with a nose radome housing an APS-6 radar 
installation in the same manner as the F-86D Sabre.  

The second prototype was eventually destroyed, but the first XF-91 
(46-680) is currently on display at the Wright Patterson Air Force 
Base Museum.  

Specification of the XF-91: 

One General Electric J47-GE-3 axial-flow turbojet, 5200 lb.st.  plus 
four Reaction Motors XLR11-RM-9 rocket motors rated at 1500 lb.st.  
each.  Dimensions: wingspan 31 feet 3 inches, length 43 feet 3 inches, 
height 18 feet 1 inches, wing area 320 square feet.  Weights: 14,140 
pounds empty, 18,600 pounds combat, 28,300 pounds maximum.  Maximum 
speed: 1126 mph at altitude (Mach 1.71), 984 mph at 47,500 feet.  
Climb to 47,500 feet in 2.5 minutes, service ceiling 50,000 feet, 
normal range 1175 miles.  The planned armament of four 20-mm cannon 
was never installed.  

Sources: 

  Fighters of the United States Air Force, Robert F. Dorr and David 
  Donald, Temple Press Aerospace, 1990.  

  The American Fighter, Enzo Angelucci and Peter Bowers, Orion, 1987.  

Joe Baugher		AT&T Bell Laboratories   	
2000 North Naperville Road      Naperville, Illinois 60566-7033