The Rocket-Boosted P-51 Mustang

In September 1944 the first operational unit of Messerschmitt Me 262 jet fighters was established under the command of the celebrated ace Major Walter Nowotny of the Luftwaffe, his unit being tasked with the operational development of tactics for the fighter and being named "Komando Nowotny" in honor of that significance. USAAF heavy bomber crews had ready been encountering the Me 163 Komet rocket interceptor for several months but the persistence of the jet over the point-defense nature of the Komet made the 262 a more significant threat in the eyes of the Allies. After Nowotny was killed in combat in November 1944, the unit became Jagdgeschwader 7 (JG 7), the first operational jet fighter unit in history. However a threat that the Me 262 posed in the threat, it was quickly realized that it was most vulnerable during its landing and takeoff phases of flight as early jet engines of the day suffered from long spool up times. If American fighter pilots could get at the 262 at its home bases, then its speed advantage was negated. 

The distinctively marked Fw 190 D-9s of JV 44

In response, not only did the Luftwaffe bases of the 262 boast more robust air defenses in the form of anti-aircraft artillery, German fighter pilot Adolf Galland's fighter unit, Jadverband 44 (JV 44) had also come up with another tactic to defend the 262s. At the time, Galland had become a very vocal critic of the Luftwaffe's tactics and operational decisions to the point of becoming quite the thorn in the side of Hermann Goering, the head of the Luftwaffe. It was thought by Goering and his command staff that if Galland were given front line command of a unit, he might be killed in action which would get him out of their hair. Galland was tasked to gather the best fighter pilots he could find to operate the 262 as JV 44 was formed in February 1945. To deal with marauding American fighters over their bases, one of Galland's men formed what was called the Platzschutzstaffel (airfield protection squadron). Equipped with Focke Wulf Fw 190 "Dora 9s" which had excellent performance at low to medium altitudes, the Dora 9s were painted with a distinctive red/white stripped pattern on the underside of the fuselage so they could be recognized and not fired up on by the anti-aircraft artillery of JV 44's base. Each time Galland sent up 262s on an intercept mission or 262s were recovering at the base, the distinctively-marked Dora 9s were airborne. 

Ground testing the Aerojet rocket-equipped P-51D

Fighting past the the Platzschutzstaffel's combat air patrols wasn't the most ideal situation as American fighter pilots also had to deal with the AAA. With the first American jet fighters still a ways from operational employment, various methods were considered to deal with the Me 262 jet menace. One of the few concepts that did make it to the flight test stage was a rocket-boosed North American P-51D Mustang. With an approximately 100mph speed advantage over the Mustang, it was felt that a rocket booster of some sort could bring the P-51D a burst of speed that would allow it to battle the Me 262 at altitude instead of over the Luftwaffe's home fields. Tail number 44-73099 was pulled from the production line for modification with an Aerojet liquid rocket engine installed in the lower aft fuselage just behind the radiator and ahead of the tailwheel. The rocket engine used two hypergolic fuels- red fumaric acid and aniline with the fumaric acid acting as the oxidizer. Both were extremely toxic and corrosive and each was housed in a pressurized, 75-gallon tank, one under each wing and much smaller than the standard Mustang drop tank. 

With the lower aft fuselage painted with a reflective material to prevent heat damage from the rocket engine, all the pilot had to do was flip a simple off/on switch on the left side of the cockpit to activate the rocket engine. North American test pilot Bob Chilton took the modified Mustang to the air for the first time on 23 April 1945 and at 21,000 feet, he fired the rocket engine and the Mustang surged forward another 100mph. The two 75-gallon tanks were sufficient for one minute of operation. Several test flight were made, but the underwing tanks, even though smaller than the standard fuel drop tank, induced a considerable amount of drag that prevented the rocket-boosted Mustang from really getting the most out of that one minute of rocket-boosted flight. By the time the USAAF took delivery of the 44-73099, the Germans had surrendered unconditionally and the operational need for the rocket boosted Mustang faded into aviation history. 

Source: Building the P-51 Mustang: The Story of Manufacturing North American's Legendary World War II Fighter in Original Photos by Michael O'Leary. Specialty Press, 2010, p166-167.