Home Aviation History Project Castor: the story of B-17 and B-24 bombers converted into flying bombs and aimed to destroy Luftwaffe’s V-1 sites

Project Castor: the story of B-17 and B-24 bombers converted into flying bombs and aimed to destroy Luftwaffe’s V-1 sites

by Dario Leone

The idea of converting existing aircraft into power-driven controlled bombs began in January 1944 at the Special Weapons Branch of the Equipment Lab at Wright Field, Ohio.

The idea of converting existing aircraft into power-driven controlled bombs began in January 1944 at the Special Weapons Branch of the Equipment Lab at Wright Field, Ohio.

As told by Steven J. Zaloga in his book American Guided Missiles of World War II, the initial concept was to use the C-47 transport aircraft or assault gliders, carrying ten tons of high explosives. The Materiel Command of the USAAF Engineering Division broadened the concept to include a variety of bombers, including the B-24 and B-17. The program formally began on Mar. 25, 1944 with preparations for twelve conversion kits. The guidance system was based around the aircraft’s autopilot. A two-man crew would pilot the aircraft during the initial phase, and once cruising altitude was reached, activate the autopilot and bail out. An accompanying control aircraft would then steer the aircraft to target using a radio-control system. The aircraft would be stripped of any extraneous weight, including gun turrets. The program received enthusiastic encouragement from the combat theater. In May 1944, Lt. Gen. James Doolittle, the 8th Air Force commander, offered the use of worn-out “Weary Willie” bombers available in Britain as the basis for flying bombs.

On Jun. 9, 1944, the B-17F and B-24J were selected as the first missile planes, now labeled as the RB-17F and RB-24J. Besides the dozen missile planes, two B-17s, one B-24, and one P-38J “Droopsnoot” were selected as the control aircraft. On Jun. 19, the project received the codename MX-541 Castor.

Project Castor: the story of B-17 and B-24 bombers converted into flying bombs and aimed to destroy Luftwaffe’s V-1 sites
After completing 80 missions with the 323rd Bombardment Squadron, Aphrodite B-17F (The Careful Virgin) was used against Mimoyecques but impacted short of target due to controller error.

The priority for Project Castor increased dramatically on Jun. 20, 1944 when Maj. Gen. Alfred Maxwell, Director of Operations of the US Strategic Air Forces in Europe (USSTAFE), telegraphed an urgent request that the Materiel Command dispatch any available experimental guided weapons to Britain with highest air priority. The Luftwaffe had initiated Operation Eisbär (Polar Bear) earlier in June, launching the new V-1 cruise missile against London. The USSTAFE commander, Lt. Gen. Carl Spaatz, was under intense political pressure to destroy the launch sites, especially the large “Heavy Crossbow” bunker launch sites that were impervious to conventional bomber attacks. The requirement was to attack the bunkers with a very heavy load of explosives within ten to twenty feet of the aim point.

Some preliminary studies had suggested that the radio-control system in use with the Azon guided bombs could be adapted for this requirement. Since the Azon only controlled the bomb along one axis, two systems were combined, one to control altitude and one to control direction. A crash program began under the codename Aphrodite with the 8th Air Force Operational Engineering Section at Bovingdon overseeing the program. Development of the Double Azon was undertaken by the 8th Air Force’s principal Azon expert, Maj. Henry “Jim” Rand. The radio link between the “Mother” and “Baby” consisted of the AN/ARW-18 transmitter on the control aircraft and the AN/ARW-1 receiver on the missile.

An experimental B-17F “robot” and “mother” ship were completed on Jun. 23, and conducted the first test flight on the evening of Jun. 24. The USSTAFE Base Air Depot 1 at RAF Burtonwood was assigned the conversion program and through the end of June, a total of ten “Babies” were converted, numbered B1 to B10. On Jul. 8, Spaatz ordered the conversion of 65 Aphrodite aircraft, with three-quarters of them loaded with 10 tons of nitrostarch high-explosive and the remainder with jellied gasoline (napalm).

The first Aphrodite mission was conducted on Aug. 4, 1944 using four Babies. Robot B1, targeting the Watten missile bunker, aborted the first pass due to cloud cover, but reached the objective on its second pass. The B-24 Mother was unable to activate the “Down” control, so Robot B1 was flown back out toward the English Channel. It was hit by flak and exploded before it could be reoriented for another pass. Robot B4 aimed at Wizernes operated satisfactorily, but cloud cover momentarily obscured the target during the final approach and the B-17 impacted about 700 yards beyond the bunker. On approach to the Mimoyecques bunker, the B-17 Mother crew felt that Robot B5 was flying too high and tried to reduce its altitude to the standard 300 feet. The controls were very crude, and as a result, it impacted about 1,500 feet short of the target. Robot B8, intended for Siracourt, experienced an altimeter failure over England. One crewman parachuted safely, but the aircraft suffered a series of uncontrolled climbs, finally stalling and crashing near Woodbridge in Suffolk.

Project Castor: the story of B-17 and B-24 bombers converted into flying bombs and aimed to destroy Luftwaffe’s V-1 sites
Drone B-17 used as a guided missile during World War II

Mission 2 was conducted on Aug. 6 using B-17F “Franklin Yellow” filled with 10 tons of Torpex explosive and B-17G “Franklin White” filled with 830 gallons of jellied gasoline and a dozen 100lb bombs. The Franklin Yellow group encountered a formation of B-24 bombers over the English Channel, which led to the Mother overshooting the Baby and experiencing control problems. Franklin Yellow went out of control, turned sharply to the left, flipped over on its back, and crashed into the sea. Franklin White seemed to be flying properly when the crew bailed out, but moments later the Mother noted that the B-17G was drifting to the right. This situation grew progressively worse with the Baby starting to do complete turns. After failed attempts to bring Franklin White under control using the second Mother, the B-17 was deliberately crashed into the sea rather than risk it inadvertently flying back to England.

The US Navy offered to assist Aphrodite by adapting its remote-control technology to the PB4Y-1 Privateer, the maritime patrol bomber version of the B-24 Liberator bomber. This remote-control system was significantly more sophisticated than the crude Double Azon. The Anvil was fitted with two television cameras, one in the cockpit to monitor the flight instruments and the other in the nose to aim the robot into the target. The Project Anvil conversions were undertaken at the Naval Air Factory in Philadelphia, and the first Anvil PB4Y-1 and two modified PV-1 Ventura motherships left the US on Jul. 7, 1944. The Anvil PB4Y-1 was dispatched on its Aphrodite mission on Aug. 12, aimed for the Mimoyecques bunker. The formation became airborne as planned, but before the crew could bail out, the 10 1⁄2 ton load of Torpex prematurely detonated, obliterating the Anvil PB4Y-1 south of Halesworth and causing extensive damage on the ground below. The pilot of the aircraft was Lt. Joseph P. Kennedy, son of the former US ambassador to Britain, and elder brother of future US president John F. Kennedy. Further Aphrodite missions were halted to determine the cause of the accident. A faulty safe-and-arming system was considered the most likely cause, a conclusion verified by more recent research.

Operation Crossbow, the campaign against the German V-1 sites, came to a close by the end of August 1944 as Allied troops overran the missile launch sites in France and Belgium. As a result, the targets for the remaining missiles shifted to other high-value, heavily protected targets. Deep-penetration missions into Germany were judged too dangerous. A second Anvil Privateer was expended at the Kriegsmarine base at Heligoland on Sep. 3, 1944, striking an area with fuel dumps and barracks on Dune Island. This was the first successful mission of the Aphrodite program.

While the improvised Aphrodite program was underway in Britain, the Castor Project continued in the United States. On Jul. 5, plans were approved to convert 500 missiles and 100 control aircraft. The first RB-17G and RB-24J were sent for conversion into Castor aircraft on Jul. 12. On Aug. 5, 1944, the Castor designations were changed with the missiles designated as BQB-17F and BQB-24J, and the control aircraft as CQB-17G, CQB-24J, and CPQ-38. This was changed again on Aug. 25 when the missiles were given assault drone designations as the BQ-7 for B-17 conversions and BQ-8 for B-24 conversions.

Project Castor: the story of B-17 and B-24 bombers converted into flying bombs and aimed to destroy Luftwaffe’s V-1 sites
USAAF B-24 Liberator bombers

The Castor aircraft used a flight control system closer to the Navy’s Anvil than to the Aphrodite’s Double Azon. The Castor system included two SCR-549A Block III television cameras, one in the cockpit to monitor the flight instruments and one in the nose to help aim the aircraft during the terminal phase. The Castor Baby aircraft were fitted with a small AN-145 or AN-146 antenna at the rear instead of the triple AN-117-A antenna array fitted on the Azon aircraft. The aircraft received a set of remote-controlled servos to operate the various flight aircraft controls including the throttle, propellers, landing gear, and cowl flaps. The Castor conversions also included an AN/TPN-1 Eureka navigation beacon that could be tracked by the AN/APN-2 Rebecca navigation aid on the control aircraft to monitor the drone even in cloudy conditions. To further aid the control aircraft, the BQ-7 was fitted with a smoke generator pod under the fuselage that emitted a white smoke trail.

The first Castor BQ-7 detachment from Wright Field arrived in Britain on Aug. 2, 1944. At RAF Burtonwood, twelve B-17 bombers were converted into BQ-7 drones as well as five CQB-17G motherships. The first drone conversion was completed on Aug. 20 and the first Castor mission was conducted on Sep. 11 against the Heligoland submarine pens. Of the ten Castor missions conducted through January 1945, only one was marginally successful. Flak, equipment failure, and poor weather plagued the program.

One special Castor mission was a plan to attack the German battleship Tirpitz in the Alten Fjord in Norway. Since normal high explosive would not be sufficient, British munition experts recommended a large hydrostatic bomb, detonated underwater. These large weapons could not be loaded into the bomb-bay of the B-17, so Boeing engineers outlined a method to remove the fairing behind the B-17 cockpit. This created an opening large enough to allow the weapons to be lowered into the fuselage from overhead. Only one aircraft was converted in this fashion, Aphrodite B3 “Gremlin Gus II.” After the payload was secured into the fuselage, the opening was faired over with sheet metal, leading to the drone’s new nickname “The Roadster.” In the event, the Tirpitz was sunk on Nov. 12, 1944 by RAF Lancaster bombers using Tallboy penetrating bombs.

The Oldenburg mission on New Year’s Day 1945 was the last Castor mission in Europe. By this stage, the 8th Air Force felt that the system was not effective under poor winter weather conditions and against targets heavily defended by flak.

American Guided Missiles of World War II is published by Osprey Publishing and is available to order here.

Project Castor: the story of B-17 and B-24 bombers converted into flying bombs and aimed to destroy Luftwaffe’s V-1 sites
B-17 bombers

Photo credit: U.S. Air Force and www.fold3.com

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