19 April 2011

Sukhoi's First Jet Bomber

The Sukhoi Su-10 in its final configuration
Following the end of the Second World War, both the Soviet Union and the West aggressively pursued jet bomber designs after the Luftwaffe had successfully introduced the Arado Ar 234 to combat in the waning months of the war. In the West, many early designs were based on layouts of high-mounted wings with wing-mounted nacelles to allow for a reasonably-sized bomb bay. Similar approaches were taken in the Soviet Union with the design bureaus (called OKBs) of Illyushin, Tupolev, and Sukhoi tapped to develop jet bomber designs to succeed the Soviet Air Force's standard bomber, the piston-powered Tupolev Tu-2. Last summer I had posted about an early Tupolev design that actually did fly, the Tu-12, that was based on the Tu-2 as a matter of expediency pending the arrival of the Tu-14 bomber. While Illyushin and Tupolev both had large aircraft design experience from their own work on twin-engine bombers during the war, Pavel Sukhoi's experience was limited to his prewar tenure at OKB Tupolev. But, given the pace of technological progress and the urgency of rearmament in competition with the West, Sukhoi was ordered on 26 February 1946 to develop a jet bomber powered by four Junkers Jumo 004B turbojets, the same jet engines that powered the Arado Ar 234. Work on Sukhoi's first bomber design began in earnest in April of that year and the aircraft received the official designation of Su-10. 

Several powerplant arrangements were considered along with the use of six engines instead of the four as originally specified. In the Soviet Union, the Central Aerohydrodynamic Institute in Moscow, or TsAGI by its Russian name, did a lot of wind tunnel and theoretical research work to support design efforts at each of the OKBs. At the time that OKB Sukhoi was working on the Su-10, TsAGI had lagged behind Germany and the West in high speed research and as a result, they lacked a significant amount of information on evaluating jet aircraft designs. As a result, much of what laid the basis of TsAGI's high speed aircraft research came about during their work in support of the development of the Su-10 in addition to what had been obtained of German design work following the end of the war. Two leading design variants were evaluated in TsAGI's wind tunnels- one design had four of the jet engines clustered in the mid fuselage and exhausting out the rear with two jet engines under the nose- this was felt to be advantageous design as it left the wings clean. The other leading variant had wing mounted nacelles with a high unswept wing with three engines mounted in clusters on each wing. 

To meet the specified target speed of 528 mph at 26,000 feet, Pavel Sukhoi decided that four engines were insufficient and that six were necessary. Since the Junkers Jumo 004 engine was the only jet engine available to the Soviet Union at the time that had reached production status, the Soviet Politburo placed high priority in reverse-engineering the engine for production- Vladimir Klimov and his OKB were already known for their piston engine designs during the war and he was put in charge of getting the German engine into production as the Klimov RD-10. Klimov's closest aide, Nikolay Kuznetsov, headed the actual reverse-engineering effort- Kuznetsov would go on to form his own engine OKB several years later. 

Inboard layout of the Su-10
With approval from the state authorities to use six engines, design work had settled on a cluster of three engines on each wing as the most efficient layout- the nacelle had two engines side by side with the third RD-10 engine below and slightly ahead of the pair. On 6 May of that year, a full scale mockup was built that was tested in TsAGI's largest wind tunnel with real RD-10 jet engines. Within two months, refinements to the design based on TsAGI's evaluation were in place as full-scale engineering began on the prototype. While two months sounds rapid, development of the Su-10 hit repeated technical hurdles, the biggest of which was that TsAGI lacked a significant portfolio of well-studied high speed airfoils. As a result, while supporting development work on the Su-10, assimilating German design work, TsAGI was also hurriedly developing its on portfolio of high speed airfoils. As a result, Sukhoi's team was constantly having to revise the Su-10 design based on developments from TsAGI. 

By October the full scale mockup had been approved by Soviet Air Force authorities and metal was finally cut for the prototype on 14 October 1946. In the first week of December, the Soviet government commission in charge of aircraft production decreed that the Su-10 would no longer use the Klimov RD-10 engine but instead use the TR-1 engine from the Lyulka OKB, the first indigenous Soviet jet engine design. Since the TR-1 was more powerful than the RD-10, the Su-10 could revert back to a four-engined design and once again Sukhoi and his team had to revise the bomber's design to accommodate the new Lyulka engines. Working at a frantic pace to meet state-decreed deadlines, Sukhoi managed to have a full set of production drawings ready by 23 December 1946 and three days later the OKB's own workshops had completed a static test airframe and production jigs and tooling for the prototype. If things weren't frantic enough as it was, the Minister of Aircraft Industry wanted the Su-10 flying for participation in the air show at Moscow on 18 August 1947! Common sense prevailed and that was one deadline Sukhoi was allowed to ignore. 

Three-view showing the layout of the Su-10 medium bomber
By 15 December 1947 the hydraulic system had been fully tested on a special ground rig (similar to today's "iron bird"), but construction of the prototype was hampered by slow progress from the various subcontractors that were responsible for some of the Su-10's systems. For example, the defensive armament system (which consisted of a manned tail turret, a remotely-operated dorsal turret and forward-firing cannon), the autopilot, the navigation suite and even the Lyulka TR-1 engines were to have all been delivered to Sukhoi's workshops for the prototype but by the end of 1947 none of those items were ready yet. During the delay, studies looked at alternative powerplant options and it was decided that the initial flight tests of the Su-10 would use the TR-1 engine but as soon as the more powerful TR-2 engine developed from the TR-1 became available, the Su-10 prototype would have its engines swapped out and then continue with the flight test program. 
These persistent delays led to the Su-10 prototype to sit in the OKB workshops missing various components- by 4 June 1948 the Soviet Council of Ministers ordered that spending had to be reduced on aircraft development programs that year and one of the unlucky programs to get canceled was the Su-10. By that summer OKB Ilyushin had already made the first flight of its Il-28 medium bomber and its performance outstripped what was projected for the Su-10. Not even rolled out, the Su-10 prototype was donated to the Moscow Aviation Institute where it was slowly reduced to parts over time as an instructional airframe. Sukhoi in the years to come devoted its efforts at interceptor, fighter, and ground attack aircraft and it wasn't until the arrival of the Su-24 Fencer in the late 1970s that Sukhoi finally had a production jet bomber. 
Source/Images: OKB Sukhoi: A History of the Design Bureau and Its Aircraft by Yefim Gordon and Dmitriy Komissarov. Midland Publishing, 2010, p93-101.

03 April 2011

The Last Operational B-17 Flying Fortresses

The Israeli B-17s originally flew without any defensive armament
On the day prior to the expiration of the British Mandate over Plestine on 15 May 1948, David Ben-Gurion declared the independence of the new State of Israel and within hours, Arab forces from Egypt, Jordan, Syria, and Lebanon invaded, starting the 1948 Arab-Israeli War, or the War of Independence in Israel. At the time of the declaration, the hastily-organized aviation assets of the fledgling state became the nascent Israeli Air Force, which in turn became part of the IDF, Israeli Defense Forces, on 26 May 1948. Initially outclassed by the Arab air forces with only a modest light plane fleet, the air war began to swing in favor of the Israelis on 20 May with the arrival of the first Avia S-199s from Czechoslovakia- the Junkers Jumo-powered version of the Messerschmitt Bf109 leveled the playing field against the Egyptian Spitfires. Despite a UN arms embargo on the participant parties on the 1948 war, resourceful Israelis and supporters worldwide (who were called "machal") insured a supply of arms through rather creative means, often involving subterfuge. Al Schwimmer was a long time flight engineer for Trans World Airlines who organized the transfer of arms to Israeli in 1948. At first Schwimmer got surplus C-47 transports transferred via Panama to form the nucleus of the IDF's air transport arm, but he soon recruited a former government purchasing agent, Charlie Winters, who at the time was based in Miami and was using three civilianized Boeing B-17 Flying Fortresses to transport produce between Puerto Rico and Florida. Winters sold the B-17s to the Israelis for $15,000 a piece and organized a team of former USAAF mechanics and engineers to make the aircraft combat ready.

Aircraft 1602 had a Mickey Mouse cartoon on its tail
The three B-17s (44-83811, 44-83753, and 44-83851) were covertly flown from the United States to an airfield in Czechoslovakia that was acting as the European terminus for the aerial supply line to Israel. Bogus flight plans to Brazil were filed to cover their tracks. Winters himself flew one of the B-17s across the Atlantic. He had arranged for a fourth B-17 as well, but this aircraft after eluding authorities in Canada managed to reach the Azores only to be impounded by the Portuguese government. At the Czech airfield of Zatek, the B-17s were further upgraded with improved instrumentation. They were loaded with bombs for the flight to Israel on 15 July 1948, but with the military situation becoming tenuous for the Israelis that summer, during their delivery flight they were diverted to hit Egyptian targets since they were carrying bombs anyway. One B-17 was to hit Gaza City, the second B-17 was assigned the Egyptian air base at El-Arish, and the third B-17 was assigned King Farouk's Royal Palace in Cairo. The first two B-17s had problems finding their assigned targets but the third B-17 did manage to bomb Cairo, which, like the Doolittle Raid's psychological effect on the Japanese in 1942, caused significant anxiety in Egypt as Cairo was felt to be immune from attacks by Israel's rag-tag air force of what was thought to be just light aircraft. The first two aircraft ended up bombing an Egyptian target in Rafiah instead but the overall effect of the raids not only damaged Egyptian morale, it served to boost Israeli morale as well. Since all three aircraft had been hurriedly made combat ready, numerous technical problems beset them on their delivery flight/first combat mission- one of the more notable issues was that the oxygen system kept quitting, which on several occasions during the flight from Czechoslovakia to Israel by way of Egypt had caused some of the crew members to pass out. 

The three Israeli B-17s at various points in their careers
All three B-17s landed safely at Ekron airfield in Israel following their highly eventful delivery flight- the leader of the flight, a former USAAF pilot named Bill Katz, was named commander of a new squadron based at the former RAF base of Ramat David that would operate the B-17s- 69 Squadron "Patishim" or "The Hammers". The three aircraft were camouflaged and serialed 1601, 1602, and 1603. At the time the IDF had been relying on converted transports as bombers, so the arrival of the B-17s represented a significant leap in offensive capability for the Israelis. The following day on 16 July the three bombers flew three combat missions together, the first one to bomb the El-Arish air base that was missed the previous day, the second mission later in the day to bomb advancing Egyptian forces in the south and that night the third mission was against advancing Syrian forces in the north. Over the next several days multiple bombing missions were flown each day against Egyptian, Jordanian, and Syrian forces. Several attacks were mounted against Arab air bases in the belligerent countries and even several missions were flown against targets in the Syrian capital of Damascus. At first the missions were flown with fighter escort by Israeli Spitfires and Avia S-199s, but as the Arab air forces' losses mounted, soon the B-17s were able to operate without fighter escort. By the time of the armistice in February and March of 1949, over 200 combat missions had been flown by 69 Squadron. With the end of the war, the squadron eventually moved to the new air base at Hazor.

One of the B-17s, aircraft 1602, was modified to carry a search radar under the nose where the chin turret was located. This aircraft through the first half of the 1950s was stripped of its desert camouflage and operated as a maritime patrol aircraft in bare metal colors in the Mediterranean. By this time, enough spares had been acquired to allow all three B-17s to be retrofitted with gun turrets and at least two of the B-17s were kept operational at any given time. By July 1956 the long serving bombers were placed in storage. 

During the 1956 War, Israeli B-17s wore yellow/black identification bands
The Suez Crisis of 1956 brought the three B-17s back out of storage that October. Israeli involvement in the 1956 war began on 29 October with Operation Kadesh, the Israeli invasion of the Sinai Peninsula. On 31 October 1956 the three B-17s attacked Egyptian positions in the Gaza Strip, but a series of mounting technical problems over the course of the war spelled the end of B-17 operations for the IDF. The bombers were finally retired in November 1958 as the last operational B-17 Flying Fortresses in action. 69 Squadron was disbanded as well, but would be reformed in 1969 as the second Israeli Air Force squadron to operated the F-4E Phantom II. Today 69 Squadron operates the F-15I Ra'am, the Israeli variant of the F-15E Strike Eagle. Al Schwimmer, the TWA flight engineer who was instrumental in organizing Israel's air force, would go on after the 1948 War to establish Israeli Aircraft Industries. His activities were called by David Ben-Gurion as the "single biggest contribution by the Diaspora towards the survival of the State of Israel". Charlie Winters was an Irish Protestant who helped the Israelis as a favor to his Jewish friends in Miami. As a result, he was charged by the US District Attorney in Miami for violating US laws and was fined $5,000 and sent to prison for 18 months. Two other Americans were also charged who aided Israel- one was Al Schwimmer, who never served prison time as he was convicted in absentia and was pardoned by President Bill Clinton in 2000. The other American in the operation was Hank Greenspun, who also never served an prison time and was pardoned in 1961 by President John F. Kennedy. 

Charlie Winters passed away in 1984 having never told his children of his role in the creation of the State of Israel. It was only after the Israeli government sent an arrangement of blue and white flowers did his family learn of his activities in 1948. His ashes were interred in the ancient Templars' Cemetery in Jerusalem. In 2008, Winters was pardoned by President George W. Bush as only the second posthumous pardon in history. 

Sources/Images: Aviation Classics, Issue 8. "The Israeli Air Force and the B-17" by Tim Callaway, p118-119. B-17G Flying Fortress in Israeli Air Force Service 1948-1957 by Alex Yofe. White Crow Publications, 2010.