29 December 2006

Flightdeck Friday - Flying Boats

Martin XPB2M-1 & JRM Mars

August 2004. While conducting a survey in Hawaiian waters of a Japanese mini-sub site, a NOAA minisub crew stumbled upon a large debris field, consisting of what appeared to be a seaplane keel and wings. Maneuvering the sub around the crew was able to clearly read the word "Marshall" still painted on the side of the wreckage. Unknown to the crew at the time, they had come across the final resting spot of the Martin Mars flying boat named "Marshall Mars, " one of the most famous of the five JRM-3 Mars flying boats operated by the Navy during and after WWII. Based on the West Coast, these aircraft ranged the Pacific, ferrying passengers, freight and when necessary, bringing wounded back to the States. An extraordinarily large aircraft, the Mars was the largest operational flying boat built and second in size only to Howard Hughes' "Spruce Goose" (the Goose weighed in at 200 tons gw while the Mars would weigh in at 165).

Throughout the 1920's and -30's, the Navy spent considerable effort in the development of long-range, sea-based patrol planes. Likewise, the commercial air travel industry also spent considerable time/effort as the "flying boats" offered the best possibilities for carrying large payloads long distances across the Atlantic and Pacific (the latter especially). Several major companies were involved -- Boeing, Sikorsky and Martin, to name a few. Boeing, of course, came to be known for its famous Clippers, which have come to define the exotic and romantic aspects of air travel in the late 1930s.

Over at Martin, considerable work was accomplished for the Navy in development of patrol aircraft, notably the PBM Mariner. While Consolidated was busy developing the soon to be famous PBY Catalina, Martin was contracted to build a huge long range seaplane which was heavily armed and envisioned as a" dreadnaught of the air." Starting with the PBM as a baseline, Martin scaled up the design to produce a prototype, the XPB2M-1 Mars.

This was a huge aircraft for the time -- weighing in at 140,000 lbs with a wingspan of 200 ft, the Mars was powered by four 2,000 hp Wright R-3350 engines turning laminated wood props. For comparison purposes, Boeing's XB-29 (which first flew in 1942) was 105,000 lbs and had a wingspan of 142 ft. The plane's interior was laid out with separate mess rooms, berths, and washrooms for officers and enlisted men. Its commander had a private stateroom and issued his orders from a desk behind the pilots' seats. A huge bomb-bay, located in the hull underneath the wings, contained racks capable of holding five 1,000-pound bombs each. When it came time to drop them these could slide out on either side along the lower edge of the wing.

Initial taxiing came to an abrupt end on the Friday before Pearl Harbor when one of the giant laminated-wood propellers threw a blade. It just missed the Martin flight engineer inside the hull and started a fire in one of the huge Wright engines (note that this was but the first time that fire would be an issue with the Mars). The stricken Mars had to be towed closer to shore to allow firemen to put out the blaze. When the smoke cleared serious damage to the starboard wing and number-three engine nacelle was apparent. Repairs took more than six months, by which time the plane's mission had undergone a complete re-evaluation.

Pearl Harbor showed that fast carrier planes made very effective bombers indeed, while German U-boats turned the Atlantic coast into "Torpedo Alley." Thoughts naturally turned to a "sky freighter" as an alternative way to ship supplies to Britain and other battlefronts, invulnerable to torpedoes. The industrialist Henry J. Kaiser suggested that, given Martin's blueprints for the Mars, he could quickly build hundreds of the planes in his west-coast shipyards. Martin's response was ambivalent. Although the company issued calculations suggesting that building the Mars in quantity would be more cost-effective than Liberty ships, Glenn Martin was not inclined to share his prize plane with another manufacturer. Kaiser joined forces instead with Howard Hughes; this was the origin of Hughes' 400,000-pound "Spruce Goose." Like Hughes, what Martin really wanted was government support for an even larger flying boat. Plans for the 250,000-pound Model 163, projected back in 1937, were dusted off and modernized. Building five hundred six-engine Model 193's could win the war, declared Glenn Martin, and company ads frequently depicted it as a postwar airliner. Meanwhile the Navy re-designated the original Mars as a transport, XPB2M-1R, and Martin began to remove its turrets and bombing equipment.

Long before either Mars transports or the Model 193 could have been ready, the tide had turned in the Battle of the Atlantic. The Mars was sent to the Pacific instead, where it built an impressive record between 1943 and 1945, carrying cargoes of up to 34,811 pounds. Particularly impressive was the plane's ability to carry ten tons of cargo on the critical California to Hawaii route.

In January 1945 the Navy ordered twenty more Mars transports, now designated JRM-1. In comparison to the original, their hulls were to be six feet longer and the split PBM-style tail replaced by a single 44-foot vertical fin. Fewer internal bulkheads and an overhead hoist would assist cargo-handling. Maximum take-off weight grew to 148,500 pounds. Recalling the China Clippers a decade before, the first JRM-1 was christened the "Hawaii Mars" in July 1945. It crashed just two weeks later in a landing accident on Chesapeake Bay. Four more JRM-1's were completed in 1945, but, in the wake of V-J Day, the Navy order was cut to six.

The Navy purchased its sixth and last Mars with Wasp Major engines, which enabled the single JRM-2 to carry an extra 18,000 pounds of cargo on the San Francisco-to-Hawaii run. The four earlier planes were eventually re-engined with Wasp Majors as well and designated JRM-3's. All five served in the Pacific, carrying military personnel, Korean-war wounded, blood plasma, and other priority cargo over the same routes as were once flown by the glamorous clippers. Like them, they were duly christened for Pacific destinations: Philippine, Marianas, Marshall, a second Hawaii, and Caroline.

A fire destroyed the Marshall Mars in 1950 not long after setting a record for carrying 308 passengers on a typical California-Hawaii run. This record would stand until the flight of the 747 almost two decades later. The other four JRM's served in the Navy (with VR-8 in the Naval Air Transportation Service or NATS) until 1956. They were then sold as surplus to Forest Industries Flying Tankers Limited, a Canadian firm, for a mere $100K. Currently, two still survive – the Philippine Mars and Hawaii Mars (the Marianas Mars crashed in an accident in 1961, and the Caroline Mars was destroyed in a hurricane a year later) and are used to drop 60,000-pound loads of water and fire retardant on Western forest fires.

General characteristics

  • Crew: four (with accommodations for a second relief crew)
  • Capacity: 133 troops, or 84 litter patients and 25 attendants
  • Payload: 32,000 lb (15,000 kg) of cargo, including up to seven jeeps
  • Length: 117 ft 3 in (35.74 m)
  • Wingspan: 200 ft 0 in (60.96 m)
  • Height: 38 ft 5 in (11.71 m)
  • Wing area: 3,686 ft² (342.4 m²)
  • Empty weight: 75,573 lb (34,279 kg)
  • Loaded weight: 90,000 lb (40,820 kg)
  • Max Takeoff Weight: 165,000 lb (74,800 kg)
  • Powerplant: 4× Wright R3350-24WA Duplex Cyclone 18-cylinder radial engines, 2,500 hp (1,865 kW) each


  • Maximum speed: 221 mph (356 km/h)
  • Range: 4,300 nautical miles (5,000 miles or 8,000 km)
  • Service ceiling: 14,600 ft (4,450 m)
  • Rate of climb: ft/min (m/s)
  • Wing loading: lb/ft² (kg/m²)