The Convair YB-60 Prototype is a piece of digital artwork by Erik Simonsen which was uploaded on June 23rd, 2018.
The YB-60, foreground, was a gargantuan beast for its time, eclipsing the B-36, background, in terms of both overall size and weight.
YB-60 49-2676 in the assembly hanger. Note the twin engine pods on the port wing, which were taken from the XB/YB-52 designs.
The standard implosion atomic weapon expected to be carried by the B-60 was the MK VI, developed from the World War II `Fat Man’ atomic bomb.
As Convair refined the design of the new bomber, so radical became the changes between the YB-36G and its genetic forebear, the B-36F, that the USAF accepted that a new designation was required, this emerging as the YB-60, which came into effect around late summer 1951. While the new designation was being decided upon, Convair was encountering problems defining the configuration of the planned operational B-60, and, therefore, in August 1951, proposed that it build the two prototype aircraft to different configuration as the final configuration was, at that time, still being refined. Following a two day deliberation the USAF accepted the new proposal from Convair on the understanding that additional costs would be borne by the contractor.
The second design iteration documentation of late summer 1951, the first under the designation YB-60, showed the aircraft retaining the same height, wing span and wing area (50.4 ft., 206.4 ft., and 5,239 sq. ft. respectively) of the YB-36G design. Length, however, was increased from 168.7 ft. of the YB-36G to 171.2 ft. The aircraft was still to be powered by 8 x YJ57-P-3 turbojet engines with the same ratings as before. Crew numbers, fuel load, bomb load, defensive armament and systems remained the same as those specified for the YB-36G. The aircraft was equipped with a relatively conventional undercarriage consisting of two quad wheel main units and a twin wheel nose unit along with a stabilizing twin wheel unit in the extended tail section, the latter enabling the aircraft to retain a more or less level posture for extended periods during takeoff. Landing roll was reduced by use of a 67.5 ft. diameter braking parachute.
The maximum internal capacity of 42,106 gal. JP-3 fuel was housed in ten fuel tanks; 2 in the outboard wing sections (7,616 gal.), 2 in the wing outer panels (4,470 gal.), 2 in the inboard wing (8,816 gal.), 2 in the wing centre (10,660 gal.) and 2 in the wing central sections (10,244 gal.); the outboard and wing centre tanks being self-sealing. As well as fuel, 96 gallon of 1010 grade oil and an undisclosed volume of water/alcohol mixture for the engines was carried.
USAF documentation showed that projected performance and weight figures were identical to those of the YB-36G, despite the extended length fuselage of the YB-60. As with the YB-36G design of May 1951, all figures were contractor estimates that were not substantiated by the USAF WADC (Wright Air Development Centre).
The USAF described the YB-60 as a “long-range, high altitude, very heavy bombardment” bomber aircraft with a five-man crew – pilot, co-pilot, bombardier/radar operator, radio operator/tail turret operator and navigator. All five crew were housed in a pressurised, heated and ventilated cabin in the forward section of the aircraft with emergency oxygen supplied in the event of cabin depressurisation. The bombardier/radar operator operated the K-3A bombing navigation system which was equipped with a Y-3A optical/radar bombing sight, all carried over from the B-36F. The twin M24A1 20 mm cannon defensive armament, located in the tail with 400 rounds of 20 mm ammunition per gun, was remotely controlled and directed by the AN/APG-32 fire control radar, also carried over from the B-36F, with the gunner/radio operator located in the forward cabin.
As was the case with the YB-36G, the main difference between the late summer 1951 YB-60 design iteration and the B-36F was the wing and empennage in the latter being swept back, with eight YJ57 engines in four twin podded nacelles – two suspended beneath each wing. The new wings featured “retractable single slotted type trailing edge flaps”. The nose compartment of the YB-60 was redesigned for the higher operating speeds that were possible courtesy of the new wing design and jet power. All of the B-36F defensive gun turrets were removed with the exception of the tail turret noted above. The planned extended tail section would, as previously noted, allow the aircraft to retain a horizontal posture for extended periods during take-off.
As the intended powerplant was the J57-P-3 turbojet being developed for the Boeing XB/YB-52, Convair, when designing the YB-36G, was allowed to incorporate the Boeing designed twin engine pod and nacelles developed for the XB/YB-52, these being carried over to the YB-60 design.
The J57 was developed in an attempt to overcome the high fuel consumption associated with early turbojet engines. In short, it was hoped to overcome the short-range associated with turbojets, allowing acceptable range to be achieved together with the benefits of turbojet performance in areas such as speed. The alternative was to switch to turboprop engines when a suitable powerplant was available, which, although addressing the range problem, came with the undesirable side effect of reduced speeds, although as was demonstrated by later operational turboprop powered aircraft, this speed reduction may have been of a smaller than anticipated margin.
Development of the J57 was conducted in parallel with that of the YB-52. By the first weeks of 1951, available bench test engines had accrued a mere 550 hours of running. However, manufacture of production standard J57-P-3 engines commenced in time to allow both the YB-52 and YB-60 to conduct their maiden flights in the first half of 1952, assuming the aircraft were ready. That said, J57 engines were less than abundant in the early months of 1952, the last of the initial eight such units required for the first flight of the YB-60 being delivered to Fort Worth only on the 6th of April that year.
The YB-60 electronics suite consisted of AN/ARC-3 VHF command, USAF combat interphone, AN/ARN-6 radio compass, AN/ARN-12 marker beacon, AN/ARN-5B glide path, AN/APN-14 range receiver, AN/APN-9A Loran, AN/APX-6 IFF, AN/APG-32 gun laying radar, AN/APS-23 Search radar, an ECM Equipment suite – AN/APR-4 receiver, AN/APR-9 radar receiver, AN/ALA-2 panoramic indicator, and AN/APT-4, -6, -9 jamming radar. Some elements of the electronics and ECM suites were carried over from the B-36F.
The main area in which the YB-60 excelled over the XB/YB-52 was in its immense load carrying ability. Bomb load outs could include either a single 43,000 lb. class weapon or three 22,000 lb., or four 12,000 lb., or twelve 4000 lb., or twenty eight 2,000 lb., or seventy two 1,000 lb., or 132 x 500 lb. bombs. The 4,000 lb., 2,000 lb., 1,000 lb., and 500 lb. weapons were Box Fin’ bombs of World War II generation. Interim conical fin 2,000 lb., 1,000 lb., and 500 lb. weapons were also expected to be carried.
Atomic weapons included the MK VI Aerial Bomb which was developed in 1949 as an improved variant of the first generation atomic weapon, the so called `Fat Man’ bomb, dropped on the Japanese city of Nagasaki in August 1945. The MK VI weapon was 10 ft. 7 in in length, diameter was 60 in, weight was 8,500 lb. and explosive yield was in the range of 1 kiloton = 1,000 tons of TNT. Between 1951 and 1955, several modifications were introduced resulting in Mod 0 to Mod 6, the weapon being carried by a number of delivery platforms – the B- 29, B-36, B-47, B-50 and the B-52. Had the B-60 entered production then this weapon would have been the standard `implosion’ atomic weapon carried by that aircraft.
The MK 17 Thermonuclear H-Bomb (Hydrogen Bomb) entered service on B- 36 bombers in 1954, and served until 1957 when they were replaced by MK 39 H-Bombs. The cumbersome MK 17 had a casing three and a half inches thick, was 24 ft. 10 inches in length, had a diameter of 5 ft. 2 inches and weighed in at 41,400 lb. This powerful weapons yield was in the megaton class (1 MT = 1 Million tons of TNT). When the B-36 was phased out the MK 17 was withdrawn from the inventory as it was too large to be carried by the smaller B-52.
DESIGN ITERATION 11 JULY 1952
Before its official cancellation Convair unveiled a new design iteration of the YB-60 to which specification the second prototype was being built. This design placed increased emphasis on defensive armament while overall performance did not increase, and in many areas deteriorated, further widening the gulf, in regards to performance, between the Fort Worth design and the XB/YB-52.
The design iteration documentation of summer 1952 showed the aircraft retaining the same height, wing span and wing area (50.4 ft., 206.4 ft., and 5,239 sq. ft. respectively) of the previous design iterations, but length was further increased from 171.2 ft. of the summer 1951 design to 175.2 ft. The aircraft was still powered by 8 x YJ57-P-3 turbojet engines with the same ratings as before. There were, however, major design changes to the aircraft which would now have a defensive armament of ten x M24A1 20 mm cannon housed in five remotely controlled turrets with a combined ammunition load of 3,600 rounds. The tail turret accommodated 400 rounds for each of the two 20 mm cannon and the two upper forward and two lower aft turrets, which were retractable, accommodated 350 rounds for each of the combined eight 20 mm cannon. The two turrets located on the lower aft fuselage and the two upper forward fuselage turrets were to be remotely controlled from the two aft and two forward fuselage sighting stations in the pressurised cabins. Secondary control of the two upper forward turrets was to be conducted by the APG-32 “evasive” nose radar. The tail turret was to be remotely controlled by a “duel radar system”, the primary being the AN/APG-41 gun laying radar, which replaced the APG-32.
At 72,000 lb., bomb load remained the same as before, as did the load out for various bomb sizes.
The maximum internal fuel capacity of 41,462 gal. was slightly reduced, the fuel now being JP-4, still housed in ten wing fuel tanks; 2 in the outer wing panels (4,770 gal.), 2 in the wing outer outboard (7,616 gal.), 2 in the inboard (8,816 gal.), 2 in the centre (10,660 gal.) and 2 in the wing centre sections (9,600 gal.); the outboard and centre being self-sealing. Oil capacity was increased to 125 gal. of PWA 521-A synthetic, accommodated in the nacelles.
As with the previous designs iterations, all figures were contractor estimates that were not substantiated by WADC.
Whereas in previous design iterations the crew of five was housed in a single pressurized compartment in the forward fuselage, the crew, now increased to nine, was split between two pressurised cabins with heating and ventilation. The forward pressurised cabin accommodated the pilot, co-pilot, bombardier/radar operator, navigator/gunner, and the forward gunner, and the aft pressurized cabin accommodated the radio/ECM operator, tail gunner/APG-41 operator and the two aft under fuselage turret gunners.
The electronics suite consisted of AN/ARC-3 VHF command, AN/ARC-27 UHF command, AN-ARC-8 liaison, USAF combat interphone, AN/ARN-6 radio compass, AN/ARN-12 marker beacon, AN/ARN-5B glide path, AN/ARN- 14 range receiver, AN/APN-9A Loran, AN/APX-6 IFF, AN/APG-41 gun laying radar, AN/APG-32 radar (evasive), AN/APS-23 Search radar, RF-3A/AP phantom target, ECM equipment suite – AN/APR-4 receiver, AN/APR-9 radar receiver, AN/ALA-2 panoramic indicator, AN/APT-6 x 2 sets and one AN/APT- 9 jamming radar. The ECM suite also included 178 lb. of chaff to be dropped at each bomb release point.
The increased weights of the new design equated to reductions in performance in many areas. Ground run during an unassisted take-off was increased from 6,710 ft. to 8,000 ft., with an altitude of 50 ft. now being attained in a distance of 9,700 ft., some 1,569 ft. more than that of the summer 1951 design. Maximum speed was reduced from 448 knots to 441 knots, this being attained at an altitude of 39,250 ft., whereas the 1951 design attained its maximum speed of 448 knots at 35,332 ft., the 1952 design capable of attaining 433.5 knots at this altitude – all figures being at maximum power. Combat speed was now set at 436.2 knots at maximum power at an altitude of 43,400 ft., whereas the 1951 design attained its combat speed of 446 knots at an altitude of 35,000 ft. Stalling speed was increased from 115.3 knots to 121.3 knots in “power-off, landing configuration” at take-off weight. Figures for climb rate were stated as 1,540 ft./pm at sea level with normal power at take-off weight and 3,555 ft./pm at sea level at combat weight and maximum power, down from the previous design figures of 1,570 ft./pm and 3,815 ft./pm respectively. Ceiling was stated as 36,650 ft. at 100 ft./pm at take-off weight and normal power or 44,650 ft. at 500 ft./pm at combat weight and maximum power. These figures were down slightly from the 1951 figures of 37,300 ft. and 46,200 ft. respectively.
Combat radius was also reduced, now being stated as 2,538 nm when configured with a 10,000 lb. bomb load and flying at an average speed of 406 knots in a duration of 12.68 hours. Ferry range was stated as 5,380 nm when configured with 38,590 gal. of JP4 fuel at maximum take-off weight and flying at an average speed of 406 knots in a duration of 13.39 hours. It should be noted that much of the above performance figures were estimates by Convair and “not substantiated by AMC” (Air Material Command).
While the weights for various configurations and flight conditions differed considerably from the previous iteration of summer 1951, the only major difference in overall dimensions was in the aircraft being 4ft longer than the summer 1951 iteration and 6.7 ft. longer than the May 1951 YB-36G design.
The USAF did not release any detailed mission profiles for the YB-60, but a number of “Formula” mission profiles were released by the contractor, these, it being pointed out, not being substantiated by WADC. The following summaries of mission profiles are for the summer 1951 YB-60 design.
For profiles termed `Radius Missions I & II’ the aircraft would take-off from base and, under normal power, climb to the unspecified `initial cruise altitude’ (this, it is inferred, being the optimum cruising altitude) where it would cruise at optimum speed and power for a substantial part of the flight to the target area where it would conduct a six minute bomb run at normal power then drop bombs before conducting a further six minute bomb run at normal power to drop bombs, presumably at a secondary target. This would be followed by a six minute evasive phase under normal power, following which the aircraft would set course for the return cruise flight to base. Fuel allowances, not included in the above missions, were 5 minutes fuel consumption at normal power settings for the ground warm-up and take-off, a further six minutes consumption at normal power for evasive maneuvers and an allocation of 10% as an “endurance reserve” and for landing back at base.
For the profile termed `Range Mission I’, the aircraft would take-off and climb to the `initial cruising altitude’ from which it would then cruise to the target area “at long range speeds” where it would commence a six minute bomb run at high speed under normal power settings. Once the bombs had been dropped the aircraft would return to optimum cruise altitude and speeds for the return flight to base where it would land. The range free fuel allowances were the same as for Radius Missions I & II, with the exception that no allowance was considered for evasive action maneuvers.
`Radius Mission III’ involved take-off and climb to cruising altitude (above 40,000 ft.) for long range flight to an area some 500 miles from the intended target, at which point the aircraft would descend to an altitude of 40,000 ft. for a high speed run to the target under normal power settings. Once the bombs were dropped the aircraft would conduct evasive maneuvers under normal power settings for six minutes and then cruise at 40,000 ft. for 500 miles of the return portion of the mission before climbing to optimum altitude for the long range cruise to base. The range free fuel allowances for this mission type was the same as those specified for Radius Missions I & II.
With `Radius Mission IV’ the aircraft would be at 45,000 ft. when it reached a point 500 nm from the target. Remaining at this altitude the aircraft would conduct a high speed run to the target under normal power and drop its bombs, conduct six minutes of evasive maneuvers at normal power followed by a high speed run, still at 45,000 ft., to a point 500 nm out from the target, where it would “climb to long range flight path” under normal power for the cruise flight back to base. The range free fuel allowances were the same as those specified for Radius Missions I & II and III.
`Radius Mission V’ would have involved take-off and climb under normal power to a specified “high speed flight path” where the aircraft would cruise under normal power settings to the target and drop its bombs, which would be followed by six minutes of evasive maneuvers. USAF Documentation then states that the aircraft would climb to a “high speed flight path” for the return to base, however, it did not state that the aircraft would previously have descended from the “high speed flight path” near the target. It could, however, be inferred by the intended document readership that an altitude descent occurred during the evasive maneuvers. The range free fuel allowances for this mission type was the same as those specified for Radius Missions I, II, III & IV.
The last of the 1951 documented mission scenarios was referred to as `Range Mission VI’, whereby the aircraft would take-off and climb under normal power to its flight path altitude and cruise “to a point where 90% of internal fuel has been consumed”, the aircraft then being landed. This mission scenario is assumed to be a ferry mission; range free fuel allowances being the same as those specified for Radius Mission I.
For the final design iteration the `Formula’s’ for the respective Radius and Range mission scenarios were more or less the same as before, but with some slight differences such as allocations for evasive maneuvers which were generally down to about 2 minutes compared to the six minutes of the previous design iteration.
FROM FLIGHT TEST TO CANCELLATION
While there had been no official competition for selection of a new strategic bomber to replace the B-36, neither Convair nor Boeing were under any illusions that their aircraft were being closely compared to each other in terms of performance, capability, cost and the ability to adapt to future operational scenario changes.
The Boeing aircraft always seemed to be the favoured choice for the USAF, the YB-60 appearing more like the unwanted child of that service. When the first YB-60 conducted its maiden flight from Carswell AFB, Fort Worth, Texas, on 18 April 1952, it was in the shadow of the YB-52, which had conducted its successful maiden flight three days earlier on the 15th of the month. In the event the YB-60 maiden flight, which lasted 66 minutes and was conducted with a Convair crew, the aircraft being flown by Convair pilot Beryl A. Erickson, was dogged by inclement weather, a portent of things to come. Two more flights were conducted before the end of April, and although these were somewhat more successful than the first flight, overall results of the early test flights were mixed, showing the basic design to have reasonable handling characteristics, although it was clear, even at this early stage, that in most important respects the design was inferior to the rival Boeing design, and as flight testing continued further results confirmed that the aircraft was generally inferior to the YB-52, in regards to flight performance. This inferiority, combined with a number of problems encountered during the test program, not least of which were control problems due to buffet, eroded any real hopes that the aircraft would progress to operational service. Problems were encountered with the rudder, flutter, engine surge and electrical systems, and, as testing continued, problems with aircraft stability increasingly surfaced, this being attributed to strong aerodynamic forces interacting with the aircraft control surfaces.
In the event, the YB-60 program was officially cancelled by the USAF on 14 August 1952, by which time the second YB-60, 49-2684, was more than 90% complete. Convair’s fortunes were further dented when in August 1952, the same month that the YB-60 was cancelled, the USAF announced its intention not to procure any more B-36 bombers other than those already in production. This stemmed from the awarding of a contract to Boeing for production of 70 B- 52 bombers, which was now seen as the future airborne delivery platform for Strategic Air Command.
For the YB-60, the writing had been on the wall for some months before as the USAF appeared committed to the smaller B-52, which was not only proving to be the better performer in almost every respect other than load carrying ability, but showed greater potential to differing operational scenarios such as the later switch from high altitude nuclear delivery to low altitude delivery as improving Soviet air defence capabilities meant that medium and high altitude bombers would find it very difficult to survive on deep penetrations of the Soviet Union and Soviet controlled territory.
While the B-52 was able to adapt reasonably well to operating at lower altitudes, the YB-60, as would have been a production B-60, was encumbered by an unacceptable speed reduction when operating at low altitudes, this brought about by structural strength limitations. Even when operating at higher altitudes, the B-60 was encumbered by the buffet problems noted above.
Even with the advent of jet power for strategic bombers, one thing remained clear in the early 1950’s, as it had been since the dawn of powered combat aviation, a larger, slower aircraft was more vulnerable to interception than a smaller faster aircraft, which put the YB-60 at a disadvantage when compared with its Boeing rival. The XB/YB-52, while being a very large aircraft in its own right, was smaller in overall dimensions in comparison to the YB-60, both designs equaling out to become similar in empty and basic weights by the time of their final design iterations. In regards to bomb load the XB/YB-52, at 25,000 lb., was severely outclassed by the 72,000 lb. capability of the YB-60. However, in overall performance most of the advantages, as noted above, lay with the XB/YB-52, which was between 84 and 90 knots faster than the YB-60 dependent on what iteration of the later was used as reference. Similarly, combat speed of the XB-52, at 516 knots, was significantly higher than that of the YB- 60 which attained 446 knots for the 1951 iteration and 446 knots for the final iteration. At 2,538 nm, for the final YB-60 design iteration, combat radius was lower than that of the XB-52, which had a radius of 3,070 nm (both designs being configured with a 10,000 lb. bomb load). At 46,500 ft. the XB-52 had a slightly higher operating ceiling than that of the YB-60, which came in at 46,200 ft. and 44,650 ft. for the 1951 and 1952 design iterations respectively; the XB-52 and both YB-60 design iterations being at combat weight and maximum power.
With a defensive armament of a quad 0.5 in machine gun installation in the tail, defensive armament of the XB-52 was inferior to the remotely operated twin 20 mm cannon in the tail turret of the summer 1951 YB-60 design iteration. In the final YB-60 design irritation defensive armament was increased to ten 20 mm cannon in five turrets, harking back towards the B-36 that it was designed to replace.
Only the first YB-60, 49-2676, flew, with a total of 24 flights in 81 flights hours being completed; 20 flights in 66 hours by the contractor and 4 flights in 15 hours by the USAF, with flight, testing officially terminating in January 1953, several months after the official cancellation Despite the program cancellation almost two years previously, the USAF did not officially take the two YB-60’s on charge until 24 June 1954; both aircraft being allocated for immediate disposal and scrapped, this apparently commencing that same month.
There had been an attempted reprieve from scrapping as Convair tried, in vain, to interest the USAF in operating at least one of the two aircraft as a flying test bed for emerging technologies such as turbo-prop engine developments. However, funding was in even shorter supply than interest for such a proposal and the USAF association with the giant from Fort Worth was effectively over, by which time program costs had reached some $14.3 million. In contrast to the cancelled YB-60 program, the B-52 production run spanned over 700 aircraft in a number of variants culminating in the B-52H which continues in service as a strategic bomber/cruise missile carrier in the second decade of the 21st century.