Convair GEBO II Arrow wing configuration

SCALE 1/144 of this aircraft would still expend a large amount of costly hardware… the pod and everything in it as well as two of the engines originally attached to the manned aircraft. But as this aircraft was intended to carry nuclear weapons, the probability was that it would only need to fly a single operational mission. After that mission was done, the crew could rest easy and watch the colourful and only somewhat horrifyingly radioactive sunsets.

The bomb/fuel pod was substantially larger than the fuselage of the aircraft, and contained not only the turbojet, bomb (weighing 6,000lb) and fuel, but also radar. As originally envisioned it would simply drop ballistically as a free-fall bomb after release, using the onboard bomb aiming equipment to improve accuracy. But it was soon decided that once separated from the aircraft it would continue on under power for a short distance as a standoff weapon, continuing to use the turbojet in the tail. For control it had a trio of tail fins; the ventral one could fold to the side to provide ground clearance for takeoff. Presumably at least part of the explanation for the drop from 10,000lb to 6,000lb for the ‘special’ warhead was, since the bomb was not to be dropped but to remain within the pod, fins, outer casing, radar fusing and the like would have been removed.

This design was considerably sleeker than the earlier concept, being accurately described as “pointy”. The flat underside of the aircraft mated flush with the flat topside of the pod, creating a fairly low drag body. The vertical stabilizer projecting above the rear fuselage of the aircraft was substantially smaller than the prior design, due to the bigger moment arm of the longer fuselage. The manned aircraft’s internal turbojet was fed from two inlets located to each side of the upper fuselage; pressure recovery for these inlets may have been a concern, especially at high angles of attack such as during landing.

For the carrier aircraft, the GEBO II design shown here would use not the B-36 but the B-36G, later redesignated the B-60. The B-60 added swept wings to the B-36 and removed the piston engines, replacing them with four individually podded turboprops (see Boeing B-47 Stratojet & B-52 Stratofortress: Origins and Evolution for more on the B-60). The GEBO II bomber would take off equipped with eight engines but return with only one, having separated from four of them and thrown away three more. Where GEBO reportedly analyzed 10,000 configurations, GEBO II upped that number to 100,000.

Following the GEBO II studies, Convair refined its work. The updated Convair design, first shown to the USAF in late January of 1951, was generally similar to the earlier studies: a relatively small aircraft with highly swept delta wings, an underslung bomb/ fuel pod larger than the aircraft’s fuselage, carried to altitude and range underneath a swept-wing turboprop-powered B-60. The Air Force assigned this new configuration the MX-1626 programme designator in mid-February 1951. The MX-1626 was intended from the outset to be configurable as either a bomber or a recon platform.

It would not have been simply dropped from the B-60’s bomb bay; instead a trapeze system would deploy the supersonic bomber well below the carrier aircraft prior to release. This would have placed the bomber outside of the turbulent airflow that was closely associated with the underside of the B-60, a lesson learned during the development of the McDonnell XF85 Goblin parasite fighter. Unlike the Goblin, inflight recovery of the MX-1626 does not seem to have been contemplated, nor does it seem likely to have been a feasible – not to mention sensible – proposition.

The pod contained one turbojet, the fuel for the outbound leg and either the nuclear weapon or reconnaissance equipment, along with navigation radar in the nose. The pod also had three tail fins for stabilization; the lower fin could fold to provide ground clearance while stowed within the carrier aircraft’s modified bomb bay. For reconnaissance variants, the data recorded during the flight would be stored within the manned aircraft on tape and/or film; the recon equipment itself, such as cameras, would be expended. The nose of the pod had an empty space that would be used as a passageway for the nosegear… the gear retracting into the nose of the aircraft, with a separate set of doors on the lower surface of the pod. The composite aircraft would have little ground clearance, but the manned aircraft alone would have a tall stance.

The manned aircraft had two turbojet engines mounted roughly at mid-span in long nacelles, the nacelles buried within the wing rather than located above or below. This would have complicated maintenance and increased the weight of the spars that would have to encircle the engines, but the overall drag would have been somewhat reduced compared to a design with engines suspended from pylons. Mounting the engines that far out put them outside of the carrier aircraft’s bomb bay, allowing them to operate cleanly while the MX-1626 was still retracted into the carrier.

Perhaps oddly, the pod was meant to fall ballistically after separation from the aircraft; it does not seem that it was intended to use its own turbojet to increase delivery range. As with the previous supersonic bomber configurations, the upper surface of the pod was flat, mating to a flat undersurface of the aircraft fuselage (which had a distinctly triangular cross-section). This would reduce drag while the pod was attached to the aircraft: while the frontal area was greater than if the pod and the aircraft fuselage were discrete, separate structures, the turbulent interference between two closely coupled cylinders would have created far more drag.

The wings themselves were 65° delta wings with a simple symmetrical airfoil, similar to the first supersonic bomber configuration. The aircraft did not have the area ruling that would give the F-106 its distinctive ‘wasp waist’, as the area ruling concept for supersonic drag reduction was being tested at that time, but was not yet well characterised. The overall configuration of the MX-1626 was quite reminiscent of the later Lockheed A-12, though substantially less refined and elegant. It was an interesting case of convergent evolution in action. Considerable effort and expense went into testing the MX-1626 configuration, using not only Air Force wind tunnels but NACA and Navy tunnels as well. Further, metal models of the configuration were built and launched skywards by solid-propellant sounding rockets to test stability at supersonic speeds.

The MX-1626 was capable of launching itself from a runway without the use of a carrier aircraft. This would come with a substantial performance penalty, greatly reducing range. The composite aircraft would be fitted with jettisonable tricycle landing gear which would fall away immediately after liftoff, saving considerable weight. The landing gear was of distressingly low stance, giving almost no ground clearance; it would have been vital that the runways be kept in good working order as the smallest bump could have led to catastrophe. Whether self-launched or dropped from a carrier plane, the MX-1626 was incapable of landing with the belly pod still attached. Once airborne, the aircraft would need to expend the pod, destroying the expensive equipment on board.

And for all the effort, the performance of the MX1626 was surprisingly modest, with a maximum speed of only Mach 1.5. That may have been respectable for 1951, but within relatively short order it would seem positively lacklustre. This was something of the end of an era in conceptual aircraft design. Up until this point ‘staged’ aircraft such as this, carried to distance by a larger, slower aircraft, were seen as the only way to achieve certain missions. But in the early 1950s airto-air refuelling technology and techniques improved by leaps and bounds, and soon it was clear that it made far more sense to achieve great range via inflight tanking. But staged aircraft were not quite done.

In December 1951 the design was refined in important ways. It was no longer to be a parasite aircraft; advances in in-flight refuelling meant that the B-36G/B-60 would change from a carrier aircraft to an aerial tanker. Additionally, the turbojet in the pod was deleted as unnecessary, considerably simplifying the design while lowering weight and cost of the expended components. The aircraft now had only two turbojet engines, none expendable. A third crew member was added for the first time to operate newly added defensive systems.