The theory behind using the floating wingtips had originated with Dr. Richard Vogt, a German scientist who had come to America after World War II.
Vogt proposed increasing the range of an aircraft by attaching two "free floating" panels to the wingtips to carry extra fuel.
He believed this could be accomplished without undue structural weight penalties if the extensions were free to articulate and self-supported by their own aerodynamic lift.
In addition, the panels would effectively increase the aspect ratio of the overall wing (while they were attached), providing a significant reduction in wing drag.
Therefore, as the theory went, the extra fuel was being carried “for free”, by the more
efficient wing and the additional fuel increased the range of the aircraft.
Other, potential uses for this concept quickly became apparent.
The one that sparked the most interest was for a bomber to carry two escort fighters, one on each wingtip.
The Germans had apparently experimented with the idea during late 1944 and early 1945.

During 1949, initial U.S. experiments had used a Douglas C-47A and Culver PQ-14B.
These tests involved a very simple coupling device:
a single-joint attachment that permitted three degrees of freedom for the PQ-14.
A small ring was placed on a short boom attached to the right wingtip of the C-47.
Only local structural reinforcement was required since the PQ-14 would be supported by its own lift.
A rearward lance was mounted on the left wingtip of the PQ-14, and the PQ-14 would position itself slightly ahead of the C-47 and essentially 'back" the lance into the ring.
No locking mechanism was required since drag would keep the aircraft in place.
To uncouple, the PQ-14 would simply speed up.
The first attempt at coupling was made on 19 August 1949 over Wright Field.
Problems with wingtip vortex interference were encountered, forcing the engineers to reevaluate the concept.
The solution was to move the ring further away from the C-47’s wingtip, and on 7 October 1949, a successful coupling was made with Major Clarence E. 'Bud" Anderson at the control of the PQ-14B.

At the same time as the C-47-PQ-14 experiments took place, a full-scale program was initiated using a B-29 to “tow” two straight-wing F-84 fighters.
Republic Aviation Corporation was awarded a contract to design, build, and evaluate the combination under project TIP TOW.
Two F-84D-1-REs (serial numbers 48-641 and 48-661) were modified for the initial TIP TOW tests under the designation EF-84D.

The wingtips of the EF-84Ds were modified so that they could be attached to flexible mounts fitted to the wingtips of a specially modified EB-29A (44-620s3).
This idea proved to be high dangerous, although several successful linkups were made.
Tragically, midway through the planned test series, the entire three-plane array crashed as a unit on 24 April 1953, killing everybody on all three aircraft.
TIP TOW was immediately cancelled.
The cause was subsequently traced to one of the EF-84Ds going out of control during the link-up and flipping over onto the wing of the B-29.

A parallel project was undertaken using a pair of swept-wing RF-84F-5-REs (51-1848 and 51-1849) attached to wingtip hook up assemblies on the JRB-36F (49-2707 - the initial FICON testbed).
The B-36 was formally assigned to the Tom-Tom project on 8 May 1954.
Interestingly, the Tom-Tom moniker was derived from the first names of two men, Major General Tom Gerrity and Convair contract manager Tom Sullivan, which is why it is not written in all caps.
The B-36 system included previsions to launch and retrieve the fighters in flight, and to provide fuel, pressurization, and heating air to the parasites while coupled.
The wing structures of the B-36 and F-84s were substantially strengthened to tolerate the stress of coupled flight.
After the TIP TOW crash, tests continued for a few months with the RF-84F/RB-36F/RF-84F array.
Only a few hookup attempts were made, and wingtip vortices and turbulence made this operation considerably more dangerous than the average operational pilot could accomplish. The first hookup, using only the left-hand fighter, was made on 2 November 1955.
In what became the final Tom-Tom flight, on 26 September 1956, Beryl Erickson found the F-84 he was piloting oscillating violently up and down while attached to the B-36 wingtip.
Fortunately, part of the attachment mechanism broke and the F-84 fell away from the bomber before any serious damage was done.
Since experiments with in-flight refueling techniques seemed to offer greater promise for increased fighter ranges with far less risk to the lives of aircrews, the Tom-Tom experiments were cancelled.



References :

VALKYRIE – North American’s Mach 3 superbomber

Goleta Air & Space Museum

Nevertheless, the floating wingtip concept was strongly supported by the ARDC during 1956 as a likely method of achieving the desired range for WS-110A.
It should be noted that the WS-110A concepts took off with the wing panels attached - and there were no plans to reattach after the separation during flight - so many of the difficulties encountered during TIP TOW and Tom-Tom would not have been encountered.
In September 1956, an ARDC study group reported that floating wingtips appeared to be a "very promising means of extending the subsonic range of aircraft from 30% to 100%.

Not surprisingly the preliminary designs submitted by Boeing and North American in mid-1956 were quite different, bur at the same time very similar.
Both aircraft would weigh some 750,000 pounds and use "floating wing panels" (a term coined by Boeing but applied to both designs) to house additional fuel.
Each of the 190,000 pound (loaded) panels was the size of B-47 medium bomber and would carry fuel for the trip to the target.
When the fuel was exhausted, the panels would be jettisoned –
without them, the main aircraft would be capable of dash speeds in excess of Mach 2.

The Model 724-1 was the first Boeing design to use floating wingtips for range extension.
The basic airplane was very similar to the previous Model 713-1-169, although it was
209.5 feet long with a wingspan of 118.3 feet.
The difference was the addition of two large fuel tanks carried on their own wing section outboard of the basic aircraft.
Each of the fuel tanks was 8 feet in diameter and 62 feet long, and housed a landing gear to support it during taxi and takeoff.
The dedicated wing section spanned 55 feet, but was mounted obliquely on the fuel tank to continue the sweep of the main wing.
The bomber would take off with the floating wingtips attached, using the fuel contained within their tanks during the first part of the journey.
The extensions would be jettisoned before the bomber entered enemy airspace and were not recoverable.

Boeing documentation in March 1956 described the Model 724-13 as :
"a straight wing, canard type airplane weighing 300,000 pounds powered by four turbojet engines with afterburners carrying a bomb load of 10,000 pounds and a crew of four."

However, the overall design was very unusual.
Although described as "straight wing," in reality the design featured a trapezoidal plan form spanning 20.6 feet with a General Electric X275 engine mounted under each wingtip.
Two other engines were mounted on the sides of the fuselage under the trailing edge of the wing.
No conventional horizontal tail surfaces were fitted, the design relying totally on the canard for pitch control.
The fuselage was 156.6 feet long, with the wing mounted about two thirds of the wry back.
The basic airplane had an empty weight of 106,710 pounds, with a maximum takeoff weight of 225,000 pounds and a maximum flight weight of 300,OOO pounds.
The fuselage had a diameter of 12.5 feet, and a 10,000-pound weapon could be carried in the single bomb bay.
Four crewmen sat in an unusual arrangement;
the pilots were side by side, the bombardier-navigator behind the co-pilot and the "battle director" behind him.
Comparatively large areas inside the fuselage were dedicated to electronics, primarily ECM equipment, and the bomb-nav system.


The Model 724-1001 fuel tanks were fairly good-sized aircraft themselves.
The floating wingtips had a span of almost 81 feet, and the fuel tanks were 6.6 feet in diameter and nearly 76 feet long.
Each fuel tank had a horizontal and vertical stabilizer, and was fitted with a tri-cycle landing gear.
Each had an empty weight of 24,500 pounds and an all-up weight of 131,000 pounds.
Again, the tanks were nor recoverable, the landing gear being provided to facilitate takeoff
only.
Operationally the floating wingtips would be empty at takeoff and filled during the first aerial refueling.
The fuel in the wingtips would then be used first so that they could be jettisoned before
entering enemy airspace.

By July 1956, Boeing had progressed to the Model 724-15.
This 370,000-pound airplane returned to the basic concepts, with a conventional
empennage instead of a canard and a more conventional wing planform.
A crew of four and up to 10,000 pounds of bombs, along with another 10,000 pounds of ECM equipment, and expendable counter-measures (the defensive missiles again being deleted).
The Model 724-1,5 was 156.6 feet long and spanned 93.5 feet.
The airplane was powered by four 26,425-lbf General Electric X275A turbojet engines in individual pods under the outer portion of the wing.
The four-man crew sat behind a retractable ramp that was installed ahead of the windshield to provide better aerodynamics at high speeds.
The same offset crew arrangement used in the Model 724-13 was retained.
An air-to-air refueling receptacle was located just ahead of the retractable windshield ramp. The main landing gear retracted into the fuselage just behind the weapons bay.
The airplane had an empty weight of 135,500 pounds and a maximum takeoff weight of 350,000 Pounds.

Each of the Model 724-1003 floating wingtips was 75.8 feet long and had an oblique wing that spanned 90.8 feet.
Since the wing continued the same sweep as the main airplane, each wingtip panel was different, although the fuel tanks themselves were identical.
Each had an empty weight of 31,000 pounds and an all-up weight of 180,000 pounds.
The wingtip panels were not reusable, did not have an engine, and were meant to be dropped prior to the airplane entering enemy airspace.
They were equipped with landing gear to make ground handling easier and to support themselves during taxi and takeoff.

Boeing also proposed an alternate design.
The Model 724-16 had more powerful General Electric X279A engines, different floating wingtips, a higher flight weight (425,000 pounds), and a 4,000-pound increase in useful military load.
This airplane was 175.25 feet long and had a wingspan of 93.5 feet.
Instead of the retractable ramp of the Model 724-15, the -16 had an articulating nose:
the forward 30 feet could swing down approximately 15 degrees to provide pilot vision during low-speed operations - shades of the supersonic transport a decade later.
The articulating nose held the aerial refueling receptacle, all of the radar antennas, and some of the electronic equipment.
The crew arrangement was also different, with the two systems operators facing backward directly behind the two pilots, all in individual escape capsules.

In March 1955 the North American design had a gross takeoff weight of over 450,000 pounds, was 182 feet long, and spanned 88 feet.
The airplane would cruise at subsonic speeds to the target, and then accelerate to its Mach 2.75 dash speed for the final penetration.
The airplane burned a high-energy chemical fuel in four General Electric X2Z5 engines located in the aft fuselage, fed by an intake on either side of the fuselage ahead of the wing.
Twin vertical stabilizers and a large nose-mounted canard provided directional stability.
The crew sat in a flush cockpit and used a periscope for visibility.
Interestingly, documentation indicates that the design had cannon mounted in a tail turret for defense.
This approach was abandoned in favor of conventional JP-4 for the other designs.
The canard also changed shape, although it remained in the same unsatisfactory location.

By April 1955, the design had evolved considerably.
The same basic airplane was fitted with a set of floating wingtips to carry additional fuel, resulting in a gross weight of over 650,000 pounds.
The fuel was specified as JP-4 instead of the high-energy fuel used previously.
The basic airplane was essentially the same size, and each fuel tank was just over 61 feet long with a 40 foot wingspan.

The design continued to evolve and by July 1956, the floating wingtip idea had increased in gross weight to approximately 700,000 pounds.
At this point, the airplane was only 762 feet long, with a basic wingspan of 92 feet.
Two 1,500 gallon drop tanks could be carried under the wing at mid-span in addition to the floating wingtips.
By now, six General Electric X279A engines of 27,07o lbf each were required for power, but JP-4 fuel was still being specified.
The floating wingtip panels were each 92 feet long with a span of almost 49 feet, resulting in a complete airplane spanning 190 feet.

A large canard was located in front of the cockpit, severely hindering the forward visibility at high angles of attack (such as landing and take-off).
This is generally the design that is depicted when discussing the early North American concepts.
North American compared the design with both a B-36 and B-52, with some interesting results.
For instance, the B-36 had four bomb bays that totaled 69 feet in length and could accommodate 84,000 pounds of bombs;
the new design had a weapons bay somewhat less than 20 feet long that could accommodate only 10,000 pounds.
The B-52 fell somewhere in the middle with a 29-foot-long bomb bay that could accommodate
25,000 pounds.
The B-36 had a far greater wingspan than either of the others (230 feet versus 185 for the B-52 and 190 for the new design), but unlike the others the B-36 did not have outrigger landing gear and could operate off of much narrower runways. (The B-36 had a track of 46 feet, compared to 135 feet for the floating wingtip airplane and the outrigger-gear on the B-52.).