You’ll be familiar with the concept of an Airborne Aircraft Carrier if you’re an Avengers fan. The superhero franchise’s ‘Helicarrier’ is essentially a normal aircraft carrier with the addition of four large deployable, ducted fans, stealth technology, a number of superheroes and Samuel L Jackson at the helm.
Yet this fictional platform could have actually been a reality, as in the early-1970s, the US Air Force (USAF) contracted Boeing to investigate and develop a concept for an actual flying aircraft carrier – using a Lockheed C-5 Galaxy or a Boeing 747 as the mothership instead of an actual navy ship. The company conducted a study to determine the feasibility of operating an Airborne Aircraft Carrier (AAC), which could launch up to ten micro-fighters and recover them in flight!
A History of Airborne Aircraft Carrier Concepts
The US has explored this concept multiple times over the last century. In the early 1930s, the US Navy experimented with launching and recovering biplane fighters from the USS Akron (ZRS-4) and the USS Macon (ZRS-5) rigid airships. These fighters were stored internally and deployed and recovered using a trapeze mechanism. Both airships were destroyed during separate storms by 1935.
During the 1950s, the US again revisited the possibility of developing and employing an AAC. This time, the USAF explored the concept of repurposing a Convair B-36 Peacemaker for the role under the Fighter Conveyor (FICON) programme. This project sought to equip the long-range strategic bomber with a Republic F-84 Thunderflash in its bomb bay, using a similar trapeze mechanism to launch and recover the ‘parasite’ fighter. Testing of the FICON concept began in January 1952 and it saw limited air force service from 1955 until 1956, when the programme was cancelled.
It wasn’t long before the concept was again revisited. In September 1973, Boeing – having been contracted to study the feasibility of an AAC with air-launched and recoverable micro-fighters – published its Investigation of a Micro-Fighter/Airborne Aircraft Carrier Concept.
The report detailed “the results of an exploratory investigation to determine the size, performance and feasibility of a micro-fighter design such that a number of vehicles could be transported or air-launched and recovered by a C-5-class carrier aircraft.” It identified five potential micro-fighter designs and outlined possible initial operational capability (IOC) dates of 1980 and 1985, respectively.
Interestingly, the company concluded that the concept could work by using a heavily modified C-5A Galaxy or Boeing 747 as the mothership. “This investigation has studied the feasibility and usefulness of an airborne airbase and has found it to be technically feasible and potentially valuable to the nation as a rapid deployment multi-purpose strike system. It has the potential for intercontinental response, with large combat forces, before an aggressor can full [mobilise] for invasion of neighbouring countries,” the report said.
Boeing’s evaluation of the concept was informed by the results obtained by previous efforts to develop AACs with the emerging fighter and transport aircraft technology of the 1970s. The platform would have created new airborne rearming, multi-sortie and force multiplying capabilities, along with the ability for a new form of in-flight refuelling.
The C-5A Galaxy and Boeing 747 Motherships
Lockheed’s C-5A Galaxy heavy-lift strategic transport and the Boeing 747 long-range wide-body commercial airliner were identified as the ideal carrier aircraft as they were both relatively new aircraft. Neither type was under the threat of obsolescence and they both had projected service lives beyond 2000, which they both achieved comfortably. Both aircraft are still operational today, with relatively new variants of each type in service.
The C-5A and 747 were compared against each other during the study, with attention being paid to their performance, airframe growth, possible use for alternate applications and how easily they would be to modify. It determined that the C-5A’s structural arrangement would be the easiest to modify, but at the expense of much of its cargo capabilities.
In the report, Boeing outlined that the AAC’s design criteria should have dual launch and recovery bays, two in-flight refuelling booms and the ability to carry ten micro-fighters. Alongside this, the platform was to also have the ability to operate as transports, troop carriers and tankers. The platform also had to be able to operate at high speeds to provide clear launch and recovery operations.
“The operational employment of strike fighters operating from airborne aircraft carriers is indicated by this study to be technically feasible. Furthermore, the system concept offers the potential of great national benefit in a political world that leans toward a low profile American exposure overseas while being responsive to [the] diverse needs of our allies,” it said.
During operations, the fighters would be recovered by making initial contact with a refuelling boom. The “retracting action of the telescoping boom then pulls the fighter into the trapeze index and lock fittings. Verification of lock-on brings umbilical power to the fighter while the [in-flight refuelling] boom is completely retracted and moved aside to its park position. Engine shutdown follows and initiates the hoisting cycle by the trapeze.
“Operation of ten fighters in combat situations from a high altitude base requires [pressurised] crew compartments and hangar decks. The launch and recovery bays become air locks to transfer the fighters between environmental extremes,” the report added.
In terms of micro-fighter handling on-board the AAC, the aircraft could be serviced and rearmed. Boeing estimated that a complete turnaround, including the rearming of one micro-fighter, would take ten minutes. It also outlined that 44 personnel would be employed on each mothership, comprising 12 AAC crew members, a micro-fighter squadron of 14 personnel, and 18 supporting specialists.
The AAC concepts were designed to carry payloads of approximately 200,000lbs (90,718kg) and support multiple sorties carried out by their micro-fighter squadrons. The fighters would have provided the capability to conduct combat air patrols (CAPs), interception and low-level strike missions from the AACs. The motherships were projected to remain on station for up to eight hours, supporting multiple sorties.
Boeing found that the C-5A Galaxy would be easier to modify, but that the 747F AAC had better performance. The latter boasted increased operational ranges, speeds, cruising altitude and endurance than the former, but required more weight for the desired AAC configuration – as noted in the figure below.
“Both designs can be made to carry [ten] fighters with space for on-board rearming, but the C-5A loses some of its capability to carry outsize cargo. In-flight [refuelling] at both launch and recovery stations, a requirement for rapid recovery, would require extensive modification to the C-5A. These considerations led to [the] selection of the 747F as the baseline for further studies,” the report stated.
In total, five micro-fighter designs were created for the study, as illustrated below. As with the mothership designs, the micro-fighters were compared with each other and their operational requirements. Each concept was tested in wind tunnels by the Air Force Flight Dynamics Laboratory (AFFDL) – now known as the Air Force Research Laboratory (AFRL).
The five designs followed the same criteria. Each platform had to be developed using 1975 technology and have a wingspan of 17.5ft (5.3m) due to the 747F’s launch bay restraints. The micro-fighters must also feature a high-g cockpit with IIPACS displays and controllers, a shock absorbing skid system and drag chute for use as an emergency landing gear, and flight controls that offered zero static margin in pitch and neutral directional stability.
The aircraft’s basic armaments included two 20mm M-39 cannons, with a capacity of 400 rounds, along with two internally carried, tube-launched short-range missiles and/or two AIM-7F Sparrow medium-range radar homing air-to-air missiles on wing pylons. They would have also been able to carry electronic countermeasure and reconnaissance equipment in wing-mounted pods. Smart bombs would have been employed as the platform’s primary air-to-ground munition.
The micro-fighters were also required to have an internal fuel load of 2,500lbs and an in-flight refuelling receptacle. Each platform would have been powered by a single GE Aviation YJ101-GE-100 afterburning turbofan engine. Variable geometry (sweep wing) designs must also have been flyable at launch and recovery speeds in a folded configuration.
“Studies were conducted with five baseline fighters to establish their capability to meet the requirements identified in the operational analysis. Basic comparisons were made for intercept and strike performance. Subsonic cruise performance was sensitive to vehicle configuration. Cruise specific range for variable sweep is approximately 50% better than other designs but supersonic and low altitude performances are essentially equal.
“The clipped “Arrow” [design] benefits from endplate fins to increase span loading. The Delta provided simplicity and [was] light weight. The high aspect ratio wing selected for [the] ‘Canard’ and ‘VITAC’ concepts were [penalised] with high wing loading due to the span limitation.”
Boeing selected two of these five conceptual designs – the ‘Delta’ and the Variable Sweep Wing (‘VSW’). The former was chosen due to its simple design and the latter was selected because it possessed greater overall performance. One factor that the company scrutinised during the selection process was the micro-fighter’s ability to defend itself against a Mikoyan-Gurevich MiG-21 (NATO reporting name: Fishbed), as well as in conduction air-to-air and air-to-ground missions.
Programme’s Demise and the Future
Essentially, the programme stalled when the research entered a design and development stage. The AACs and the associated micro-fighters were never prototyped or produced. Though it was feasibly possible to operate, the USAF did not proceed with the programme. The US effectively shelved the idea of employing AACs in the mid-1970s and the idea of having a large aircraft carrying a number of micro-fighters fell into the pages of fiction.
However, in 2015 the US Department of Defense once again took interest in the concept. The Defense Advanced Research Projects Agency (DARPA) Gremlins programme sought to develop and demonstrate using a small, low-cost, recoverable unmanned aircraft system (UAS) that can be equipped with a variety of payloads and undertake multiple missions. The platform is semi-autonomous and can be air-launched and recovered from a mothership, with a single controller operating up to eight aircraft at once.
Dynetics was awarded a contract to develop the X-61A Gremlin – an experimental prototype of the platform – in April 2018. The X-61A completed its first flight above the US Army's Dugway Proving Ground near Salt Lake City, Utah, in November 2019. The test included a captive-carry mission aboard a Lockheed C-130A Hercules and the prototype was air-launched successfully and flew for an hour and a half.
The Gremlins will integrate with existing reconnaissance, strike and transport aircraft, providing new unmanned swarming, force multiplying capabilities, along with the ability to conduct limited suppression/destruction of enemy air defence (SEAD/DEAD) and intelligence, surveillance and reconnaissance (ISR) missions.
The UAS weighs 1,500lb (680kg), has a mission radius of 29 miles (46km) and a four-hour loiter time. It can reach speeds of up to 457mph (735km/h) and after its recovery, it can be turned around in 24 hours. A single mothership will be able to carry eight Gremlins.
Although it won’t be like the ‘Helicarrier’ from the Avengers and it is not to the same scale as Boeing’s concept in the early-1970s, the Gremlins programme seeks to finally deliver the AAC capability that the US military has been exploring for nearly a century – and one that could see extensive employment in the future.