In June 1996, Major Bryan J. Benson- a C-141 transport pilot, wrote an article for the School of Advanced Airpower Studies at Maxwell Air Force Base on the feasibility of modifying current transport aircraft so that they may also fulfill the dual role of transport and weapon truck. The article was archived at the Federation of American Scientists website.
Its certainly not a new idea to modify (sometimes heavily) a large frame aircraft for the purpose of delivering ordnance. Today we have the AC-130 series gunships, P3 and P-8A anti-submarine/ maritime patrol aircraft that can carry torpedoes and even anti-ship missiles. In the past, the Douglas DC-3 was adapted in Vietnam for fire support. And as far back as the Second World War, Germany designed passenger and transport aircraft with the idea of bomber conversion in mind as a means to get around the military buildup restrictions following World War One.
In 1974, the USAF even test launched a Minuteman 1 ICBM from a C-5 Galaxy in an Air Mobile Feasibility Demonstration. Secretary of Defense Harold Brown considered developing a dedicated cruise missile carrier in the late 1970s after the cancellation of the B-1A, but ultimately decided to back modification of the B-52 airframe for the role.
When this article was written, cost was very much an issue in the post- Goldwater Nichols military reconfiguration and Clinton Administration cutbacks. Heavy bombers are specialized and expensive to operate. Having a common airframe that could accomplish most of two different roles was (and I suspect still is) attractive.
While cost-effectiveness is still highly valued for obvious reasons, there are more reasons apparent today to consider the Bomber Transport as a concept:
Political Acceptance. Strategic bombers are highly visible politically. Having an innocuous-looking transport aircraft in a host country may be much more politically acceptable for that country's leaders, while still keeping a potent and dual purpose aircraft in theater.
Subterfuge: Russia is developing the rail-mobile RT-23 Molodets ICBM for the express purpose of being tactically much more difficult to find and target. Having legions of transport aircraft that are able to be converted within a short time to carry 30 to 70 ALCMs each would make for a considerable strategic riposte. An enemy could be similarly surprised by transport aircraft they have already seen and dismissed via commercial transponder settings while looking for bombers running in EMCON mode.
Resource Streamlining: While some components of modern heavy bombers are common to all High Value Heavy Airframe Assets, most require their own dedicated supply chain for maintenance and repair. Additional base and tanker tasking and even available ramp space may also be factors. This would be especially true at the initial deployment stages at a new expeditionary base.
Availability: There are far fewer heavy bombers in the active inventory than there are allocated to airlift- 76 B-52 Stratofortresses, 63 B-1 Lancers, and 20 B-2 Spirits as of August 2013. With the number and volatility of current geopolitical hotspots as well as the increasing age of the B-52, There is a considerable risk to inter-theater procurement issues. The piecemeal deployment of single, two or three ship bomber sorties to various world flashpoints; while politically valuable, would prove of limited value in the initial stages of an armed conflict; the point at which enemy intelligence estimates are likely to be their most accurate.
By the time additional assets could be moved from, say, Dyess or Barksdale, the battlefield will have changed, and targets that could have been struck en masse in the initial stages will be forced to make due with the coordination of larger numbers of multi-role and strike aircraft like the F-16C or F-15E. While able to handle most of these capabilities, those airframes are typically tasked with other types of sorties.
Volume saturation: With the advent of formidable and numerous anti-missile assets, the potential damage value for any single cruise missile is diminished. For an example, the Russian Federation has opened availability to such weapons systems as the SA-12, SA-22, and SA-21 to various allies and adversaries to the US and NATO. In Benson's article, it is stated that the C-5 has the potential to carry between 48 and 60 ALCMs. With a squadron capable of launching over seven hundred cruise missiles in a single sortie (barring stockpile concerns), particularly well-defended targets once again become more viable through the overwhelming of an enemy's air defense capabilities.
There are four primary mission types assigned to heavy bombers like the B-52H, B-1A and B-2A: Suppression of Enemy Infrastructure, Halting of Invading Armies, Defeat (or Destruction) of Enemy Air Defenses, and Destruction of Critical Mobile Targets. In Benson's article, during the first phase of the campaign, it is suggested that Transport Bombers be considered for striking stationary targets at standoff distance, leaving mobile targets for the dedicated bombers which have the electronic countermeasures and training to get closer to the fight.
The technical challenges in converting a transport fleet into that which can carry and deploy munitions yet lose none of its proclivity as an air transport seem straight forward and not especially costly when measured against the development of a new dedicated airframe like the B-21. In Benson's article, a number of slipstream-penetrating techniques were considered for launching the weapons. All required the redesign of the loading ramp, pressure and petal door assemblies, though he was confident it could be done without the loss of the aircraft's air transport capabilities. Four airframes were considered for Fiscal Year 2005: The C-17 Globemaster, C-5B Galaxy, KC-10 Extender, and B-747-400 air cargo version of the Jumbo Jet.
A number of methods for deploying missiles into an aircraft's slipstream for launch were explored. The two most feasible would seem to employ the rear cargo egress doors. One would be nearly identicle to the current standard Container Delivery System used for cargo air drops and the Low Altitude Parachute Extraction System. Missiles would roll back off the ramp similar to the 1974 Minuteman ICBM test launch. The cargo bay would need time to depressurize, and the rate of launch would be limited to single weapons to allow for safe extraction of the weapons.
In my opinion the most reasonable launch method would be a self-contained pneumatic launch tube module similar to the Mk-41 Vertical Launch System used aboard US Destroyers, Cruisers and Aegis Ashore assets. So what else is needed to launch a missile?
Transports already produce the electrical power and air-cooling capabilities as well as a mil standard 1553B universal data bus for programming the weapons. What transports do NOT provide are mission planning and offensive avionics. That is, target acquisition, launch information and terminal guidance for programming the missiles. These electronics can be palletized and 'strapped on'; plugged into the aircraft. This technique is already done with other avionics as a stopgap measure for temporary aircraft upgrades for a fleet that cannot be afforded to entirely leave service for full hard-wired plant upgrades. Transport Bombers would likely use off-board sensors and fire control via data links. This is handy but compared to an on-board system is prone to communication outages and Electronic Warfare attack.
So if a Transport Bomber program were implemented, how would it be possible to detect when any external changes to the aircraft are minimal to non-existent? Currently, the USAF bomber force practices missile launch runs on specific targets. For example, the Russian arctic bases and naval shipyards in Murmansk during the 2016 POLAR ROAR flights, and B-52H sorties into the South China Sea from Andersen Air Force Base, Guam for simulated launches against Chinese targets. The flight tracks of these sorties are very different than a typical transport run, and would likely take place well-away from commercial flight lanes. Plotted, these tracks look like emergency diverts; the aircraft suddenly banking away from their previous course in an about-fact back to their base or onward to another. It may be possible to view these sudden course changes if the aircraft are squawking in commercial mode as a point of mission subterfuge. Transport bombers would also be presumably flying in combat formations, and not in the characteristically long 'wagon trains' indicative of airlift operations.
Actual Transport Bomber sorties would require a lot of munitions. Depending on the model, even more than purpose-built heavy bombers. These transports would come with pre-loaded ordnance into their launch modules from a CONUS base, or would require to be loaded in theater at a forward operating base. The former situation would most likely see an intercontinental sortie. Transports loaded in-theater might be noticeable by satellite with weapons being loaded into the aircraft sitting in the arming pit or near weapons storage magazines. While its common for transport aircraft to airlift weapons, seeing the long, cigar shapes of ALCMs, JASSMs or JSOWs would make for an obvious flag for possible transport bomber activities. Another clue may come in the form of a SECDEF air reserve component activation to increase air crew and aircraft utilization rates. Especially during an emerging crisis overseas in an area without significant nearby air assets, and flashpoints that are landlocked and far from significant naval air support.
TELINT and other forms of SIGINT could detect the presence of non-standard electronic emissions and signals like those used in the avionics bombers use for missile programming and guidance.
The transport bomber concept is by no means perfect; and is by nature a stopgap measure that will intercept a lot of criticism. But the necessity is the mother of invention. And at the moment, the rewards appear to far outweigh the risks. I believe it is possible that this doctrine is actively kept around as an option at the very least. But due to the more clandestine applications of the system, is probably very well compartmentalized in airlift commands and classified to a suitably high level.