v1.0.0 / chapter 1 of 2 / 01 oct 08 / greg goebel / public domain
* The A-10 owed its birth to two influences: the inadequacies of close airsupport aircraft used in Vietnam, and the need to counter Soviet armoredmight in Europe. The development program that emerged from these roots was amodel of clean organization and management, and it produced an unusual andvery neatly thought-out product. This chapter describes the origins anddetails of the A-10.
* During the Vietnam War, the US Army and the US Air Force were often atloggerheads over the issue of close air support. The Air Force regarded CASas their mandate and were opposed to any Army attempts to take over that rolefor themselves, while the Army felt the Air Force was not responsive to theirneeds.
The Army was not allowed to operate armed fixed-wing aircraft -- though therewas a little cheating on this point -- but the service did acquire armedhelicopter gunships. The Air Force did obtain quantities of the old butexcellent piston-powered Douglas A-1 Skyraider or "Spad" strike aircraftoriginally developed for the Navy, which earned the appreciation of theground-pounders by its ability to carry a big warload, dish out and takepunishment, and remain on station for an extended period of time.
USAF brass had mixed feelings about the Spad, regarding piston combataircraft as a thing of the past, but the "fast-mover" jets preferred by theservice were nowhere near as adequate in the CAS role -- they came in quick,dropped their warloads, and left, since they generally lacked both theendurance and the resistance to damage to let them hang around on thebattlefield. They were also much more expensive in every respect than theA-1. The bias against the Spad was not completely unreasonable, however,since the enemy continually improved their anti-aircraft capabilitiesthroughout the war, and the Skyraider suffered accordingly. Something moresophisticated really was needed.
Late in the war the USAF shifted the CAS mission to the jet-powered VoughtA-7 Corsair II or "SLUF (Short Little Ugly 'Fella')". The A-7 had beendeveloped against a US Navy requirement for a carrier-based strike fighter toreplace the Douglas A-4 Skyhawk. The SLUF was an excellent aircraft, but itwas designed for the strike-interdiction role, not for the battlefield CASmission. It was not really the equal of the Spad for actually getting rightdown in the mud with the troops and turning the tables against the blackhats.
Well before that, in mid-1966, the USAF began to put together an "AttackExperimental (AX)" program to develop a CAS aircraft that could do the jobfar better than the Skyraider. An initial request for proposals (RFP) wasissued in March 1967. The AX, as defined in the RFP, was to match theSkyraider in warload and endurance, but was to be substantially faster whilebeing extremely maneuverable, with a tight turning radius to keep in ontarget, and was to have short takeoff and rough field capability. The AX wasto be highly survivable through the use of armor and redundant systems,including twin engines, while being cheap to buy plus easy to operate andmaintain from forward bases. One of the more interesting features of the AXwas that it was to be armed with a fast-firing cannon, most likely a versionof the General Electric (GE) M61 "Vulcan" 20-millimeter six-barreledGatling-type gun.
* While the industry worked on proposals, the AX definition shifted slightly.The North Vietnamese began to introduce armor in combat in 1967 and 1968, andduring the Six-Day War of 1967 the Israelis had shown that their French-builtMirage IIIC fighters could kill Arab heavy tanks using the aircraft's twin 30millimeter DEFA automatic cannon, punching through the thin armor on the top.
Even during the Vietnam War, the US Army regarded the service's primarymission as dealing with a Soviet armored offensive into Western Europe. NATOwas outnumbered by the numbers of troops and tanks that the Warsaw Pact couldfield, and into the early 1960s the US Army favored the nuclear option as thebest equalizer, fielding thousands of tactical nuclear weapons. Over time,the uneasiness over heavy reliance on the nuclear option as a defenseincreased. Going nuclear even with very small tactical nuclear weapons wasan obvious first step towards a full-blown nuclear exchange that coulddestroy civilization; even supposing that could be avoided, major use oftactical nuclear weapons would still wreck the countries the US was supposedto be protecting; and in general relying strictly on the nuclear option gavevery little flexibility for dealing with a crisis.
The Army was putting a lot of effort into acquiring weapons that reallycouldn't be used without direct authorization from the Commander-in-Chief --the President of the United States -- and no competent field commanders feelcomfortable relying on tools that there is no assurance they can actuallyuse, or for that matter train to use in any realistic way. In effect,tactical nuclear weapons were worthless except as a deterrent. Theinflexibility of the nuclear option meant the US needed to look to morepowerful conventional weapons as equalizers to Soviet armor. The Air Forcebegan to see the AX less as a counterinsurgency weapon than as an antiarmorweapon, and accordingly adjusted the requirement to feature a more powerfulcannon. This was not a minor change, since the cannon was almost certainlygoing to be very big and the aircraft would have to be effectively designedaround it.
* In the meantime, the various manufacturers were wrestling with the RFP,particularly the speed and endurance requirements. The only solutionprovided by traditional engine technology was turboprop engines, sinceturbojets simply didn't have the fuel economy, but turboprops requiredheavy-duty reduction gearboxes and variable-pitch propellers that made themmore complicated and heavier than turbojets. The use of propellers alsoimposed restrictions on engine placement. Although turboprops worked fine oncargolifters, the more the designers looked at turboprops for the AX, themore troublesome they seemed.
Douglas had actually tried to develop a turboprop follow-on to the Skyraidernamed the "A2D-1 Skyshark" in the 1950s, and it had been one of the worstaircraft ever built, plagued with continuous development problems. It neverentered production. To be sure, the turboprops available in the early 1950swere notoriously troublesome in themselves, but even the much more matureturboprops available in the late 1960s didn't seem to fit the AX very well.
Fortunately, there was a new option, in the form of the "high-bypassturbofan". The first practical turbofan engines, which were basicallyturbojets with a fan attached to increase airflow, were introduced in thelate 1950s and provided substantially improved fuel economy relative toturbojets. The problem with these early turbofan engines from the point ofview of the AX was that they were only efficient at fairly high speeds, andthey would not be efficient at the relatively low speeds of a close-supportaircraft, orbiting a battlefield and pouncing on targets.
However, these early turbofans had low "bypass ratios", or ratio of exhaustairflow provided by the fan to the airflow provided by the turbojet engine"core". The early Pratt & Whitney TF33 turbofan used on the B-52H, forexample, had a bypass ratio of 1:1. Engine designers realized they coulddesign a more fuel-efficient turbofan by raising the bypass ratio to 6:1 ormore. Such high-bypass engines weren't suitable for fast combat aircraftsince such engines produced a low-velocity exhaust, but they were fine forcargolifters and airliners.
The high-bypass turbofan provided good fuel economy at low speeds, and so itbegan to seem like a very attractive option for the AX. There was thedifficulty that most of the early high-bypass turbofans were big enginesdesigned for big aircraft, but there was no inherent obstacle in scaling downthe technology for smaller aircraft. The USAF helped accommodate the use ofhigh-bypass turbofans by tweaking the AX requirements toward a higher speedthat was more in line with the optimum for such engines and by not demandingtrue "short takeoff or landing (STOL)" performance, which would have imposedadditional overhead such as thrust reversers.
* The Air Force issued a final RFP for the AX in May 1970. The aircraft wasto be highly survivable and very maneuverable, allowing it to get "up closeand personal" with adversary forces even in bad weather and low cloudceilings. It was to have a speed of 650 to 750 KPH (350 to 400 KT) and beable to operate with maximum warload from rough forward airstrips with alength of 1,200 meters (4,000 feet). It was to be able to carry a warload of4,300 kilograms (9,500 pounds) to a combat radius of 460 kilometers (250 NMI)with a loiter endurance over the battlefield of two hours. Warload would ofcourse include the heavy cannon envisioned by the Air Force, but the aircraftwas to have no less than ten pylons for external stores.
The AX was to be simple, not merely to make it survivable and easy tomaintain on the front lines, but to make it cheap to buy and operate. TheAir Force had run into major problems with the development of the GeneralDynamics F-111 strike aircraft and the Lockheed C-5 Galaxy cargolifter duringthe 1960s, and the service was sensitive about getting into another expensiveboondoggle. In addition, the USAF was trying to get their "FighterExperimental (FX)" program rolling. The FX program was focused ondevelopment of a new air-superiority fighter and would eventually produce theMcDonnell Douglas F-15 Eagle.
The FX was top priority, and of course the CAS mission took second place.The USAF wanted to keep AX costs down, which was one of the reasons therequirements for STOL operation were relaxed, and in fact the serviceinformed the vendors that meeting the cost ceiling was more important thanmeeting operational specifications.
The Air Force also issued an RFP in 1970 for the AX's primary weapon, the"GAU-8", which was to be a 30 millimeter cannon with a rate of fire of 4,000rounds per minute. The Air Force intended to perform a competitiveevaluation of both the aircraft and the cannon, selecting two finalists fromthe field of competitors for each. This was a lesson learned by the AirForce from the F-111 program, where the service had basically committed tothe type even before flight of a prototype and ended up deeply regretting it.
The deadline for proposals against the AX RFP was 10 August 1970, and sixcompanies submitted their concepts, including Cessna, Fairchild,Boeing-Vertol, Lockheed, General Dynamics, and Northrop. Four companiessubmitted proposals for the GAU-8 cannon, including General Electric,Philco-Ford, Hughes, and General American Transportation (GAT).
The two finalists for the AX competition, Northrop and Fairchild, wereannounced late in 1970. Both were to build two prototypes each, the Northropdesign being designated the "YA-9A" and the Fairchild design being designatedthe "YA-10A". Northrop was awarded a $28.9 million USD contract for theirtwo YA-9As, while Fairchild was awarded a $41.2 million USD contract fortheir two YA-10As. Fairchild got more money because the company sold the AirForce on the idea of building the YA-10A closer to production specificationthan the YA-9A.
The YA-10A was actually to be built by Fairchild's division on Long Island inNew York state. The division had originally been Republic Aircraft, buildersof the famous P-47 Thunderbolt of World War II, the F-84 Thunderjet of theKorean War, and the F-105 Thunderchief of the Vietnam War; it had been boughtout by Fairchild in 1965.
The two finalists for the GAU-8 competition, GE and Philco-Ford, wereannounced later. They were each awarded a $12.1 million USD contract forprototype development. Since AX prototype development couldn't wait on GAU-8development, the AX prototypes would be fitted with the GE M61 Vulcan.
* While the two sets of AX prototypes were being built, the US Congressreviewed the program. The Army was also developing the Lockheed AH-56Cheyenne heavy helicopter gunship -- a program that would be eventuallycancelled, though the modern Hughes AH-64 Apache would arise from its ashes-- and the US Marines had acquired the British Harrier jump-jet as theMcDonnell Douglas AV-8A for the close-support mission. Why, Congress askedwith some good reason, did the military need athird close-supportplatform?
Much to their credit, despite their mixed feelings about the CAS mission, AirForce officials fought effectively for the AX, pointing out that the Cheyenneand the Harrier had their uses, but that neither machine was the close-up,long-endurance mudfighter that the ground-pounders demanded when they wereunder pressure from the black hats. Congress bought the argument, butstipulated that once a winner was selected from the AX competition, it wouldbe put through an extended and rigorous evaluation of its combat value beforebeing approved for production.
The first Northrop YA-9A prototype performed its initial flight on 30 May1972, with the second prototype following on 23 August 1972. The YA-9A wasof fairly straightforward configuration, something along the lines of anoversized single-seat version of a jet trainer. It had a high-mountedstraight wing with an Avco Lycoming ALF 502 turbofan in each wing root; aconventional tail assembly; and tricycle landing gear, with the main gearretracting into the engine nacelles. The Soviets would adopt a broadlysimilar configuration in their answer to the AX, the Sukhoi Su-25 "Frogfoot".
The first Fairchild YA-10A performed its initial flight on 10 May 1972, 20days before the first YA-9A, with Howard "Sam" Nelson at the controls. Thesecond prototype performed its initial flight on 21 July 1972. The YA-10Awas unusual, maybe even a bit bizarre, in appearance. It had a low-mountedstraight wing; twin GE TF34 turbofans mounted in pods above the rearfuselage; a twin-fin tail; and tricycle landing gear, with the main gearsemi-retracting into fairings in the wing, a scheme that seemed to have beenretrieved out of time from the 1930s. There is an old saying in aircraftdesign that "if it looks right it flies right", and the YA-10A seemed to haveone strike against it from the outset.
Both manufacturers worked out bugs in the prototypes until they were handedover to the USAF "Joint Test Force (JTF)" on 24 October. The JTF consistedof teams from a range of Air Force commands to judge different aspects of theaircraft. The evaluation involved both practical tests and "paper" studiesof the aircraft, and it was very thorough. Both the YA-9A and the YA-10A gothigh grades, and in fact on the average they exceeded requiredspecifications. There was political pressure to select the YA-10A since theNew York aviation industry was ailing at the time and the AX contract wasseen as a possible "make or break" deal for Fairchild, and by implication itsLong Island Republic division.
Fairchild had shot itself in the foot badly a few years earlier. In themid-1960s the Army had conducted a competition for a battlefield observationhelicopter; Hiller's offering had stood a good chance of winning, but HowardHughes sidelined Hiller through a set of behind-the-scenes intriguesinvolving irregular and possibly corrupt practices, with the Hughes 500winning the deal. The end result proved to be such a mess that the Army wasforced to start over and run the competition again, but Fairchild brasswanted nothing to do with it. The company had clearly been treated badly,but they lost bigtime when Bell won the competition, almost by default, withtheir Model 407 / Kiowa light helicopter. The AX seemed like it might beFairchild's last chance.
However, selecting a winner for the competition strictly on the basis ofpolitical pressure was crooked as well, and also not in the military's bestinterests over the long term. They needed the best weapon available for thetroops in the field. Selecting the "best" in this case was troublesome,since the YA-9A was more maneuverable, while the YA-10A was easier tomaintain and judged more survivable.
The deciding factor turned out to be Fairchild's wisdom in building aprototype closer to production specification: their machine could be putinto production and service well ahead of the Northrop contender. Theevaluation also showed that the YA-10A's wing was much easier to rearm thanthe YA-9A's high-mounted wing, which was a significant factor in missionturnaround time. The YA-10A was selected as the winner of the competition on18 January 1973. This led to award of a $159 million USD contract for theproduction of ten "A-10A" pre-production "development, test, and evaluation(DT&E)" machines. Since there would be only one production variant of theA-10 -- though other variants would be flown -- the A-10A is simply referredto as the A-10 in this document.
An option was included for an initial batch of 48 production machines, but asper the instructions of Congress, the go-ahead for production would have towait until full evaluation of the type was completed. The GE TF34 turbofanwas selected for the A-10. The competitive evaluation had considered themerits of a YA-9A with GE engines and a YA-10A with Avco Lycoming engines.The Avco Lycoming ALF 502 was substantially less powerful than the TF34, butit was much lighter and the company was developing an uprated version thatwould be competitive with the TF34. However, the TF34 was a solution inhand, having been originally developed against a US Navy requirement for acarrier-based antisubmarine warfare aircraft, which emerged as the LockheedS-3 Viking. Only minor modifications were required to fit it to the A-10;the Air Force was in a hurry and didn't want to take risks, so the servicechose the TF34.
* In the meantime, the "shootout" between the GE and Philco-Ford candidatesfor the GAU-8 cannon was underway, with ground firings beginning on 15January 1973, three days before Fairchild won the AX competition. GE hadbeen working on heavy cannon since 1968, and coupled with their massiveexperience with the Vulcan they had a substantial edge over Philco-Ford. GEwon the cannon competition in June 1973, receiving a $23.7 million USDcontract for 11 "GAU-8/A" preproduction cannon, including three for groundtesting and eight for installation into preproduction YA-10As.
As per the instructions of Congress, the two YA-10As continued to performtrials through the rest of 1973 and into 1974, if at a less stressful pacethan had taken place during the original competitive flyoff. Congress wasstill waffling on acquisition of the A-10, and the preproduction build wascut to six aircraft in mid-1973. Although some legislators wanted to axe theA-10 and concentrate on acquisition of the A-7D, the Air Force pushed back,and in September 1973 Congress gave provisional approval for the continuationof the A-10 program, as long as second fly-off competition was conductedbetween a YA-10A and an A-7D.
The contest duly took place at Fort Riley, Kansas, in April and May 1974, andproved what the Air Force basically already knew: the SLUF might be muchbetter for strikes into the enemy's rear, but the YA-10A was much better forgetting down in the dirt with the grunts. In one particularly devastatingtest, the YA-10A flew to a remote field location and loitered on station fortwo hours; the A-7D was only able to hang around for seven minutes. In July1974, the green light was given for construction of 52 more A-10s, includingthe four preproduction aircraft that had been dropped the a year before andthe 48 options specified in the original contract.
* The first preproduction aircraft was rolled out in late 1974, put throughpreliminary ground tests, then broken down and flown from Long Island toEdwards Air Force Base (AFB) on a Lockheed C-5A cargolifter. It wasreassembled to perform its first flight on 15 February 1975. It was followedby the first flight of the second preproduction machine on 26 April and thethird on 10 June.
The second and third preproduction machines were the first to be fitted withthe GAU-8/A cannon. The preproduction machines proved slightly overweight,but trial attacks with the cannon on old US M-48 tanks and Soviet T-62 tanks,obtained from Israel, were to put it simply, awesome. The GAU-8/A cannon allbut tore the targets to shreds. The Air Force was not inclined to worryabout the fact that the A-10 was slightly overweight.
After completion of the ten preproduction machines, manufacture offull-production machines was shifted to a Fairchild plant in Hagerstown,Maryland. The first production A-10 performed its initial flight in October1975 and was handed over to the USAF on 5 November. While early productionaircraft were still used for test, trials, and evaluation, aircraft werebeing delivered to line units by March 1976.
* Although the A-10 was given the name "Thunderbolt II" in honor of itsRepublic ancestor of World War II, the name was unimaginative and never stuckwith the flight crews, the military-aerospace media, or the public. It hadacquired a much less dignified but more appropriate name even before theflight of the first preproduction machine.
All Republic fighters before the A-10 had been "earth lovers", reluctant toget off the runway, and so, as the story goes, the original straight-wingedF-84 Thunderjet had acquired the nickname "Groundhog" or just "Hog". Whenthe much improved swept-wing F-84F was introduced, it became known as the"Super-Hog", and the F-105 Thunderchief was sometimes called the "Ultra-Hog",though it was more generally referred to by the nickname of "Thud".
In the summer of 1973, USAF Major Michael Major (apparently no relation tothe "Major Major" character of the novel CATCH-22) proposed in an article inan Air Force publication that the A-10 be referred to as the "Warthog". Itfit so perfectly, capturing the aircraft's appearance and personality, thatit caught on quickly and permanently without any real competition. Inpractice it is usually shortened to "Hog".
* With a name like "Warthog", nobody would ever think that the A-10 could bedescribed as "pretty" aircraft, and it isn't. It looks strictly forbusiness. As mentioned, it has a low, straight wing, a twin-fin tail,tricycle landing gear, and twin TF34 turbofans perched above the rearfuselage. It has a boxy fuselage with the pilot perched high in a bubblecanopy, and is built mostly of conventional aviation aluminum alloys, withthe notable exception of some major titanium-armor assemblies. Fuselageelements are made up of flat panels or simple cylindrical or conical piecesto simplify manufacturing and maintenance. The muzzle of the GAU-8/A cannonsticks slightly out the nose, and the wing bristles with stores pylons for awide range of external stores.
The wing has three spars, making it very robust, and has a large area to givelow wing loading, resulting in good low-speed handling and maneuverability,as well as heavy load capability with a relatively short takeoff run. Unlikeits Republic predecessors, the A-10 is by no means an earth-lover, though ofcourse the amount of runway it needs increases with warload. With a typicaloperational combat load, the A-10 can take off in about 1.1 kilometers(3,600 feet) and land in 350 meters (1,150 feet), less than half that neededby a fighter jet.
Its short-field performance is enhanced by four large "Fowler" type flapsthat extend well behind the wings. There is one flap on the inner section ofeach wing, one flap in the middle section, with an aileron for roll controlon the outer section. The ailerons are "split", with top and bottom halvesthat can move independently, to act as airbrakes. This scheme provideseffective braking without the aircraft to pitch up or down, a commondifficulty with fuselage-mounted airbrakes. The outer panels of the wingshave a dihedral of 7 degrees, while the wingtips are slightly turned down toimprove aileron response and reduce air vortexes that impair flightefficiency.
There is a fuel tank in the inner section of each wing, but Fairchildengineers were reluctant to put fuel tanks in the outer sections since theywould make too good a target for ground fire. The bulk of the fuel tanks arein the center fuselage between the wings, which provides them with someprotection and also makes fuel trim a minor issue. Total internal fuelcapacity is 7,598 liters (1,650 US gallons), ensuring that the A-10 didn'tneed to carry external tanks on a combat sortie. Fuel lines are routedthrough the tanks to shield them from ground fire. In operational practice,a Warthog pilot uses up the fuel in the wing tanks first.
The aircraft is topped off on the ground through a single-point refuelingconnection in the hinged nose of the left wing landing gear fairing, whichalso contains a self-test control panel. There is a boom refueling socket inthe nose, not featured in the YA-10As.
Incidentally, the A-10 is designed for short-field operation more than forrough-field operation. Its tricycle landing gear, all of which retractsforward, feature single wheels with low-pressure tires mounted on simplestruts without shock-absorbing mechanisms. More serious "farm tractor" typelanding gear would have increased cost and weight. The A-10 was intended tooperate from relatively short strips of highway, runways that had beendamaged by runway-dibber bombs, or leveled dirt or grass strips, but notgenerally from an unprepared flat patch of ground.
The use of the old-fashioned wing fairings for the main landing gear helpedreduce the complexity of the landing gear mechanism, which otherwise wouldhave had to rotate 90 degrees to lie flat in the wing. It also eliminated theneed to "cut out" the wing spars to accommodate the landing gear, which wouldhave weakened the wing. Since the wheels don't retract completely, this alsoprovided some degree of protection to the airframe during a wheels-uplanding. The additional drag of the fairings was judged irrelevant given theWarthog's low flight speeds.
The tailplane has three spars and is very strong. The twin fins on the tailnot only provide redundancy, allowing the aircraft to be controlled even if afin is shot away, it also provides continuous control response during violentmaneuvers, since even if one rudder is blanked out by "dead air", the otheris almost always still functional. Incidentally, the leading edge of thetailfins of production aircraft has a curved figure, while the leading edgeon the YA-10s was angled.
* As mentioned, the GE TF34 turbofan was developed for the US Navy S-3 Vikingsub-hunting aircraft. The initial "TF34-GE-2" engine went into productionfor the S-3A in 1972. This variant provided all the performance required forthe A-10, but it was too expensive, and so GE worked to develop a simpler,cheaper version for the Warthog, designated the "TF34-GE-100". It eliminatedsome of the features required for maritime operation and used some cheaperassemblies, at the expense of a slight loss of thrust and slight increase infuel consumption.
Each TF34-GE-100 provides about 40.5 kN (4,130 kgp / 9,100 lbf) thrust, andhas a thrust-to-weight ratio of over 6, along with excellent fuel economy,increasing the Warthog's "time on station". The engine is also very quiet bycombat jet standards, a definite advantage for the low-level front-line CASmission since an adversary may not hear it coming. In addition, the TF34 wasdesigned to be easy to maintain and to be reliable, though the reliabilitystill left something to be desired, leading to the introduction of animproved "TF34-GE-100A" variant in the early 1980s. The TF34 is tolerant offoreign object ingestion.
The positioning of the engine pods above the rear fuselage helps improve thesurvivability of the machine, with the wing providing protection againstground fire and the twin fin tail masking the exhaust from heat-seekingsurface-to-air missiles (SAMs) -- the fact that high-bypass turbofans likethe TF34 have a relatively cool exhaust flow also helps. The high positionalso protects the engines from foreign object ingestion while operating offof dirt strips. In addition, the engine placement reduces the impact ofgun-gas ingestion, in which gases released by firing the cannon choke offairflow to the engines. Considering the armament of the A-10, gun gasingestion was a major potential problem.
The engine position does have its liabilities. At high angles of attack, thewing can block airflow to the engines, and so a short leading-edge slat wasinstalled between the fuselage and the main gear fairings. The slat opensautomatically under hydraulic control at high angles of attack. The YA-10Asuncovered the problem and were fitted with fixed slot surfaces as an interimfix. Apparently the use of the leading-edge slats introduced anotherdrawback in turn, in that it reduced indications to the pilot that theaircraft was going into a stall. A small strip was attached to the leadingedge of each wing to ensure that the descent into a stall wasn't so abrupt.
The engine exhausts are canted up nine degrees to the centerline of theaircraft. This arrangement provides some protection against heat-seekingSAMs, but it was apparently done to reduce changes in aircraft pitch withchanges in thrust. The engine pods feature large access doors -- in fact,they mostlyare access doors -- to permit easy maintenance in fieldconditions and reduce the need to pull the engines. There is a Garrettauxiliary power unit (APU) in the fuselage between the TF34s for enginestarting and ground power.
* The Warthog's pilot sits in an armor-plate "bathtub" that providesimpressive security against ground fire. The bathtub consists of titaniumplates, ranging from 1.27 to 3.81 centimeters (0.5 to 1.5 inches) thick,bolted together, with the assembly having a total weight of 544 kilograms(1,200 pounds), almost half the total weight of armor on the aircraft. Thetub's interior is lined with "ballistic" nylon armor to block the "spalling"of titanium fragments due to hits on the exterior.
The pilot gets into the A-10 using a pull-down telescoping stepladder builtinto the left side of the fuselage, which was not included in the YA-10As.He (or she -- in recent years some femmes have become Hog drivers) sits highunder a bubble clamshell canopy that hinges open from the rear. The A-10 wasoriginally fitted with the McDonnell Douglas IE-9 Escapac ejection seat, anearly "zero-zero (zero height, zero speed)" ejection seat that was thestandard at the time. This was later upgraded to the more effective andcomfortable McDonnell Advanced Concept Ejection Seat II (ACES II)."
The dashboard is simple by combat jet standards, consisting of analog dialsand other indicators, with the exception of a TV display in the upper rightcorner for guiding Maverick missiles -- more on this below -- and a Kaiser"head up display (HUD)" capable of displaying simple flight and targetingsymbology. These two displays were about the only thing in the cockpit thatmight have been particularly unfamiliar to, say, a Republic F-84 pilotbrought forward in time. Aircraft avionics were equally simple, though theywould eventually become more sophisticated.
There wasn't even an autopilot, which would prove to be a substantialinconvenience for Hog pilots who had to relieve themselves in flight.Ordinary radios and a TACAN beacon-navigation system were initially fitted,but TACAN is a line-of-sight system and unsurprisingly turned out to be anineffective way for a pilot to find his way around in low-level flight. Hogpilots ended up trying to fly while figuring out where the hell they werewith a map on the lap. As a result, the Air Force bit the bullet and had astandard AN/ASN-141 inertial navigation system (INS) built into the last 283A-10s produced. The INS data was displayed on the Kaiser HUD. Since thisrequired an update of the HUD electronics anyway, the opportunity was used tocompletely revise the system to provide much more sophisticated and capablesymbology.
In addition, the simple classic air-pressure altimeter initially fitted tothe Hog proved of limited value while jinking around the hills of Germany incrummy weather, and so late-production A-10s replaced the original altimeterwith a standard AN/APN-164 radar altimeter. The INS and radar altimeter wereeventually retrofitted to older Hogs in the field.
Even an INS and a radar altimeter were nothing particularly special. Theonly halfway whizzy item fitted to the Hog was a Martin Marietta "AN/AAS-35Pave Penny" pod on the underside of the right side of the nose on a pylon.The Pave Penny is a "laser spot tracker", which picks up the spot on which alaser target designator is focused and relays its position on the pilot'sHUD. The Pave Penny pod isnot a laser target designator, and in fact itdoesn't even have laser rangefinder capability. Although laser spot trackersare generally fitted into the end of an aircraft's nose, the blast effect ofthe GAU-8/A cannon made this impossible. Incidentally, the Pave Penny podwas not fitted to initial A-10 production. It was introduced in 1978 andretrofitted to aircraft in the field.
That was it as far as the Hog's internal sensors went. The A-10's onlybuilt-in active countermeasures device was an Itek AN/ALR-46 radar warningreceiver (RWR), with antenna bumps in the nose and tail, plus a little "planposition indicator (PPI)" radarscope-like display on the upper left corner ofthe dashboard to show the direction and location of radar-emitting threatssuch as tracked air-defense vehicles. The AN/ALR-46 was later upgraded tothe more capable AN/ALR-64 and AN/ALR-69 models, capable of dealing with agreater range of emitters.
The Hog was also fitted with passive countermeasures, in the form of fourAN/ALE-40 chaff-flare dispensers, one under each turned-down wingtip and onein the end of each main landing gear fairing. Each AN/ALE could carry 480flares or chaff cartridges, organized in 16 batteries of 30 each. Thedispensers were manually activated by the pilot.
* The fuselage is basically wrapped around the GAU-8/A Avenger cannon, whichwas intended at the outset to be the A-10's primary weapon. A detailed studyconducted in the planning stages of the AX project showed that a heavy cannonwas the most cost-effective means of killing armor. It was also the bestweapon for frontline fights when friendly and hostile forces were mixedtogether, since picking out one from the other meant getting up close anyway,where the standoff range of an ASM was negated.
The GAU-8/A is basically a Gatling-type mechanically-driven machine gun, withseven barrels rotated by two hydraulic motors to fire sequentially. Itsbasic principles and even many of its fine details would be perfectlyfamiliar to Dr. Richard Gatling, who invented the scheme in the 1860s.Gatling actually tinkered with electrical motor drive for his guns in the1890s and his experiments were resurrected by GE engineers after World WarII, using an actual Gatling-built gun to perform a proof-of-concept that ledto the M61 Vulcan.
The GAU-8/A is 5.06 meters (19 feet 10 inches) long and weighs, fully loaded,1,830 kilograms (4,029 pounds). Incidentally, strictly speaking thedesignation "GAU-8/A" only means the cannon itself, with the cannon and ammodrum and so on making up the "A/A 49E-6 Gun System", but for the sake ofsimplicity this distinction will not be made further in this document.
The cannon can fire at selectable rates of 2,100 or 4,200 rounds a minute (35to 70 rounds a second), and can continue to fire at the 2,100 rate if one ofthe hydraulic motors fails. It is highly reliable, in part as a legacy ofDr. Gatling's thorough development of his design. The cannon can firearmor-piercing / incendiary (API), high explosive / incendiary (HEI), andtraining rounds.
The API round fires a steel-cased shell with a penetrating rod made of highlydense "depleted uranium (DU)", with the rod smashing through the shell onimpact and then hopefully through the armor beneath. It destroys by kineticenergy but also has incendiary effect as its designation implies, sinceuranium will burn and the core ignites on impact. The HEI round is moreconventional, with a shell containing a high-explosive / incendiary filling,and the training round is a simple steel slug with an aluminum cap.
All the types of ammunition have aluminum-alloy cases, providing asignificant savings in weight compared to steel or brass and increasing theaircraft's ammunition capacity. The shells have plastic "driving bands" tomate them to the cannon rifling, which reduces barrel wear. Differentmixes of ammunition can be loaded, but the most common mix is apparentlyfive API for one HEI.
The GAU-8/A can fire 40 rounds in a second, placing them in a tight circle,with five or six being enough to take out heavy armor. The shells travel atMach 3 and, at least originally, no need to lead or shoot high to hit atarget -- they get there almost immediately on basically a straight line.The accuracy is such that a Hog firing on a target from a range of 1.8kilometers (4,000 feet) will place 80% of the rounds in a circle about 12meters (40 feet) in diameter around the target. Although the Hog didn't havea "fly by wire" flight control system (FCS), it was designed with a"Stability Augmentation System (SAS)" to add an extra degree of steadiness asa gun firing platform.
One of the major problems in development was that unburned propellant fromthe cannon cartridges would accumulate in front of the muzzle and thenexplode in a distinctly unnerving and potentially dangerous fireball. Theproblem was solved by adding potassium nitrate to the propellant to encouragemore complete combustion. The US Navy knew this trick from the use of heavynaval guns.
The GAU-8/A generates considerable muzzle blast, which is why it the muzzleis positioned at the most forward part of the aircraft -- anything that wasalongside that blast might take a beating. The cannon generates 40 kN (4,080kg / 9,000 lb) of recoil when fired -- apparently it's something of a jolt tothe pilot, and in any case the firing muzzle of the cannon is on the exactcenterline of the aircraft to ensure that the recoil doesn't throw off firingaccuracy.
To ensure that firing the cannon doesn't cause the Warthog to pitch up, thecannon is mounted below the aircraft's center of gravity and is boresightedalong a line 2 degrees below the aircraft's line of flight. One of theeffects of this is another one of the many peculiarities of the A-10: thenose gear is mounted on the right side of the nose, not on the centerline,since the cannon barrel is in the way. The oversized ammunition drum is setwell back into the fuselage, just forward of the wings, to reduce the effectof changes in trim as the heavy ammunition load is expended. The position ofthe drum also helps protect it from hostile fire that could set off theammunition, destroying the aircraft in a blaze of fireworks.
* While the GAU-8/A was intended to be the A-10's primary weapon, asmentioned earlier its design specifications specified that it be able tocarry a hefty external warload, and it has a total of eleven stores pylons --four on each wing and three in parallel on the belly.
The two outer pylons on each wing have a maximum load capacity of only 450kilograms (1,000 pounds) each, while the two inner pylons on each wing have amaximum load capacity of 1,135 kilograms (2,500 pounds) each. The centerlinebelly pylon has a maximum load capacity of 2,270 kilograms (5,000 pounds)while the two off-center belly pylons have a maximum load capacity of 1,590kilograms (3,500 pounds) each.
It is impossible to put a load on all three belly pylons at the same time --a load can be carried on the centerline pylon, or loads can be carried on thetwo off-center belly pylons, but it's strictly one or the other. A verticalstrake was fitted under the belly just forward of each wing root to ensuresmooth airflow around the belly pylons for clean stores separation.
The outermost pylon on each wing was intended for defensive warloads, with adual launch rack for AIM-9 Sidewinder AAMs on one wing and an AN/ALQ-119 orAN/ALQ-131 radar jamming pod on the other. Up to three 2,536 liter (600 USgallon) external fuel tanks can be carried, with the centerline pylon and themost inboard pylon on each wing being "wet". External tanks are mostly usedfor ferry flights; the A-10's endurance is good and there's usually no needfor external tanks in operations.
* The A-10 was designed for the battlefield CAS role and carries offensivestores appropriate to the task. General purpose HE bombs, usually withretarder tails, cluster bombs, and unguided rocket pods are useful properwarloads for front-line battlefield support missions. In principle, smallerlaser-guided bombs (LGBs) would also be useful in that role; although theWarthog wasn't designed with a laser target designator, ground forces hadsuch things and could use them to precisely spot targets for strikes with alow chance of dumping the bomb on friendlies.
However, LGBs seem to be an unusual store at best for an A-10, and the Hog'smost characteristic external store is the AGM-65 Maverick ASM, and in fact asmentioned earlier the cockpit has a TV display specifically included tosupport the missile. The Maverick provides a degree of "stand-off"capability to allow the Warthog to pick off dangerous targets before movingin to closer range to fight with the cannon. The Mavericks are usually amix of AGM-65B weapons, with a magnifying TV seeker, and the AGM-65D with animaging infrared (IIR) seeker. The AGM-65D is several times more expensivethan the AGM-65B, but it provides the Hog with a degree of night-combatcapability, with the pilot using the IIR seeker as a night vision device.
Although some press photos have displayed A-10s loaded up with an impressivearray of bombs and other stores, the Hog is traditionally not loaded up withanywhere near its maximum external load, since that would basically destroythe maneuverability that is the aircraft's main means of defense. A heavywarload for a Hog would be four Mavericks, two cluster bombs, plus twoSidewinders and a jammer pod.
* As mentioned, the Warthog was designed with survivability in mind. TheUS military had got a rough course in the subject over Vietnam, and as the F-105 Thunderchief carried the brunt of the USAF's strikes against North Vietnam early in the war, Republic Aviation had learned a lot from thatcourse as well.
The F-105 was a tough aircraft and sometimes came back home with good-sizedchunks ripped out of it, but in other cases minimal damage, even possibly asingle hit from a rifle-caliber bullet, could bring it down. The lessonultimately learned was that the notion of survivability had never really beencomprehensively addressed in the design stage. To be sure, Republic hadalways built their aircraft strong, rugged, and heavy, and items such ascockpit armor and self-sealing fuel tanks were often included, but thatwasn't the same thing as sitting down at the outset and thinking outeverything that needed to be done to make an aircraft as survivable aspossible.
Once that mindset took hold, aircraft designers realized that they had beendoing some odd things, such as installing redundant hydraulic, electrical, orcontrol lines right next to each other. The redundant systems did generallyprovide some backup against a straightforward failure of one system, but ifthe lines of both systems were close together then one hit could knock bothof them out. In other words, some fairly simple consideration of layout ofthe hydraulic, electrical, control, and other systems could do a great dealto improve the odds that an aircraft could make it back home safely aftersoaking up a number of hits. The A-10 has dual hydraulic, electric,pneumatic, and flight control systems (with a backup manual flight controlsystem) designed along such lines, with the duplicate systems separated fromeach other and running through protected ducts.
Of course, Fairchild-Republic engineers had also considered obvious means ofimproving survivability. The cockpit armor bathtub, engine position, enginefire extinguishing system, and countermeasures gear have been mentioned. Inaddition, the fuel tanks are self-sealing; filled with folded ("reticulated")foam panels to slow down leakage of fuel and propagation of fire through atank; and surrounded by rigid foam to soak up leakage and block the spread offire. Aside from the cockpit tub, the lion's share of the armor on the A-10protects the fuel system.
Some of the other features are less obvious. The flight and control surfacesof the A-10 are generous, not merely to improve maneuverability but to allowthe machine to be flown after it has lost big chunks of itself. For example,it can lose half of a wing, half of the tail assembly, one of the engines, orthe nose and still keep on flying. The airframe has structural redundancy sothat such damage does not necessarily lead to the structural failure of otherparts of the aircraft. Another interesting feature is that the landing gearis "normally open", kept retracted by continuous application of power andcoming down automatically if the hydraulic system fails.
The USAF conducted extensive survivability tests on the A-10 by firing atotal of 707 23 millimeter rounds at major subassemblies mounted on testjigs, along with 108 rounds of smaller calibers, such as a burst of 7.62millimeter ammunition against the windscreen.
* One of the clear weaknesses of the A-10 at the outset was that it wasessentially a day weapon, and an adversary was clearly not going to be soconsiderate as to restrict his attacks to daylight hours only. In late 1977,Fairchild began talks with the USAF about a possible "Night / All Weather(N/AW)" variant, and in April 1978 the company began work on a demonstrator,using company and USAF funds and some material assistance from interestedpotential subcontractors.
The first development A-10 was converted to the NA/W configuration in 13months, performing its first flight on 6 May 1979. It was a two-placetandem-seat aircraft, with a "Weapons System Operator (WSO)" in the backseat. Unsurprisingly, given how carefully thought-out the A-10's design hadbeen, the design had provisions for a back seat from the outset.
The second seat was placed above the ammunition drum in a raised positionthat gave the WSO a good forward view; a few avionics boxes had to berelocated to a fairing on the end of the cockpit. There were separatecanopies for each crewperson, each canopy hinging open to the right side.The tailfins were increased in height to compensate for the aerodynamicinterference of the two-seat cockpit.
The tandem cockpit was the most visible change but not necessarily the most significant. The NA/W Hog featured a significantly enhanced avionicssuite, including:
The pilot's Kaiser HUD was modified to interface with the new avionics, andthe WSO in the back seat had two CRT displays to see through the FLIR, radar,or LLLTV. The WSO had simple flight controls to allow him to take over ifthe pilot was disabled.
Although the N/AW Hog carried a sophisticated kit of gear, it was still muchcheaper than that fitted to a fast-mover strike aircraft like the GeneralDynamics F-111. For example, the F-111 used its radar with an automaticterrain following system to allow it to zip over the ground at high speed,but the NA/W Hog crew didn't need automatic capability since their aircraftwas so much slower. The WSO could navigate using the INS and relay cues tothe pilot, who would be watching radar, FLIR, and LLLTV inputs on his HUD.
The N/AW Hog was extensively flight-tested from 1979 into 1982. Theevaluation was very successful. Fairchild promoted a production version thatwould have the sensors from the two pods integrated into extended mainlanding gear fairings; an improved one-piece clamshell canopy; and armor sidepanels for the rear cockpit. The company also outlined concepts for bothsingle-seat and two-seat Hogs with various capability subsets. However, theAir Force was then working hard on the new Lockheed LANTIRN targeting podsystem for their strike aircraft, and USAF officials felt that it would be agood cheap-and-dirty solution to give a standard A-10 the night /all-weather attack capability. LANTIRN was supposed to be available in 1985and so there seemed to be no need for the N/AW Hog.
The biggest problem with the N/AW two-seater was said to be finding thequalified aircrew to fill the back seat -- training and maintaining aircrewis not cheap and it would have been a major expense. The Hog would get thecapability to fight at night and in bad weather, but it would take aboutanother two decades -- longer than even Hog enthusiasts thought the machinewould remain in service. The Air Force did order 20 "A-10Bs" in 1981, whichhad the tandem cockpit but were strictly two-seat trainers with no advancedavionics. Congress shot the order down, since pilots said the Hog was verystraightforward to fly and there seemed to be no great need for a conversiontrainer.
