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Aircraft engine starting
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Aircraft engine starting
Ground crew disconnecting an air start hose from a Boeing B-52 Stratofortress

Many variations of aircraft engine starting have been used since the Wright brothers made their first powered flight in 1903. The methods used have been designed for weight saving, simplicity of operation and reliability. Early piston engines were started by hand. Geared hand starting, electrical and cartridge-operated systems for larger engines were developed between the First and Second World Wars.

Gas turbine aircraft engines such as turbojets, turboshafts and turbofans often use air/pneumatic starting, with the use of bleed air from built-in auxiliary power units (APUs) or external air compressors now seen as a common starting method. Often only one engine needs be started using the APU (or remote compressor). After the first engine is started using APU bleed air, cross-bleed air from the running engine can be used to start the remaining engine(s).

Piston engines

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Hand starting/propeller swinging

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A 1918 sketch of ground crew receiving instruction on hand starting

Hand starting of aircraft piston engines by swinging the propeller is the oldest and simplest method, the absence of any onboard starting system giving an appreciable weight saving. Positioning of the propeller relative to the crankshaft is arranged such that the engine pistons pass through top dead centre during the swinging stroke.

As the ignition system is normally arranged to produce sparks before top dead centre there is a risk of the engine kicking back during hand starting. To avoid this problem one of the two magnetos used in a typical aero engine ignition system is fitted with an 'impulse coupling', a spring-loaded device which delays the spark until top dead centre and which also increases the rotational speed of the magneto to produce a stronger spark. When the engine fires, the impulse coupling no longer operates and the second magneto is switched on.[1] As aero engines grew bigger in capacity (during the interwar period), single-person propeller swinging became physically difficult‍—‍‌ground crew personnel would either join hands and pull together as a team, or else a canvas sock would be fitted over one propeller blade, the sock having a length of rope attached to the propeller tip end.[2][3] Note that this is different from the manual "turning over" of a radial piston engine, which is done to release oil that has become trapped in the lower cylinders prior to starting to avoid engine damage. The two appear similar, but while hand starting involves a sharp, strong "yank" on the prop to start the engine, turning over is simply done by turning the prop through a certain set amount.

Accidents have occurred during lone pilot hand starting, whether due to high throttle settings, brakes not having been applied, or wheel chocks not being used, all resulting in the aircraft moving off without the pilot at the controls.[4] "Turning the engine" with the ignition and switches accidentally being left "on" can also cause injury if the engine starts unexpectedly when a spark plug fires‍—‍‌if the switch is not in the "start" position, the spark will occur before the piston hits top dead center, which can force the propeller to violently kick back.

Hucks starter

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Main article: Hucks starter
The Shuttleworth Collection's working Hucks Starter positioned with their Bristol F.2 Fighter

The Hucks starter (invented by Bentfield Hucks during WWI) is a mechanical replacement for the ground crew. Based on a vehicle chassis the device uses a clutch driven shaft to turn the propeller, disengaging as the engine starts. A Hucks starter is used regularly at the Shuttleworth Collection for starting period aircraft.[3]

Pull cord

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Main article: Recoil start

Self-sustaining motor gliders (often known as 'turbos') are fitted with small two-stroke engines with no starting system, for ground testing a cord is wrapped around the propeller boss and pulled rapidly in conjunction with operating decompressor valves. These engines are started in flight by operating the decompressor and increasing airspeed to windmill the propeller. Early variants of the Slingsby Falke motor glider use a cockpit mounted pull start system.[5]

Electric starter

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Main article: Starter (engine)
A Supermarine Spitfire at readiness with a trolley accumulator connected

Aircraft began to be equipped with electrical systems around 1930, powered by a battery and small wind-driven generator. The systems were initially not powerful enough to drive starter motors. Introduction of engine-driven generators solved the problem.[6]

Introduction of electric starter motors for aero engines increased convenience at the expense of extra weight and complexity. They were a necessity for flying boats with high mounted, inaccessible engines. Powered by an onboard battery, ground electrical supply or both, the starter is operated by a key or switch in the cockpit. The key system usually facilitates switching of the magnetos.[6][7]

In cold ambient conditions the friction caused by viscous engine oil causes a high load on the starting system. Another problem is the reluctance of the fuel to vaporise and combust at low temperatures. Oil dilution systems were developed (mixing fuel with the engine oil),[8] and engine pre-heaters were used (including lighting fires under the engine). The Ki-Gass priming pump system was used to assist starting of British engines.[9]

Aircraft fitted with variable-pitch propellers or constant speed propellers are started in fine pitch to reduce air loads and current in the starter motor circuit.[citation needed]

Many light aircraft are fitted with a 'starter engaged' warning light in the cockpit, a mandatory airworthiness requirement to guard against the risk of the starter motor failing to disengage from the engine.[10]

Coffman starter

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Main article: Coffman engine starter

The Coffman starter was an explosive cartridge operated device, the burning gases either operating directly in the cylinders to rotate the engine or operating through a geared drive. First introduced on the Junkers Jumo 205 diesel engine in 1936 the Coffman starter was not widely used by civil operators due to the expense of the cartridges.[11]

Pneumatic starter

[edit]

In 1920 Roy Fedden designed a piston engine gas starting system, used on the Bristol Jupiter engine in 1922.[3] A system used in early Rolls-Royce Kestrel engines ducted high-pressure air from a ground unit through a camshaft driven distributor to the cylinders via non-return valves, the system had disadvantages only overcome by conversion to electric starting.[12]

In-flight starting

[edit]

When a piston engine needs to be started in flight the electric starter motor can be used. This is a normal procedure for motor gliders that have been soaring with the engine turned off. During aerobatics with earlier aircraft types it was not uncommon for the engine to cut during manoeuvres due to carburettor design. With no electric starter installed, engines can be restarted by diving the aircraft to increase airspeed and the rotation speed of the 'windmilling' propeller.[13]

Inertia starter

[edit]

An aero engine inertia starter uses a pre-rotated flywheel to transfer kinetic energy to the crankshaft, normally through reduction gears and a clutch to prevent over-torque conditions. Three variations have been used, hand driven, electrically driven and a combination of both. When the flywheel is fully energised either a manual cable is pulled or a solenoid is used to engage the starter.[14]

Gas turbine engines

[edit]

Starting of a gas turbine engine requires rotation of the compressor to a speed that provides sufficient pressurised air to the combustion chambers. The starting system has to overcome inertia of the compressor and friction loads, the system remains in operation after combustion starts and is disengaged once the engine has reached self-idling speed.[15][16]

Electric starter

[edit]
Main article: Starter (engine)

Two types of electrical starter motor can be used, direct-cranking (to disengage as internal combustion engines) and starter-generator system (permanently engaged).[17]

Hydraulic starter

[edit]

Small gas turbine engines, particularly turboshaft engines used in helicopters and cruise missile turbojets can be started by a geared hydraulic motor using oil pressure from a ground supply.[18]

Air-start

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Main article: Air-start system
Cutaway view of an air-start motor of a General Electric J79 turbojet

With air-start systems, gas turbine engine compressor spools are rotated by the action of a large volume of compressed air acting directly on the compressor blades or driving the engine through a small, geared turbine motor. These motors can weigh up to 75% less than an equivalent electrical system.[15]

The compressed air can be supplied from an on-board auxiliary power unit (APU), a portable gas generator used by ground crew or by cross feeding bleed air from a running engine in the case of multi-engined aircraft.[19]

The Turbomeca Palouste gas generator was used to start the Spey engines of the Blackburn Buccaneer. The de Havilland Sea Vixen was equipped with its own Palouste in a removable underwing container to facilitate starting when away from base.[20] Other military aircraft types using ground supplied compressed air for starting include the Lockheed F-104 Starfighter and variants of the F-4 Phantom using the General Electric J79 turbojet engine.

Combustion starters

[edit]

AVPIN starter

[edit]

Versions of the Rolls-Royce Avon turbojet engine used a geared turbine starter motor that burned isopropyl nitrate as the fuel. In military service this monofuel had the NATO designation of S-746 AVPIN. For starting a measured amount of fuel was introduced to the starter combustion chamber then ignited electrically, the hot gases spinning the turbine at high revolutions with the exhaust exiting overboard.[21]

Cartridge starter

[edit]
Mass cartridge start of Hawker Sea Hawk aircraft
Main article: Coffman engine starter

Similar in operating principle to the piston engine Coffman starter, an explosive cartridge drives a small turbine engine which is connected by gears to the compressor shaft.[22]

Fuel/air turbine starter (APU)

[edit]
Main article: Auxiliary power unit

Developed for short-haul airliners, most civil and military aircraft requiring self-contained starting systems these units are known by various names including Auxiliary Power Unit (APU), Jet Fuel Starter (JFS), Air Start Unit (ASU) or Gas Turbine Compressor (GTC).[21] Comprising a small gas turbine which is electrically started, these devices provide compressed bleed air for engine starting and often also provide electrical and hydraulic power for ground operations without the need to run the main engines.[23] ASUs are used today within the civil and military Ground Support to serve Aircraft on main engine start (MES) and pneumatic bleed-air-support for the Environmental Control System (ECS) cooling and heating

Internal combustion engine starter

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Riedel two-stroke starter motor of the Junkers Jumo 004. Note the pull-cord handle

An interesting feature of all three German jet engine designs that saw production of any kind before May 1945 (the German BMW 003, Junkers Jumo 004 and Heinkel HeS 011 axial-flow turbojet engine designs) was the starter system, which consisted of a Riedel 10 hp (7.5 kW) flat twin two-stroke air-cooled engine hidden in the intake, and essentially functioned as a pioneering example of an auxiliary power unit (APU) for starting a jet engine — for the Jumo 004, a hole in the extreme nose of the intake diverter contained a D-shaped manual pull-cord handle which started the piston engine, which in turn rotated the compressor.[24] Two small petrol/oil mix tanks were fitted in the annular intake.[25]

The Lockheed SR-71 Blackbird used two Buick Nailheads as starter motors, which were mounted on an AG-330 Start Kart trolley, later with Chevrolet big-block 454-cubic-inch (7.44 L) V8 engines.[citation needed]

In-flight restart

[edit]

Gas turbine engines can be shut down in flight, intentionally by the crew to save fuel or during a flight test or unintentionally due to fuel starvation or flameout after a compressor stall.

Sufficient airspeed is used to 'windmill' the compressor then fuel and ignition are switched on, an on-board auxiliary power unit may be used at high altitudes where the air density is lower.[16]

During zoom climb operations of the Lockheed NF-104A the jet engine was shut down on climbing through 85,000 ft (26,000 m) and was started using the windmill method on descent through denser air.[26]

Pulse jet starting

[edit]
Main articles: Pulse jet engine and Argus As 014
Sectioned AS 014 engine on display at the London Science Museum

Pulse jet engines are uncommon aircraft powerplants. However, the Argus As 014 used to power the V-1 flying bomb and Fieseler Fi 103R Reichenberg was a notable exception. In this pulse jet three air nozzles in the front section were connected to an external high-pressure air source, butane from an external supply was used for starting, ignition was accomplished by a spark plug located behind the shutter system, electricity to the plug being supplied from a portable starting unit.[27]

Once the engine started and the temperature rose to the minimum operating level, the external air hose and connectors were removed, and the resonant design of the tailpipe kept the pulse jet firing. Each cycle or pulse of the engine began with the shutters open; fuel was injected behind them and ignited, and the resulting expansion of gases forced the shutters closed. As the pressure in the engine dropped following combustion, the shutters reopened and the cycle was repeated, roughly 40 to 45 times per second. The electrical ignition system was used only to start the engine; heating of the tailpipe skin maintained combustion.[27]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Aircraft engine starting

View on Grokipedia
from Grokipedia
Aircraft engine starting is the critical process of initiating and in an aircraft's powerplant to achieve self-sustaining operation, typically involving the use of specialized starters to rotate the or to a speed sufficient for ignition and establishment. This procedure is essential for safe operation, as improper starting can lead to engine damage, fire hazards, or failure to achieve takeoff power. For reciprocating engines, starting systems evolved from manual hand-cranking to modern electromechanical devices, with direct cranking electric starters being the most prevalent; these use a 12- to 24-volt motor drawing 100 to 350 amps to engage reduction gears and a that drives the until it reaches self-sustaining speed, after which the starter disengages automatically. Inertia starters, another common type, employ a accelerated by an or hand crank to store , which is then transferred to the via gears for rotation. Starting procedures for these engines limit cranking to 1 minute followed by a 1-minute cooldown, with longer rests after multiple attempts to avoid overheating components. Turbine engines, including turbojets and turbofans, require accelerating the to generate sufficient for , often using electric starters, air turbine starters powered by 30 to 50 psi , or hybrid starter-generators that function as generators post-start to reduce weight. The starting sequence involves three main phases: motoring the engine to purge fuel vapors, igniting the fuel-air mixture with spark or chemical means, and accelerating to idle speed while monitoring for issues like or hot starts. Factors such as ambient , altitude, and fuel volatility significantly influence starting success, with low temperatures increasing required fuel flow and spark energy—typically 0.02 to 12 joules per spark—to ensure reliable ignition up to 50,000 feet. Air turbine starters provide higher torque at lower weight than electric alternatives, making them suitable for larger engines. Safety protocols emphasize pre-start checks, such as verifying battery charge and , and adherence to time limits—like a 30-second maximum ignition attempt—to prevent overtemperature or starter overload. Advances in starting systems, including digital controls for adaptive fuel scheduling, continue to enhance reliability across diverse operating environments.

Starting piston engines

Hand starting and propeller swinging

Hand starting, also known as propeller swinging or hand-propping, involves a pilot or ground crew member manually rotating the aircraft's to initiate the pistons' compression and subsequent ignition in the cylinders of a radial or inline engine. This method relies on the , often augmented by impulse couplings that provide a spark at low speeds, to fire the mixture after priming the cylinders with . The process requires the engine to be set with the mixture control in the lean position, throttle closed or cracked slightly, and magnetos switched on, ensuring no unintended upon startup. Historically, hand starting was the predominant technique from the early 1900s through the , as most aircraft lacked onboard starting systems. It was widely used in fighters, such as the equipped with a 130 horsepower Clerget , where members would swing the after priming and adjusting the ignition timing to overcome the engine's compression resistance. This manual approach allowed rapid preparation for combat sorties but demanded coordinated effort between the starter and the pilot, who monitored controls from the . By the , it remained standard for civil and , reflecting the era's emphasis on lightweight, simple designs without electrical dependencies. The technique entails positioning one propeller blade at approximately the 10 o'clock position to align with the compression stroke of the top , gripping the blade mid-span without wrapping fingers around the , and delivering a firm downward pull while stepping backward to clear the arc. protocols include securing the with chocks or tiedowns, confirming the pilot's readiness with a verbal "clear" signal, and verifying that the selector is off for solo operations to prevent runaway. These steps minimize exposure during the swing, which typically requires multiple attempts if the engine does not catch immediately. Despite its simplicity, propeller swinging posed substantial risks, particularly in early when standardized procedures were absent. Kickback from cylinder compression could violently reverse the , fracturing the swinger's arms or hurling them into the blade path, while unintended engine firing risked strikes causing lacerations, amputations, or fatalities. In the nascent years of flight, such hazards contributed to frequent ground injuries among and pilots, exacerbating the high overall rates of the and . Later analyses of incidents, such as those from 1965 to 1979, recorded 69 hand-starting s resulting in 13 deaths and 56 serious injuries, underscoring persistent dangers even with improved awareness. The method's perils prompted its gradual replacement by mechanical starters in the 1930s, transitioning aviation toward safer, automated initiation systems.

Pull cord starting

Pull cord starting, also known as starting, is a manual initiation method employed for small two-stroke engines in ultralight , paramotors, and vintage setups. The mechanism features a sturdy cord wound around a sheave or directly linked to the engine's , typically integrated with a spring assembly that retracts the cord after each pull. This setup allows a single operator to generate sufficient rotational speed—often 200-300 RPM—by yanking the cord sharply, engaging pawls or a to turn the without direct propeller contact. Key components include the sheave, spring (e.g., part 939 078), and (e.g., part 952 799), designed for durability in lightweight applications. This starting technique is prevalent in compact engines such as the , 503, and 582 series, which power modern ultralights like the Quicksilver or early gliders, as well as paramotors for . These engines, producing 40-65 horsepower, benefit from the system's simplicity in weight-sensitive designs where electrical alternatives would add unnecessary mass. Historically, pull cord systems gained traction in the 1920s and 1930s for , building on hand-starting precursors for solo operations in remote or basic environments. The starting procedure begins with priming the —squeezing the fuel bulb to fill the bowl and activating the primer 2-3 times—followed by setting the choke to full, closing the , and enabling the ignition. The operator then pulls the cord through by hand 2-3 times to clear the , clears the arc, and delivers a firm, full-length pull to spin the until ignition occurs, typically after 3-5 attempts in cold conditions. Once running, the choke is gradually reduced as RPM stabilizes around 3,000-4,000. Advantages include minimal weight (under 5 pounds for ) and from batteries or electrical , ideal for ultralights prioritizing portability and low . However, disadvantages encompass limited cranking , which can strain users on higher-compression models, and the of sudden snap-back if the cord binds or the engine kicks. protocols emphasize wearing protective gloves to guard against rope burns or snap-back injuries, verifying a clear 10-foot radius around the , and conducting a full pre-start to prevent unintended starts. In ultralight contexts, positioning the into the wind and briefing any assistants further mitigates risks during this hands-on process.

Hucks starter

The Hucks starter was invented by Captain Bentfield Charles Hucks, a pioneering British aviator and pilot who became the 91st licensed pilot in Britain in 1911 and served as a for the Aircraft Manufacturing Company (). Developed around 1917–1918 during the final stages of , it addressed the growing dangers and impracticality of hand-starting increasingly powerful piston engines by manually swinging the , a method that risked injury to as engine compression ratios rose post-war. Hucks, who died in the 1918 influenza epidemic before seeing widespread adoption, received a U.S. patent for the device in August 1919; it was introduced to the in 1920 and built initially by , seeing extensive use in the for starting large radial piston engines on across the and beyond. In operation, a truck-mounted Hucks starter approached the stationary aircraft, where ground crew adjusted a telescopic drive shaft—capable of vertical and horizontal movement—to engage a claw adapter with the propeller hub's splined "dog" fitting. The truck's engine then powered a chain- or gear-driven shaft to rotate the propeller at cranking speed, turning over the engine until it fired; an overrunning or spring-loaded clutch automatically disengaged the drive once the engine reached sufficient RPM, with a bungee cord or similar mechanism retracting the shaft for safety. This ground-based system delivered the high torque needed for large-displacement engines, enabling one start per minute under ideal conditions and serving as a mobile auxiliary power unit before onboard electric starters became standard. Key components included a rugged chassis, often based on the or TT with its four-cylinder engine and worm-drive differential for reliable low-speed torque; an adjustable X-frame or universal-jointed shaft for precise alignment; the engaging claw bar; and a folding platform for crew access, with the drive system incorporating chains or gears from the truck's . Safety features, such as the automatic disengagement, prevented overspeed damage to the starter or propeller, though manual coordination was essential to avoid misalignment. The Hucks starter found primary applications in starting radial piston engines on interwar military aircraft, such as the Bristol F.2B Fighter and designs, and remained in use into for training and operational planes by Allied and Axis forces alike, including U.S. (NACA) facilities, Soviet units for cold-weather starts, Japanese Army aircraft like the Ki-43, and German operations on the Eastern Front. It was particularly valued for its ability to handle high-compression engines that manual methods could no longer manage reliably, supporting airfields worldwide until the early . Despite its effectiveness, the Hucks starter had limitations as a weather-dependent system, struggling in mud, snow, or high winds that could hinder positioning or , and it required skilled coordination between the driver and the handler to ensure safe engagement. These factors, combined with the rise of compact onboard electric and pneumatic starters, led to its phase-out by the mid-1940s in favor of more independent and versatile methods.

Inertia starter

The inertia starter is a mechanical system designed to initiate the rotation of engines by storing in a heavy , which is then transferred to the engine through a and reduction gearing mechanism. The , typically weighing 50 to 100 pounds, is accelerated to speeds of 2,000 to 3,000 RPM either manually via a hand crank or electrically using a small motor, allowing for transfer without the need for continuous power input during cranking. This design incorporates multiple spur and planetary gears for speed reduction, a multiple-disc for engagement, and an overrunning mechanism to prevent back-driving once the engine starts. Developed in the post-World War I era, inertia starters gained popularity during the and as a reliable alternative to manual propeller swinging and external ground equipment like the Huck starter, particularly for radial, V-type, and inline engines up to 1,300 cubic inches in displacement. They were commonly used on such as the de Havilland Gipsy-powered models, including the Moth series, where often hand-cranked the to build up speed before engagement. By the , these starters supported engines up to 2,000 horsepower in military applications, valued for their simplicity and emergency hand-cranking capability. Post-World War II advancements in electric technology largely supplanted them due to automation demands. Operation begins with the spin-up phase, where the is accelerated—manually by inserting a crank handle and rotating it at 75 to 80 RPM for efficient , or electrically via a 12- or 24-volt system with remote control. Once sufficient speed is achieved, the operator advances the spark timing and pulls an engaging rod or lever to connect the to the through and gears, transferring to crank the engine at high initial speed for fuel priming and ignition. Upon engine firing and self-sustained rotation, the overrunning automatically disengages to avoid damage, completing the sequence without further intervention. A key advantage of the starter is its independence from batteries or external power sources during the critical cranking phase, relying instead on stored calculated as E=12Iω2E = \frac{1}{2} I \omega^2, where II represents the flywheel's and ω\omega its , providing high proportional to minimal weight. This results in low current draw if electrically assisted, consistent performance regardless of engine size or environmental conditions, and thousands of starts with minimal maintenance due to factory lubrication and no ongoing electrical demands. For automated spin-up, some variants integrated small electric motors, though manual operation remained prevalent in earlier models. Despite these benefits, inertia starters carry significant drawbacks, including substantial overall weight that impacts aircraft payload and the labor-intensive manual cranking required for ground crews, often necessitating multiple personnel for larger engines. Improper lubrication or overload can lead to clutch slippage or gear wear, and their limited continuous cranking ability compared to direct electric systems contributed to their replacement by more efficient electric starters after .

Electric starter

Electric starters for piston engines are direct-cranking systems that use a high-torque, series-wound (DC) motor, typically rated at 12 or 24 volts, to rotate the engine until it reaches a speed sufficient for ignition and self-sustained operation, usually around 100-200 RPM. The motor draws 100 to 350 amps from the aircraft battery during engagement and is connected to the via reduction gears and a or overrunning clutch, which automatically disengages once the engine starts to prevent damage from reverse rotation or overspeed. These starters became the most common method for initiating engines after , replacing manual and inertia systems in and due to their reliability, ease of use, and integration with onboard electrical systems. They are standard on modern single- and multi-engine aircraft, such as the or Piper Cherokee, where the pilot activates the starter via a momentary switch after priming the engine and setting the mixture and throttle. Historically, adoption accelerated in the with improvements in battery technology and motors, enabling solo starts without ground assistance. Operation involves the pilot closing the , advancing the to full rich, and switching on the ignition before engaging the starter, which spins the to draw in fuel-air for compression and spark ignition. The system includes a to connect battery power and safety interlocks to prevent while the is running. is generated by the series-wound motor's design, where field windings produce a strong for high starting force, following the relation τ=Pω\tau = \frac{P}{\omega} ( as power divided by ), ensuring sufficient cranking even in cold conditions. Disengagement occurs automatically via the overrunning when speed exceeds starter speed, typically within 10-30 seconds of cranking. Advantages of electric starters include rapid activation, minimal operator effort, and compatibility with units for ground starts, making them ideal for remote operations. They provide consistent performance across temperatures, unlike battery-dependent systems that may falter in extreme cold without pre-heating. However, high current draw can strain batteries, necessitating cooldown periods (e.g., 30 seconds after 10 seconds cranking) to avoid overheating, and they add weight (typically 10-20 pounds) compared to manual methods. In larger radial engines, heavy-duty variants deliver up to 200 ft-lbs of . Modern enhancements, such as brushless motors in , improve efficiency and lifespan.

Coffman starter

The Coffman starter was a pyrotechnic starting system developed by American inventor Roscoe A. Coffman in the early for large engines in and armored vehicles. Coffman applied for a patent in 1935, which was granted in 1942, and the device saw its first use in 1936 on the . It gained widespread adoption during , powering engines in fighters such as the and (with engines), as well as the Grumman FM-2 Wildcat and . The system's mechanism relies on cordite-filled cartridges, akin to oversized shells in 4-gauge size, loaded into a rotary breech with a multi-shot typically holding 3 to 6 rounds for repeated attempts. Upon firing, the cartridge's produces gas around 1,000 psi, which drives a through a steel pipe; the engages a starter with the engine's ring gear via helical splines, delivering rotational force to the . An exhaust valve then releases the , and a spring resets the for the next cycle, while periodic firing helps clear carbon buildup in the chamber. Operation begins with the pilot or crew pulling a toggle to align and load a fresh cartridge into the breech via a spindle and cam mechanism. A then electrically ignites the cartridge's primer, propelling the gas at up to 600 ft/s to spin the engine to about 250 RPM—enough for the to take over. This impulse provides a rapid burst, particularly effective in cold weather down to -30°F, without relying on batteries or external power sources. Key advantages of the Coffman starter included its compact, lightweight design—lighter than inertial or full electric systems—and ease of cartridge storage and use in remote or austere environments. It avoided battery drain and offered superior cold-start performance compared to alternatives. Safety considerations involved secure cartridge storage to mitigate misfire risks and allowing a 10-minute cooldown after failures, alongside handling the noxious smoke produced during firing. By the 1950s, the system was phased out in favor of more reliable electric starters, though it persisted in some post-war trainers like the de Havilland Chipmunk.

Pneumatic starter

A pneumatic starter for aircraft piston engines utilizes compressed air to rotate the crankshaft, facilitating ignition and initial operation of the engine. This system typically consists of high-pressure air tanks charged to 300-500 psi, control valves, and an air motor—either a vane or type—geared to the engine via reduction gears. The is often recharged by a ground-based cart or an onboard , ensuring readiness for multiple starts. The operation begins with the pilot or opening a to release into the starter motor, which spins the engine up to approximately 150 RPM. Once sufficient rotation is achieved, fuel and ignition are introduced to sustain , and an automatic shutoff disengages the starter to prevent overrun. In some designs, such as the Heywood system, air is distributed directly to selected cylinders through an air distributor driven by the , allowing bidirectional starting and aiding cold-weather operation with glow plugs. This method was particularly suited to radial engines due to their size and torque requirements. Pneumatic starters gained prominence in the and for their reliability in early , appearing in engines like the Kinner R-5 radial and Deschamps diesel V-12, and continued into military applications through the 1950s, including variants of the DC-3 (such as the Soviet Li-2) where ground carts provided the air supply. They remain in use on some heritage and radial-engine aircraft today for their robust performance. Key advantages include operation across extreme temperatures without battery degradation and elimination of spark risks in fuel-laden environments, making them ideal for radial engines in settings. However, the onboard air tanks add significant weight, limiting their adoption in lighter . Variants include integral turbine starters, where a small air-driven is mounted directly on the engine accessory gearbox for compact integration. Similar principles extend briefly to air-start systems for gas turbine engines, using for compressor rotation.

In-flight starting

In-flight starting of engines relies primarily on windmilling, where the 's forward motion drives airflow through the , rotating the engine to create compression for ignition without ground-based starters. This technique is critical for single-engine experiencing recoverable failures, such as fuel starvation from switching tanks or , allowing pilots to attempt restart while toward a suitable site. The standard restart procedure prioritizes maintaining best glide speed (typically 65-75 KIAS for light aircraft) while troubleshooting. For example, in the , pilots select the fuel selector to BOTH, set the mixture to RICH, open the fully, activate the electric , and position the to BOTH; if the is windmilling due to fuel exhaustion, the engine often restarts automatically within seconds once fuel flow resumes. Similar steps apply to the Piper PA-28 series: switch the fuel selector to the fullest tank, turn on the electric , enrich the mixture, open the , and select BOTH on the ignition, while checking for fuel in the lines if power does not return immediately. Pilots must monitor for backfires, which can occur if the engine floods, and avoid prolonged cranking to prevent overheating. If an electric starter is available and battery power permits, it may provide a brief boost to initiate rotation, though this is less common in flight due to power limitations. If the has stopped rotating, a controlled dive may be necessary to accelerate to sufficient airspeed for windmilling, often around 130 KIAS in , though exact speeds vary by model and must not exceed Vne. This maneuver trades altitude for momentum, emphasizing the need for ample height above terrain—ideally 5,000 feet or more—to execute safely. Risks include substantial altitude loss (potentially 1,000-2,000 feet during the dive and restart attempt), exacerbation of fuel starvation if lines remain empty, and the danger of engine fire from unburned igniting during . WWII-era training materials for U.S. and Allied pilots stressed these hazards, advising immediate glide establishment and cautious system checks to avoid secondary failures. Historical examples illustrate the method's origins and challenges in high-performance piston fighters. During , pilots of aircraft like the frequently performed windmilling restarts after inadvertent fuel cutoff from dropping external tanks without switching to internal supplies; a steep dive to 150-200 knots would spin the , followed by and adjustments for relight, often succeeding if executed promptly above 10,000 feet. Such procedures were detailed in RAF pilot notes, highlighting the technique's role in combat survival despite risks like structural stress from high-speed dives. In modern , systems in select piston engines—such as the AE300 diesel used in DA42 variants—enhance reliability by automatically managing fuel delivery, , and propeller pitch during restarts, reducing pilot workload and minimizing errors like improper mixture settings. These electronic controls can initiate and optimize in-flight relights more efficiently than manual systems, particularly for temporary interruptions, though manual intervention remains standard in most carbureted or fuel-injected gasoline engines.

Starting gas turbine engines

Electric starter

Electric starters for gas turbine engines employ high-power DC or AC motors, typically rated between 10 and 50 kW, to provide the necessary for cranking the rotor. These motors, often designed as starter-generators that switch functions post-start, are mounted on the engine's accessory gearbox and connected via reduction gears to achieve the required speed and multiplication. In modern systems, AC motors may incorporate variable frequency drives to optimize performance across varying engine speeds, enabling precise control without mechanical slippage. For example, switched reluctance starter-generators rated at 30 kW have been developed specifically for aircraft applications, delivering high at low speeds while minimizing weight. During operation, the electric starter accelerates the high-pressure (N2 spool) to approximately 20-30% of its rated speed, establishing sufficient for ignition and light-off. This process integrates with the (ECU) or electronic engine control (EEC), which sequences starter engagement, introduction, and using sensors for RPM, temperature, and pressure to prevent hot starts or stalls. Power is supplied by batteries, ground power units, or units, with the starter disengaging automatically via an overrunning once the reaches self-sustaining speed, typically around 50% N2. The provided by the motor relates to its power output and rotational speed through the fundamental equation: τ=Pω\tau = \frac{P}{\omega} where τ\tau is torque, PP is power, and ω\omega is angular velocity; this ensures adequate cranking force, often modeled as linearly decreasing with RPM for DC motors. Electric starters are widely applied in small turboprop engines, such as the Pratt & Whitney PT6A series, where battery-powered DC motors initiate compressor rotation for reliable starts in general aviation aircraft. They are emerging in larger jet engines through electrified architectures, leveraging high-capacity batteries or ground power for hybrid systems that reduce reliance on pneumatic sources. These applications benefit from precise speed regulation via electronic controls, eliminating hot gas exposure risks associated with other methods, and supporting more electric aircraft trends. Historically, electric starting gained significant traction in gas turbine engines after the , driven by advancements in and that enabled lighter, more efficient integrated starter-generators. This shift aligns with broader trends toward reduced hydraulic and pneumatic systems, as seen in concepts like the Versatile Electrically Augmented Turbine Engine (VEATE), which repurposes accessory drives for enhanced starting and operability. In contrast to lower-power electric starters for engines, those for turbines demand higher energy densities to overcome inertia.

Hydraulic starter

Hydraulic starters for gas turbine engines utilize pressurized hydraulic fluid to drive a motor that rotates the engine's compressor section to a sufficient speed for ignition and self-sustained operation. The primary components include a hydraulic pump sourced from a ground support cart, auxiliary power unit (APU), or onboard accumulators, which generates fluid pressure typically ranging from 3,000 to 4,000 psi, and a hydraulic motor—often a vane or piston type—mounted on the engine's accessory gearbox pad. Additional elements consist of control valves, fluid reservoirs, and an overrunning clutch to prevent back-driving once the engine accelerates. In systems like the F-16 Fighting Falcon, two brake/JFS accumulators charged by the aircraft's Hydraulic System B provide the initial fluid pressure, with an emergency power unit (EPU) serving as backup if system pressure falls below 1,000 psi. The starting process begins with the activation of a start switch, which routes pressurized fluid to the , spinning the via a geartrain in the accessory drive gearbox (ADG) to approximately 15-25% of operational speed—such as 20% RPM in the F-16's . At this point, is introduced and igniters fired, allowing light-off within 10-20 seconds; the starter automatically disengages via a or once the reaches self-sustaining speed (around 50% RPM), preventing overload. Fluid is then recirculated in a closed loop, with accumulators recharging as the engine-driven pumps come online. This method contrasts with air-start systems, which rely on for but may offer less smooth acceleration. Hydraulic starters are commonly applied in military fighter aircraft, such as the F-16, where they integrate with the aircraft's dual hydraulic flight control systems for enhanced , enabling two independent start attempts from pre-charged accumulators without external ground power. They have been favored in high-performance jets since the , evolving from early adaptations in piston-era to modern self-contained units suitable for engines. Advantages include high for compact, lightweight design—critical for fighters—and auto-disengagement, providing reliable operation in extreme conditions with unlimited re-engagement cycles up to 10,000 starts. However, potential disadvantages encompass fluid leak risks requiring regular and sensitivity to , which can degrade performance if not addressed.

Air-start systems

Air-start systems, also known as pneumatic starting systems, utilize to initiate rotation of the engine's compressor sections in gas turbine engines, serving as the predominant method for starting large commercial jet engines. The is sourced from a ground-operated air cart via hose connection, the 's (), or cross-bleed from an already running on the same . These sources typically deliver air at pressures ranging from 30 to 40 psi to ensure sufficient for acceleration. This approach evolved from earlier engine pneumatic systems but adapted for the higher power demands of turbojets. The core mechanism involves an air turbine motor (ATM), or air turbine starter, mounted on the engine's accessory gearbox. High-pressure air drives a wheel within the starter, which is mechanically linked through reduction gears to the engine's high-pressure compressor shaft (N2 spool), accelerating it to approximately 20% of its rated speed. Once this motoring speed is achieved, fuel is introduced into the , followed by ignition to achieve light-off, allowing the engine to self-sustain and accelerate to . This process provides reliable, high-torque starting without the weight penalties of electric alternatives. The starting procedure typically begins in "crank" mode, where the starter engages to motor the without or ignition, purging the system and confirming rotation before advancing to the full start sequence. Pilots monitor temperature (EGT) during light-off to ensure it remains within limits, aborting if anomalies occur. Systems operate in either continuous start mode, requiring manual intervention for termination, or automatic mode, where the manages and for optimized performance. This method is standard on aircraft like the and A320 families, tracing its origins to the 1940s with the advent of engines, where air-start units replaced heavier electric starters for operational efficiency. Potential issues include hot starts, caused by insufficient airflow during acceleration leading to excessive EGT rise, often due to incorrect fuel scheduling or starter malfunctions. Additionally, ground-supplied air can introduce contaminants such as or if is inadequate, potentially damaging compressor blades or causing uneven .

AVPIN starter

The AVPIN starter is a monopropellant system employed primarily in gas turbine aircraft engines, where (IPN), designated as AVPIN with the C₃H₇NO₃, undergoes to generate high-temperature, high-pressure gases that drive a starter . The decomposition initiates via homolytic cleavage of the weak O-NO₂ bond, producing (NO₂) and an isopropoxy radical (CH₃CH(O•)CH₃), which further breaks down into smaller molecules such as acetone, , and additional nitrogen oxides, releasing and expanding gases without requiring external oxygen. This reaction, represented simplistically as C₃H₇NO₃ → gases + , occurs in a dedicated where a metered quantity of AVPIN (typically 0.5–1 liter per start) is injected, electrically ignited by a high-energy spark, and directed through a to impinge on the blades of an air connected to the engine's accessory gearbox. The resulting accelerates the to approximately 25% of operational speed (N₂), sufficient for self-sustaining ignition once is introduced. Developed in the early by British engineers for rapid engine starts in high-performance jets, the AVPIN system was first integrated into Rolls-Royce Avon-powered aircraft such as the FGA.9 and interceptors, enabling quicker spool-up compared to earlier cartridge-based methods. It provided a self-contained alternative to ground-based air supplies, ideal for dispersed military operations, and was used in various RAF and export variants through the era, including the . The system's monopropellant cartridge design, akin to pyrotechnic cartridge starters but using liquid decomposition instead of solid propellants, delivered high in a compact form, with the achieving peak acceleration in under 10 seconds. Key advantages of the AVPIN starter include its independence from external power sources, lightweight construction (typically under 20 kg for the unit), and ability to operate in austere environments without infrastructure, making it suitable for forward-deployed fighters. However, significant hazards arise from AVPIN's extreme volatility, low (around 20°C), and ; decomposition products include (HCN) gas, which acts as a potent tear agent and respiratory irritant, alongside risks of spontaneous ignition or if mishandled during storage or transfer. Incidents of engine damage or personnel exposure were reported in operational use, necessitating strict safety protocols like armored tanks and scavenging vents. By the late , the AVPIN system was largely phased out in favor of electric and air-start alternatives due to these safety concerns, issues for the specialized , and advancements in reliable electric motors that eliminate while maintaining start reliability. Preservation efforts for like the have involved to electric starters, with the last operational AVPIN starts occurring around 2006 in RAF Canberras.

Cartridge starter

A cartridge starter is a pyrotechnic system employed to initiate gas turbine engines in by combusting solid cartridges to generate high-pressure hot gas, which drives a connected to the engine's shaft. These systems are breech-loaded, with cartridges inserted into a chamber akin to an enlarged mechanism, where ignition produces gas that expands through nozzles to impinge on blades. The design typically incorporates a single-stage impulse , reduction gearbox (often with a 14:1 to 15:1 ratio), and a to engage the engine rotor while preventing back-driving. Propellants such as ammonium nitrate-based formulations (e.g., MXU-4/A) are common, burning to yield gas at temperatures around 2400°F and pressures up to 700 psi, with a to regulate flow and prevent overpressurization. Many units are dual-mode, allowing pneumatic operation from external air sources as an alternative to cartridge use. In operation, the cartridge is electrically ignited, rapidly combusting to produce a burst of gas that spins the turbine at speeds up to 67,500 RPM, delivering torque (e.g., up to 680 lb-ft) through the gearbox to accelerate the compressor from standstill to light-off speed (typically 20% of operating RPM) in 8-10 seconds. Fuel is then introduced for ignition, and the starter disengages once self-sustaining combustion occurs; systems are designed for single attempts per cartridge, with sequential loading possible for retries if multiple cartridges are available. Burn rates are controlled (0.05-0.15 inches per second) to match temperature extremes from -65°F to +160°F, ensuring consistent performance without turbine overspeed. These starters found applications in early post-war jet aircraft, such as the (powered by engines) and its U.S. variant, the Martin B-57, as well as fighters like the North American F-100 and F-105. They remain in use for certain military platforms, including the B-52 and some unmanned drones requiring independent starts. Advantages include their compact size and light weight—often one-quarter to one-half that of equivalent electric starters—enabling reliable operation in remote or austere environments without external . They provide high initial torque for rapid acceleration and quick-start capability, ideal for tactical scenarios. However, drawbacks encompass the one-shot nature, necessitating cartridge replacement after each use, potential residue accumulation from , and risks of hot gas system or malfunctions due to high temperatures exceeding 1900°F. Safety features include interlocks to prevent firing with an open breech, protection that diverts gas flow, and rings to absorb failure energy. Cartridges have limited (typically 5-10 years) due to propellant sensitivity to moisture or phase changes in variants, requiring regular inspection and disposal. Misfire protocols involve immediate venting of residual pressure, manual cartridge extraction, and verification of system integrity before reloading to mitigate explosion risks. This design draws analogy to the Coffman starter for piston engines but employs turbine drive for gas turbines rather than direct piston impulses.

Auxiliary power unit starters

Auxiliary power units (APUs) are compact gas turbine engines integrated into aircraft, primarily designed to supply and electrical power for starting the main propulsion engines, as well as supporting onboard systems during ground operations. These units operate independently of the main engines, enabling self-sufficient engine starts without reliance on external ground equipment in many scenarios. APUs are typically mounted in the tail cone of the for balance and accessibility, functioning as a dedicated power source that enhances operational flexibility for commercial and . In terms of design, feature a multi-stage , , and configuration, often employing two-shaft architectures for efficient power extraction. For instance, the 131-9A APU incorporates a two-stage to extend operational life and includes a single starter/generator that utilizes electrical power from batteries or ground sources to initiate rotation. This electric starting mechanism drives the to a self-sustaining speed, after which the APU generates at pressures around 30-50 psi suitable for main engine pneumatic starting. Variants like the PW980 for employ similar two-shaft gas turbine designs, optimizing for both pneumatic output and electrical generation up to 120 kVA. Electrical APU models, such as the APS5000, prioritize shaft power over , directly coupling to generators for all-electric starting systems on like the 787. The starting sequence begins with activating the APU using onboard batteries, which power the electric starter to accelerate the to ignition speed, typically within 30-60 seconds. Once stabilized, the APU's is ducted to the main 's air turbine starter, spinning the engine to a light-off speed where ignition occurs, followed by to idle. This pneumatic cross-feed process ensures controlled starts, with the APU maintaining stable pressure throughout. In electrical variants, the APU instead supplies shaft power to motor the main engine directly, bypassing requirements. APUs also integrate with air-start systems by providing the necessary pneumatic source for cross-bleed starts between engines. APUs are standard on all modern airliners, including the equipped with the PW901A and the using the PW980, where they not only facilitate engine starts but also deliver electrical power for and ground air conditioning. Beyond starting, these units support environmental control systems (ECS) by supplying conditioned for and ventilation during turnaround times. Their multi-role capability reduces dependency on airport ground power units, streamlining operations at remote locations. Efficiency considerations include fuel consumption rates of approximately 100-200 lb/hr under typical ground loads, varying with size and environmental conditions, which underscores their role in minimizing overall fuel use when integrated with ECS for optimized airflow management. Modern designs like the 131-9A in high-efficiency mode further reduce burn by up to 13% through advanced diffuser and control software. Historically, evolved in the 1950s as compact gas turbines replacing less reliable ram air turbines for consistent ground power, with delivering its first unit in 1950 for applications, paving the way for widespread adoption in by the 1960s. Over 100,000 units have since been produced, with more than 36,000 in service across diverse fleets, reflecting iterative improvements in reliability and multifunctionality.

Internal combustion engine starters

Internal combustion engine starters utilize a small auxiliary engine, typically a or diesel unit rated at around 50 horsepower, that is mechanically geared to the main gas turbine shaft via reduction gears. This setup allows the auxiliary engine to be started either manually with a crank or electrically using a small battery-powered motor, providing a self-contained means to initiate of the main engine's . Such systems were developed as lightweight alternatives to early air turbine starters, with estimated weights around 85 pounds for the entire reciprocating unit. During operation, the auxiliary engine cranks the gas turbine to 10-15% of its normal operating speed, sufficient to generate adequate through the for stable once and ignition are introduced. At starter cutoff speed, the main engine accelerates under its own power, disengaging a to shut down the auxiliary engine and prevent overload. This ensures reliable ignition without dependence on external ground or subsystems. These starters found applications in early helicopters and some , particularly for remote operations or as backup systems, exemplified by their use in the Sikorsky S-55. They are rarely employed in contemporary designs, having been largely supplanted by more efficient auxiliary power units that integrate starting, electrical, and pneumatic functions. The primary advantage of starters lies in their operational independence from the aircraft's primary electrical or pneumatic systems, enabling starts in austere environments without ground support. However, their drawbacks include added weight from the engine, fuel supply, and gearing, as well as increased mechanical complexity and maintenance requirements compared to modern alternatives.

In-flight restart

In-flight restart of gas turbine engines is a critical procedure performed after a , where the process ceases, often due to fuel starvation, ingestion of foreign objects, or environmental factors like . This process relies on either windmilling the engine using relative airflow or sources to achieve self-sustaining , enabling relight at altitudes typically below 30,000 feet where air density supports sufficient speed. Regulations mandate that must demonstrate reliable in-flight restart capabilities within defined and altitude envelopes to ensure safe continuation of flight or diversion. The primary method, windmill restart, uses the aircraft's forward motion to drive airflow through the , spinning the and turbine sections without external assistance. Pilots initiate this by advancing the to idle, closing the fuel control valve to prevent , and descending or diving to increase airspeed to approximately 250 knots (IAS), which generates enough ram air pressure to accelerate the high-pressure spool (N2) to 15-20% of maximum speed. Once N2 stabilizes at this threshold, is reintroduced, and ignition is selected (often automatically via in modern ); the crew then monitors low-pressure spool (N1) acceleration, exhaust gas temperature (EGT), and flow for signs of light-off, which may take up to 30 seconds. If no relight occurs within this window, the is cut off again to ventilate the and avoid overtemperature. Assisted restarts supplement windmilling when available, using compressed air from an operational engine via cross-bleed, the (APU), or () to drive the engine starter. This method is particularly effective at lower altitudes below 200 (approximately feet), where starter torque overcomes higher drag in denser air, and is preferred if one engine remains running to minimize altitude loss during the procedure, which can exceed 5,000-15,000 feet depending on initial conditions. Key risks include hung starts, where ignition occurs but the engine fails to accelerate to RPM due to insufficient or fuel scheduling, potentially leading to excessive EGT and damage to hot-section components like blades. Hot starts from over-fueling can similarly exceed material limits, necessitating immediate fuel cutoff per Quick Reference Handbook (QRH) checklists, which outline sequenced actions to mitigate these without delaying the overall restart envelope. Success depends on factors such as altitude above 10,000 feet for adequate ram air but below maximum certified limits, airspeed within the manufacturer's envelope (e.g., holding or cruise speeds), and prompt response to limit altitude loss to under 1,500 feet in optimal scenarios. These procedures apply to commercial jet airliners like the and Airbus A320, as well as military fighters, where higher speeds (e.g., 450 knots) enable windmilling in combat scenarios. For Extended-range Twin-engine Operational Performance Standards (ETOPS) flights, reliable relight capability is essential for twin-engine operations over remote areas, ensuring diversion to an adequate airport within 180-240 minutes. A notable case is Flight 9 in 1982, where all four engines on a flamed out after ingesting at 37,000 feet; the crew descended to 13,500 feet, cleared the ash via windmilling, and successfully relit engines one by one, landing safely in . In contrast, piston-engine in-flight restarts are simpler, relying on windmilling without complex spool monitoring.

Starting other aircraft engines

Pulsejet starting

Pulsejet engines are categorized into valved and valveless types, each employing distinct starting mechanisms to initiate their intermittent cycles. Valved pulsejets, such as the used in the German during , rely on external acceleration to provide the necessary initial airflow, as they cannot self-start from standstill. These engines feature reed valves at the that open during the intake phase and close during , but starting requires to approximately 200 mph (320 km/h) via a steam-powered on an inclined ramp approximately 50 meters (164 ft) long, enabling ram air to enter the and sustain the pulsation once fuel is ignited. In contrast, valveless pulsejets use aerodynamic valving through the engine's geometry—a convergent-divergent tailpipe—to direct flow, allowing some designs to start without mechanical valves or high-speed launch; for example, certain resonant configurations employ or rocket assist to initiate the cycle. Starting methods for pulsejets generally involve establishing initial airflow to mimic forward motion, followed by ignition to trigger the oscillatory . On the ground, a blast of is directed into the to force atomization and initiate the pressure waves, often using a , , or high-pressure bursts at 60-90 psig, while (such as or ) is supplied via aspiration or injection. Ignition is typically achieved with a positioned in the , though historical or simplified setups may use a hot tube—a heated element to ignite the mixture—or a propagating flame from an external torch. For in-flight or scenarios, ram air from vehicle motion at speeds equivalent to 200-300 mph can replace the ground air supply, as seen in WWII applications where the V-1's pulsejet ignited automatically upon reaching operational airflow. Challenges in pulsejet starting stem primarily from the need for high initial airflow to overcome cold-start inertia, as the engine's intermittent combustion requires sufficient momentum to establish resonance without sustained low-speed operation. Cold starts are particularly difficult below 60°F, often necessitating fuel additives like 25% ether for better vaporization, and the process demands precise air-fuel ratios (approximately 12-13:1 by mass) to avoid flameout. These issues limited pulsejets to disposable or short-duration roles in WWII, such as the V-1, where reliability was secondary to simplicity and low cost. In modern applications, particularly for , pulsejets are started using electric blowers to provide the initial air blast, enabling hobbyists to ignite small-scale engines (e.g., 5-10 lb ) without catapults, though and inefficiency restrict their use to controlled environments. These systems do not support sustained operation at low speeds, as airflow must exceed a critical threshold to maintain pulsation. The underlying physics relies on , where the engine's pulsation frequency approximates the fundamental mode f≈c2Lf \approx \frac{c}{2L}, with cc as the and LL as the effective tube length, ensuring pressure waves reinforce combustion cycles. Like their counterparts, pulsejets demand external airflow initiation but achieve it through pulsed rather than continuous combustion.

Ramjet and scramjet starting

Ramjets are air-breathing engines that rely on the vehicle's forward motion to compress incoming air via , requiring an initial airflow speed of at least Mach 0.5 to generate sufficient pressure for and production. Unlike turbojets or engines, ramjets produce no static and cannot initiate operation from standstill, necessitating external to reach this threshold. This dependency stems from the absence of rotating stages, making auxiliary essential for startup. Common starting methods for ramjets include rocket boosters, turbine-assisted combined-cycle engines, or air-launch from carrier aircraft to achieve the required velocity. In integrated rocket ramjet systems, such as the supersonic cruise missile, a solid-propellant accelerates the vehicle to supersonic speeds (approximately Mach 2-3), after which it separates, allowing the liquid-fueled to ignite and sustain cruise at up to Mach 2.8. Air-launched configurations use the carrier's momentum or an onboard to provide initial , bypassing the need for ground-based internal starters. These approaches ensure seamless transition without onboard cranking mechanisms. Scramjets, or supersonic combustion ramjets, extend this principle to hypersonic regimes, demanding even higher initial speeds—typically Mach 4 or above, with optimal operation between Mach 5 and 6—for sustained supersonic airflow through the . Combustion occurs without decelerating the air to subsonic speeds, enabling efficiency at velocities where ramjets falter, but startup requires hypersonic boost from external sources like boosters or ground test facilities simulating high-Mach conditions. The X-43A demonstrator, for instance, was air-launched from a B-52 via a modified booster, achieving separation at near-Mach 7-10 before ignition, highlighting the reliance on such integrated launch vehicles without internal starting hardware. Key challenges in and starting include managing inlet shock waves to prevent unstarts—where shock trains propagate upstream, disrupting —and ensuring autoignition in high-speed, low-residence-time flows. Inlet designs employ oblique shocks and isolators to compress air while maintaining stability; for scramjets, flameholder struts can cause up to 25-30% blockage, leading to oscillations at 45-60 Hz if not mitigated, as observed in Mach 3.7 tests. ignition often requires additives like for rapid autoignition (within 0.001 seconds) in supersonic conditions, as alone struggles with stability; the X-43A used a 20% silane- mix injected at 4,500 psi (31 MPa) to overcome heat loss and risks during 10-11 second burns. These issues demand precise cowl actuation and CFD-optimized geometries to avoid buzz or extinction. Looking ahead, reusable and systems are poised for integration into hypersonic aircraft and launch vehicles, leveraging fueling for higher and cost-effective access compared to rockets. Optimization of inlets, isolators, and addresses unstart vulnerabilities, enabling sustained operations in reusable platforms like dual-mode scramjet-RLV concepts, which could drastically reduce mission expenses while supporting multi-mission strike and .

Modern and emerging technologies

Advanced electric starting systems

Advanced electric starting systems for gas turbine engines in modern leverage high-voltage electrical architectures to provide efficient, lightweight alternatives to traditional pneumatic methods. These systems typically employ 270 V DC motors or 400 Hz AC starter/generators, powered by lithium-ion batteries or the 's main electrical bus, enabling seamless integration with the overall more-electric (MEA) design. Auto-transformer units (ATRUs) play a key role by converting variable-frequency AC from engine generators or units (APUs) into stable 270 V DC for starting, ensuring high (typically 97-98%) and reduced in the power supply. A prominent example is the , which uses variable frequency starter/generators (VFSGs) connected directly to the engine gearbox for both starting and power generation. Each engine features two VFSGs rated at 235 V AC variable frequency, drawing power from the APU or ground sources to crank the engine to self-sustaining speed. This configuration eliminates dedicated pneumatic starters and associated ducts, contributing to significant overall system weight reductions compared to pneumatic setups by removing heavy tubing, valves, and heat exchangers. Additionally, these systems support efficient starting sequences that minimize ground time and fuel burn during turnaround. Operationally, advanced electric starters are managed by systems, which sequence the start process—including motor engagement, fuel introduction, and ignition—based on real-time sensor data for optimal performance and fault protection. FADEC monitors parameters like rotor speed (N2) and temperature to prevent issues such as hot starts, automatically adjusting power delivery for reliable ignition. Some designs incorporate , where the starter/generator operates in generator mode post-start to recapture kinetic energy and recharge lithium-ion batteries, enhancing energy efficiency during multiple starts or APU operations. Adoption of these systems accelerated in the and , driven by MEA initiatives like the 787's entry into service in 2011, which pioneered no-bleed electric architectures for engine starting and other subsystems. Ongoing advancements focus on improving , with recent starter/generators achieving up to 12.75 kW/kg through high-speed permanent magnet designs and advanced cooling, supporting lighter, more compact installations in next-generation . Despite these benefits, challenges persist in thermal management, as high-power densities generate significant heat during cranking, requiring liquid cooling or advanced heat sinks to maintain component reliability under extreme conditions. from high-voltage switching in VFSGs and ATRUs also demands robust shielding to prevent disruptions to and communication systems.

Hybrid-electric starting methods

Hybrid-electric starting methods integrate electric motors with auxiliary power units (APUs) or other hybrid power sources to initiate aircraft engine operation, offering a bridge between traditional pneumatic or pure electric systems and fully sustainable propulsion. These systems typically employ an electric motor to provide the initial rotational torque for engine cranking, followed by seamless transition to hybrid power from combined battery, supercapacitor, or small turbine sources, enhancing reliability and reducing dependency on ground support equipment. Key designs include electric motor-APU hybrids, such as Safran's eAPU60, which features a gearbox capable of driving starter generators at voltages like 28V DC or 270V AC while supporting combinations for multi-mode power delivery. Series hybrid configurations incorporate supercapacitors for high-power bursts during startup, storing from the main engine or APU to assist in rapid acceleration without excessive battery drain, as explored in optimization studies for propeller-driven hybrids. These designs build on advanced electric starting systems by adding regenerative capabilities, where excess from the engine is captured post-startup to recharge storage elements. In applications for next-generation sustainable aviation, hybrid-electric starting has been tested in past demonstrators like the (canceled in 2020), a modified that integrated a 2 MW alongside engines to explore decarbonization, including efficient ground and in-flight initiation sequences; current efforts continue through Airbus's ZEROe hydrogen-electric concepts, which incorporate hybrid starting for projected entry in the . Such systems can reduce overall fuel use by 10-20% in hybrid configurations through optimized starting and partial , particularly in short-haul operations where startup energy demands are significant. Operationally, the process begins with the spinning the engine to ignition speed, often using battery or power for the initial phase, before transitioning to hybrid mode where a small or APU sustains rotation until self-sustaining operation. This multi-stage approach minimizes peak power requirements and enables compatibility with variable-frequency electrical architectures. Market projections indicate robust growth, with the sector valued at approximately $2.92 billion in 2025 and expanding at a CAGR of over 33% through 2030, driven by demand for efficient starting technologies in electrified fleets. Advantages encompass silent ground operations due to the absence of pneumatic bleed air noise, providing quieter airport environments and reduced community impact. They also serve as emergency backups, with the APU or stored energy enabling in-flight restarts without external assistance, while energy recovery mechanisms—such as regenerative braking during deceleration—improve overall system efficiency by recycling up to 20% of startup energy in some prototypes. Development efforts are advanced through initiatives like the EU's Clean Sky program, which funds hybrid-electric propulsion research focusing on integrated starting solutions for radical aircraft configurations to achieve 20-30% emission reductions. Similarly, NASA's Electrified Aircraft Propulsion (EAP) program supports hybrid system maturation, including starter-generator technologies and flight tests as of 2025 for commercial viability by the mid-2030s.

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