The Aircraft Carrier Catapult: Flinging a Jet to Takeoff Speed in Two Seconds

Why a carrier needs a catapult at all

Takeoff is a work-energy problem. To fly, a jet must reach a speed where its wings generate enough lift to carry its weight — on the order of 70 m/s (~140 knots) for a fully loaded strike fighter. On land the aircraft simply accelerates down a two-kilometer runway on its own engines. A supercarrier's flight deck offers barely 90 meters of catapult track, and jet engines alone cannot bridge that gap.

The work-energy theorem sets the terms: the launch force F acting over stroke d must supply the kinetic energy, F·d = ½mv². Rearranged, the average acceleration needed is a = v²/(2d). For v ≈ 72 m/s and d ≈ 90 m, that is ~29 m/s², about 3 g, sustained for ~2.5 seconds. A 30-tonne jet therefore needs an external push delivering on the order of 78 megajoules of kinetic energy in that stroke — and the piston, shuttle, and tow gear absorb more still. That is the catapult's job.

Carriers cheat the math further by steaming into the wind. A ship making ~30 knots into a natural breeze adds wind-over-deck, so a jet that needs ~140 knots of airspeed to fly can leave the track at only ~110–120 knots relative to the deck, trimming how much the catapult itself must supply.

The steam catapult: a piston in a slotted cylinder

The classic launch engine is a giant single-shot steam cylinder. Steam from the ship's boilers or reactor steam plant is bled into a wet accumulator (the "receiver") and held at up to roughly 500–520 psi (~35 bar), saturated near 240°C. Under the flight deck run two parallel slotted cylinders, each perhaps 90–100 m long, each holding a piston. On the C-13-1 catapult of the Nimitz class the power stroke is on the order of 300 feet.

When the catapult officer fires, a large quick-acting launch valve dumps the accumulator's steam behind the pistons. Pressure acts on the piston faces and drives them down the track. Each piston connects, through the slot in the top of the cylinder, to a shuttle that rides in the deck. The engineering trick is sealing that slot against high-pressure steam while a blade passes through it: a continuous stainless-steel sealing strip lies over the slot and is momentarily lifted by the piston's connecting element as it sweeps past, then reseals behind it — a flexible, self-closing seal running the length of the cylinder.

Above deck, the shuttle grabs the aircraft. Modern jets use nose-gear tow: a launch bar on the nose landing gear drops into the shuttle, while a holdback fitting at the rear of the nose gear ties the aircraft to the deck. The pilot runs the engines to full power; the holdback keeps the jet stationary until the catapult builds tension. A repeatable-release holdback bar (or, historically, a calibrated frangible link) lets go at a precise preset tension, so the aircraft breaks free only when the catapult is truly driving it — not before, not late.

Governing numbers: force, stroke, and the shape of the shove

Everything hinges on F·d = ½mv². With the stroke d fixed by the deck, the required average force scales directly with launch mass and with the square of end speed. That is why the catapult must be tuned to each aircraft: the catapult officer sets the steam pressure (or, on EMALS, the commanded force) from the jet's weight and the wind-over-deck, using a launch bulletin. Too much energy overstresses the airframe; too little is lethal.

The shape of the acceleration matters as much as its size. For a fixed end speed and stroke, a perfectly constant force gives the lowest possible peak — exactly a = v²/(2d) ≈ 3 g. Steam cannot hold that: as the piston travels and the steam volume expands, pressure and force fall, and the launch valve can only partly compensate. The result is a peak-to-average acceleration ratio around 1.25–2, so the airframe's tow point sees transient loads well above the 3 g average. Those peaks drive structural fatigue and set limits on the lightest and heaviest aircraft a steam catapult can safely fling.

The steam catapult is also thermodynamically profligate — only around 5% of the steam's energy ends up as aircraft kinetic energy; a single launch consumes several hundred kilograms of steam, with the rest lost to condensation, friction, and the water brake. It is heavy, water-hungry, and maintenance-intensive, but for seventy years it was the only system rugged enough to do the job at sea.

Stopping the piston: the water brake

At the end of the stroke the aircraft flies off — but the piston, shuttle, and their fittings are still hurtling forward at ~140 knots and carry a large slug of kinetic energy that must be killed in a few feet. That is the job of the water brake. A tapered spear (plunger) on the front of the piston rams into a water-filled cylinder at the forward end of the track. As the spear drives in, it forces water out through a shrinking annular gap; the tighter the gap becomes, the harder the water resists, producing a rapidly rising retarding force that brings the piston from full speed to a stop in a very short distance.

This is a fluid-dynamic energy sink: the ordered kinetic energy of the piston is converted into turbulent, viscous water motion and heat. It is punishing hardware — a violent, repeated hydraulic hammer — and the spear, water-brake cylinder, and seals are among the most fatigue-loaded parts of the whole machine. Once the piston is stopped, a retract (grab) system hauls the shuttle back to the starting position, the sealing strip reseats, and the catapult reloads steam for the next shot, typically on the order of a minute later.

EMALS: replacing steam with a linear motor

The Gerald R. Ford class (CVN-78) replaces steam with the Electromagnetic Aircraft Launch System (EMALS), built by General Atomics. Its heart is a linear induction motor (LIM) — essentially an electric induction motor unrolled flat. A long stator along the track carries polyphase windings; energizing them in sequence produces a traveling magnetic wave that sweeps down the track. Attached to the shuttle is a conductive carriage (the armature); the moving field induces currents in it, and the interaction of those induced currents with the field — the Lorentz force — drags the carriage forward. The force depends on the slip between the wave speed and the carriage speed, and only the stator segment beneath the carriage is energized at any instant, which keeps losses down.

Because the drive is electrical, the acceleration can be commanded and measured in a closed loop. EMALS holds a nearly flat force profile, so its peak-to-average acceleration ratio is about 1.05 — far gentler than steam's. For the same end speed it subjects the airframe to lower peak loads, reducing fatigue and extending service life, and it can be dialed down for a light drone or up for a heavy jet with equal precision.

The catch is power: the LIM needs tens of megawatts for a couple of seconds, which the ship's grid cannot supply directly. EMALS therefore uses a pulsed-power energy store — four rotating disk alternators (flywheels), each storing on the order of 121 MJ at up to ~6,400 rpm. They are spun up between launches over ~45 seconds and then discharged in 2–3 seconds through power-conditioning electronics that synthesize the traveling wave. The system can deliver up to ~122 MJ to the aircraft per launch at peak powers on the order of tens of megawatts, at overall efficiency near 60% — an order of magnitude better than steam.

Trade-offs, failure modes, and history

The dominant fear is the "cold cat shot" (soft launch): if steam pressure is set too low, the accumulator leaks, or the weight is entered wrong, the jet leaves the deck below flying speed and settles into the sea ahead of the ship — a documented, sometimes fatal failure. This is why holdback release, launch-valve timing, and the weight-and-wind computation are treated with such rigor. Other steam-side failure modes include holdback fittings that release early or late, erosion and wear of the sealing strip under repeated steam blasts, and fatigue cracking around the water brake.

EMALS trades those for electrical and control complexity. On the lead ship, early reliability fell short of requirement — the U.S. Director of Operational Test & Evaluation reported a mean number of launches between critical failures far below the specification, and the shared energy-storage and power-conversion groups meant one catapult could not be repaired while the others kept launching. Maturing that reliability has been a central engineering challenge of the class.

The steam catapult itself is a British invention: Royal Navy engineer Commander Colin C. Mitchell developed the slotted-cylinder steam catapult and proved it aboard HMS Perseus in 1950–51, after which the U.S. Navy adopted it as the C-11 (first U.S. steam launch, 1954), the C-7, and later the C-13/C-13-1 designs that launched carrier jets for two generations. Throughout, the pilot's role during the stroke is almost nothing: hands are held clear (grabbing the "towel rack") so a startle reflex cannot disturb the controls, the throttle is held at the detent, and the ~3–4 g forward jolt (+Gx, "eyeballs-in") is simply ridden out until the jet is flying and control returns. The catapult, steam or electromagnetic, does the work in the two seconds it has.

Steam catapult (C-13-1, Nimitz class) vs. electromagnetic catapult (EMALS, Ford class)

Attribute	Steam catapult (C-13-1)	EMALS (Ford class)
Working principle	Steam pressure drives a piston in a slotted cylinder	Linear induction motor; traveling magnetic wave drags a carriage
Energy store	Wet steam accumulator (up to ~500–520 psi) fed by boilers/reactor	Four rotating disk alternators (flywheels), ~121 MJ each
Acceleration profile	Force falls as steam expands; peak-to-average ~1.25–2	Commanded, nearly flat; peak-to-average ~1.05
Airframe stress	Higher peak tow loads and fatigue	Gentler, tunable — extends airframe life
Launch weight range	Heavy manned jets; poor at very light loads	Light UAVs up to ~100,000 lb aircraft, all tunable
Efficiency / crew	~5% thermodynamic; steam, water, many operators	~60% electrical; smaller crew, more electronics