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Ejection Seat Seminar report ’07
Almost since the first days of flight man has been concerned with the safe escape from an aircraft which was no longer flyable. Early escape equipment consisted of a recovery parachute only. As aircraft performance rapidly increased during World War II, it became necessary to assist the crewmen in gaining clear safe separation from the aircraft. This was accomplished with an ejector seat which was powered by a propellant driven catapult - the first use of a propulsive element in aircrew escape. . Since then, this collection of componentry has evolved through several generations into today's relatively complex systems which are highly dependent upon propulsive elements. Ejection seats are one of the most complex pieces of equipment on any aircraft, and some consist of thousands of parts. The purpose of the ejection seat is simple: To lift the pilot straight out of the aircraft to a safe distance, then deploy a parachute to allow the pilot to land safely on the ground.
2. A LITTLE HISTORY OF AIRCRAFT ESCAPE SYSTEM
The first operational use of a propulsive element to assist an aircrew to escape from an aircraft apparently occurred during World War II. It appears that the country to receive credit for the first operational system was Germany, as it is known that approximately 60 successful ejections were made from German aircraft during World War II. It is interesting to note, however, that the first aircraft ejection seat was designed and tested (successfully) with a dummy in 1910 by J. S. Zerbe in Los Angeles, California. This was one year before the first parachutist successfully, jumped from an aircraft. Another country involved in early ejection seat work was Sweden. Initial experiments were made by SAAB in 1942 using propellant powered seats. The first successful dummy in-flight ejection was on 8 January 1942. A successful live ejection was made on 29 July 1946.At the end of World War II both the British and Americans acquired German and Swedish ejection seats and data. This information and equipment added impetus to their efforts. The first live flight test in England occurred on 24 July 1946 when Mr. Bernard Lynch ejected from a Meteor III aircraft at 320 mph IAS at 8,000 feet, using a prototype propellant powered seat. On 17 August 1946 First Sergeant Larry Lambert ejected from a P61B at 300 mph IAS at 7,800 feet to become the first live in-flight US ejection test.
3. BASIC COMPONENTS
To understand how an ejection seat works, you must first be familiar with the basic components in any ejection system. Everything has to perform properly in a split second and in a specific sequence to save a pilot's life. If just one piece of critical equipment malfunctions, it could be fatal. Like any seat, the ejection seat's basic anatomy consists of the bucket, back and headrest. Everything else is built around these main components.
Here are key devices of an ejection seat:
This early propulsive element has been called a gun or catapult and, is in essence, a closed telescoping tube arrangement containing a propellant charge to forcibly extend the tubes, thereby imparting the necessary separation velocity to the "ejector seat" and its contents .The rocket is a propulsive device in the seat. The catapult remained as the initial booster to get the seat/man mass clear of the cockpit, while the rocket motor came on line, once clear of the cockpit, to act in a sustainer mode. The restraint system for the crue member is the protective devices to avoid injury while ejecting the seat. Harness straps can be tightened and body position can be adjusted to reduce injury from the forces encountered during ejection. Leg lifting devices and arm and leg restraints are provided to prevent limb flail injuries due to windblast forces. The limb restraints
do not require the crew to hook up as they enter the aircraft and do not restrict limb movement during normal flight operations. Parachute helped the pilot to land safely on ground.
4. EJECTION-SEAT TERMS
Bucket - This is the lower part of the ejection seat that contains the survival equipment.
Canopy - This is the clear cover that encapsulates the cockpit of some planes; it is often seen on military fighter jets.
Catapult - Most ejections are initiated with this ballistic cartridge.
Drogue parachute - This small parachute is deployed prior to the main parachute; it designed to slow the ejection seat after exiting the aircraft.
Egress system - This refers to the entire ejection system, including seat ejection, canopy jettisoning and emergency life-support equipment.
Environmental sensor - This is an electronic device that tracks the airspeed and altitude of the seat.
Face curtain - Attached to the top of some seats, pilots pull this curtain down to cover his or her face from debris. This curtain also holds the pilot's head still during ejection.
Recovery sequencer - This is the electronic device that controls the sequence of events during ejection.
Rocket catapult - This is a combination of a ballistic catapult and an under seat rocket unit.
Underseat rocket - Some seats have a rocket attached underneath to provide additional lift after the catapult lifts the crewmember out of the cockpit.
Vernier rocket - Attached to a gyroscope, this rocket is mounted to the bottom of the seat and controls the seat's pitch.
Zero-zero ejection - This is an ejection on the ground when the aircraft is at zero altitude and zero airspeed.
5. PHYSICS OF EJECTING
Ejecting from an airplane is a violent sequence of events that places the human body under an extreme amount of force. The primary factors involved in an aircraft ejection are the force and acceleration of the crewmember. To understand the forces in the ejection we want to know the following.
Frames of Reference - refers to the orientation of the object in relation to some reference. This way up/down, left/right, and front/back can be defined so others understand the position. In ejections, the following convention is used:
The primary vector acceleration axes are defined relative to the crewman's spinal axis (+Gz, positive spinal, eyeballs down; -Gz, negative spinal, eyeballs up; +Gx, positive fore-and-aft, eyeballs in; -Gx, negative fore-and-aft, eyeballs out; +Gy, positive lateral, eyeballs left; -Gy, negative lateral, eyeballs right).
Forces and G's - Newton's second law states that the force on a body is a function of the mass it contains and the acceleration it undergoes. It is represented in an equation as
Force = Mass * Acceleration [F=MA].
The acceleration is usually measured in terms of the G, or gravity, force equivalent. For each 32 feet/second^2 or 9.8 meter/second^2, one experiences 1 G of acceleration. A rocket assisted seat has a G rating of 5-10, while a pure catapult seat would be in the 10-20 G range.
G's and speed - To determine the speed of the seat at any point in time, one solves the Newton equation knowing the force applied and the mass of the seat/occupant system. The only other factors that are needed are the time for the force to be applied and the initial velocity present (if any). This all works together in the following equation:
Speed (final) = Acceleration * Time + Speed (initial)
[V(f) = AT + V(i)]
Initial velocity may involve the climb or sink rate of the aircraft, but most likely involves velocity resulting from a previous ejection force. For example, in most current seats, the ejection in a two step process where an explosive catapult removes the seat from the aircraft then a rocket sustainer gives final separation. So to solve this seat system, the Newton equation would be solved twice. Once with a V(i) of zero for the catapult and a second time where the initial velocity would be the speed at which the seat left the catapult.
Seat speed, aircraft speed, & aircraft size - All the above parameters, force, mass, time, and seat sequencing, need to be considered when the system is applied to an operating aircraft. A seat speed needs to be high enough to give a reasonable separation distance between the occupant and the aircraft. At the same time, the operating time needs to be short enough to move the person out of danger and allow all actions to take place. But as speed goes up and time goes down, the G force may become excessive. Therefore distance and time have to be balanced to provide a system that will operate swiftly, provide adequate separation,
and not impose an undue G load on the seat occupant. This relationship is given in the following equation:
Distance = 1/2 * Acceleration * Time^2 + Speed (initial) * Time
[D = 1/2AT^2 + V(i)T]
Pilot size and weight - No discussion has been made about the occupant of the seat. This is important since the mass of the pilot will ultimately have an effect on the acceleration. There are three things determining the mass to be ejected and two of them are essentially constant. These are seat mass, equipment mass, and pilot/occupant mass. The seat mass is composed of the seat itself, any pyrotechnics that eject with it, the survival kit, and the parachute.
These weights can vary greatly. For the Martin Baker H-7 seat as installed in the F-4 phantom they were as follows, seat = 193 pounds (88 kg), survival kit = 40 pounds (18 kg), and parachute = 20 pounds (9 kg). Looking at the McDonnell Douglas ACES II seat, the seat weight drops to about 150 pounds (68 kg) with the other factors remaining essentially constant. For seats used in some aircraft, weight is even less as the survival kit may be deleted since the aircraft is only used for flight test or over land where rescue is immediately available.
Equipment mass is what the pilot brings on board. The clothing worn by the occupant does not count, however the G suit, torso harness, life preserver, and helmet would. Depending on the aircraft and the occupant that may be 30-50 more pounds (14-22 kg) of weight.
The pilot mass is the largest variable since the seat mass will be determined by the aircraft and the equipment mass determined by the mission. When preparing for the addition of women pilots, the United States Air Force revised its pilot weight data and found a 5th percentile pilot to be a weight of 103 pounds (47 kg) and a 95th percentile pilot to be 205 pounds (93 kg). This difference in mass will produce a significant difference in the forces involved
Pilot position and seat actuation - As noted above, wind blast is a factor during an ejection. From the beginning, efforts were made to keep limbs and the head in place during the ejection event. The first step was using the inertia reel straps and having the pilot lock the harness prior to activation of the seat. As seats became more automatic, gas pressure from seat activation was used to retract and lock the reel. This helped to insure the hips and torso were tight against the seat.
The head and arms received attention next. Seats designed on both sides of the Atlantic settled on the face curtain as a means of seat activation and protection for the occupant. Hands grasped a set of handles mounted at the top and pulled down. The extended curtain helped hold thehead back and gave some wind blast protection. The arms were also tight against the body and, with muscles under tension, less likely to be out of position. A similar idea was the use of the center pull handle. By grasping in a two hand grip, the hands and arms are again inside the body and protected from wind blast. The final option is the side firing levers. Some designers feel that moving the hands up to a face curtain or into the center for the handle wastes valuable time. Therefore side firing handles
put the hands and arms in an anatomically stable position and also reduce reaction time when the need to eject arises.
The final appendages to consider are the legs and feet. Some seats, such as the Escapac and ACES II, attempt to passively control them through the use of high sides that keep the knees together and prevent the legs from abducting. Martin - Baker and Stencel have favored the use of a strap and garter assembly that attaches to the aircraft, passes through pulleys attached to the seat, and connects to the ankle of the occupant. As the seat moved up the rails, the cords tighten and pull the feet and legs into the bottom of the seat.
6. THE WORKING OR EJECTION SEQUENCES
A typical ejection sequence includes the following functions which occur generally in the order listed below:
6. 1. Seat activation: Seats are activated through different methods. Some have pull handles on the sides or in the middle of the seat. Others are activated when a crew member pulls a face curtain down to cover and protect his or her face.
6.2. Canopy or hatch jettison: Prior to the ejection system launching, the canopy has to be jettisoned to allow the crewmember to escape the cockpit. There are at least three ways that the canopy or ceiling of the airplane can be blown to allow the crewmember to escape:
6.3. Seat ejection/crewmember extraction: In modern ejection seat there is a two stage propulsion system for Seat ejection/ crewmember extraction. The catapult remained as the initial booster to get the seat/man mass clear of the cockpit, while the rocket motor came on line, once clear of the cockpit, to act in a sustainer mode. When combined into a single unit, this propulsive element was termed the rocket catapult.
6.4. Drogue parachute deployment: This small parachute is deployed prior to the main parachute; it designed to slow the ejection seat after exiting the aircraft. Once out of the plane, a drogue gun in the seat fires a metal slug that pulls the drogue parachute, out of the top of the chair. This slows the person's rate of descent and stabilizes the seat's altitude and trajectory. After a specified amount of time, an altitude sensor causes the drogue parachute to pull the main parachute from the pilot's chute pack. At this point, a seat-man-separator motor fires and the seat falls away from the crewmember. The person then falls back to Earth as with any parachute landing.
6.5. Seat man separation: Test flights and operational experience showed that some aspects could be safely automated. One of the main ones was the removal of the occupant from the seat. Multiple methods have been used ranging from simple to complex. The simplest is gravity. The lap belt and shoulder restraints are released and the seat will drop away of its own accord. In some cases this is assisted. The original Escapac seat used a rubber bladder system and a bottle of nitrogen. When
the time delay expired, a bell crank rotated pulling retaining pins from the straps and puncturing the bottle. High pressure gas inflated bladders in the bottom and back of the seat, pushing the occupant away.
There was a small probability of collision following the split. Therefore, in later versions of the seat, the gas assembly was replaced by a downward firing rocket at the top of the seat to insure a positive separation distance.
6.6. Recovery parachute deployment and inflation: The altitude of parachute deployment is an important issue. Above 10 000 feet there is insufficient oxygen in the air (reduced altitude thins the air) to maintain consciousness. If the parachute opens too high, the occupant may become hypoxic and pass out. To alleviate this problem, many seats added a barostatic sensor to the parachute assembly. If below the preferred height, chute deployment would occur without delay. If above the appropriate height, a delay was initiated until the altitude conditions were met. One would think that the delay would not be needed since supplemental oxygen could be included as a part of the seat. However, masks can be ripped off by wind blast. Therefore altitude restrictions and opening the chute at the correct height is still important.
6.7. Parachute Descent and Landing: This phase of the ejection sequence is critical to the outcome of the entire process of escape and yet 90 per cent of all non-fatal injuries associated with escape occur during landing. Although the techniques of landing by parachute are easily taught and simulated by jumps from training towers, the incidence of sprained or fractured ankles is estimated to be 50 per thousand descents . The correct
procedures for parachute landing are taught aircrew during several phases of their training.
7. THE ACES II EJECTION SEAT
The Advanced Concept Ejection Seat (ACES) was designed to be rugged and lightweight compared to earlier systems. It also was designed to be easy to maintain and updatable.
It includes the following features:
The ACES II is a third-generation seat, capable of ejecting a pilot from zero-zero conditions up to maximum altitude and airspeeds in the 250 knots (288 mph / 463 kph) range. The peak catapult acceleration is about 12gz. The ACES II has three main operating modes, one each for the low speed/low altitude, medium speed, and high speed/high altitude.
Deployment is delayed by the sequencer until the seat-man package reaches either Mode 2, or Mode 1 conditions, whichever comes first.
Seat modes are selected by the sequencer based on atmospheric conditions, and the modes vary depending on differences in the conditions such as apparent airspeed and apparent altitude.
7.1 Recovery Sequencing Subsystem
Seat functions are normally activated by the Recovery Sequencing Subsystem which consists of the environmental sensing unit , and the recovery sequencing unit, an electronic box located inside the seat rear on the right hand side. The environmental sensing unit consists of two altitude compensated dynamic pressure transducers, and two static pressure
transducers. The dynamic pressure sensor (pitot tubes) are located on or near the headrest and read the air pressure as the seat exits the aircraft. The pressure differential between them and the ambient (static) sensors behind the seat is compared by the recovery sequencing unit to determine what operating mode the sequencer should select.
7.2 Ejection control handles
Firing of the seat is normally by pulling one of the ejection control handles mounted on the seat bucket sides. (On ACES seats fitted to F-16s and F-22s the ejection control handle is located in the center of the front of the seat bucket) The side pull handles are mechanically linked so that raising one will lift the other as well. Raising the handles actuates a pair of initiators via mechanical linkages.
7.3 Stapac Package
One particularly unique feature to the ACES II is the STAPAC package. STAPAC is a vernier rocket motor mounted under the seat near the rear. It is mounted on a tilt system controlled by a basic pitch-rate gyro system. This system is designed to help solve one of the great problems inherent to ejection seat systems. Center of mass/Center of gravity is extremely important in terms of keeping the thrust of the booster rocket from inducing a tumble. Rocket nozzles for the main boost of a seat are aligned to provide thrust through the nominal center of gravity of the seat-man package. The STAPAC provides a counter force to prevent extreme pitching in cases where the CG is off by up to +2 inches. The yellow flag is a safety pin preventing accidental firing of the STAPAC.
7.4 Survival kit
Another unusual feature is related to the survival kit. In most ejection seats the survival kit is a rigid fiberglass box that makes up the seat inside the seat bucket. The ACES II survival kit is a soft pack that is stored under a fiberglass seat lid that is hinged at the front. When the pilot separates from the seat, the straps that connect him to the survival kit lift the seat lid up and forward. The seat kit then slips free from the rear end.
7.5 Inertia Reel Harness Assembly
The Inertia Reel Harness Assembly is located in the center of the seat back just below the headrest. The inertia reel fulfills two functions: (1) it acts like the shoulder belt in a car, restraining the pilot against a 2gx forward (-x) motion. (2) upon ejection, it retracts the pilot to an upright posture to minimize the possibility of spinal damage due to spinal misalignment upon catapult ignition. On the left side of the seat bucket there is a handle which allows the crew member to manually lock the reel prior to intense maneuvers or landing to prevent possible injuries.
7.6 Drogue System
The Drogue System consists of a hemisflo chute, a small extraction chute, and the Drogue Mortar. The drogue mortar is fired in Mode 2 and Mode 3 to slow and stabilize the seat-man package. This is intended to prevent or limit the injuries to the crewmember as he/she is exposed to the windblast after exiting the aircraft. The mortar fires a 1.2 Lb slug of metal that draws the extraction chute out which by means of a lanyard deploys the drogue chute. The extraction chute is packed in a small wedge-shaped
container on the upper left rear of the seat covered with metalized fabric. The lanyard is also covered in the metalized fabric. The drogue mortar is below this, and the drogue is packed in the metal covered box below this. The lid to the drogue is retained by a small plunger unit that is held in place by machining on the slug and released when the mortar fires.
7.7 Safety Lever
The seat is made safe by means of a Safety Lever on the left side of the seat bucket which prevents the seat from being fired when the lever is in the up/forward position. When it is down/back flat against the side of the bucket, it allows the seat to be fired.
7.8 Emergency Manual Chute Handle
The Emergency Manual Chute Handle is located on the right hand side of the seat bucket, and functions to fire the main chute mortar and initiate seat separation in case of failure of the electronic sequencer. Unlike other seats, the manual chute handle is inhibited in the aircraft and prevents the systems from functioning while the seat is still in the rails. In the event of ground egress, the crewman would have to unstrap the two shoulder harness connections, the two seat kit connections and the lap belt prior to egressing the aircraft. Given the 0-0 capability of the seat, in any case requiring extremely rapid egress, ejection would be a viable alternative.
7.9 Emergency oxygen system
The emergency oxygen system consists of an oxygen bottle attached to the seat back, an automatic activation lanyard, and a manual pull ring. As the seat rises up the rails, the lanyard activates the oxygen bottle and allows the crewman access to oxygen as long as he is still connected to the seat. During an in-flight emergency that does not require ejection, the oxygen bottle provides breathable air for enough time to return the aircraft to 10000 feet or below where the atmosphere is thick enough for the pilot to breath.
The ejection seat has evolved into a complicated system with subsystems. Seat improvement has improved the odds of survival, and expanded boundary limits for successful ejection. The ability of the seat to monitor environmental factors has allowed better control inputs, improving seat stability. The incidence of ejection injuries is reduced by employing a complex acceleration profile. The profile is impulsive and of high amplitude at the beginning and end of the acceleration period, while relatively smooth and of low amplitude during the interposed major time segment.
The next generation of escape systems will use controllable propulsion systems to provide safe ejection over the expanded aircraft flight performance envelopes of advanced aircraft. Continued research will only enhance the capability of future ejection systems. Current research efforts are being directed toward solving the problems associated with high speed and high altitude ejections. So we can expect that in the future more advanced ejection seats will evolve which will be more safer and will save many valuable lives.