Pogo oscillation
Encyclopedia
Pogo oscillation is a potentially dangerous type of self-excited combustion oscillation
in liquid fuel rocket engine
s. This oscillation results in variations of thrust
from the engines, causing variations of acceleration on the rocket's structure, giving variations in fuel pressure and flow rate. Pogo places stress on the frame of the vehicle which can be severe. Although the term is frequently written POGO, it is not an acronym, but in fact a reference to the bouncing of a pogo stick
.
increases back pressure
against the fuel
coming into the engine, reducing engine pressure, causing more fuel to come in and increasing engine pressure again. Flexing of fuel pipes can also induce fluctuations in fuel pressure. If the cycle happens to match a resonance frequency
of the rocket then dangerous oscillation
s can occur through positive feedback
, which can in extreme cases tear the vehicle apart.
Another situation in which pogo oscillation will occur is when the engine is moving upward with fluctuating speed. Owing to inertia
, if the speed of the vehicle suddenly increases, the movement of the fuel inside the fuel tank tends to 'fall behind' and is forced into the turbopump, a situation somewhat similar to the slosh
of liquid inside a tanker. This creates excess pressure
to the turbopump and causes unintended excessive fuel to be delivered. This in turn creates excessive thrust and causes the vehicle to accelerate which leads to further increase in turbopump pressure and an unintended increase in fuel delivery. This can set up a vicious circle
, and can result in structural failure in the vehicle.
The most famous pogo oscillation was in the Saturn V
first stage, S-IC
, caused by the cruciform thrust structure. This structure was an "X" of two I-beams, with an engine on the end of each beam and the center engine at the intersection of the beams. The center of the cruciform was unsupported, so the central F-1
engine caused the structure to bend upwards. The pogo oscillation occurred when this structure sprang back, lengthening the center engine's fuel line below (which was mounted down the center of the cruciform), temporarily reducing the fuel flow and thus reducing thrust. At the other end of the oscillation, the fuel line was compressed, increasing fuel flow – causing a sinusoidal thrust oscillation during the first stage ascent.
engine of the second stage, S-II
, of the Apollo 13
lunar mission. Fortunately in this case the engine shut down before the oscillations could cause damage to the vehicle. Later events in this mission overshadowed the pogo problem. Pogo was also the cause of some of the serious problems experienced by the unmanned Apollo 6
test flight. One of the Soviet Union
's N1-L3 rocket test flights suffered pogo oscillations in the first stage on November 23, 1972. The launch vehicle reached initial engine cutoff, but exploded 107 seconds after liftoff and disintegrated. There are other cases during unmanned launches in the 50s and 60s where the pogo effect caused catastrophic launch failures.
However, modern vibration analysis methods can account for the pogo oscillation to test that it is far away from the vehicle's resonant frequencies. Suppression methods include damping mechanisms or bellows in propellant lines. The Space Shuttle Main Engine
s each have a damper in the LOX
line, but not in the hydrogen
fuel line.
Oscillation
Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...
in liquid fuel rocket engine
Rocket engine
A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
s. This oscillation results in variations of thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....
from the engines, causing variations of acceleration on the rocket's structure, giving variations in fuel pressure and flow rate. Pogo places stress on the frame of the vehicle which can be severe. Although the term is frequently written POGO, it is not an acronym, but in fact a reference to the bouncing of a pogo stick
Pogo stick
A pogo stick is a device for jumping off the ground in a standing position with the aid of a spring, used as a toy or exercise equipment. It consists of a pole with a handle at the top and footrests near the bottom, and a spring located somewhere along the pole...
.
Origin
In general, pogo oscillation occurs when a surge in engine pressurePressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
increases back pressure
Back pressure
Back pressure refers to the resistance to a moving fluid by obstructions or tight bends in the confinement vessel along which it is moving, such as piping or air vents, against its direction of flow....
against the fuel
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...
coming into the engine, reducing engine pressure, causing more fuel to come in and increasing engine pressure again. Flexing of fuel pipes can also induce fluctuations in fuel pressure. If the cycle happens to match a resonance frequency
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
of the rocket then dangerous oscillation
Oscillation
Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...
s can occur through positive feedback
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...
, which can in extreme cases tear the vehicle apart.
Another situation in which pogo oscillation will occur is when the engine is moving upward with fluctuating speed. Owing to inertia
Inertia
Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. It is proportional to an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to...
, if the speed of the vehicle suddenly increases, the movement of the fuel inside the fuel tank tends to 'fall behind' and is forced into the turbopump, a situation somewhat similar to the slosh
Slosh dynamics
In fluid dynamics, slosh refers to the movement of liquid inside another object . Strictly speaking, the liquid must have a free surface to constitute a slosh dynamics problem, where the dynamics of the liquid can interact with the container to alter the system dynamics significantly...
of liquid inside a tanker. This creates excess pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
to the turbopump and causes unintended excessive fuel to be delivered. This in turn creates excessive thrust and causes the vehicle to accelerate which leads to further increase in turbopump pressure and an unintended increase in fuel delivery. This can set up a vicious circle
Vicious Circle
Vicious Circle is an album released in 1994 by L.A. Guns. Most of the songs have Phil Lewis on lead vocals, but the track "Nothing Better to Do" features Kelly Nickels on lead vocals, and "Tarantula" is instrumental. MC Bones drums on several songs. Lewis and Bones also played together in the band...
, and can result in structural failure in the vehicle.
The most famous pogo oscillation was in the Saturn V
Saturn V
The Saturn V was an American human-rated expendable rocket used by NASA's Apollo and Skylab programs from 1967 until 1973. A multistage liquid-fueled launch vehicle, NASA launched 13 Saturn Vs from the Kennedy Space Center, Florida with no loss of crew or payload...
first stage, S-IC
S-IC
The S-IC was the first stage of the Saturn V rocket. The S-IC first stage was built by The Boeing Company. Like the first stages of most rockets, most of its mass of over two thousand metric tonnes at launch was propellant, in this case RP-1 rocket fuel and liquid oxygen oxidizer...
, caused by the cruciform thrust structure. This structure was an "X" of two I-beams, with an engine on the end of each beam and the center engine at the intersection of the beams. The center of the cruciform was unsupported, so the central F-1
F-1 (rocket engine)
The F-1 is a rocket engine developed by Rocketdyne and used in the Saturn V. Five F-1 engines were used in the S-IC first stage of each Saturn V, which served as the main launch vehicle in the Apollo program. The F-1 is still the most powerful single-chamber liquid-fueled rocket engine ever...
engine caused the structure to bend upwards. The pogo oscillation occurred when this structure sprang back, lengthening the center engine's fuel line below (which was mounted down the center of the cruciform), temporarily reducing the fuel flow and thus reducing thrust. At the other end of the oscillation, the fuel line was compressed, increasing fuel flow – causing a sinusoidal thrust oscillation during the first stage ascent.
Hazard
If the oscillation is left unchecked, failures can result. One case occurred in the middle J-2J-2 (rocket engine)
Rocketdyne's J-2 rocket engine was a major component of the Saturn V rocket used in the Apollo program to send men to the Moon. Five J-2 engines were used on the S-II second stage, and one J-2 was used on the S-IVB third stage. The S-IVB was also used as the second stage of the smaller Saturn IB...
engine of the second stage, S-II
S-II
The S-II was the second stage of the Saturn V rocket. It was built by North American Aviation. Using liquid hydrogen and liquid oxygen it had five J-2 engines in a cross pattern...
, of the Apollo 13
Apollo 13
Apollo 13 was the seventh manned mission in the American Apollo space program and the third intended to land on the Moon. The craft was launched on April 11, 1970, at 13:13 CST. The landing was aborted after an oxygen tank exploded two days later, crippling the service module upon which the Command...
lunar mission. Fortunately in this case the engine shut down before the oscillations could cause damage to the vehicle. Later events in this mission overshadowed the pogo problem. Pogo was also the cause of some of the serious problems experienced by the unmanned Apollo 6
Apollo 6
Apollo 6, launched on April 4, 1968, was the Apollo program's second and last A type mission—unmanned test flight of its Saturn V launch vehicle. It was intended to demonstrate full lunar injection capability of the Saturn V, and the capability of the Command Module's heat shield to withstand a...
test flight. One of the Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....
's N1-L3 rocket test flights suffered pogo oscillations in the first stage on November 23, 1972. The launch vehicle reached initial engine cutoff, but exploded 107 seconds after liftoff and disintegrated. There are other cases during unmanned launches in the 50s and 60s where the pogo effect caused catastrophic launch failures.
However, modern vibration analysis methods can account for the pogo oscillation to test that it is far away from the vehicle's resonant frequencies. Suppression methods include damping mechanisms or bellows in propellant lines. The Space Shuttle Main Engine
Space Shuttle main engine
The RS-25, otherwise known as the Space Shuttle Main Engine , is a reusable liquid-fuel rocket engine built by Pratt & Whitney Rocketdyne for the Space Shuttle, running on liquid hydrogen and oxygen. Each Space Shuttle was propelled by three SSMEs mated to one powerhead...
s each have a damper in the LOX
Lox
Lox is salmon fillet that has been cured. In its most popular form, it is thinly sliced—less than in thickness—and, typically, served on a bagel, often with cream cheese, onion, tomato, cucumber and capers...
line, but not in the hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
fuel line.