How rocket engines works park 1 Thrust and Efficiency.
The F-1 rocket engine, five of which powered the first stage
of the Saturn V, was absolutely colossal. It used a gas generator to drive a
turbine which powered its fuel pumps. This turbine produced 55,000 horsepower,
equivalent to the max combined horsepower of 62 Porsche 918 supercars. This
power was used to pump a total of 40,000 gallons of fuel per minute which was
pushed into the combustion chamber at over 1000 psi or about 70 atmospheres of
pressure.
The controlled explosion in the combustion chamber reached a
temperature of almost 6,000 degrees Fahrenheit. Hot enough to boil iron. The
exhaust gas was accelerated through the12 foot wide nozzle to a speed of mach
7.5 and produced 1.5 million pounds of force.
Rocket engines are
some of the most sophisticated and powerful machines mankind has ever created.
In this series of videos we will be looking in depth into how these impressive
devices work. In this first part we’re talking about thrust and efficiency. A
rocket engine’s job is to create thrust. It’s really their only job. And to
create thrust all rocket engines use Newton’s third law of motion. This law
tells us that every action has an equal and opposite reaction.
So the action of the
engine accelerating the rocket fuel out the back of the rocket has the opposite
reaction of pushing the rocket forward. The amount of thrust a rocket engine
produces is equal to the speed of the exhaust multiplied by the mass flow rate
of the exhaust. And the mass flow rate of the exhaust is equal to the mass flow
rate of the fuel being fed into the engine. By the way mass flow rate just
means how much mass goes through a system in a certain amount of time. Most of
the time in these videos we’ll describe this using the unit kilograms per
second. If a rocket engine consumes 1000 kg of fuel per second and the exhaust velocity is 3000
m/s than it will produce 3 million newtons of thrust.
This equation shows
us that if we want a rocket engine to produce more thrust so we can lift bigger
rockets we have to either increase the amount of fuel it uses or increase the
exhaust velocity. Which brings us into the topic of rocket engine efficiency.
Just like with a car higher efficiency means being able to do more with less
fuel. In rocketry fuel efficiency isn’t given in miles per gallon, but in a
term called specific impulse which is often shortened to Isp. Specific impulse
is a measure of how much thrust an engine will produce by consuming a certain
amount of fuel. Which sounds a lot like exhaust velocity from the equation we
just looked at.
Specific impulse is
usually measured in seconds. This has become the standard world wide since the
second is a universal unit of time. It is possible to measure it in a unit of speed
such as m/s by multiplying it by 9.8 m/s/s. When measured this way engineers
usually call it effective exhaust velocity. Effective exhaust velocity isn’t
always the same as actual exhaust velocity. In engines where 100% of fuel is
fed into the combustion chamber effective exhaust velocity and actual exhaust
velocity are the same, but in many engines use a portion of the fuel for other
tasks such as running fuel pumps or actuating hydraulics. In these engines the
effective exhaust velocity can be significantly lower than actual exhaust
velocity. Specific impulse and effective exhaust velocity are the real measure
of fuel efficiency and can be compared between all types of rocket engines.
There are a lot of ways to increase efficiency in rocket engines. Fuel choice,
engine cycles, chamber pressure, and nozzle shape all affect it.
We’re going to take a
look at all of these topics and how they influence the design of rocket
engines, but first we need to get some more basics out of the way. In part two
we’ll be talking about flow and pressure. This has been Liam from Space Is Kind
Of Cool. Thanks.
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