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The first time an aircraft flew, little did anyone know that it would evolve into various shapes with varying working principles serving different purposes.
As such, a jet and a rocket have distinct functions, yet it is hard to differentiate between them due to the many different aircraft.
Both jets and rockets follow Newton’s Third Law of Motion; they eject high-velocity exhaust gasses backward to produce thrust in the forward direction, a.k.a. every action has an equal and opposite reaction. However, jets use air from the atmosphere to produce thrust, while rockets carry oxygen.
Apart from the fact that both jets and rockets fly and have similar working principles, they were introduced to carry out vastly different functions. While jets can operate in the atmosphere, rockets are primarily made to shoot off into space to do the 'harder' work, like launching satellites into orbit. We’ll later discuss the differences and functions of jets and rockets in detail.
As an aviation enthusiast with an undergrad degree in aerospace engineering, I’ll talk about how efficient jets and rockets are and compare them with their engines, safety, and speed.
Difference Between Jets and Rockets
A jet engine has an inlet, compressor, combustion chamber, turbine, and nozzle. Some jet engines also consist of fans known as turbofans. A jet engine takes in air through the inlet and compresses it in the compressor before injecting it with fuel in the combustion chamber. The hot air-fuel mixture then drives the turbine, and the exhaust gasses eject via the nozzle producing thrust.
On the other hand, a rocket engine consists of fuel, an oxidizer, pumps, a combustion chamber, and a supersonic nozzle. The oxidizer and fuel are linked using two pumps. The oxygen in the oxidizer and the fuel travel to the pumps and are mixed before they are blasted into the combustion chamber. The mixture is ignited, and the exhaust gasses jet off the nozzle.
Both jets and rockets use the same principle to produce thrust, but while a jet relies on air in the atmosphere to get its oxygen to burn, rocket engines carry their oxygen in the oxidizer. This is because rockets are mainly used to operate in space (vacuum) while jets stay in the Earth's atmosphere.
Another major difference between jets and rockets is their fuel-burning rate. A jet has a longer burning rate, such as Boeing 747 burns around 48,445 gallons of fuel in 12 hours. In contrast, a rocket, such as the Saturn V, burns 205,400 gallons of propellant within 3 minutes.
Are Jets Faster Than Rockets?
No. It’s the other way around: Rockets are faster than jets. A typical jet engine can fly up to a speed of 0.25 km/s, while a rocket needs to achieve a speed of 8 km/s to get into a low orbit around Earth.
The fastest supersonic jet aircraft is the SR-71 Blackbird, which flies at 3540 km/h at Mach 3.4. However, The Space Shuttle can speed up to 29,000 km/h, which is about 18,000 miles per hour. Rockets require greater speeds to break away from the Earth's gravitational pull. The minimum speed needed to break away from the effects of gravity is known as escape velocity.
A jet is slower than a rocket because a jet needs to compress the incoming air before sending it to the combustion chamber. At supersonic speeds, i.e., Mach 2.5 or above, compressing air via a compressor is no use. As a rocket doesn't require any incoming air and already has liquid oxygen in the oxidizer, it can travel way faster than jets.
Are Jet Engines More Efficient Than Rocket Engines?
There are two ways to determine whether jet engines are more efficient than rocket engines. The first metric of comparison is the “thrust-specific fuel consumption' which determines how fuel efficient an engine design is. It gives a number that tells us how much fuel is consumed per unit of thrust.
We know a rocket consumes ten times more fuel than a jet engine to produce the same thrust. According to this metric, a jet engine is far more efficient than a rocket engine.
The second comparison metric is the “Specific Impulse." It is inversely proportional to the thrust-specific fuel consumption and tells us how efficiently a reaction mass engine generates thrust. Specific impulse is defined as thrust produced per unit, the product of the mass flow rate of propellant, and the standard gravity:
The higher the specific impulse, the more it uses the amount of propellant efficiently. Jet engines don’t have to carry oxygen, so they have a higher specific impulse than rocket engines.
Rockets have a specific impulse value of 100 to 700 seconds, while jet engines have it in thousands. According to this metric, jet engines are more efficient than rockets.
Are Rockets Safer Than Jets?
There is more risk involved in space travel than in air travel. This is because jets have been around longer than rockets, and there's a degree of apprehension around rocket travel.
Around 530 people have journeyed to space from which about eighteen couldn't return. There have been various deaths due to different reasons:
- In 1971, a capsule decompressed caused three Russians to die from suffocation.
- In 1986, the Challenger shuttle exploded 71 seconds after initializing flight, killing seven people.
- In 2002, seven people died when the Columbia shuttle was on its way back to the Earth’s atmosphere.
Around three percent of 650 astronauts have died in space due to one accident or another. The total mortality rate is around 4%, meaning four astronauts die for every 100.
Traveling via a jet is considerably safer because a commercial jet has a fatality rate of 0.035 for every 100,000 miles traveled. In other words, it has a 0.0001% mortality rate, making it safer than rockets.
According to Professor Spiegelhalter, an expert in analysis and studying risks at Cambridge University, one would have to travel up to 7500 miles on a commercial jet before encountering a plane crash. There is a one-in-a-million chance of dying in a plane crash on a commercial jet compared to a light aircraft or rocket.
Why Are There No Rocket-Powered Airliners?
Using a rocket engine instead of a jet engine or a rocket engine in addition to jet engines on a plane would definitely increase the amount of thrust generated and, in turn, the plane's speed.
However, a rocket uses up its stored fuel and oxygen quickly to provide high amounts of thrust in the first few minutes of flight so it can launch into space. A rocket-powered airliner would function in the same way; it would initially travel at very high speeds, after which it would end up in a glide, which is not very efficient for a commercial airliner that has to travel long distances in less time. A glide also means the rocket-powered plane would require a longer runway during landing.
A rocket-powered airliner also consumes a lot of fuel and would be very heavy because it carries its own oxygen. In this case, it won't be able to carry as many passengers as a regular jet does.
Rocket fuel is also different from the fuel in jets; it is explosive and risky when you have so many passengers on board, unlike rockets.
Do Jet Engines Work in Space?
A jet engine can fly because of the air in the Earth’s atmosphere. Not only does it require incoming air to produce thrust, but a jet also needs air to produce lift.
If you have looked at an airplane's wings closely, you would have observed that the top portion of the wing is rounder than the bottom. This way, incoming air has a greater distance to travel on top of the wing than at the bottom. This increases the speed of the air at the top and thus reduces the pressure.
In fluid dynamics, we call this phenomenon Bernoulli's equation: the speed of a fluid is inversely proportional to its pressure.
The pressure difference produces an upward force on the wing known as lift. The pressure decreases with altitude, so jets can't operate after a certain height. If you keep going higher, you reach a certain point where there is no air, just a vacuum. Jets can't operate in a vacuum because there is no air to produce lift.
On the other hand, a rocket doesn't need air to produce lift. It only needs to generate a large amount of thrust initially to take off at a speed large enough to break free from the Earth's atmosphere.
Unlike a rocket engine that uses burnt fuel to produce thrust, a jet engine needs incoming air to compress, burn, and then accelerate it through a nozzle to produce thrust. It doesn't use burnt fuel as its reaction mass.
In a nutshell, rockets carry oxygen in the oxidizer to burn fuel and propel into space. In contrast, jet engines use air from the Earth's atmosphere to produce thrust and lift.
Jets vs. Rockets: Which One is Better?
Neither is better than the other. Both jets and rockets have different functions; a jet can only fly in the Earth’s atmosphere, while a rocket can only operate in space.
If you want to travel to the U.S from China, you better book an international flight on a commercial jet rather than hopping onto a Falcon 9, a partially reusable rocket first launched in June 2010 by SpaceX (although it won't be easy).
If you are an astronaut who has to go up to the International Space Station on a mission, you need to board a rocket to get up there, not a jet.
It depends on your flight requirements - terrestrial or extraterrestrial - to decide which is better for you.