Physics

The Difference between a Rocket and a Jet



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During the first half of last century a great deal of experimentation into revolutionary methods of propulsion became a duopoly between rockets and jets, both vying for dominance in the pre-WWII years. The Chinese in 1232AD used rockets as a weapon of war against invading Mongols that consisted of a hollow tube of gunpowder to shoot arrows of fire into advancing hoards however the German V2 became the first true rocket of war capable of thundering at supersonic velocity up to 60 statute miles above the planet before reigning down on an unsuspecting Great Brittain with absolutely no warning or even time to sound a warning.

While perhaps not a true rocket as we would know it today, the first experimentation with rockets began as far back as 100BC where a Greek inventor experimenting with the mechanical interaction between heat and water developed a primitive steam powered rocket. Spherical in shape with two L-shaped vents on opposite sides this device hung over a fire to heat the water inside and as pressure began to build the cylinder began rotating. In contrast jet propulsion did not become a reality until the 1930's where British mechanical engineer Sir Frank Whittle and German Hans von Ohian developed their own jet engines. The significance of this invention was not realised until close to the end of WWII with the German Messerschmitt 262 became operational albeit not soon enough to make a difference to the German side.

Rocket and jet propulsion do have some common characteristics based on the physics of motion but this is about the limit of similarities. Put simply a defining difference between rocket and jet propulsion is the method through which propulsion is achieved. Rockets typically rely on a controlled chemical reaction that, while capable of producing significantly more thrust than a jet, is relatively short lived and requires a great deal more fuel' than a jet propulsion system.

Rocket fuel is unstable and highly dangerous in comparison to the aviation grades of hydrocarbon fuels. German scientists developed and tested a rocket-powered Messerschmitt ME163 (Komet) which used a mix of hydrazine hydrate methanol with a high grade hydrogen peroxide hey called T-Stoff. Mixed in the burn chamber these two fuels generated terrific thrust that hurled the ME163 to a new world speed record of more than 1 000kph (623mph) in 1941, and 1130kmh (702mph) in 1944 war of course negated any attempt of recognition for this incredible feat. The combination of C-Stoff and T-Stoff resulted in many explosions, fatalities and disasters moreover the short duration of less than eight minutes limited the overall operational effectiveness. Allied pilots unfortunate enough to encounter a fully armed Komet were hopelessness outclassed however with few pilots skilful enough to effectively engage allied aircraft the ME163 had little effect.

Jet propulsion is achieved using a gas turbine where air is sucked into the engine using a high-speed fan, a compressor increases pressure which in turn is sprayed with a fuel ignited with some form of electric spark that discharges through the exhaust. The burning gasses expand rapidly to generate thrust. Limitations in the science of metallurgy in the early days of get propulsion meant the fast moving components lasted around 10 hours. When the ME-262 became operational, allied pilots again found themselves literally defenceless as these marauding predators streaked through formations faster than escort fighters could deal with. Few were shot down however with Germany exhausted of trained pilots and fuel as well as a tactical mistake in developing the 262 as a bomber instead of a fighter again limited the effect.

Modern rocket fuel is typically powder-based solid forms or liquid fuels combining liquid oxygen (LOX) with liquid hydrogen for the fast chemical reactions needed to produce thrust. Jet engines use a kerosene-based Jet-A or AVTUR fuel and compared to the short duration of rocket-propelled flight a jet engine is superior. We could close with this final and decisive difference in characteristics between jet propulsion and rocket propulsion: A rocket engine will function in the vacuum of space while a jet engine will not.

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