Hello there, I decided to write this after I prepared a short ten minute segment on rocket technology for a podcast. In this article, I will be writing about the basic physics that underlie rocket propulsion and will also elaborate upon some of the basic types of rocket engines.
The Physics of Rocket Propulsion
The physics of rocket propulsion are actually quite simple and are based on Newton’s laws of motion, the third one in particular. That one states that for every action, there is an equal and opposite reaction in a kinematic context. Rocket engines are propelled by means of a reaction to rapidly moving, large volume jet of gas in the opposite direction. This jet of gas is produced by the chemical process of combustion in most cases.
This is in fact very similar to how jet engines work, but the difference is that jet engines can only work when there is an external oxygen supply, while rockets can work anywhere due to having an onboard supply of oxidiser.
The combination of oxidiser and fuel and the combustion that follows is the main mechanism by which rocket engines produce that all-important propulsive jet. Some of the arrangements by which rocket engines achieve this will be discussed later.
A brief history of Rocketry.
Now while Newton was the first one to place the law of action and reaction in a mathematically rigorous framework I think it is important to note that the idea of jets of gaseous substances as means of propulsion goes back a long long way. I, personally, see Hero of Alexandria’s rotating steam powered device, called the Aelophile, which used jets of steam to drive a solid body around an axis.
It may be possible to procure a classroom model of this device and see it in action.
The first true rockets (as in containing their own oxidizer) were built by the Chinese before the tenth century BC and were powered by Gunpowder. There are reports of rockets being used in combat not only by the Chinese but also the Mongol hordes of Ghengis Khan who took the technology with them on their conquests.
Rocket technology reached Europe in the middle ages thanks to the Mongolians and later the Ottoman Empire. It was incorporated into artillery practises of the era. Iron-cased rockets were first successfully employed by the kingdom of Mysore against the soldiers of the English East India Company in the Anglo-Mysore Wars of the late Seventeen Nineties. This kind of rocket was further developed by William Congreve, the Comptoller of the Royal Arsenal in London; he developed long stabilizing tail-poles to improve accuracy and developed stronger rockets and new propellant mixtures to improve range.
Congreve Rockets look like this.
William Hale came up with the idea of slightly vectored thrust which improved control of direction further. This negated the need for a tail-pole and thus contributed to rockets being lighter and more aerodynamic.
As with so many other things in science, science fiction was where the idea of space travel using rockets first began to take root; Jules Verne’s classic novel “From the Earth to the Moon” comes to mind.
Scientists like Konstantin Tsiolkovsky in Russia/the USSR, Herman Oberth in Germany and Robert Goddard in the USA worked to provide the science of racketeering a rigorous footing, eventually it was Robert Goddard who built a very successful rocket (for the standards of its day, in any case) based on design principles such as a dedicated combustion chamber for solid fuelled rockets and the use of an asymmetrical hourglass shaped nozzle to accelerate propulsive gases to beyond supersonic speed. He and Tsiolkovsky also speculated about the idea of multi-stage rockets, which basically have multiple, self-contained rocket engines.
To go on a little tangent here, multistage rockets enable range to be extended and the ability to discard spent units make it progressively easier to accelerate the payload to ever increasing velocities. This is critical in achieving the velocity required to escape the Earth’s gravity, which eventually made space exploration using rockets possible.
Robert Goddard launched the world’s first liquid fuelled rocket in 1926, several companies around the world started to dabble in rocketry and there was some significant activity until the second world war broke out, including advances in the design of various engine components and methods to implement the principles of rocketry.
The photograph above is of Goddard and his liquid fuelled rocket.
In World War II, advances were made in the military deployment of rocket technology, including large unguided rockets like the Soviet Katyusha and sophisticated guided missiles like the German V-Two. Messerschmitt also built a rocket plane called the Me-163 Komet, which by all accounts was a pretty unsafe machine to handle, given unstable propellant components which had a tendency to explode.
Following the war, rocket development, while still being focussed on the military use of rockets, found offshoots in space exploration programmes, the Redstone rocket programme in the USA and the R7 derivatives of the V2 in the USSR laid the foundations for propelling objects into space. The solving of the problems of atmospheric re-entry made space flight a reality, finally.
The space age had arrived.
Types of Rocket Engines – A Brief Introduction
Right, as mentioned earlier, there are several extant ways by which rocket propulsion may be achieved, and I am now going to try and offer very brief explanations of the defining features of each.
Solid-propellant rockets use a solid state combination of fuel and oxidizer called the grain, this fuel does not spontaneously combust and therefore must be ignited by other means, such as a small powder charge. Obviously it is difficult to stop and start the combustion of solid propellants when required and they also produce less thrust than other types of rocket engine and are therefore not as efficient, however, they can be stored for long periods of time and without too many safety risks.
Liquid propellant rockets bring together oxidizer and fuel in liquid form, either through the use of pumps or using pressurized gases. Some fuel-oxidizer mixtures ignite spontaneously when formed, and rockets that use these kinds of fuels don’t need a dedicated ignition system, if one is needed, however, electric sparks or a small amount of solid propellant can be used. These rockets offer more thrust and control, at the cost of increased rocket complexity and storage concerns.
Electric rockets use electricity to produce thrust, they are not as powerful as the other two types of rocket engines that have been described yet and therefore, as far as I’m aware, is restricted to propulsion after a spacecraft has already managed to escape the earth’s gravity. These use things like ions, jets of plasma and the control of electric and magnetic fields to accelerate gas that is conducive to manipulation by such fields.
One other kind of rocket that’s been envisaged uses heat from a nuclear reactor to heat liquid hydrogen to extremely high temperatures, producing a rapidly expanding gas jet that can produce a lot of thrust.
I guess that is it for this article, I hope you enjoyed reading it. You could always peruse wikipedia or Encyclopaedia Britannica for more information, it is quite an addictive thing to read about 😉
That is it from me.
-Ankur ‘Exploreable’ Chakravarthy