It is rocket science, but I’m trying to make it a bit easier to understand (for my own sake as well as yours!) Here’s my attempt to boil it down to the basics with a handy recipe for a rocket.
1x Newton’s 3rd Law (every action has an equal and opposite reaction)
1x Combustion chamber
Fuel (enough to fill chosen container)
Optional extras: Fins, Launch pad/Launch Tower, Parachutes
1) Choose a container, preferably something aerodynamic, (long skinny tubes works well for this). Ensure container is able to hold pressure and heat – things will heat up later.
2) Pick your fuel type. You’re looking for something that burns rapidly. There are two options when thinking about selecting your fuel. Fuels that release heat when they break-up, or fuels that use two different substances which release heat when they combine. (Technically these are known as mono-propellant and bi-propellant fuels respectively). The problem with using a single fuel that releases energy when it breaks up, is that it is generally more unstable than the bi-propellant option. With the bi-propellant, you can use relatively stable component fuels which either react when mixed or can be safely mixed and later ignited to kick-start the reaction.
Example: The V2 rocket, developed by Germany during WWII (the first liquid fuel long-range rocket ever produced) uses the bi-propellant method. It used liquid oxygen and ethanol as its fuels. At launch, the fuels were allowed to mix and ignited to begin the reaction.
3) Create a nozzle at the base of your container. You want to ensure that this is the only hole in the container as this is where the burning fuel will be ejected. By creating a focus point for the release of exhaust gases you can control the direction of the force, and thus the movement of the rocket in the opposite direction.
4) Prepare for launch. You only have a limited amount of fuel within the body of the rocket so you need use it wisely. The exhaust gases must be ejected from the nozzle as fast as possible in order to create the force needed to propel the rocket forwards. The nozzle should be cone-shaped.
Relevant aside: If you imagine a garden hose without any attachments, the water comes out of the hose, may arc out slightly away from the hose and then fall to the ground. If you attach a nozzle to focus the flow of water, then even with the tap on at the same level, the water will travel further away from the hose before falling to the ground. We are looking for a similar effect with our rocket, but it turns out that the optimum design for expelling gas is to have a convergent-divergent nozzle. While in the engine, the gases are pushed together by the convergent part of the nozzle which speeds them up. But once expelled from the engine, you want to ensure that none of the gas will escape sideways (because this would move the rocket sideways instead up upwards). It turns out that the divergent bell-shape is the best way of ensuring the maximum effect of the rocket engine because unlike water, gas escaping through a hole at high pressure would go in all directions. The bell-shape helps refocus the direction of the energy from the gas.
5) You don’t want to light all your fuel at once because this would just create a fireball of all your hard work. To get round this problem we use a combustion chamber, basically a space where a limited amount of fuel can burn at any one time. This should placed just above the nozzle. Continue adding fuel to replace the fuel that has burnt and been ejected as exhaust gases.
Congratulations! That’s a rocket!
Fins – If you’re keen that your rocket should go up in a reasonably controlled manner, you might look at adding some fins to body of the rocket. You’ll need a minimum of three, evenly spaced around the rocket. If the rocket begins to spin or turn, a force will act on the fins which will counteract it, allowing your rocket to maintain balance.
Launchpad/Launch Tower – If you ‘re launching a small rocket (anything from a centimetre to a few metres high) you probably want a launch tower. This will keep your rocket pointing in exactly the right direction (the fins will only work when the rocket is travelling fast enough).
For bigger rockets, shuttles etc a launch pad is recommended as this will allow you to get up close to the rocket pre-launch to do any checks or repairs. You may also want to include a fire trench at the base of the rocket to direct the hot expelled gas away from the rocket as it launches to prevent it reflecting back and melting your rocket.
Parachutes – If your rocket isn’t designed to make it into orbit and you want it back in one piece you’ll need some parachutes to slow its descent back to earth. Without these you could end up with a piece of crumpled metal, lodged deep in the ground.
2 Responses to “Recipe for a rocket“
PhysBrainsays:February 8, 2011 at 7:06 pmReplyThe converging-diverging nozzle is really one of the most fascinating parts of the rocket. You don’t necessarily need it to produce thrust, but with it, you get a significant performance boost. The reason for the converging-diverging nozzle has to do with accelerating the fluid to the maximum velocity possible prior to leaving the rocket, and thereby imparting the greatest change in momentum to the rocket for a given amount of fuel.
When the fuel ignites in the thrust chamber, the temperature rises causing the gas to expand rapidly. This also increases the pressure inside of the chamber. It’s at this point that the high pressure gas notices the low pressure region just outside the nozzle and decides to head in that direction. Before the hot gasses can escape, however, they have to flow through a small opening. As you alluded to with your garden hose analogy, when you make the opening smaller, the fluid tends to speed up as it passes through. Now, ordinarily, the gasses would speed up to get through the narrow opening, and then slow down again when the opening widens up. However, something very interesting happens if you can accelerate the gas to the speed of sound just as it passes through the narrowest part of the nozzle. Once the gas hits the speed of sound, the physics change just slightly so that the gas will actually continue to accelerate as the nozzle widens up. The shape of the nozzle bell has a big influence on how much more acceleration you can get; however, the optimal shape changes depending on whether the rocket is firing at sea-level or in the vacuum of space or somewhere in between.
Thus, a well designed nozzle will give you the maximum amount of thrust out of a given fuel and chamber pressure. (Note: You can always get more thrust from higher chamber pressure, but at some point your thrust chamber will melt and/or explode.)
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Good recipe, liked it, but cannot try it.