Wednesday, 24 August 2011

Fuel Concerns

Before I start going into details about nozzle design and dimensions I had better say a few things about fuel choice. I was minded to tell you about this when I visited a friend recently. Michael is a technical horologist. Together with his wife Maria, they run The House of Automata . Michael asked me what fuel I am using...I suddenly realised that as yet I hadn't made any mention of this all important subject.  

The list of substances that can be used as rocket engine fuels and oxidisers is a long one. Few of these are easily obtainable. For an amateur rocket project we can discount such exotic animals as hypergolics, acids and amines. Likewise peroxides. An out of control fire or explosion would not endear us to our neighbours, or indeed the authorities.

That leaves commonly available gaseous and liquid hydrocarbons or alcohols as fuels. We have the choice of gaseous oxygen or oxides of nitrogen as oxidisers.

Availability is not the only desirable property for a fuel and oxidiser to possess. A good fuel should be safe to store and handle. As the fuel will also be used as a coolant, it should have a good specific heat capacity.This latter requirement rules out the use of gaseous fuels in a safe rocket engine. The choice then comes down to petrol (gasoline) and kerosine in the hydrocarbon camp and methanol, ethanol or IPA representing the alcohols.

As for the choice of oxidiser, gaseous oxygen for welding is widely available and easily obtainable. It requires no special handling or storage requirements beyond those dictated by common sense. This is in contrast to LOX which is a non starter in a semi residential workshop setting. That said, cryogenics are not neccesarily out of the question. Nitrous oxide is readily available due to its use in car performance enhancement. It is stored in liquid form under pressure in cylinders. It is also relatively safe and easy to handle. When released under control of a regulator the liquid rapidly changes state to a gas.

Gaseous oxygen seems to be the best option at this stage. Its ease of availability, handling and storage contribute to moving the project forward in a timely fashion. That said, I wouldn't rule out the use of nitrous oxide. I will be able to make a much more informed decision once the engine is built and hot fire experience has been gained.

Petrol, kerosine and the alcohols, methanol, ethanol and IPA have been widely used as rocket engine fuels. For comparison, here are some performance figures for these three fuels, when burned with gaseous oxygen:-


O/F ratio = 2.2
Isp = 255
Flame temperature = 3200 Celsius (5800 Fahrenheit)


O/F ratio = 2.5
Isp = 260
Flame temperature = 3170 Celsius (5740 Fahrenheit)

Alcohol (Methanol)

O/F ratio = 1.2
Isp = 240
Flame temperature = 2810 Celsius (5090 Fahrenheit)

 These fuels are readily available in almost every community. Hydrocarbons give good performance when burned with oxygen, as can be seen. Unfortunately they make poor coolants. Kerosine and petrol have a tendency to "crack" at high temperatures and leave sooty, solid carbon deposits. These can block coolant and injector channels, thereby causing problems. I am trying to construct a safe and reliable engine and so hydrocarbons are out, at least for the time being.

As shown, alcohol gives decent performance with a lower flame temperature. It is also chemically stable when in contact with the hot walls of the chamber cooling jacket. The flame temperature could be reduced further by increasing the ratio of fuel to oxygen, that is to say producing a rich mixture. This would be the only option with hydrocarbon fuels. With alcohol, the flame temperature can be reduced further and the cooling properties greatly increased by diluting the alcohol with water, with which it mixes in all proportions. 

 In 1948 the Aeronautical Research Council published report 2816. Authored by ABP Beeton, it dealt with "The Calculated Performance of Ethyl Alcohol - Water Mixtures as Rocket Fuels with Liquid Oxygen". It states that a combination of 100% ethyl alcohol and LOX gives an Isp of 250. By contrast, a 70% ethyl alcohol - water and LOX combination results in an Isp of 240. For the 100% ethyl alcohol combination the combustion temperature is given as 3000 celsius. With 70% ethyl alcohol the temperature is 2820 celsius. Thus it can be seen that the hot gas temperature is some 180 Celsius lower for a decrease in Isp performance of only 4%.

To conclude, it can be seen that gaseous oxygen is a good choice of oxidiser for the Thunderchild Project. It is readily and cheaply available due to its use in welding. Similarly, ethanol is a good choice of fuel. It is easily available and relatively inexpensive. It gives good performance and is a good coolant. Dilution with water increases the cooling capacity and eases the cooling problem for a very small decrease in performance.  

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