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 http://www.thehouseofautomata.com/ . 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:-

Kerosine

O/F ratio = 2.2

Isp = 255

Flame temperature = 3200 Celsius (5800 Fahrenheit)

Petrol

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.  

Exemplar

An exemplar is defined as an excellent model or typical example, worthy of imitation. In my last post I spoke about manufacturing a banjo type fitting. Now, it struck me that there may be many who have never heard this term and have no idea what a banjo fitting is. A recent visit to the Royal Air Force Museum at Cosford, in Shropshire, gave me the chance to rectify the situation.

The Cosford museum is well worth a visit if you are in the West Midlands with an afternoon to spare. You can read about it here:-  www.rafmuseum.org.uk/cosford/

The collection includes the National Cold War Exhibition, including excellent examples of the "V" bomber type in the form of a Vulcan, Valiant and Victor. Cosford perfectly complements the Southern Branch of the Royal Air Force Museum in Hendon, North London.

The Cosford Museum has an excellent collection of British Rocket Engines, including a De Havilland Sprite RATO unit and a De Havilland Spectre. It is also possible to see the Saunders Roe SR 53 aircraft, the mixed power interceptor that the Spectre was designed for. Both the Spectre and Sprite were Kerosine/HTP units. Incidentally, you can see a cutaway of the Sprite in the second edition of Sutton, on page 36.

Both engines included good examples of banjo fittings and I took some photographs to illustrate the type:-

This is a banjo fitting on the top of the Sprites' Hydrogen Peroxide tank. The purpose of this fitting and its' associated pipeline was to charge the tank with compressed air from the air distributor valve. The fitting is composed of a cylindrical outer portion that is fixed to the tank by means of a hollow "banjo bolt". This has a perpendicular drilling that allows the hole in the centre of the bolt to communicate with the cylindrical portion. By this means the fluid entering the cylindrical portion is delivered through the bolts' central hole. The cylindrical portion is sealed at the top and bottom with Dowty Washers. Copper or aluminium crush washers are used in some instances. This photograph also shows a frankly beautiful example of TIG welding.

The next example is taken from the De Havilland Spectre:-

The fitting in this picture is of an AGS type. As a matter of fact, it is an AGS 1130 banjo body and an AGS 1135 banjo bolt. AGS stands for "Aircraft General Standard". This is a standard for various aircraft detail components. It was used on British Aircraft for many years. Threaded components were based around the BA system for smaller than 0.25 inch and BSF for 0.25 inch and larger. For fluid power systems, the BSP parallel thread was used.

Here is an AGS 1132 banjo body:-

And here is an AGS 1136 banjo bolt:-

This image clearly shows the central hole communicating with the perpendicular hole. These images come from the website of LAS Aerospace Ltd, on which you can see and learn more about AGS parts:- http://www.lasaero.com/

I am going to be in the workshop all next week so with any luck the next photographs I show you should be of parts that I have made. I  am still working on the posting detailing the nozzle design calculations, so do look out for that.