Injection, Injection

At long last.

The fuel injector of a rocket engine is without doubt the single most important component in the entire system. This is because it exercises influence over all of the closely coupled key performance arbiters. These are:-

• Combustion efficiency
• Combustion stability
• Heat transfer and cooling

If this were not enough, the unit also has to endure the aggressive, high temperature environment of the combustion chamber whilst conforming to a specified form factor.

Over the years injector design has combined approaches of both meticulous research and "try it and see what happens". I have travelled a similar road. Fortunately today there is a vast literature on atomisation, mixing and sprays, both rocket specific and general. Hence my experimental efforts have been guided at each stage by reference to published works.

Injectors for rocket engines fall into two main categories:-

• Impinging Stream
• Coaxial Jet

These two types are further broken down into a number of distinct subsets.

An important subset of the Impinging Stream type is the so called Shower Head injector; this may be regarded as an impinging stream injector in which the impingement angle is zero. Impinging stream types are further subdivided by impingement component and pattern. Types include fuel on fuel, fuel on oxidiser, oxidiser on oxidiser and combinations of these. Patterns used are many and various, including doublet, triplet, pentad, and numerous combinations of these. The main mechanism of stream break up and atomisation in the impinging stream type is momentum exchange. Impinging stream types are primarily used in systems where both fuel and oxidiser are liquids.

Coaxial injectors are the chief choice when one of the propellants is in a gaseous state. They are split into two main groups, shear and swirl. In the shear type, a low velocity central liquid flow is stripped and atomised by a higher velocity annular gas flow. The main mechanism of atomisation in this type is induced Kelvin-Helmholtz instability. The most important performance controlling variable in the shear coaxial unit is the gas/liquid relative velocity. Surface tension of the liquid resists the deforming force of the gas momentum, hence high gas/liquid Reynolds Numbers are common in efficient shear coaxial injectors. Shear coaxial injectors are almost always liquid centred.

The swirl coaxial injector imparts a tangential velocity to either the gas or liquid flow, and in some cases to both. They may be liquid or gas centred. Centrifugal force due to tangential velocity causes liquid ejected from the injector to form a cone shaped, thin sheet spray. Under the influence of the centrifugal force ligamentation and droplet formation rapidly occur in the thin swirling sheet. The key formation mechanism here is Rayleigh Instability, with the centrifugal force standing in for gravity. These droplets are then further broken down by the gas flow induced Kelvin-Helmholtz type mechanism of the shear injector. In the case where both gas and liquid are swirled, momentum exchange also plays a part in the spray formation.

 High gas to liquid Reynolds numbers are again a feature of this type of unit. Swirl atomisation is governed by the efficiency with which the potential energy of the flowing stream is converted to a tangential velocity. Key to this is the minimisation of friction in the flow path.

A quick survey of amateur liquid rocket engines on the internet showed the proliferation of the impinging type injector. My view was that the multiplicity of small holes needed would be difficult to drill accurately at the angles required, as well as being difficult to deburr effectively. This would lead to stream misimpingement and hydraulic flip as well as many broken drills. The impinging stream type injector would also be more difficult to manifold.

As the present project is using a gaseous oxidiser, I decided to go with a Coaxial injector design. I felt that this would be easier to manufacture and be more amenable to performance enhancing modifications. It would also be a unique feature of the Thunderchild Project.

Here is a photograph of an early mock up to illustrate the coaxial principle:-

Next I'll look at how this principle was developed.



An Update

First of all let me apologise for not having posted anything in the last few months. Rest assured, things have been moving steadily onward with the Thunderchild Project!

I have been very busy in the workshop and with nozzle and injector design. This coupled with the generalities of everyday life has meant  I have had little time to post.

Workshop wise I have been re-acquainting myself with screwcutting in the lathe. As well as being important to the project generally, my acquisition of threading tools for the Harrison has allowed me to radically re-design and improve the injector. I've also had some excellent input on the injector design front from Dr. John Chinn, of the University of Manchester Institute of Science and Technology (UMIST). Dr. Chinn is an authority on atomisation and sprays and was kind enough to take time to clarify a few points for me.

To try to get some idea of the efficiency of the injector design, I've been looking at a technique called "Shadow Sizing". Essentially this involves taking a photograph of the spray against a brightly illuminated graticule. It is then possible (with some effort) to gauge droplet size, distribution and number. I've been trying to work out a scheme to implement this, and thinking about software to automate the particle counting/sizing task. As an aside, I have put this notion forward for the BBC Radio 4 "So You Want to be a Scientist" search for amateur research ideas. I don't hold out much hope for it, as there usually seems to be an environmental, or "primary school nature table science" type of bias to the chosen entries. Still, you never know.

The design of the nozzle and convergence is well advanced, and this will most likely be the next component to be machined. The extra area required to achieve the design L* figure was calculated using the formula for the frustum of a cone. I will post the full nozzle and convergence design calculations in due course.

Some time ago I told you about the new tilting rotary table, well here it is:-

And here, tilted:-

Last but not least...the British Reaction Research workshop is moving! Yes, I have bought a new, larger house, and the space for the workshop is insulated and has access from inside the house. It will be much more comfortable in the colder months...I still get chills when I think about machining the components for the chamber last winter, when it was minus 10 degrees celsius outside!

Inevitably, it will take some time to get the workshop up and running again. I will give you some pictures of the move and set up, and I'll try to use some of the time to start giving you the much vaunted injector details. Do stay tuned.



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.



Fasteners

I just wanted to mention that when I returned home I had received a delivery of stainless steel fasteners and a new piece of workshop equipment.

I had bought a selection of metric stainless bolts, mainly M8 and M14. These are destined to play a part in the further development of the injector.

I found a new supplier for stainless fasteners. I used to use Stagonset. According to various forums, they have gone out of business. The former employees have started up again, calling themselves Westfield Fasteners. You can find them at:- http://www.westfieldfasteners.co.uk/

As for the workshop equipment, I now have a tilting rotary table. It is a Vertex one which I got from RDG tools:-
 http://www.rdgtools.co.uk/
 I had decided to get one as I was struggling to drill perpendicular, conjoined holes with my existing one. Once I get a chance to clean the grease off it and take a photograph, you'll get to see it.

Back in the Real World

Once again I have returned from one of my globetrotting escapades. After a few days to decompress, I will be able to get back into the workshop. Work on the rocket motor will resume, as will regular postings to let you know what is going on.

Whilst I was away, I did some work on the design of the convergence and nozzle. The plan is to get it fabricated this time home. I will post the design equations for it in due course. I have also been thinking about an idea for a banjo type fitting that will be based on commonly available hardware. At the moment the design of the motor includes various BSP to Swagelok fittings. These are quite expensive, so if I could devise a means of replacing them it would be a definite plus.

Time permitting, you should also start to see a few postings regarding injection. I think this is the holy grail; whenever I find a new site devoted to rocket engine construction, my first thought is to find out what their injector is like. So in the next few weeks you'll be able to read about the design, evolution and testing of mine. An evolution that is still ongoing, I hasten to add.

Plenty there to keep me busy and to hopefully be of interest to the likeminded. Thank you for following and keep watching this space. I will endeavour to use the one between my ears.

The Practical Chamber

As previously mentioned, I decided to use nominal bore hydraulic pipe to form the tubular section of the chamber. Nominal bore hydraulic pipe is readily available. It is manufactured to verifiable, laid down standards governing pressure rating and dimensional tolerance. 

Confusingly it is specified by a non dimensional figure (in Imperial units) for the diameter. A schedule number is used for the wall thickness.

The practical chamber had to include the volume required to deliver an L* of 1.52m (60 inch). However, as the nozzle and convergence were to be separately fabricated, the volume of the tubular portion had to be reduced accordingly.

 In addition I needed to include the external jacket with a coolant gap of 3.2 mm (0.125 inch). The task reduced to finding a combination of nominal bore pipe diameters that represented a good compromise in the light of these requirements. 

I finally decided on a combination of 1.5 inch, schedule 10 nominal bore pipe for the internal tube and 2 inch, schedule 10 for the external tube. This was the best compromise I could achieve, the coolant gap becoming the final arbiter. Below are the key dimensions of both pipe sizes:-

1.5 inch Schedule 10 

External diameter = 48.26 mm (1.9 inch)

Internal diameter = 42.72 mm 1.68 inch)

Wall thickness = 2.77 mm (0.109 inch)

2 inch Schedule 10

External diameter = 60.32 mm (2.37 inch)

Internal diameter = 54.79 mm (2.16 inch)

Wall thickness = 2.77 mm (0.109 inch)

For a small chamber, the volume of the convergent section is generally assumed to be about 1/10 that of the tubular section. So the volume of the tubular section needs to be reduced by 1/10 compared to the theoretical value. It will be recalled that the proposed value for Vc was 0.121 x 10^-3 cubic metres. The volume of the tubular section, less 1/10, is thus 0.109 x 10^-3 cubic metres. The volume of the convergent portion is then 0.012 x 10^-3 cubic metres.

Examination of the internal diameter of the 2" tube and external diameter of the 1.5" shows that the coolant gap will be 3.265 mm (0.128 inch). The theoretical chamber diameter and length are 50.3 mm (1.98 inch) and 61 mm (2.4 inch) respectively. The external diameter of the 1.5 inch tube is 48.26 mm (1.9 inch) and the internal is 42.72 mm (1.68 inch).

 I decided that this internal dimension would be acceptable for the chamber. The external diameter of 48.26 mm (1.9 inch) still gives sufficient area for the injector design I plan to use. The length of the chamber was extended to 75 mm (2.95 inch) to compensate for the slightly smaller diameter. This gave a chamber volume of 0.107 x 10^-3 cubic metres. I decided that lengthening the chamber to 75 mm was as far as I was prepared to extend it. I had to bear in mind the fact that I was increasing the heated area and hence exacerbating the cooling problem. The volume of the convergence will be adjusted to 0.014 x 10^-3 cubic metres to ensure an L* of 1.52 m (60 inch). Finally, the contraction ratio Ec comes out at 4.25 as opposed to the theoretical 5. This will still be more than sufficient.

The exact chamber dimensions are far less critical than the nozzle dimensions in terms of performance. It is felt that the practical chamber represents a good compromise between the theoretical dimensions and the limitations of the constructional method chosen.

Thunderchild

When I had finished fitting out the workshop, and began construction of the engine in earnest, I decided a name was required. I chose the name "Thunderchild". As literary readers will remember, this was the name of the Royal Navy Ironclad in H.G. Wells' "The War of the Worlds".

The engine as designed will be a 222 N (50lbf) thrust unit. As the engine is relatively small, is my first attempt, and will, I'm sure, be incredibly noisy, "Thunderchild" seemed appropriate.

As you will no doubt have guessed, I decided to build the engine as a series of flanged sub assemblies. Although Kryzcki warns against this in his treatise, it is certainly not without precedent. Whilst researching prior to beginning construction, I saw several examples of one piece chamber/nozzle type units on the internet. I suspected this would involve a great deal of machining to convert a section of round bar into a tapered tube. I decided to try to find a more efficient constructional method. The combustion chamber is essentially a pipe; all that was needed it would seem would be to make a suitable pipe. A unit based on separate sections would make for easier fabrication. It would also allow different injector, nozzle and chamber configurations to be trialled.

The combustion chamber dimensions were calculated using the relations given in Sutton. The parameter used to define the chamber volume is the Chamber Characteristic Length, denoted by L*. This is the ratio of chamber volume to nozzle throat area (Huzel and Huang). It is a substitute for determining the propellant stay time in the chamber.


Values of L* have been predefined for various propellant combinations, and are published in the literature. For a gaseous oxygen/hydrocarbon combination, a value of 1.52 metres (60 inches) is indicated. This figure is quite large, but for a small motor will ensure adequate space for atomisation, mixing and complete combustion of the propellants. The volume of the chamber can be found from:-

Vc = L* At (1)

Where:-

Vc = Chamber volume

At = Throat area

Nozzle theory and thermodynamic relations determined the throat area as 79.40 sq.mm (0.123 sq.inch). I will show these calculations in a later post.

Substituting the knowns into equation (1) produces:-

Vc = 1.52 x 7.94 x 10^-5 (2)

Vc = 0.121 x 10^-3 cubic metres (7.7 cubic inches)

The chamber volume considered in the definition of L* must include the volume of the convergent portion. The theoretical figures resulting from the calculations here give dimensions for a chamber inclusive of a convergent portion. I decided early on that the tubular section of the chamber and the nozzle/convergence would be fabricated as separate units. This has implications for the final exact choice of dimensions for the tubular portion but I will explain these later on. 

The next piece of information required to progress the theoretical chamber sizing calculations is the Contraction Ratio, Ec. This is the ratio of the chamber to throat diameter.

In order to give a usable injector face area, I decided to use an Ec of 5. Figures for Ec greater than 3.5 also enhance stability and energy utilisation efficiency (Sutton). In addition, a value of this order is recommended for thrust chambers smaller than 333 N (75lbf) (Kryzcki).

Hence if Dc = 5Dt,

Dt = 10.06 x 10^-3 m (0.396 inch)

Dc = 50.3 x 10^-3 m (1.98 inch)

Therefore the chamber area Ac is:-

Ac = pi (50.3 x 10^-3/2)^2 (3)

Ac = 1.98 x 10^-3 sq. m

To evaluate the chamber length:-

Lc = Vc/Ac (4)

Substituting the knowns into (4):-

Lc = (0.121 x 10^-3)^3/(1.98 x 10^-3)^2 (5)

Lc = 61 x 10^-3 m (2.4 inch)

The theoretical dimensions for the combustion chamber have now been derived from nozzle theory, thermodynamic relations and published values for Characteristic Length. To summarise, these are:-

Chamber Diameter Dc = 50.3 mm (1.98 inch)

Chamber Length Lc = 61 mm (2.4 inch)

Chamber Area Ac = 1980 sq. mm (3.06 sq. inches)



Next we'll look at the compromises neccesary to convert these theoretical figures into something that can actually be made. I'll tackle this in the next post. Along the way I will explain how the volume required for the convergent portion will be accomodated.











Combustion Chamber:- Some Machining Details

While I was away I spent a lot of time looking into the calculations relating to the combustion chamber. These dealt with structural integrity and heat transfer. These musings will be posted in due course. In the meantime here are some details of the combustion chambers' component parts during machining.

This picture shows a combustion chamber flange being bored out, to suit the nominal bore hydraulic pipe forming the chamber inner tube. The flange is made from a section of 304 stainless steel round bar. Concentricity was assured by chucking the part on the central hole to turn and face it to size.

This photograph depicts the flange brought to size. The 3.2mm wide step has been machined. This maintains the spacing between the inner and outer tubes.

Another view of the same part, this time showing the gasket side.

Clean up cuts being made. The edges of the part have been chamfered. The 45 degree chamfer has been generated on the edge of the internal bore, ready for welding the inner tube on.

Here is a section of nominal bore hydraulic pipe being cut on the bandsaw. The tube was then machined to exact size and its edges chamfered to 45 degrees. It was then ready to become the inner tube of the chamber.

Here is the fit up of the inner tube and flange after machining. The next task was to drill and tap the fixing holes in the flange face. The face was drilled and tapped to M6 in 8 positions on a PCD of 62mm. Of course 8 positions means a hole every 45 degrees. I centred and fitted a spare 3 jaw chuck to the rotary table to do this on the mill/drill. Unfortunately I have no pictures of the drilling and tapping operation, but here you can see the rotary table and chuck arrangement fixed to the mill/drill. 

Coming next will be the design calculations for the chamber. Keep watching.

Returned

Well, it has been some time since my last post. I have been away on one of my work related jaunts. All I can really tell you is that I was in the Southern Hemisphere. Suffice it to say that I played my own small part in the spread of civilisation, delivered through the medium of British Engineering Skill.

I returned through London to find Blighty wearing her summer finery...everything is lush and green. Shakespeare put it best:-

"This royal throne of kings, this sceptred isle,
This earth of majesty, this seat of Mars,
This other Eden, demi-paradise,
This fortress built by Nature for herself
Against infection and the hand of war,
This happy breed of men, this little world,
This precious stone set in the silver sea,
Which serves it in the office of a wall
Or as a moat defensive to a house,
Against the envy of less happier lands,--
This blessed plot, this earth, this realm, this England..." 

I can say no more or no better than that.

Combustion Chamber Completed

Today I finished the combustion chamber. The unit was chucked in the Harrison to skim the welds. I gripped it on the inner surface of the inner tube. This provided the maximum contact area, given that the unit wouldn't sit flush due to the flange weld bead.

This also meant that I had to spend some time truing the unit in the chuck, using a DTI. Once this was done it was a relatively simple operation to skim the weld bead. Due to the less than ideal chuck grip, I took very small cuts to minimise the cutting forces on the unit.

Once the bead was reasonably flush, I blended the weld into the flange using incrementally increasing grades of emery paper. Here is the chamber in the chuck of the Harrison during the skimming operation:-

On completion of the first face I reversed the unit in the chuck and skimmed the second bead. I had a major moment of anxiety at this point. Despite taking care to take small cuts, the tool dug in - I'm not sure why, I can only assume I misfed it. The chamber shifted in the chuck and one of the jaws damaged the flange face. My heart was in my mouth as I carefully removed the unit from the chuck. Fortunately the damage looked much worse than it was. I was easily able to blend it out using emery. After a few minutes it was barely noticeable. Returning the chamber to the chuck (and after changing my underpants) I finished skimming and blending the bead - using even smaller cuts than before!

 I have to say that I am very pleased with the blended welds. I think they will be more than capable of withstanding the tensile forces exerted on the chamber. Here is one of the flanges after blending:-

I decided to give the outer surface of the cooling jacket a coat of heat resistant black paint. As mentioned previously, most rocket chamber heat transfer is by convection, but it will do no harm for this hot component to be as good a radiator as possible. I'll post some pictures of the painted chamber shortly. Until then here is the completed unit masked up awaiting paint:-

In the next few posts I'll be going into the engine design rationale, and start introducing a few equations. I bet you can't wait.

Welding the Combustion Chamber

Today I finished welding together the combustion chamber for the rocket engine. The time spent on trialling the welding set-up proved its worth; the whole process went without a hitch.

Before I assembled the chamber, I gave the outer surface of the inner tube a coat of black high temperature paint. The idea being to improve its performance as a heat radiator. Most of the thermal transfer in a rocket engine takes place by convection, but every little helps. I welded the outer tube in position first.

The assembled chamber was clamped together through its central hole using clamp bars and a threaded rod.

 I then tacked the outer tube to the flanges in approximately eight places each, as per the fabrication trials. The tacks were diametrically opposed to minimise distortion. These were joined up with short runs of bead, again as per the trials. I detected no discernible distortion of the chamber once the outer tube was fully welded.

Here is the chamber with the outer tube finished:-

The Swagelok nuts on the coolant fittings protected the threads whilst the chamber was moved around.

With the outer tube completed, it was time to start on the inner tube. This had to be welded to the flanges at each end. The image of the pre-assembled chamber in the Fabrication Experiments post reveals the welds have to be done close to the inner tube edges.

The pulse TIG really proved its worth here. The welds were completed without incident. I kept the pulse settings the same as for the outer tube, with an average current of 70A.

Here is one of the inner tube to flange welds:-

Again, no discernible distortion was detected post welding. The chamber will be returned to the lathe next, for a tidy up. This will involve polishing the unit to remove any heat discolouration and small nicks caused by handling. The flange faces will be restored to  flat, mating surfaces by skimming the inner tube to flange beads.

The chamber is now able to act as a foundation element for the injector and nozzle. It has a hot gas enclosure which is surrounded by a cooling jacket. Fittings are provided for coolant inlet and outlet. The cooling jacket and hot gas side tubes also form structural members to withstand the chamber pressure forces.

Today has been a productive one, and I am very happy with the way the welding went. The chamber looks pleasingly like the original sketches I did for it. These are in one of my notebooks, and were drawn in the summer of 2009. This is the joy of designing, calculating and constructing. A problem exists. An idea forms in the mind. A rough sketch is made. The idea takes shape. Trials begin, problems are solved, the concept evolves. And then one day a thing of paper becomes a thing of metal. A tangible object that serves a real purpose, its form, one with its function, the confluence of equations, experience and expedience.

United Kingdom 2011 Budget Boosts British Space Industry

Before I start this post, I should make it clear that this blog has been conceived to serve a higher purpose - that of the acquisition and furtherance of technical knowledge. In the words of Leonardo Da Vinci:-

"The acquisition of any knowledge whatever is always useful to the intellect, because it will be able to banish the useless things and retain those which are good. For nothing can be loved or hated unless it is first known."

Whilst I do not intend to sully myself unduly with such base concerns as politics, the following story is most definitely worthy of note.

On the 23rd of March the Chancellor of The Exchequer, The Rt. Hon. George Osborne MP, announced the United Kingdom 2011 budget. The budget had some good news for British Science, and in particular the British Space Sector.

The BBC reports that the Budget includes a raft of measures to increase the competitiveness of the UK Space Industry, which is currently worth some £7.5 billion per annum, and is growing at the rate of 10% per annum.

The measures take the form of a £10 million injection of funding to support new technologies used in spacecraft systems, and a change to the 1986 Outer Space Act, which affects insurance and underwriting concerns.

The £10 million on offer will be matched by industry and is part of £100 million on offer to British Science as a whole. It will be matched by industry to start a National Space Technology Programme.

The 1986 Outer Space Act is the primary piece of legislation governing British Space activities. At present, liabilities are essentially unlimited and this makes it more expensive for UK companies to compete with their international competitors. Mr Osborne has asked The UK Space Agency to assess how best to change this.

Additionally, licensing arrangements will be clarified for Space Tourism companies, such as Sir Richard Bransons' Virgin Galactic venture. This should encourage such companies to base themselves in the UK.

Mr. Ian Godden, chairman of ADS (an umbrella group representing British Space Companies) stated:-

"The space sector is an unsung success story, supporting 70,000 jobs in the UK and generating £7.5 billion per year to the economy. Industry and government have in place a shared plan to grow this to £40bn and this additional investment will assist in achieving that aim"

Whilst I welcome the injection of funding from the Coalition Government, and the recognition of the contribution made by UK Space and Aerospace companies to the success of Britain that this implies, I feel almost bound to say that it is too little, too late. I speak as one who has still not forgiven the UK establishment for the  cancellation of the TSR2 in the 1960s, and the shocking and destructive scrapping of the Black Arrow satellite launch system in the early 1970s. Now, if my own small investigations are anything to go by, £10 million is going to be a drop in the ocean for the UK Space Sector, even with the Industry contributions.

Fabrication Experiments

I am still trying to get my head around the dynamics of the blog format; I am much more accustomed to writing in a chronological, linear fashion, where causal events follow on in a natural progression. You see, things have been going on in the workshop and I want to tell you about that. However, I feel that I should be enlightening you on the design of the engine...I also have to take into account the fact that all you've seen so far are pictures of tools. Now, though there is a certain voyeuristic pleasure in this, I have to admit that if I came to a Rocket Engine blog, I'd want to see pictures of shiny rocket engine components...You can see my dilemma.

Well, anyway, its my blog so I can write it how I like. So in this post you can find out about some welding and machining trials I've been doing. And see a few shiny rocket engine components. 

I have to say, its been a pleasure to be in the workshop over the past few weeks. The snow came in mid November last year and stayed, with little reprieve, until after Christmas. It wasn't much fun in there when it was -10 Celsius outside, let me tell you, even with the heating on. Fortunately Spring has now resolutely sprung. The new warmth is most welcome!

I mentioned I had a new welder in the last post. I realised quite early on that welding was going to be a prerequisite for the construction of a safe and viable rocket engine. The inclusion of welding in the armoury of fabrication techniques solved many of the contentious design problems at a stroke. TIG welding was an obvious choice given my need to make clean, precise welds on relatively small components. I also went on a welding course to gain skills and knowledge. I did my course with Mark Ellis at Varis Training in Morayshire:- http://www.varis-training.co.uk/ The course was money well spent, what Mark doesn't know about welding would fit in a thimble.

Prior to Christmas I manufactured all the parts for the rocket engine combustion chamber. Here is a picture of the trial-assembled unit:-

Everyone loves a picture of something next to a rule, don't they? Metric at the top, Imperial at the bottom. I will go into the detailed design of the chamber in a later post. Suffice it to say it has an L* of 1.5 metres (60 inches) and is double walled, the external wall forming the cooling jacket. I've decided to go for a split design, in which the chamber, injector and nozzle are separate flanged units that can be bolted together. Sealing will be by gaskets manufactured from ceramic paper.

The chamber is entirely made from 304 stainless steel. The flanges are machined from round bar and the inner and outer tubes are lengths of nominal bore hydraulic pipe. The inner surface of the inner tube is of course the hot gas side. The dump cooling water flows in the gap formed by the inner tube outer surface and outer tube inner surface. This gap is 3.2 mm (0.125 inch) and is maintained by a lip machined on the inner face of each flange. Eight blind holes are tapped M8 to a depth of ~12mm (~0.5 inch) for fixing purposes.

Here is another view:-

As I said I'll go into the detailed design with all the juicy equations in a later post. All you need to know now is that I have to weld this unit together. Ideally with negligible distortion. The dimensions are slightly oversize to allow for post weld machining.

The requirement for pulse TIG welding should be fairly apparent from the pictures. The outer tube has to be fillet welded to the flanges - fairly close to an edge in each case, and at the back of the tapped holes. Similarly the inner tubes need to be welded to the top faces of the flanges - again right on the edge of the tube and close to the mounting holes.

Pulse TIG works by using a Pulse Width Modulated welding current. When the pulse is high a large current flows, creating the weld pool. Then when the pulse is low, the lower current "freezes" the pool. The effect of doing this at a frequency of around 25Hz is to effectively make the pool stay where you put it. The tendency to melt the edge off a workpiece is greatly reduced. The second benefit of pulse is to effectively "focus" the arc; it becomes tighter and more precise.The waveform results in an average current that should be something like what you'd be using if you weren't in pulse mode. So you can see that pulse TIG seems to be the solution for welding components like my chamber. That is one of the main reasons I invested in the new welder.  

I decided to do some fabrication trials to find the optimum pulse setting for my application. I should also mention that I needed to weld the coolant inlet and outlet fittings onto the cooling jacket (outer tube). I had initially intended to make some stainless bosses, tapped to 1/4 inch BSP, and weld these on. I decided on the simpler option of just welding modified swagelok fittings directly to the jacket. You can see the end result in the first photograph. I didn't use pulse TIG for this, though in retrospect I wish I had. That said the result is tidy enough and fit for purpose. Here it is:-

I didn't need to bevel the edges of the tube for the fillet weld. I made a mistake when machining the part but I decided to use it anyway as it won't make much difference to the weld.

I was quite concerned about doing the fillet welds between the flanges and outer tube. As you can imagine it took a substantial amount of painstaking work to make the chamber components. I didn't want to ruin it by carelessly ploughing on with the welding. So I decided to trial the pulse parameters on some test pieces. This would not only prove the welder but also hone my torch technique. I cut some sections of 304 round bar and nominal bore pipe to simulate the flange and tube arrangement. The picture below shows the weld I managed to achieve after three different attempts at altering current and pulse settings, as well as developing a good workpiece and torch position.

I found the best way to achieve a good weld was to tack at about eight equidistant points and then join these up with short runs of bead. This minimised the amount of torch manipulation required. Here is the test assembly as tacked:-

After these fabrication experiments I feel more confident about joining the combustion chamber. Admittedly, the flanges in the chamber have less metal in them, due to the central hole. I will most likely need to reduce the current slightly to account for this smaller heatsink.

So much for the welding. You will have heard me talking about using BSP fittings and threads in the design. I decided early on to try to avoid any sort of elastomeric sealing arrangement as far as possible. I felt this would both simplify construction (no "O" ring grooves to machine) and promote safety and reliability. BSP threaded components are ideal for this as they seal by virtue of interference on a tapered thread.

I made a mild steel test version of my injector design last summer. It was machined from BS230MO7 steel. This material is known for its machinability. 304 stainless steel on the other hand, is not. The design of the injector requires the tapping of two holes, one to 3/8 inch BSP and one 1/4 inch BSP. This was fairly straightforward in the free cutting steel.

I decided to trial cut some BSP threads in 304 stainless. I used a piece of 25mm 304 round bar for the tests. As expected, it was much harder to cut the threads using my HSS taps. A great deal of torque was required. I normally tap by hand in the lathe, but in this instance I had to tap under power to achieve the torque required. The thread produced was quite serviceable, but I could almost feel the strain that the taps were under. I will post some pictures of these tests in the next few days.

My findings justified obtaining some new tooling. I got some "Blue Ring" spiral flute machine taps, in 1/4 inch and 3/8 inch BSP. I used Drill Service Horley, in Horley, Surrey:- http://www.drill-service.co.uk/

I've yet to trial these but you'll get to know about it when I do.  







More Workshop Details

It has been some time since my last post, in which I said I would start giving details on engine and component design. Rest assured that in the intervening period I have been working on this. Additionally I've managed to get some workshop time in.

I have also acquired some new pieces of kit. I thought it might be good to show these, along with some general views of some of my other equipment. I got a new TIG welder. Last year I bought a Sealey 175 amp TIG unit, but I quickly realised its shortcomings - no foot pedal to control the welding current and no pulse facility. So I got myself a 200 amp unit from R-Tech Welding Equipment in Gloucester. I went for the TIG 201, which you can read about here:- http://www.r-techwelding.co.uk/ 

This unit has a pulse feature that allows fine control of the heat input to the work. This makes welding close to an edge easier, as the arc tends to "stick" where you put it. When working close to an edge there is a tendency for things to run away, as the heat has nowhere to go. You can watch your meticulously machined part melting into a congealed mess before your eyes. Here is the unit set up and ready to go:-

If that wasn't enough, I am also now the proud owner of a tailstock turret for the lathe. This nifty little fixture sits in the tailstock by means of an MT3 arbour. Mounted to this is a rotatable head. This has sockets machined into it for attachments like a jacobs and collet chuck, tap holders, die holders and a live centre. The idea being to save time and make the work more fluid, due to not having to stop to change tooling over. Just a twist of the turret and you've presented the relevant tool ready for the next machining operation. I got this unit from Arc Euro Trade in Leicester:- http://www.arceurotrade.co.uk/

Here is the unit:-

You can see the turret fitted to the tailstock of the Harrison. The tap holder is carrying an M8 taper tap, just in case you were wondering.

Home again

I have just returned from one of my work related jaunts. My temporary sojourn took me into the Southern Hemisphere. It feels very good to be home again, and eventually I will be able to get some workshop time in.

All my previous posts have been concerned with the setting up of my workshop equipment. The work that I did on restoring my lathe took place, in the main, in September to December of 2009. In the summer of that year I did a great deal of research into fuel injection schemes for the rocket engine. My next few posts will detail this. I'm also going to be saying a bit about the overall design of the engine and how this has evolved over time.

Up and Running

So now I had my lathe all ready to run. I had installed a 240 volt 16A mains supply into my workshop, fed from the main consumer unit in my house to a standard IP44 CEE 16A blue socket. This was needed as the M250 manual quoted a maximum start up current of 15A. The standard UK ring main is only rated for 13A.

I removed the control panel from the lathe and gave it a quick dose of looking at. Whilst somewhat untidy, it was nevertheless safe. I made a mental note to rewire the panel; it would be fine for an initial test though. I connected a length of blue 2.5mm square "arctic" flex to the appropriate point and terminated this with a blue plug. This flex is designed for outdoor use and can carry 25A maximum. I replaced the panel and plugged in. Here you can see the panel as found with my addition of the arctic flex.

Sharped eyed readers will notice a component missing. When I plugged in the lathe, the lamp worked and the coolant pump motor ran, but the main motor did not. I checked to make sure that all of the interlock microswitches were in the correct state. Still nothing. Now, despite the main and coolant motors being 240 volts, the microswitches and control relays are all 110 volts. This is common industrial practice. Microswitches are not double insulated and as there is more chance of the operator coming into contact with them they are fed from a 110 volt supply. This has its centre tapping earthed to the machine frame. That way the maximum voltage the operator will see, between the frame and a conductor, is 55 volts.

I discovered that this 110 volt supply was absent. I suspected the supply transformer and sure enough its primary turned out to be open circuit. Hence the gap above - I took the photo after removing the offending item. Here is the original transformer.

And another view.

As can be seen, Harrison had every eventuality covered, despite this machine being a 240 volt variant. They clearly just used one transformer type throughout. The primary has tappings for 380 and 440 volts, voltages normally found in ship systems. I knew that Harrison supplied to HM Forces, and later discovered the M250 was common on Royal Navy vessels. The 12 volt secondary was unused.

I now had to find a replacement transformer of the same rating and form factor. Obviously a direct equivalent wasn't a problem; I didn't need all the tappings for the different mains inputs, nor did I need a 12 volt secondary. The trusty manual showed the original transformer was 50VA rating and manufactured by Romarsh. Enquiries quickly revealed that this was an obsolete product. I suppose if I had wanted to be a purist I could have had one made, but I just wanted to get the machine working.

I found an off the shelf 240 - 110 control transformer manufactured by a great little company called Douglas Electronic Industries LTD, in Lincolnshire. You can find them here:- http://www.douglas-transformers.co.uk/

 The part number was GST50 240/110. The secondary was centre tapped and the dimensions fit the bill. I called them and they told me to send them a cheque and the transformer would be in the post that day. Perfect! 

If you are reading this and you have a 3 phase machine, do not despair. A 415 - 110 volt transformer is also available. The secondary wiring would be as described above, with the primary connected across two of the phases.

I  cleaned and rewired the panel using new equipment cable and fitted the new transformer. Here you can see the finished article.

Note the earthed secondary centre tap. The newly spruced up panel was refitted and a successful test of all the machines functions ensued. It was a great moment.

How (not) to Build a Workshop

In 2006 I fulfilled an ambition I'd held since I was twelve..I became the proud owner of a lathe. Now I could progress my rocket project. Or so I thought. The lathe I had bought was a fairly inexpensive Chinese import. It was of a type that boasted a mill/drill head incorporated into the headstock.

 You get what you pay for, and I soon realised the machines' shortcomings. I decided it was a better option to buy a second hand, high quality British lathe than to spend more on modifications to make my Chinese lathe into the machine I needed it to be. 

My requirements for a lathe were:-

Geared headstock
Screwcutting gearbox
Power crossfeed
Coolant system
Camlock chuck mount
Quick change toolpost
Single phase motor

I spent a good deal of time deliberating before I made a purchase. My initial port of call was the Myford range. Reckoned by many to be the ultimate amateurs lathe. I found it to be somewhat over priced for what is essentially an outmoded design.

I decided to go for a Harrison M250. The Harrison company has a pedigree going back to the halcyon days of the Industrial Revolution. It is situated in Heckmondwike, West Yorkshire. The county of my birth and the cradle of British Engineering. So called model engineering lathes often sacrifice some functionality in return for compactness and affordability. Whereas the M250 is a scaled down industrial quality machine, designed for small one off production work. It has many constructional features more usually associated with precision toolroom machines. They were in full production throughout the 1980s and 1990s and were much favoured by schools and colleges. There was also a factory single phase version. You can read more about the machine here:-
http://www.lathes.co.uk/harrison-m/

I found a genuine factory single phase M250 on a secondhand machine dealers site, here in the UK. My new (to me) machine arrived wrapped in polythene and bolted to a pallet. I had to chop out sections of the pallet with a circular saw, to make room for my engine crane. Once this was in I lifted the lathe off its' pallet and fitted the base with resilient, adjustable machine mounts.

 Once on terra firma I inspected the machine thoroughly. As I'd expected from the photos I had seen prior to purchase, it was in superb condition, basically just needing a good clean. I set to this and also changed the headstock, screwcutting and saddle gearbox oils. The well illustrated manual was excellent, giving recommended lubricants and equivalents. I also fitted a new 50 volt 60 watt bulb to the machine lamp. I got it here:- http://www.lightbulbs-direct.com/ 

Inspection of the headstock gearbox revealed very little wear. The machine seemed hardly to have been used. This was also borne out by the condition of the paint on the pedestal; generally the coolant (especially the soluble variety) tends to lift the paint. The finish was more or less intact. I ran through the manual and adjusted the motor mounts and all the gibs in the ways. I decided not to use soluble coolant - it starts to smell and I have always questioned the wisdom of allowing a water based fluid to sit on precision steel surfaces. It causes corrosion, no matter what the manufacturers may say. I used Castrol Ilocut 486. Shining like a new pin and adjusted to perfection the machine was now ready to be tested.

While all this was going on, I had also invested in a mill/drill and a metal cutting bandsaw. The mill/drill and the saw were both of Far Eastern origin. In contrast to the purchase of my first lathe, I had been able to visit the dealer and inspect the machines thoroughly. I'm pleased to report that they were both well built, sturdy and workmanlike bits of kit. They have given me excellent service so far.

I now had the makings of a decent machining capability. Next I'll relate what happened when I switched the new lathe on for the first time.