Wednesday, 6 April 2011

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.

Tuesday, 5 April 2011

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.

Sunday, 3 April 2011

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.

Saturday, 2 April 2011

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:- 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:-

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

Friday, 1 April 2011

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:- 

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:-

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.