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.
Sunday, 31 July 2011
Wednesday, 27 July 2011
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.
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.
Thursday, 9 June 2011
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:-
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.
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.
Tuesday, 24 May 2011
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:-
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:-
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.
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.
Wednesday, 18 May 2011
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.
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.
Tuesday, 17 May 2011
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:-
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..."
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.
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.
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.
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