britishrocket's bipropellant GOX/ethanol thread

[frankenhealey]

Mar 12, 2015 10:59:49 GMT -5 britishrocket said:

And here is a screenshot of Profilab displaying the temperature in my electronics lab (which my wife continues to refer to as the dining room):-

Carl,

I don't understand a lot of this thread but I do understand the sentiments above. My wife also has alternate names for things when I'm having a manic building phase. Some of them are

Parts cleaner = Dishwasher
Workbench = Dining room table
Assembly area = Sitting room floor
Thermocouple test chamber = Oven
Engine Test Safety Zone = Hiding behind the dustbins

Cheers,

Ian

[britishrocket]

Hello Ian,

Thanks for that! My wife has similar acronyms. Please tell me what you didn't understand as my intention is to make things as plain as I can.

[britishrocket]

I've been thinking about chamber and nozzle design a good deal recently. My motivation was to try to come up with a different method of fabricating a thrust chamber. A method that doesn't involve machining the chamber and nozzle from solid, which I've always thought was wasteful. That said, it certainly works, just check out Anders's long running thread.

So I just wanted to try something different, to see what was possible. You'll remember from my post on cooling that I posited a notional tube bundle chamber for my calculations. What if it wasn't purely notional? I decided to see what it would take to build a tube bundle thrust chamber.

My initial thoughts were based around bending the tubes to shape and stacking them together around a circular manifold, possibly using some sort of temporary centralising mandrel or jig. The idea was to braze the tubes, but my attempts at brazing did not go very well. I had much more luck TIG welding them. After a few false starts to get the TIG parameters right I produced a reasonable weld:-





I didn't use any backing gas here and got away with it; in a production unit I would have to set up an internal argon shielding flow in the tubes. However, cheered by this result I next looked at methods of manifolding the tubes and anchoring them to a manifold. Using 10mm dia. 2mm thick wall metric hydraulic tubing meant it was possible to turn down a portion of the tube to 8mm:-





I used a collet to hold the tube, as seen. Next I envisaged the manifold ring to have a series of holes on a PCD, into which the tubes would be welded. I decided to make a rough mock up of such an arrangement using just two tubes, to see how they sat together and generally get a feel for the problem.

I used a square block of mild steel, cleaned up in the 4 jaw chuck. I then drilled two holes in it of the correct size and pitch for the tubes. I countersunk one end of these holes. This would mate with the chamfer on the tubes so they'd sit flush to the block/manifold. The four pictures below show cleaning up the block faces, drilling the holes, the countersunk portions and the tube/block fit up.




Centre drills are very rigid and are great for starting drilled holes (as well as for creating centres!):-








The countersunk ends of the holes:-








And the tube to block fit up:-







Ideally the hole for the tube on the right should have been countersunk fractionally deeper, but you get the idea. The number of required holes and the PCD they'd need to be on for the diameter of the projected chamber would put them closer together than shown here. The gap betwen the tubes in this mock up is about 0.25mm. The number of tubes and the PCD required for the chamber I have in mind would put this gap at just over 0.05mm. I might need to play with the diameter a little to get an adequate gap for welding. This would also feed into the tube bending calculations to ensure that the throat diameter was correct.

Here is a close up of the machined end of one of the tubes. The chamfer is 45 degrees and this then mates nicely into the 90 degree countersink:-






Finally, here is a photograph of the opposite side of the block showing the tube exits. In the design under consideration this is where the fuel/coolant would exit and cross over to go down the next tube. I machined the block thickness to 20mm so that the ends of the tube would be flush with it. The holes would need to have a countersink to correspond with a chamfer on the tube end to get a nice weld. I tried to do this by hand on one of the holes and made a pig's ear of it; why is it always the last operation that wrecks your part?! Anyway, it isn't too much of a worry and I think you can see what I'm trying to achieve.






I'm back at work now so there will be no more practical updates for a few weeks. You might be lucky though and get a lot of meandering nonsense with some highly suspect calculations thrown in. I bet you can't wait.

[Johansson]

Very interesting to see some machining pics of the engine!

I got an idea from reading a book about liquid rocket history. Wouldn´t it be possible to machine a plug with the exact size of the thrust chamber/nozzle internals, then hand wind a thin walled copper tube around it in a lathe and silver solder the tube coil together into a solid walled thrust chamber?

The plug would need to be made in a material that doesen´t stick to silver solder and machined in two parts with the split in the throat section so it can be removed afterwards. This would make a very rigid and internally cooled TC that would be easy to make with basic tools.

Cheers!
/Anders

[britishrocket]

Hello Anders,

It is really good to hear from you my friend. Thank you for the comments. I guess you are talking about GP Sutton's book on rocket history? The idea you describe is a really good one. I wish I had better gear for silver soldering and brazing; I have heard some people can do TIG brazing, and I got some SIF silcopper rods and tried it, but I couldn't get it to work at all.

Hope the turbine bike is going well!

Good wishes,

Carl.

[britishrocket]

Just a thought Anders. The method you proposed to make a thrust chamber might be achieved by using MACOR as the mandrel material. This is the acronym of Machineable Glass Ceramic. It is the stuff the tiles on the Space Shuttle were made out of. You can get it in all the standard sections, and it apparently machines a bit like aluminium. It is quite expensive but I think that could probably be justified. Invented by DuPont as I recall.

So you could make your two piece shaped mandrel out of this as it would easily put up with the heat of silver soldering or brazing.

Carl.

[Johansson]

Hi Carl,

The bike build is going very well thank you.

Interesting material that MACOR, I´ll look it up.

Perhaps the coil will be stable enough so the soldering can be done after the core has been removed, that way the core can be 3D-printed for easier and more precise manifacturing.

Cheers!
/Anders

[britishrocket]

Hello Anders,

Glad to know the turbine bike is going well. MACOR is a very interesting substance. It is definitely of interest to anyone involved with gas turbines or rockets, because it can withstand very high temperatures, but can be turned, milled etc like more common materials.

3D printing would be a good idea to make the core, it seems a good way to make cores for investment casting too. A friend of mine was looking at producing a kit to build a 1/4 scale model of a Rolls Royce Merlin engine. He had the gearbox output shaft 3D printed and it was a joy to behold. You could even print turbine blades or a whole turbine wheel. 3D printing with metal exists as a sort of 3D sintering. Then there is Mr. Unreasonable Rocket, who has 3D printed a whole rocket thrust chamber and injector...I feel a bit old fashioned slaving over a lathe and welding gear!

On the subject of advanced manufacturing techniques, a couple of things have been on my mind lately. The first is electroforming, i.e. creating a metal form by electrolysis. You could do this to close out cooling channels for example. The second is EDM. I am a member of a group here in the UK called SMEE - The Society of Model and Experimental Engineers. They have a sub section called the Digital Workshop Group, one of whom has demonstrated a homebuilt EDM unit. I'm thinking the thing might be to combine this with one of the small x-y-z engraver or routing kits you can get now to make an EDM machine.

Regards as ever,

Carl.

[britishrocket]

Some more information on telemetry and control...

On my return from work I found the digital video recorder and the four CCD CCTV cameras I had ordered waiting for me. The DVR has a USB interface and so will be easy to integrate into the overall telemetry system. The idea is to have a camera at all of the main points of interest, with at least one on a pan and tilt unit. You can have all the sensors you like, but you can't beat a recording of a camera looking at a bank of gauges.

I wouldn't mind betting that a lot of us who post here will have heard of the "Maker" movement. An affiliation of individuals who combine engineering and design to come up with interesting and novel projects. This movement has been gathering momentum over the years, and the major players have started to take notice. ONe result of this is that NI LabVIEW have now released a copy of their industry leading telemetry and control software in a "Home bundle" edition. This is a fully featured version of NI LabVIEW available for home users at a price of 49 US dollars. This equates to about £33, which is an absolute bargain. More details can be found here:- sine.ni.com/nips/cds/view/p/lang/en/nid/213095

The design of the tube bundle engine is progressing well, and I am going to be posting more details of the mock up tube arrangement both here and on the blog. The tube bundle concept will require lots of indexed holes on a PCD, so to that end I have decided to convert one of my rotary tables to stepper motor operation, under control of an Arduino Uno. I'm using the method shown by Gary Liming in the US Digital Machinist magazine. So I will post more on this as well.

[britishrocket]

More on telemetry...

It seems fairly obvious that a device as potentially fractious as a home built liquid fuelled rocket engine would be best viewed from a safe distance. Indeed, Krzycki suggests viewing the rocket exhaust plume by means of a conveniently positioned mirror.

Fortunately things have moved on since 1967. From the outset, a video system was always going to be a part of the telemetry set up I had in mind. Nowadays, small, reliable CCD cameras and USB digital video recording systems (DVR's) can be had for a modest outlay.

I purchased a four channel USB DVR and four miniature CCD cameras from Lightinthebox.com. The cameras are also equipped with a small microphone. Each camera has a flying lead terminated in three RCA type connectors. The colour coding used is as follows:-

Red = 9VDC power

Yellow = Video signal

White = Audio


The DVR features four video inputs through male BNC connectors, hence a BNC to RCA adaptor was required for each one. There is also one audio input. This is not a great worry as I do not anticipate hearing the engine being a problem.

Each camera measures ~ 20mm (0.75") square. They are thus small enough to fit into a tight space.

Here is a photograph of the DVR unit with all four cameras connected:-






In and amongst this snake's wedding it is possible to see the four cameras, DVR and the RCA to BNC adaptors.

The DVR comes packaged with software to view and record the camera images. The images can be viewed in quad format, or each channel can be viewed separately. Motion detect record is also possible. Here is a screenshot of the software running, with some frankly resistable images of my kitchen:-




And here is a close up of one of the cameras, the lens through the lens, so to speak:-






When these cameras are spread around the test stand, they will greatly improve the safety of operation, as well as acting as a back up and cross reference to any data acquisition set up. Transducers may well tell the story, but a recording of a camera pointing at a bank of gauges is still hard to beat.

And finally...on the subject of telemetry, National Instruments have just released a home version of their industry leading LabVIEW software, aimed at the Maker Movement. It boasts elements compatible with Arduino, and is being offered at a bargain price. Google "NI LabVIEW Home bundle" and you will see what I mean.

More to come, keep watching.

[britishrocket]

I suspect that it will be abundantly clear to you, my astute reader, that the production of a tube bundle rocket thrust chamber will require the fabrication of a foundation element with a multitude of holes equally spaced on a PCD.

Of course this means indexing. The usual way to achieve this is to use a rotary table with dividing plates. This time honoured method, whilst working extremely well, is also rather tedious and is prone to human error - especially when I am doing it!

Therefore I decided that it would be worthwhile spending a little time motorising my Vertex 6 inch rotary table to automate the indexing process.

After some internet research I discovered Gary Liming's excellent Step Index project. Gary writes for the US magazine "Digital Machinist". Gary also has a very good website with a build log for the project.

Step Index is an Arduino based system with the firmware currently at version 2.3. The system can work with fixtures with a variety of gear ratios. Initially it was designed for a 3:1 set up, but has since been re-written to cope with 40:1 and 90:1. Most rotary tables are 90:1, as is mine. The Arduino source code can be downloaded from the Digital Machinist site, as can a Read Me file explaining how to make modifications to the code to add more ratios or to make a specific value the default.

Gary gives a comprehensive explanation of the operation of his system on his site, so I will only briefly outline it here.

An Arduino Uno board is at the heart of the system. This is fitted with an LCD keypad shield. I got mine from Sain Smart, but there are many other sources for this part. The Arduino displays system modes and status via the LCD. A stepper driver board based on the Toshiba TB6960 IC is clocked by the Arduino, thereby driving the stepper motor. The motor I used is from Zapp Automation here in the UK, type SY57STH76. This is a NEMA 23 frame, 2A, 1.8 degree per step motor with a holding torque of 1.89Nm. I also obtained a 12V, 5A switch mode supply to run the lot.

The photograph below shows the system connected up for bench testing:-







The buttons on the LCD shield are used to scroll through and select menu items. I will piggyback larger, more user friendly ones when I fit the system into a die cast case.

The menus are as follows:-

Ratio - Allows the user to select the ratio of the fixture being used. The software can be modified to include as many as are required, but 3:1, 40:1 and 90:1 are included as standard.

Temp - Facility to read motor and driver heatsink temperature, using two sensors connected to the Arduino's analogue inputs. I did not use this function.

Step - Enables user to specify a number of divisions. The motor is then incremented the correct number of steps for each division.

Angle - Similar to above except that the user inputs the angle the fixture is to be rotated through.

Run - The motor will run continuously with speed set by the user.

Jog - The motor can be nudged a preset number of steps.

The photographs below show the Arduino Uno board and LCD shield, the stepper drive board and the switch mode PSU.







The momentary push buttons can be seen here, as can the multi-turn potentiometer used to set the screen contrast.





Stepper driver board. DIP switches allow the setting of current characteristics and microstepping settings. The power IC and heatsink are on the underside.





The switch mode PSU. The preset potentiometer above the terminal strip allows the output voltage to be set exactly.

The next step is to prepare the rotary table. This unit is a Vertex HV6. It will need to be finessed somewhat before the motor can be fitted. I will also have to make the motor mounting hardware.

Once assembled the new improved rotary table will greatly facilitate the machining of the engine components.

[britishrocket]

Today I got the stepper motor connected to the rotary table input shaft for a preliminary test. The mechanical link between the stepper shaft and table shaft was made with a bellows type aluminium coupling. The stepper drive shaft is 12mm diameter, whilst that of the motor is 6.35mm. i.e. 1/4 inch. I managed to find an off the shelf bellows coupling with these bore dimensions.

Due to the relatively high gear ratio of 90:1, I took the decision to dispense with micro-stepping of the motor, and I altered the appropriate line in the software to reflect this, as well as setting the dip switches on the driver circuit.

Here is a picture of the set up. As can be seen, the motor was supported by a machine table clamp during the test:-










The motor ran the table with no trouble at all. I ran the table in Step, Angle, Jog and Run modes. I am not sure if the motor will be able to drive the table for actual machining, as opposed to just positioning, but it certainly did not seem to lack torque.

I didn't attempt any accurate measurement of the angle the table turned through in this test. That said, going by the graduations on the side of the table, everything seemed to be in order.

[britishrocket]

Yesterday evening I started machining the motor mounting hardware for the indexer. Started with square piece of 6082 aluminium, squared this up on the mill. The dimensions are 56mm x 56mm, the NEMA 23 standard motor face dimensions.

With the piece in the 4 jaw, I machined a 38mm diameter bore to act as a locating feature for the 38mm diameter register on the front of the motor.

I also machined a 48mm register for the tube that will join this square portion to the round portion at the opposite end.

Here is the part thus far:-

[britishrocket]

Yesterday I turned up the tube portion of the mounting bracket. This started out as a piece of 2" dia. x 10swg 6082 tube. I turned down the outside diameter in the 4 jaw to be an exact fit in the recess of the square plate.

Here are some photographs of the plate and tube trial assembly, and the fit up of the motor and the parts made so far:-

[britishrocket]

Round portion of the mounting bracket is now complete. Took a section of 6082 2 1/4 inch bar, turned it down to 56mm in the 4 jaw. Bored hole to 21mm for slipping over the rotary table drive shaft sleeve, then generated the 3mm deep recess to suit the tubular section. Here are some photographs of the round component and all the parts so far made:-