Tuesday, 29 May 2012

New thruster and vehicle design

Today I finished off machining a new thruster from 304 Stainless. Machining the stainless was quite dificult compared to the aluminium and mild steel I am used to. The new small boring bar was good but I went through 2 inserts in what was not allot of machining which was all I had. I really need to get good set of carbide tools as the ones I have dont seem to be lasting a long time (cheap ones from ebay). I broke too many inserts by not being patient enough.

I am happy with the engine as a first attempt but its heavier than I wanted (600 vs 400g) as I was conservative with wall thickness. I had designed for 3mm wall thickness but after spending hours working on it I was paranoid about cutting through the wall so I made it 5mm to be safe. The flange is also quite a bit thicker than I need it but I made it thicker so I could experiment with different clamping forces on the copper gasket. I allowed 1Kg for each engine in the vehicles design. Also I never seem to be able to get the bolt holes perfectly arrayed and there are always small mis-alignments (only goes together in one position).

I also made some parts to mount it to the test stand. This was a bit of a challenge because the inlet is on top of the injector plate as the thruster is designed to be bolted to a plate. I ended up making an adapter which makes the thruster all one size (size of the flange) so it can slide in a pipe which is fixed to the stand. There is then an inner free pipe which transfers force to the load cell. Both have slots milled in them at the load cell end at so the feed line can escape.

I really wanted to do a test later this week but Scott and Buren are not free until the weekend. After that I will start on a final revision. Hopefully we will get some good data this time.

 I am starting to build up momentum on the hovering vehicle front. We completely redesigned the frame after the original concept proved tricky to build. The old design had a frame made from sections of right angle aluminium which formed a triangle with plates to make it more rigid.

The tank was epoxied to the brackets which I was never particularly keen on. The whole thing was pretty complex and I was worried that it wouldn't be rigid enough so we came up with a new design which is much simpler and will be allot easier to build.

The new tank is now held between two plates with three thin aluminium tubes which clamps the plates to the tank. Electronics are not shown but I would like to have a second plate bellow the lower one so the electronics box can go between it and the upper plate.

The thrusters are brought in really close which makes landing gear not particularly effective unless they are a long way out which would be heavy. For initial tests we want to have the vehicle hanging so I am not even bothering with landing legs at the moment. One idea is a foam disc as shown in black in the concept picture, however I think it would be likely to tip over when landing. I am worried that the tether will mess with the control system. 

The vehicle will be initially pressurised to 500PSI leaving a safety factor of 1.9 which is much  less than that of the nirous tank we were going to use. If the vehicle had a bad fall the tank could rupture which would be really really bad. I think we will need to find a place other than the workshop to test. Padding around tank might help but I still want to be comfortable that in the worst case know-one will be injured. 

I started making the base plate yesterday and am fairly happy with how it turned out. It was pretty difficult to make as I had to plasma cut out the rough shape, mill it flat then linnish. I think I will try to find someone with a press brake for the rest. I would like to get a prototype frame mostly finished by the end of the weekend.

Saturday, 19 May 2012

I haven't been getting as much done as I would like to over the last two weeks but things are progressing and its about time I wrote an update.

I have been mainly working on the control system and have also been talking to a few people from UNI and other places about it but have gotten really mixed advice about how to develop it, which has really confused and bogged me down, to the point where I am not sure what to work on.

Choices are:
Do a really detailed simulation in CADAC++
Investigate a more optimal controller (LQR has been suggested) and use a more powerful micro controller.
Keep doing what I was planing on doing

What I was planing on doing is using a simple PI (turns out I don't need the D) and using simulink to program an arduino to demonstrate a proof of concept tethered stabilised vehicle; hanging or to the ground if that is too difficult.

I think the problem is my goals are not clear enough. The entire reason I started this project is to learn and programming a detained simulation in CADAC and learning about optimal control sounds like fun, however, if I did everything at a university/professional level I wouldn't get anywhere because it would take too long. I really need to decide if I want to do everything to a high standard or if I just want to get something that works. I could easily see my simulator talking to the deadline I set in June and I would probably be less motivated to work on it. Considering the amount of time and money I have put in and that I probably wont be able to work on it next year as I will be working full time I think I just need to get something working rather than study everything in depth.

 I have also started on an electronics box for all the flight electronics:

I gave some thought on how to package everything so that it would be still functional after a decent impact. Scott had the idea of mounting everything inside one box which was padded inside another. I liked the idea but it is a bit complicated so I just decided to pack everything in foam and have several levels of electronics stacked on top of each other. The top level isn't finished as I am waiting on some switch mode power supplies. Annoyingly I ordered some but they were only step up and wouldn't transform down. I am slightly worried that they will create noise which could interfere with the magnetometer on the IMU.

Also the miniature boring bar finally came yesterday which means I can continue work on the stainless thruster. I made a few attempts to make my own out of 1/8" square tool steel stock but none worked well enough. 

Wednesday, 2 May 2012

On Tuesday we tried a 30 second fireing of the 100N engine measuring thrust and chamber pressure. We diluted the peroxide to %85 so we wouldent have any issues with the silver melting over the long run. A few seconds into the test one of the nitrile gaskets broke which resulted in most of the peroxide leaking out and the engine pulsing wildley.

Despite the engine only running propely for a few seconds we good some good data. I still havent calbrated the load cell propelly but it looks like it produced about 60N. I am not sure if this is because the engine hadent warmed up yet, or because the throat needs to be enlarged more. Engine pressure was 120PSI at its peak which is alright but I would like it at 200PSI.

So we really need a new design of seal. I have been looking at Armidillo’s engines and it seems that they use copper gaskets. I suppose it makes sense that a somewhat flamamable rubber seal (nitrile) which is heated to 800 degrees and exposed to peroxide would fail. The odd thing is that the gasket worked fine for the last test which was much longer than this one, although it did look slightly burnt/warped. Looking at the video it seems that the gasket actually popped out rather than burning through. Also when we took the egnine apart it wasent actually broken. My guess is that the engine bolts wernt as tight as it was for the first test.

In other news I found a great new tank which will make for a perfect flight tank on the hovering rocket as it is holds nearly the same volume as the heavy nitrous one (10L) and only weights 2Kg! Its designed for compressed air in automotive applications and is only rated to 200PSI but I figured it would have a generous safety factor so I got one to test. It also has holes on its top and bottom which means we could do without vacuum filling which is another bonus.

We hydro tested the tank and found it to burst at 950PSI. My previous method of hydro testing is hydraulic pump, however it runs on oil and makes a mess so I tried another method of using air from our high pressure cylinders and a regulator. I figured that because the tank was filled with water the failure wouldent be particularly catastrophic, however I think that some of the nitrogen dissolved in the water because when the vessel ruptured it made quite a loud sound. It was in the peroxide bunker (where we used to concentrate the peroxide) so I dident do any damage but it gave everyone quite a fright. The resulator method was good because I could slowly increase the pressure as opposed to the jack which required allot of force and usually vessels fail when applying force and when you are not looking at the gauge.

I still want to test one more tank to failure to ensure the failure mode is the same before we incorporate it into the vehicle. The vessel seems to have failed at one of the leg welds which is a bit unexpected as we all thought it would fail at the cap welds, but makes sense as the stress in the cylindrical section is double that of the hemispheres and the leg weld would have weakened a small part of it. The crack seems to have travelled down the tank to the other leg which would have been the next point of stress concentration. When you look inside you can see discolouration from the leg weld. I will film the next test with my high speed camera which should give us a better idea of what happend.

Assuming the next vessel fails at around 950PSI I will feel comfortable using the tank at 500PSI which is what we were going to use the nitrous tank at. The tank has 5 fittings in it, 4 on one end and one on the other. The 4 fitting end will probably go on the top of the vehicle. I am slightly worried about what would happen if the vehicle fell from a decent height and the tank took the impact causing it to rupture. I think we will meed to find a larger location to test at. I was actually thinking that we could pressurise the vehicle remotely so we dont have to be anywhere near it.

As for the vehicle, my goal is have it ready for a tethered test by the 25 of July  which is about 11 weeks away. My original goal was 6 weeks but that would be the middle of exams, and the holidays will provide a good time to bring everything together. I am confident we can get everything together before then.