Wednesday, 27 June 2012

Weekend test, vehicle progress

I tested the new catalyst pack on Saturday and although thrust still isn't what I want it to be (10Kg) it is a definite improvement over the last pack.

Roughness is defiantly an issue but I am not so concerned with it as getting higher flow rates through my pack. I applied about half as much compression as the last pack but it seems like that was still too much. 

I decided to start testing on a plastic sheets after I heard about a incident where peroxide leaked into cracks between two concrete slabs. Apparently pressure built up underneath and caused an explosion which killed a person. This meant that I needed to test on an incline so the sheets didn't catch on fire....

Engine with injector removed and pack removed from engine:

Both the distribution plate moving and the bits of teflon tape are of concern. I have probably gotten a bit slack with cleaning the old tape off fittings. The plate rotated because it was only pressed in place and not held up against anything. Its difficult to get the top of the pack to line up with the injector, especially when you are assembling a new pack as it shrinks so much. In future I will take it out and add spacers to the bottom. I think the roughness/oscillations are mostly due to the head space.

The pack fared very well and was not at all stuck together. The combination of the stainless mesh and the top and bottom distribution plates gave the pack good strength and seems to work well.

I used about half as much compression in this pack as the last one but I think that was still too much. I really need to add a pressure gauge to the press so I can quantify exactly how much force I am applying. That might be a task for the weekend. I am hopping that with less compression I can get an even higher flow rate.

If not then I think that I will need to go to a larger diameter pack for more flow rate. I really like the 20mm size pack because I can get 60mm square pure silver mesh easily from which I can get 9 discs. The original vehicle design called for a 30Kg vehicle with 10Kg of fuel. With the new tank/design I should be able to get the weight down to about 12Kg without optimising its structure. There is a lot of room for weight saving if necessary as things like the base plates and solenoids could easily be drilled/milled to save weight. I could also make my own aluminium fittings instead of using the heavy stainless ones if it came to it.  If I can get more thrust out of the engine then that will mean more flight time but if not I would rather just use what I have and get something flying. 

Thursday, 21 June 2012

Progress this week has been slower than I would have liked because I am working full time over the uni holidays. Testing didn’t happen last Sunday because I went for a long run in the morning and couldn’t really walk/stand up. Monday I dident have a car after work and Tuesday I did but got to the workshop and realised that I forgot the keys and after getting home just ended up working on other things. Wednesday night I did manage to get into the workshop and spent a few hours modifying the brass thruster to use copper gaskets instead of the rubber ones. This involved making a new injector as the old one was too big. The new one isn’t pretty/ well aligned but will work. Testing with the brass truster will be good because I will be able to measure chamber pressure and also because there is a flow distributor on the top.

So time is the biggest issue at the moment. I would love to just settle for less sleep but the problem is I drive for 3 hours to and from work and it gets dangerous with little sleep. I could probably do it for a while but I can get pretty cranky when I drink too much coffee and I refuse to sacrifice relationships for some engineering project (however interesting). The upside of the drive is that I get to listen to lots of books and entertain my other hobby, philosophy. I have tried listening to text to voice of rocketry blogs and journal articles things but that quickly gets frustrating.

This  weekend I would like to fire the previous cat pack compressed and if that doesn’t work adding more layers of silver/stainless/.

Also on Wednesday I took apart the previous pack to transfer it to the new pack and made some interesting observations. Firstly the top few layers which had almost no compression and were packed with spaces in them looked very un-broken in (in the other pack the top layers were very broken in). I will try to remimber to take some photos next time I am at the workshop. By broken in I mean the colour change that happens to silver when it is exposed to peroxide. I thnk that this has something to do with the amount of compression since the other discs which were at least together seem pretty stuck together although this time in lots of 5 as they dident stick to the silver mesh. I have a theory that one of the mechanisms by which compressing pack makes it more effective is that it makes it much more able to hold peroxide (sponge like) due to surface tension until it can react. This might also make a more porous, inter-connected structure. This is like several really thin plates closely spaced vs the same number of plates spaced far apart. I could probably test this by seeing how much water a pack vs the individual layers hold. A more effective way might be to test the decomposition rate of two layers stuck together vs. two separate.

If I cant get the thruster working satisfactorily by this weekend I have promised myself that I will get more scientific with future tests. To date I have been really just going with my feelings based on what I have read but there are lots more variables that I initially appreciated so I will need to get a little more consistent and directed with the tests.   

Saturday, 16 June 2012

New pack testing

Todau I assembled a new catalyst pack with the following composition:

SS = Inert Stainless Steel Mesh
SM = Silver Mesh
AC = Anti-Chanelling ring

10 x SS
5 x s
1 x ss

5 x s
1 x ss
5 x s
1 x ss
for i in range (2):
   1 x ss
   5 x s
   1 x ss
   5 x s
   1 x ss
   5 x s
   1 x ss
   5 x s
   1 x ss
4 x ss

I tried 2 runs of this new pack today in the stainless engine with a 8.5mm throat. The first one was really just a break in run with 800ml of %90. I started with a really slow flow rate and increased. A few times liquid peroxide squitrted out so I backed off the flow rate. The exhaust of the first pure silver pack was clear at this stage (after 800mL) however the exhaust of this new pack was still quite cloudy so I loaded another 2L and tried again. After some time there was no liquid in the exhaust but it was still really cloudy. I was clear that the pack wasn't completely catalyising so I decided to stop the test as to not waste the rest of the peroxide. The thrust seemed promising, about 7Kg but it was difficult to determine accurately as I was only pulsing the engine and it was bouncing around allot.

It seems like this new pack can handle a much higher flow rate than the pure silver one, although this could just be because it is not catalysing fully. I didn't actually compress the pack as I was assembling it, although it did compress slightly when putting the injector on the engine. Tomorrow I intend on adding another 25 silver layers (interleaved with stainless as above) and compressing firmly before trying again. If I get time I might try a run with just compressing (before adding 25 extra laters) to see what difference that alone makes.

Now that the pack can handle higher flow rates it would be really useful to add a jet/oriface to limit the flow of peroxide; this will help limit flooding of the engine at startup, although hopefully once the pack breaks in this won't be as much of an issue.

I have started just filling up the run tank directly (by taking off the valve/stem) instead of using the vacuum system. It is actually quicker and I believe safer as it reduces the number of handling steps and chance for impurities to get into the system. Also we can fill the tank in the clean area instead of outside.

Saturday, 9 June 2012

Catalyst Pack Dissection

Today I dissected the catalyst pack and made some interesting observations.

The first thing I noticed is that all the layers of mesh seemed to be fused together. I could peel the top 10 or so layers off individually (with considerable effort) but after that they were all stuck together.

Eventually I managed to break the pack up where the anti-chanelling rings were by levering them.

There is a "tunnel" through the bottom 1/3rd of the pack but this doesn't exit the pack. The pack also seems to get progressively more degraded the farther down which I suppose is expected. Its really off the the tunnel doesn't exit the pack, but this could just be because it hadn't been used enough. The mechanism for the tunnelling seems to be bits of silver breaking off and impacting farther down.

The fact that all the layers are fused would explain why I can't get the flow rate I need through the pack, as the pressure drop would have increased significantly.

For the next catalyst pack I will use interspaced stainless mesh which should stop the fusing problem. I thought that the function of the stainless mesh was only to give the pack rigidity and that because my pack was so small I cold do without it but its other function is to stop the silver layers sticking togethere. I will also have a proper retaining ring at the bottom which should stop the bottom layers extruding through the bottom ring. Sets of 5 silver intersapced with stainless mesh seems to be the standard so thats what I am going to use. I have read about engines using 80 mesh so I am going to use the 70 mesh which is better value (more area for less weight) than the 40 mesh. The downside is that it is not as strong but hopefully the stainless mesh should take care of the strength.

I am still aiming for a limited test flight on the 26th of next month. Once I have the engine sorted I will continue with the vehicle but I don't think it will take long to finnish off the first prototype. I think we will probably have to make a second as I am not expecting everything to fit first time. I suppose the first vehicle will really be more of a mock up and we will probably figure out how to save a bunch of weight from it. Also I still don't have a final design for plumbing but it will be allot easier to plumb everything once I have something to work with. I was actually thinking of using stainless hard lines which would be lighter and more compact. Also the teflon-stainless lines don't have the best bending radius. Once exams are over I am hoping Buren and Scott will be able to put in more hours which should also speed up construction.

Friday, 8 June 2012

More Tests

For the I have conducted three tests over the last few days but the engine still doesn't seem to work properly; the problem being lack of thrust.

From the last test it was clear the flow rate of peroxide into the engine wasn't enough as if we assume complete decomposition and a modest 4Kg of thrust (was probably more likely 3.5) the theoretical run time pretty much matches up with the measured run time.

At first i thought the solenoid could be the cause so on Wednesday I tried 2 tests, the first with 2L of peroxide and the second with 1L. I forgot to press record on the first test so hence the second. Looking at the real time graph of the first I could see that the thrust hadn't changed much (possibly a very small amount) so I decided to increase the pressure to 750PSI. This increased the thrust to about 5Kg on full (still with the larger solenoid). The flow rate appeared to be about the same. Unfortunately the results from this test didn't save properly, I believe because I disconnected the controller without stopping data being recorded. So the smaller solenoid seems not to be the cause.

I actually wanted to do this test on Monday night Everything was set up and ready when I Accidentally shorted a wire and broke the usb server. I went and bought another but annoyingly the new one was also broken. I took it back and even more annoyingly there was no more in stock so I got a usb cable extender (usb to cat5) which seems to fix a few problems I have had with the data recording and which I will be using from now on.

Here are plots of the first and second test. The first test was actually longer in duration but was done in parts (test 2 was one continuous run):

Having eliminated the solenoid next I thought that the throat could have been too small so I enlarged it from 7 to 8.5mm. Design says an 8mm throat for 10Kg for 300PSI but I made it a little bigger just to be sure. I tested again last night with the standard 500PSI and 2L however enlarging the throat seems to have made the problem worse with thrust being less than before and quite rough. This time I was careful to not make any mistakes with the data recording. My guess is that because the throat wasn't limiting flow (its still being limited) the larger throat means the engine can't maintain pressure. Because the engine was so rough I don't think the flow rate estimate can be compared with the smooth tests.

Plot of most recent test:

Plot of first

I think the most likely cause now is the catalyst pack. Except for when the pack is cold I have never been able to flood it. This seems like a good thing but probably means it is too big. The fact that enlarging the throat causing the chamber pressure to drop (probably) to the point where the flow was rough and thrust decreased and that this didn't increase the flow rate (probably) means that the pressure drop over the pack is too big. More evidence for this is seen in the thrust increasing with feed pressure increase (although not necessarily). The design I was using called for 111 screens of 40 mesh 21mm in diameter. I used 80 screens of 80 mesh and 70 of 40 mesh. The extra mesh would have increased the pressure drop significantly. I need to do more reading on pack design; I thought it was relatively straight forward but it doesn't seem to be.

For the next test I plan on reducing the size of the catalyst pack. I will also need to make another thruster as I want the results to be comparable, but I could just use the brass one (enlarging the throat to 7mm). To eliminate the feed system up to the engine I will put a pressure transducer in the inlet; if it is close to 500 its probably not the problem.

On another note, something I have wanted to build for a while now is a liquid nitrogen generator. A while ago I got a quote for a model that can make 20L per day but it was $65000 US which is a bit outside of my price range.... Most of the smaller generators seem to work on a Stirling cryocooler which uses a sealed Stirling cycle with helium as the working fluid. Cryocoolers themselves are not that expensive, you can get a used cooler capable of a few watts off eBay for less than $1000. This person actually made his own using a cryocooler in combination with a nitrogen filter (filters nitrogen from air) and a dehumidifier. Presumably the same setup could be used for making liquid oxygen by changing the nitrogen filter for an oxygen one as oxygen condenses as a lower temperature. Another way I thought of getting the temperatures required would be to out several pelters in series. This would be much cheaper but considering the losses and the fact that pelters get less efficient at lower temperatures the system would be horrendously inefficient. I really want to give it a go but I already don't have enough time and I need to stick to my goals lest I do a Space Shuttle......

Sunday, 3 June 2012

Thruster Test + Vehicle Components

Yesterday we tested the new engine, and we got some good load cell data but disapointingly the engine only produced about 3Kg of thrust. There could be a few reasons for this but I think we the most likely thing is that the engine isn't getting enough peroxide and we need a bigger solenoid. Other factors which could have caused less recorded thrust are that the thruster was firing on a slight angle and could have gotten slightly jammed in its sleeve but I tried to calibrate the cell recreating these and it is fairly clear that something is not working properly. I would like to try another test this week with using a larger solenoid. Playing with the engine is still pretty novel and I pulse the engines a few more times than I need to. I noticed a small amount of leakage from the engine which I think is because the engine flange is too smooth and the copper ring doesn't bite into it enough.

For the above test we used 1L of %90. We were actually going to use %85 but at the time we couldn't figure out how much water to add (its harder than it seems). Here is a excel spreadsheet my friend Robert Walker wrote to do it, I thought someone might find it usefull.

I was quite happy with how the test went and how everyone performed running the test but there definitely room for improvement on the logistics side. Firstly it was raining pretty heavily, and although everything was under a tent the water did make the test more difficult the it would have been otherwise, so no more testing in the rain. The tent should be for protecting everything if it rains, not for testing in the rain. It made the suit fog up and we couldn't really hear Buren read out the fill steps so he mostly went alone. Following on from that we need a better method of communicating with the person  filling. I will getting a radio with a VOX headset. The filler should also wear a sweatband or something on his head so he can wipe any moisture on the inside of the suit.

We actually planned on doing two tests but when filling for the second test Buren found the tank only sucked a small amount of peroxide before it wouldn't take anymore which gave us all quite a fright as it could have meant that there was decomposing peroxide in the tank. We used the small amount of peroxide and vented and I decided not do try again but instead to disassemble and check everything. As it turned out the reason the system wouldn't take any more peroxide was because it had sucked a bunch of air initially (the tube wasn't properly in the peroxide container) but I thought it was better to be safe. Also we hadn't changed anything since the first test and the went fine so I don't think any contaminants could have gotten into the system. To deal with this I will install a pressure gauge on the system so in the unfortunate event there is peroxide decomposing in the system we know as it is happening.

We also need to stop doing nigh tests as it doesn't make things any easier.

I have also been working on components for the vehicle, and have now finished the three base plates. I managed to get the plates into the lathe with some difficulty as I couldn't find any other way of making decent holes in them.