Saturday, 7 January 2012


I am currently at the workshop working on some stuff while the sparger is going its thing. I have moved the setup outside and now monitor it remotely via a camera. Having it outside does have the advantage that I can only sparge at night when know one else is around.

Now that I have a steady supply of peroxide I ave started thinking about what to work on next. What I would like to work towards is a hovering vehicle similar to the first vehicle armadillo made, with 4 small thrusters  each with a pulsed solenoid for throttle control. I have always been interested in the control side of things, and rather than building something that goes high or fast I would greatly prefer the challenge of building a actively controlled vehicle. People seem to waste so much time and money on rockets that they never recover.

I am thinking of going for a design with three thrusters in a triangle. Although one less thruster would slightly simplify the design (and save weight) it will complicate the control as the thrusts will not be orthogonal to each other. It shouldn't complicate it greatly as I should be able to write a function which I will give cartesian vectors and will return the required throttles. I have done quite a bit of work with omni-drive robots and this should be quite similar, although there is an extra (coupled) degree of freedom. Getting that to work in practise is another matter entirely.

Pretty much the entire vehicle needs to be designed around the tank('s) as there are very few options available, short of getting something custom made. There are plenty of long skinny tanks for hybrids etc, but there are not manny med-high pressure (1000 PSI) tanks around that are short and fat. (I want the vehicle to be as flat as possible to make it difficult to flip over). There are some really nice 1.1L (1.43Kg %90 peroxide) composite tanks designed for paintball but as I will explain later thats not quite enough volume. The next size up is a 10Lb now bottle which is what armadillo used on their first vehicle and is what I will probably go with. Nitrous tanks never seem to list their volume, only how much now they can hold which i suppose makes sense but is annoying. Considering the density of nitrous oxide the tanks seem to be able to hold about 8.5L although I know that you can not completely fill nitrous bottles so it may be more. I am going on the assumption that a 10Lb bottle will be able to hold 10Kg of peroxide although it should be able to hold more. The bottle itself weighs 6.8kg so lets say 7.

We are up to 17Kg with just tank and a fuel load of fuel. Lets say that each thruster (including consumable catalyst) weighs 1Kg and each  solenoid weighs another 1Kg thats 23Kg. Then there is just structure, batteries, electronics, wiring and plumbing. I am hopeing to keep the whole vehicle under 30Kg. Some of the propellant tank will be used for storage so each thruster will need to be about 100N.

If we assume a modest ISP of 80s, with max thrust we will have a fuel flow rate of 0.375Kg/s, with means that at max thrust and assuming only 8Kg of fuel it will be able to fly for 21.3 seconds. I would be happy with 10 seconds of flight so this scale of vehicle seems to be good, and with 8kg of peroxide per flight I should be able to get in a few flights per sparge batch. I don't want to build something that has outstanding performance, my main objectives are price and simplicity.  I am tempted to try a gimbal, but that introduces a lot more complexity. I would like to go for the smallest vehicle feasible but I don't think I will be able to get more than a few sounds out of the paintball tank. Although that ratio of tank mass/fuel is about the same I don't think it will be easy to build a light enough thruster and I am already going with the smallest solenoid available. Buren is doing up a model of a basic vehicle with should give us something to start from.

So I guess the first step is engine. Convientely  the 100N nitrous-ethanol thruster was designed for 100N thrust, so it will make for a good test engine while I wait for parts to repair the lathe. All I need is a small sleeve to provide an internal edge for a catalyst pack to but up against, a piece of aluminium tube would work well. Really what I need to start work on is a test stand.

I have been doing some experiments with different methods of using potassium permanganate as a catalyst. A silver catalyst has its advantages but everyone who uses them seems to spend so much time trying to get them working properly, and it seems to be more art than science. Poisoning is a big issue especially if you get different brands of peroxide. It seems like although it is slightly more work using a consumable catalyst is the way to to. I would like to avoid using a liquid catalyst (injected into the chamber) if I can, because that would mean a whole extra set of plumbing. Stacked discs of something coated in a consumable catalyst seems to be the most popular design for cat packs, but I would like to use a granular catalyst that I can just sandwich between mesh. I haven't seen any other engines use this design, but it seems to have the advantage that you can just pour in the catalyst instead of delicately arranging lots of discs.

Today I tried making some granular catalyst by letting kitty litter absorb a permanganate solution, then evoperating the water leaving a porous granular material with permanganate inside of it. I should add that the idea of using a absorbent material and evaporating the water came from an acquaintance of mine (thanks!).

I tried two different types of kitty litter, a clay based one (Brown and on the left) and a gel based one (clear/Blue and right). The clay wasn't particularly strong to begin with but after baking it became even softer and clumped together when saturated so the final product contained quite a lot of soft power.   The gel one was hard to begin with and was able to absorb more solution giving a product darker in colour which would indicate  more permangenate (or maybe it lust looks like that because it was clear to begin with). The gel one seemed to be more reactive (both were more than good enough to use for a cat pack) when exposed to %55 peroxide. I would also like to try  the same thing with silica gel. If it is as good as the gel kitty litter it would make an excellent catalyst  material because unlike the litter gel it would make a more uniform material with constant pressure drop between batches (the litter is jagged and not uniform). I think the gel kitty litter is actually silica gel, but it is probably a lower grade than that used for dehydration, which means that dehydration gel might be able to absorb more. There is also a paper based kitty litter that I would like to try. I am slightly worried that the catalyst might just absorb moisture but i suppose that if its operating in a engine there won't be any water to absorb.

1 comment:

  1. Hi John,
    I'm really interested in your project. Two quick comments I'm having reading this post.

    First, you probably want a test stand before a vehicle. Getting an instrumented test stand to test your motors, get an idea of their performance, try different kinds of arrangements for the catalyst, etc. is much more work than it may look at first sight.

    Second, I'm predicting that the consumable catalyst, in a monoprop motor, is not going to last long, at all. I'm looking forward to see your results, but I doubt you could pack enough permanganate or any other catalyst in some porous material, to last 10sec or so in the flow rates of peroxide you're talking about (the catalyst will probably be washed out quickly). Also, assuming it can last long enough for each of your tests, it's probably going to be a pain to replace it between every two tests (open each motor, clean/fill it with new catalyst, close it and make sure it's closed tight...). Considering your project overall, you probably want to make *lots* of incremental tests to get the control system to work, so motors that don't require much service between tests are probably preferable.

    Have fun, be safe, and keep blogging about your progress!