Yet Another Liquid Motor

[ernie wrenn]

Can you move the jet to the very end of your injector? This will stop the frosting. Second idea, let the nitrous and water merge together at a 45' angle, this should start a siphon on the water side instead of building pressure as the high pressure co2 will create a scavenging effect.

ernie

[stevep]

Hey Ernie,

Thanks for the suggestions. Yes, I could change the center post along the lines you describe: use a bigger diameter bolt so the center hole could be much larger (corresponding to the I.D. of the other tubing in the chain) and just drill the very tip to the 0.073 orifice I want. That would certainly move the freezing to the very tip.

Regarding using a siphon effect to draw the liquid into the stream, I'm sure I'd get *some* liquid, the question would be how much and how easy would it be to calibrate such that I get the right amount? If I decide to go back to a coax injector, I'd be game to give it a try.

As it happens, I've been drilling some 0.021" (#75 drill) holes without too much difficulty, so my next test will be with an impinging style injector....

BTW, Thanks for suggesting the CO2--I'm so glad I'm spending $2/lb instead of $8 for nitrous as well as not having to worry about flammability.

--Steve

[ernie wrenn]

I can supply you with different jetting sizes. We stock from .013 to .140 and you can make a holder with a stainless fitting or give me some spec's and I can make it for you.

Take a look at the web site.

Ernie
www.compucarnitrous.com

[stevep]

Thanks, Ernie, I had a look but didn't see any photos or drawings for your jets. Just to be clear, this injector plate would have 8 orifices (assuming they're 0.021" diameter) arranged around a 1" diameter circle. That's pretty small--the whole injector plate that fits inside the chamber is only 1.5" in diameter. Also, in combustion chambers, typically anything that sits proud of the plate gets burned away--that's why most injectors for rockets are flat plates with holes drilled. The back sides of the plates are milled with channels for fuel/oxidizer which also serve to cool the plate.

--Steve

[ernie wrenn]

10-4... Trying to learn about the NO2 rockets. I have read and watched about the builds and it fascinates me the power they make.

I have several racing go carts for upcoming projects and a few small test pilots with enough liquor they will drive anything......Chris.... That is the initials but I will not name names.

ernie

[stevep]

Ernie,

There are two ways people typically use nitrous in rockets: in hybrids, there is a solid fuel (wax, plastic, rubber, even paper) in the form of a tube in the combustion chamber and nitrous is just shot down the middle of the tube eroding the solid fuel such that it liquifies (or disintegrates into fine particles) and then burns.

What Anders and I are up to is commonly called a "bi-propellant" meaning that two liquid fuels are combined in the combustion chamber. (Confusingly, this term also applies to what Carl is doing which is to combine a gas and a liquid).

In hybrids, the injector can be very simple since all it needs to do is turn nitrous from a liquid into a gas so it can then (in the presence of heat) dissociate into nitrogen and oxygen. About the only issue is to get the hole the right size so that the ratio of nitrous to fuel (wax, plastic, whatever) is roughly correct. And, as I mentioned above, typically there is no throttling involved. Also burn times are typically short--5 seconds or so. These are the "hobby" rockets I've been referring to--these are still commercially manufactured and flown; also, lots of people have designed and built workable hybrids--it seems to be a popular project in colleges and there are tons of examples on YouTube.

In a bi-prop, not only do we need to turn the nitrous from liquid into gas, we also have to turn the liquid fuel into a fine mist and mix it thoroughly with the nitrous, while at the same time maintaining the proper ratio of nitrous to fuel. Therein lies the complexity with bi-props; relatively few people design/build these, so there's still plenty to figure out.

Hope this helps,
--Steve

[ernie wrenn]

I get it now...

If you take your nozzle plate, drill your plate pattern and use drop in jets to adjust the flow.

ernie

[stevep]

Ernie,

Yes, you could drill out a bunch of large holes, thread them for the jets and screw in the jets. However, most jets that I've seen look like bolts--they have a hex head. Those will (a) take up a lot of space reducing the number of jets you can put in a circle or other pattern, and (b) probably get melted off since they will sit proud of the plate. However, McMaster-Carr has some that are allen-screw types--you can thread them inside a tube, for example, but in my case, they're small enough they'd fit completely inside the injector plate. Look up part #2943T887. They have orifices as small as 0.010".

However, they're $8-10 each and in that size, I'd probably need at least 20 of them ($$$) and they wouldn't fit in the required circle. At that price, I can break one bit drilling every hole and come out ahead money-wise (and well ahead space-wise). Not that I'm ready to start trying to drill 0.010" holes, mind you. But even with my current 0.021" holes, I still need 8 (which is pretty expensive per injector unless I can re-use them) and (so far) I'm not breaking a bit on every hole.

--Steve

[ernie wrenn]

Steve

If I can help let me know...

[britishrocket]

Hello Steve,

Regarding metering orifices/jets. What you could do is get hold of some small diameter socket headed grub screws and then bore out the centre to the diameter you want. I did this with some M6 and M5 grub screws, and a job lot of 2BA ones I had. It worked well and was inexpensive. I made a small threaded boss to hold them in the chuck of the lathe while I drilled the holes.

[stevep]

Carl,

I'm not sure I understand what you're proposing, or, at least why you think it will solve the problem of drilling small holes. I can buy grub screw jets from McMaster (see part number given above). And I can drill small holes in aluminum -- at least small enough for the moment. Maybe you didn't realize I meant "grub screw" when I said "allen-screw types" of jets?

How's your work coming?

--Steve

[stevep]

Having tentatively decided to use an impinging rather than coaxial injector design, my first thought was to simply impinge a bunch (8?) of fuel streams on one nitrous stream. This was easy to test using parts from the coax injector--all I had to do was make a new face plate and drill two holes for the fuel stream (two being good enough to test proof of concept).

First I took some scrap pieces and tried various approaches to drilling 0.021" (.5mm) holes at 45 degrees and 60 degrees. Realize that I'm doing this on a drill press, not a mill. That makes setup for me *much* harder since there's no way on a drill press to raise/lower the table and maintain x-y position, nor is there normally any way to be sure that the alignment is such that when you tilt the table the axes won't be skewed. Damn that round column :-)

So...first problem is to overcome these issues. My answer was to figure out the required table height given the workpiece, clamping arrangement, and drill bit length(s) so that I would *never* need to adjust the table height. Next step was to deal with the "skewed axis" problem. I marked a straight line on a scrap piece of aluminum which I clamped to the table such that the line ran through the point of the drill when the table was in the horizontal position. That point is (normally) in the center of the table. Then I tilted the table to 30 degrees (to get a 60 degree angle between the two orifices) and noted where the point of the drill was relative to the line, this second point being some distance away from the center of the table. I adjusted the scrap aluminum until the line ran through the drill point in both the horizontal and tilted position. (I chickened out on 45 degrees and opted for the much less steep 30 degrees.) This gave me a reference line that, had I been using a mill, would be parallel to the slots in the table, *as long as I didn't allow my drill press table to swivel*.

The workpiece itself was scribed with a line running through the centers of the holes I would be drilling. I then clamped it to the table (on top of the scrap piece) with its line exactly coinciding with the line on the scrap piece.

The face plate for the injector is 1/4" (6mm) thick which seemed too thick to drill with a #75 (.021", .5mm) drill, especially on an angle. I therefore drilled from the back side (using an end mill to create a flat-bottomed hole) until I had about 0.08" (2mm) of material left. I then scribed the front side to indicate where I should start the orifice hole relative to the hole on the back.

Drilling of the orifice proceeded as follows (after some trial and error, and, yes, a broken bit or two): Using a 1/4" (6mm) end mill, nick the work piece so that there is a tiny flat spot where the hole is to be drilled. Yes, you have to do this, because even a
spotting/center drill will shift a tiny amount when you try to drill on an angle. The starting hole will then not be aligned when you bring the drill bit to bear and you'll snap the bit. So, make the flat spot, *then* use the center drill to start the hole.

Next I had to reposition the work piece since the end mill was much larger in diameter than the drill bit itself. Here I was careful to keep the line on the workpiece aligned with the line on the scrap piece. I used a broken #75 drill bit to aid in the positioning (along with a magnifying visor to see what the heck I was doing).

Now I'm not in favor (generally speaking) of using center drills to start holes. Center drills (the ones with the "pip" on the end) are meant for creating holes for turning between centers and the angle on the point is 60 degrees. Drill bit points are typically *not* 60 degrees so there's a mismatch there. Also those pips tend to break off quite easily. So what you want to use is known as a "spotting drill" which has the proper angle on the point and doesn't have a pip. Trouble is, I haven't been able to find spotting drills that make a tiny enough starting hole for a #75 drill, but I could get a center drill that would. OK, center drill it is.

Once the starting hole was made, I changed to the #75 drill and with the lightest touch I could muster (two fingers on the lever for the quill) I "peck drilled" the hole. You pretty much have to peck at it or you'll wind up with swarf clogging the drill, and once clogged, it will break in short order. About a dozen pecks for a 0.01+" deep hole. I drilled it dry because even a single drop of oil totally obscured my view of what was going on.

Given that I was drilling only two holes in my test injector plate, I just had to rotate the workpiece 180 degrees, and repeat the above process.

There are minute burrs around the edges of the holes that need to be cleaned up. I found some dremel bits that I thought might work and the best turned out to be a 1/16" inch (.062", 1.6 mm) engraving bit which looks like a ball-nose end mill. I found that simply twirling it by hand was the best way to knock off the burrs and clean up the holes.

OK, so I had my first test plate drilled and attached to the rest of the injector. Went to try it out and discovered I was out of CO2. OK, that's 20 lbs of CO2 used so far. Got a second bottle and tried out the new injector. It seemed to work well--the holes lined up pretty well and the CO2 didn't freeze the water. However, I couldn't assess the drop size because the single stream of CO2 just slammed the drops against the plastic leaving a big wet spot surrounded by some fine mist. I had to cut the stream about 18" (45 cm) away from the impingement point to avoid the problem and that's just too far downstream to be useful. So...time for a smaller orifice for the CO2.

That was easily arranged--I just drilled out a new center post to 0.04" (1 mm), and drilled just the tip to that diameter, leaving the rest about twice that diameter. Tried it out and discovered that even with just the tip drilled to that small a diameter, the CO2 qickly chilled enough to create solid CO2 pellets and clog the opening. However I was able to run it long enough to see that I was on the right track--now I could cut the stream at about 8" (20 cm) and get a decent impression of the drop size (which, BTW, was looking like a very uniform 200 microns).

OK, time to abandon the quick and dirty injector and build one more suited to impingement testing. The new one is similar to the coaxial one--a block of aluminum acts as a manifold for the CO2 and water with interchangeable face plates for the orifice holes. As I mentioned above, my original thought, now abandoned, was to use a central stream of nitrous impinged upon by a number of streams of fuel. My new approach is to use a triplet or split triplet design in which two jets of fuel impinge on one jet of nitrous (F-O-F) or two jets of fuel on two jets of nitrous (F-O-O-F). The full injector would then have a number of these elements spaced around it to achieve the flow necessary for the designed thrust level. However, for testing I just have to make one element (either FOF or FOOF).

In making this new injector, I got smart and did a run of several blanks for the plates, cutting them to size, drilling the back sides, and drilling the holes for the attachment screws, so all I have to do when I want a different plate is to drill the orifices.

[stevep]

Injector #1: F-O-O-F, .021 fuel diameter, .021 ox diameter (#75 drill, 0.53mm)

Having arrived at a workable test injector, I did several tests just to make sure it was more or less doing what I wanted. First up was the CO2 test. Clogged almost immediately. This was a surprise since I'd sucessfully tried that orifice size before with CO2 and it seemed to work (see photo above). Cleaned up the passage leading to the injector plate, rounding all edges, and tried again. Same result. Tried using shallower orifices by drilling more out of the back side of the injector plate. Slightly better. Futzed some more and gave up. May need to revisit this--is it because I've got two orifices whereas in the test above I had only one? Is my passage to the orifices not clean (smooth) enough?

Injector #2: enlarged #1's ox orifices to .026" (#71 drill, 0.66mm)
Seemed slightly better, but still clogging after a few seconds

Injector #3: enlarged ox orifices to .0292" (#69 drill, 0.74mm).
Much better. Still occasionally clogged, but usable for short tests.
Water flow was good and water streams impinged nicely on the two CO2 streams.

At very low water flow (20 psi (1.4 bar) or so), I got a very fine mist only a couple of inches from the impingement point. Nothing froze, and varying the flow of CO2 had no noticeable effect on the flow of water. OK, that's way better than the coax injector.

Cranked up the water flow with 100 psi (6.9 bar) and now I had to cut the stream 6-8" from the injector to see individual drops. Not so good. Worked my way down to 30 psi (2 bar) and got decent results about 4" from the impingement point, but the drop size still seemed to be around 200 microns.

OK, so this leaves me with either the water travelling too fast (either making the velocity too close to CO2 velocity or the water just having too much momentum) or the orifice size is too large (even at .021") to get a really fine spray.

Since I needed to get some stuff from McMaster anyway, I decided to order a couple of the "grub screw" type orifices with .010" holes (the smallest they offer). I could have attempted to drill those holes myself, but it seems easier for experimentation purposes to just buy a couple to play with. While I was at it, I also ordered a couple of fogging nozzles just to see what they produced--they're supposed to produce a nice fine mist at a fair volume of water, but I couldn't get any drop size specs from them, so we'll just have to see. If they produce a really fine mist (say 50 microns) with a usable flow, then I can see using one of them for fuel in the middle of the injector and shooting streams of nitrous through the resulting mist.

I've also been reading up on fogging/misting nozzles that don't require super-high pressure to work. For instance, see U.S. Patent #7198201. These require fabrication that I'm not prepared to attempt, but it's interesting to see what people have come up with. Commercially available ones that I've seen so far which are specified to produce very fine sprays (less than 50 microns) don't allow flows anywhere close to my needs of 36 gph (gallons per hour; 136 L/hr), however.

--Steve

[Richard OConnell]

Looks like you've made some progress and learned a few things. Most of this stuff is way over my head but its always a good read and fun to ponder

[stevep]

Richard,

Yeah, I feel like Edison who is said to have remarked "I haven't failed, I now know 20,000 ways not to make a light bulb". I have learned a tremendous amount about the behavior of nitrous, gotten some orifice construction under my belt, learned a lot about drop size evaluation, and so on. There's just a lot to learn on a project like this. Fortunately, it's been fun :-)

--Steve