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Armadillo Aerospace News: New mill, engine work, vehicle work

Published by Sigurd De Keyser on Thu Apr 6, 2006 10:36 pm
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New Mill

Our new Haas VF-3 CNC machining center (http://www.haascnc.com/details_VMC_NEW.asp?ID=241#VMCTreeModel )is installed, and we love it! I wouldn’t have normally purchased something this expensive for Armadillo, but my wife got it for me as a birthday present (isn’t that sweet!).

A 20HP enclosed machining center with flood coolant and an automatic tool changer. Ahhhhh. No more chips strewn all over half the shop.

Machining the manifold side of one of our new injector heads used to take six hours and many tool changes on our old Sharnoa mill, but it now gets done in 45 minutes, with the only operator actions being to help clear chips a couple times and place clamps on the final cutout pass. With more optimal tool selection and pushing it harder, I could probably cut that in half if we ever needed to make dozens and dozens of engines, for, oh, say, an OTRAG cluster.

The old mill had a fourth axis rotary table that I used to mill channel wall chambers, but I got a tilting rotary table on the new machine to give full five axis control, for the primary use of drilling arbitrary injector angle patterns. We may have gone a little weak on this with the little TR110 (http://www.haascnc.com/details_rotary_trunniontables.asp?ID=459#RotaryTreeModel ) model, because we can’t push very hard on it while machining, but it works great for the injector holes made with tiny tools.

One thing I was rather surprised at was how different the G-code was between the old SDC-850 with a Tiger IV control and the new Haas control. I expected the modern control to be a superset of the what I was used to, but practically everything outside of the half dozen most basic features is different. The G codes other than G0 / G1 / G2 / G3 seem to be essentially randomly assigned, and mean completely different things. I was especially surprised to find that there were some features on the old CNC that actually aren’t present on the modern one, like general purpose variables with arithmetic, and some things that were more convenient, like looping without having to create subroutines.

I’m still writing all the G-code by hand, but now that I have a common, modern control, I will probably get some basic CAM software. The 2D graph mode on the control is very helpful, but a full rendering with depths and tool diameters will be better.

1000 inches per minute rapid transit is frightening. I leave it on 25% most of the time because having it chuck around hundreds of pounds of machinery and look to all the world like it is going to pile drive your tool right into the bed, only to drop to machining speed 0.1” above the surface is stressful. After I know everything is working well I will sometimes put it at full speed, but I still cringe after tool changes. Something that surprised me: my old mill did rapids proportionally, and always did Z raises first if the destination Z was higher, which seemed eminently sensible to me. The new mill (and evidently this is standard practice in the industry now) moves all the axis at the max speed, so if you are rapid moving to another point that isn’t on an 90 or 45 degree path, the tool will actually make a 45 degree move, then a straight move. I cut an unintended hole in a part before I learned this. If I had had the rapid at 100% it would have surely broken the tool.

I’m starting to do finish passes on some of our machined parts, and I finally got around to buying a chamfer mill so I can start automating the edge breaking on the parts instead of just having Tommy do it on the wire brush wheel.


Engine Work

We have generally had a very frustrating month, because we kept melting a lot of wiring and various other things on our test stand. A couple times it was due to fuel leaks on the engine plumbing (we were in good company with that this month – hi Elon…), which we started addressing by making a sealing plate we can bolt under the injector to allow us to pressure test everything. We tried putting a plumbing plug in the nozzle so we could also check the graphite and phenolic seals, but it blew right out at a very low pressure. Even after we got all the leaks fixed, we were still burning things on the test stand just due to the hot gas mass flow around it. It looks like 1500+ lbf engines are just too much for our current testing arrangement, so we only got short runs in between putting out fires. We are going to do some longer runs in a horizontal orientation, and the really long runs on the actual vehicle, while it is suspended in the air attached to huge concrete blocks to keep it from going anywhere.

We also had a lot of trouble with our flow meters, so our data has not been good lately.

Now that everything is being done with the new mill, I am using a 1/16” diameter diamond coated carbide end mill to do the injector hole drilling now. This is happy to plunge into aluminum at a 45 degree angle at 10 inches / minute / 7500 rpm with no deflection at all. On the latest injector, after the hole is drilled I mill a 0.002” path around it to get a very clean wall and break off the exit chip. I can also now tailor the holes to be other shapes. The lox holes are slightly stretched circles to get the 20% larger area they need to flow the correct amount with an equal number of holes as the fuel manifold. For unlike impinging, oblong holes are better than larger holes, because if you have a smaller diameter jet impinging with a larger diameter jet, the outside edges of the larger jet will just go past the impinging jet. An oblong hole can have the same forward area, and meet up exactly.

The experiments with top mounted injectors so far have been: (all with an L* of 50, 3” throat, 6” ID chamber)

#1: Unlike impinging 45 degree to 45 degree (90 degrees total) head with spot drill cone left on. Fuel on the inside, so any overspray would be directed towards the chamber wall. The streams came out messy because of the spot drill marks, and performance wasn’t great.

#2: Unlike impinging 30 degrees to 30 degrees with the spot drill marks faced off. Excellent performance, but it burned through.

#3: Exactly as #2, but with tapered manifold inserts to keep the propellant velocity higher and improve cooling. Severe erosion after a short run, burn through was imminent.

#4: Straight down showerhead injector. Extremely poor performance, and even had some trouble starting due to the lack of atomized propellant. When we did straight shot injectors from the side they performed much, much better than this, probably because they still impinged together in the center, and they were shooting crosswise to the direction of gas flow. Performance could probably be better if we arranged to have more than just two rings of holes, but that would complicate the manifolding a lot.

#5: Like impinging (30 on 30) with radial fans impinging edge on with the opposite propellant. I couldn’t get the fans closer than about a half inch, so the fuel and oxidizer couldn’t mix until at least an inch away from the injector face, and at low pressure drops they probably didn’t directly impinge at all. Performance was moderate, and the injector didn’t show any signs of heat damage.

#6: Unlike impinging 45 degrees on 0 degrees, with the lox shooting straight down and the fuel shooting inwards from the outer ring, plus film cooling holes added at a 10 degree angle to the chamber wall. This was done with the new hole drilling process, with the lox holes oblong for better mixture ratio. Excellent performance, no signs of injector melting, and the film cooling seems to reduce the chamber erosion rate significantly.

We have additional injector designs to try if necessary (a like impinging 45 on 0 and 0 on 45 for a quadlet effect was the next on the list), but it looks like #6 will do the job for us.

The latest injectors also have integral gimbal arms.machined on the flange, which is really neat.

We moved to a phenolic spacer to replace the machinable ceramic ones that kept breaking. It has worked fine so far, but we haven’t been able to do any really long runs.

We rebuilt the test stand blast deflector to use graphite plates, and it is essentially not eroding at all, just pitting a bit so far.


Vehicle Work

We sent in our experimental permit.application for this vehicle to AST, but they have already come back with a bunch of things we need to fix on it.

We found that the gimbal mounting points we had made by hand weren’t very square, so we machined a combination engine mount / gimbal mount plate completely from a thick plate of aluminum. Combined with the integral gimbal arms on the engine flanges, we are now guaranteed good alignment.

When we are ready to do ground liftoff hover tests at our remote site, we are going to be using a new tether system. Shock loads are a huge issue with tethers. If a vehicle can accelerate for 20’ before hitting the tether, it will break almost anything hard. Energy absorbing tethers are likely to get cooked by rocket exhaust. Our solution is to have a fairly stout wire cable going from one vehicle leg to a pile of extremely heavy anchor chain. If the vehicle goes runaway, it will just start picking up 100 pound chain links one at a time, putting a gradually increasing force on the vehicle. Pulling on a single leg will arc the runaway vehicle into the ground quickly. It will be a heck of a crash, but it will happen within 40’ of the launch point.

We have started making parts for the 65” vehicle. We are planning on making everything with the mill, avoiding any hand fabrication, so we can churn out more vehicles quickly when necessary. We have a new landing shock / ground sensor design that gives a much stronger side load support and fully encloses the ground contact spring.


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