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Vehicle, throttled biprop, pump, servo regulator

Vehicle

June 15, 2005 notes:

 

Vehicle

 

Work is proceeding well on the vehicle:

 

http://media.armadilloaerospace.com/2005_06_06/vehicle.jpg

 

Since this picture was taken, we have basically finished all the plumbing, and most of the wiring. Our first leak check was on Tuesday, which turned up a couple things that need to be fixed.

 

I have decided to go ahead and build a second copy of this vehicle. Doing so will be a lot easier with the original sitting next to it, rather than crumpled up in a post-crash mess… We did a top-to-bottom inventory of all the parts that go into it, and it turned out to be a longer list than expected – four pages. It will be interesting to have everything in a big pile, then turn it into a rocket.

 

Building wiring harnesses always takes longer than you expect, especially when working with shielded aircraft cables. We are still working with AMP metal shell circular connectors for this vehicle, but I am actively working to move our next iteration to higher end connectors that will be sealed as well as shielded. I was originally looking at MIL-C-5015 Amphenol / Cannon connectors, but I was strongly urged by a helpful aRocket contributor to use the more modern MIL-C-38999 Series III connectors instead, for sensible-sounding reasons. These connectors are several times as expensive, but more annoying is the fact that you can’t just “click and buy” the connectors. I have been waiting a week for a distributor to get back to me on my second quote.

 

Throttled Biprop

 

We calibrated several fuel / lox throttle points, and did a long run throttling the engine up and down several times. The throttled cases were intentionally very rich for cooling safety, but the fuel flows were higher on the variably throttled run than on the calibration runs, perhaps indicating that we have a lash problem with our fuel valve actuator.

 

http://media.armadilloaerospace.com/2005_06_06/throttledBiprop.mpg

 

We were preparing to test a new style of injector that moved the fuel injection point in from the side to the same radial distance as the lox injection. The positioning should help, but I also managed to get 4x as many 1/32” holes drilled as we used to use 1/16” holes, which should positively be an improvement.

 

http://media.armadilloaerospace.com/2005_06_06/injector1.jpg

http://media.armadilloaerospace.com/2005_06_06/injector2.jpg

http://media.armadilloaerospace.com/2005_06_06/injector3.jpg

http://media.armadilloaerospace.com/2005_06_06/injector4.jpg

 

Unfortunately, we had another chamber burn through before we could test it. We have a pretty good idea what caused this one – the weld at the base of the lox injector cracked, leaking lox into the chamber from there and allowing it to burn and melt off some aluminum. When molten aluminum falls from the top of the chamber, it can pick up a lot of heat and smack into the throat converging section, allowing a burn-through in the otherwise steady-state cooled area. We have had weld cracking problems in cases like this where we have had to face off a welded seam to allow a flat plat of some sort to bolt directly below it. Better V-grooving of the weld area would help, but for the new engine I made both the fuel injection ring and the lox injection ring together from a single billet of aluminum, so there are no welds to crack at all. We have also changed our fuel and lox plumbing to tangent entry instead of perpendicular entry, which should improve distribution. On water testing, we could see that there is still a noticeable difference in flow out of the channels around the circumference, so our next engine iteration will provide much more manifold space.

 

We are getting a professional hardcoat put on this engine, but it should be ready to test this Saturday. We are also going to start out with a fuel additive. We have switched over to ethanol for future work due to the slightly better handling and performance, and we tested both DOT-5 silcone brake fluid and ethyl silicate. The brake fluid has reportedly been used by others, but at least the brand we tried (Russel performance) seemed to settle out somewhat after sitting for a little while. The color stayed evenly distributed, but there was a puddle of different refraction at the bottom of the flask. Ethyl silicate seems to be completely soluble up to very high concentrations, so we are going to use that. We are going to start out with a 2% solution, but we hope that we will be able to do without it. We want to make sure we get at least one good run first.

 

Pump

 

We built and tested a third-generation free-piston pump. As a development aid, we used 4” diameter commercial air cylinders for the chambers. This automatically gave us a convenient, well-sealed piston, and the actuator rod is an excellent visualization aid. They are very heavy, with huge slabs of aluminum on each end, and steel tie rods and actuator rods, but when we get around to making a lightweight version we can still take the cylinder and piston, and just weld caps on each end.

 

The big piloted solenoids on the last pump gave us a fair amount of trouble, because the vents wouldn’t always close reliably when the piston was all the way at the top and all the pressure had completely vented. For the new design, we use a single 3-way ball valve on each cylinder that flips the chamber from pressure to vent. By arranging the two cylinders properly, we were able to use a single pneumatic actuator to cycle the valves on both cylinders simultaneously. This is a really neat arrangement.

 

We did vibrate off a nut on the linkage of our pneumatic valve actuator linkage during testing – oscillating motion brings in issues we haven’t faced before. The actuator used snap rings to contain its rotating parts, which is probably a good idea. Our 3-way ball valves are also not perfect, because there is an overlap period where pressurant can travel directly to the vent. Interestingly, this seems to be more of an issue on the valve that we had to rotate the ball to get the orientation correct. Some 3-way ball valve seats aren’t designed for pressure in all directions. What we ideally would want is a ball valve that had smaller plumbing ports than the ball, so there was no overlap period at all. I think I have seen some valves that claim this.

 

Overall, I think this free-piston design is just better than the pistonless pump concept.  We don't need any sensors, we waste less pressurant, we have a single actuator instead of four, and two valves instead of four.  We still have the same four check valves.  Warmer gas could be used with a piston separating it from the working fluid.


The weight savings offered by a pump versus heavy propellant tanks is related to the ratio of the pump cycle time to the total burn time.  You get the best benefit if you cycle fluid through a small pump really fast.  XCOR is driving their pump so fast they had to solve a lot of vibration problems.  We are using long cylinders and a long cycle time at the moment because it makes understanding what is going on easier, but when we make a lightweight version we are probably going to have the cylinders be half or a quarter the length, and the cycle times proportionately shorter.  This ceases to be a benefit when your valve actuation times become a significant fraction of the cycle time, when your trapped gas volume becomes significant compared to your displaced volume, or when the gas used for actuating the valves becomes significant to the displaced volume.  I'm pretty sure you want cycle times under a second, perhaps well under.  If you are using off the shelf check valves and vent valves, cylinder weight won't be worth optimizing away too much, but if you make very lightweight integrated valves you would probably wind up down at the speeds that XCOR is running.

Like a pistonless pump, a free-piston pump can't self prime, it requires some head pressure to force liquid into the chambers.  I considered connecting the two chambers (especially since we already had actuator rods sticking out of them...), but the packaging gets messier, and the pressure output then has huge dips in it as the piston pulls away.  Pistonless / free-piston pump chambers each act as accumulators during the transition to the other chamber, so it is easy to get smooth output pressure.

Even though we are having great success with this, I don't think we are going to use it in any currently planned vehicle.  We are going to be making 3' spherical propellant tanks from spun 5086 aluminum hemispheres, and they just aren't going to be that heavy for our operating pressures, even with a 2x safety margin.  We are going to be bursting a trial tank fairly soon to see how closely it hits our expectations.

It only starts looking attractive if you can also significantly reduce the weight of the pressurant bottle.  Flowmetric's plan to use a separate pump to pump liquid helium up to high pressure before vaporizing it sounds troublesome to me, but with a piston you can deal with hotter gas, so using cooled Tridyne might be easy and workable, or you could always go to the conventional gas generator or chamber tapoff with a differential driven piston.

 

http://media.armadilloaerospace.com/2005_06_06/pump1.jpg

http://media.armadilloaerospace.com/2005_06_06/pump2.jpg

http://media.armadilloaerospace.com/2005_06_06/pump3.jpg

 

http://media.armadilloaerospace.com/2005_06_06/pumpTest.mpg

 

Servo Regulator

 

Our next big vehicle and engine tests are going to need a much larger regulator to maintain pressure than anything we have used before. Because of the nature of the motors and tanks we were using, blowdown pressurization made good sense for the monoprop vehicles, but we need tight pressure control for biprop work.

 

We built a manifold with a high pressure ball valve, a pressure regulator, and a very large relief valve so we could experiment with direct servo regulation from the high pressure bottles. This also has the advantage of allowing us to start or stop the pressurization of the main tanks remotely, which we can’t do with a fixed regulator. I started testing using regulated pressure at modest levels, but after getting the basic proportional – differential control loop coded, I was able to run it directly from a high pressure bottle. It works pretty well, and doesn’t suffer from any droop at changing supply or demand, but the ball valve seems to have significant control lash in it, cracking open at 15%, but not shutting completely off until it goes back down to 5%. We haven’t determined yet if this is some aspect of the seats in the valve, or if our actuator mounting has significant lash. We are going to try making an adapter using lash-free helical beam couplers.

 





 






 
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