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Servo regulator, Throatless engines, Hold down test

Servo regulator

Aug 4, 2005 notes


Despite not having time to do an update for a while, we have been steadily working…


Servo regulator


When we last worked with it, the setup showed what seemed to be a valve lash problem – flow would begin when the high pressure ball valve reached 15% open, but it wouldn’t shut off until it was closed all the way back to 5%. Since we had fabricated our own actuator to valve adapter, we thought we might have allowed too much lash into the coupling. We built a new mount using helical beam couplers with zero lash, but that turned out not to help. The coupling seems tighter, with the valve following every little jitter of the actuator, but the flow behavior seems to be an aspect of the seals in the ball valve, not the linkage between the actuator and the valve.


This cracking problem is only really an issue at very low flow rates, so we were able to do some flow tests at roughly the performance levels that our single-man space shot vehicle will use. With a single large nitrogen bottle feeding the servo regulator, we did the following test:


2700 psi initial bottle pressure

60 gallons of water at 230 psi and 215 gpm flow rate

1800 psi final bottle pressure

2” plumbing, 1” valve


The small fittings at the bottle valve became the limiting factor as the pressure dropped below about 2200 psi, with the servo valve eventually going wide open and still not quite being able to keep up. Our flight vehicle pressurant tanks will manifold directly out of bottle necks with a -10 fitting, so they won’t become flow limited at all. When our new 36” hemispheres arrive, we will be welding up the full tankage and pressurization system for the big vehicle and doing water flow tests in preparation for testing a 5,000 lbf class engine.


Speaking of spheres, here are a couple pictures of the tear area on the burst one:





Throatless engine


I was recently looking at the table in Sutton regarding losses due to small chamber to throat contraction ratios, and they weren't as significant as I had remembered them.  A chamber with no contraction ratio at all will lose 20% of its thrust due to pressure losses from accelerating gasses in the straight section, but the Isp loss is only 1.5%.  The text mentions "throatless rockets" being used in some missile applications to minimize chamber length and dry mass at the expense of Isp. The text doesn't say if these were liquids or solids, but I assume they were solids.

However, this does open up the question of building liquid engines like that.  If L* remained constant, you would have an extremely long engine that would probably be impossible to cool, but I could imagine the accelerating, high speed flow could reduce required combustion stay times significantly.  A 1.5% Isp loss is utterly meaningless for our purposes, so a configuration that traded that for fabrication benefits could be quite useful.


We fired a few crude throatless lox / ethanol chambers, and the results were surprisingly encouraging. With a very crude injector (a spray nozzle for the lox and four straight horizontal jets for the ethanol), we measured a 190 Isp from a 12" long straight pipe combustion chamber.  It melted in a couple seconds, but this was still very impressive. With a 3:1 expansion cone added, performance should increase about 15% to around 220 Isp.  That would be right at theoretical values, and MUCH better than we have been seeing in our engines so far.


Side note: it turns out that our flow distribution to cooling channels and injector ports has been Really Bad with our previous designs.  The test that demonstrated this dramatically for us was to cut off the top of an engine so the cooling channels were exposed, and flow water through it. With our original manifolds, there was a 2:1 difference in height between the highest and lowest flowing channels, and the high point moved around the engine as flow rates changed. We are now using taper milled manifolds that maintain a constant flow velocity around the ring, and flow rates are essentially the same. Unfortunately, this doesn’t seemed to have helped the engines in any perceptible way.

Our second revision to the engine didn’t work out so well. We wanted to incorporate two of our new tapered flow manifolds with the same injection points we have been using on the other engines, but I wasn’t about to try machining them out of stainless to weld onto expendable pipe stages. Instead, I machined the engine top parts out of brass, and we tried brazing them together and onto a section of straight pipe.


This didn’t work out for two independent reasons. Most obviously, because the brass top section had a smaller ID than the stainless pipe below it, the step section served as an excellent flameholder, and the pipe burned itself off right under the injector before the rest of the pipe even started glowing. Second, the burn-off wasn’t completely even, so we flowed water through the injectors and found out that almost half of the LOX holes weren’t flowing anything at all. We cut the engine up and found that brazing flux had snuck in and plugged most of them up.





Watching the disposable chambers glow red hot gives us lots of good information on the evenness of our burning and the required L* of the engine, but we decided to go ahead and make a regen cooled pipe engine, because we could just make the entire thing out of aluminum. A 2” aluminum pipe with only light external machining can be a slip fit inside a 2.5” tube section, which lets us make these engines without any boring at all.





The first test engine was a bit shorter than the expendable ones, with a 12” total engine pipe length, and 9.5” of cooling channels below the injector. 20 cooling channels of 3/16” width, tapering from 0.030” deep at the nozzle end to 0.060” deep at the injection point.


Isp performance wasn’t good. There was about 30% less L* on this motor, but it may turn out that our “crappy, cobbled together” injector for the expendable engine was actually a lot better than the 20 hole version we moved to. We have been using very low velocity injectors because we had a lot of smooth running engines even without high pressure drop, and I feared that relying on the pressure drop for stability would give us problems when deep throttling, However, the hand-made injector did have four very high velocity streams impinging reasonably accurately, and that may have been the key to the performance, more so than the straight tube nature of the engine.


Since we didn’t have much else to do with the first regen tube engine, we made a couple more full throttle runs, then did a run without any ethyl silicate mixed in the fuel. At about the 25 second point, fuel flow rose and the engine note changed. We had burned through one of the cooling channels. That was the type of back-to-back confirmation that I was looking for, showing that the ethyl silicate really is making a difference, and we aren’t just putting it in as a random hope. We are currently using 1 oz of ethyl silicate per gallon of ethanol.


The next test engine has much higher injection velocity, and 3” more chamber length. The great thing about these engines is that it only takes me two nights to machine the parts, so we can test two engines a week if necessary.


We got to see a new failure mode on this one – the internal chamber buckled, then melted through. We have been making the outer jacket a tight press fit over the inner chamber, but the thermal expansion of the chamber, coupled with the extra pressure drop, apparently caused the inner chamber to buckle instead of stretching the jacket.


The next test engine will use a pressure drop intermediate between the last two, use a looser fit outer jacket (inner machined to 2.345”) to allow a bit of thermal expansion, and will bring the propellant impingement points farther out towards the side. I used to have issues with wandering drill bits during injector drilling, but now I am spotting everything with a larger diameter carbide spotting drill, and manually applying cutting fluid to every hole as the mill runs. The 1/32” holes in the last engine came out perfectly straight.


If this line of tube engine development works out, we can make a 5,000 lbf engine with very little more effort than the test engine.



Hold down test


We swapped out the valve actuators on the vehicle from 1.5 second to 0.5 second speeds. On the previous vehicles we had moved to slower valves to reduce the bobbing at hover effect that we got from the overshoots due to latency between moving the valve and vehicle acceleration, but now that all the motor drives use PWM for variable speed, I can control this much better in software. Having valves that shut off fast is good in general – 1.5 seconds feels like a very long time when you notice something is on fire and you want to shut down the engine.


We moved the flow meters from the test stand to the vehicle in preparation for calibrating throttled mixture ratios in-situ. This involved some contorted plumbing, and we are still having some leakage problems. I finally got around to finding a supplier (Aircraft Spruce stocks them) of the little conical seal caps you can put on AN fittings to improve sealing, so hopefully that will help. Plumbing 1” and over is much more difficult to seal than smaller stuff. We are moving more and more towards welding practically everything together. It is tempting to move to elastomer seals for the fuel side sometimes.


Another thing we have started to do is put some red food coloring in our ethanol when it has been mixed with ethyl silicate, so we don’t accidentally forget it, mix too much in, or confuse fuel drained out of tanks with pure fuel. This has the side benefit of leaving evidence where fuel leaks have been, even if the ethanol has evaporated away.


We welded the gimbal arms on the latest converging / diverging engine, and mounted it up under the vehicle. We fired up the engine and gimbaled it around for a few seconds, which should be more heat load than the vehicle will see during liftoff and landing on an actual flight. There was a little bit of fire on the bottom of the legs’ silica insulation, which we think was some of the RTV that holds it on burning, but it could also have just been methanol from the engine that came out during the initial very rich throttle up. The main heat shield and the rest of the legs were cooler than we expected, we probably worried more than necessary.


The one surprise was that the vehicle clearly pulled the support chains taut, which we didn’t expect at the pressure we were running. With propellant, the vehicle was over 400 pounds, and at the pressure we ran it, the engine should have made under 350 pounds of thrust. My working theory is that the big flat heat shield allowed the vehicle to get some “ground effect” lift since it was only 3’ off the ground.






I spent a while double checking the GPS integration with the new electronics box orientation, and getting the startup self-test functioning with the new electronics layout. Since the exact same electronics box runs both the test stand and the vehicle, we have had a few problems with forgetting to add the command line parameter to specify the vehicle valve calibrations. To fix that, I made a stub connector for one of the sensor values that isn’t used on the vehicle (the load cell sensor) that just ties the signal line to ground, so I read a hard zero from that channel. I now use that to fail the self test if the state of that line doesn’t match the specified configuration.


The vehicle is ready to fly, as soon as we are comfortable with the throttling and reliability of our engines. Worst case, we can just make an engine with really crappy Isp and still do our flights while we are figuring out how to make a reliable, higher Isp engine.




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