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Engine, Gimbal, Pump


May 22, 2005 notes




The design that we have been following so far uses a separate chamber to vaporize all the lox with a small amount of burning fuel before directing it out holes at high speed to atomize the alcohol. The problem with this is that you basically have a small rocket engine on top of your main rocket engine, and we have managed to hard-start the preburner twice, resulting in some bent metal. One rather obvious thing we found is that watered alcohol has worse ignition characteristics than pure alcohol. It still isn’t really difficult, but if you have marginal ignition with straight alcohol, adding water (for better cooling) may push you over the edge to the point where you have enough ignition delay to cause a bang.


We decided to build another preburner that didn’t have a bottom distribution plate at all. There is a central tube with the preburner spray nozzle and spark plug on top, and a set of radial lox injector holes near the bottom of the tube. This just exits directly into the main chamber, while the fuel still comes in radially from the outer edge of the chamber at the top of the cooling jackets. An advantage of this arrangement is that there are no problems with melting the gox injection plate, since it isn’t there any more. All surfaces are cooled by lox, so we can run the burner injector at a higher rate and we don’t need to worry about throttling that down separately when we reduce the lox flow. We had just finished setting up a tiny 1/8” ball valve for throttling the burner fuel, but now we can go back to just using a solenoid instead of a closed-loop actuator.


Ignition has been nice and reliable with the new burner (we also made changes in the spark plug position and gox flow for the burner), but unfortunately our Isp with the existing cooled chamber has gone from not-good to basically crap. The hot gox jet and splash action had been contributing a significant amount to the combustion process, and the new design is just less efficient (but safer). Another sort-of obvious thing we realized was that for any given O:F ratio rich of stoichemetric, the lower your Isp, the more heat you are going to be pumping into everything outside the rocket engine, because more of the fuel is burning outside the chamber instead of inside, and the thermal energy doesn’t have an opportunity to be turned into kinetic energy by the nozzle. We started to melt some wires on the test stand during our runs.


Just about any injector will give high performance if you can give it enough chamber volume for complete combustion. We built a very large stainless steel chamber to run under the preburner and a couple different options for fuel injection. The theory is that there should be a range of constant temperature from the bottom of the chamber up to some point above it that corresponds to full temperature being reached in the chamber. With a steel chamber, you should be able to tell this from the glow colors. Our test didn’t go perfectly due to a leak past our improvised fuel injector ring resulting in some burning at the top flange, but it looks like the current injector needs an L* (characteristic length, the ratio of chamber volume to throat area) of about 60, while the current cooled chamber only has an L* of about 35.


That would be about 4” longer on the chamber section, which will be pushing our in-house fabrication capabilities, but more importantly it will be increasing the total amount of heat that the coolant needs to absorb. I suspect that the combination of higher total temperature plus more area to cool will push us beyond what we can manage at this thrust level. Load on the coolant gets lower as engine thrust increases due to volume / area scaling, so we might have to go up to a single bigger engine and roll control thrusters for the test vehicle instead of two smaller engines.


The alternative is to build a better injector. We may try an unlike impinging injector with our preburner torch just acting as a continuous igniter instead of a lox vaporizer. The conventional wisdom is that you can’t throttle such an injector very much, and it will be more prone to combustion instability problems if the injector pressure drop gets low, but we may just give it a try anyway. We will have a large pressure drop at the throttle valves upstream, so it may not be too horrible to let the pressure drop across the injectors fall down. We also don’t care all that much if the Isp suffers while throttled, as long as the performance remains smooth.


Another little lesson: we were rebuilding our blast deflector, but we ran out of the furnace cement we used to keep the high density fire bricks in place, so we put some in with RTV on the back. That worked for a while, but on the third run after things had heat soaked, most of the bricks flew off the stand, except the one directly under the exhaust plume, which was held in place until the run ended, at which point it slid down to the base of the blast deflector. The backs had the powdery white surface of burned RTV. I got a five gallon bucket of the furnace cement for the future.










The new actuator linkage (u-joints and hinges instead of ball joints) and PWM software control work great:









In general, I have been anti-pump for low cost rockets. They add complexity, and if you optimize for them you wind up with a much more fragile vehicle than if you were pressure fed. Especially when we were using peroxide and had access to the incredibly cost efficient, robust, and pretty good mass ratio fiberglass tanks we were using, I didn’t think pumps made any sense for us. Still, as I observed the work that Flometrics and XCOR were doing, I began to think that it wouldn’t be all that hard to put one together.


Now that we are working with lox, I am having a harder time finding large pressure vessels for tanks that are anywhere near as convenient as the ones we were used to. I have only just started the search, so I’m not too worried yet, but using a pump with a trivial tank is less of a stupid idea for us than it used to be.


For a year or so, I have been saying that we might just go off and build a pump to get the experience. I recently had to buy some piloted ¼” solenoid valves to increase our engine purge capacity over the direct acting NOS solenoids we were using, so I went ahead and picked up some big ½” piloted solenoids that would be suitable for pump chamber controls. Supplier note: we had been getting these lines of solenoids from Snap-Tite, but it turns out that they were just the US distributor for Jefferson, who has now opened a direct office. I’m still ticked at Snap-Tite about the 100 piece minimum order for Watmizer solenoids, so I am happy to take my business directly to Jefferson. All good so far. $110 for the ½” piloted solenoids, $86 for the ¼” piloted solenoids, and $47 for direct acting 1/8” solenoids.


Initially we built a Flometrics style (www.rocketfuelpump.com) pistonless pump. They use closed loop control with level sensors to cycle the valves, but I just used fixed timing, allowing liquid to fly out the vent valve without trying to shut it just-in-time. Initial results were pretty good, pumping garden hose flow up to 200 psi.




On normal 12v power, these solenoids are rated for 225 psi, but they open up to 350 psi. They are continuous duty at 12v, so you could get higher pressure switching ability by driving them at 24v. Since the duty cycle in a pump is inherently periodic, they could probably still run forever. The NOS direct acting solenoids are an example of doing this, they run 6v (someone said they are actually 3v) coils at 12v, allowing them to open much larger orifices against 1000+ psi than they otherwise would be able to. The downside is that they draw 8 amps and get pretty hot after 30 seconds, and they burn up if you leave them on continuously for a few minutes.


The inlet splashing issue concerned me for a pistonless pump. Our initial chambers were a bad choice for this design, being fairly squat, but even with a narrow chamber you are always going to be either wasting pressurant gas by closing the vent early, or splashing some propellant out by closing late. You also have swirl issue on pushing the fluid out, and you need to worry about pressurant compatibility / dissolving in the working fluid. I really didn’t see why you wouldn’t want a free-floating piston in the chambers. I then realized two additional benefits of using a piston: The vent and pressure valves on top could be combined into a single three way valve, because it is safe to leave the piston pressed up against the top of the chamber. You also don’t need closed loop control with pistons at all – you set your cycle times based on the time required to fill a completely emptied piston chamber. If the flow is throttled downstream of the pump, the chambers simply don’t empty all the way before they need to refill. Avoiding any level sensors looks like a large advantage to me.


I also realized that it would be a good idea to have a cavitating venturi after a chambered pump to prevent an excessive flow that could drain one of the chambers dry, resulting in a momentary total flow stoppage. With a properly sized CV, pressure would just start dropping progressively if too much flow was being allowed.


We upsized some check valve plumbing and made version 2.0 of the pump, with larger chambers containing polyethylene pistons. Each chamber holds about half a gallon





The parameters I use to control the valves are: Pressurize time: the time allowed to push the fluid out of the chamber under pressure. Isolation time: the delay between closing the vent and opening the pressure valve and vice-versa, to prevent pressure from rushing right out the vent. Crossover time: the time that both chambers are under pressure to insure a smooth transition. The total cycle time and fill time are derived from those values.


It turns out that crossover time isn’t needed at all with free pistons or pistonless pumps. In a system with pistons connected to each other that operate in lock step, when one piston finishes its power stroke, there is no pressure at all on the output, so you need another piston to provide some push to avoid a severe drop in pressure. With a free piston (or no piston), when you close the pressure valve a chamber just becomes a pressure accumulator, decaying in pressure instead of having the pressure go to zero. In fact, we get upwards spikes in outlet pressure when both pistons are pressing, due to outlet flow losses being divided between two plumbing paths. Even with zero commanded crossover time, we still get positive thrust kicks due to the actuation time of the valves. I will probably try some negative overlap values to actually put a gap between the cycles later.


Some isolation time is necessary to keep from getting a burst of high pressure out the vents just as they are closing. This behaves as expected without a piston, but with a piston the piloted solenoids have a bit of a problem – if the piston gets all the way to the top and any residual pressure is allowed to completely bleed down before the vent solenoid closes, it won’t close because there is no differential pressure to operate the pilot. As soon as the pressure valve opens, the vent will close, but it still lets out a puff of high pressure gas. Using actuated ball valves instead of piloted solenoids would fix this. You can see/hear this behavior clearly in the video:




Water hammer effect is visible on the feed hose at high flow rates. I like flometrics’ idea about submerging the entire pump inside the propellant tank and just having open check valves for inlet.


The issue we were left puzzling over is that there seems to be different flow rates between the two chambers. At 8 gpm you can see it a bit, but as the flow rate goes up to 20 gpm, the outlet pressure differences get larger and larger. At first we thought it might have been the plumbing after the outlet check valves, which had an extra set of fittings on one side, but replacing the outlet plumbing with a big box manifold didn’t reduce it at all. We added pressure taps on each chamber, but the battery ran down on the flight computer as we were about to test it. Either the outlet check valves are different, or the piston is binding a bit in one of the chambers.





The other obvious thing is that as the flow rate goes up, our regulator can’t keep up with the flow. We have been planning on making a servo regulator that uses closed loop control on a high-pressure ball valve to regulate pressure directly from a high pressure tank. We may try that next week.



Some pics of the vehicle fabrication:






We also finally bought a hydrostatic tester of our own, and we had to break something to test it out:




This was a 316 SS tube of initially 1.65” ID with 0.036” wall. It failed at over 3500 psi (after yielding quite a bit), which would be 80 ksi stress at the original wall thickness, or 103 ksi at the thinned 0.028” wall at time of rupture.




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