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Film cooledn engine work, Rocket crane truck, Vehicle pictures

Engine work, Rocket crane truck, Vehicle pics

February 24, 2008 notes:


Film cooled engine work


We got set back up for full-throttle horizontal testing, something we haven’t done in over a year. We used to do tests at the shop in the loading dock bay, but we eventually got people complaining about it, so we are now set up on the back of the crane truck, and test out at the airfield where we do our flight testing.


The injector that we flew the vehicle on last month burned through the oxidizer manifold when run at full throttle for an extended period. The hover test was only at about 60% throttle, and had seen some initial erosion, so that wasn’t too surprising. The chamber wasn’t damaged.


We made another injector that was basically all we could do to help the single ring of elements cooling – we thinned down the deck to increase heat transfer, reduced the area of the center lox manifold to decrease the heat going into the cryogen, and decreased the element impingement angle to move the impingement point away from the face.


This engine lasted longer, but the Isp was down. We were able to do several engine start sequences with it, and collect a lot of 200 hz sampling data, but we did eventually burn through the lox manifold on that as well. With the lower Isp, the chamber wasn’t getting very hot at all.


We gave up on the single ring injectors, and resigned ourselves to using a fuel-ox-fuel manifold like the successful engines of the past year and a half. There are three construction options – spokes in the element pattern and tricky manifold welds like we have used in the regen engines of last year, external crossover pipes, or buried welds in the manifold. I am very concerned that spoke holes in the element patter will give hot streaks on the film cooled wall, and external fuel bridges cause uneven manifold fill rates and difficult fabrication, so we decided to just do the buried welds and proof test very carefully.


Spacing the element exit points apart in the last engine seemed to have killed our Isp, so I took extra steps to get them very close in the next injector. Because of some construction details on the new designs, I can drill the element holes from the top side of the injector with a sacrificial plate on the chamber side, resulting in clean exit holes that can be very close together without the need for flat spot faces.


We used a new spun chamber from AMS for this engine, and Phil polished it for best smoothness. This chamber is more concentric, but it is still a 3.5” throat, and AMS doesn’t think they will be able to spin the tube all the way down to 3.0”. We will see how the next couple go, but machining out of billet may turn out to be a better option.


When we fired this engine, the Isp was great (we are only doing relative comparisons by comparing chamber pressure with a constant orifice are and feed pressure), but the combustion wasn’t completely stable. The plume jumps around a bit, and there was a high pitched whistle. The pressure transducer graph was absolutely level (until the transducer died late in the test, another good sign of instability), so the instability was well above the transducer bandwidth. After about 20 seconds at full throttle, we saw stainless burning in the plume and shut it down. This time, the injector face was still perfect – there was heat discoloration, but it was the chamber throat that had started eroding. We did another short firing with this engine, but we are making two changes for next time:


We are cutting 3” off this chamber’s barrel section and putting the same injector back on. I doubt that will help the instability, but the film cooling won’t have to absorb as much heat, and it may reduce the combustion efficiency, which would also reduce the cooling needs. If we get the same chamber pressure with the shortened chamber, it means we had more L* than necessary (at least for full throttle operation, deep throttle would be different), which would be good information.


I am going to change the impingement angles on the elements to hopefully fix the instability, and also hopefully keep the combustion farther away from the walls. This may reduce the Isp, but we’ll see. Isp is of secondary concern right now, so if we get an engine that lights 100% reliably and can burn indefinitely, we will go back to flying vehicles.


One annoying aspect of firing the engine horizontally is that we are having more issues with initial lox density. On our modules, the lox tank is all “uphill” from the valve and engine, so we have full density lox right in front of the valve. Using Pixel up on the crane truck as our run tanks gives us a nice gas / low density lox trap. We can’t just dump a whole lot of lox through the engine, because it makes a big puddle in the horizontal engine and might run back into the fuel manifold. We found out that our manifold purges definitely do NOT sweep out liquid pooled in the chamber very well. We added an external lox line conditioning valve right at the engine to let us dump it outside. We tried first with a big solenoid, but it was clear that wasn’t even close to good enough for the 20’ of feed line we have, and we eventually wound up using a spare main propellant valve teed off the main one. When I let this run for a while, the engine starts fast and smooth. If I don’t let it go long enough, the engine starts smooth, but then has a big drop in thrust for a second as it is consuming the “fluffy” lox that had risen up and collected at the top of the plumbing.






























Rocket crane truck


We had to do it.


We pulled up the crane feet, took off the brakes, and let Joseph and Russ ride in the truck cab with the engine firing. We built a little control box so they could control the engine while I monitored the telemetry from our normal position.


A 1500 lbf engine doesn’t move a 50,000 lb crane truck very fast. If we put it on the back of James’ little pickup…







Vehicle pics


We did a test assembly of our four module bodies to make sure everything fits properly. After we finalize the next generation engine, we will pull one of the modules off for a set of single module tests, then go straight to four module tether tests to prove out differential throttling.





Matt made some nice renderings of the “six pack”, our current plan for commercial suborbital use. Don’t try to read too many technical details into these concept renderings, there are lots of things not explicitly drawn.






Half the team will be at Space Access ’08 next month, so you can hit us up with detail questions there…


On a final note, we did not get selected for the Air Force Research Laboratory phase II project we were in contention for, but the NASA team we are working with are very amiable, and that may lead to larger things in the future. Our crop of potential commercial customers continues to shift around, but it looks like some things will be finally closing on that side soon.




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