750-3 Engine Module Development Update

(This is a combination of Joel’s ARocket post and some more details from me — Jon)

750-3 Engine Development
At the time of my last detailed update, back in June, we had just flown our 60 second hover on our first aluminum chamber (the -2AS). A while after the flight, we disassembled the engine, and noticed that a critical weld joint had cracked and was leaking a little. The chamber also showed some sign of thermal warping. It hadn’t buckled, but it looked like it was getting hotter than we wanted. Shortly thereafter, the brazed aluminum chamber (the -2AB) also showed similar problems after an even smaller number of firings. We had never had an engine actually buckle in flight, and weren’t sure if it would damage other hardware, so we decided it was time to revisit our cooling design.

Our initial cooling design for the 750-1 and -2 engines had been closely derived from the cooling design for the 500lbf engines. We had figured that this would be overkill for the lower-pressure we were using in the 750′s, and when we had time and more thermal data, we’d go back and tweak the design to get a better balance of cooling efficiency versus pressure drop. The thermal warping in the two aluminum chambers made this a high priority, so earlier in the summer, I sat down with Brooks Clarke, one of our interns, and dove into making a much more thorough thermal analysis model. We ended up using a spreadsheet with all the fluids equations to drive a parametric thermal FEA model to simulate the heat flow, with some perl scripts to link the two. It wasn’t fully automated, but the scripting made it so that I could come up with high level scenarios and have Brooks do the number crunching.

First LLC Flight Engine Flash
Xombie’s green flash

The end result was that we were able to come up with a cooling channel geometry that looked substantially better than our old geometry, but with only 40% the original pressure drop. When we measured the pressure drop, that part of the calculation was within a few psi of what the model predicted, though the thermal stuff is harder to model. In addition to the change in cooling channel geometry, I had also thinned out the wall substantially, rolling all those changes into two aluminum 750-3 chambers.

Unfortunately we ran into a buckling issues on these two engines. The first aluminum one failed due to some debris (a shard from an igniter o-ring) blocking the IPA injector ring in one section, leading to mixture ratio excursions and hot spots. The second one failed between firings, which led us to make some changes to make our shutdown sequence more thermally benign.

While we were testing these two chambers, we had started a third engine, to try out a new chamber construction method that used an o-ring joint between the head cap and the rest of the chamber, instead of a weld or braze joint. This was the engine we ended up flying with for our first LLC window, last month. We had been interchanging copper and aluminum chambers (our machinist can almost crank out a copper one in a day at this point), and had gone back to the copper for the first LLC attempt, figuring it would deal with the heating issues better. Unfortunately, it also had buckling problems, in-flight (as you can see from the green-plume-of-death in the picture to the right). This was surprising, because the copper chamber had been fired a bunch by the time we flew, including a long-duration flight the day before, but we made a critical error in not inspecting it before the Sep 16 LLC flight. We’re not sure, but there’s a chance that the buckling had already started before the flight.

In spite of the buckling problems, some of the changes worked well enough that we didn’t want to just roll back to the original 750-2 design. The aluminum saddle worked great. The o-ring sealed head-cap and chamber design had shown no issues at all, and removed a tricky (and time consuming) welding step (particularly on the aluminum chambers). The pressure drop was indeed much better.

Xoie's Engine Module
Xoie’s Engine Module

Things seem a lot happier now. We realized that after we changed our cooling groove pattern (to both improve cooling flow and reduce pressure drop) that we hadn’t really re-characterized the engines. We think what was really happening was that the rules of thumb we had come up with for our mixture ratio controller (that we had used for our the earlier designs) were no-longer correct, and we were running extremely lean. And at the same time our cooling is worse at lower throttles, because less fuel is flowing through the cooling channels. Those factors together combined to soften the chamber wall and allow the pressure in the regen channels to completely dimple in the chamber until it burst through.

We had two aluminum versions of the no-weld chamber design come in a few days after our first LLC window. Ben drilled new locations for the thermocouples in our testing jacket so it could better measure cooling groove temperature at multiple locations, and gathered a whole bunch of new data. I did some more detailed analysis on the chamber buckling strength, and we decided to make the chamber walls a little thicker again for our next rev. We also decided that we really wanted the aluminum weight savings for Xoie (our Level 2 vehicle), and so went back to aluminum for the Xombie chamber, instead of waiting for new copper chambers to be machined.

So far, everything is looking really good – the chamber on the Level 1 vehicle (which had been used for testing the last few weeks) shows no sign of any heat issues. In fact, we’re used to using force (and a gear puller) to remove or insert a fired chamber into the jacket – this is the first one that slides in and out as smoothly as a brand new one, which is a really good sign.

The 750 is now misnamed, as well. As Ben put it last week after testing – “our engine goes to 11″. We originally designed the 750 for about 250 psi chamber pressure with 400 psi tanks. We didn’t quite get the pressure drop we wanted, but we got close, so we decided to focus on the other end – better tanks. At 550 psi we can run at 900+ lbs thrust, which is good because we takeoff weight has been the concern for a fully-fueled Xoie (for Xombie duration was the bigger focus).

All told, while the last few months of engine development have been rather involved, at the end of the process I think we have a much better engine than we started with. Thermally the design is showing a lot more robustness, we’ve put several minutes on our latest aluminum chambers with no sign of damage, we’ve cut the pressure drop substantially, made the chambers a lot easier to fabricate, and now have better design tools and a lot more design intuition to back them up.