I’ve been too busy to do much blogging lately, but I just saw this on twitter a bit over an hour ago: XCOR and ULA Demonstrate Revolutionary Rocket Engine Nozzle Technology, and wanted to make some comments.
First off, I wanted to congratulate my friends at XCOR and ULA. This engine work that ULA and XCOR have been doing is something I’ve been watching from the sidelines for some time now, and it’s cool to see them making progress. As Jeff Greason pointed out during and after the Augustine Committee’s work, the US rocket industrial base is in bad shape, and getting new blood and new ideas injected into it is critical.
Second off, I’ve been an advocate of aluminum rocket engine fabrication for several years now. It’s worth noting that while I was still at Masten we ended up doing almost every one of our successful Xombie/Xoie flights using aluminum chambered engines (and I think we’re still the only company to ever fly a reusable rocket engine made of aluminum). Aluminum has a ton of advantages especially for cryogenic engines (ie Methane or LH2 fueled ones), but even for non-cryo ones as well. A quick list includes:
- Low density and high strength-to-weight allows you to get a very lightweight engine without having to push margins or analysis anywhere near as far as with more traditional materials.
- Low-cost and easy availability of many alloys with good mechanical and thermal properties. Once you’ve tried to source a high-strength copper alloy for a medium-ish sized rocket engine you’ll know why this matters.
- Easy, quick, and cheap to machine, even if you want to do tricksy things with the cooling groove geometries.
- There are a ton of manufacturing process options that are semi-unique to aluminum that give you a lot of tools for optimization of the design without excessive costs. Some of these knobs allow you to optimize either for maximum heat flux into the coolant (for expander cycle engines) or minimum heat flux into the coolant while still keeping the wall cool.
- High thermoconductivity (about 50-60% of pure copper’s thermal conductivity) allows you to keep walls cooler–which is kind of necessary with it’s low softening temperature.
- If you can keep it cool enough for long-duration operations (which you usually can for low-moderate pressure engines), thermal stresses can be much lower making it easier to make engines that can stand hundreds or even thousands of cycles
The list definitely goes on from there (like making feasible an alternative engine cycle that I was supposed to have blogged about months ago), but that gives you an idea. The manufacturability/availability issues were enough to get me an opportunity to try them out at Masten, and the work we did for the Xombie/Xoie engines vindicated the choice. For an upper stage engine, the benefits are even more compelling. One of the things I’ve always looked for are manufacturing technologies/choices that allow you to cheat on the cost vs. performance curve. With a small alt.space company, you’re not going to be able to spend the same amount of engine optimization as a bigger aerospace company, so any technologies that allow you to approach “big boy” performance while still being something that a 1-3 person propulsion team can do is worth pursuing.
I think this technology is especially relevant to RL-10 follow-on type efforts like what ULA and XCOR mention they are collaborating on in this announcement. Using the right combinations of manufacturing processes (and there are probably several ways of skinning the cat), you can increase heat flux into the coolant (which allows you to get more power out of the engine or higher chamber pressure), lower the weight of the engine assembly, substantially reduce the manufacturing/inspection/rework cost and complexity compared to a tube-wall nozzle, improve the reusability of the engine, and at the same time allow robust enough margins that a small team can have a realistic shot of delivering a world-class engine.
While I am very happy for XCOR and ULA, I do have to admit to being somewhat jealous that I haven’t had a chance to be involved in this aluminum nozzle technology effort. I spent a lot of time at Masten working on coming up with approaches for making scalable, low-cost, high-performance manufacturing approaches for aluminum nozzles, with just this sort of application in mind, but we were never able to get the sort of outside traction ($$$) it would take to actually validate our concepts (past what we did for the Xombie/Xoie/Xaero/Xogdor engines). Since leaving and starting Altius I’ve been trying to push the ideas even further. In fact, this past month I came up with a completely new approach that if it works (I’d give it about a 75-80% chance of working) could be amazing, not only for rocket engines but also for 3D printing, and many other applications as well. Imagine a process that would make a full-density part with lithium-aluminum strengths, where minimum hole size for internal channels was small enough that you could basically make metal foams, that would allow you to build-in electronic components and sensors, but without the size limitations of most other additive manufacturing processes, which could be scaled up for large thin structures (on the scale of an F-1 rocket engine or an Apollo CSM-sized transpiration-cooled heat-shield).
Anyhow, I hope that some day we’ll get to see some more details on what exactly XCOR/ULA doing for the manufacturing process, and I also hope that we’ll see an RL10-class engine flying some day with an aluminum nozzle (and maybe even chamber). Congrats guys!