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!
Jonathan Goff
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Super-naive question: What materials are traditionally used for rocket nozzles?
Fair question. For regen cooled nozzles, most I’ve seen have used either stainless steel or superalloy tubes, and a few have used machined copper with nickel plated closeouts. RL10 and SSME at least as I’ve seen them have been the stainless steel or superalloy tubes, as is Merlin-1C.
The copper ones tend to be heavier, but they obviously have good thermal conductivity, so that helps. The stainless or superalloy ones tend to have very low thermalconductivities (going off the top of my head, I think most are in the 17-25W/m*K range, compared to 160-200W/m*K for most aluminums and 300-350W/m*K for most typical rocket-grade coppers). What that means is that for the same thickness of wall, the stainless or inconel tubes are going to have 8-12x the temperature rise of an aluminum wall…
I could go into more details, but am at home with three little distractions today.
~Jon
Next question, for those that haven’t made their own rocket engines before:
If aluminum is so awesome, why isn’t everyone using it already? What are the disadvantages?
Brandon,
Good question. I think it’s a combination of reasons. First off, there were manufacturing difficulties in the early days. I think the first tube-wall nozzle actually used welded aluminum tubes, but they had some sort of problem (where’s my “History of Liquid Propellant Rocket Engines when I need one”. AFAIK, the only mainstream engine that used an aluminum chamber and nozzle was the Bell Agena engine. Part of the problem with rockets is that there were very few groups that actually did most of the mainstream rockets, and once they had a basic manufacturing technique down, they didn’t tend to do a lot of experimentation past that.
Another part of the equation is that aluminum works best either when you have a really cold coolant, or when the chamber pressures are modest, or both. There are some recent tricks that I think might allow extension for both chambers and nozzles all the way into the Merlin engine chamber pressure range (a bit north of 1ksi, and with LOX/Kero–Kero’s not that great of a coolant), but they’re relatively newer. Most rocket engines in the past have tried to push the chamber pressure a lot higher, which when combined with 60s/70s era materials and processes meant that Aluminum only worked for a small subset of engines.
Another possibility is that a lot of rocket people came over from the jet world, where the logic may have been “lets make the chamber out of something that has good high temperature strength, and cool it just enough that it doesn’t burn through”. Not “let’s start out with something with pretty decent thermoconductivity that has to be kept cool, and figure out how to keep the wall to that much lower temp range”.
The big issue is that you have to work aggressively to keep the wall cool, which can involve coatings, tricksyness on the coolant side geometry, thinning the walls, and other things, many of which hadn’t originally been thought of (or even possible) when rocket engine development was already stagnating in the late 60s/early 70s.
I could probably give a better, more thorough answer in a paper, but I’m a bit frazzled right now. Been playing “Mr Mom” for the past few days. I apologize. If Doug Jones ends up popping on here, he might have some other ideas of why aluminum hadn’t really come into its own before now.
~Jon
I was watching some hot rod show where they make an entire car out of aluminum.. as I watched them machine sq foot parts out of solid blocks, I had to wonder how much that cost and how much money there is sitting on the sidelines waiting for someone to say “hey, wanna buy a rocket ship?”
Tricksy Hobbitses and their aluminum nozzles!
LR40 ran many thousands of seconds had an aluminum tube wall motor.
http://www.gkllc.com/history/rmi/AIAA-2001-3838_History_of_RMI_Super_Performance_90_Percent_H2O2-Kerosene_LR-40_RE-pitch.pdf
Jon, thanks for the detailed reply. Much appreciated. I’m in the space biz, but just a software guy, so I enjoy learning more about the loud and firey side of our industry. 🙂
We used alu chambers for years… I don’t understand that “revolutionary” hype… grmpf
Bruno
Having spent a lot of years in the jey engine industry, I can tell you that the brute force approach (high enough thermal strength properties to not worry abuot burn through- i.e. super alloys) has real benefits when planning & executing time & cost sensitive development programs even if it costs you in performance or mass.
Up until quite recently (last 15 years) it has been a truely tedious process to design and analyse actively cooled hot components. Quite a number of early engine tests on new designs are run specifically to see if the cooling schemes for combustion chambers, turbine ducts and turbine blades & vanes all work. If anything doesn’t, the result is a burn through and massive headaches all around. A round of redesigns then ensues. This is the equivalent of pouring vast gobs of money down a hole. Jet engine guys are loathe to start out with active cooling as it is a royal pain in the , er, posterior. If we can make it out of materials that passively withstand the temperatures, we will.
The 60s was a period of short development cycles and, compared to today, limited analytical resources. Codes didn’t exist to do the CFD (Computational Fluid Dynamics) work that is regularly performed today. And still it ocassionally comes out wrong.
Jet engine guys aim for simplicity and even if a lot of the guys working for the big rocket engine companies have never worked on a jet engine, their culture is based on the gas turbine industry and they have built in cultural biases that are really hard to overcome.
New companies, like Masten and XCor, have no previous culture and have engineers who have never worked on gas turbines. They are open to new approaches and have analytical power that earlier teams could never dream of. It is now possible to design systems that simply weren’t possible 25 years ago simply because you couldn’t analyse them.
Getting the most out of low temp alloys (like aluminum) requires a lot of computer design time, both in generating funky shapes that transfer heat the way yo want while being as strong as you want as well, analysing them so you are reasonably sure it will all work and then finding a shop that can machine those funky shapes for you. You need to do all that with a minimum number of redesign cycles if you want to run a successful development program. A lot of those technologies are pretty recent.
Paul
How do you think the thrust-to-weight and Isp will compare with the ~ 50-to-1 and ~ 465-seconds for the RL10?
Will this XCOR engine be an expander cycle like RL10 or a gas generator cycle engine with ~ 440-sec Isp like the J-2?
P&W is supposedly asking for $1-Billion in development costs for RL10c for an IOC beyond 2018. If this XCOR engine is potentially competitive with a 1-3 person team or a 50 person team for dramatically lower development costs, then why wouldn’t ULA, NASA, or the Air Force be more aggresive with funding?
Why wouldn’t SpaceX or Blue Origin take the same low cost approach to develop LH2 upper-stage engines?
This looks like a press release that is really set up to lower P&W’s price using a bluff versus showing the capability for XCOR to really develop a new engine.
Anom,
Total speculation on my part, but my guess is the chamber will come in a lot lighter, but not sure if the pump will–expander cycle turbopumps can have really good Power to Weight ratios. At worst it would be competitive with the existing RL-10, but with some real potential to beat it.
Re engine cycle, XCOR has an “expander-like” cycle they’ve been touting for their piston-pump engines, I’m guessing they would use that.
I don’t know how big the XCOR propulsion team is at this moment, but it’s probably in the 5-10 range. It’ll be interesting to see how other players respond. I know that SpaceX was interested in a LOX/LH2 engine, but don’t know how high of priority it is. Also, P&W isn’t going to lie down on this (I don’t think).
As I said, it’ll be interesting to see where things move.
~Jon
Jon,
Thanks for the detailed reply.
I would think that XCOR would initally focus on an LH2 engine of 1,500-lbs thrust similar to their existing Lynx engines. This engine could be clustered and used for in-space propulsion using re-fueling from propellant depots.
I thought that XCOR and ULA were big propelant depot fans, so this application looked easier and more attractive for them than an expensive upper stage engine.
Paul Roberts, since rocket engines have to use active cooling anyway, we may as well grasp the nettle firmly and get maximum benefit from it. I always scratched my head when looking at the old XLR-11 engine, which had such lousy cooling geometry that they had to add 20% water to their ethanol to prevent burn through. With good geometry, we can run a kerosene engine at higher O:F and higher pressure with a poorer coolant and no film cooling. In some cases, at least, the existing rocket engine technology has found local optima, and there are better optima elsewhere in the design space.
Exactly where they are, I’m not saying 🙂
Piston-pump engines have the ability to rapidly vary thrust, more so than turbopump engines? I’m wondering if they want to target these for (say) lunar landers.
I was going to ask if the aluminum gets coated, but I see you mentioned that already. I also wonder how useful aluminum metal matrix composites would be here.
@ Doug Jones
I agree that new optima are needed and that this is a really cool development. I was just commenting on why such a paradigm may have developed given the jet engine genesis of many rocket engine designers. 🙂
Paul
Paul Dietz,
I’ve talked with XCOR before about piston-pump powered lunar landers. At the time we were still on LOX/Methane, but if they can do LOX/LH2 that might also be very interesting.
As for the coating, I know that Masten and Swiss Propulsion Lab coat their engines, but I couldn’t tell from the pictures if XCOR was or not. Coatings can help, but I’m not 100% sure if they’re always needed–we never tried without. There are also a lot of knobs you can turn with the various coatings.
~Jon
OT, Jon, but speaking of propellant depots, there is a satellite propellant / spare parts depot going up in four years. Actually it’s more like an automated propellant / spare parts truck, but it is intended to be resupplied by a reusable capsule. Sounds sweet, and validates your work. You’ve probably heard of this, but thought I’d comment anyway. I am surprised not to have seen this on Rand’s blog before I saw it today.
Yours,
Tom
Yeah, I saw about that one. One of our frequent commenters here works for MDA. I’ve been following this with interest for some time now. It’s not a full-blown depot, but hopefully people will start getting more used to the idea of satellites and space hardware not being one-time use items that can’t be touched once they are put in orbit.
Re: MDA/Intelsat
I posted this in the Iridium thread…
SpaceNews also has the response from Satellite manufacturers to Intelsat and MDA’s contract. Unanimously dismissive.
Favourite comment: “Asked what Orbital might do to prepare its satellites for in-orbit servicing, [David W.] Thompson [chief executive of Orbital Sciences] joked: “We’d probably weld that fuel cap on.” Nice bastards.
About coatings… they can reduce heat flux to up to 20%. But in all cases the health of your engine should not solely rely on it. If your engine only survives with coating than you have a bad design 🙂
Bruno
>>Actually it’s more like an automated propellant / spare parts truck,
Yes, that’s a better analogy.
>>but it is intended to be resupplied by a reusable capsule.
Well, not quite. We won’t be reusing the tankers, but they will bering new propellant and new tools to the servicer.
I’m the lead engineer on the team designing the tools to do the work at the satellite. Consider: These valves were designed to be serviced by a skilled human, with two dextrous hands, and then sealed up and lockwired to prevent unintentional opening. Then they have been left in a high orbit, with lots of radiation and a low flux of really corrosive crap passing through the innards. of the valves
My job has had it’s moments over the last couple of years trying to figure out how to do this with one arm, a really tight mass constraint and a command delay from Earth. I can’t tell you how we’ve done it, but I do think we’ve got ‘er licked! We’re building hardware now and hope to be testing by the end of the year. The entire team has been pushed hard to do this.
Paul
“My job has had it’s moments…”
For someone not in the space business I get the impression that’s the most expensive thing about space, because the environment is so different, everything has to be done from scratch, even the simplest of items and tasks needs to be built or done so much differently to how it’s done on Earth. You’re left hoping that once it’s been done, that’s been done right and that that particular approach is goings to be applied over the next decades and won’t have to be relearned or tinkered with at further expense.
On a different note and drifting way off the topic of the tread, I was wondering if space agencies should look at different approach to how some of the servicing is done in space.
My idea is to use a tent-like lightweight bag to bring whatever’s being serviced into (call it a space workshop) pressurise it, but, to keep weight down, just to a fraction of a PSI. The aim is to provide an environment in which astronauts can work that would require cheaper, less bulky, and more dexterous space suits. One of the reasons for their bulk is for thermal protection, inside a space workshop temperature would be uniform, perhaps mechanical counterpressure suits, could be used with any additional standard protective clothing over the top.
Paul,
Fascinating!
> We won’t be reusing the tankers, but they will bering new propellant and new tools to the servicer.
Sounds like StrategyPage got it wrong. OTOH, if you could work with SpaceX on a reusable Dragon tanker … it would be like the Air Force tankers which can carry freight or people or fuel, and maybe even take a turn as a gunship now and then. Anything for more frequent flights, right?
I will be praying for your success.
Yours,
Tom
Bruno,
True, you do want your engine to not instantly rip itself apart if the coating fails. That said, using the coating as part of what guarantees your engine’s longevity seems to be legit. I mean, a 20% heat flux reduction (if you’re using the coating as a TBC) means 20% less dT across the walls, which means 20% less thermal stresses. That can often make the difference between something that at least will get you home in one piece, and something that can be cycled 1000 times and just yawn at you and say “that’s the worst you could do?”
~Jon
Paul Roberts,
A year ago, had I not gone off and started my own company, we were hoping to present a study on servicing the Iridium Satellite constellation. We didn’t have a solution to the “how to take the safety wire, covering caps and other stuff off robotically” yet, but we were at least going to point out the details of the problem. Yeah, it’s amazing how hard it is to compete with two 100+ DOF hands robotically.
~Jon
Jon,
I’ll confess to being at least as interested in your hinted-at manufacturing approach as in the XCOR-ULA engine project. I’m no rocket jock, but it seems to me one could make a regen chamber or nozzle of aluminum fairly straightforwardly by machining it in two coaxial pieces. Machine the cooling channels on the outside of the inner of the two chamber/nozzle pieces. Machine the outer piece with an inner profile that matches the outer profile of the inner piece. Secure the outer piece immovably, align the inner piece co-axially, spin this up to a suitable RPM, then jam the inner into the outer and bond them with an inertial friction weld. Poof! Instant regen chamber/nozzle. The cooling channel machining of the inner part could be easily done on a 4-axis CNC mill; the rest of the machining for both parts could be done on a CNC lathe. A suitable inertial friction welding rig could be quickly knocked together out of catalog parts. For all I know, XCOR may well be doing something very like this now for their existing LOX/kerosene engine. It sounds as though you have something rather more exotic in mind. Any additional hints available?
@ Andrew W
The minute you imagine bringing people to do any task on orbit, you have chnaged your problem from a multi hundreds of millions problem, to a multi billions (and maybe tens of billions) problem. People on site are simply a non-starter for a commercial enterprise. After that I can talk about how hard it would be to actually do that work that you describe or to seal off the satellite, but it could never get that far because people in space are just so incredibly expensive.
@ Tom DeGisi
Yep, they got it wrong. 🙂
Using someone else’s verhicles doesn’t allow the business case to close, so that’s not goinjg to happen. At least not for now. If we evolve into a true depot, then maybe, but that’s far, far down the road.
@ Jon Goff
>>We didn’t have a solution to the “how to take the safety wire, covering caps and other stuff off robotically” yet, but we were at least going to point out the details of the problem.
It is stunningly difficult to do this given the mass constraints we have. This, I know. 🙂
>>Yeah, it’s amazing how hard it is to compete with two 100+ DOF hands robotically.
Ya think?? 🙂
I do believe we will be patenting all of this, so once we do, I can talk about how we’ve approached it. But I can’t until it’s all sewed up.
Paul
>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.
If you can use aluminum alloy to design combustion chamber, why not consider some low-cost copper alloy? It can still feature better mechanical and thermal properties than any aluminum alloy (even high-strength heat/corrosion-resistant) and beat the last two points: high thermoconductivity to keep walls cooler and make reliable reusable engines.
Note also that throttling the engine makes it harder to implement reliable cooling even using cooper alloys.
Alexander
“people in space are just so incredibly expensive.”
Yep, and as the cost of robotic systems drops, the cost of people, in terms of the resources they need to survive in space, remains constant.
But if commercial operations in space are to move beyond the limited applications we have now, a much greater human presence in space becomes inevitable.
As always it comes back to the cost of space access.
Dick,
Regarding the first part, you have the basic idea for how to do a regen nozzle section. There are more exotic versions, but yeah you basically form channels into an inner liner, and have an outer jacket or closeout that fits over the inner piece forming the cooling grooves. The question is, do you bond that closeout on in some manner, or do you leave it unbonded. Both have advantages and disadvantages. If you’re bonding the two together, friction welding *might* work, but I’d be worried about bending/breaking the lands between the grooves, as well as making sure you actually get all of them to bond. There are many other ways of skinning that cat though.
As for my particular solution, hints will come later. First I’m trying to do a few proof of concept tests on my own dime. If this works the way I think it does, I’ll probably file a provisional patent and then start talking about it. The good news is that this solution, if it works the way I think it does, has really broad areas of application. My challenge is just pulling together the time and money to try it out. That said, I’m hoping to have a proof of concept tried out within the next few months.
~Jon
Paul,
re: humans in space…I think that a lot of the traditional assumptions about the relative cost of humans vs. robots has been driven by NASA’s approach to doing things. Sure, you could probably have made a robotic ISS that could do the same amount of untended experiments as ISS did, and done it for $1-5B. But I think you also could’ve gotten most of the capabilities of a manned ISS equivalent (via Bigelow or somesuch) for the same ballpark.
That said, for this particular market–robotic servicing of satellites–launch costs would have to go way down before man-on-the-site would actually make economic sense. Having run some of the numbers for missions like this, you end up having to do a lot of things mission-wise that wouldn’t line up well with having to deal with people there. This for now is definitely an application for robotics.
Looking forward to seeing the approach you guys came up with.
~Jon
Alexander,
We actually started with low-cost copper alloys, and moved away from them. The aluminum was substantially cheaper, and much easier to source and machine. Getting copper alloys of the right sorts, in the right sizes is fairly challenging, and most of them machine like pencil erasers. And you can’t use coating tricks, and the density is substantially higher.
I know that there are plenty of ways of making high strength copper-chamber engines–that’s the canonical way of doing things, and in fact the way Masten started out. I just think that aluminum is a lot nicer when you’re within the bounds it can handle (which my guess is anything up to around 1000-1200psi, depending on propellant and how aggressive you get with manufacturing). As I was saying in the post, the lower density gets you much lighter engines without having to try as hard, and there are some tricks that an all aluminum engine can get you that a traditional copper w/electroplated nickel closeout can’t.
As for throttleability, that depends strongly on a lot of variables (such as how long it has to do low throttle, how sensitive you are to performance hits during that portion, and how in general you do the engine configuration). We did fine with pretty deep throttling on our aluminum chambers, though admittedly the pressures were relatively low.
~Jon
I’m curious — how do they end up bonding the aluminum? There have been significant advances in that in the last couple of decades; I wonder if techniques like FSW are used here.
Paul D,
I’m curious too. Now, this being XCOR, and with all the praises they’ve sung in the past about how a chamber-saddle-jacket type arrangement gives you longer life due to only thermally yielding once, they might not be bonding the chamber inner wall to the jacket at all. *I* would bond them, and I’ve got several approaches (one or two we looked at at Masten, and the new one I came up with last month at Altius), but I don’t know personally what XCOR used.
~Jon
This is one of the few places on the internet where it’s worthwhile to read the comments. It’s interesting that the LR40 used aluminum chamber walls. I wonder if the AR2-3 did as well? Both of those rockets used hydrogen peroxide for oxidizer. The oxidizer to fuel ratio was something like 7 or 8 so there was plenty of peroxide flowing through the cooling jacket.
Thanks Jacob! To be honest I’m getting rusty on my rocket lore, but wouldn’t be surprised that aluminum was used with a peroxide motor. Not all aluminums are peroxide compatible, but I bet you can find a good aluminum that has reasonably good properties and is aluminum compatible. And yeah, having lots of coollant helps keep the walls cooler. I wish I still was in a position to experiment with new rocket technologies, but alas I don’t have a test setup, and my company’s gone more in a space robotics direction.
~Jon