Random Thought: RS-68 Inflatable Nozzle

It’s interesting how you can hear a part of an idea, file it away in the back of your brain, and then all of the sudden have something pop into your mind months later.

A couple of months ago, my arch-nemesis from ATK (ok, he’s actually a friend of mine, we just tend to disagree about…work related issues…) posted a rather interesting article about inflatable nozzle extensions that was done by Goodrich back around the time of the Apollo program. Basically, the idea was that they wanted to see if they could put a huge deployable nozzle extension on the bottom of a J-2 engine, so that it could have a much longer nozzle without paying a huge weight penalty for a long interstage section. The nozzle was made of a stainless steel “airmat” mesh, with the outside of the bell sealed off with a high temperature silicone rubber, and the whole thing was inflated with the turbine exhaust gas (which also acted as a warm-gas transpiration coolant for the nozzle extension). They did some testing with the concept including deployment tests and such, though they didn’t get an opportunity to hot-fire altitude test the concept.

The RS-68 has a fairly short nozzle (21.5 expansion ratio) so that it won’t get flow separation at sea level, so it can have enough thrust to lift the vehicle off the pad (since in some Delta configurations all the thrust is provided by RS-68s). However this lower expansion ratio severely hurts the vacuum Isp of the system. The RS-68 has a vacuum Isp of only 420s (similar to the J-2 which had an Isp of 421s), compared to the ~450s of the SSME or RL-10. The SSME has a much higher chamber pressure, so it can have a longer nozzle extension (~75:1 if I’m recalling correctly) without flow separation at sea level. The RL-10 only operates at high altitude, so it doesn’t have to worry about flow separation at all, and can have a really large expansion ratio (84:1).

Interestingly enough, if you could put a large enough inflatable nozzle extension on the RS-68, you could boost its Isp into the 430-445s range. And unlike the sliding nozzle extension like on the RL-10B-2 used on the Delta IV, this doesn’t involve any sliding seals. You’d probably want to go with a regen cooled nozzle upgrade while you’re at it, which would also help with things.

If this technology pans out, it could probably increase the performance of Delta IV vehicles by a substantial margin, while also bringing ideas like DIRECT back into the realm of feasibility.

It’d definitely be fun to see if the idea could be dusted off (now that the patents have expired).

Just a random thought.

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Jonathan Goff

Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
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8 Responses to Random Thought: RS-68 Inflatable Nozzle

  1. OrenT says:

    IIUC, the original J2 concept was to deploy the inflatable nozzle with helium after staging and just before the engine is started.

    For using this on the delta you would need to extend the nozzle in mid-flight – with the engine running. This will introduce difficulties, though probably nothing that can’t be solved.

  2. Gabriel says:

    Brilliant idea, perhaps some technology could be borrowed from the Bigelow inflatables. After all, these babies need to be hardened to resist meteoric impact.

  3. Jon Goff says:

    Yeah, they were planning on inflating the nozzle before ignition. And yeah, figuring out how to inflate it safely while the engine is firing may end up being a challenge (I don’t know for sure). Of all the schemes for extending nozzles while the engine is running though, this is one of the few that doesn’t involve sliding seals and other things like that. But it’ll be interesting to see what happens to the plume while it extends. Of course, if it extends quickly enough, the transient may be relatively minor…

    But *lots* of ground testing would likely be done first. It’d probably be a good idea to try this out on a much, much smaller rocket engine first, so we can get lots of experience under our belt before trying it on something big. If anyone has a six-figure wad they’d like to send our way, I’m sure I could talk Dave into letting us try it on our vernier engines. 🙂


  4. mz says:

    Of course, the level of extension depends some on how much room there is…

    I wonder how rigid the extracted part is when gimballing… But I guess all info is in that pdf, maybe I’ll read it another day.

    I also wonder what a toroidal aerospike version of RS-68 would look like. 🙂
    Maybe the “why no aerospike for shuttle” question needs some more going back to still…

  5. Brad says:

    Inflatable nozzles! Wow, that’s new.

    Instead of using an inflatable nozzle on something like the RS-68 of a Delta IV, wouldn’t the real potential of an inflatable nozzle be for super-high expansion ratio engines of Earth Departure Stages? Just how high an expansion ratio could be reached using an inflatable nozzle? And how much ISP improvement would result?

  6. David Stever says:

    Here’s a dumb question from outside the biz- how is nozzle shape effect the ISP (which I assume is a unit of output or power of the engine/stage)? Does the shape and size of the throat also effect the output?

  7. Jon Goff says:

    While you definitely could use it as an extension for in-space engines (as was the original purpose), you’ll get a whole lot less benefit for the same amount of complexity. Most in-space engines already have expansion ratios between 80-200, so adding a bit more will only bump the Isp up by a couple of seconds. A booster stage though, particularly a low-to-moderate pressure gas generator pump fed engine is going to get a much bigger benefit out of it. You could theoretically take a space nozzle, and truncate it fairly early on and add an extension like this to make packing a lot tighter. But you need an engine cycle that produces gas for keeping the nozzle inflated, and for keeping the extension transpiration cooled. Most upper stages are pressure fed, or closed cycle turbopump fed.

    It’s not a crazy idea, but gas generator fed booster stages like Merlin-1C or RS-68 would benefit more.


  8. Jon Goff says:

    Isp is a measurement of fuel efficiency. In badly mangled dead-monarch units, it’s pounds of thrust per pound-mass of fuel consumed per second. Ie if you have a 500lbf engine, with an Isp of 240s at full throttle, you’re burning through 2.083 lb/s of propellant (ie 0.945kg/s in SI).

    Isp is affected mostly by nozzle expansion ratio (the ratio of nozzle outlet area divided by the throat area), and somewhat by the actual geometry of the nozzle (a cone with a 15 degree half angle is more efficient than one with a 30 degree half-angle for what should be fairly obvious reasons–more of the hot gas is traveling in the “business” direction). For a constant nozzle expansion ratio, throat diameter doesn’t affect Isp hardly at all. But it does affect thrust. All other things equal, doubling the throat area will double thrust….

    Does anyone know of a good introductory, say on-line text describing all this to help David out? Most of the canonical references (Huzel and Huang, or Sutton) tend to be more geared toward engineers…


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