Thrust Augmented Nozzles

While I sometimes use this blog to talk-up some of the stuff we do at MSS, I also sometimes like to mention interesting technologies and projects being done by other groups. This post is about a potentially very interesting engine technology being pioneered by Aerojet, “Thrust Augmented Nozzles” (link to a free copy of a paper they wrote on the topic here).

I stumbled on this while trying to track down some old Aerojet papers about a sort of forced flow separation control technique that they researched back in the late 50s. I had noticed that most of the papers that cited the research talked about how Aerojet’s had concluded that the approach didn’t yield any net benefit, however the way they discussed it made me somewhat suspicious of their conclusion. You can sometimes get a sort of telephone effect with academic citations–where someone will read someone else’s review of some obscure and hard to locate article, and instead of reading it themselves, they’ll just summarize the summary, and before long who knows what the original article said. To make a long story short, I had good reason to be suspicious that there was something of that sort going on with this paper (especially since the two abstracts I was able to find online for their research seemed to directly contradict all the claims I’ve seen in citations of their work elsewhere). Anyhow, I couldn’t find any good online sources for the papers, so I was digging around on Aerojet’s site to try and find some contact information for someone there at Aerojet so I could see if they could somehow get me a copy. Which was when I stumbled across their work on Thrust Augmented Nozzles.

The simplest way of describing a thrust augmented nozzle is that it is a sort of rocket afterburner. As you can see in the picture below (from the paper linked above), there is an injector ring located in the expansion section of the nozzle where additional fuel and oxidizer is added to the stream and ignited by the core chamber flow.


The basic idea is that you can use the extra propellant injected and burned in the nozzle section to raise the nozzle outlet pressure to the point where you are no longer overexpanded. This propellant that’s injected lowers the effective Isp of the system while it’s being injected (it’s probably similar to having a throatless rocket motor with a shorter effective expansion ratio since you’re injecting the propellants partway down the nozzle), but it greatly increases the system thrust at takeoff without adding very much mass at all. They were able to demonstrate thrust augmentation levels of up to 77% (which was limited by the maximum flow capacity of the test-stand they were using for the tests in 2005), and claim that augmentation levels above 250% appear to be feasible. In fact, they point out that the maximum level of augmentation is only limited by the expansion ratio, with higher expansion ratios requiring a higher level of augmentation to operate at sea level.

So what are the implications of this technology for a rocket design?

  1. It allows you to make a rocket that gets excellent Isp in vacuum and excellent T/W ratio on the ground (though your Isp is going to be mediocre on the ground, and your T/W ratio with only the core flow isn’t going to be anything to write home about either).
  2. This allows you to get away with a much smaller engine overall (or fewer engines), while still getting a big Isp benefit for a lot of the exoatmospheric acceleration portion of the launch due to the much higher than usual expansion ratio.
  3. You can achieve a very good T/W ratio at takeoff, and good vacuum Isp without requiring high pressures in the chamber or in the thrust augmentation injectors. The tests Aerojet ran used 500psi chamber pressure and a TAN injector pressure much lower (probably around 200psi) for their tests. The lower pressure requirements mean easier thermal design on the main chamber, lower pump power (and hence performance) requirements, and overall a potentially more robust system.
  4. Because you shut off the thrust augmentation at some point during the boost, you don’t actually need to throttle down the main combustion chamber anywhere near as far to limit peak G’s on high mass ratio stages.
  5. It turns out that the thrust augmentation doesn’t need to use the same propellants as the main chamber. This is a potential way to do a tripropellant engine without as many of the same complications. For instance, you could take a LOX/LH2 engine like the RL10, and add in a LOX/Kerosene TAN injector in the nozzle. Aerojet’s studies showed that for a modest increase in GLOW (of about 50%), that a tripropellant SSTO design using their nozzles could probably acheive nearly 2.5x the payload of a pure LOX/LH2 SSTO, even if they lowered the core chamber pressure for the engines by half.Below is a picture of their test engine firing using a tripropellant GOX/GH2 main and LOX/Kero TAN injector setup. In spite of the challenges of burning kerosene completely with LOX even under the best of circumstances, notice the color of the plume in the thrust augmentation picture. For those who’ve seen normal LOX/Kero biprops, that is truly impressive!

As with any technology, there are a few drawbacks:

  1. The technology is patented by Aerojet. Depending on how Aerojet treats this patent, this could be a relatively benign drawback or could be a showstopper.
  2. Thrust augmentation requires very rapid and efficient propellant mixing in order to work well. This may imply using Aerojet’s bonded platet manufacturing technologies for at least the injector, and that could get expensive.
  3. Your nozzle is now more complicated, and your engine has more components (though probably less than a completely separate sustainer engine would).
  4. In order to really benefit from this technology, you want long nozzles with high expansion ratios, which may not package well into a given state.
  5. It isn’t readily clear how you would use this for a VTVL design, as you have to do a second burn at low altitude (for landing) that

So, overall the concept is rather interesting, though definitely not a panacea. That said, there are a couple of rocket concepts that could likely benefit very strongly from such a technology (if it scales well and doesn’t turn out to have any showstoppers at larger sizes). While more detailed analyses would be required to really determine if there’s benefit to be had, here they are:

  1. A future ULA hybrid of the Atlas V and Delta-IVH could be done that used a modified RS-68 using this technology. Imagine an RS-68 with regen cooled nozzle (extended to a very large expansion ratio, say 60:1 or even 80:1), using LOX/Kero pressure fed drop tanks (instead of solid fueled strapons). You’d get the extra thrust of solids, while getting a better mass ratio overall, and much better Isp on that core engine.
  2. A similar idea might work for a DIRECT-like launcher. Ditch the SRBs and all the expense and complexity coming from those, and just go with simple monolithic pressure fed drop tanks. You’re probably talking on the order or 300psi in the tanks to make them work, they’ll get much better mass ratio than the solids, while giving a much more efficient burn overall. Might even be enough of a boost from thrust augmentation to allow you to only need two RS-68s for the EDS launches (making it a Jupiter-222-TAN instead of a Jupiter 232)?
  3. Air-Launched “Assisted” SSTO launchers like the Space Plane or Frequent Flyer concepts developed by Dan DeLong at Teledyne Brown back during the 80s. These designs used winged LOX/LH2 SSTO stages that had all the propellant in tanks in the main fuselage. With the addition of Kero you could possibly fill the wings with propellant at little increase in mass (wet wings are a 50 year old technology), and trade a slightly shorter LH2 tank for a slightly longer LOX tank. All in all, you could probably make the design a lot less marginal than the all LH2 version. You have to deal with kerosene in the form of jet fuel for the air launch carrier. More importantly, you’re only slightly increasing the GTOW of the stage, which means you’re more likely to be able to launch it off of a reasonable sized carrier aircraft (might be possible to put a minimal cargo into LEO off the bottom of a White Knight 2).
  4. SpaceX’s Falcon 9 might be able to get away with better payload using only 4-5 modified Merlin engines instead of the current 9. That would make integration a lot easier, and decreases the odds of catastrophic engine failure taking down the vehicle.

Anyhow, a lot of this is still theoretical. Aerojet has fired such an engine several times, but there’s lots of scaleup and follow-on testing that needs to be done before this is ready for the primetime. But I figured that it was worth mentioning as one of those neat technologies that wasn’t getting much attention. It also kind of puts to lie the exaggeration that rocket technology hasn’t improved at all in 40 years and therefore we’ve already seen the best there is to see…

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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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22 Responses to Thrust Augmented Nozzles

  1. meiza says:

    Great and simple idea.

    I bet this will never reach flight test status. It’s like the aerospike.

    There are some complications: you either need additional or bigger turbomachinery or then additional tanks.

    Could be cool to calculate the performance of a Delta IV with strap-on kerosene tanks though.
    The savings in ground infrastructure could be significant if the rockets would always operate without the heavy solids.
    A single Delta IV medium+ GEM 60 solid booster weighs 34 tonnes, more than the empty core which is 27 t. And there are many solids on the rocket always.

  2. Monte Davis says:

    the exaggeration that rocket technology hasn’t improved at all in 40 years and therefore we’ve already seen the best there is to see…

    I’ve asserted that in some discussions, denied it in others… which is usually nature’s way of telling you to define the trade space better.

    Put the question as: do chemical rocket engines have “headroom for improvement” quantitatively comparable to what we saw, e.g., in aircraft engines from the state of the art in 1905 to the best radial engines of 1945 — or to the best turbojet/fan/prop engines today?

    If you approach it from physics and chemistry and thermodynamics, the answer is “no.” Barring near-magical breakthroughs, we’ve been near the practical limits (and within sight of the theoretical limits) of energy/kg in propellants and heat tolerance in combustion chambers for quite a while. No order-of-magnitude improvements in T/W coming there (although with the exponents involved, 5% or 10% can still be very much worth striving for.)

    But if you approach it from reliability, operating lifetime/MTBF, and maintenance — everything that would give us the practical RLV — the answer is “oh my, yes, there’s headroom galore.”

    Tech junkies focus on the T/W delta and the new speed records that the jet engine brought in the late 1940s and 1950s — but from the airlines’ POV, it was at least as important (probably more) that it so quickly turned out to be much more reliable/ maintainable than IC engines. T/W mattered, yes — but it was the jet engine’s ratio of money-making passenger seat-miles to total cost of ownership that really transformed commercial aviation.

    So you end up with a dual perspective: rockets are already thermodynamically more efficient than jets — i.e. convert chemical to kinetic energy “better” — simply because they run hotter and use less (and simpler) structure for the conversion. But so far, we’ve managed to translate very little of that into seat-mile : TCO ratios.

  3. Jon Goff says:

    Meiza,
    Well, it’s simple in concept, but the actual execution of getting thorough combustion in such an environment isn’t so simple…

    As for if it’ll ever be flight tested? I’m a little more upbeat about that possibility. An aerospike really has to be designed into a vehicle from the start (since it is so interdependent with the vehicle aerodynamics). Which means that if the aerospike didn’t work out, you may now have a useless vehicle. That’s a bit high risk for most folks. OTOH, something like this (especially if you’re trying a bipropellant TAN instead of a tripropellant version) might be interchangeable with more traditional nozzles–ie if the technology turns out to have a lot of problems in flight testing, you might not be as screwed. I’ll leave it at that.

    Yeah, they do somewhat gloss over pressurization of the TAN propellants. But would it be a whitepaper about a New! Cool! technology if they didn’t gloss over at least one inconvenient detail? The good news though is that since those propellants are injected at such relatively low pressures (2-5x lower than the chamber pressure), pumps or pressure feed systems for them shouldn’t be too tough or heavy. In fact, for some engine cycles you might be able to get a lot of that for free. The increased mass flow creates more heat in the nozzle, and for expander cycle engines, that translates into more available pump power. Also, since LOX/Kero is about 3x as dense as LOX/LH2, you really don’t need that much extra pump power even for very large augmentation ratios. A 300% augmentation of an RL-10 with either LOX/Kero or LOX/subcooled-propane might only increase the required pump power by 30-50% (which is a lot lower than the actual increase in available pump power that you’d likely see with such a scheme). Just some thoughts.

    Lastly, yeah, getting rid of big heavy solids and replacing them with pressure fed tanks would be a big win I think. It would likely be somewhat easier also than doing a propellant crossfeed.

    Of course, one last option would be to take a vehicle like the DeltaIVH CBC, and give it three tanks (one LOX, one LH2, one Kero) by shrinking the LH2 tank, stretching the LOX tank, and adding a small Kero tank at the bottom. That would likely be a big improvement over the existing design.

    ~Jon

  4. Rand Simberg says:

    …that it so quickly turned out to be much more reliable/ maintainable than IC engines.

    Just a nit, but I think you mean piston engines. Turbofans are internal combustion engines.

    Good post, Jon. I have further comments.

  5. Jon Goff says:

    Monte,
    Two thoughts. First off, this concept while it won’t lead to an order of magnitude improvement in T/W could easily lead to a 2-3x improvement, which is still pretty darned impressive in my book.

    However, I agree with you that the more important implication of this technology is that it can allow you to get the performance you need while allowing for much more robust, lower pressure, more reliable booster engine systems. With this approach you no longer want or need staged combustion and really high chamber pressures for booster engines–much simpler gas generator or much more benign expander cycles could be used. As I say before–definitely not a panacea–I’m sure there are some designs that wouldn’t benefit much or at all from it. But the ability to say go with a low pressure expander cycle, but still get the T/W ratio you want and still get the performance you want…that’s powerful. Also, since this can greatly increase your T/W ratio, it allows you to be more robust, higher FOS on all your other engine subsystems.

    Just a thought.

    ~Jon

  6. Anonymous says:

    This sounds too good to be true.

    Why wasn’t it considered before? Tripropellant ssto were all the rage in the late 80’s and early 90’s – some of the delta clipper proposals had them. Yet this scheme is better than aerospike engines!

    Why would vtvl be difficult?

    Also, how would the calculations change if methane is used? I’m thinking of applications for Mars.

  7. Jon Goff says:

    Anonymous,
    As I’ve pointed out, there are some drawbacks. It isn’t a free lunch. That extra propellant you’re burning in the nozzle doesn’t have the benefit of using as much of the nozzle expansion ratio, and has to burn really quickly. This makes the injector tricky, and causes you to lose some Isp at the low-end. According to the paper, they had to leverage quite a bit off of scramjet research they had done in-house in order to design the injector correctly. So this idea might not have even been possible 20-40 years ago.

    As for aerospikes, I think the guys have a patent out for applying this technology to those as well. While it doesn’t give you the same advantages you get for other systems, it could potentially increase the T/W.

    As for why it doesn’t give you everything for VTVL, imagine you have a high mass ratio VTVL vehicle. For the final touchdown, you want to be able to do a little bit of a hover and/or a gentle descent. There’s a reason why you want throttleability. That implies running the engines at a much lower thrust level than you need for liftoff. In some cases it could be as low as 10-20% of liftoff thrust.

    With a TAN concept, that would require you completely shutting off the thrust augmentation. However once you’ve done that the jet is now so overexpanded it will separate (potentially violently). Now, if you combined this with a forced flow separation device, or combined it with an aerospike or E/D nozzle…maybe you could get it to work…but that complexity is now going up pretty quickly. Of course, you could also have a central fixed engine with TAN concept with several smaller sea-level optimized “verniers” around the outside…but there are lots of complicated tradeoffs there. Can you keep the engine-out/intact abort capabilities while still getting enough benefit out of your central TAN engine to justify the added complexity? I have some ideas on how to approach the problem, and while a little added complexity can often more than be worth it, there is a point where things start getting to complicated to be reliable.

    As for your last question, I’m not really sure. LOX/Methane should work just fine as either the TAN propellants or the main chamber propellants (or both). It’ll burn easier than LOX/Kero so your TAN injector design is easier. Other than that…I’m not really sure what the differences in the system would be for different propellant combos.

    ~Jon

  8. Monte Davis says:

    Rand: you’re right, of course. I was tacitly thinking of IC as synonymous with all the challenges of “batch” injection, carburetion, ignition, venting, and timing that became “continuous,” so to speak, with the transition from pistons to jets.

  9. Anonymous says:

    Jon, thanks for the long response!

    Good point about the injector only being around recently.

    On landing for vtvl I guess that the vernier engines would make most sense.

    One additional thought: could the injected propellant be used as cooling fluid during reentry? A water wick, but using rocket propellant instead?

    Seer

  10. Anonymous says:

    I wonder if you could inject a monopropellant, instead?

  11. Anonymous says:

    An interesting application of this technique would be to have a turbopump burning methane via expander cycle on the nozzle.

    When the propellant is injected into the nozzle it will raise the temperature of the nozzle with the higher thrust, and hence the expander cycle would have a greater heat capacity to utilize with the turbine.

    An advantage would be that in high-thrust mode the turbopump would also be augmented to higher power levels, at least enough to inject the nozzle manifold propellants without specific-power losses for also delivering props to the combustion chamber.

    With no gas generator or burner, and the already-relaxed chamber pressure requirements with this technique, the turbine-turbopump machine would be small and very efficient. The advantage in weight savings for tankage, especially with bigger vehicles by not having to pressurize them would be worth the complexity. And for ease-of-use, re-startability, and re-useability, the expander-cycle I think is the best thermo-cycle out there for forced induction rocket motors.

  12. Anonymous says:

    Jonathan, thanks for sharing. This can indeed solve some classical problems, specially if you use a low pressure fed system. Not sure whether the patent is a show stopper or not. Most of the components are prior art and maybe the particular application is already described somewhere else. I will dig for that…

    Bruno/SPL

  13. porfiria says:

    some old Aerojet papers about a sort of forced flow separation control technique that they researched back in the late 50s.

    Any technologies described, or even just suggested, in these papers cannot still be covered by a patent.

  14. Jesse says:

    >>porfiria said…

    >>Any technologies described, or
    >>even just suggested, in these
    >>papers cannot still be covered
    >>by a patent.

    Note that he said he “stumbled across” the TAN stuff while looking for the other stuff. I don’t think the two are related.

    As far as the patent stuff goes, they would be allowed to patent something related to the old stuff if, for instance, they had taken the old idea and worked on it to upgrade/improve it in such a way that it had significant (non-obvious) changes.

    – Jesse D

  15. Chris In A Strange Land says:

    The SSTO thing is easy. Just use it on a seperate pair of fixed boost engines.

  16. Anonymous says:

    Looks like an effective path to a “tri-fuel” engine for SSTO. The F-1 had a somewhat similar system for reinjecting the turbopump exhaust around the nozzle about halfway down the bell. Apparently this improved performance a little and cooled the nozzle extension.

    However for the D-IV, which already has thrust near to its structural limit, crossfeed and regenerative cooling might be more practical upgrades.

  17. Anonymous says:

    Regarding the possibility of injecting methane in the nozzle, would not the low density of methane make it less effective than a heavier fuel such as kerosene?

  18. Jon Goff says:

    Anonymous,
    Yeah, subcooled propane would be better. Almost identical performance, no residue, no coking (if you freeze out the mercaptans), a density of around kerosene, and you can use it to run an expander cycle pump for the Thrust Augmentation if you want to.

    ~Jon

  19. Robert Clark says:

    Thought you might like this since it mentions TAN:

    Hovering capability for the reusable Falcon 9, page 2: Merlin engines in a pressure-fed mode?
    http://exoscientist.blogspot.com/2015/07/hovering-capability-for-reusable-falcon.html

    If this works then it might provide a general method of giving any rocket stage vertical landing capability.

    Bob Clark

  20. Jonathan Goff Jonathan Goff says:

    Bob,
    Interesting idea, though I don’t think it would actually work in practice. There are plenty of ways to make Merlin-1D throttleable enough for hovering, they’re just all more complex, and would either require the landing engine to be unique, or a lot of added complexity per engine. I’m still more a fan of hovering than not, but I can understand why Elon’s doing things the way he’s doing–his philosophy seems to be start as simple as possible, and only add complexity once it’s prove necessary.

    ~Jon

  21. Peterh says:

    Regarding a low chamber pressure mode for an engine, I believe the low throttle on the Merlin engine at sea level is dictated by flow separation. If the nozzle exit plane pressure falls too far below ambient the flow becomes unstable and control is lost.

  22. Peterh,
    Correct–or more specifically, it’s uncontrolled and potentially asymmetric flow separation that’s the issue. You could theoretically use TAN-like injectors to force flow to separate cleanly and axisymmetrically at a specified point in the nozzle. But you wouldn’t need a lot of flow, especially if you were say injecting a little bit of oxidizer into a fuel-rich plume. But that’s a lot of added complexity that may not be necessary. I say it’s worth seeing how reliable you can make a hover-slam approach be. A few early failures isn’t the end of the world–though ultimately I’d only give SpaceX a 50/50 shot of getting recovery reliability above 95% without the ability to hover.

    ~Jon

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