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?
- 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).
- 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.
- 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.
- 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.
- 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:
- 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.
- 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.
- Your nozzle is now more complicated, and your engine has more components (though probably less than a completely separate sustainer engine would).
- 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.
- 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:
- 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.
- 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)?
- 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).
- 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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