I’ve been a bit busy to weigh in on this for a while, but being a propulsion engineer myself, I can see *several* potentially interesting technologies that could be invested in.  To me, I don’t think the reason they’re suggesting investing in HLV technologies instead of building an HLV right now has anything to do with whether or not we can build one with existing technology.  My guesses at the reasoning are more along the lines of:

1-If they build an HLV right now and keep Orion going, they would either need to greatly increase the budget, stretch timelines out so far that it would drive the costs way up, or sacrifice the other parts of the plan that are what they’re actually interested in (tech demonstrators, ISS utilization, commercial crew maturation).
2-If they punt on actually building an HLV until after this first five-year chunk, then they’re no longer conflicting with the most immediate tech demonstrators or the commercial crew development efforts, which would allow them to build such vehicles without requiring a big increase in NASA’s budget.
3-By punting for a while, they both may have better technology options to start with, allowing for a cheaper, lower manpower HLV, but there will also be a much better baseline to start with for a new Orion vehicle.  By that point there will likely be at least two commercial capsule providers out there, and possibly some new tricks up their sleeve, which may make Orion a much smoother, better vehicle overall.
4-By punting on the actual HLV development until after the technology demonstrations, it may be clearer what capabilities you really need for the HLV.  Going ahead with one right now either has to assume that the tech demonstration will work (risky) or assume it won’t work (risky via overconservatism).  Waiting until you know more about if depots and high-Isp in-space propulsion systems are reasonable allows you to pick an HLV better matched to the new technology.

Going back to the technologies, if you understand that this isn’t a question of “we can’t do it with the technology we have today”, but more of a “we don’t want to do an HLV for other reasons, but what stuff could we do to make an HLV more affordable when we do have the budget to move on it”, then it becomes a lot clearer.  To me, the goal of any of the booster engine R&D should be to take technologies that have potential for drastically better cost/performance ratios, and mature them to the point that they could be rolled into an HLV effort when it is time to move.  TAN is one option, since if done right you could get enough thrust that a shuttle derived HLV wouldn’t need SRBs anymore to takeoff, and commercial EELV-class vehicles could either reduce their engine size, or increase their propellant load drastically for the same weight booster engine.  It might also allow you to get good performance with a lower pressure booster engine.  Other options include stuff like pistonless pumps, or some of the other pump concepts I’ve seen that while they may not have quite the performance of staged combustion, have most of the performance at a tiny fraction of the complexity/cost.  Flow separation control is another cool trick that can allow you to get more performance out of lower chamber pressure engines.  Imagine being able to build an engine that had better T/W ratio than an existing staged combustion engine, had no turbomachinery, and similar or better mission averaged Isp, but cost about a tenth as much?  That would be a game-changing set of technologies in my book.

There may be some other work relating to getting domestic high-thrust LOX/hydrocarbon engines ready.  There’s also the high thrust expander cycles you mentioned, which would be beneficial to EELVs as well as HLVs.  Depending on the engine size, it might even be interesting to further develop the Mid-Air Recovery idea that LM was investigating for their Atlas V engines.  Being able to recover the engines from an HLV launch and reuse them a few times without having to deal with salt-water corrosion issues is a great way to reduce some of the big marginal costs of a flight.  For upper stages, putting some money into ACES/Raptor related technologies might not be a bad investment either–find something that can give you some commonality between your HLVs and other stages flying (or make it so your HLV is just a family member of an upgraded EELV-class launcher family).  Or doing at least a little funding on reentry technologies could also be interesting (to allow for easier reuse of currently expendable booster stages, low the hurdle for true-RLVs, and also possibly make Orion-like vehicles easier to build and more capable down the road).  Some concepts like transpiration cooling and electromagnetic TPS/aerobraking are both really interesting.

Anyway, I’m an ideas guy and a propulsion engineer, so I may have a different view of this than most, but I for one think this is a reasonable approach.  To me, cutting back drastically on the amount of people who are needed to launch an HLV, while also using new technologies that allow you to get better system-level performance out of lower-cost, lower-complexity hardware is potentially game changing in my book.

Drop Tanks to Enable SSTO?
I don’t like drop tanks. While they do definitely make the rest of the SSTO easier, they come with several significant drawbacks:

1. Large expendable hardware on an otherwise RLV somewhat misses the point.  While rocket tanks are relative cheap compared to the rest of the stage, they’re still pretty expensive compared to the propellant cost, or even the likely maintenance cost of the rest of the vehicle.
2. Dropping stuff on people is usually considered somewhat anti-social.  That means you’re stuck launching over water like the ELVs (there might be one or two over land paths where the drop zone would be in a safely unpopulated area that isn’t a national park, but it’s unlikely).  Launching overwater means aborts have a much higher chance of costing you the launch vehicle.  It also likely limits you to existing ranges, and ties you in with their practices that may not be very conducive to RLV operations.
3. You still get some of the drawbacks of TSTOs.  You still have to design and analyze two vehicles instead of one.  You still have to design and analyze a separation system.  You still have to integrate two stages together at the pad instead of dealing with just one.  You still have to figure out how you handle aborts (if possible) while the drop tank is on.
4. Drop tanks reduce the “fluffiness” of the orbiter stage, which may complicate reentry TPS considerations (though maybe not enough to matter).
5. The dry weight to orbit savings might not be as much as you think.  Propellant tanks are pretty lightweight for pump-fed vehicles.  Mechanical connections, separation systems, plumbing, pressurization systems, quick disconnects all add quite a bit of mass.   How much mass do you really save compared to just building slightly bigger tanks on the SSTO side?
6. Drop tanks probably complicate the aerodynamics.  If you have one drop tank, you now have something like a Biamese vehicle (I’ll get to those in a minute), which has much more complicated aerodynamics, harder abort environments, etc.  If you go with a lot of smaller tanks, the aerodynamics becomes easier, but your scar weight to structures, plumbing, etc becomes a lot heavier.

That’s not saying they don’t have some advantages.  Any reduction in dry mass can go a long way towards making an SSTO shift from marginally infeasible to marginally feasible.  Expendable tanks can be designed to lower safety factors since they don’t need to consider fatigue issues like RLV tanks do.  Less dry mass means a lighter landing weight, which decreases the amount of landing propellants required and the weight of landing gear (or the wing and landing gear weight if you’re doing an HTHL design).

It’s not that they don’t have advantages, but the disadvantages are enough that I’m not convinced it pays for itself.

Biamese or Parallel Staged TSTOs
Even though my boss Dave is fan of Biamese approaches, I’ve never been.  He gives me crap about far-out stuff like FLOC, I give him crap about Biamese.

While once again there are enough benefits for Biamese vehicles to make them sound interesting, I think the drawbacks once again win out:

1. The engines have to be able to operate from launch altitude all the way to vacuum.  While altitude compensation can often help TSTO RLV designs, and while there are some techniques like TAN and Flow Separation Control that might make it easier to have an engine operating over that wide of a regime, you’re still making some pretty big compromises to both stages (or you’re making compromises to the biamese concept and losing some of the benefits).  The upper stage now has to have far more thrust than it would’ve wanted anyway.  That means more weight that the upper stage didn’t need.  It also needs some way of operating its space-rated engines at low altitudes.  This means altitude compensation (which tends to be heavy and complicated), high pressures (which makes the engine harder to develop and more complicated, or something like TAN or Flow Separation Control, which it might not have otherwise needed.  If the booster and upper stage engines are the same (as they would be in a purist Biamese design), the booster engines now have to have a much higher expansion ratio than they would have otherwise.  Once again this leads to higher pressures or other complications that they might not have needed otherwise.
2. A nearly 50/50 mass split is not very optimal for staging.  Especially when you figure in that the first stage wants to get back to the launch site.  It isn’t heinously bad, but it does mean you end up having moderately high delta-V requirements on both stages (since the first stage is likely going to be far enough downrange that you’ll need to do some sort of boostback).
3. TPS requirements for the two stages are vastly different.  For a purist Biamese vehicle this means the booster is lugging around a lot of weight and complexity it really doesn’t need.
4. If you don’t do a purist Biamese design (where the two stages are really identical and interchangeable), you’re back to designing two separate stages, but now with all sorts of unneccessary constraints.  The more divergences you make to simplify things (like going with a lower expansion ratio on the booster stage, or going with a lighter TPS on the booster stage), the more the two designs become different, and the more and more you start losing any real benefit from the process.
5. Parallel staged vehicles have uglier aerodynamics.  Aerodynamic design and analysis for supersonic vehicles can be very complicated and expensive.  I’ve never seen a TSTO Biamese design that didn’t look like it would be a bear to analyze and design the control system for.
6. Biamese RLVs tend to lead to compromised structural design.  Rocket vehicles are most weight efficient (and easiest to design and fabricate) when they are bodies of revolution.  In order to get good mechanical connections, most Biamese vehicles I’ve seen end up being lifting bodies, which starts driving either really weird propellant tank shapes (with added weight and fabrication complexity) or really inefficient structures (where the propellant tanks fit inside a more complicated shell.
7. Because of the different operating modes of the stages, you’re really stuck still designing and analyzing three different vehicles (the two together, the first stage independently and the upper stage), not just one stage.
8. You do potentially reduce the number of engines you need to make, and may allow you to design some subsystems only once, but now they’re being designed to meet more constraints.  Many times it’s easier to design two slightly different subsystems with 10 constraints each than one with 15.  You can still reuse a lot of the design and analysis work if you do things right, but each of the two designs are easier.

I guess to me it boils down to the fact that jacks of all trades really tend to be compromised kludges by the time they make it into operations.  In a Biamese system, both stages are carrying stuff they don’t need, and are being designed to more constraints than were necessary.  I really don’t see how that will lead to a cheaper system than one that has the two stages scaled the way that performance and operations want them to scale, and that can be more custom-suited for the task they’re being asked to perform.  Hybrids tend to give you the worst of both worlds.

Sea Dragon
For those of you not familiar with this concept, the Sea Dragon was an old Aerojet design by Bob Truax for putting 1,000,000lb of payload into orbit on a single TSTO launch vehicle (whose first stage might be recoverable). The design was a Big Dumb Booster, with pressure fed tanks made of maraging steels, built more like a submarine than a rocket vehicle. The first stage engine would’ve been something like 70x higher thrust than the F-1 engine on the Saturn-V. You can get more details here.

I’m a tiny bit more torn on this one than the others, but I still think it makes sense in today’s world.  It might make sense at some future date, but not right now.

Here’s my big concerns:

1. Where’s the demand?  I don’t think we currently as a species launch a million lb or payload into orbit in a year.  Until other systems like RLVs get the cost down and the flight rate up there’s never going to be enough demand for more than one or maybe two of these per year.  While the marginal cost of one of these would be pretty low, the fixed costs and development costs aren’t going to be trivial, and they have to be amortized over those flights, cutting into any cost advantage the design might have.  Now, if RLVs do get the cost down to the point where you start having enough demand where Sea Dragon could make sense, you run into a different problem–the Sea Dragon is no longer competing against expensive existing ELVs, it would be competing against RLVs.  Sea Dragon may get stuck only launching payloads where the integration costs of launching them separately and putting them together in orbit outweigh the cost diffrence between the two.  Now, we live in a world where even though most stuff gets shipped in tiny intermodal containers, there are still Super Guppies and Belugas that get used occasionally.  In an RLV centric world, there may still be situations where a Super Heavy Launch Vehicle might be useful enough frequent enough to justify its existence.  But we’re nowhere near that point in time.
2. What’s it going to be like developing and testing a 70 Mlbf rocket engine?  Pintles are a pretty cool, pretty scalable combustion system, but will they really scale up to something 70x bigger than has ever been built before?  We have no idea what unknowns lurk between here and there.  Maybe pintles will turn out to work fine without any problems, but we’re pushing far past what has ever been done in the past.   But pintles tend to get worse c* as they scale up, will they still have adequate performance at those scales?  Nobody knows, and nobody will know until they start.  That’s scary.
3. Testing an engine this big is going to be mindbogglingly expensive as well.  Every second the engine would be going through about 5 Falcon-1′s worth of propellant.  That’s only $110k/s of propellant (the upper stage uses much more expensive propellants, so even though it’s only 8Mlbf, it’s still likely going to cost a lot), but that’s not counting anything else.  You’re talking about $25M per full-duration burn test.  With how expensive the payloads would likely be for a vehicle this big (see below), you’re likely going to need to do a lot of tests.  Just the injector testing alone for something like this would likely run you into the $1B+ range.  If you did even a fraction of the number of runs typically done in a rocket engine project, you’d be talking about billions of dollars up front.  And where are you going to test a monster that big?  You’d pretty much have to do it out at sea a long ways.  How are you going to vacuum test an engine the size of the upper stage engine?  I guess you can get away with not doing the full nozzle extension tests, but that’s still putting a lot of risk into the first few flights.
4. Development costs would be insane.   Between testing the huge engines, and doing at least one or two flight tests, you’re likely talking several billion dollars to develop–if it’s done on a commercial basis!  The marginal cost of one of these things in 2009 dollars is likely going to be in the $1B range, so that starts adding up fast.  It’ll be a long time before there’s enough demand to justify putting up that kind of money.
5. Payload costs per launch would likely be very high.  While I full-heartedly agree that relaxing mass constraints can reduce the cost of space payloads, it’s only one part of the cost involved.  Being able to go with welded steel and an FOS of 3 may reduce some design and fabrication costs a lot, you’ve still got the fact that unless you’re launching bulk commodities, you’re designing hardware that has to operate in a very harsh environment, that still needs to be fairly complex, and which very few will be built.  People talk about stuff like being able to launch an ISS in a single launch.  While avoiding all of the EVAs and integration stuff would take a lot of work out of ISS, you’d still be talking about a several billion dollar payload.  How would you get launch insurance?  How often can you afford to fly a vehicle that costs $500M-1B to launch and has payloads that will tend to cost several times more?

Unlike the other two ideas, I’m not convinced that super heavy launch vehicles will never have a place in the rocket world.  I’m just not convinced we’re even within visual range of such a time where they make sense.  We’re still back not too far past the Wright Flyer stage of launch vehicle design.  We’re nowhere near the point where a Beluga or Super Guppy makes sense.

Dubbed HC Boost, the technology development program is aimed at providing an improved, home-grown alternative to the Russian RD-180, the only other viable current-production hydrocarbon rocket engine. Unlike the RD-180, however, the US engine would be designed to be re-usable for up to 100 missions, have up to 15% better performance and would operate for up to 50 missions between engine overhauls.

Now, the RD-180 is a very high performance engine. It’s combustion chamber pressures are actually quite a bit higher than the SSME. When you combine this with the statements in this paragraph (my emphasis again):

The last US-designed and produced hydrocarbon engine was the Rocketdyne RS-27, based on 1960s technology and now out of production. The HC Boost engine, on the other hand, is expected to have higher operability, faster turn time, a longer-life thrust chamber, turbopumps and a new design nozzle.

It really does seem to suggest that they might be talking about thrust augmented nozzles. By going with nozzle thrust augmentation, you could get that same high lift-off thrust, and good sea-level Isp without requiring anywhere near as high of a chamber pressure. Which would necessarily lead to less demand on the thrust chamber and pumps.

A similar engine concept I’ve talked about with a few friends in the propulsion business is if you took the Merlin-1C upgrade that’s been hinted at by SpaceX, and applied that extra pump power they’re looking at creating to providing higher flow, lower pressure propellants to a TAN section (say by diverting some of that extra high pressure flow into some sort of a jet pump), you’d be able to get away with a much larger expansion ratio nozzle (with good sea-level Isp), while also nearly doubling the thrust. I really like the idea of a Falcon 1f that can put say 5000lb of payload into LEO for little more than the cost of a Falcon 1e….It’s a powerful concept, though I would understand if for now a company like SpaceX is focusing on getting Falcon-1 and -9 flying reliably first. First get it working and working reliably, then add the afterburners…

Which reminds me, I need to go back and do a more detailed article on some of the additional concepts and technical information I’ve found about Thrust Augmented Nozzles at some point.

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…