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Limits of air augmentation in rocket engine design


sevenperforce

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16 hours ago, DerekL1963 said:

If you have to keep making your engine more complex and heavier (and costing performance as the individual nozzles aren't as efficient as a single aerospike type combustor) in order to magically overcome real engineering concerns...  that's usually prima facie evidence that you're headed off in the wrong direction entirely if your first goal was to increase efficiency.  Not to mention that (multiple swiveling motors) engine will have even worse heating problems and will still have plume recirculation issues.  And also not to mention, airflow is also a function of speed and altitude, not just of the gimbal position of the internal motors.

Hmm...we may be conceptualizing the design a little differently, because I'm not sure those things apply.

First of all, the individual nozzles should have peak efficiency; rocket engines scale quite well within reasonable limits such as these, and using many smaller engines/nozzles may even allow pressure-feeding instead of turbopumping, for a net reduction in weight. I'm a little confused by your mention of a "single aerospike type combustor"; in most of the aerospike designs I've seen, numerous small nozzles end up being the rule. Consider this and this cylindrical design and this linear design. Granted, there are also designs which appear to have a single toroidal combustion chamber, but these seem to be the exception, and I can't imagine that it would be easy to control or adjust.

Gimballing on a single axis is pretty simple compared to the multiaxial gimballing on most rocket engines. When they are angled all the way "out", there won't be any plume recirculation issues because they are functioning like an inside-out aerospike:

straight_down.png

That's also how they'd be oriented for vacuum operation (though in vacuum you'd obviously see more expansion away from the wall). In this configuration, they operate identically to conventional thrusters without any loss in efficiency; the only drawback is the added weight of the ducting. There are no heating issues here that conventional rockets don't already have to deal with.

Gimballing the nozzles inward would be done only so far as was necessary to take maximal advantage of the airflow. You're absolutely correct that airflow is a function of speed and altitude, but the purpose of inward gimballing is to direct the rocket exhaust toward the point of maximum ram compression, so that the airstream is mixed with the exhaust most efficiently. At launch, the thrusters would be angled similarly to how they are angled in vacuum operation, but as airspeed increases, they would be gradually angled further and further in. Only once the forward airspeed becomes a substantial percentage of the exhaust velocity would the thrusters start to be angled back out; this would coincide with the transition to exoatmospheric orbital insertion.

If there was a point in the launch sequence at which reducing the oxidizer supply would allow partial ramjet operation with net increase in efficiency, this could be done easily, but it wouldn't depend on that. That might also depend on the load size and overall mission profile.

16 hours ago, DerekL1963 said:

No matter what airflow you choose, you're going to have massive heating problems.   Airflow alone won't cool those nozzles, nor cool the exhaust sufficiently that you won't cook the downstream portion of your shroud - you'll need active cooling.   Especially for the period when you'll have insufficient or zero airflow, E.G. once you're at sufficiently high altitude or in vacuum.

I wasn't planning on airflow cooling the nozzles. It's possible that they'd be large enough to have 3D-printed flow channels for regenerative cooling; if not, making them out of a niobium alloy and using radiative cooling would also work. Using many small nozzles allows for a high surface-area-to-volume ratio. And sure, the downstream portion of the shroud is effectively a secondary rocket bell and would need regenerative cooling, but that's no surprise, and hardly outside of what conventional rockets already deal with.

The intake can be made of the same PICA-X type material used for the heatshield on the Dragon V2.

Heat dissipation is a major problem with ramjet and scramjet designs because they have very low thrust-to-weight ratios, meaning they must spend a long time in-atmo at high speeds in order to build up velocity. In practice, this is what dooms them more than any other issue. Air augmentation is different; while ramjets and scramjets focus on using atmospheric oxygen to reduce fuel consumption, air augmentation focuses on using the atmosphere to augment thrust for the same amount of fuel. Because of this, air-augmentation designs can have T/W ratios approaching those for bare rocket engines, and would be able to accelerate rapidly enough that cooling is much less of an issue.

7 hours ago, fredinno said:

Russia still uses Liquid for ICBMs. Actually, you'd wonder why no one bothers to have a air-augmented ICBM, especially for rail/submarine transport.

 As somebody pointed out earlier, the Russians did do substantial development of a truck-mounted air-augmented ICBM that would have been vastly smaller than anything the US was able to field at the time, but they scrapped the project because the inventor kicked the bucket.

7 hours ago, fredinno said:

How much more expensive are air-augmented? Might be a decent SSME alternative.

Adding an air-augmentation shroud to existing ELV engine designs is not really cost-effective for the fuel savings. Fuel is cheap. But there's nothing about air-augmentation-design that requires exotic or extremely expensive tech; designing an optimized AA engine from the ground up is just a lot of testing and development and fine-tuning. Once development is done, the manufacturing and materials costs would not be significantly different than existing liquid-fueled rocket engines. At that point, there would be no reason not to use it on SSTO ELVs, but the huge cost savings would be SSTO RLVs. I need to crunch the math to figure out the upper limit on delta-V for thrust augmentation for a high-bypass-ratio central AA engine.

7 hours ago, fredinno said:

Use a second set of engines for high-altitude burns- a conventional engine. This would also be used for steering and landing. The Air-augmented engines only are used in the early boost stage of flight.

As I pointed out above, there's no need for a second set of engines. A central-bypass engine can point its thrusters straight down to "ignore" the central airstream altogether; there's no loss of efficiency in vacuum operation. The central airstream is used exactly to the degree that you need to use it.

Edited by sevenperforce
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15 hours ago, DerekL1963 said:

You need to be careful of extracting lessons from KSP.  It's first and foremost a game, and only in a very distant second place comes being a low fidelity engineering simulator.   That being said, this is being proposed and discussed as a propulsion system for an RLV, where engine weight matters a great deal there as it dominates vehicle weight by a wide margin.    In this case, even if it was an ELV, weight matters because you're spending so much of it for a system that's really only useful for a short period of time.   Dead weight doesn't impact payload on the first stage as much as it does on later stages, but it does have an impact.

You might want to tell Elon Musk that.  While the Merlin engine is relatively light (less than 1 ton), reports vary on how much fuel reserve is maintained for return, but it is somewhere between 30 and 60 tons.  If there was no need for return, and spacex was willing to design yet another rocket engine, they could afford to build an engine heavier by a factor of 4 to 10 times heavier (the upper stage can't afford to be heavy).  Of course, the dead weight is displacing fuel you could have (for an equal TWR), so there are real issues with the delta-v, but it is surprising just how little it matters (compared to upper stages where any dead weight kills you.  Just look at the rocket equation and it's obvious ("dry weight"="dry weight + all upper stages")

In Harvester's initial description of KSP to the Orbiter forums, he was trying to figure out ways to reduce the issues of orbital mechanics for game players.  While he did find one way to make it easier (make the planet *much* smaller, which made all the rocket parts less than authentic), most of the lessons are pretty close [unfortunately, apropos to this thread, airbreathers are so inaccurate to not worth mentioning any experiences in this forum, SRBs are pretty poorly modeled as well.  And don't ask about throttling].  But the lessons of the rocket equation are pretty good, and the issue with engine weights flow directly from the rocket equation.

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21 hours ago, sevenperforce said:

It would likely be possible to design an intake/nozzle shroud that can be simply slid forward or backward relative to the central rocket so that it is perfectly matched to airspeed, air density, and ambient pressure. But I'm primarily looking at the central-bypass approach anyway, simply because it offers a higher bypass ratio and intrinsic altitude/pressure compensation. but quadruple your potential thrust augmentation.

That. Right there. That sums up this discussion. I'm not a rocket scientist but I seriously doubt anything is simple at supersonic and hypersonice speeds. Something tells me that if it were that easy to deal with the consequences, rocket designers in the past would gladly of that large tank full of oxidizer that is either cryogenic, highly corrosive, extremely toxic and quite possibly all at once. And yet, somehow, that was a price Von Braun and Korolev were willing to pay. Maybe modern technology makes it easier but I doubt it will ever be easy.

Of course, history is full of geniuses who thought “out of the box” and revolutionized an industry with an idea, so simple, it's amazing nobody else ever thought of it. But remember, for everyone of those revolutionaries, there are thousands of nameless failures, forgotten by history, who learned the hard way that and this is why we do things the way we do them.

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1 minute ago, Kerbart said:

That. Right there. That sums up this discussion. I'm not a rocket scientist but I seriously doubt anything is simple at supersonic and hypersonice speeds. Something tells me that if it were that easy to deal with the consequences, rocket designers in the past would gladly of that large tank full of oxidizer that is either cryogenic, highly corrosive, extremely toxic and quite possibly all at once. And yet, somehow, that was a price Von Braun and Korolev were willing to pay.

Well, "simple" is relative. I wasn't suggesting that executing this design would be simple, only that the motion would be translation, which is a "simple" motion in comparison to swiveling, rotating, pivoting, or any other number of more complex transformations.

Also, it should be noted that this doesn't eliminate the need for oxidizer. An air-augmented rocket carries all of its own fuel and oxidizer, though it has the option of burning fuel-rich during the limited range of airspeeds where secondary combustion is mechanically efficient.

7 minutes ago, Kerbart said:

Of course, history is full of geniuses who thought “out of the box” and revolutionized an industry with an idea, so simple, it's amazing nobody else ever thought of it. But remember, for everyone of those revolutionaries, there are thousands of nameless failures, forgotten by history, who learned the hard way that and this is why we do things the way we do them.

Well, people have certainly thought of it before, since they've built air-augmented rockets on several occasions. I don't think anyone has thought of a central-bypass AAR; at least, I wasn't able to find it in any patent literature. But it wouldn't necessarily have been possible before now, simply because we lacked the right materials. For a central-bypass AAR to enable SSTO RLVs, the air intake will need to be lightweight, strong, heat-resistant, and reusable. In the past, we didn't have any materials with more than two or three of those qualities, but materials science has advanced quite a bit in the past couple of decades.

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