farmerben

That is a good starting point. But an air augmented engine will have a unique flight profile. More research and modelling would be needed to determine exactly what that is. I would guess that in the low atmosphere one would want to go a bit slower. We would want our thrust bonus to exceed the gravity losses of a slower climb, and otherwise stay slow to reduce drag. In the upper atmosphere (~20-70km) we would want to pitch more horizontal and throttle up. This maneuver would happen later than a scramjet design, but earlier than a pure rocket.

I agree with all those concerns. I don't know if the complexity, gimbaling, and cooling considerations can be adequately solved. But this idea of a simple skirt around the rocket engines seems to have a lot of merit compared to side mounted ramjet boosters. It should improve efficiency across all altitudes and airspeeds. The P51 Mustang channeled bypass air around its exhaust pipe and got a noticeable increase in thrust at 500mph. The drag created by air intakes must be contrasted with the drag created where a cylindrical fuel tank ends a meter or more before the exhaust plume begins with a conventional rocket. Combustion is difficult to achieve at high altitudes and airspeeds, but heating bypass air works basically the same way. Fuel may be cheap, but good fuel tanks are not. The question is how much a skirt would cost in terms of weight and money compared to the fuel tanks you would need without them. If a relatively thin titanium alloy skirt could withstand the temperatures, without active cooling, it might be a better deal.

Thanks Wjolcz! It is useful to know there is a name for it and somebody thought of it. It appears few tests have been done with the concept. I suspected the biggest problem would be vibration or cooling considerations of the primary rocket engine, or overheating issues with any practical skirt. The problem of large sized air intakes and extra weight does not seem all that serious to me. If it is only used with the first stage for the first minute or two and then detatched, and is reusable, then the weight does not seem like a problem. We should in fact save a lot of weight in fuel for the first stage. All the NASA designs I have seen involving ramjets are for SSTO's. In that context I understand why the weight might not be worth it. But in a multistage design this is not an issue.... you detach the parts when you run out of atmosphere. If the parts are reusable and do not require active cooling it seems like a bargain. It also seems to me that Whiplash style air ducts would be stable, in that a change in pitch will increase the relative airflow on the side the vessel is pitching into.

Take a close look at the Whiplash engine in KSP. The engine has a skirt with radial air intakes to mix bypass air with the exhaust plume. The bypass air is not fed directly into the combustion chamber, but mixed with the exhaust. The heating of the bypass air inside the skirt increases thrust. If the radial air intakes had ramps to control input air, an afterburner could be used, but increased thrust is possible even without an afterburner. My question is: Is it practical to combine this type of bypass air skirt with a regular rocket engine?