Due to nature of space-rocket launches using "conventional" rocket fuel (fuel+oxidizer, e.g. hydrogen+oxygen), every kilo of fuel that has to be carried impacts the rocket's range in a very nonlinear way (in short, more fuel on board needs more fuel to lift it).
Therefore, any fuel/weight saving at early stages of rocket flight correspond to the significant increase in rocket's ability (max height or max payload). That's why for example, Virgin Galactic uses WhiteKnight jet powered plane to lift the SpaceShip to relatively meager height of 20 km, even if it is only 1/5 of the final height of 100 km.
It is obvious that turbofan engines have pretty low ceiling with respect to spaceflight, but why not continue the flight (e.g. start of the SpaceShip's ascent) with the ramjet engines, which have the following properties:
- Very simple design, much less (none?) moving parts than the turbojet/turbofan
- Still makes the use of the environmental air, not needing oxidizer, therefore much less weight to carry
- With increasing speed the ramjet needs regulation of amount of air at intake, which would be naturally compensated by increasingly thinner air during ascent, perhaps not needing regulation at all?
- Unlike solid fuel rockets it can be turned off if necessary
- It is far less efficient than turbofan, but due to use of environmental air, still much more efficient than fuel/oxidizer combination?
So, why no one is using ramjets for the second stage of the launch? Perhaps even with "classic" design, solid first stage (to get to the minimum velocity for ramjet operation), ramjet second stage and then liquid fuel third stage?
EDIT: I checked the question (and answers) which was reason for this question was marked a duplicate. I strongly disagree - the previous question asks about jet engines in general, and the answer is quite turbofan-specific. I know the difference between various types of jet engines, therefore I was asking specifically about the ramjet engine, and I also outlined its advantages (turbofan engines, commonly referred to as "jet engines" used in commercial aviation are quite different beast compared to the ramjet engine!) For one, the size, the main disqualification for turbofans in the answers of the previous questions is due to fan part of the turboFAN engine.
Therefore, no, this question is not a duplicate. I asked specifically because there is very little discussion on ramjets used for rocket propulsion. I would kindly ask that the "duplicate" tag is removed, since the question the tag refers to is only distantly related.
SECOND EDIT: I was asking specifically about ramjets for the second stage of the launch, please read the title of my post before directing me to the answer why ramjets cannot be used for the first stage!
And, thanks to @Hobbes, we know that NASA has design that incorporates that exact sequence of propulsion techniques, so my question again: Why no commercial operators are using it - since differently from NASA they are very cost-aware. Or shold be.