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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.

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  • $\begingroup$ Design for a reusable rocket using ramjet and turbojet stages: nasa.gov/offices/ipp/centers/dfrc/technology/… $\endgroup$ – Hobbes Sep 14 '16 at 16:50
  • $\begingroup$ @Hobbes this is actually great answer. It shows that such design is much more efficient and cheaper to operate, so my question is right on the spot: why no one is using it commercially? $\endgroup$ – xmp125a Sep 15 '16 at 20:12
  • $\begingroup$ Re your edit: the second, more highly-voted answer on that question does address ramjets. Read all the upvoted answers, not just the first one. $\endgroup$ – Nathan Tuggy Sep 15 '16 at 20:35
  • $\begingroup$ The second stage/main engine of the Ferranti Bloodhound SAM was a ramjet. $\endgroup$ – Thumbnail Sep 15 '16 at 23:02
  • $\begingroup$ @NathanTuggy That second answer is very brief, though, and deals with a different case. The OP is suggesting use as a second stage. I don't understand why this was closed. $\endgroup$ – kim holder Sep 15 '16 at 23:26
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What is the use of a second stage using ramjet in a height where the remaining air is very, very thin? The Falcon 1 first stage is used up to a height of 90 km, the second stage reaches a height of 200 km where the satellite is put into the orbit.

You would need three stages, the first with a rocket engine to get a speed where a ramjet may be used, the second with a ramjet engine up to the height where the remaining air is to thin for ramjet operation and an additional third stage with a rocket engine to get into a low orbit at 200 km height.

A hypothetical example, the ramjet is used from 13 km height and 19 % of the ground air pressure up to 36 km height with 1 % pressure. The first stage is restricted to only 13 km, the second stage adds only 23 km and the rest of 164 km is left to the third stage. Lets look for the speed, if the ramjet operates from mach 2 to mach 5 or from 0.66 km/s to 1.7 km/s. We have to reach a speed of 7.8 km/s for a low orbit. The third stage has to deliver the remaining 6.1 km/s.

I looked into Wikipedia for some speed values for a ramjet.

Needing three stages instead of two adds complexity and cost and the reliability is smaller, that means the risk to loose the payload due to a malfunction of the whole rocket is higher. The third stage has to contribute more speed and height than the second stage of a conventional rockert solution, therefore needs more fuel and a eventually a stronger rocket engine. The first stage may be a little cheaper, but the third stage gets more expensive and we have the additional cost of the second ramjet stage. I doubt that the total cost is lower.

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    $\begingroup$ @Uwe Due to nonlinearities, caused by chicken-and-egg (payload and fuel) dependencies, the 23 kilometers of added range are nothing to sneeze at. Saturn V rocket had 3 stages, first stage was about 80% of weight, and lifted the rocket "only" 67 km high. Turbofan/ramjet combination would result in much much smaller first (now third) "true rocket" stage, even if it seems miniscule on paper! $\endgroup$ – xmp125a Sep 15 '16 at 20:18
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    $\begingroup$ If the first stage is smaller, this is true only for the propellant tanks. The rocket engines need the same thrust for a lift up. With smaller tanks and less fuel, you don't save much money. The cost for engines and electronics are the same. Saturn V had three stages not only for a low earth orbit, bur also for the lunar injection. 2 stages may lift a satellite to a low orbit. $\endgroup$ – Uwe Sep 16 '16 at 7:40
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    $\begingroup$ @xmp125a: You focus waaaaaay too much on altitude. The altitude is not the hard part of going to space. The hard part is the speed. You'd need to be able to get your ramjet up to speeds much faster than have ever been demonstrated for it to be worthwhile adding it as a stage. Once you've got the speed, getting the additional altitude is easy. However, you talk of "23 kilometers of added range" and such as though that means anything. It really, really doesn't. You can accelerate harder once out of the atmosphere, but getting out of the atmosphere is the easy part. $\endgroup$ – CBHacking Sep 22 '16 at 21:19
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    $\begingroup$ @xmp125a: "Conventional" (non-supersonic-combustion) ramjets have a pretty strict upper boundary on speed, one that is low enough that you'd need a third stage nearly as powerful as a modern second stage. Above that speed (in the neighborhood of mach 5), sub-sonic combustion is no longer possible - you can't slow down the incoming air enough - and while supersonic-combusting ramjets (or scramjets) are beginning to be demonstrated in small numbers, they appear to be far harder to design, operate reliably, and control. $\endgroup$ – CBHacking Sep 24 '16 at 19:40
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    $\begingroup$ @CBHacking ok, that makes sense. Saturn IV was travelling close to mach 7 at the end of the first stage burn, and falcon 9 apparently at mach 6, so if mach 5 is the ramjet limit, then this is definitely one of the reasons. Thanks. $\endgroup$ – xmp125a Sep 26 '16 at 5:32
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Thanks to many people answering and commenting, I think it could be summed up as follows:

1) For suborbital flight, height is important. Ramjets could be used for that. NASA even researched ramjet as a propulsion option.

2) BUT, if your goal is to reach orbit, the height does not matter that much. You can lift your space vehicle to an altitude, and it will fall down like a rock. What matters in orbital and interplanetary flight, is the velocity - (see escape velocity), so the large part of the energy invested is used for accelerating the vehicle, not purely lifting it. In that case, ramjets get significantly less useful due to the narrow range of velocities.

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    $\begingroup$ Specifically in reason 2, the relatively narrow speed range of ramjet usefulness - around Mach 1 to 5 - when you need to get to Mach 25. $\endgroup$ – Organic Marble Dec 23 '16 at 13:14
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    $\begingroup$ And the energy required to go from M5 to M25 is not 5x but 25x because kinetic energy goes as V squared. $\endgroup$ – Philip Ngai Apr 16 '18 at 21:21
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    $\begingroup$ @OrganicMarble: On the other hand, if your ramjet can convert into a scramjet at mach 5... $\endgroup$ – Sean May 1 at 21:43

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