Yesterday, there was an off-topic question on Space.SE, asking if a ramjet-missile could place itself in orbit. Impossible for a ramjet, obviously, as it requires a high speed flow of air and is too slow to reach orbital velocity.

However, this got me thinking about a ramjets altitude limitations:

How high could a ramjet-powered aircraft or missile possibly go? And how does the speed the jet is traveling at effect the highest possible altitude?

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    $\begingroup$ Jets of all kinds need enough oxygen outside. The highest a jet has ever flown was 37,650 metres (123,520 ft) by a MiG-25 in 1977. That was however a very parabolic flight. In leveled flight jets can fly at 90,000 ft (27 km) highest, as yet. $\endgroup$ – Giovanni Sep 14 '20 at 11:35
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    $\begingroup$ Conventional ramjet or scramjet? $\endgroup$ – Zeiss Ikon Sep 14 '20 at 11:36
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    $\begingroup$ You might want to look into Shcramjets as well. $\endgroup$ – Polygnome Sep 15 '20 at 8:13
  • $\begingroup$ Is bringing your own air considered cheating, for the purposes of this question? $\endgroup$ – Flater Sep 15 '20 at 10:55
  • $\begingroup$ @Flater yeah i'd say that is cheating... that would kinda eliminate one of the primary reason why we use ramjets at all (especially in missiles), the fact that they do not need to carry any air themselfes, which leaves more room for burning fuel... $\endgroup$ – finnmglas Sep 15 '20 at 10:59

Here's a quick estimate for this.

The maximum altitude for level flight is when the engine can't produce the thrust required to fly fast enough to generate the lift required to balance the weight.

Ramjets work best at around Mach 3 and can operate up to Mach 6

Lift is proportional to speed squared and air density. So doubling the speed allows the plane to fly in air that is a quarter of the density.

The equations for calculating air density with altitude are moderately complex but there are a couple of tables here that show that the density at 32km is about eight times less than at 20km.

So if the SR-71 flew at Mach 3 and 25km, and if you could make a SR-71 that could fly at Mach 6, it might manage another 5 or 10 km in altitude.

Obviously that's a massive simplification because if switching engine technologies would have improved the SR-71's speed, they would have done it. Also because the SR-71 engines were ramjets when it was flying fast.

A ram jet won't get you into orbit because orbit is not just a matter of flying high enough but of reaching orbital speed - the speed where the curve of your naturally falling trajectory will miss the earth due to the earth's curvature. It's about Mach 25. And (as Ross points out) even if you could reach Mach 25, you can't circularise the orbit with an air breathing engine because that requires thrust at the highest point.

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    $\begingroup$ At those altitudes you also have to take temperature into account. A fixed Mach number is not going to give you a fixed TAS due to the temperature dependency of the speed of sound. $\endgroup$ – Bianfable Sep 14 '20 at 13:53
  • $\begingroup$ @Bianfable - yes, it would really help if someone who understood the calculations better could work it out more accurately. $\endgroup$ – Robin Bennett Sep 14 '20 at 13:58
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    $\begingroup$ I could do the calculations more accurately, but I don't think that would really answer the question. This is more of an answer to how high could the SR-71 fly if it could go to Mach 6?, but the original question was about a ramjet in general (or a "ramjet-missile"). $\endgroup$ – Bianfable Sep 14 '20 at 14:19
  • $\begingroup$ You can't reach orbit with an airbreathing engine. Even if you can get fast enough and high enough, you can't circularize your orbit once you're outside the atmosphere if your engine requires air. And a ramjet just can't go fast enough to escape Earth entirely. $\endgroup$ – Ross Presser Sep 14 '20 at 18:40
  • $\begingroup$ Wouldn't air density be a limiting factor with respect to generating thrust? Where is that ceiling? $\endgroup$ – user2647513 Sep 15 '20 at 0:40

Ramjets can fly at least this high:

Scramjet, X-43A flown in 2004: 110,000 feet / 33 km.

Ramjet, RJ43-MA-11 on a BOMARC B missile, 1961, 100,000 feet / 30.5 km (photo caption, p. 31).

(Ramjet, XRJ47-W-5 tested by NACA in 1955: 73,000 feet / 22 km.)

Ramjet, extrapolated from data for flights where the vehicles were still accelerating when they ran out of fuel: 131,000 ft / 40 km. (One flight coasted to 151,000 feet, but that's probably not what's being asked here.)

...if ramjets can continue to accelerate in an environment where the air density is under 20 g/m3, then perhaps they can sustain powered flight to 40 km, where the density is only 4 g/m3.

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    $\begingroup$ The SR-71 (with engines in ramjet mode at altitude and speed) routinely flew higher than 22 km -- it was operational at about 81000 (roughly 25 km) at Mach 3 for many years. $\endgroup$ – Zeiss Ikon Sep 14 '20 at 16:01
  • $\begingroup$ Yes, but I found only unresolved debate about whether its "turbo ramjet" engines really count as ramjets. $\endgroup$ – Camille Goudeseune Sep 14 '20 at 16:08
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    $\begingroup$ Bomarc missile (B series) had a maximum cruising altitude of 100K ft. It was initially launched with a rocket engine, but titled over and cruised with a pair of ram jets $\endgroup$ – Forward Ed Sep 14 '20 at 18:35

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