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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
    Commented Sep 14, 2020 at 11:35
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    $\begingroup$ Conventional ramjet or scramjet? $\endgroup$
    – Zeiss Ikon
    Commented Sep 14, 2020 at 11:36
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    $\begingroup$ You might want to look into Shcramjets as well. $\endgroup$
    – Polygnome
    Commented Sep 15, 2020 at 8:13
  • $\begingroup$ Is bringing your own air considered cheating, for the purposes of this question? $\endgroup$
    – Flater
    Commented Sep 15, 2020 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... $\endgroup$
    – publicdomain
    Commented Sep 15, 2020 at 10:59

3 Answers 3

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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
    Commented Sep 14, 2020 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
    Commented Sep 14, 2020 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
    Commented Sep 14, 2020 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
    Commented Sep 14, 2020 at 18:40
  • $\begingroup$ Wouldn't air density be a limiting factor with respect to generating thrust? Where is that ceiling? $\endgroup$
    – user2647513
    Commented Sep 15, 2020 at 0:40
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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
    Commented Sep 14, 2020 at 16:01
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    $\begingroup$ Yes, but I found only unresolved debate about whether its "turbo ramjet" engines really count as ramjets. $\endgroup$
    – Camille Goudeseune
    Commented Sep 14, 2020 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
    Commented Sep 14, 2020 at 18:35
  • $\begingroup$ SR-71 engines may act as turbojets or ramjets depending on airspeed, I'd say. Blessings + $\endgroup$
    – Urquiola
    Commented Nov 12, 2021 at 14:54
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[1 Orbital Airplane, Science&Vie, Jul 1986]

Thrust and efficiency of Lorin Ramjet (Report by Sänger and Bredt) increase with the square of speed, with the square root of combustion temperature. Ramjet start working around 300 km/ h, studies considered 300 m/ sec airspeed. Yield circa 12'000 HP per square meter of main bulkhead area. At higher altitudes, (calculations were made for a top speed of 0.9 Mach and 12 km ceiling, intended in a pursuit airplane, engineers thought Lorin Ramjet would not work above 18 km, because of poor combustion), thrust and propulsion power decrease, slightly less than density of air, but propulsion coefficient and efficiency increase because of lower temperature of fresh incoming air.

At ground level and 300 m/ sec airspeed, thrust is 3'000 kg per square m of bulkhead area. Range increases from 367 km at ground level, in a case, Skoda-Kauba P.14?, proposed in https://www.enginehistory.org/Rockets/LorinRamjet/LorinRamjet.shtml to 1'100 km at 18'000 m. Duration of flight for a given fuel load was maximum at 14'000 m height, decreased slightly above this, for a 6'000 kg total airplane weight, 1'000 kg payload, 2'400 kg of fuel, wing area 30 m2. Athodyd operating at higher temperatures losses efficiency, smaller athodyd impose higher working temperatures.

Propulsion power increases with third power of speed. They advised 2 m diameter Ramjets. Minimum speed at which diving is not needed to accelerate, at 12'000 m height, was 430 km/ h. Test of this tube on conventional airplanes were conducted at 100 m/ sec over a Do-17Z, then 200 m/ sec airspeed. For the design considered, speeds were 1'100 km/ h at ground level, and 950 km/ h in mid stratosphere.

I've suggested having a Pulse Jet inside or side by sides of the Ramjet, Athodyd tube diameter modified accordingly, pulsejets provide power at zero airspeed, and could carry Ramjet to its start speed. Intake duct angle was determined being best at 10º. Inside a duct, heat form outer parts of pulsejet would be added to thrust; if a double duct wall is installed, same would happen with heat of outer side of ramjet. OK? Blessings +

Skoda-Kauba P.14 Ramjet Fighter Airplane, as in 'German Jet Genesis', D Masters

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    $\begingroup$ So the short answer is "18 km, theoretically." $\endgroup$
    – Camille Goudeseune
    Commented Oct 29, 2021 at 21:16
  • $\begingroup$ Perhaps, but about added information, you never know when it will be useful. Besides this generic comment, info I've added comes from WW II times, situation has changed, 'Hypersonic Ramjet' may not differ too much from these Lorin tubes tested. Thanks. Blessings + $\endgroup$
    – Urquiola
    Commented Nov 2, 2021 at 17:53
  • $\begingroup$ Ramjet Thrust is produced by passing the hot exhaust from the combustion of a fuel through a nozzle, it accelerates flow, the reaction to this produces thrust. To maintain flow through nozzle, combustion must occur at a pressure higher than pressure at the nozzle exit. In a ramjet, high pressure is produced by "ramming" external air into the combustor using the forward speed of the vehicle. $\endgroup$
    – Urquiola
    Commented Nov 12, 2021 at 14:45
  • $\begingroup$ The external air brought into propulsion system becomes the working fluid, like in a turbojet. In a turbojet engine, the high pressure in the combustor is generated by a piece of machinery, a compressor. There are no compressors in a ramjet. Therefore, ramjets are lighter and simpler than a turbojet. Ramjets produce thrust only when the vehicle is already moving; ramjets cannot produce thrust when the engine is stationary or static. $\endgroup$
    – Urquiola
    Commented Nov 12, 2021 at 14:47
  • $\begingroup$ Since a ramjet cannot produce static thrust, some other propulsion system must be used to accelerate the vehicle to a speed where the ramjet begins to produce thrust. The higher the speed of the vehicle, the better a ramjet works until aerodynamic losses become a dominant factor. $\endgroup$
    – Urquiola
    Commented Nov 12, 2021 at 14:47

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