# Tag Info

## Hot answers tagged ramjet

19

A jet engine compresses air, heats it by mixing it with fuel and burns it, and lets the heated air escape at the end, where it accelerates to more than its initial speed in a convergent-divergent nozzle because the density of the heated gas is lower, thus needing a higher volume at the same pressure. By converting the kinetic energy of the flow into ...

13

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 ...

13

Tip jet rotors are still being developed, none of them are ramjet powered as far as I know. The Dragonfly DF1 is tip rocket powered, while the Pegasus uses air jets powered by a conventional engine. Ramjet technology has not improved, so the cases for using them on rotor tips has not changed. They are still too heavy, thirsty and noisy.

9

Short answer: No. Or at least, nothing that's been demonstrated that I'm aware of. Whether there have been experimental R&D aircraft or developments, I'm not sure - but I can't find any. The simplest reason is just because there's no need for them - they offer no real advantages over a conventional internal combustion or jet engine, so as with so much ...

9

A rocket-powered missile carries both fuel and oxidiser and oxidiser is the heavier part. For example a gram of jet fuel needs almost 3.4 grams of oxygen to fully burn! Increasing the fuel load requires also increasing the power, which means even more fuel and this limits the practical range of a missile. Ramjets are air-breathing, so they don't need to ...

7

Ramjets could be miniaturized, but at some point, air viscosity and boundary layer thickness will make it impossible for them to work. Simply put, the intake hole will get so small that air will find it easier to flow around the whole thing and ignore all of its complicated inner structure. In more technical terms, as the intake shrinks, intake velocity ...

7

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:...

6

The ramjet is conceptually the simplest jet engine. It is a duct where air is burned creating a hot jet that provides thrust, it is known aero thermodynamic duct as it is no more than a duct where a thermodynamic cycle is performed. They are no more used for airplanes, as it can not provide thrust at zero airspeed -since he does not have any compressor ...

6

Scramjets There's only a couple flying examples. The early Soviet-born Kholod, the HyShot series, and the recent ISRO ATV all flew small scramjet engines, although none were designed for net thrust. By far the most [publicly] successful vehicles have been the X-43A and the X-51. Both were fully controllable, independent flight vehicles (rather than just ...

5

How long does the lithium take to evaporate and then to react with oxygen? Air in a scramjet is moving extremely fast (supersonic by definition!), so there is very little time left for mixing and combustion if you desire to create thrust from the the combustion heat energy. Your lithium engine will most likely trail a bright contrail of burning lithium but ...

5

Well the Pratt & Whitney J58 engines which powered the SR-71 worked in a similar fashion, using a bypass system which allows the engine to operate as a ramjet at high speeds and a turbojet engine at lower speeds. By the very nature of a ramjet, it relies of stagnation pressure at the intake and inlet throat to pressurize the airflow prior to combustion. ...

4

Pressure is the greatest just before the combustor in both jet and ramjet engines - therefore air would have against the pressure differential for this to happen (turbine only has therefore an indirect role)

4

I'm reading a book right now that talks briefly about flight testing the Hiller. As I recall, it was tough to get both engines started at the same time, and if one of them cut out, was a pretty violent affair. It was also an environmental disaster, with horrible fumes and noise. It was an interesting experiment, but it had no real world application at all. ...

4

René Lorin invented the ramjet in 1908, and his ideas were tried in flight for the first time from 1941 onwards. Different sizes were tried; below are two pictures, one of a Do-17 Z with a small Lorin engine (source) and below that one of a Do-217 with a much larger version (source). Speed increases of up to 150 km/h were recorded, but efficiency was poor. ...

4

Are you asking about current production or through out time? The SR-71 was powered by a Pratt & Whitney J-58 engine which operated as a partial ram jet at high speeds. At lower speeds it operated like a turbojet changing its internal shape as it increased its speed. The SR-71 was capable of departure under its own power from the ground. You can find ...

3

Take a look here but the short answer A ramjet, sometimes referred to as a flying stovepipe or an athodyd (an abbreviation of aero thermodynamic duct), is a form of airbreathing jet engine that uses the engine's forward motion to compress incoming air without an axial compressor. In other words it uses the thrust it generates to compress the on coming ...

3

The thrust of a ramjet depends on the dynamic pressure of the airframe since this determines how high the pressure inside the ramjet can become. Thrust grows therefore initially with the square of speed. The efficiency of heat engines depends on the temperature and pressure ratio of their cycle, and again speed helps to improve that. The specific ...

3

Primarily the pressure differential between the stagnation pressure at the inlet and the ambient static pressure. Combustion takes place in an isobaric environment with the enthalpy of the exhaust gases being converted to kinetic energy by exiting the nozzle per Bernoulli’s Principle. Therefore the pressure differential between the combustion section and ...

3

Yes, they have a limit at around 2. And this can only be achieved at Mach 1, as Peter Kampf points out. Increasing the inlet to diffuser area ratio to extreme values will just lead to your engine being filled with slow moving air. The mass and volume of air in your engine will increase, but neither the pressure nor the rate of airflow will increase ...

2

Another way to realise a liquid fuel ramrocket engine, in theory, would be to incorporate the rocket motor into the ramjet air intake centerbody. As the aircraft approached ramjet operating speed, the ratio of fuel to oxidiser being fed into the rocket motor would be increased so that some of the fuel would be expelled from the rocket motor and mixed with ...

2

Jan Hudec’s answer here is pretty good, but the I’d add that it’s more than just oxidizer, it’s the idea with a ramjet that: 1) You can use the air from the atmosphere itself as a working mass. It’s also the same reason airliners and other airplanes don’t make use of rocket engines for propulsion. As opposed to a rocket engine which requires the vehicle ...

2

It's a slightly odd question, as for the most part a ramjet isn't an alternative to a rocket engine for an ASM, but an alternative to a turbojet or turbofan engine. On the whole, rockets are normally used in short-range weapons (typically on semi-ballistic trajectories), whereas cruise missiles (utilising aerodynamic lift for flight) use various types of ...

2

Simplicity of propulsion translates to cost savings, but there are tradeoffs in using a ramjet in a missile. The reason most aircraft-carried missiles do not use ramjet engines is because such a missile has to be accelerated to a sufficient speed for the engine to begin producing thrust. "A ramjet equipped aircraft requires another type of propulsion to ...

2

Why is that the ramjets do not require [variable geometry] to maximize pressure recovery? Turbofans, in aircraft that have variable geometry intakes, such as in an F-111 or F15, operate from standstill, at the start of the runway, to about Mach 2.5. That means, they go from having no shock, to having quite a large (strong) shock. The intake needs to provide ...

2

The Aurora D8 doesn't have traditional jet engines at all. The design incorporates a very large bypass fan which due to its size is not mechanically linked to the engine core. It's more like a turbo-prop engine, but driving a ducted fan instead of a propeller. The fan is specifically designed to cope with the flow disturbance in the boundary layer. Indeed, ...

2

It is simply the rate at which fuel can be burned. For instance, the Saturn V's first stage carried 1.37 million kg of liquid oxygen (along with the kerosene fuel), which it burned in about 165 seconds. Imagine the size of the jet intake you'd need for a comparable amount of air.

2

The Carnot condition notwithstanding, whether a ramjet actually operates at a higher "hot end" temperature than a turbojet is questionable, as well as whether a comparable percentage of thermal energy in the exhaust stream is converted to kinetic energy. Further, just because there's no turbine, doesn't mean there's nothing in the hot part of the ...

1

Well, that would depend on the design parameters, but for example this ideal ramjet would, with stoichiometric combustion process, provide best thrust per unit mass flow at a speed of approx. 2.4 Mach ...the best operating range of a hydrocarbon-fueled ramjet is $2 \leq M_0 \leq 4$ . The parameters used are $\tau_\textrm{max} = 10$ , \$ a_0 \approx 300\...

1

Replacing the turbines with ramjets won't work. If the airplane had ramjet(s), it would still need enough thrust from non-ramjet engines to reach about Mach 0.5, because ramjets can't accelerate from a standstill, and they make hardly any thrust below that speed. See the Nord Griffon, for example.

1

Back in the 1980's I saw a small ram jet from the 1950's to be used in model boats, planes and cars. (1.25-2" diameter and 16-20" length.) The accompanying literature suggested using a baby food jar as a fuel tank. It used gasoline. I expect anything today would utilize a bit more sophisticated (and safer) positive control of the fuel. It was ...

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