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46 votes
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How efficient is a Scramjet?

A scramjet is more efficient than a rocket engine for the same two reasons that all air-breathing engines are more efficient than (chemical) rocket engines: Chemical rocket engines need to carry both ...
Jörg W Mittag's user avatar
24 votes
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Is the compressor required on jet engines? Can air be rammed into the turbine?

Your questions At what point (if any) does air stop getting sucked into the turbine and is just getting rammed in? On a CFM56-7B engine (Boeing 737 NG) pressure is provided by the fan, the low ...
mins's user avatar
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17 votes
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How high can a ramjet go?

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. ...
Robin Bennett's user avatar
13 votes

Is it possible for a ramjet to start from 0 velocity?

No, a ramjet can't be started at zero speed. Yes, there are missiles that use ramjet propulsion without any rotating parts: they use a rocket to accelerate the missile to high enough speed for the ...
Zeiss Ikon's user avatar
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12 votes

Can free fall velocity be used to start a ramjet?

A well streamlined body with high enough sectional density can easily fall fast enough to reach ignition speed for a suitable ramjet design (some of the heavier gravity bombs of the Second World War ...
Zeiss Ikon's user avatar
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10 votes
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What are the advantages of using a ramjet in an air-to-surface missile?

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 ...
Jan Hudec's user avatar
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8 votes

How efficient is a Scramjet?

Scramjets use super compressed air from the atmosphere, while a rocket engine carries its own oxidiser. This main difference means that all weight and volume required for the oxidiser and reaction ...
Koyovis's user avatar
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8 votes

Is it possible for a ramjet to start from 0 velocity?

No. A ramjet relies on stagnation pressure at the inlet in order to compress air prior to combustion. They are typically not effective until the speeds get quite high, somewhere in the Mach 2+ range. ...
Romeo_4808N's user avatar
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8 votes

Can free fall velocity be used to start a ramjet?

The inlet of the engine is indeed a source of drag, but it can be covered by some aerodynamic louver until the aircraft picks enough speed. B-36 had the deployable louvers over jet engines to reduce ...
h22's user avatar
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7 votes

How high can a ramjet go?

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-...
Camille Goudeseune's user avatar
7 votes
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Are there any planes that have ramjet or scramjet engines?

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. ...
Hephaestus Aetnaean's user avatar
7 votes
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What is the minimum size of a ramjet?

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 ...
Therac's user avatar
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5 votes

Could liquid alkali metal fuels be used for ramjet/scramjet propulsion?

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 ...
Peter Kämpf's user avatar
4 votes

Are there any planes that have ramjet or scramjet engines?

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 ...
Peter Kämpf's user avatar
4 votes

Are there any planes that have ramjet or scramjet engines?

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 ...
Dave's user avatar
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4 votes

How can a modified ramjet produce static thrust without a turbine?

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 ...
Romeo_4808N's user avatar
  • 74.1k
4 votes

What mechanism or design aspect prevents jet blast from escaping the front of a Ramjet or scram jet?

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 ...
Francis L.'s user avatar
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4 votes
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Could small ramjets on the tip of propellers allow for supersonic spin of turbine blades?

Spinning fast is not a problem. In fact, for a turbine-driven engine, it's easier: turbine is an inherently fast-spinning device. So much so that tips of the compressor fans are typically supersonic: ...
Zeus's user avatar
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4 votes
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Could ramjets be implemented on sub/super-sonic aircraft wings due to the shockwaves generated by them?

Regardless of the shape of the wing, ramjets only outperform turbojets at supersonic airspeed. Ramjets work because the speed of the incoming airflow is such that there is no rotating compressor/...
Koyovis's user avatar
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3 votes

Is the compressor required on jet engines? Can air be rammed into the turbine?

Like any kind of vacuum system, vacuums cannot “suck“ anything. They just remove air from an enclosed space. This causes a pressure differential between the evacuated space and the surroundings. The ...
Romeo_4808N's user avatar
  • 74.1k
3 votes

Is there a limit to a subsonic ramjet's pressure ratio?

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 ...
Abdullah is not an Amalekite's user avatar
3 votes

What are the advantages of using a ramjet in an air-to-surface missile?

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 ...
Romeo_4808N's user avatar
  • 74.1k
3 votes

What mechanism or design aspect prevents jet blast from escaping the front of a Ramjet or scram jet?

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 ...
Romeo_4808N's user avatar
  • 74.1k
3 votes

What is the effect of Mach number on efficiency, thrust and specific fuel consumption of a ramjet engine?

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 ...
Peter Kämpf's user avatar
3 votes

Could ramjets be implemented on sub/super-sonic aircraft wings due to the shockwaves generated by them?

Looking at the Wikipedia article on ramjets, we have: Although ramjets have been run as slow as 45 metres per second (160 km/h), below about Mach 0.5 (170 m/s; 610 km/h) they give little thrust and ...
Eugene Styer's user avatar
  • 3,709
2 votes

What are the advantages of using a ramjet in an air-to-surface missile?

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 ...
Juan Jimenez's user avatar
2 votes

Do ramjets use variable geometry supersonic inlets?

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 ...
Penguin's user avatar
  • 4,483
2 votes

What are the advantages of using a ramjet in an air-to-surface missile?

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 ...
Matt Bowyer's user avatar
2 votes

Is the compressor required on jet engines? Can air be rammed into the turbine?

It is certainly possible to 'ram' air into a turbine. The maximum pressure that can be achieved is called the 'stagnation pressure', which is the point at which all incoming kinetic energy is ...
Sanchises's user avatar
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2 votes

Can ramjets reach higher thermal efficiencies than gas turbines?

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 ...
Zeiss Ikon's user avatar
  • 17.1k

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