Everything Science Forum

Does anyone know what is a Scramjet they say it is going to replace air planes and may be space craft.

But looking at Wikipedia it does not say much about it.

http://en.wikipedia.org/wiki/Scramjet


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A scramjet (supersonic combustion ramjet) is a variation of a ramjet with the key difference being that the flow in the combustor is supersonic. At higher speeds it is necessary to combust supersonically to maximize the efficiency of the combustion process. Projections for the top speed of a scramjet engine (without additional oxidiser input) vary between Mach 12 and Mach 24 (orbital velocity), but the X-30 research gave Mach 17 due to combustion rate issues. By way of contrast, the fastest conventional air-breathing, manned vehicles, such as the U.S. Air Force SR-71, achieve slightly more than Mach 3.2 and rockets achieved Mach 30+ during Apollo.

Like a ramjet, a scramjet essentially consists of a constricted tube through which inlet air is compressed by the high speed of the vehicle, a combustion chamber where fuel is combusted, and a nozzle through which the exhaust jet leaves at higher speed than the inlet air. Also like a ramjet, there are few or no moving parts. In particular there is no high speed turbine as in a turbofan or turbojet engine that can be a major point of failure.

A scramjet requires supersonic airflow through the engine, thus, similar to a ramjet, scramjets have a minimum functional speed. This speed is uncertain due to the low number of working scramjets, relative youth of the field, and the largely classified nature of research using complete scramjet engines. However it is likely to be at least Mach 5 for a pure scramjet, with higher Mach numbers 7-9 more likely. Thus scramjets require acceleration to hypersonic speed via other means. A hybrid ramjet/scramjet would have a lower minimum functional Mach number, and some sources indicate the NASA X-43A research vehicle is a hybrid design. Recent tests of prototypes have used a booster rocket to obtain the necessary velocity. Air breathing engines should have significantly better specific impulse while within the atmosphere than rocket engines.


(http://upload.wikimedia.org/wikipedia/commons/thumb/5/55/X-43A_%28Hyper_-_X%29_Mach_7_computational_fluid_dynamic_%28CFD%29.jpg/697px-X-43A_%28Hyper_-_X%29_Mach_7_computational_fluid_dynamic_%28CFD%29.jpg)


However scramjets have weight and complexity issues that must be considered. While very short suborbital scramjet test flights have been successfully performed, perhaps significantly no flown scramjet has ever been successfully designed to survive a flight test. The viability of scramjet vehicles is hotly contested in aerospace and space vehicle circles, in part because many of the parameters which would eventually define the efficiency of such a vehicle remain uncertain. This has led to grandiose claims from both sides, which have been intensified by the large amount of funding involved in any hypersonic testing. Some notable aerospace gurus such as Henry Spencer and Jim Oberg have gone so far as calling orbital scramjets 'the hardest way to reach orbit', or even 'scamjets' due to the extreme technical challenges involved. Major, well funded projects, like the X-30 were cancelled before producing any working hardware

I guess the major technical issue with both ramjets and scramjets is the minimum required velocity for the air compression.

Scramjets are a lot like rockets, except they don't carry an oxygen tank.  They rely on the fact that the oxygen in the air is so compressed when it hits the engine that it can scoop up the compressed air and use it instead.

Ramjets are similar, but they do need to carry some fuel.  The SR-71 Blackbird had a configurable turbine/ramjet engine system that would switch into being a ramjet when the appropriate air compression level was reached.

http://www.aerospaceweb.org/question/propulsion/q0175.shtml

So in other words those jets have to be very high up for it to work where the air compressed oxygen is high,

And those jets will not work low where compressed oxygen is low.

No, they need to be moving fast enough that the force of the craft flying through the air compresses the air to a point where it can be used as fuel.  This is even more difficult at higher altitudes because the air becomes thinner, so even more velocity is required to reach the right compression.