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LACE or combined cycle tuborocket vs (sc)ramjet

Posted by: Minthos - Tue Oct 28, 2008 3:59 pm
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LACE or combined cycle tuborocket vs (sc)ramjet 
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Space Walker
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Post LACE or combined cycle tuborocket vs (sc)ramjet   Posted on: Tue Oct 28, 2008 3:59 pm
It occurs to me that since the drag created by compressing and slowing down air in ramjets increase with velocity and at some point become greater than the thrust generated by combustion, it would be more efficient to use some alternative way of compressing the air than by relying solely on air resistance.

By having an adjustable air inlet, it should be possible to control intake air pressure and thus drag to the current speed and altitude. A turbo can compress the air to desired pressure for combustion, and an intercooler can be added to increase combustion efficiency at the expense of added weight and drag. The power to drive the turbo and intercooler should be taken from a secondary source, for example a turboshaft engine operating at a constant speed and near constant intake pressure (using the same adjustable intake).

This is obviously a heavier and more complex design than a standard turbofan engine, but it should be able to produce net thrust at a wide range of altitudes and velocities, and the idea is that it might be viable for a reusable first stage of a staged launcher or suborbital spacecraft.

Obviously the main advantage over rockets is the absent need of oxidizer, and the intended advantage over tubofans is the higher operating speed, while the main disadvantage to both, looks to me to be weight.

What do you think? Could it be viable, could it compete with pure rockets or traditional turbofans? How fast and high could it fly, how low would the thrust/weight ratio be, what kind of specific impulse could it achieve? Could the trust/weight ratio be better than for a (sc)ramjet? And finally could it be used to improve the payload fraction of launch vehicles while still keeping the first stage fully reusable(implying powered landing)?

Certainly this is not a new idea in itself, more an interpretation of ideas currently being explored by others. Since it can operate from zero velocity it shouldn't be too expensive to develop and test a small prototype..


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Post    Posted on: Tue Oct 28, 2008 8:40 pm
Turbos only function with Subsonic air. To get to the compressor you will have had to slow the air way way down causing large amounts of drag.

Scramjets don't slow the air nearly as much. Ramjets get to run everything a lot hotter and lighter than turbines.

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Post    Posted on: Wed Oct 29, 2008 2:48 am
Aha, thanks. So we'll still be limited to the same mach numbers as a ramjet.


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Post    Posted on: Wed Oct 29, 2008 7:12 am
The SR-71 supposedly changes the configuration of its engine cones so as to duct the air past the turbine at high speeds turning the jet engine into a ramjet.

The Mig-25 without these features was redlined at mach 2.8 even though it could pull mach 3.2.

From Wiki:
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However, at speeds above mach 3, the force of the engine sucking fuel through the pumps overwhelmed the pumps' ability to limit the flow. At this point, the engines effectively became ramjets, as air began to bypass the low pressure compressors, accelerating out of control until the pilot could regain throttle control through using firewalls or compressor stall, or the tanks ran dry. However, a more probable series of events would be the eventual destruction of the engine as the suction force of the compressors began to pull various engine parts through the ignition chamber and turbines.


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