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Re: Proposals for air breathing hypersonic craft. I

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>> Look at the SR-71 Blackbird's engine intakes. Those big cones >> occluding the apertures provided 64% of the plane's full-throttle >> thrust ("Skunk Works," Ben Rich). > >Er, not exactly. The compression from the inlets (not just the >spikes) produces subsequent expansion which produces thrust, but only >when the rest of the engine is working and the airplane is moving.

One needs to be very careful with one's words here. No net thrust can be generated by just an expansion following the inlet compression. This would be a violation of the 2nd law of thermodynamics. The theoretical possible best, an isentropic compression followed by an isentropic expansion back to the free stream pressure would yield no net thrust (or drag) at all. When most folks hear the word thrust used in talking about an engine, they assume it is net thrust that is being discussed.

Now, if you care to talk about the stream thrust of the flow through the engine, that is something entirely different. ________________________________________________________ Ed Ruf Lifetime AMA# 344007 ( snipped-for-privacy@EdwardG.Ruf.com)

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Reply to
Ed Ruf
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There are two schools of thought:

  1. Fuel cooled structure where the fuel temperature is heated to on the order of 1000F before injecting it into the combustor.This is used in the P&W engine in in the AF HySet and also in the now cancelled NASA ISTAR programs. Tests conducted last summer in direct connect combustor tests at NASA LaRC were perfomed at Mach 3.5 enthalpy with fuel temps a low as 800F.
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    these and many of the heat sink tests done in HyTech a fuel heater such as this was utitilzed to heat the fuel to proper conditions.
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  2. Running a fuel rich gas generator to heat and partially crack the fuel prior to entering the scramjet combustor. This is the approach utilized in the JPU-APL designed dual combustor ramjet (DCR) in the Darpa/ONR HyFly program.

________________________________________________________ Ed Ruf Lifetime AMA# 344007 ( snipped-for-privacy@EdwardG.Ruf.com)

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Reply to
Ed Ruf

This is one of those "Everybody knows that. It's so obvious you don't even have to mention it." assumptions that are easy to make. And I made it. My apologies.

Ed's quite correct. The only way to produce real thrust is by putting energy from an external source into it (i.e. JP-7 in the engine). However, you can get "apparent thrust" (the stream thrust of the flow) from the compression and expansion of the total flow.

I also have to point out that a certain amount of the flow that gets compressed in front of the engine goes around, not through, the engine. The ratio of air around the engine to air through the engine is the bypass ratio. Typically, engines for fighters (not SRs) are low bypass ratio engines and engines for airliners are high bypass ration engines. The SR-71 bypass ratio varies from low to way high.

Mary

Reply to
Mary Shafer

It is also worth noting that the "apparent thrust" can be changed merely by changing the cross-sectional area of the chamber behind the inlet, which holds the compressed air: make it fatter, and the apparent thrust becomes larger, in proportion. There is no limit to this: if you wanted, you could make it 200% of the net thrust, not just 64%.

About the only place where this number is really important is in the design of the engine structure: if it's larger, the structure has to be stronger. This is much the same as the design of any other pressure vessel: to be able to hold the same pressure, the larger it is, the stronger it has to be.

Does that bypass air get fuel injected into it? I've seen the engine described as part-ramjet.

Reply to
Norman Yarvin

I get the impression you don't understand how that works, right?

There are two modes of operation and the engine can run in either one or in both, sort of. In the conventional mode, the air that comes in goes through the engine, just like a conventional jet engine. In the ramjet mode, most of the air goes around the combustion chamber to the afterburner, where fuel is injected (just like a normal afterburner). Did I mention that air can come in the inlet doors as well as the inlet itself?

Somewhere on the web is a site with diagrams of both modes. Let's see what I can find. Well, this isn't what I wanted, which is from the Dash-1, but it's interesting and informative. Try:

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However, the Dash-1 is on the Web somewhere, although I can't find it readily, I did find a whole bunch of really interesting pages as I googled around.

Mary

Reply to
Mary Shafer

Sorry for the sloppily-phrased question. I was wondering whether this bypass air was part of the ramjet-like aspect of the engine (and thus had fuel injected into it), or whether it was something else. (The answer seems to be "mostly the ramjet-like aspect, although not exclusively".)

It's at:

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in particular

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which diagrams the conditions under which the bypass tubes open. As well as opening at high Mach numbers to provide ramjet action (not purely ramjet, as the air does go through the first four compressor stages before entering the bypass tubes), they also open at low power levels.

Also of interest is:

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which diagrams the airflow through the various bleed ports in the engine pod, at several different speeds.

Reply to
Norman Yarvin

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