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Lockheeed SR71 Inlets

Thu Feb 13, 2003 3:06 am

I have read much about the A12/YF12A/SR71 family, in particular the amazingly complex inlet plumbing for the J58's. However, I have never been able to understand how more than half the thrust can originate from the inlets at cruise Mach. Does it suck itself through the air?
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RE: Lockheeed SR71 Inlets

Thu Feb 13, 2003 6:18 am

More or less yes.
The shape of the inlets provides an enormous amount of compression to the air, which is what thrust really is.
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RE: Lockheeed SR71 Inlets

Thu Feb 13, 2003 8:34 am

Sounds about right, though it uses a very different intake system, we get 63% of thrust at Mach 2 from them on Concorde.
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RE: Lockheeed SR71 Inlets

Thu Feb 13, 2003 12:56 pm

I recommend Ben Rich's "">Skunk Works.

The SR-71's inlet cones can move back and forth along the axis of the engines to change the area of the inlet and to and to compensate for the angle of the oblique shock wave that forms at the tip of the cone. At takeoff, the inlets are wide open and in cruise the inlets are at a minimum. I guess you could claim an efficiently-designed inlet minimizes the amount of work necessary to compress air, leaving more power for thrust?

In his book, Mr. Rich refers to "starting" an engine. This is where you have supersonic flow throughout the engine and energy loss to the flow is less. However, this is a delicate state in which to operate and small gusts and surges can cause an "unstart" ... where gas flow in the engine becomes subsonic and is accompanied by a large drop in thrust from that engine, possibly a flameout and a well-shaken flight crew. Their solution to this problem was to make the other engine have a "sympathetic unstart" so the thrust on both engines drops and the crew can deal with the problem instead of everything being thrown around and rattled to pieces.
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RE: Lockheed SR71 Inlets

Thu Feb 13, 2003 9:40 pm

Hi, Minuteman, I always understood that turbojets would not run on supersonic airflow. I have since located the following,, and quote

"In a turboramjet engine a turbojet with afterburner is used to get to minimum ramjet speed and altitude.

The incoming air is then partially or completely bypassed around the turbine portion and ducted into the afterburner, so it functions as a true ramjet.

This is the type of engine flown on the Blackbird family of aircraft. With the Blackbirds, as speed increases, more and more of the ram-effect compressed air bypasses the turbine section of the engine and goes directly to the afterburner.

At Mach 3+ cruise most of the thrust comes from the afterburner acting as a ramjet, and most of the air necessary for the combustion which produces this thrust comes from ram effect at the inlet. (Many references erroneously state that at cruise most of the thrust come from the inlets, which is a misunderstanding of a statement made by Kelly Johnson. Inlets are actually a source of drag, not thrust.)

What do you think?

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RE: Lockheeed SR71 Inlets

Mon Feb 17, 2003 12:31 am

I would agree that thrust is actually created by the inlets. I would also agree that it is counter-intuitive. I had to write out the momentum equation to convince myself that it was actually true. Odd as it may sound, during diffusion in the inlet, the fluid exerts a force in the forward direction on the inlet, or a thrust force. Think about an axial compressor as an analogy. It compresses the air (obviously) but it also produces thrust. It's a simple application of Newton's Action-Reaction Law, you have lots of little airfoils pushing the flow backward, so the flow is in turn going to push those airfoils forward. Now for the SR-71 I guess I'm going to have to take people's word for it about what percentage of thrust is produced by the inlets.

In normal supersonic operation, the inlet cone is positioned so that the oblique shock coming off of it enters the inlet. It then proceeds to bounce off the inlet walls until, compressing and slowing down the flow. By having the oblique shock bounce off the walls, the flow must go through a series of them, each one compressing and slowing the down a little bit. Eventually after the flow passes through all the oblique shocks, it has a higher pressure and is traveling just over sonic speed. Before it heads off to any other part of the engine, the flow goes through a weak normal shock to raise the pressure a bit more and bring the speed down to subsonic. This subsonic compressed flow can either enter the compressor or continue around the engine core and go straight to the afterburner/ramjet burner. (Reducing the flow to subsonic speed is very important since the Blackbird's ramjet is not a scramjet.) Note that some of the air (albeit a small percentage at high speed) always goes through the engine core; the Blackbird does not operate as a pure ramjet at any time.

An "unstart" occurs when the pressure gradient across the inlet changes due to some disturbance. The oblique shock then no longer enters the inlet, which means the engine loses all the compression/thrust that the oblique shocks in the inlet contributed. As was mentioned, Ben Rich and Co. devised the "sympathic unstart" in the unaffected engine to alleviate the problem.
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RE: Lockheeed SR71 Inlets

Mon Feb 17, 2003 12:35 am

Hello all,

Nothing really to add here, just wanted to pay compliments to a great post.

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RE: Lockheeed SR71 Inlets

Thu Feb 20, 2003 1:01 am

The on-line web site for the Smithsonian's magazine Air and Space has an excellent article on the inlet system of the SR-71.

Go to

Or go to Paul Kucher's excellent site
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RE: Lockheed SR71 Inlets

Thu Feb 20, 2003 6:34 am

I second Minuteman's motion of reading Ben Rich's Skunk Works. I couldn't put it down until I was done. It covers a broad scope of the "mystery planes" that we see and hear about.


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