SR-71

"You can tell a Finn but you can't tell him much" (youcantellafinn) 04/08/2014 at 11:39 • Filed to: PLANELOPNIK

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Have a 5 minute video of Colonel Rich Graham talking about the Blackbird and what it took to fly it. Crazy that the engines only created around 34,000 pounds of thrust each on the SR-71. To put this in perspective, the engines on the Boeing 787 start at 64,000 pounds of thrust each.

Jayhawk Jake > You can tell a Finn but you can't tell him much 04/08/2014 at 11:50

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The engines on a 787 are a totally different animal than the SR-71

In fact, the engines on the SR-71 are totally different from any other engine in existence. At high speed they shut off airflow to the core and become ramjets. It's pretty insane.

Thrust numbers are irrelevant anyways, the SR-71 is much lighter than a 787. Thrust to weight would be more interesting to compare.

Mike_Smith > Jayhawk Jake 04/08/2014 at 11:54

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How does a ramjet work without airflow?

Paul, Man of Mustangs > Mike_Smith 04/08/2014 at 11:59

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The ramjet is outside of the turbojet. The air flows around the turbojet, pressurized by the sheer airflow from high Mach speeds, then fuel is dumped into the afterburner, giving the effect of a ramjet.

Jayhawk Jake > Mike_Smith 04/08/2014 at 12:00

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It's not shutting off airflow, it's shutting off flow to the core of the engine. The air flows around the engine core

It's complicated and even I don't have a total understanding of it, but this picture shows what's happening

Basically there's a series of bypass doors near the entrance to the compressor (behind the spike). As mach number increases, these doors open until they are fully opened, which means all the air coming in the inlet bypasses the engine proper with the outer duct around the engine acting as a ramjet

HammerheadFistpunch > Mike_Smith 04/08/2014 at 12:00

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Supersonic air is captured by the inlet spike (which retracts with speed to keep the wave front where it should be) and diverts it around the turbojet then...well...basically just dumps raw fuel into the superheated airstream and BOOM. MASSIVE thrust. its basically an air fed rocket motor

like so

GTI MkVII > You can tell a Finn but you can't tell him much 04/08/2014 at 12:01

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Though the weights and intended purposes of both planes are vastly different, heres another thrust stat to put it in even more perspective.

The GE-90's on 777's each produce 90,000 lbs of thrust. They're massive and massively powerful.

Saracen > You can tell a Finn but you can't tell him much 04/08/2014 at 12:07

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Twice the max thrust, sure, but the Blackbird's max takeoff weight is 1/3 of the 787's. The J58's also weigh half that of the 787 engines, and are designed to cruise at supersonic speeds.

Regardless, it's still enough to propel it past its "official" top speed Mach 3.2 (one pilot said they hit Mach 3.5 during a mission and the plane was still accelerating smoothly.)

Mike_Smith > Jayhawk Jake 04/08/2014 at 12:31

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Kewl.

Axial > Jayhawk Jake 04/08/2014 at 16:44

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It's not actually bypassing the compressor completely; see those arrows in the last diagram running along the spike and leading into the checkers? That's going through the engine.

What retracting the spikes does is force more air through the bypass because at higher speeds, that compressed air gets super hot and can cause engine failure. So instead, they restrict airflow to the compressor and send more of it to the afterburner.

vicariousILive > Jayhawk Jake 04/08/2014 at 16:53

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I think the doors opening and closing to manage the input shock wave is an awesome feature. Rather than just retracting the ramjet cone you can increase the pressure by closing the gate and therefore increase your body angle and thereby increase the shock wave angle.

Jayhawk Jake > Axial 04/08/2014 at 18:49

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Right, I guess I should have clarified but I wanted to keep it simple. Obviously something goes into the engine otherwise you'd have no power

Axial > Jayhawk Jake 04/08/2014 at 19:35

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Well, it's not like they couldn't go into a dive to restart the engines, and you don't need any moving pieces for a ramjet, I just wanted to clarify that the point of the spikes isn't actually to create a ramjet proper but rather to decelerate the incoming air and essentially manage engine temperature. Engine temperature is actually the performance limiter for the SR-71 and related aircraft.

Jayhawk Jake > Axial 04/08/2014 at 19:42

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Yup, although temperature limits have increased with new materials.

The problem with just stopping the engines would be aircraft power, but there could be an alternate solution like running an APU or stored battery power

Axial > Jayhawk Jake 04/08/2014 at 19:45

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Hmmm, I forgot about that one. Shame on me, the engineering gods are not pleased.

gmporschenut also a fan of hondas > You can tell a Finn but you can't tell him much 04/08/2014 at 21:49

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The starting motor was 2 V-8s linked to a common shaft to spool the J58s to 3200 rpm!