Is It True That the SR-71 Blackbird Uses Barely Any Fuel At High Speed (Mach 3+)?

Question

i read somewhere some time ago than when the blackbird is cruising at mach 3 or above it uses little or no fuel because it is going so fast that the air itself is being compressed and ignited without little or no fuel

does anyone know if this is true?

personally i cant see how this would work, i mean its true that a ramjet becomes very efficient at very high speed but it cant be so efficient that it uses no fuel because if that was the case wouldnt we all be flying on supersonic aircraft?

Answer 1

There's a huge difference in fuel consumption at sea level vs. very high in the atmosphere.  The drag is very low at very high altitudes where the air is really thin. So, yes, I can see that it would use much less fuel cruising at very high altitudes at Mach 3+, compared to sea level flight.

Answer 2

Ramjet " self sustaining" means that there is no need for active compression (from a rotating compressor) and that no energy has to be extracted from an exhaust turbine to drive the compressor.

In fact, in the case of the SR-71, most of the thrust comes from the air intake; the nozzle in the rear is there mostly to dump the ram-compressed air that passes through the engine (and to do that, energy has to be added in the form of fuel being combusted), if one draws the specifics of the pressure/flow diagram across the engine, this becomes quite clear although somewhat counter intuitive.

Answer 3

it would use barely any fuel compared to what it uses at a lower altitude

the engines on the 71 are near-ramjet-like at mach 3 and at 80,000 feet. they do indeed require lots less fuel (but they still do comsume some) thanks to the ramjet-like qualities of the intake and engines, reduced drag at high altitudes and the low drag fueselage. (the aircraft was pretty much 75 percent fuel tanks, 20%engines 2 percent crew and 3% recon hardware)  the problem was that it would guzzle up lots of fuel at lower altitudes and lower speeds. Did I mention it leaks fuel until they speed up and friction caused enough heat that the titanium expands and plugs the leaks?

I wouldn't say I know about the ramjet in detail so read the other answers or google/wikipedia it.

I'd like to add a little about flying supersonic with ramjets.

we are not flying supersonic just yet commercially because ramjets are only efficient-no, make that ramjets don't work until at mach 2 +. you need another form of propulsion to get the aircraft up to that speed. The SR71's engines are truly wonderful in the sense that they combine turbojet and ramjet. Unfortunately, they also drink a different jet fuel, the JP7. With proper research and/or the interest those engines or similar ones using jet A or otherwise would have been implemented onto airliners but those engines are also very maintenance intensive and costly, not to mention loud. They were also very very fuel guzzling at lower altitudes. At the time, titanium was the material of choice for the SR71 to withstand the heat caused by friction with the air at Mach 3. It leaked JP7 while on the ground and the leaks only stopped once the fueselage heated up and expanded. Each mission started with an aerial refueling to top up the tanks.

Supersonic capable aircraft usually will exhibit weird behavior or become unresponsive at lower speeds. SR71 is a good example. The Concorde too. You cannot just stick a couple of the engines onto a 747 and firewall the throttles. You need to strengthen the airframe, use light but heat resistant material that can resist up to 3000 degrees celcius (temp encountered by SR71), you need adequate soundproofing,

some sort of oxygen producing machine in case of decompression because otherwise the aircraft would never be allowed to fly over 60,000 feet due to the time it would take to get down to a altitude with breathable air,

supersonic boom supression because otherwise it will not be allowed to fly over land,

larger fuel tanks or aerial refueling capability which will never be implemented any time soon onto commercial airliners because of the dangers involved, not to mention the protesting passengers.

All new changes will add weight, and with the rising gas prices and deteriorating economy resulting in loss of ridership, the revival of the project is a bust. But if you just happen to have a few hundred billion lying around, be my guest.

Good Luck

Answer 4

The SR-71 used fuel that costs the "same as vintage scotch", and used quite a bit of it. I have never seen any burn figures, but it took quite a few KC-135 tankers to support the missions.

Because it flew so high, there was not a lot of forward resistance, but it still required a lot of thrust to go forward, and fuel burn was required for this. You cannot just run air through an engine and get thrust.

Regards,

Dan

Answer 5

Well, an aircraft that goes so fast it needs no reation to sustain it sounds a LOT like perpetual motion to me.  Fundamentally, it's not true.  Drag increases with the SQUARE of velocity, so the faster it goes, the more thrust is needed to sustain lift.  Nah, it uses fuel during all phases of flight, but like you said, the ramjet is efficient, but not 100% efficient.

Answer 6

Not true.

In order to self ignite, the air has to be preheated to several thousands degrees C.

The air  in J-58 intake  was heated to 425 degrees C  at 3.2 Mach.  That was the limit of the engine.

In fact the SR-71 was very inefficient  as it gulped fuel at enormous rate.  The afterburners were needed at above 0.9 Mach,  so that it carried  startling  38 tonnes of fuel.

A real aerial fuel truck.

Answer 7

I believe you have it right.  From what I've read about the SR-71 the engines were unbelievable power plants and in their function, ahead of their time.  They functioned like a turbojet at lower speeds and once at mach 3 functioned more like a ramjet.  This occurred once the shock wave of the incoming air entered the engine intake and acting like a compressor stage in normal jet engine.

At this point the engine would have been functioning at a more economic level because some of the work in the propulsion cycle, the air compression, is self sustaining. It would have as you said burn far less fuel at speed.

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Answer 8

The SR-71 is quite efficient in cruise, thanks to the extremely high altitude and the ramjet effect of supersonic flight at that altitude.

There are other problems with supersonic flight that currently outweigh the advantage of fuel economy at very high altitudes, so don't expect to see it any time soon in commercial air travel.