SR-71 with modern materials

**eocoolj** · 15 Oct 10,, 01:53

I have read in a few places that the limiting factor in the top speed of the SR-71 was not engine power but the temperatures that the plane was capable of handling. I am not sure if this refers to the skin, airframe, engines, or other parts of the plane. In my single materials science class in college, they were constantly extolling the virtues of all the modern materials that have been recently developed.

My question is, if either retrofitted or completely rebuilt SR-71s today with the best, most modern materials available, would they be capable of flying faster? If so, how much faster. And what materials would we use? We would be using exactly the same engines (aside from any upgrades to the alloys used in their construction).

Or haven't we come that far in terms of high temperature capability in the past 50 years?

**Gun Boat** · 15 Oct 10,, 08:26

The SR-71 was way ahead its time and I believe it for the most part forfilled its design requirments. You could probably make her more efficient but any increase in performance resulting form the use of modern materials would see a massive cost increase.

To improve her overall performance ie speed and range would be a big task even today. I very much doubt she could go any higher but she may be able to go a bit faster. But that would require an expotential power increase. Works the same in cars. To achieve a top speed of 100km/h you may need 100hp, to achieve 200km/h you may need 200hp then to reach 300km/h you need 500hp. Also, if you increased the design requirements above the originals she may not look anything like the aircraft we know as the SR-71.

Just my 2 cents.

**Stitch** · 15 Oct 10,, 17:19

The only "modern materials" that I can think of that would have a better heat tolerance than the ones already used in the SR-71 (which was pretty much ALL titanium, except for the engines) would be some pyroceramics; even today, the best combination of strength-to-weight ratio for an aircraft would still probably be titanium. In fact, some of the literature I've read said that strength of the titanium airframe of the SR-71 actually INCREASED every time the plane flew a mission due to the anealing process of heat-soaking the airframe for several hours, and then cooling it down again. IIRC, the problem the SR-71 had, and the reason it was eventually retired, was that all of the sub-systems were getting quite long in the tooth, especially the avionics. They had also stopped making the J58 engines back in the '60's, so there was a limited supply of those. There WAS some talk back in the '90's, before the -71 was retired for the second time, of replacing the avionics with a "glass" cockpit but, as usual, the funding wasn't available, so the idea went nowhere.

As for how fast the SR-71 could actually fly, some unclassified "sources" claim the SR-71 was easily capable of Mach 4.0 flight, but that there was never any reason to go that fast, Mach 3.2 was sufficient; the airframe itself was cleared for flight beyond M4.0, but the limiting factor was the compressor inlet temperature on the J58's. The efficiency of the J58's actually dropped off quite rapidly above M3.2; the duct/inlet system on the SR-71 was actually optimized for flight at M3.2. SR's occasionally exceeded this speed during op's, but that was usually only to evade SAM missles.

**highsea** · 15 Oct 10,, 18:27

Sure, we could build a faster SR-71.

We've already exceeded the speed of SR-71 in atmospheric flight by a wide margin in the hypersonic vehicles.

**Skywatcher** · 15 Oct 10,, 18:33

Speaking of high speed aircraft, how much of a performance boost the XB-70 would have gotten if it entered service and was still around in the present day (damn you, McNamara!)

Stuart Slade estimated 3.75 Mach for something using modern technology (or its equivalent) and the maximum speed was higher for more modified versions. .

**Stitch** · 15 Oct 10,, 20:43

Originally Posted by Skywatcher

Speaking of high speed aircraft, how much of a performance boost the XB-70 would have gotten if it entered service and was still around in the present day (damn you, McNamara!)

Stuart Slade estimated 3.75 Mach for something using modern technology (or its equivalent) and the maximum speed was higher for more modified versions. .

Imagine the XB-70 with six J-58's, instead of J-93's! 34,000 lbs. of thrust each, instead of 29,000!

**2DREZQ** · 15 Oct 10,, 22:32

At altitude and cruising at Mach 3.2 only 17% of the thrust was actually produced by the turbine engines. Any faster and a 100% bypass to pure ramjet would have rendered them deadweight (Although, not being an engineer, I'm not at all certain that doing so would have actually been possible.)

**USSWisconsin** · 15 Oct 10,, 23:00

I beleive some new technologies like ion implantation and ceramic coatings would be useful in upgrading the Blackbird. Aviation Week talks about the still classified replacement for the Blackbird - the Aurora as they call it - with far greater capabilities than the SR-71

**Pakmiran** · 16 Oct 10,, 00:12

Man, the SR-71 is my closest thing to plane porn. The idea was brilliant---high altitude recon plane---the design was sexy and the execution was pretty awesome too.

As for the engines, if you're gonna use a ramjet why not just give it a disposable rocket-based booster? Or a launch-vehicle type system to get it up to speed?

I heard one of the problems they had was that the seals on the fuel gaskets were optimized for low-pressure environments, and hence leaked like sieves on the ground. Might be a rumor.

**maximusslade** · 16 Oct 10,, 03:20

What I've been told that it wasn't the seals that leaked. Since space and weight were at a premium on the airframe, voids in the airframe were used as tanks. I do not mean that a gas tank was stuck into the void, the void itself was the tank. Also in order to prevent excessive stress on the skin of the plane the skin was installed with "grow room" for the skin to expand while experiencing high temperatures during flight. This meant that on the ground the plane leaked fuel, so it would have to take off then after it warmed up, it would refuel in flight.

**Stitch** · 16 Oct 10,, 04:20

Originally Posted by maximusslade

What I've been told that it wasn't the seals that leaked. Since space and weight were at a premium on the airframe, voids in the airframe were used as tanks. I do not mean that a gas tank was stuck into the void, the void itself was the tank. Also in order to prevent excessive stress on the skin of the plane the skin was installed with "grow room" for the skin to expand while experiencing high temperatures during flight. This meant that on the ground the plane leaked fuel, so it would have to take off then after it warmed up, it would refuel in flight.

Correct. Did you happen to notice the "warped" nature of the wings, they looked like corduroy? That was actually a very elegant solution to perplexing problem: how do you compensate for thermal growth in an airframe? The striations in the wing "absorbed" the thermal growth of the airframe (laterally), and prevented the SR from popping wing rivets as it heated up in flight. Yes, the SR "leaked like a sieve" on the ground when it was cold (hence all the pictures of the SR on the ground with puddles underneath it), but once the airframe was heat-soaked, everything sealed up nicely. SOP was to take off with half a load of fuel, pop up to angels 30, refuel from a KC-135Q (full load), and continue on up to mission altitude (typically 80-85K); at speed and altitude, everything sealed up, and the plane grew by 6 inches.

**Jimmy** · 16 Oct 10,, 05:40

Originally Posted by USSWisconsin

I beleive some new technologies like ion implantation and ceramic coatings would be useful in upgrading the Blackbird. Aviation Week talks about the still classified replacement for the Blackbird - the Aurora as they call it - with far greater capabilities than the SR-71

Aurora is a name applied to so many imaginary programs that the AF or Navy could actually release any truth behind any of it, but it would be completely ignored.

**Stitch** · 16 Oct 10,, 06:54

Jimmy's right; the "Aurora" name has been thrown around for so long now (decades, in fact), that it has ceased to have any real meaning.

**Pakmiran** · 16 Oct 10,, 14:20

Originally Posted by Stitch

and the plane grew by 6 inches.

Lucky plane.

**2DREZQ** · 16 Oct 10,, 22:33

Originally Posted by Pakmiran

Lucky plane.

Fully loaded
Hot
High
Fast
would you expect it NOT to?

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