Why was the U-2 so different from the SR-71?

Question

Both were designed for the same goals: taking photographs at high altitude, avoiding radar. Both were designed by the same company with a few years apart. Yet one is a "jet glider", with large wingspan, small chord, subsonic, and the other is a ultra high speed, delta wing airplane.

Why did these two planes took such different approaches to perform similar tasks? What would be the pros/cons of each? The SR-71 was developed later, so it could be seen as a better approach, yet it was retired, while the U-2 is still in use.

Answer 1

Because the requirements were different. When U-2 was developed, the requirement was for an aircraft which would fly high at 70,000' due to the (mistaken) belief that the Soviets would be able to detect and engage aircraft only below that altitudes.

Once the U-2 entered service however, it became obvious that they could detect U-2 (they complained about the overflights, though misidentifying the aircraft), and once an aircraft was shot down, it became obvious that U2 isn't going to be of much help.

SR-71, which came later, was the result of a requirement which expected it to not only fly higher, but more importantly, faster. Interestingly, the Soviets went the same way with the Mig-25R.

The main reason for the retirement of spy aircraft was the advent of reconnaissance satellites, which are immune to being shot down (not that it is impossible, but no one has done it except in tests). Spy aircraft are still operated only over areas where the threat from anti-aircraft systems are extremely limited or non-existent. At the time of its retirement, the USAF accepted that even the SR-71 is not invulnerable:

In congressional testimony, Air Force Chief of Staff Gen. Larry D. Welch identified the increased survivability of reconnaissance satellites, SR-71 vulnerability to the Soviet SAM-5 surface-to-air missile and the cost of maintaining the SR-71 fleet.

As for why U2 has outlived the SR-71, the main reason is the operating costs- the operating costs of SR-71 is quoted to be anywhere from \$85,000 to \$200,000 per hour, while the U-2 costs much less than that (incidentally, U-2s operating cost is less than that of its proposed replacement, the RQ-4 Global Hawk).

From the same source as above:

The Air Force decision to retire the Blackbirds in 1990 is based on several factors. ... The cost factor is the most significant to the Air Force because it limits expenditures in other areas. Reagan Administration Air Force Secretary Edward C. Aldridge Jr. estimated that the money used to operate the SR-71 fleet could operate and maintain two tactical fighter wings.

Answer 2

First off, the U2 was an earlier design, and engineered around the philosophy that the primary threat to a reconnaissance aircraft were cannon armed fighters. Kelly's thinking was that if he could design an airplane which could fly far higher than existing turbojet powered fighters, the aircraft could not be shot down and could conduct reconnaissance missions with impunity.

Then came Sputnik and the dawn of the missile age.

Lockheed had quietly begun to investigate higher and faster flying design which could nullify the threat of missiles using solid fuel boosters and sustainer motor to reach the altitudes the U2 flew at but the U2 continued to fly over the USSR and with great success.

And it all worked great until May, 1960 when a CIA pilot named Francis Gary Powers was shot down over the Soviet Union by an SA-2 surface to air missile.

At this point there was an impetus to acquire a newer faster aircraft for strategic reconnaissance. The idea being that the plane could fly so high and so fast that a ground launched missile system would not pack the energy needed to overtake and shoot down the new aircraft. This was the genesis of the A-12 Oxcart program, which struggled through a 7 year long development and flight test program before entering service with the CIA; a two seat advanced derivative of the A-12 went into service with the USAF as the SR-71 Blackbird. Officially there have never been overflights of the Soviet Union by either the A-12 or the SR-71 due to the political risks but Blackbirds did officially overfly North Vietnam, China, Eastern European communist bloc countries, Lybia, Egypt and North Korea and were shot at multiple times using advanced Soviet made SAM systems - all without a single loss.

Answer 3

The main idea was completely different behind the 2 designs. The U-2 was designed in an era when the main threat to aircraft were anti aircraft guns and gun carrying point interceptors.
Neither could (at the time, and to a large extend still) reach the operational altitude of the U-2. The Soviets tried many times to shoot one down, but it wasn't until Gary Powers (due to a navigational gamble he took to get out of the USSR in time to make a daylight landing at his intended destination, which had him fly near a Soviet SAM site) was shot down by the newly fielded strategic SAM capability of the USSR (designed against American bombers) that the idea was shown to have outlived its usefulness.

The SR-71 was then rushed through design and production to be completely different, flying not just very high but also very fast. AND its operational concept relied far more on side looking sensors so the aircraft could remain outside the target country or area and look in from the relative safety of friendly airspace (under the assumption, which was never broken by the USSR or China, that the USSR would not shoot at aircraft outside of its airspace.
Later the D-21 project intended to add a deep penetration drone to the back of the SR-71 which could overfly China or the USSR and later be recovered. Several of these drones were lost, causing top secret technology to fall into enemy hands, and the D-21 program was shut down.