The retired aircraft that is serving as a sign for the Mojave airport deserves some notice. It was absolutely critical to the safe landing of the space shuttle. Here is why.
No other aircraft put as much stress on the tires, brakes, and landing gear as the space shuttle did. Only the Concorde came close, touching down at about the same speed and weight. But the Concorde had four tires on each main gear truck as opposed to the space shuttle’s two so the load on each tire was much less.
In the early design of the space shuttle, the landing weight was calculated to be about 150,000 lbs. and the landing gear designed accordingly. In actual practice, due to weight growth and other factors, the shuttle rarely weighed in at less than 200,000 lbs. at landing. When returning with a payload – say a SpaceHab or SpaceLab – the weight could be considerably more. Maximum normal landing weight could be up to 230,000 lbs. In an abort landing, we were prepared to touch down at 258,000 lbs. All this on the same gear that was designed for 150,000 lbs.
Just to make it more fun, the ‘derotation’ of lowering the nose to touch down on the nose wheel tires caused even more loading. Rolling on all tires on the runway, the shuttle is 5 degrees or so nose down. At the instant of nose gear touchdown, the downward pressure on the main tires is increased by the aerodynamic forces pushing down on the top of the wing.
Speed is critical to tire performance; at speeds over 225 kts, a kind of reverse curvature in the rubber tire sets in that can quickly heat the tire to failure. Normal landing speed for the shuttle was targeted at 195 kts but for some heavyweight flights, the big glider had to touch down at 205 kts. With piloting variations, some landings approached the critical 225 kts touchdown speed.
Runway surface can make a difference; desert runways like Edwards are smooth but runways in wetter climates are grooved to shed rainwater quickly. The shuttle landing facility at KSC had groves and we found that landing there chewed up the tires.
And sideways forces from a crosswind at landing could cause even more damage to the tires.
In fact, for all the early landings, the post landing inspection of the tires showed more damage that we expected. More damage than we were comfortable with. Consternation over the tire and brake situation resulting in tests, redesigns, improvements to the tires, the brakes, and the runway surfaces.
We used several main tools in testing the shuttle tires to these extreme conditions they would experience during landing. First, the big tire dynamometers at Wright-Pat AFB in Dayton Ohio. Tires and wheels were pressed hard onto bug drums of steel spinning to simulate touchdown speeds. These dynamometers gave great data. But they didn’t do side forces and since the drums are curved, the effects were not quite like real flight.
Second, we used the test track at Langley Research Facility. A big carriage with the main gear tire was propelled to high speed and ‘touched down’ on a section of track a quarter mile long that had the surface – grooved or smooth – that was of interest. A great improvement; but it lacked the full rollout length and was limited again in side forces.
Analysis and these partial mode tests revealed a lot but the real landings were still incurring more damage than we predicted. So finally it was decided to actually fly the tires.
A Convair 990 transport was converted with an armored bay that could carry a shuttle main gear truck and one or two tires. We could land at high speed, hydraulically force the tire onto the runway with as much force as required, sideslip or take crosswind to well documented levels, and see what happened. Many many runs reviled the data was needed. Final adjustments were made to the tire manufacturing; improvements to the brakes; changes to the runway surface; and most importantly, limits on speed, crosswind, and weight were all confirmed using real flight data.
In space flight, it is almost impossible to test ‘combined environments’. We get one or two dimensions. Analysis has made great strides and computer models can inform engineering decisions greatly. But when complete accuracy is really important, the only real test is a flight test. No matter how much it costs.