# Why did the space shuttle use bias-ply tyres instead of radials?

Aircraft and automobile tyres can be divided into two main types, based on the orientation of the tyres’ body plies (the layers of tough fabric that form the main structural body of the tyre); radial tyres are built with the body plies laid perpendicular to the bead and tread (so that the body plies extend straight out radially - hence the name - from the tyre bead, straight across from one side to the other at a 90º angle beneath the tread, and straight in radially to the bead on the other side of the tyre), while bias-ply (or simply bias) tyres have the body plies laid at an oblique angle.

Radial tyres are the more common type by far, for very good reason; they are stronger than bias-ply tyres and wear much more slowly.

I was, thus, rather surprised to learn that the tyres used on the space shuttle’s main landing gear were of the older, weaker bias-ply type:

Wheels and Tires

[...]

The main landing gear tires are 44.5 by 21 inches and have 16 cord layers in a bias-ply design. They are normally inflated with nitrogen to a pressure of 370 pounds per square inch (psi). The maximum allowable load per main landing gear tire is 132,000 pounds. With a 60/40 percent tire load distribution, the maximum tire load on a strut is 220,000 pounds. The main gear tires are rated at 225 knots maximum ground speed and have a life of one landing. [Shuttle Crew Operations Manual; my emphasis.]

The space shuttle’s main landing gear would seem to be one of the worst possible places to use bias-plies instead of radials; with each tyre carrying up to 59.9 tonnes (66 short tons) and having to cope with touchdown speeds in excess of 200 knots, it would seem imperative to use the strongest tyres available. The magnitude of the stresses placed on these tyres was so great that, while most aircraft tyres can make it through dozens to hundreds of flights before having to be changed, the shuttle MLG tyres were rated for a grand total of one use before replacement. Additionally, with only four MLG wheels per shuttle orbiter, even a single MLG tyre failure would significantly impair the vehicle’s braking capability, which is most emphatically not a good thing for something going as fast as a landing shuttle orbiter. Given all this, one would think that radial tyres would be the obvious choice, hands down, for the shuttle’s main landing gear, since their greater strength and slower wear would considerably decrease the chance of a tyre failure upon landing - yet the shuttles instead used bias-ply tyres. Why?

• I am going to go out on a limb and suggest that the tires are only used once, also they may have taken weight into consideration and if a blow out, the steel shrapnel? Just a thought. – Skyhawg Jan 23 at 0:04
• It's worth noting that most aircraft tires were bias-ply during the shuttle era. It's only in recent years that radials have started to be common on airliners, like the Boeing 787 or Airbus A350. – Bret Copeland Jan 23 at 0:28
• @Bret Copeland is this do to FAA being slow to adopt? – Skyhawg Jan 23 at 0:48
• It's interesting to see that NASA got realistic on tire life. When I worked on the Shuttle in the mid '80s, the tire life was molded into the sidewalls as 'Good for 6 landings'. They were never used for more than one landing. It was obvious from one look at the tires after a landing that they were done. – Gerry Jan 23 at 12:39
• The natural advantage of radial tires is their behaviour when making turns and when rolling on very uneven surfaces. Neither of these is a significant factor for shuttle landings. So the real question to answer is, why would anyone ever want to use radial tires for the shuttle? – Ray Butterworth Jan 23 at 14:29

Simply put, for the weight bearing factor bias-ply is a lighter tire, and when building a spaceship weight is the top concern.

The reason they were thin and single use also had to do with weight, according to NASA:

Weight: Since weight is of extreme importance, the tires are made with a minimum amount of tread to conserve weight, allowing for larger payloads. A few pounds may not seem to make much difference, but when you add up all of the ways to decrease weight throughout the shuttle it can have a significant impact.

Due to their multiple layers, bias ply tires generally have the advantage when it comes to load-carrying capacity. Because there are a uniform number of plies to support the weight of a vehicle in both the tread face and the sidewall, bias ply tires are generally better when hauling heavy loads. This is why trailer tires, tractor tires, and heavy equipment tires are often bias ply; they can better support heavy loads. Radials are available with high load ratings, but the weight-carrying capacity requires adding many more plies to a radial tire, making them very stiff.

The important line here being "adding many more plies" which will also make the tire heavier (reference above note on weight). It costs in the neighborhood of $10,000 to put 1 lb into space, so if you can save a few pounds on each tire, it does not matter that they cost \$5,660 each; you save by replacing them every time if they are indeed lighter.

even a single MLG tyre failure would significantly impair the vehicle’s braking capability, which is most emphatically not a good thing for something going as fast as a landing shuttle orbiter.

The shuttle has no issue coming to a full stop with a blowout as was the case with the shuttle Discovery in 1985 during the landing of STS-51-D.

• Re: STS-51-D, Discovery had the advantage of being able to use one of the longest runways anywhere in the world (and most of the ones that are longer than the SLF's runway are like that as a consequence of being at high-altitude airports), and, even so, NASA considered the risk from tyre bursts and/or brake failures during a landing at KSC to be so severe that they shifted the next eight flights to land at Edwards instead while they modified the orbiters to increase safety during landing. Also, what if a shuttle had had to abort during ascent (or make an unplanned emergency deorbit)... – Sean Jan 23 at 21:41
• ... and land heavy at an emergency site, with a shorter runway than the primary landing sites, and lose one or more tyres during a heavy, fast landing on a (relatively) short runway, and go off the end or side of said runway due to the resultant loss of braking force? And, finally, given how much weight they managed to shave off the shuttles over the course of the programme through various improvements, I have difficulty believing that they wouldn't be able to compensate for heavier radial tyres by finding a way to trim weight somewhere else. – Sean Jan 23 at 21:44
• @Sean, most of the emergency landing sites had runways of at least 11,000 feet, with 12,000+ being common. (And one of the post-Challenger enhancements was a braking parachute, which presumably weighs less than better tires while providing benefits in more situations.) – Mark Jan 23 at 22:43
• @Sean the likely could have accounted for the increased weight of a radial but chose not to since they could sell that payload space to customers instead. Radial tires are not necessarily stronger so there is no need to lug the weight around. – Dave Jan 23 at 22:48
• Also, the sidewalls of a bias-ply tyre give and absorb a lot of the shock. – SEoF Jan 24 at 14:25