Why is an aircraft carrier's arresting cable made from steel?

Question

Steel seems to be what modern aircraft carrier arresting cables are made from.

It's strong, provides some elasticity, and serves its purpose well. However, it does need to be changed after every so many traps due to metal fatigue (about every 125 landings from what I've heard).

Why not use a more modern material such as Kevlar? Wouldn't a material such as Kevlar reduce the frequency which the cable must be changed? It's lighter, more flexible, more durable, can tolerate more temperature extremes, is not susceptible to corrosion, and it might even be cheaper - since it's made from nylon threads. It also has the added advantage of being less dangerous in the rare event of a cable break under load.

This BBC Documentary (no longer available) posed the same question, but didn't really provide any answer.

Why are modern aircraft carrier arresting cables still made from steel?

Answer 1

The single most important thing for an arrestor cable is that it works reliably, compared to this incremental differences in weight or durability are fairly minor considerations.

One of the major advantages of kevlar is that it has a very high strength to weight ratio, however all this really achieves in this context is that the cable is thinner and lighter and I would guess that the weight of spare arrestor cables is vanishingly trivial in the context of the total payload of an aircraft carrier.

Another consideration is that Kevlar has much lower stiffness than steel, which will be further exacerbated by the differences between a fibre rope and a braided steel cable. This means that it will extend further under the same load. Not only does this mean that it allows the aircraft to travel further before it stops but you also need to fundamentally redesign the whole arrestor system to account for the very different material properties of the wire and then the new design would need to be tested, evaluated and certified.

Also, while kevlar is considered abrasion resistant by the standards of textiles it is nowhere near as resistant to abrasion as high strength steel cable. Consider that industrial steel cables may have a design life in the order of decades (think of suspension bridges) whereas high strength polymer climbing ropes are typically retired after a few years, mostly because of the cumulative effects of abrasion, dirt and UV exposure. Also steel is almost entirely unaffected by contamination by spilled fuel, solvents, lubricants, hydraulic fluid etc.

Similarly the arrestor hooks on aircraft are made form high strength steel, which is much harder than Kevlar so even a small burr or sharp edge (as you miht get form landing or combat damage) could cut through a kevler rope like knife whereas a steel cable is much more resistant to sharp edged shearing.

125 loading cycles is nowhere near where metal fatigue would be an issue, especially in steel, so it seem more likely that the life of the cable is based on direct wear and local overloading from the arrestor hook. Steel being relatively hard while having reasonable ductility is much better at tolerating this sort of wear and tear than most other materials.

It's also worth noting that overloading beyond the yield stress (but before total failure) will often produce a noticeable kink in a steel cable whcih is a warning sign that it shod be replaced. This gradual failure mode (ie damage is measurable before it becomes critical( is one of the major reasons why steel is preferred for safety critical structures ie you get some waring of impending failure.

However the most compelling answer is that steel cables are proven to work well for the job and so there is no compelling reason to redesign the whole system to use a new material for the sake of fairly marginal (if any) benefits in performance.

It's also worth noting that textiles are use for the arrestor nets for emergency landings where it is known or suspected that the landing gear on a particular aircraft is compromised. Here the light weight and greater stretch is a distinct advantage as it mitigates potential damage to the air-frame.

Answer 2

The short answer is the cables are made from steel because that design has been tested and found suitable for the purpose.
The military, and aviation in general, tends to keep using a working solution once we have one because designing new solutions is expensive and time-consuming.

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From a materials science perspective there's no reason you couldn't make an arresting cable out of Kevlar: It has adequate tensile strength to be used in this application. To do so however you would have to perform the same kind of fatigue testing on it that was done on the steel cables, determine its useful service life, and then add an appropriate margin of safety after which it's removed from service so the cable doesn't snap and send a very expensive fighter jet careening off the end of the deck (to say nothing of whipping the pendant around to slice deck personnel in half).
You would also have to consider wear - breakage of the individual strands, damage from friction, grit, contamination from oil/fuel/other materials, etc. - and its effects on Kevlar, or any other proposed replacement material.

It's possible, after all that testing, that Kevlar would be found unsuitable due to a short fatigue life (more downtime of the deck while changing cables), higher per-unit cost, or any number of other reasons.

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Note that there is at least one patent for a hybrid-type arresting cable with a composite (Kevlar or Nylon) core and a steel outer wrap. We may see it used in next-generation arresting gear, or the numbers may get crunched and the "traditional" steel cable found to be a better solution when all factors are considered.

Answer 3

Its a good question. The answer why is part technology, part economics, part life cycle reality and part bureaucracy. In short, it may be possible to build arrester pendants out of other materials than hemp cored steel rope, but life is never that simple.

First off is reliability. For the entire history of CATOBAR aircraft carriers in naval aviation, the ships have relied on arresting gear using steel rope cross deck pendants to arrest landing aircraft. With 100 years and literally millions of traps under its belt and few, if any, reliability, cost or maintenance issues, there exists no impetus to change from that design.

But for a moment, lets assume we wanted to design a new cross deck pendant making use of, say, a kevlar rope or more exotic materials eg woven carbon nanotubules, etc.

LIFE CYCLE CONCERNS

• Inception: Is there a pressing reason that offers enough benefits to change away from the existing steel rope technology to a more modern alternative? Is there an increase in reliability, cost savings, safety, etc. that make it worth spending a tremendous amount of time and money to improve the existing solution.

• Development: Requires time and money to develop a new cross deck pendant alternative. Can it do all of the things the current steel rope can? Is it flexible? Does it interfere with existing carrier infrastructure? Are there drawbacks to the new design such as a increase brittleness or a need for high maintenance to work properly? Does it offer a long shelf life in harsh environemnts such as sea spray, etc and still work properly?

• Manufacturing: Does the new design require expensive and exotic raw materials? Is it more difficult to manufacture? Or does it require a much more stringent manufacturing environment or elaborate quality controls which are more difficult to realize and present operational hazards if not met as opposed to manufacture from steel rope? Does it require the use of toxic chemicals which are hazardous to personnel in the plant or present an environmental hazard to dispose of properly?

• Service: This one can open up a host of problems which are never fully anticipated and can often show up years or even decades after the product is introduced to the fleet. This can come in the form of difficulties transporting, stowage aboard ship, degradation during use, unanticipated structural failures for the reasons listed above, or operational hazards to the flight deck crews, aircraft of the ship. A new design like this would undergo years of testing, often wringing out these kinds of problems, but it can result in an issue which makes further development unwarranted.

• Disposal: Inevitably the pendant must be retired and subject to destruction. Can it be recycled? If not, does it have to be shredded and incinerated? Or buried in a landfill? Does this process create toxic chemicals which are hazardous to humans or the environment?

COSTS

A new cross deck pendant design is going to cost millions and this expenditure is really unwarranted if the only reason for doing so is that other, more exotic materials exist which are stronger.

BUREAUCRACY

The Navy is, by far, the most stodgy and difficult to convince of unorthodox technology of any of the branches of the military. They often don't know what they want and this dramatically compounds development times and costs for procurement of weapon systems.

Consequently, it appears that hemp cored steel rope will be used as arrester pendants for decades to come. It is possible that other endeavors will make use of exotic material ropes which, once they have been developed through a mature life cycle will have proven more useful for the Navy to evaluate for this purpose. A similar thing happened with the Electromagnetic Aircraft Launch System (EMALS) which the Navy is developing as a replacement for the steam catapults currently used on its CATOBAR carriers from an existing linear motor technology which has been used in amusement parks for over a decade now. But even this system has been fraught with development problems and does not quickly translate over to use aboard an aircraft carrier.

Answer 4

Many of the "benefits" you listed just don't matter:

It's lighter, more flexible, more durable, can tolerate more temperature extremes, is not susceptible to corrosion, and it might even be cheaper - since it's made from nylon threads. It also has the added advantage of being less dangerous in the rare event of a cable break under load.

• Lighter: This is carried on an aircraft carrier, a few thousand pounds more or less is no big deal
• More flexible: Steel is flexible enough, being more flexible is optimizing something that does not need optimization.
• More Durable: Maybe a mildly redeeming quality, but Kevlar weaves can get small metal particles trapped in them which cause long term abrasion and weaken the cable.
• Temperature extremes: Arresting cables are in the same environment as humans: roughly 0°F to 120°F. Steel and Kevlar are both fine until around 300°F.
• Corrosion resistance: Steel is just fine against seawater, jet fuel, and common solvents. Kevlar is slightly susceptible to abrasion from small metal and dirt particles getting in the weave.
• Cheaper: When it comes to jet planes and military operations, small cost differences are not a consideration.