Kevlar 49 is a aramid fiber used in aerospace applications. Tensile strength: 3,600 MPa.
Kevlar 49 is the aerospace grade of Kevlar, the famous synthetic fiber that is about as strong as steel but roughly five times lighter. On rockets and spacecraft, it shows up as the tough shell wrapped around high-pressure tanks and solid rocket motors.
Quick facts
- Type: A para-aramid fiber (poly-paraphenylene terephthalamide) — the high-modulus, or stiffest, reinforcement grade in the Kevlar family.
- Invented: By chemist Stephanie Kwolek at DuPont in 1965, and sold commercially in the 1970s.
- Density: About 1.44 g/cm³ — roughly one-fifth as heavy as steel.
- Tensile strength: Around 3,000 MPa (about 435,000 psi), the force it can take when pulled lengthwise before breaking.
- Stiffness (Young’s modulus): About 112,000–124,000 MPa — the highest of the common Kevlar grades, meaning it stretches very little under load.
- Heat: Does not melt and resists flame. Instead of melting it slowly breaks down (decomposes) at roughly 450°C; for steady, long-term use it is kept well below that, since it starts to lose strength at a few hundred degrees.
- Cold: Stays stable at cryogenic (extremely cold) temperatures without becoming brittle.
What it is and how it works
Kevlar 49’s strength comes from its molecules. They are long, rigid chains that line up almost perfectly parallel to the length of the fiber and lock together with dense hydrogen bonding (weak chemical links that, in huge numbers, add up to a strong grip). The result is a fiber that is extremely stiff and strong when pulled, with very little stretch.
The fiber is rarely used on its own. Instead it is wound or layered into epoxy resin to form a composite — a material that combines fibers and a glue-like binder. A good example is a composite overwrapped pressure vessel (COPV), which is a gas tank. A thin metal or plastic liner holds the gas in and stops leaks, while continuous Kevlar 49 filaments are wound tightly around it under tension. The fiber carries almost all of the pressure load. Think of it like wrapping many turns of strong thread around a balloon: the thread, not the balloon skin, does the real work of holding it together. The same idea is used for solid rocket motor cases, where the wound aramid shell contains the pressure of the burning fuel.
Why it matters
In spaceflight, weight is the dominant cost. Every kilogram of tank or motor case is a kilogram of payload you cannot carry. Because Kevlar 49 is far lighter than metal for the same strength, it lets engineers replace heavy metal tanks and motor cases with composite parts that are about 40–60% lighter while handling the same loads. On the Space Shuttle Orbiter, Kevlar-49/epoxy tanks alone removed roughly 752 pounds compared with all-metal versions. Its stability in extreme cold also suits vehicles that carry liquid oxygen, liquid hydrogen, and high-pressure helium.
There is an important trade-off, called stress rupture: a Kevlar-49 tank held under pressure for a long time can fail unexpectedly. This delayed failure depends on stress level, temperature, and time, and it is imperfectly understood. NASA reviews of Shuttle tanks found that the real stress on the fibers had been underestimated — in some cases the bursting stress was about 23% lower than assumed — which made earlier predictions of tank life too optimistic. The fiber is also relatively weak in compression (when pushed rather than pulled) and can absorb moisture, so it is often combined with carbon or glass fiber in hybrid composites.
Where it is used
- Space Shuttle Orbiter tanks: Kevlar-49/epoxy COPVs stored high-pressure gases and cut Orbiter weight by about 752 pounds.
- Trident missile motor case: Built with Kevlar 49/epoxy filament winding for a light, strong rocket motor case.
- Spacecraft gas tanks: Studied by NASA against S-glass and graphite fibers, giving 40–60% weight savings over titanium vessels.
- Missile structures: Kevlar 49 fabric used in laminated shielding and structural parts.
- Aircraft and aerospace composites: Used as reinforcement, often blended with graphite or glass to fine-tune the material.
As performance demands have grown and tank life needs to be better understood, carbon-fiber COPVs have become more common, while legacy Kevlar tanks call for careful life management.
Poly-paraphenylene terephthalamide (PPTA)
| DENSITY | 1 kg/m³ |
| TENSILE STRENGTH | 3,600 MPa |
| STRENGTH-TO-WEIGHT | 2500000 kN·m/kg |
| MAX SERVICE TEMPERATURE | 250 °C |
| THERMAL CONDUCTIVITY | 0.04 W/m·K |
| THERMAL EXPANSION | -2.0 µm/m·K |
| CATEGORY | Aramid Fiber |
| DESIGNATIONS | Kevlar 49, Kevlar 149 |
| MANUFACTURER | DuPont / Teijin Aramid |
| DENSITY | 1 kg/m³ |
| TENSILE STRENGTH | 3,600 MPa |
| MAX SERVICE TEMP | 250 °C |
| THERMAL CONDUCTIVITY | 0.04 W/m·K |
| THERMAL EXPANSION | -2.0 µm/m·K |
| CORROSION RESISTANCE | Excellent |
| WELDABILITY | N/A |
| MACHINABILITY | Specialized |
| COST RATING | High |
