strap-on
Of a booster rocket, that is mounted on the outside of a launch vehicle (typically around the first stage) to provide an additional boost at lift-off, ultimately separating after the first few minutes of ascent.
strap-on: a throwaway rocket bolted to the main stage
A strap-on booster is a solid or liquid-fueled rocket motor mounted externally to the primary launch vehicle, clustered symmetrically around the first stage to deliver extra thrust during the initial seconds of flight. The Space Shuttle Orbiter carried two solid rocket boosters (SRBs) strapped to its external tank; each produced 12.5 million pounds of thrust at sea level and burned for approximately 125 seconds before separation and parachute recovery downrange.
The engineering appeal of strap-ons lies in their simplicity and modularity. Rather than design a larger core engine with proportionally heavier structure, launch providers bolt on identical boosters as needed to meet payload and trajectory requirements. Ariane 5 flew with two solid strap-ons for heavy-lift missions; Delta IV Heavy achieves its capacity through core clustering rather than external boosters. Strap-ons burn in parallel with the first stage, not in series, so their flame remains a constant presence in the first few minutes of ascent.
The term derives from the literal mechanical attachment: the booster cases are strapped, bolted, or welded to the vehicle structure via load rings and hold-down brackets that must transmit lateral bending loads and engine-induced vibration. Separation occurs through ordnance charges or explosive bolts when burn time ends and the booster's thrust drops below a threshold; relying on gravity alone would leave dead weight aloft. Recovery of strap-ons (common with liquid boosters, less practical with solid cartridges) adds to program cost but recycles expensive hardware.
Failure modes and operational limits
Asymmetrical booster failure presents the greatest risk: if one strap-on on a multi-booster vehicle ignites late, burns unevenly, or separates prematurely, the resulting side load can exceed the structural limit of the first stage and core engines. Flight control systems have finite authority to compensate. Structural cracks in strap-on casings (found through ultrasonic inspection) and O-ring degradation in solid motor joints have historically grounded programs. External attachment also increases aerodynamic drag and staging complexity, which is why some modern heavy-lift candidates (New Glenn, Vulcan Centaur) rely instead on core clustering or larger core engines.
Strap-ons occupy a middle ground in launch economics: they are cheaper than re-engineering the core stage but costlier and more complex than a single-core vehicle. Their use signals a deliberate trade-off between development schedule and vehicle capability. For operators flying variable payloads, the modularity of strap-on count (two, three, or four on the same core) reduces the need for multiple launch vehicles, albeit at the penalty of increased ground processing, range time, and range safety complexity.