Cold Launch

Cold launch ejects a missile from its tube or silo before the rocket motor lights, usually with a gas generator or piston, so the missile is already clear of the launcher, sometimes tens of meters up, when its engine ignites. Hot launch, the alternative, ignites the motor inside the tube and lets the missile fly itself out on its own exhaust. The distinction sounds like a minor engineering choice. It shapes the design of silos, ships, and submarines in ways that matter.

Why eject first

Cold launch buys several things. It protects the launcher, since the rocket's blast and heat never touch the tube, which means the tube survives to be reloaded and does not need heavy blast-venting plumbing. It suits confined or shared launchers, a big reason many vertical launch systems and silos are cold: you can pack tubes closer and reuse them. And it is essential underwater. A submarine-launched ballistic missile is cold-launched, blown out of the tube by gas or steam, floating up through the water, and igniting only after breaching the surface, because firing a rocket motor inside a submerged tube is not survivable. The image of a Trident or Bulava missile erupting from the sea and then lighting its motor a moment later is cold launch in action.

Russian and Chinese air-defense and ballistic systems favor cold launch heavily, the S-400 and many silo-based ICBMs eject and then ignite, tilting the missile toward its target during the brief unpowered rise. There is even a tactical benefit: a cold-launched surface-to-air missile can be kicked upward and pointed before ignition, saving the fuel a hot-launched missile would burn turning after it clears the tube.

Why hot launch survives

Hot launch is mechanically simpler, the missile just flies itself out, with no ejection system to build and maintain, and it avoids the risk that an ejected missile fails to ignite and falls back on the launcher, a hazard cold systems must engineer against. Many Western vertical launch systems, including the widely used Mk 41, are hot-launch, venting the exhaust through uptake channels built into the cell. Both approaches work, and navies and missile designers choose based on the platform: submarines force cold launch, and beyond that it is a trade among launcher survivability, packing density, complexity, and the specific missile.

Does the launch method change the missile's performance?

Marginally, and mostly at the start of flight. Cold launch can improve efficiency by pointing the missile before ignition and by not wasting the first seconds of thrust escaping a tube, which slightly helps range or reaction time. But the difference is small next to the missile's own design, and the launch method is chosen far more for what it does to the launcher and the platform than for what it does to the missile.