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Delta II: Countdown 101
Delta II rocket launch
Delta rockets have been carrying NASA spacecraft aloft since the 1960s, and today's Boeing Delta II has a long history of successful launches.

Image at Right: Clouds of exhaust form around a Boeing Delta II launch vehicle as it blasts NASA's Swift spacecraft on its mission from Complex 17A, Cape Canaveral Air Force Station. Image credit: NASA

Through the Virtual Launch Control Center, you can share in the excitement of each new launch with live coverage directly to your computer via the Web. Coverage usually begins two hours prior to liftoff, and you can use the countdown events below to track the prelaunch milestones and learn about the Delta II countdown process.

Live launch commentary is also provided on NASA TV.

Here are some countdown milestones and key events that take place after the countdown begins. Keep in mind that event times and lengths are approximate and subject to change.

Illustration of a Delta II Rocket

Drawing of the Delta II rocket. Image at Left: The Delta II Launch Vehicle.
Image credit: The Boeing Company

+ View Delta II Component Definitions

T-150 minutes
  • Launch countdown begins
  • Personnel not involved in the count clear the launch area; warning horn sounds
T-146 minutes
  • First stage helium and nitrogen pressurization begins
  • Second stage helium tank pressurization begins
T-140 minutes
  • Turn on launch vehicle guidance system
T-130 minutes
  • First stage fueling begins
T-95 minutes
  • Weather briefing
T-87 minutes
  • Launch team polled for readiness to begin loading
    liquid oxygen (LOX)
T-80 minutes
  • Blast danger area is cleared
T-75 minutes
  • Liquid oxygen loading begins

T-60 minutes
  • Vehicle C-Band radar transponder beacon checks begin
T-30 minutes
  • Perform engine gimbal steering checks (slew checks)
T-20 minutes and holding
Built-in 20-minute hold
  • Weather update
  • Launch team polled for readiness

A Delta II rocket sits on the launch pad. Image at Right: On Launch Pad 17-B, Cape Canaveral Air Force Station, Fla., the Delta II rocket carrying the Deep Impact spacecraft looms into the night sky. Image credit: NASA

T-20 minutes and counting
  • Top off liquid oxygen to 95 percent
  • Top off helium and nitrogen pressurization system
  • Range Safety Radio Command Carrier on
T-14 minutes
  • Range Safety Command Receiver checks
T-10 minutes
  • Pressurize first stage fuel tank
T-7 minutes
  • Top off liquid oxygen to 99 percent
T-6 minutes
  • Verify observed weather is "go" for launch
T-5 minutes
  • Spacecraft to internal power
  • Launch enable switch to "on"
T-4 minutes and holding
Built-in 10-minute hold
  • Launch Conductor receives "go" to release from hold
T-4 minutes and counting
  • Arm launch vehicle and Range Safety devices
  • Launch vehicle to internal power
T-3 minutes
  • Spacecraft Mission Director reports "go" for launch

Liftoff of Delta II rocket. Image at Left: Emerging through the smoke and steam, the Delta II rocket carrying NASA's Deep Impact spacecraft lifts off. Image credit: NASA

T-2 minutes
  • Raise liquid oxygen tank to flight pressure
T-80 seconds
  • Top off first stage liquid oxygen to 100 percent
T-70 seconds
  • Range report "go" for launch
T-60 seconds
  • Launch enable switch "on"
T-30 seconds
  • Liquid oxygen fill and drain valve closed
T-10 seconds
  • Arm launch vehicle ignition system
T-7 seconds
  • Go for main engine start
T-2.5 seconds
  • Command main engine start
T-0 minutes
  • Liftoff of the Delta II rocket!
NASA's John F. Kennedy Space Center