Text Size
NASA - Delta II: Countdown 101
February 15, 2007

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

You can share in the excitement of the live countdown via NASA TV or 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.

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.


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-15 minutes and holding
Built-in 20-minute hold
  • Weather update
  • Launch team polled for readiness
T-15 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 (mission dependent)
  • 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
  • NASA launch manager reports "go" for launch
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!


Illustration of a Delta II Rocket Drawing of the Delta II rocket.
The Delta II Launch Vehicle. Depending on the payload, the size of the fairing can vary, and the number of strap-on motors can range from three, four or nine. Image credit: The Boeing Company
› View Delta II Component Definitions

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

A Delta II rocket sits on the launch pad. 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

Liftoff of Delta II rocket. Emerging through the smoke and steam, the Delta II rocket carrying NASA's Deep Impact spacecraft lifts off. Image credit: NASA
Image Token: 
Image Token: 
Page Last Updated: October 28th, 2013
Page Editor: Holly Zell