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NASA - Countdown 101: Delta IV
February 28, 2007

Countdown 101: Delta IV

The Delta IV is a rocket operated by United Launch Alliance. The vehicle is capable of carrying single or multiple payloads weighing between 9,480 to 28,620 pounds. Once in space, the Delta IV can place spacecraft in polar, sun-synchronous, geosynchronous, geosynchronous transfer and low-Earth orbits.

At the heart of the Delta IV is the first stage Common Booster Core, which features a liquid oxygen- and hydrogen-powered RS-68 engine. The second stage engine is a Pratt & Whitney RL10B-2, also fired with the same propellant combination.

The Detla IV can be outfitted in various configurations to lift medium, medium-plus, and heavy satellites and spacecraft: Delta IV Medium: One Common Booster Core first stage topped with a second stage engine. Delta IV Medium-plus: One Common Booster Core with two or four strap-on solid rocket motors and topped with a second stage engine. Delta IV Heavy: Three Common Booster Cores joined together and topped with a second stage engine.

ere 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-315 minutes
Launch Countdown Begins

T-290 minutes
Begin cryogenic propellants loading
Begin Common Booster Core liquid oxygen (LOX) tank loading

T-220 minutes
Begin Common Booster Core liquid hydrogen (LH2) tank loading

T-180 minutes
Weather briefing

T-175 minutes
Begin second stage LOX tank loading

T-160 minutes
Begin second stage LH2 tank loading

T-85 minutes
Perform telemetry RF link checks

T-70 minutes
Perform Command Receiver Decoder testing

T-50 minutes
Begin RS-68 engine spin start pressurization
Perform launch pad gantry swing arm functional test Perform
RS-68 engine gimbal steering "slew" check

T-25 minutes
Weather briefing

T-8 minutes
Arm launch pad ordnance

T-5 minutes and holding
Enter 15-minute planned hold
Weather update
Transfer spacecraft to internal power
Launch Conductor Readiness Poll Mission Director Readiness Poll

T-5 minutes
Transfer launch vehicle to internal power

T-4 minutes
Arm safe and armed switches

T-3 minutes
Common Booster Core LOX secure at flight pressure and flight level
Common Booster Core LH2 secure at flight pressure and flight level

T-2 minutes
Second stage LOX securing started
Vehicle hydraulic pressure verified at 4000 psi

T-90 seconds
Second stage LOX secure at flight pressure and flight level
LOX ground support equipment secure

T-80 seconds
Second stage LH2 securing started
Range reports go for launch

T-50 seconds
Second stage LH2 secure at flight pressure and flight level

T-45 seconds
Launch enabled on 
Main power turned off 

T-25 seconds
Solid Rocket Motor Thrust Vector Control System Blowdown

T-15 seconds
Launch vehicle igniters armed
Radially Outward Firing Initiators ignition

T-4.5 Seconds
RS-68 main engine ignition

Liftoff of the Delta IV rocket

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Page Last Updated: March 5th, 2015
Page Editor: NASA Administrator