The Space Shuttle’s ascent into orbit was a complex process, often perceived as a slow start. “Takeoff” encompasses the entire initial phase of its vertical and then horizontal climb towards space. This intricate dance of thrust, gravity, and atmospheric forces determined how quickly the vehicle gained speed and altitude.
The Physics of Liftoff
At liftoff, the Space Shuttle’s speed was essentially zero. This changed rapidly as the Space Shuttle Main Engines (SSMEs) and the two Solid Rocket Boosters (SRBs) ignited, generating a combined thrust of approximately 6.7 million pounds (about 30 million newtons). The SRBs alone provided around 85% of the total thrust during the initial two minutes of flight, with each booster delivering up to 3.3 million pounds of force (14.7 meganewtons). This tremendous power countered the vehicle’s substantial liftoff mass, which could be around 4.5 million pounds (over 2 million kilograms).
The thrust-to-weight ratio expresses the relationship between thrust and the vehicle’s weight. For the Space Shuttle, this ratio was approximately 1.5-to-1 at liftoff, meaning the generated thrust was about 1.5 times greater than the vehicle’s weight. This ratio produced an initial acceleration of roughly 0.5 Gs, which, when combined with Earth’s gravity, meant astronauts felt about 1.5 Gs of force at the very start. While the Space Shuttle appeared to lift off slowly due to its massive size, it rapidly gained speed. For instance, it reached approximately 100 mph within eight seconds of clearing the launch tower.
Acceleration Through the Atmosphere
As the Space Shuttle ascended through the lower atmosphere, its speed continued to increase significantly, and the acceleration intensified. By about 30 seconds into the flight, the Shuttle was traveling at roughly 500 mph, and by 55 seconds, its velocity approached 900 mph. Astronauts experienced increasing G-forces during this phase, typically peaking around 2.5 to 3 Gs. This acceleration was carefully managed to prevent excessive stress on the vehicle and its crew.
“Max Q,” or maximum dynamic pressure, occurred when the combination of the vehicle’s increasing speed and the still-dense atmosphere created the greatest aerodynamic stress. For the Space Shuttle, Max Q typically happened at an altitude of about 10.8 to 11.5 kilometers (around 36,000 feet), at a speed of approximately 1550 to 1600 km/h (960 to 994 mph). To manage these forces, the Space Shuttle Main Engines were momentarily throttled down to about 65% to 72% power. Approximately two minutes after liftoff, at an altitude of about 45 kilometers (28 miles), the two SRBs separated from the External Tank. At this point, the Shuttle was traveling at speeds ranging from 3,000 to 3,512 mph (4,800 to 4,979 km/h). The jettisoning of the SRBs significantly reduced the vehicle’s mass, allowing the remaining SSMEs to continue accelerating the Shuttle towards orbit.
Achieving Orbital Speed
After the SRBs separated, the three Space Shuttle Main Engines continued to burn for another six and a half minutes, drawing propellant from the large External Tank. This continuous thrust was not just about gaining altitude; it was also about accelerating horizontally to achieve the speed necessary to stay in orbit around Earth. The Space Shuttle was not simply going “up” into space, but rather accelerating to a velocity that would allow it to continuously “fall” around the Earth.
To achieve a stable Low Earth Orbit (LEO), the Space Shuttle needed to reach an approximate speed of 17,500 mph (about 7.8 kilometers per second or 28,000 km/h). This speed allowed it to counteract Earth’s gravitational pull and remain in orbit. The SSMEs typically burned for a total of about 8.5 minutes to bring the Shuttle close to orbital velocity. Following the main engine cutoff and the separation of the External Tank, the Orbiter’s smaller Orbital Maneuvering System (OMS) engines performed brief burns to circularize the orbit and fine-tune the final orbital insertion. This sequence of acceleration and engine firings culminated in the Space Shuttle achieving the speed required to continuously circle the Earth.