Energy in Space Exploration

Brandon Wu
May 26, 2018

Submitted as coursework for PH240, Stanford University, Fall 2017


Fig. 1: Discovery Space Shuttle Liftoff. (Source: Wikimedia Commons)

Space travel has always been fascinating for both scientists and engineers alike. Not only does space travel require many calculations with the utmost precision, but an enormous amount of energy is also required. This energy all comes from chemical combustion, allowing the rocket engines to provide propulsion for the space shuttle. These space shuttle launches can often be spectacular to watch, as seen in Fig. 1.


Rockets work using Newton's laws of motion. Chemical reactions in the rocket cause the rocket to expell propellant, and because of the mass being pushed backwards, the rocket is propelled forwards. Rockets can accelerate at up to 25 times the acceleration of gravity (25 g = 245 m/sec2). However, space shuttle rockets accelerate between 1.2 and 6 times acceleration of gravity. [1] Rockets can generate a thrust-to-weight ratio of 75:1. By comparison, turbojet engines, used mostly in planes, have a thrust-to-weight ratio of 5:1. [1] While achieving a great amount of thrust for its weight, most of a rocket's weight is fuel. The solid rocket booster, or SRB, weigh about 1.3 million pounds at launch, with the fuel weighing 1.1 million pounds, leaving the rocket itself weighing about 192 thousand pounds. [2] To get a sense of how much thrust these rockets are actually producing, a Boeing 737 engine generates about 20 thousand lbs of thrust, while SRB from a space shuttle launch generate about 3.3 million lbs of thrust. [2,3] These rockets are providing most of the thrust during the launch, and once the shuttle reaches 150,000 feet, will detach and return to Earth to be reused. [2]

Fig. 2: External Tank of Space Shuttle. (Source: Wikimedia Commons)

Space Shuttle Main Engines

Along with the rockets serving as propulsion, the space shuttle also has its own main engines, abbreviated as SSME. Shuttles will have three main engines. The three SSME combined will provide another 1.5 million lbs of thrust. [2,4] Along with providing thrust, the engines can also move around to control the pitch, yaw and roll of the space shuttle. The fuel for these three engines is located in the external fuel tank, which is pictured in Fig. 2. At 154 feet long, the external tank is the largest component of the space shuttle, also serving as skeleton for the shuttle itself and the rockets used for launch. These engines will be throttled down and back up at different time intervals throughout the launch, before being completely shut off just before the shuttle enters orbit, traveling at 17,000 mph. [2] Once the engines have been shutoff, the external tank detaches and falls back to earth.


Space shuttles are magnificent feats in engineering, unthinkably heavy and using massive amounts of fuel, going insanely fast. The sheer power is amazing and quite a sight to behold.

© Brandon Wu. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] G. P. Sutton and O. Biblarz, Rocket Propulsion Elements, 9th Ed. (Wiley, 2016).

[2] "The Space Shuttle's Return to Flight," U.S. National Aeronautics and Space Administration, July 2005.

[3] "Type Certificate Data Sheet A16We," Boeing Company, 1 Sep 10.

[4] N. Nguyen, "Space Propulsion Technology and Energy Expenditures," Physics 240, Stanford University, Fall 2011