|Fig. 1: Artist rendition of SLS launch with LOX/LOH SSME-derived engines and SRBs shown, courtesy of NASA.|
NASA recently announced its Space Launch System (SLS) in an effort to shift its focus back to manned deep space travel and further research beyond low earth orbit. The system aims to propel astronauts and payloads to celestial bodies by means of chemical rockets. Namely, five RS-25 Space Shuttle Main Engine (SSME)-derived engines operating on liquid oxygen/liquid hydrogen supplied by its core for the initial stage, a J2-X engine for the upper stage, and two solid rocket boosters (SRB) for initial testing purposes will power the heavy-lift platform. 
Although the fully operational launches are projected to be decades away, some public opinion may question why there is still heavy reliance on the bulky conventional rocket technology that traces its origins back to the Mercury and Apollo programs--when Americans first ventured into space and towards the moon. Nuclear reactor-based rockets and thrusters have been proposed for future lengthy travels. However, what will be discussed here is not the advocation of one means of propulsion over another, but rather some explanations on the different choices of propulsion available. Their implications and energy consumption will be considered.
Before different propulsion systems can be considered, a little introduction to the principles of rocketry and spaceflight must be touched upon for the unfamiliar reader. The operation of a propulsion system is not unlike that of untying the neck of an inflated balloon. At the most basic level, a combination of pressure difference (where the stored high pressure gas in the balloon acts much like a conventional solid rocket booster's high pressure chamber) and transfer of momentum (where the air exits the neck and similarly where the gaseous exhaust exits the rocket) results in a reaction force propelling the rocket in the direction opposite of the fluid flow. In reality, the system is much more complicated since rockets use converging-diverging (C-D) nozzles to correctly expand gases to supersonic speeds at the atmospheric back pressure--which is a function of the altitude. [2,3] In nuclear rocketry and electrical propulsion, the momentum transfer idea still holds. For the highly theoretical drives such as nuclear pulse propulsion, a whole different kind of pressure principle applies. Additionally, control and electromechanical systems make the endeavour much more complex, lending to its colloquially connotative name of rocket science.
It is unnecessary to go into those details. So long as the basics of propulsion are understood, then the following exploration of space propulsion technologies will be more appreciable.
America's most recent manned ventures into space have been primarily on the wings of their recently retired Space Shuttles of the Space Transportation System. The typical gross lift-off weight can be estimated to be around 4.5 million pounds, based on the launch of STS-114.  The twin solid rocket boosters that propel the orbiter during the initial stage generates a combined thrust of 6.6 million lbf at sea level, and when taken into consideration of the SRBs' operation duration, the power output is equivalent to about 40 GW of power.  Compared to the 22-34 thousand lbf of thrust found in each of the CFM56 engines on Boeing 737s, this shows the immense acceleration needed to lift the vehicle and its payload into orbit.  The SRBs are solid chemical rocket boosters. The propellant is made of a mix of atomized aluminum fuel, polybutadiene acrylic acid acrylonite binder, and an iron oxide catalyst with an epoxy curing agent. The oxidizer of choice is ammonium perchlorate. The specific impulse, a measure that can be thought of as thrust divided by propellant mass per unit time, is on the order of 250 seconds at sea level. However, theoretical limits calculated by changing propellant concentrations and introducing different fuel-oxidizer mixture processes (almost pushing the rocket into a quasi-hybrid categorization) puts the specific impulse at 325.9 seconds.  Improvements for higher specific impulse could reduce weight and energy consumption, given that the thrust stays the same. Apart from the SRBs, the Space Shuttle Main Engines (SSMEs) provide another 1.5 million lbf of thrust at sea level. 
Of course, there are other means of propulsion that weigh less and consume less power. One such engine is the nuclear reactor thermal rocket, where thermal energy from an onboard nuclear reactor is used to expand the liquid propellant to the design pressure and speed instead of the chemical reaction heat from solid fuel. The Nuclear Engine for Rocket Vehicle Application (NERVA) project had actually explored these for earth-to-orbit propulsion. A test engine was designed to operate at 1.5 GW and provide a specific impulse of 825 seconds.  However, despite the threefold increase in the specific impulse for only the fraction of power consumed, the thrust produced by these nuclear thermal rockets is in the range of 75,000 - 200,000 lbf based on the prototypes built (with the upper limit running at 5 GW). Theoretical considerations necessitate engines at lift-off weights of upwards of 1.5 million pounds. 
Ion thrusters are even more skewed to the specific impulse side of the spectrum versus chemical rockets. These electric-based engines, despite being able to produce specific impulse values of over 6000 seconds, requiring power on the order of tens of kilowatts per engine, and being very lightweight, produce thrusts on the order of less than a pound force per engine. This is obviously not anywhere near adequate to be the main propulsion system in an Earth-to-orbit launch. In addition, ion thrusters (which are usually of the non-electrohydrodynamic kind) do not operate effectively in Earth's thick atmosphere. A vacuum is generally needed for their design condition use.
As space exploration develops, so too will the propulsion technology that will carry humans ever closer to the stars. It is important to keep in mind, though, that different engines have distinct characteristics that suit them for a specific role. There is not a one-size-fit-all model at present. Yet, there's a constant push towards newer propulsion and energy technologies, and perhaps one day there will be multi-role propulsion systems that can bring pure thrust and high specific impulse systems together.
© Nghia Nguyen. 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.
 "Preliminary Report Regarding NASA's Space Launch System and Multi-Purpose Crew Vehicle," National Aeronautics and Space Administration, January 2011.
 R. W. Fox, P. J. Pritchard and A. T. McDonald, Introduction to Fluid Mechanics (Wiley, 2009), p. 628.
 P. K. Kundu and I. M. Cohen, Fluid Mechanics (Academic Press, 2008), p. 731.
 "The Space Shuttle's Return to Flight: Mission STS-114 Press Kit," National Aeronautics and Space Administration, July 2005.
 "Space Shuttle Main Engine Orientation," Rocketdyne Propulsion & Power, June 1998.
 "Type Certificate Data Sheet," U.S. Federal Aviation Administration, A16WE-REV45, September 2010.
 R.L. Zurawski and D.C. Rapp, "Analysis of Quasi-Hybrid Solid Rocket Booster Concepts for Advanced Earth-to-Orbit Vehicles," National Aeronautics and Space Administration, Technical Paper 2751, August 1987.
 W. H. Robbins and H. B. Finger, "An Historical Perspective of the NERVA Nuclear Rocket Engine Technology Program," National Aenautics and Space Administration, NASA-CR-187154, July 1991.
 J. L. Finseth, "Rover Nuclear Rocket Engine Program: Overview of Rover Engine Tests Final Report," National Aeronautics and Space Administration, NASA-CR-184270, February 1991.