Section 2.11 - Comparison Among Methods
Transport methods can be sorted by performance, status, and cost measures. The following sections describe the measures and make a first attempt to rank the various methods. Technology is constantly progressing, so any selection of methods needs to be updated with the most current data, or projections for future planning purposes.
Comparisons Among Methods
Any propulsion method that expels matter or energy to produce thrust obeys conservation of momentum, which is defined as mass times velocity. The exhaust momentum is equal to and opposite change in vehicle momentum. Given a finite supply of materials for the exhaust, mass is fixed. Therefore higher velocity leads to higher total momentum, and thus better performance for the vehicle. Exhaust velocity is measured in meters/second. Some systems, like 72 Ionospheric Current Engine, do not use exhaust for momentum exchange, but have a consumed supply - plasma generated for contact to the ionosphere in this case. The effective exhaust velocity is then calculated from the thrust force and mass flow rate.
Specific impulse is a derived measure traditionally used for launch from Earth, and then extended to space. It is defined as pounds thrust per pounds/second of fuel consumed, which simplifies to units of seconds. When multiplied by one Earth gravity (9.80665 m/s^2) it converts to exhaust velocity, which is the preferred measure since it is in SI units.
Thrust to Weight Ratio
In past rockets, this has been done by using different type of fuel for different stages in a rocket. In the early part of the flight, air drag is important, so a dense fuel is preferred. A dense fuel means smaller fuel tanks, and hence less area to create drag. Thus the Saturn V used liquid oxygen/kerosene and the Shuttle uses solid rockets for the first stage, both being dense fuels. Both use liquid oxygen/ liquid hydrogen for the second stage. This has the highest performance in use for a chemical rocket fuel.
The Pegasus rocket uses an aircraft to get above the bulk of the atmosphere. A sub-sonic jet engine has about ten times the performance of a chemical rocket, mostly because it does not have to carry oxygen to burn.
Many, many propulsion combinations are possible in getting to Earth orbit and beyond. A large part of space propulsion design is choosing which methods to use and when to switch from one to another.