ST401385@BROWNVM.BITNET (03/06/86)
The metric unit (newton seconds thrust per kilogram propellant)
which is measured in meters per second, is properly defined as the
exhaust velocity, Ve, *not* the specific impulse.
Ve = g Isp
Where Isp is specific impulse, and g is the acceleration of gravity.
This is true in any system of units.
However, you are right that Ve is a much more convenient way to
specify propellant characteristics, since you can plug it right into
the rocket equation,
Delta V = Ve ln(Mi/Mf)
Where Mi/Mf is the mass ratio.
Specific impulse (and exhaust velocity) are most useful for
systems where the energy and the reaction gas are the same.
In this case, it is *always* to your advantage to increase the
specific impulse (or exhaust velocity). For
objects like nuclear engines, where the energy comes from a different
source than the reaction gas, it is a less useful concept.
--Geoffrey A. Landis, Brown University
Reply to: ST401385%BROWNVM.BITNET@WISCVM.ARPAlcc.todd@LOCUS.UCLA.EDU (Todd Johnson) (03/09/86)
The metric unit for specific impulse is SECONDS! I know, not only did I do all of my engineering degree in a country which has gone metric but I am currently plowing through: Rocket Propulsion & Spaceflights Dynamics by Cornelisse et al. Cornelisse is University of Delft, Holland. In any system specific impulse is defined as: Isp = F dt / m go Sorry about the subscripts. The "go" should be the standard acceleration due to gravity at the equator which is near enought to 9.81 metres per squared second to make no never mind. F is force in newtons, dt is time in seconds, m is mass in kilograms. And the whole mess cancels out to seconds. Specific impulse is a very useful RELATIVE measure of propellants. The true use of specific impulse is for Earth oriented activities - e.g.: getting to Earth orbit or dropping a ballistic missile on your nearest enemy. It relates the expense of lifting the propellant to the capability of the propellant to produce lift. Now that we've got that straightened out, why doesn't someone tell me how the NASA simulations of hypersonic flow through a De Laval nozzle is progressing. I was given to believe that this fundamental work will be applied towards figuring out how to sustain a flame at hypersonic speeds so that we may have scramjets. It is quite likely, however, that the best of this information is classified. If it isn't, how about letting us know?