> >> The unit of effectiveness of a rocket fuel is (arguably) seconds. > >> (thrust.seconds per mass of fuel). > > Isn't thrust a force? If so: Ns/kg =3D kg m^2 s^-3 * s / kg =3D m^2 s^-= 2 ?? > > Oops... brain fart :) > > Ns/kg =3D kg m s^-2 * s / kg =3D m/s, which probably can be interpreted a= s > the final velocity to which a unit of mass can accelerate something (no > gravity, no friction, etc.) "seconds" is arrived at by misuce of units, but it's generally accepted "in the trade". The measure of goodness of a fuel is how many seconds one unit mass will deliver one unit of thrust, So eg if 1 lb of fuel delivers 300 pound.seconds of thrust then the "specific impulse" =3D Isp =3D 300 pound.seconds/pound_of_propellant =3D 300 seconds. Right? Bzzzt. Wrong. The "error" comes from the misuse of "pounds" in day to day use in the imperial system. In any consistent system of units, in a gravity field of g strength, F=3Dg.m (NB F=3Dma) where F is in units of force and m is in units of mass. Joe man in the street has been encouraged to think that the unit of fouce is the pound, which is correct, and that the unit of mass is the pond or pound-mass, which is WRONG. The unit of mass in the fps system is the slug, with a weight of g pounds or about 32 pounds. Asking for a slug of tomatoes, or 1/32nd of a slug of tomatoes or 1/64+ of tomatoes goeth not down well with Mr Green the greengrocer. Along the way the fact that "g" is actually in the equation and has units has gone unnoticed. Plus it all in and we get s x kgf/kgm =3D g.kgm /kgm =3D s.g =3D s.m.S^-2 =3D m/s =3D velocity, a= s Gerhard said. This is known as the fuel's "characteristic velocity", is approximately meaningless while not without meaning , and is a factor od g greater than the Isp. BUT in mks units if Isp =3D 300 (and people don't add "seconds" then characteristic velocity =3D 300 x 9.8 ~=3D 300 x 10=3D 3000 but in fps unit= s it's 300 x ~=3D32 =3D 9600 f/s Which is a shame. Isp is units system independent and also has a very real everyday meaning in terms of payload and thrust and more, whereas characteristic velocity charges with system (and value of g) so you have to know and or quote the units system (or find yourself lithobraking at the darndest of times). So, sometimes "bad units" that everyone understands, can be very useful. Until you arrive near Mars, anyway. Russell McMahon Back to work --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .