RussellMc wrote:

>>>> The unit of effectiveness of a rocket fuel is (arguably) seconds.
>>>> (thrust.seconds per mass of fuel).
>=20
>>> Isn't thrust a force? If so: Ns/kg =3D kg m^2 s^-3 * s / kg =3D m^2 s^-=
2
>>> ??
>>=20
>> Oops... brain fart :)=20
>>=20
>> Ns/kg =3D kg m s^-2 * s / kg =3D m/s, which probably can be interpreted
>> as the final velocity to which a unit of mass can accelerate
>> something (no gravity, no friction, etc.)
>=20
> "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,

This definition could serve in SI units just as well, and the unit would
be seconds. But the value, of course, would depend on the units of mass
and thrust one uses.

> 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?
>=20
> Bzzzt.
> Wrong.
> The "error" comes from the misuse of "pounds" in day to day use in the
> imperial system.

> [...]

> Isp is units system independent=20

Bzzt. Wrong again, I think :)

Isp as stated by you ("lbf * seconds / lb" where pound-force and pound
cancel themselves out) requires a unit system where g has a value of 1
(in the chosen unit system's acceleration unit). In this case g would be
1 lbf/lb.=20

(This works of course only if you use lbf/lb as acceleration unit. If
you chose to use ft/s^2, as often the case when using Imperial units, g
doesn't have the value of 1 anymore (~32 ft/s^2) and things get ugly
from there on.)


> ... whereas characteristic velocity charges with system (and value of
> g) so you have to know and or quote the units system=20

You have to do that with Isp just the same, just that the value of g is
"coded" into the units of force and mass, to be 1.

> So, sometimes "bad units" that everyone understands, can be very
> useful.=20

The problem may be the "understanding" part. It took a lot of work of
quite a number of quite smart people to come up with a consistent system
of measurements. Using other systems, one is often required to recreate
parts of their work -- with much less time and much less mental
resources, and the corresponding outcome... :)

> Until you arrive near Mars, anyway.

Exactly... Isp does have a hidden g in its value. It's hidden in the
definition of lbf.

Gerhard
--=20
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