2003-02-18
Blumrich's book and In Search of Ancient Gods were written
in the 70s. Even though SI was around since 1960, most engineers at the
time did not use it. They still used "technical" metric, where the
kilogram was both a unit of force and mass. The point is confusion still
exists when both the kilogram is used as a force unit and a mass
unit.
That may or may not be the source of any error.
This site disagrees with your definition of specific
impulse. The last section states:
The specific impulse is:
Isp = ueq/ge
where
Isp = specific impulse
ueq = total impulse / mass of expelled propellant
ge = acceleration at Earth's surface (9.8 m/s2)
And since we are approximating the speed of a gas with a constant velocity; the momentum of the escaping gas is:
p = mv
where
p = momentum (kg m/s)
m = mass (kg)
v = velocity (m/s)
Notice how masses cancel out and therefore we the Isp is just the velocity of the exhaust gas (Ve) divided by the gravitational attraction of the Earth (ge).
If the velocity of the gas is in metres per second, and
that is divided by the gravitational attraction in metres per second squared,
then the resultant unit is seconds.
Am I missing something here?
John
----- Original Message -----
From: "Gene Mechtly" <[EMAIL PROTECTED]>
To: "U.S. Metric Association" <[EMAIL PROTECTED]>
Cc: "U.S. Metric Association" <[EMAIL PROTECTED]>
Sent: Monday, 2003-02-17 19:24
Subject: [USMA:24885] Re: Nasa's Germans used
metric
> > ... Blumrich is quoted in the book In Search of
> > Ancient Gods ...
> > "... I discovered that Ezekiel's spaceship has very
> > credible ...
> > Specific impulse = 2,080 sec ...
>
> Blumrich's unit (sec) is an error. The SI unit of specific impulse is
> the same as exhaust velocity, meter per second (m/s), not seconds.
>
> The error arises from dividing pounds force by pounds mass without
> proper distinction between force and mass. The error: pound/pound = 1.
>
> > "... Weight of construction = 63,300 kg..."
>
> Once again, Blumrich fails to properly treat the kilogram as the SI
> unit of "mass". The newton is the SI unit of "weight" (a force, as are
> weight's companion forces; lift, thrust, and drag in aerospace dynamics).
>
> Blumrich would be well advised to stick to questions of structural designs
> and not to dabble in problems of aerospace dynamics.
>
> Gene.
>
