isp from MKS units

I have a question that may have an obvious answer but I have lost too
many brain cells for it to be obvious to me.


isp is given as the thrust produced/quantity of fuel/sec used.
Pounds/(Pounds/sec) gives units of seconds for ISP. Do the same in
MKS (metric) units and you get:

Newtons/(Kg/sec)=Kg*m/sec^2/(Kg/sec)=meter*seconds.

Why do I never see isp expressed in MKS units?

"Parallax" wrote...

Why do I never see isp expressed in MKS units?

Do a Google search on (Ns/kg "specific impulse") and you'll see plenty of
examples.

Jim Davis

In article , Parallax wrote:
I have a question that may have an obvious answer but I have lost too
many brain cells for it to be obvious to me.


isp is given as the thrust produced/quantity of fuel/sec used.
Pounds/(Pounds/sec) gives units of seconds for ISP. Do the same in
MKS (metric) units and you get:

Newtons/(Kg/sec)=Kg*m/sec^2/(Kg/sec)=meter*seconds.

Why do I never see isp expressed in MKS units?

First off, your units are wrong in your MKS equation: it should have come
out m/sec, not m*sec ....

Next, the "English" Isp uses pounds mass, not pounds force. So the
equivalent metric version would be Kg/(Kg/sec) rather than Newtons;
resulting in "seconds" as the unit.

Or, alternately, you *ALWAYS* see Isp expressed in MKS units - it's
what the "S" in "MKS" is.

In article ,
Parallax wrote:
isp is given as the thrust produced/quantity of fuel/sec used.
Pounds/(Pounds/sec) gives units of seconds for ISP. Do the same in
MKS (metric) units and you get:
Newtons/(Kg/sec)=Kg*m/sec^2/(Kg/sec)=meter*seconds.
Why do I never see isp expressed in MKS units?

If you read European papers, you will sometimes see it written that way.
(Computed that way, it is the effective exhaust velocity of the engine.)

However, there is some case for keeping Isp in seconds regardless, even
though that means introducing an arbitrary factor of g (9.81m/s^2) into
the way it's computed in metric units. Engineering figures of merit are
often computed in fairly arbitrary ways, and consistency -- preserving the
ability to compare results -- is more important than strict ideological
purity or direct relation to physical properties. (Consider EPA mileage
ratings, which use a test procedure that is now known to be overly
optimistic in predicting real mileage, but which continue to use that
procedure because it gives a consistent basis for comparison.)

Isp in seconds also has the incidental advantage that it is independent of
the choice of unit systems.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

In article ,
Michael K. Heney wrote:
Next, the "English" Isp uses pounds mass, not pounds force. So the
equivalent metric version would be Kg/(Kg/sec) rather than Newtons;
resulting in "seconds" as the unit.

Proper metric practice never, ever uses the kilogram as a unit of force --
it is always mass. Just because some versions of the Imperial units are
stupid about this is no reason to repeat the mistake.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

I wrote:
Newtons/(Kg/sec)=Kg*m/sec^2/(Kg/sec)=meter*seconds.
Why do I never see isp expressed in MKS units?

If you read European papers, you will sometimes see it written that way.

Except that, of course, I missed the use of "*" instead of "/" as it
should be. Oops.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

Parallax wrote:

I have a question that may have an obvious answer but I have lost too
many brain cells for it to be obvious to me.

isp is given as the thrust produced/quantity of fuel/sec used.
Pounds/(Pounds/sec) gives units of seconds for ISP. Do the same in
MKS (metric) units and you get:

Newtons/(Kg/sec)=Kg*m/sec^2/(Kg/sec)=meter*seconds.

Why do I never see isp expressed in MKS units?

Newtons per (kilogram-per-second) to me seems to boil down thus:
kg m s^(-2) per (kg s^(-1)) ---
kg m s^(-2) times (s kg^(-1)) ---
m s^(-1),

not metre-seconds as you have it.
But metres-per-second is the dimension of the effective jet velocity,
aka the propellant expulsion speed.
It probably has several other names.

I'm curious whether any merit has been seen
in reusable first stages that lift straight up just a few km,
then fall straight down again, using half their delta 'V'
to match speed with the ground just as they get to it,
maybe decelerating a little later and more roughly
if at their apogee they weren't able to get rid of the upper stages.
Expelling a whole lot of very cool reaction mass.


--- Graham Cowan
http://www.eagle.ca/~gcowan/boron_blast.html --
fireproof fuel, real-car range, no emissions

(Parallax) wrote in message . com...
I have a question that may have an obvious answer but I have lost too
many brain cells for it to be obvious to me.


isp is given as the thrust produced/quantity of fuel/sec used.
Pounds/(Pounds/sec) gives units of seconds for ISP. Do the same in
MKS (metric) units and you get:

Newtons/(Kg/sec)=Kg*m/sec^2/(Kg/sec)=meter*seconds.

Why do I never see isp expressed in MKS units?

your missing g. Basicaly it all cancels out and your left with
seconds. Its a funny way of doing things, but its the way its done.

The effective exit velocity is Ve=Isp*g -- in the units used for g. ie
if you use MKS for g then Ve is in m/s

Greg

Parallax wrote

I have a question that may have an obvious answer but I have lost too
many brain cells for it to be obvious to me.


isp is given as the thrust produced/quantity of fuel/sec used.
Pounds/(Pounds/sec) gives units of seconds for ISP. Do the same in
MKS (metric) units and you get:

Newtons/(Kg/sec)=Kg*m/sec^2/(Kg/sec)=meter*seconds.

Why do I never see isp expressed in MKS units?

You forgot g, the attraction due to gravity. It is there to change kgf to
Newtons. Isp is simply the time one kilo/lb of fuel will produce one kilo/lb
of force.

The dimension of Isp is time. Makes sense when you think about it, a big Isp
will provide the same force for longer.

The unit of Isp is typically the second. The second is a MKS unit as well
as a unit in many other systems.

Isp is the force generated times the time the force is generated divided by
(g times the fuel used).

It works out like this:

Force * time / (g * mass)

Eg kg m s^-2 * s /(m s^-2 * kg)
= kg m s^-2 s m^-1 s^2 kg^-1
= s


--
Peter Fairbrother

"It's all right in practice,
but in theory it'll never work."

(Henry Spencer) writes:

I wrote:
Newtons/(Kg/sec)=Kg*m/sec^2/(Kg/sec)=meter*seconds.
Why do I never see isp expressed in MKS units?

If you read European papers, you will sometimes see it written that way.

Except that, of course, I missed the use of "*" instead of "/" as it
should be. Oops.

Boy, you're really racking up those ICH T-shirts, aren't you. I can't
figure out if that means you're having a good day or a bad day.