isp from MKS units

(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?

A pet peeve. I always use m/s for specific impulse:
specific impulse (Ns/kg = kg m/s^2 x s / kg = m/s).

The unit of impulse is force x time;
the unit of specific impulse is force x time per unit mass.

In English units the correct units are ft/sec, or
lbf x sec / lbm =
(slugs x (ft/sec^2)x sec/lbm) x 32.174 lbm /slug
= ft/sec

Thus 300 lbf/lbm/sec
= 9652 ft/sec
or 2942 m/s
= 300 kgf/kgm/s (kgf is a ******* term, as Henry
points out.

I guess people have found "sec" convenient, since it appears
to be the same number in both English and metric units.
However, this usage is wrong and has caused serious engineering
mistakes.

Best regards,
Len (Cormier)
PanAero, Inc.
(substitute len for x)
( http://www.tour2space.com )

(Henry Spencer) writes:

Isp in seconds also has the incidental advantage that it is independent
of the choice of unit systems.

What if one is working in the "Furlong / Fortnight / Stone" system? :-I


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

"Len" wrote in message
om...
(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?

A pet peeve. I always use m/s for specific impulse:
specific impulse (Ns/kg = kg m/s^2 x s / kg = m/s).

The unit of impulse is force x time;
the unit of specific impulse is force x time per unit mass.

In English units the correct units are ft/sec, or
lbf x sec / lbm =
(slugs x (ft/sec^2)x sec/lbm) x 32.174 lbm /slug
= ft/sec

Thus 300 lbf/lbm/sec
= 9652 ft/sec
or 2942 m/s
= 300 kgf/kgm/s (kgf is a ******* term, as Henry
points out.

I guess people have found "sec" convenient, since it appears
to be the same number in both English and metric units.
However, this usage is wrong and has caused serious engineering
mistakes.

This is one of those things that has been brought up and argued
hundreds of times since I have been reading these groups. You
and most of the people I respect here seem to be in agreement
on this matter. Due to a self inflicted education in this subject,
I tend to think in seconds for specific impulse.

Could you explain again, in small words and short sentences,
what problems are caused by using seconds for specific impulse.
The way I think, it is difficult for me to want to change units
unless I have a clear understanding of why I should want to do so.
In this business, as in my day job, consistently using an amatuerish
definition brands one as ignorant to the pros. I wouldn't mind being
able to hide it a bit better.

Best regards,
Len (Cormier)
PanAero, Inc.
(substitute len for x)
( http://www.tour2space.com )

In article ,
Gordon D. Pusch wrote:
Isp in seconds also has the incidental advantage that it is independent
of the choice of unit systems.

What if one is working in the "Furlong / Fortnight / Stone" system? :-I

Then one deserves what one gets. :-)
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

(Henry Spencer) wrote in
:

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.

Imperial units aren't the only ones stupid about this. I see plenty of
references to kgf in spacecraft specs, especially Russian ones.

--
JRF

Reply-to address spam-proofed - to reply by E-mail,
check "Organization" (I am not assimilated) and
think one step ahead of IBM.

In article , Gordon D. Pusch wrote:
(Henry Spencer) writes:

Isp in seconds also has the incidental advantage that it is independent
of the choice of unit systems.

What if one is working in the "Furlong / Fortnight / Stone" system? :-I

FFS...

--
-Andrew Gray

"johnhare" wrote in message .com...
"Len" wrote in message
om...
(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?

A pet peeve. I always use m/s for specific impulse:
specific impulse (Ns/kg = kg m/s^2 x s / kg = m/s).

The unit of impulse is force x time;
the unit of specific impulse is force x time per unit mass.

In English units the correct units are ft/sec, or
lbf x sec / lbm =
(slugs x (ft/sec^2)x sec/lbm) x 32.174 lbm /slug
= ft/sec

Thus 300 lbf/lbm/sec
= 9652 ft/sec
or 2942 m/s
= 300 kgf/kgm/s (kgf is a ******* term, as Henry
points out.

I guess people have found "sec" convenient, since it appears
to be the same number in both English and metric units.
However, this usage is wrong and has caused serious engineering
mistakes.

This is one of those things that has been brought up and argued
hundreds of times since I have been reading these groups. You
and most of the people I respect here seem to be in agreement
on this matter. Due to a self inflicted education in this subject,
I tend to think in seconds for specific impulse.

Could you explain again, in small words and short sentences,
what problems are caused by using seconds for specific impulse.
The way I think, it is difficult for me to want to change units
unless I have a clear understanding of why I should want to do so.
In this business, as in my day job, consistently using an amatuerish
definition brands one as ignorant to the pros. I wouldn't mind being
able to hide it a bit better.

Hi John:

One example that I remember is an early version of a
trajectory program that is now widely used. This earlier
version indicated that an airbreathing, airlifting booster
was getting to mach 3 with only a 3 percent expenditure of
fuel. As a consultant I said, no way. Then they (NASA)
found a mistake in the program in which the fuel flow was
off by a factor of, you guessed it, "g."

There is no problem with a well established erroneous term
--as long as you always remember that it is erroneous. I
don't expect to change this bad usage. I just want to sound
a note of caution to those who may not realize the potential
pitfalls of bad usage.

I'll have to admit that it is very convenient to divide
lbf by sec to get lbm/sec -- or kgf by s to get kg/s.
It sometimes seems superfluous to multply lbf or kgf by
"g" to get legitimate units of force--and then multiply
the widely used term of "sec" to get legitimate units
of specific impulse. As long as you realize what you
are doing, not bothering to multiply both numberator and
denominator by "g" seems like a reasonable shortcut.

Best regards,
Len (Cormier)
PanAero, Inc.
(substitute len for x)
( http://www.tour2space.com )