Author Topic: Difference between lbf and lbf.s  (Read 1012 times)

Offline Andrew9141

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Difference between lbf and lbf.s
« on: 12/25/2017 11:37 PM »
Whats the difference between pounds of force and pounds of force seconds.

It's my understanding that if you have a rocket that is producing exactly one pound of force thrust then it could hover a one pound object.

I recently just bought a model rocket and it is rated at a maximum thrust of 2.25 pounds force seconds. How can I understand this in a way that I can practically apply it in the example just above? Is there any correlation between the two different thrust measurements or is it like comparing apples and oranges?
« Last Edit: 12/25/2017 11:37 PM by Andrew9141 »

Offline QuantumG

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Re: Difference between lbf and lbf.s
« Reply #1 on: 12/25/2017 11:48 PM »
uhhh imperial units. lbf is a measure of thrust, lbf-s is probably referring to the total impulse.
I hear those things are awfully loud. It glides as softly as a cloud. What's it called? Monowhale!

Offline envy887

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Re: Difference between lbf and lbf.s
« Reply #2 on: 12/26/2017 01:27 AM »
lbf-s is total impulse, that is, the product of average thrust (in pounds-force) and burn time (in seconds).

Offline testguy

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Re: Difference between lbf and lbf.s
« Reply #3 on: 12/26/2017 01:41 AM »
Sorry I just have to but my ballistics hat on.  Total impulse is an integration of thrust from t zero to the time when the thrust returns to zero.  This is total time.  Burn time definition is generally different than total time in that is usually defined from the when thrust achieves 10 percent of maximum ignition thrust to the time when burning ends usually determined by the tangent bisector on the tail off of the thrust curve.  It takes a period of time to vent the combustion chamber of gases so total time and burn time are not one in the same.









Offline strangequark

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Re: Difference between lbf and lbf.s
« Reply #4 on: 12/26/2017 01:45 AM »
lbf.s is a unit of impulse, not force. Without knowing more your motor could be providing 2.25 pounds for one second, or 5.5 pounds for half a second, or 0.00225 pounds for 1000 seconds, or any other combination of force and time that multiplies to 2.25.

Force doesn’t even necessarily need to be constant, in which case the impulse, in lbf.s, would be the integral of thrust over time.

Offline Darren_Hensley

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Re: Difference between lbf and lbf.s
« Reply #5 on: 12/26/2017 08:09 PM »
What size engine will the rocket take?

https://en.wikipedia.org/wiki/Model_rocket_motor_classification

The chart clearly indicates the diameter, Burn duration, and delay in seconds to recovery method deployment(Parachute/streamer)

A "C 6-2" motor is 3/4" dia, burns for 6 seconds, and will wait 2 seconds for inertia bleed off and deploy a separation blast to deploy a parachute.  See Estes model rocket forum.
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