# power conversion

• posted

Hi, I am looking for a way to convert pound seconds to watts or watt hours or some equivalent. every conversion formula I see deals with pound feet rather than time....

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That's because impulse in pound-seconds is a momentum quantity, while pound-feet and watt-seconds are both measurements of energy. There isn't a direct conversion with a single numerical factor.

-dave w

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Keep your units straight... Power is "foot pounds per second"

1.0000000000 ftlb/s = 1.35581794833 Watts or 1.0000000000 Watt = 0.737562149277 ftlb/s

Watt hours deals with Energy, not Power.

Tater Schuld wrote:

• posted
1 pound per sec = 1.355818 watts. Hope this helps
• posted

Are you sure Newton seconds isn't what you're after?

Impulse is the accumulated force applied over a time interval.

Force is measured in pounds in the imperial system, and newtons in the metric system.

A newton is the force required to accelerate one kilogram of mass by one metre per second per second. 1 N = 1 kg x 1 m/s^2

A pound is the force exerted on one slug of mass by the earth's surface when the mass is resting at some particular place on the earth's surface. 1 lb = 1 slug x 32 ft/s^2 (yuch! :-) )

A watt is a joule per second. It is the accumulated energy flow over a time interval. A joule is the amount of energy exerted when a force of one newton is applied over a displacement of one metre. A watt probably won't do you much good for force and impulse conversions.

Conversion?

1 pound of force = 4.44822162 Newtons.

1 pound-second of impulse = 4.44822162 Newton-seconds of impulse

Best regards, Dwayne

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If you'd like to work through the conversion process to figure out how to get from imperial power to metric power, the bare-bones mks unit definition of a Watt is:

1 Watt = 1 kg x 1 m^2 / s^3

1 Watt = 1 newton x 1 metre

1 Watt-hour = 1 newton x 1 metre x 3600 seconds/hour

1 Watt = 1 foot-pound/second x 1.356

Best regards, Dwayne

• posted

Last year sometime, I was asked to do a conversion to energy terms (for comparisions to engine horsepower) to help the layperson get a grasp on the 'enery' involved in some 'big' motors. As others have said, impulse is not directly converted to energy. When I made the conversion last year, I did it based on the rocket performance, where total energy is the sum of the kinetic energy and potential energy. So, E = 1/2mV^2 + mh, where m=mass, V=velocity, and h=height (AGL). It's not perfect, but it'll get you in the ballpark.

For what, it's worth... Chuck

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How does that come about? A Watt is a unit of power. Power is the rate at which work is done divided by the time interval. Work is defined as force * displacement or W = Fs

Since there is no displacement in the rating of a rocket motor, I don't think it is possible to figure the power(and horsepower) of a rocket motor.

Now, if you take an actual rocket flight and analyze the distance the rocket traveled while the motor was boosting, you can figure the power and work, but it will always be different for different sizes, weights, shapes of rockets.

Pax

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I see i need to expand more.

mind you, this is just fantasizing.

X-prize assumes 2 flights in a 2 week period. so *if* I was to clone a full scale merc redstone, I'd need to generate enought H2/LOX to sustain a burn of 78000 pounds for 155 seconds.

in two weeks (ignore the fact that the redstone used kerosene)

ok, not if I wanted to do this the cheap way, I'd use solar panels to electrolyze water into the stuff I need. sunlight averages about 1000 watts per square meter. solar panels are about 10% efficient (so about 100 wats per M^2)

lost yet? keep up, not an electrolyzer is about 5-10% efficient. down to about 10 watts per meter squared.

i am factoring other losses as well, but I was looking for the equivalent wattage of a redstone flight, so I could guestimate the ammount of acreage of solar panels I'd need......

Note: this is just fantasizing, I'd hate to have to invest in that much silicon. maybe a fresnel solar funace to genrate steam?

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for future reference and anyone looking for a great site to convert just about anything....check this out

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That is one foot-pound/sec = 1.355818 Watts.

This reminds me of the nitwits on Mythbusters equating the rocket motors they mounted to a car to an equivalent number of horsepower. Can't be done except in very specific circumstances. An engine running at a given RPM and horsepower will create a certain amount of torque.

Horsepower = 33,013 foot-pound/minute

Once you have the torque you can then compute the amount of force generated where the tires meet the road. (You need to know gear ratios and tire radius.) You can now equate the force generated by the rocket motor to horsepower. But only at a specific RPM, gear ratio, and tire radius.

Dan Cordes wrote: > 1 pound per sec = 1.355818 watts. Hope this helps >

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-- Erik Ebert

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Bertrand R. Brinley in his classic book published a formula for the horsepower generated by a rocket motor:

HP = Vex * F / 375

where Vex, exhaust velocity, is in miles per hour and F, thrust, is in pounds-force. I verified that this formula is physically correct(actually, correct to an error of 0.2 parts per million).

Conversion to standard hobby terms is left to the reader. :-)

Vex = g * Isp

+McG+

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