Re: How do lbs. of thrust equate to in horsepower?

>I saw an add in a magazine for a jet engine with 37 lbs. of thrust,
>for go-karts or small boats etc. so I wanted to know how thrust equates to
>horsepower before I buy it!
                                    
                                            Thank you 
                                                  Brad Bartunek


Thanks for the interesting question. Horsepower is a unit of power (33,000 foot 
pounds/min or 550 ft lb/sec). Power measures amount of work done over a defined 
period.

Pounds of thrust is a measure of force. It is most comparable to the torque 
measurement that is often used to characterize the performance of engines. 
(Force may or may not do any work.) It probably is instructive to take your 
thrust measurement and apply it to a simplified physical situation and see what 
develops. If you have a 400 lb (sometimes a mass-like unit) kart and apply the 
37 lbs of thrust to it, how fast will it go? (You will also need to know the 
time that the rocket motor can burn in order to make any sense out of this.) 

I need to do this in metric units. 400 lb= 400/2.2 kg=181 kg. A pound of thrust 
equals about 4 Newtons. F=ma, and a=F/m,  so assuming a frictionless kart, you 
get an acceleration of 4*37/181 meters/sec_squared. So, at the end of 1 sec you 
would be going .8 m/sec; at the end of 10 sec, 8 m/sec; in 30 sec., 24m/sec. 
Converting back to English units at 30 secs:
  24m/sec= 24/(.447 mile/hr/(meter/sec))=51 mi/hr

Respectable but not a drag racer. How do you control this thing anyway?

This of course is a simplified analysis, since it did not consider frictional 
losses or air resistance. If you think about it a different way, consider how 
much force you could generate pushing a kart. I think it would be in the same 
range as 37 pounds. So back to your original question, it looks to me as though 
this engine, attached to this kart would probably be generating 1/20 to 1/10 hp. 
Hard to tell since the frictional losses may eat up some of that.

Since in real life I am a physician and work for a life insurance company, I 
feel compelled warn you that none of this analysis should be taken as an 
endorsement of any particular method of human propulsion, ESPECIALLY rocket-
powered gokarts. Commonsense would seem to argue against this strategy.

HTH, David.