General Propulsion Help Support The Rocketry Forum: ttgont

New Member
I have a very general question on propulsion. Assume an engine can deliver 20 Newtons of force for one second (N-s). Converted to pounds of thrust that is approximately 4.5 pounds, if I'm not mistaken. Anyways, does that mean that that engine has the ability to "push" a 4.5 pound mass in a foward (not vertical) direction (ignoring friction) for one second?

Thanks

teflonrocketry1

Well-Known Member
No, it means the motor will deliver a force of 4.5 pounds for one second. If there is no friction the one pound mass would continue moving forever! Another way of saying this is that the motor will push with a force of 4.5 pounds for 1 second.

Hope this helps,

Bruce S. Levison, NAR #69055

Well-Known Member
Originally posted by teflonrocketry1
No, it means the motor will deliver a force of 4.5 pounds for one second. If there is no friction the one pound mass would continue moving forever! Another way of saying this is that the motor will push with a force of 4.5 pounds for 1 second.

Hope this helps,

Bruce S. Levison, NAR #69055
So what you are saying is that it in fact will move the 4.5 "object" but only for one second. Then I suppose, speed comes in to play as to how fast, thus how far the object moved...?

teflonrocketry1

Well-Known Member
I did not say the force would move the object, I said the force will act on the object for one second. If there is no friction or other opposing force the object, it will continue to move forever regardless of its weight. The speed at which the object moves under these conditions will depend on its weight. If there is a frictional force that acts upon the object that is greater than the 4.5 pounds of thrust the object will not move. If the frictional force is less than the 4.5 pound force the object will move the speed and distance depends on the frictional force and the objects weight.

Bruce S. Levison NAR #69055

bobkrech

Well-Known Member
ttgont

How much an object is moved by a force depends on a number of things, but the basic equation is F = m * a or force equals mass multiplied by the acceleration.

Weight is a force. It's a mass x the acceleration of gravity which is 32 ft/sec/sec.

So consider your 4.5 pound-sec engine with a burn time of one second, try to move a 4.5 pound object. What happens will depend both on direction and friction.

Consider the simplest case where the weight is on a frictionless surface (like a skating rink or in space), and let's consider the acceleration that might occur to the object.

With a little rearrangement we get Thurst(#)/Weight(#) = a in units of g so 4.5/4.5 = 1 g

We previously said that 1 g was 32 ft/s/s so again assuming no friction we can use the equations relating acceleration, velocity, distance and time to figure out what's going on.

v = a * t = 32 ft/s/s * 1 s = 32 ft/s so that how fast it's moving at the end of the burn.

d = 1/2 * a * t * t = 0.5 * 32 ft/s/s * 1 s * 1 s = 16 ft so it's move 16 ft in 1 second. Once the motor stops and if there is no other losses such as friction or gravity, then the position as a function of time after the burn is d = do + v * t = 16 ft + (32 ft/s * t s)

1 second after engine burnout it's 48 ft from the starting position, 2 seconds after burn 80 ft. etc.

In the real world we have gravity, so you have to look at the direction of the forces and account for gravity and friction. Forces add vectorally so if you try to lift a 4.5 pound object (like a rocket) with a 4.5 pound thrust motor (a 4.5 pound force) the object goes nowhere because the forces are opposite and exactly cancel. If you took a motor with the same total impulse but with a 0.5 second burn time, the motor's thrust our be 4.5/0.5 or 9 pounds for 1/2 second, and the object would accelerate upward for 1/2 second at 1 g net acceleration.

Rockets should have a 5 to 1 thrust to weight ratio for stable flight, 10:1 is better.

I hope I answered your questions. You might find the following webreferences useful.

Bob Krech

https://www.lerc.nasa.gov/WWW/K-12/airplane/bgmr.html

https://www.lerc.nasa.gov/WWW/K-12/aerores.htm

https://www.apogeerockets.com/education/index.asp