I am NOT a rocket scientist

Pardon my ignorance.

On another forum, I'm looking at thrust curves that express total thrust in lb/ft/sec. I am trying to correlate to the "newtons seconds" measurements common in rocketry

Is the common 'total thrust' figures we see associated with motors expressed in newton second/meter²?

Is it correct that 492 lb/ft/sec = 732 Ns?

Thanks!

Kevin

Motor numbers are expressed in Newton-seconds.
The Total Impulse is measured in "lb-sec." or "N-sec", but there's no feet or meters involved unless your measuring horsepower I guess. Multiply lbs by 4.448 to get newtons. So 492 lb-sec = 492 X 4.448 = 2204 N-sec, K motor.


Steve G

Steve,

That's where I started. Since we're talking about a motor with only 2.5 oz of fuel I knew that assumption was askew. I can't give details outside the research forum, but we're talking black-powder motors. I'm really trying to figure just what these guys are on to.

Performance-wise these seem to be a big G or small H motor - hence my confusion.

Kevin

Nessalco said:

Pardon my ignorance.

On another forum, I'm looking at thrust curves that express total thrust in lb/ft/sec. I am trying to correlate to the "newtons seconds" measurements common in rocketry

Is the common 'total thrust' figures we see associated with motors expressed in newton second/meter²?

Is it correct that 492 lb/ft/sec = 732 Ns?

Thanks!

Kevin

Click to expand...

did you read " lbf-s"?

Thrust=how much force the motor is applying. Thrust is an instantaneous value-it changes continuously with time while the motor is burning, hence "thrust curves" which show the thrust as a function of time. Thrust has units of force, usually N for newtons, or lbf or lb for pounds-force.

Average Thrust is exactly what it sounds like; the average thrust that a motor produces over its burn. It is also measured in units of force.

Impulse, or total impulse, is (somewhat crudely) the amount of momentum that the rocket motor transfers to the rocket. It is a total; a single number that represents how much momentum the rocket will gain. It is equal to the area under the thrust curve. It is measured in units of momentum/impulse, the Ns (Newton*second), or the lbf-s (pounds force * second).

A rocket motor is described by something like this:

1115-J530-15A

The first number, 1115, is the total impulse measured in Ns. The Letter J is the impulse class; it is somewhat redundant as it means only that the total impulse (the first number) is between 640 and 1280 Ns. The number after the letter is the average thrust, and anything after that has to do with the delay time or propellant type. It's thrustcurve has units of force (N of lbf) on the left, and units of time on the bottom.

I'm not sure that the term "total thrust" means anything, and the units you have specified don't really make sense.

There is some confusion there. Pretty sure you can't get 2204 N-sec out of 2.5 ounces of black powder. That would have an ISP of 3149 sec.

Steve G

492 pound-force = 2188.525 newton

1 pound-force = 4.448222 newton
1 newton = 0.224808924 pound-force

Greg

PS. A newton is a force, whereas a newton-second is force over one second. I think of newton-seconds in terms as a "yard stick" to compare other motors, as the total energy for the motor. The average thrust value tells me the average rate the energy is being released.

I'm not a Rocket Scientist Either, but I totally get Newtons. I wish that Thrust was expressed in Pounds, not Newtons. When you hear about a Fighter Jet, and they describe the Max. Thrust in Pounds, your Mind can wrap its Head around it much quicker than when you hear about a Rocket Motor and is Average Newton Seconds or whatever. Stupid Newtons!!!
If I have a Rocket that weighs 1 Pound, I can assume that I will need a Motor that generates a Peak Thrust of at least 4-5lbs. That's easy to figure in my Head. With Newtons, if I have a Rocket that weighs 1 Pound, I can only hope that I have RockSim or similar Program to calculate my Stuff.

TopRamen said:

I'm not a Rocket Scientist Either, but I totally get Newtons. I wish that Thrust was expressed in Pounds, not Newtons. When you hear about a Fighter Jet, and they describe the Max. Thrust in Pounds, your Mind can wrap its Head around it much quicker than when you hear about a Rocket Motor and is Average Newton Seconds or whatever. Stupid Newtons!!!
If I have a Rocket that weighs 1 Pound, I can assume that I will need a Motor that generates a Peak Thrust of at least 4-5lbs. That's easy to figure in my Head. With Newtons, if I have a Rocket that weighs 1 Pound, I can only hope that I have RockSim or similar Program to calculate my Stuff.

Click to expand...

you should ask your manufacturer to put the information on their thrust curve like loki does.
https://lokiresearch.com/Documents/38mm_Blue_instruction_sheet.pdf

I just do a crude 1/4th power. if its 1000n i know its about 250lbs of thrust. this is a good motor for a 25lb rocket at 10:1 liftoff ratio.

Scientificly newtons and meters are better units of measurements. I mean, how do you know your lb is the same as the other guy that's not on the ADVP standard.

I always thought it was this: https://en.wikipedia.org/wiki/Newtons_(cookie) How many pounds can you gain from one box?