Trying to understand Newton/seconds, Impulse

Help Support The Rocketry Forum:

TangoJuliet

Well-Known Member
I never properly understood it when I was a teenager doing model rocketry, and now as a BAR I want to have a better understanding of it all. I didn't see anything in the sticky threads that really breaks it down stupid style. All of my past experience is in LPR BP motors. I know that some motors have a short burn time, but give a lot of initial oomph, others a longer burn time with less initial oomph but a higher average thrust. Teach me how to read the thrust charts. :confused2:

blackbrandt

That Darn College Student
OK, so let's put it this way.

Newtons (measures thrust): how much push something is putting out at any given point in time. This is what value the blue line has at any point in time.
Seconds (measures time): I hope you get this.
Newton*Seconds (Measures impulse): This is a measure of Thrust TIMES time. Imagine if you had a motor with a thrust of exactly 50 Newtons for exactly 1 second. The total impulse would be 50 Newtons * 1 Second=50 Newton*Seconds. If you like math, this is the integral of the equation that describes the motor burn. If you hate math, it's the total amount of power the motor puts out.
Impulse is pretty much how much total power the motor has.

If this doesn't make sense, please let me know. I'll explain it a different way.

TRF Supporter

TangoJuliet

Well-Known Member
Both of those help quite a bit actually. I do like Math, but it's been decades since I did any "serious" Math :wink:.

Rex R

LV2
one Newton is force required accelerate a one Kg mass by 1 meter per second, a Newton second is force over time, kind of like watts and watt seconds(or kilowatt, kilowatt hour).
Rex

TangoJuliet

Well-Known Member
Oh geez! Don't go throwing electrical terminology in there! I know even less about that!

Bat-mite

Rocketeer in MD
Even simpler...

Why do I care about thrust? Because thrust is what lifts the rocket. It must exceed the force of gravity on the rocket it make it go up. We recommend a 5-to-1 thrust-to-weight ratio for any rocket to ensure that it goes "up" and not "sideways." Make sense?

Why do I care about impulse? Because it determines how high the rocket will go (presuming that it is going "up"). As mentioned, impulse is the product of thrust times seconds. So, as Matt wrote, 50 Newtons for 1 second is an impulse of 50 Newton-seconds. But 50 Newtons for 3 seconds is 150 Newton-seconds. Got it? Obviously if you apply 50 Newtons of thrust for three times as long, then you are going to go much higher (not necessarily three times higher, but definitely higher).

Now, back to the thrust-to-weight ration. 5-to-1 means if your rocket weighs 5 pounds, you needs 25 pounds of thrust. But pounds are not Newtons, right? So you have to convert. It just so happens that one pound of thrust = 4.44822 Newtons. So, if I have a rocket that weighs five pounds (fully loaded with motor and parachute, ready to launch), then I need a minimum thrust of 22.2411425 Newtons (let's call it 23 Newtons).

With me so far? Good. So, an E30 (which has 30 Newtons of thrust) can safely list this rocket. So can an F40, and a G54.

(Quiz: could an E22 safely lift this rocket? Probably, but we were looking for a minimum of 23 Newtons, so we'll say no.)

Now, those letters in front of the thrust number indicate a range of total impulse. In simplest terms, an F will have twice as much total impulse as an E, and a G will have twice as much as an F. (Not hard and fast, as these are "ranges" and not "numbers.")

So, if I had my five-pound rocket, and I had the following three motors, which one would most likely go the highest?

F120, G80, H50

Did you say the H53? Then you are right, because it has a total impulse in the H range, which is twice the G range and four times the F range.

If you said the F120, because 120 > 80 > 53, then you failed, because those are the thrust numbers, not the impulse numbers.

If you have followed me, then you have probably realized that an H53 is a very long-burning motor, whereas an F120 is a very fast-burning motor. Let's look at the actual burn times and total impulse numbers.

The F120 burns at 120 Newtons of thrust for 0.45 seconds, which produces a total impulse of 56.0 N-s.

The G80 (the CTI one, not the AT SU) burns at 80 Newtons of thrust for 1.15 seconds, which produces a total impulse of 92.9 N-s.

The H53 burns at 53 Newtons of thrust for 4.44 seconds, which produces a total impulse of 234.2 N-s.

This is all WAY oversimplified, but gives you the basics. Be careful of the pitfall of thinking that if it flew on an F, then it will fly on a G. Impulse does not determine safe liftoff weight -- that is thrust. So, just because an F40 will lift a rocket safely, it doesn't mean that a G20 will lift it. But if both motors can safely lift it, then the G20 will go twice as high as the F40.

Fire away with questions.

Roger Smith
An (only slightly) more technical explanation is at:

https://www.nakka-rocketry.net/impcalc.html

BTW, "playing around" with model rockets as a kid gave me an introduction to things like Calculus. When we say that the Total Impulse is the total power of the rocket, or that it is the area under the time-thrust curve, we are talking about the integral of the thrust over time. Integrals are things you learn early in Calculus classes and are much easier to understand if you've already been exposed to them through rocketry.

After integrals, you learn about derivatives in Calculus. Again, exposure to rocketry helps. A derivative describes a rate of change. A teacher will use the slope of a curve at a specific point as an example of a derivative. But, any rocketeer knows that acceleration is a better example. Acceleration is a measure of the rate of change of velocity. It is the derivative of velocity. Acceleration will change during a rocket's flight. The change is acceleration is the derivative of the acceleration and the second derivative of velocity.

Add up the acceleration over time and you get the resulting velocity. Sound familiar? Taking the integral of acceleration over time gives you velocity.

It seems a little less obvious, but velocity is the derivative of position. It's a measure of the rate of change of position. If your velocity is zero, your position doesn't change. If your velocity is non-zero, then your position is changing. Add up your velocity over time (calculate the integral) to determine you final position.

Okay ... there you go. All you need to know to pass Calc I.

-- Roger
Everything I Needed to Know about Calculus, I learned From Rocketry

tomsteve

Well-Known Member
one Newton is force required accelerate a one Kg mass by 1 meter per second, a Newton second is force over time, kind of like watts and watt seconds(or kilowatt, kilowatt hour).
Rex
and here I thought a newton was the energy necessary to lift a ton of newts one meter off the ground!

Last edited:

TangoJuliet

Well-Known Member
"So, if I had my five-pound rocket, and I had the following three motors, which one would most likely go the highest?

F120, G80, H50

Did you say the H53? Then you are right, because it has a total impulse in the H range, which is twice the G range and four times the F range.

If you said the F120, because 120 > 80 > 53, then you failed, because those are the thrust numbers, not the impulse numbers.

If you have followed me, then you have probably realized that an H53 is a very long-burning motor, whereas an F120 is a very fast-burning motor. Let's look at the actual burn times and total impulse numbers.

The F120 burns at 120 Newtons of thrust for 0.45 seconds, which produces a total impulse of 56.0 N-s.

The G80 (the CTI one, not the AT SU) burns at 80 Newtons of thrust for 1.15 seconds, which produces a total impulse of 92.9 N-s.

The H53 burns at 53 Newtons of thrust for 4.44 seconds, which produces a total impulse of 234.2 N-s.

This is all WAY oversimplified, but gives you the basics. Be careful of the pitfall of thinking that if it flew on an F, then it will fly on a G. Impulse does not determine safe liftoff weight -- that is thrust. So, just because an F40 will lift a rocket safely, it doesn't mean that a G20 will lift it. But if both motors can safely lift it, then the G20 will go twice as high as the F40
."

That is what was missing from my complete understanding! Thank you!

TangoJuliet

Well-Known Member
Ah, Calculus... The only higher Math I didn't take in High School! But model rocketry taught me how to determine altitude using Trigonometry long before I learned it in school.

Bat-mite

Rocketeer in MD
Very glad to help. I wouldn't know any of that if no one had helped me.

blackjack2564

Crazy Jim's Gone Banana's
TRF Supporter
Oh geez! Don't go throwing electrical terminology in there! I know even less about that!
LOL....lol:

as a footnote, I was in Australia last year. All cars are rated in joules rather than horsepower. So a Brand x would be something like 45,000 joules when looking at engine ratings. Most everything there get joules ...even light bulbs!

TangoJuliet

Well-Known Member
Now... Where to gain basic electrical knowledge? :confused2: I was a Crew Chief in the USAF and learned at one time how to use a multi-meter, but that was 27 years ago when I learned, and we rarely used one for anything more than an occasional continuity check on B-52 Starter Cartridge Domes. In lieu of a ground electrical power unit (GPU), we would use a Black Powder Cartridge to spin up the engines to start them - called a "Cart Start".

scsager

Slightly burned-out old guy
Even simpler...

Why do I care about thrust? Because thrust is what lifts the rocket. It must exceed the force of gravity on the rocket it make it go up. We recommend a 5-to-1 thrust-to-weight ratio for any rocket to ensure that it goes "up" and not "sideways." Make sense?

Why do I care about impulse? Because it determines how high the rocket will go (presuming that it is going "up"). As mentioned, impulse is the product of thrust times seconds. So, as Matt wrote, 50 Newtons for 1 second is an impulse of 50 Newton-seconds. But 50 Newtons for 3 seconds is 150 Newton-seconds. Got it? Obviously if you apply 50 Newtons of thrust for three times as long, then you are going to go much higher (not necessarily three times higher, but definitely higher).

Now, back to the thrust-to-weight ration. 5-to-1 means if your rocket weighs 5 pounds, you needs 25 pounds of thrust. But pounds are not Newtons, right? So you have to convert. It just so happens that one pound of thrust = 4.44822 Newtons. So, if I have a rocket that weighs five pounds (fully loaded with motor and parachute, ready to launch), then I need a minimum thrust of 22.2411425 Newtons (let's call it 23 Newtons).

With me so far? Good. So, an E30 (which has 30 Newtons of thrust) can safely list this rocket.
So can an F40, and a G54.

(Quiz: could an E22 safely lift this rocket? Probably, but we were looking for a minimum of 23 Newtons, so we'll say no.)

Now, those letters in front of the thrust number indicate a range of total impulse. In simplest terms, an F will have twice as much total impulse as an E, and a G will have twice as much as an F. (Not hard and fast, as these are "ranges" and not "numbers.")

So, if I had my five-pound rocket, and I had the following three motors, which one would most likely go the highest?

F120, G80, H50

Did you say the H53? Then you are right, because it has a total impulse in the H range, which is twice the G range and four times the F range.

If you said the F120, because 120 > 80 > 53, then you failed, because those are the thrust numbers, not the impulse numbers.

If you have followed me, then you have probably realized that an H53 is a very long-burning motor, whereas an F120 is a very fast-burning motor. Let's look at the actual burn times and total impulse numbers.

The F120 burns at 120 Newtons of thrust for 0.45 seconds, which produces a total impulse of 56.0 N-s.

The G80 (the CTI one, not the AT SU) burns at 80 Newtons of thrust for 1.15 seconds, which produces a total impulse of 92.9 N-s.

The H53 burns at 53 Newtons of thrust for 4.44 seconds, which produces a total impulse of 234.2 N-s.

This is all WAY oversimplified, but gives you the basics. Be careful of the pitfall of thinking that if it flew on an F, then it will fly on a G. Impulse does not determine safe liftoff weight -- that is thrust. So, just because an F40 will lift a rocket safely, it doesn't mean that a G20 will lift it. But if both motors can safely lift it, then the G20 will go twice as high as the F40.

Fire away with questions.

Are you saying an E30 meets the 5:1 thrust ratio rule-of-thumb for a five pound rocket???

Tonimus

Well-Known Member
He missed a 5. 22.25ish newtons for 5 pounds of force, times 5 for 5:1 thrust ratio. 111.25 newtons.

dhbarr

Amateur Professional
He missed a 5. 22.25ish newtons for 5 pounds of force, times 5 for 5:1 thrust ratio. 111.25 newtons.
I just do weight-in-pounds times 25. 3 lb rocket? Look for 75ish. Got a something-50? Two pounds max takeoff weight, friendo.

You can dice it more finely ( do the actual conversion incl. dry weight and motor pre-burn; look at peak thrust ), but that plus one-caliber-unless-fat should get you in the ballpark for most anything up to lvl1.

I look forward to a detailed explanation of what I've misstated

Tonimus

Well-Known Member
I just do weight-in-pounds times 25. 3 lb rocket? Look for 75ish. Got a something-50? Two pounds max takeoff weight, friendo.

You can dice it more finely ( do the actual conversion incl. dry weight and motor pre-burn; look at peak thrust ), but that plus one-caliber-unless-fat should get you in the ballpark for most anything up to lvl1.

I look forward to a detailed explanation of what I've misstated
Is friendo the same thing as amigo? And what does me being fat have to do with motor calculations?

dhbarr

Amateur Professional
Is friendo the same thing as amigo? And what does me being fat have to do with motor calculations?
:-D

jeff_j_black

Well-Known Member
Yeah I wouldn't pick an E30 for a 5 pound rocket. 1.5 pounds maybe?

noffie79

Well-Known Member
I just do weight-in-pounds times 25. 3 lb rocket? Look for 75ish. Got a something-50? Two pounds max takeoff weight, friendo.

You can dice it more finely ( do the actual conversion incl. dry weight and motor pre-burn; look at peak thrust ), but that plus one-caliber-unless-fat should get you in the ballpark for most anything up to lvl1.

I look forward to a detailed explanation of what I've misstated
Now THAT is a neat little trick! I'm using it.

BDB

Absent Minded Professor
The other thing that always gets confusing in these conversations are the English and metric units: pounds vs. Newtons. Thrust is a force (F=ma), so the Newton is a better way to think about it because 1 Newton is the force required to accelerate one kg (mass) at a rate increasing by 1 m/s^2 (acceleration). It gets complicated when we talk about thrust in pounds because it is so easy to confuse force with weight. The use of pounds to describe both a mass and a force is an example of the stupidity of the English system of units.

I'm a scientist, so I'm a total shill for the metric system, but in this case I think it really is less confusing. [Though I completely agree that multiplying weight (in lbs) by 25 to get the minimum thrust (in Newtons) is a useful calculation for in-field motor selection.]

Dave A

The other thing that always gets confusing in these conversations are the English and metric units: pounds vs. Newtons. Thrust is a force (F=ma), so the Newton is a better way to think about it because 1 Newton is the force required to accelerate one kg (mass) at a rate increasing by 1 m/s^2 (acceleration). It gets complicated when we talk about thrust in pounds because it is so easy to confuse force with weight. The use of pounds to describe both a mass and a force is an example of the stupidity of the English system of units.

I'm a scientist, so I'm a total shill for the metric system, but in this case I think it really is less confusing. [Though I completely agree that multiplying weight (in lbs) by 25 to get the minimum thrust (in Newtons) is a useful calculation for in-field motor selection.]
No need for conversions.
Total weight of rocket w/ motor (lbs.) × 22.5 = min avg thrust in Newtons.
Short, stubby or less aerodynamic rockets may require a little more "umpf" . That's a technical term for more AP!

Bat-mite

Rocketeer in MD
Are you saying an E30 meets the 5:1 thrust ratio rule-of-thumb for a five pound rocket???
Woops! I converted pounds to Newtons and forgot to multiply by five. D'OH!

So, everything in my post was for a one-pound rocket.

Bat-mite

Rocketeer in MD
Woops! I converted pounds to Newtons and forgot to multiply by five. D'OH!

So, everything in my post was for a one-pound rocket.
This teaches two things:
2. Never trust anybody from Maryland. :wink:

dr wogz

Fly caster
Even simpler...
Now, those letters in front of the thrust number indicate a range of total impulse. In simplest terms, an F will have twice as much total impulse as an E, and a G will have twice as much as an F. (Not hard and fast, as these are "ranges" and not "numbers.")

The F120 burns at 120 Newtons of thrust for 0.45 seconds, which produces a total impulse of 56.0 N-s.

The G80 (the CTI one, not the AT SU) burns at 80 Newtons of thrust for 1.15 seconds, which produces a total impulse of 92.9 N-s.

The H53 burns at 53 Newtons of thrust for 4.44 seconds, which produces a total impulse of 234.2 N-s.
And one might wonder.. where did you get 0.45 seconds burn time (F120) and 1.15s burn time (G80) and...
(Divide the total impulse by the average thrust for the burn time, but you need to know the total impulse of that particular motor first.. )

One of my biggest issues is with the Aerotech catalogue. They could do a better job at explaining this.. They talk about [letter] motors, and mention (only once) the lb vs N conversion, but then list all their motor graphs (thrust over time) in lbs, not Newtons, but list their total impulse in N-s not lb-s on the charts.. So, it's kinda hard to correlate the two for a newbie; a conversion is needed.. The graphs make sense, the numbers on the motors don't.. (If they add the N equivalent to the graphs, that might help correlate the two..) All Aerotech motors come with a thrust curve graph..

Bat-mite

Rocketeer in MD
And one might wonder.. where did you get 0.45 seconds burn time (F120) and 1.15s burn time (G80) and...
(Divide the total impulse by the average thrust for the burn time, but you need to know the total impulse of that particular motor first.. )

One of my biggest issues is with the Aerotech catalogue. They could do a better job at explaining this.. They talk about [letter] motors, and mention (only once) the lb vs N conversion, but then list all their motor graphs (thrust over time) in lbs, not Newtons, but list their total impulse in N-s not lb-s on the charts.. So, it's kinda hard to correlate the two for a newbie; a conversion is needed.. The graphs make sense, the numbers on the motors don't.. (If they add the N equivalent to the graphs, that might help correlate the two..) All Aerotech motors come with a thrust curve graph..
Burn times were pulled from the thrust curves at CTI's site.

dr wogz

Fly caster
To me, that is the one piece of the puzzle that is missing. It's not readily apparent that you need / are missing a value. Makes it all fall into place.

blackbrandt

That Darn College Student
The other thing that always gets confusing in these conversations are the English and metric units: pounds vs. Newtons. Thrust is a force (F=ma), so the Newton is a better way to think about it because 1 Newton is the force required to accelerate one kg (mass) at a rate increasing by 1 m/s^2 (acceleration). It gets complicated when we talk about thrust in pounds because it is so easy to confuse force with weight. The use of pounds to describe both a mass and a force is an example of the stupidity of the English system of units.

I'm a scientist, so I'm a total shill for the metric system, but in this case I think it really is less confusing. [Though I completely agree that multiplying weight (in lbs) by 25 to get the minimum thrust (in Newtons) is a useful calculation for in-field motor selection.]
Former AP Physics student here.... Thrust, weight, and force are all in the same units. Pounds are a unit of force. They are just the imperial version of force. The imperial unit of mass is a slug. 1 pound is the force required to accelerate a 1 slug object at 1 foot per second squared.
I think the metric system is nice. It's a lot prettier.

EDIT: Thanks to NYTrunner for calling me out on that. It's 1 foot per second squared, not 32.

Last edited:

BDB

Absent Minded Professor
Former AP Physics student here.... Thrust, weight, and force are all in the same units. Pounds are a unit of force. They are just the imperial version of force. The imperial unit of mass is a slug. 1 pound is the force required to accelerate a 1 slug object at 32 feet per second squared.
I think the metric system is nice. It's a lot prettier.
Blackbrandt is absolutely right. All I'm saying is mass and weight are too easily confused when talking about pounds. When's the last time anyone reported reported the mass of his/her rocket in slugs?

In retrospect, maybe the best lesson from this thread is "Don't trust anyone from Maryland." (Though maybe I'll meet some of you MDRA guys next year at LDRS.)