Impulse/ neuton seconds

[scottluther1369]

I'm having a a hard time understanding the initial "moment of impulse on and engine" say C - 6 - 5. then the average thrust in neuton seconds? They are both meausre in Neuton Seconds. What is ithe "moment of impulse"? Can someone explan these topics to me like the kinder gardener my brain feels like? : )
thanks!
--Scott <the newbie)

 

[rstaff3]

I'm confused a bit, but here is a summary

The C indicates the range of total impulse, being between 5.01 and 10.0 Newton-seconds. An Estes C6 has 8.62 N-sec

The 6 is the average thrust during the burn, in newtons.

The 5 is the delay after the propellant has burned.

Burn time can be found by dividing the total impulse by the average impulse.

 

[powderburner]

Scott, I don't mean to insult by going too far toward the 'kindergarden' end of things, but you asked----

If you start with a time-thrust plot for a particular rocket motor, the 'convention' for graphing this data is to put thrust on the vertical axis and time on the horizontal axis.

The motor starts at zero thrust (the instant before ignition), increases to a point of maximum thrust (rarely do our hobby motors stay at that point for any significant duration), drop back down to an intermediate level of thrust for maybe half of the burn time, and finally drop off to zero thrust again as the delay charge is ignited. All very simple stuff, we've all seen the charts.

Impulse is the measure of the area under the thrust plot. This gets complicated if you want to use calculus to create the perfect mathematical description of these plotted curves, and integrate to calculate a mathematically perfect value for the area under the curve. Me, I am too lazy to go through all that, especially when our little blackpowder hobby motors are said to vary in performance by as much as + or - 20 percent. (Who cares about a 'perfect' calculation?)

You can come plenty close enough to measuring impulse by splitting up the burn time into small increments, drawing a line from the horizontal axis up to the plotted thrust data, and making note of the measured thrust at the beginning and end of each time increment (are you still with me?). Average those thrust values, multiply by the time increment, and add that area under the curve to your running total. When you get to the right-hand side of the thrust plot you will have a close estimate of the area under the curve. This is the actual impulse of the motor.

If the motor went instantly to a nominal level of thrust, stayed at that exact thrust level for a time, then stopped instantly and dropped back to zero thrust, your thrust plot would look like a rectangle. The vertical height (thrust, in pounds or Newtons) is multiplied by the horizontal length (time, in seconds) to get the area included under the thrust plot (impulse, in pound-seconds or Newton-seconds).

Note also that many of the blackpowder motors out there do not deliver the full impulse of their classification. That is, a "B" motor might deliver an actual impulse of way less than 5.0 Newton-seconds. It can produce anything between 2.5 and 5.0 and the vendor can properly label it a "B" even if it only produces 2.6 N-s. So you really have to get familiar with the actual power ratings (identified from instrumented test-stand firings) to understand and properly select your motors. The NAR website has a bunch of this info.

Does that answer your qstn?

 

[seo]

Powder-


Excuse me if I am overestimating your age, but....

Remember in college when we would calculate the area under the curve by weight? We would cut out the curve and compare it to the total weight of the graph?

Guess those things called compooters or HP calcultors do it now

Scott

 

[powderburner]

And we used it in lots of science classes . . . when we weren't fighting off the sabre-tooth tigers and trying to make fire.

As for my age, I went to college in the midst of the advent of personal calculators. Mind you, the calculators that cost $400-500 *then* you can buy for $2 today, but it was hot stuff at that time.

I also (painfully) remember spending waaaay too many hours in the 'slide-rule' classes, held in the dumpy old temporary buildings on the back-side of campus (surplus WWII Quonset huts?). These courses were still part of the required engineering curriculum, even though no one carried slide rules any more.

 

[scottluther1369]

I greatly appreciate the explanation and it has cleared up quite a bit. Thank you very much for the time to reply in such lengh! What I really have trouble understanstanding is we're going from 0 to X in and instant. This the moment of impulse? Is it the action from going from nothing to something? Then we have an average in Neutan Seconds after that. No problem there. The problem I have is understanding the works "moment of impulse" and their exact meaning. I understand the rest. Am I thinking to hard here? Thanks guys! I really do very much appreciate it. I'm trying to understand the physics behind the engines.
--Scott the newbie

 

[r1dermon]

moment of impulse is the moment of impulse....the exact millisecond that the engine starts thrusting. usually, the motor will ignite, and thrust way higher than its average impulse....so if its a 30Ns motor on average, and it lights up to 45Ns for a half second, then it would have to drop to 15Ns for another half second to make it a 30Ns motor on average. these are called the boost phase and the sustaining phase. the motor has to get the rocket moving somehow, so it gives it a huge boost spike, then once the rocket has reached a sufficient speed, the motor cuts the thrust way down, still enough to push the rocket, or at least, delay the apogee.

its kind of like a 2 stage rocket. there is a booster, and a sustainer. the booster is dropped after it lifts the entire rocket off the ground. here's what happens. the booster engine is usually a very short burn high impulse motor. its the kick in the pants motor. not a very long burn, maybe 1 second. once the rocket has accellerated to a sufficient speed, the booster drops and the sustainer lights. depending on the altitude you're trying to get, the sustainer is a long burn, low impulse motor. because you dont need all those newtons to get that rocket going, since it's already going.

 

[daveyfire]

Originally posted by r1dermon
moment of impulse is the moment of impulse....the exact millisecond that the engine starts thrusting. usually, the motor will ignite, and thrust way higher than its average impulse....so if its a 30Ns motor on average, and it lights up to 45Ns for a half second, then it would have to drop to 15Ns for another half second to make it a 30Ns motor on average. these are called the boost phase and the sustaining phase. the motor has to get the rocket moving somehow, so it gives it a huge boost spike, then once the rocket has reached a sufficient speed, the motor cuts the thrust way down, still enough to push the rocket, or at least, delay the apogee.

Do note that thrust is measured in Newtons, while Impulse is measured in Newton-seconds. Mathematically speaking, Impulse is the area under the thrust/time curve (or thrust integrated over time). A motor that ignites, produces a 45 N spike for 0.5 seconds, then drops down to 15 N for 0.5 seconds has a total impulse of 30 Newton-Seconds, yes, however a 30 Ns motor could also have a sustained thrust of 30 N for one second, or 15 N for two seconds... as described so eloquently by powderburner.

I'm not sure what you mean by "moment of impulse". Ideally, the motor ignites and comes up to pressure instantly, and you go from 14.7 psi (ambient) to chamber pressure incredibly fast. The thermite igniter in larger composite motors is an excellent example of this. Black powder motors also achieve this pressure-up quickly, because they have very little area to pressurize before thrust is generated. However, if not enough heat and pressure are delivered to the surface of the propellant, a slower buildup is often seen. If pressure does not build up and remain constant for the first few hundred milliseconds of the burn, the propellant will often go out, causing a chuff. You want the first situation to happen!

Here's <a href=https://www.smes.org/rocketclub/turkeyshoot2004/arcas_l650.wmv>video of a chuff</a>, <a href=https://www.smes.org/rocketclub/turkeyshoot2004/el_diablo.wmv>video of a normal pressure-up situation</a>, and <a href=https://www.mdra-archive.org/photos/LibertyLaunch/Video/LibertyPadCamera.wmv>video of pressure-up using Thermite</a>. The "moment of impulse" occurs in these videos all when thrust is developed. I guess what I'm trying to say is that there really is no exact "moment of impulse" -- as pressure ramps up in the motor, thrust is being produced at all pressures. The rocket begins to move when the thrust generated is greater than the rocket weight.

Hope this helps!

 

[r1dermon]

i was speaking of a 30Ns motor on average. if it burned for 1 second, half at 45Ns and the other half at 15Ns, then it would be a 30Ns motor on average, as well as in total. an E30, however, it wouldnt be a full E.

thanks for the clearup on the "technical" definitions of thrust and impulse. physics is not my bag. lol.

 

[daveyfire]

Originally posted by r1dermon
i was speaking of a 30Ns motor on average. if it burned for 1 second, half at 45Ns and the other half at 15Ns, then it would be a 30Ns motor on average, as well as in total.

Keep in mind that Thrust is measured in Newtons, and Impulse is measured in Newton-Seconds. Two different units of measurement for two different things.

 

[JRThro]

Scott,

I caught this thread 10 days after the last post, but I'd suggest going to the NAR website, specifically their <a href="https://nar.org/SandT/NARenglist.shtml">list of rocket engines</a>. If you click on a particular engine, say the Estes C6, it'll link to a pdf file of the engine's test information, including a thrust curve for that engine.

Seeing the actual thrust curves was really helpful to me. You can literally see how it's possible for a C6 to have an average thrust of 6 Newtons while a C11 has an average thrust of 11 Newtons, even though they are both C engines and they both have a total impulse of 9 Newton-seconds.

Most, if not all, black powder model rocket engines have a high peak thrust soon after ignition, and then they drop off to some lower thrust level for the remainder of the boost phase.