Motor/Impulse explination resources

[Bkdoubleu]

Last night I opened my Tripoli L2 Certification written test and although I have a long way to go before reaching that phase. While my epoxy was drying I took the first part of the test. I failed miserable when it came to motor nomenclature and hybrid motor theory. Any ideas of a good book or resource that would educate me on these subjects? Or has anyone seen a thread here that could lead me there?

 

[Bat-mite]

Someone else can speak to hybrids. But thrust and impulse are easy.

Thrust is the amount of Newtons (or pounds-force) a motor provides. Thrust is created by the motor burn, where the only place the exhaust can out is out through the nozzle. The equal and opposite reaction of the force pushes the rocket upward.

In rocket motor designations, the thrust is the number that follows the letter. So an F20 has 20 Newtons of thrust, a G80 has 80 Newtons of thrust, etc.

The thrust number tells you whether or not the motor can provide enough force to lift and propel the rocket.

Impulse is the amount of thrust multiplied by the burn time. So a G80 that burns for 1.71 seconds seconds has a total impulse of 137 Newton-seconds. Assuming the motor has enough thrust, the total impulse is an indicator (but not the only one) of how high the rocket will go.

For Aerotech motors, you need to read the label to get the burn time and calculate the impulse. Cesaroni motors give you the total impulse before the impulse letter. For example, the 5198M1101 has 1101 Newtons of thrust and a total impulse of 5198 Newton-seconds. To get the burn time, divide the total impulse by the thrust, or 5198 / 1101 = 4.72 seconds.

What do the letters mean? Impulse ranges are divided into classes indicated by a letter of the alphabet. See the chart below.

A 1.26 – 2.50
B 2.51 – 5.00
C 5.01 – 10.0
D 10.01 – 20.0
E 20.01 – 40.0
F 40.01 – 80.0
G 80.01 – 160.0
H 160.01 – 320.0
I 320.01 – 640.0
J 640.01 – 1,280.0
K 1,280.01 – 2,560.0
L 2,560.01 – 5,120.0
M 5,120.01 – 10,200.0
N 10,200.01 – 20,500.0
O 20,500.01 – 41,000.0

You will definitely want to read Modern High Power Rocketry 2 by Mark Canepa to get all your other questions answered.

 

[tim cubbedge]

One thing he did not mention is the the number following the letter is the average thrust, i.e., an F20 has an average of 20 Newtons but the maximum thrust for that motor would be 40.3 Newtons. Look at a thrust curve for a better understanding.

https://www.thrustcurve.org/simfilesearch.jsp?id=2082

 

[Bat-mite]

Good point. I don't think the Level 2 test gets into that, but it is important to know. Also, by the thrust curve, you can see where the maximum thrust occurs during the burn. Some motors have a maximum thrust off the pad, which means you can get away with a less than 5:1 thrust-to-weight ratio for that motor. But likewise, if the thrust is lower than average off the pad, then you will not get a good launch.

 

[CZ Brat]

Total Impulse = Average Impulse (number after the letter) X Burn Time
Average Impulse = Total Impulse/Burn Time
Burn Time = Total Impulse/Average Impulse

Total impulse is what determines the letter designation (See Bat-Mite's list)

Not to confuse you further, but if you're interested how the total impulse is calculated:
Motor is fired against a force meter. The software records the force at that particular moment in time at a certain sample rate (let's say 100 times/second). All those samples are added together and then divided by the sample rate. So in a hypothetical motor with a sample rate of 100/second:
Total sum of data points (5230 newtons) divided by sample rate (100/second) = Total impulse (52.3 Newton seconds)

 

[Bkdoubleu]

Thanks Guys I'll dive in. Looks like I got some homework. Very much appreciated.

 

[watermelonman]

If you need to remember total impulse ranges, start with G as 80 and double each letter from there. I think G80 is easy to remember because of the famous motor by the same name.

As for the hybrid questions, you can probably guess that the test was last given a major update when it looked like hybrids might take over the bulk of flying. It is silly, but the best way to approach it is like a HAM cram or similar. Simply memorize each answer per question rather than trying to make sense of it. There are few enough hybrid questions that you should be able to commit each of their answers to memory.

 

[marwady]

Good point. I don't think the Level 2 test gets into that, but it is important to know. Also, by the thrust curve, you can see where the maximum thrust occurs during the burn. Some motors have a maximum thrust off the pad, which means you can get away with a less than 5:1 thrust-to-weight ratio for that motor. But likewise, if the thrust is lower than average off the pad, then you will not get a good launch.

Could you also explain thrust to weight ratios.

Most say 5:1 is safe in an average scenario but I know that there are exceptions.

THX!!

Cheers!!

 

[Bat-mite]

Sure. This means that the amount of upward thrust applied to the rocket needs to be able to overcome the gravitational force by a factor of at least five.

Suppose you put a motor inside a ball, and the amount of the thrust the motor could generate on a 2-pound ball was 2 pounds of force. The ball would then hover: the amount of force pulling it down would be canceled out by the amount of force pushing it up.

If you increase the thrust to be greater than the gravitational force, then the ball goes up.

For a rocket, while the NAR safety code recommends a minimum thrust-to-weight ratio of 3:1, many fine rocketeers who have gone before me have determined that 5:1 is a much safer and more realistic number.

So, what does that mean to you? Well, first you need to be able to convert pounds of force into Newtons, because that is what rocket motors advertise. One pound of force = 4.44822162 Newtons. So for a one pound rocket, you would need 5 X 4.44822162 Newtons of thrust, or ~22.25 Newtons. At a quick glance, then, an F20, with 20 Newtons of thrust, would not be a safe motor for a one-pound rocket.

But keep in mind that what you really need to know is the amount of thrust in the first half-second or so, since that is when the rocket is on the rail and is at its heaviest weight (it gets lighter as it goes up because the propellant burns up). So you have to look at the thrust curve of the motor. If the curve starts high and dips over time, then most of the thrust occurs when the rocket is on the pad. An Aerotech G40, for instance, starts with nearly 80 Newtons of thrust, and then gets weaker over time. This motor, even though it is a 40-Newton motor, could actually lift a rocket that is about 3.5 pounds.

Remember we said the F20 was a bad motor for a one-pound rocket? Well, even though it has a lower average thrust, an Aerotech E16 could actually lift a 1.7-pound rocket, if you look at the curve.

Hope this helps.

 

[marwady]

Sure. This means that the amount of upward thrust applied to the rocket needs to be able to overcome the gravitational force by a factor of at least five.

Suppose you put a motor inside a ball, and the amount of the thrust the motor could generate on a 2-pound ball was 2 pounds of force. The ball would then hover: the amount of force pulling it down would be canceled out by the amount of force pushing it up.

If you increase the thrust to be greater than the gravitational force, then the ball goes up.

For a rocket, while the NAR safety code recommends a minimum thrust-to-weight ratio of 3:1, many fine rocketeers who have gone before me have determined that 5:1 is a much safer and more realistic number.

So, what does that mean to you? Well, first you need to be able to convert pounds of force into Newtons, because that is what rocket motors advertise. One pound of force = 4.44822162 Newtons. So for a one pound rocket, you would need 5 X 4.44822162 Newtons of thrust, or ~22.25 Newtons. At a quick glance, then, an F20, with 20 Newtons of thrust, would not be a safe motor for a one-pound rocket.

But keep in mind that what you really need to know is the amount of thrust in the first half-second or so, since that is when the rocket is on the rail and is at its heaviest weight (it gets lighter as it goes up because the propellant burns up). So you have to look at the thrust curve of the motor. If the curve starts high and dips over time, then most of the thrust occurs when the rocket is on the pad. An Aerotech G40, for instance, starts with nearly 80 Newtons of thrust, and then gets weaker over time. This motor, even though it is a 40-Newton motor, could actually lift a rocket that is about 3.5 pounds.

Remember we said the F20 was a bad motor for a one-pound rocket? Well, even though it has a lower average thrust, an Aerotech E16 could actually lift a 1.7-pound rocket, if you look at the curve.

Hope this helps.

THX!!

Very informative.

 

[CZ Brat]

To add to Bat-Mite's post.

The 5-1 ratio is a good rule of thumb which was devised back when sims were not readily available and as good as they are now. What ACTUALLY matters, is if the rocket is going fast enough when it leaves the rail/rod, that there is enough airspeed across the fins in order to actually be able to control the direction of the rocket. The 5-1 rule is assuming "standard" length rods.

A simulator (like Open Rocket or RocSim), will tell you how fast the rocket will be going when it leaves the rail/rod. It already knows how much impulse the motor will be pushing on the rocket (including initially, which the 5-1 rule doesn't account for) and it knows the rail/rod height (because you tell it how long it is). You then just need one rule of thumb, which is the velocity when the rocket leaves the rail/rod. For me, I use 45 FPS in <10MPH winds and 60 in 10-20 MPH winds. There have been times the velocity and 5-1 rule would stop me from launching on a 36" rod, but a 48" rod gave me a good velocity leaving the rod, even though the 5-1 still said "don't launch".

 

[Bat-mite]

Absolutely! 5:1 is a means to an end. The true goal is sufficient speed off the rail.

 

[JohnCoker]

Basic motor statistics and a glossary are on ThrustCurve.org:
www.thrustcurve.org/motorstats.shtml
www.thrustcurve.org/glossary.shtml

There is also my article on flight physics and simulation:
jcrocket.com/flight-physics.shtml

 

[CORZERO]

View attachment 282721

 

[Bkdoubleu]

View attachment 282721

I love resources. Thank you!!!!!!!!

 

[Bkdoubleu]

Basic motor statistics and a glossary are on ThrustCurve.org:
www.thrustcurve.org/motorstats.shtml
www.thrustcurve.org/glossary.shtml

There is also my article on flight physics and simulation:
jcrocket.com/flight-physics.shtml

Thanks for the expansion of Thrustcurve. good article btw. I didn't understand the math but I do understand writing and it was well written. thanks