Thrust to Weight & Getting Off The Pad Calculation

[NattyDread]

I have a Cesaroni K261, long burn mellow. Average thrust is 259.8N so if I convert to pounds, I get 57.73Average thrust pounds. The rocket sims at 13.7 pounds fully loaded. So that puts my ratio at 68rocket weight pounds to 57.7average thrust pounds. I suppose a lot of people don't check this?? But I know I have not been on this side of the calculation. Thoughts on getting off the pad or any other issues????

I suppose there is a bit of wiggle room in this high power "rocketry rule"?

For people not sure of my dilemma, you want average thrust to be 5 times your rocket's fully loaded weight. The long burn motors are cool cause they burn for 8 to 9 seconds, but have low average thrusts.

 

[CoyoteNumber2]

Check the initial thrust.

 

[TRFfan]

Like most moonburners, it has an initial thrust of 350 newtons.

 

[TRFfan]

https://www.pro38.com/products/pro54/motor/MotorData.php?prodid=2021K261-P

 

[mikec]

K261 white longburn has a maximum thrust of 83.2 pounds so you have about 6:1 TTW at liftoff. Even so, I wouldn't fly this motor/rocket combination because the longburns tend to be somewhat wobbly due to thrust/mass asymmetries, even with a better TTW. YMMV.

 

[Buckeye]

I have a Cesaroni K261, long burn mellow. Average thrust is 259.8N so if I convert to pounds, I get 57.73Average thrust pounds. The rocket sims at 13.7 pounds fully loaded. So that puts my ratio at 68rocket weight pounds to 57.7average thrust pounds.

Huh? How does your rocket change from 13.7 pounds to 68 pounds?

I suppose you are trying to say that your thrust-to-weight ratio = 57.7/13.7 = 4.2

Run a simulation.

 

[TRFfan]

K261 white longburn has a maximum thrust of 83.2 pounds so you have about 6:1 TTW at liftoff. Even so, I wouldn't fly this motor/rocket combination because the longburns tend to be somewhat wobbly due to thrust/mass asymmetries, even with a better TTW. YMMV.

Yes thats true.

 

[TRFfan]

Huh? How does your rocket change from 13.7 pounds to 68 pounds?

I suppose you are trying to say that your thrust-to-weight ratio = 57.7/13.7 = 4.2

Run a simulation.

13.7*5=68.5

 

[Nick@JET]

Another thing to note : low thrust to weight ratios don't fly well in wind or angled launch rails! I love long burns but they can also trail off which allows the rocket to go off vertical while that motor is still burning, and of course this is being propelled AWAY from you, which also leads to higher apogee separation speed!

So looking at the burn graph I see approx 340 Ns at beginning of burn. 340/25 = 13.6lbs max rocket loaded weight for a 5:1 ratio. There have been others fly at even lower than 5:1 and maybe they will chime in, but it is a risk. Looking at Pro38.com motor specs that motor total weight with hardware is 1931 grams or 4.25lbs, so 13.6lbs - 4.25 = max 9.35lb rocket weight without motor.

You said you're at 13.7 fully loaded so I would go for it. You can always use thrust curve app for quick sim on that motor.

Just from my experience I would suggest if you want to fly longburns at a 5 or 6:1;

1. Revisit the rocket items and do everything you can to shed weight safely, is there anything on there that is a waste? Oversized quicklinks, over epoxied, nose weight etc. I feel a slow lift ratio rocket is built with that motor in mind.

2. Fly only with perfect conditions, which means you'll have to get rid of "go fever" (took me 1.5 years for the right conditions to fly an H45, lol)

3. No launch rod angle

I love long burns!

 

[blackjack2564]

Use a 10 ft rail. Fly on calm day......

It will be fine, although tall skinny rocket will tend to cone for awhile, due to offset thrust. [moonburner core to side]

 

[NateLowrie]

I have a Cesaroni K261, long burn mellow. Average thrust is 259.8N so if I convert to pounds, I get 57.73Average thrust pounds. The rocket sims at 13.7 pounds fully loaded. So that puts my ratio at 68rocket weight pounds to 57.7average thrust pounds. I suppose a lot of people don't check this?? But I know I have not been on this side of the calculation. Thoughts on getting off the pad or any other issues????

I suppose there is a bit of wiggle room in this high power "rocketry rule"?

For people not sure of my dilemma, you want average thrust to be 5 times your rocket's fully loaded weight. The long burn motors are cool cause they burn for 8 to 9 seconds, but have low average thrusts.

I have gone down to 4:1 thrust to weight at times and been just fine. It's just a rule of thumb though and you should do a proper simulation. What you are really looking for is the speed coming off the rail. In general, you want the speed off the rail to be 4-5 times the wind speed you are launch in and at least 40 fps, preferably 50+ fps. Rocksim or OR can tell you that value real quick for any motor and launch rail length combination.

If you are on the low side of rail speed (say 40fps) than you have a few choices:

• Use a longer rail, which will result in increased velocity leaving the rail because your motor is still accelerating the rocket along the additional length of rail.
• Wait for the right wind conditions. 40fps off the rail would be good up to 8mph and marginal up to 10mph. Anything over 10mph you don't launch.

Given the higher initial thrust of the motor, if you use a longer rail 8' or 10' you should be fine. Run the sim though to confirm.

 

[NattyDread]

Nate: on rocksim, on the launch sim window, I see values for thrust, acceleration, acceleration total, velocity. In your explanation, you say, "speed off the rail". I assume I am looking at velocity?

I have gone down to 4:1 thrust to weight at times and been just fine. It's just a rule of thumb though and you should do a proper simulation. What you are really looking for is the speed coming off the rail. In general, you want the speed off the rail to be 4-5 times the wind speed you are launch in and at least 40 fps, preferably 50+ fps. Rocksim or OR can tell you that value real quick for any motor and launch rail length combination.

If you are on the low side of rail speed (say 40fps) than you have a few choices:

• Use a longer rail, which will result in increased velocity leaving the rail because your motor is still accelerating the rocket along the additional length of rail.
• Wait for the right wind conditions. 40fps off the rail would be good up to 8mph and marginal up to 10mph. Anything over 10mph you don't launch.

Given the higher initial thrust of the motor, if you use a longer rail 8' or 10' you should be fine. Run the sim though to confirm.

 

[NateLowrie]

Nate: on rocksim, on the launch sim window, I see values for thrust, acceleration, acceleration total, velocity. In your explanation, you say, "speed off the rail". I assume I am looking at velocity?

Yes, velocity = speed in a given direction. What velocity values do you get with 6', 8', and 10' rails.

 

[Nathan]

Yes, velocity = speed in a given direction. What velocity values do you get with 6', 8', and 10' rails.

Good point. But then since velocity is a vector, something like "40 fps" describes a speed scalar and not a velocity vector since it doesn't include the direction component. I suppose that "40 fps off the rail" could be considered a velocity since it implies a direction.

 

[markkoelsch]

Use a 10 ft rail. Fly on calm day......

It will be fine, although tall skinny rocket will tend to cone for awhile, due to offset thrust. [moonburner core to side]

+1 to Jim. You can do it. A longer rail is your friend here as it will give you more velocity under guidance.

Look at the first second of thrust and see what the thrust is there as that is where you will be off the pad.

 

[JohnCoker]

The upcoming ThrustCurve.org has initial thrust listed (if there are any simfiles for the motor).
thrustcurve.herokuapp.com/motors/Cesaroni/2021K261-P/

And of course the motor guide will only choose motors that can accelerate the rocket to a safe velocity in the length of the guide you specify.

 

[EeebeeE]

I would not use initial thrust if the motor has a huge, but extremely short spike followed but a drop down to the average or slightly above. To be on the safe side, you might want to look at the average thrust of the first 0.25 seconds or so. Most moonburners like the K261 however just have a long decay. I think the K261 burns above 261 NS the first 4 seconds of the burn. I start with loaded rocket weight and determine the minimum 5:1 T/W ratio first.

1 Newton = 0.2248 pounds. 1 pound = 4.4484 Newtons.

There are two simple formulae you can use:
(5 x Rocket weight)/0.2248. In your case (5 x 13.7 lbs.)/.2248 = 68.5/0.2248 = 305 N of initial thrust.
or
5 x 4.4484 x 13.7 lbs. = 305 N

Dividing takes one less keystroke. :eyeroll:


If you have an RSO who doesn't want to do the math or look at the thrust curve, you should probably have the info with you that shows you will be OK.

 

[JohnCoker]

I would not use initial thrust if the motor has a huge, but extremely short spike followed but a drop down to the average or slightly above.

"Initial thrust" is the average thrust over the first half second. This is much more relevant for rules of thumb than average impulse.

 

[EeebeeE]

"Initial thrust" is the average thrust over the first half second. This is much more relevant for rules of thumb than average impulse.

Perhaps, but that spike is factored into the average or the first 0.5 sec. of initial thrust. If you have a motor that has an average of say 250 NS, but in the first 0.1 sec.spikes to 500 NS, then drops immediately back to 250, then its average for the first 0.5 seconds is going to be considerably higher than what it really puts out over that duration.

Realistically, we should consider median initial thrust vs. average initial thrust as a better indicator. In the example above, the average could be 325 or so NS. The median however would be 250.

Think of it this way... The average household income in Redmond, WA, home of Bill Gates, is a helluva lot higher than the median household income. Which data point is the better indicator of wealth in Redmond, an average skewed way out of proportion by the income of one of about 50,000 residents, or the income of the resident with the 25,000th highest income?

 

[JohnCoker]

Perhaps, but that spike is factored into the average or the first 0.5 sec. of initial thrust. If you have a motor that has an average of say 250 NS, but in the first 0.1 sec.spikes to 500 NS, then drops immediately back to 250, then its average for the first 0.5 seconds is going to be considerably higher than what it really puts out over that duration.

Realistically, we should consider median initial thrust vs. average initial thrust as a better indicator. In the example above, the average could be 325 or so NS. The median however would be 250.

I only know how to calculate the median for a population. (It's definition depends on discrete value samples.) The thrust curve is a continuous quantity fitted to a fixed number of points, generally with none within the first half second. I think calculating the median would be valid if the data was a high-frequency set of samples (although a smoothing filter would probably be better), but what we have is a handful of points chosen by humans to fit inflection points in the curve.

 

[markkoelsch]

Perhaps, but that spike is factored into the average or the first 0.5 sec. of initial thrust. If you have a motor that has an average of say 250 NS, but in the first 0.1 sec.spikes to 500 NS, then drops immediately back to 250, then its average for the first 0.5 seconds is going to be considerably higher than what it really puts out over that duration.

Realistically, we should consider median initial thrust vs. average initial thrust as a better indicator. In the example above, the average could be 325 or so NS. The median however would be 250.

Think of it this way... The average household income in Redmond, WA, home of Bill Gates, is a helluva lot higher than the median household income. Which data point is the better indicator of wealth in Redmond, an average skewed way out of proportion by the income of one of about 50,000 residents, or the income of the resident with the 25,000th highest income?

This why I said to use the average thrust of the first second.

 

[Buckeye]

...you should do a proper simulation. What you are really looking for is the speed coming off the rail. In general, you want the speed off the rail to be 4-5 times the wind speed you are launch in and at least 40 fps, preferably 50+ fps. Rocksim or OR can tell you that value real quick for any motor and launch rail length combination.

This^

Seriously, I never computed a TTW ratio in my life. That is a relic of the 1960's. Flight simulations are far superior, tell you everything you need to know, and run on your phone. I like Thrustcurve for quick and dirty checks at the field.

Plus, simulations could care less about initial thrust vs. average thrust. The entire thrust profile is properly used! :bangbang:

 

[JohnCoker]

This^

Seriously, I never computed a TTW ratio in my life. That is a relic of the 1960's. Flight simulations are far superior, tell you everything you need to know, and run on your phone. I like Thrustcurve for quick and dirty checks at the field.

Plus, simulations could care less about initial thrust vs. average thrust. The entire thrust profile is properly used! :bangbang:

Indeed. That is why the ThrustCurve.org motor guide runs a quick and dirty simulation to verify speed off the guide and estimate apogee.

I've been long suggesting we drop the 5:1 rule of thumb in favor of simulation. However, people do seem to like its convenience, not to mention the RSOs who don't have time to run a sim. I think the 5:1 rule of thumb is here to stay, so Mark and I are trying to at least make it more relevant by using the initial thrust rather than the average.

 

[Buckeye]

Indeed. That is why the ThrustCurve.org motor guide runs a quick and dirty simulation to verify speed off the guide and estimate apogee.

I've been long suggesting we drop the 5:1 rule of thumb in favor of simulation. However, people do seem to like its convenience, not to mention the RSOs who don't have time to run a sim. I think the 5:1 rule of thumb is here to stay, so Mark and I are trying to at least make it more relevant by using the initial thrust rather than the average.

At the launches I attended over 16 years, not once did an RSO attempt a 5:1 calculation on my rocket. They just look at motor retention and structural fitness and ask about electronics. Maybe it's because my rockets are boring 3FNC types and look "reasonable" at first glance. :wink: Maybe other flyers get closer attention than me. Topic for another day.

So, I am not so sure about convenience. The rocket needs to be weighed. Unfortunately, far too many hobby rocketeers struggle with pounds, Newtons, and kilograms, so the conversion thing takes time. If you are using initial thrust, then you need to look up the graph of the thrust curve and not just read the average thrust from the motor code. In all that time, you could just as easily run the Thrustcurve Motor Guide. Better yet, if the rocket looks "iffy", the flyer himself should bring the supporting data.

Edit: Just saw the discussion about additional averaging, or oddly the median (?) during the first 0.5 or 1.0 second. Then for sure Thrustcurve is faster!

 

[markkoelsch]

At the launches I attended over 16 years, not once did an RSO attempt a 5:1 calculation on my rocket. They just look at motor retention and structural fitness and ask about electronics. Maybe it's because my rockets are boring 3FNC types and look "reasonable" at first glance. :wink: Maybe other flyers get closer attention than me. Topic for another day.

So, I am not so sure about convenience. The rocket needs to be weighed. Unfortunately, far too many hobby rocketeers struggle with pounds, Newtons, and kilograms, so the conversion thing takes time. If you are using initial thrust, then you need to look up the graph of the thrust curve and not just read the average thrust from the motor code. In all that time, you could just as easily run the Thrustcurve Motor Guide. Better yet, if the rocket looks "iffy", the flyer himself should bring the supporting data.

I would question your RSO then.

 

[Handeman]

At BattlePark, our RSOs have a graph with 3:1, 5:1 and 7:1 lines on thrust vs. weight. As long as your average thrust is over 5:1, you're good to go on TTW. If you're under 3;1 that is almost always a no-go, if it's under 5:1 then we take a closer look. it doesn't mean you can't fly, it just means it will take a while for the RSO, LCO, BOD, etc. to decide if, where, and/or how we will let you fly.

 

[dhbarr]

At BattlePark, our RSOs have a graph with 3:1, 5:1 and 7:1 lines on thrust vs. weight. As long as your average thrust is over 5:1, you're good to go on TTW. If you're under 3;1 that is almost always a no-go, if it's under 5:1 then we take a closer look. it doesn't mean you can't fly, it just means it will take a while for the RSO, LCO, BOD, etc. to decide if, where, and/or how we will let you fly.

Pic or doc? Sounds handy

 

[terryg]

+1, It sure does!

 

[Buckeye]

+1, It sure does!

Despite my objections to TTW, I made an Excel chart since I was bored! Easy. Handeman's probably looks something like this.



View attachment ratios.xlsx

 

[Handeman]

Despite my objections to TTW, I made an Excel chart since I was bored! Easy. Handeman's probably looks something like this.

View attachment 319307

Yes, our scales are Avg. Ns and lbs. and it is a lot more granular with grid lines, but that's essentially it. We've got them laminated and attached to clip boards so they don't blow away.