Thrust to Weight Ratio

[Qwerty]

I'm just getting started in rocketry and encountered a number of blogs that recommend a minimum 5:1 thrust to weight ratio between the rocket and engine. These blogs also say the weight should be the rocket and its engine. So, I took a look at the Initiator and found it weighs 14 ounces and converted that to Newtons. Then I looked at the engine recommended for my first flight with that rocket and found that it indicates mass, not weight. Is the data for that rocket (and the others recommended for the Initiator) really mass, not weight? If so, then I should multiply its mass times gravity (W=MG) to get its weight in Newtons... right? Thanks for any help coming my way.

 

[Raythain]

Absolutely easiest thing is to throw it into a simulation program, like RockSim or OpenRocket. You can give it the length of your rod/rail and it’ll work the math out for you. You can even get a velocity off the rod/rail and you’ll have an even better idea of how/if you’re stable at launch.

If you want to do it by hand, just do all your units in metric, and things work out simpler. (Mass of rocket (kg) + Mass of motor (kg)) * gravity (9.81 m/s) gives the weight of the rocket.

Look at the motor data from somewhere like thrustcurve.org and you can get the thrust data. That’ll give you your T/W ratio.

 

[user 30299]

All you need to do is take the motor's Newtons and divide it by 4.45 and that gives you the thrust in pounds.

The F20, that your engine link is to, shows 40.9 Newtons for maximum thrust - so 40.9/4.45= 9.19 pounds
of max. thrust. But that's just for the maximum thrust.

Some motors have a gradual thrust increase at the beginning, and some have a burst of thrust at the start.
Thrust-to-Weight ratio is just one part of the proper motor selection. So always take a look at the motor's
thrust curve. Does the motor have the thrust you need at the start of the launch.

And to play it safe, you may want to use the Average Thrust, which the manufacturer's provide, for
checking your ratio.

 

[rcktnut]

Mass and weight in your home and on earth for that matter is the same thing. If you weigh your rocket the same place you weigh your motor, mass and weight same thing. Best thing as stated above get a sim program, Open Rocket is free. I use a calculator also to convert grams etc. to lbs./ oz etc. like QFactor. No metric for me!!

 

[Steve Shannon]

I'm just getting started in rocketry and encountered a number of blogs that recommend a minimum 5:1 thrust to weight ratio between the rocket and engine. These blogs also say the weight should be the rocket and its engine. So, I took a look at the Initiator and found it weighs 14 ounces and converted that to Newtons. Then I looked at the engine recommended for my first flight with that rocket and found that it indicates mass, not weight. Is the data for that rocket (and the others recommended for the Initiator) really mass, not weight? If so, then I should multiply its mass times gravity (W=MG) to get its weight in Newtons... right? Thanks for any help coming my way.

Yes. Many people here use metric mass units as if it were weight, but you're exactly right, grams are mass, not weight. Multiply kilograms by 9.81 meters/sec/sec and you will have converted the mass to the actual weight in Newtons, which matches the force of the thrust, which is commonly expressed in Newtons.
Welcome to rocketry!

 

[OzHybrid]

Motor thrust can vary between batches as specified in NFPA1125 for the same motor designation...... So don't worry about gravity being 9.81m/s^2 Use 10. It's close enough for rough calculations. It's not worth making your rough calculation any more difficult than necessary.
Any scales you will use to weigh things will have been calibrated somewhere on Earth against a known test mass. If you have good scales, they might let you recalibrate your scales where you are. So when you weigh something on scales, it's indicating an equivilant mass......
Any innacuracy in it does not really matter in the real world.

The thrust to weight rule of 5:1 is a general rule. It is set to allow, in general terms, a normal rocket to get to flying speed by the time it stops being guided by the guide rod or rails. The point where it is truly on its own. In the Tripoli rules you could go as low as 3:1 if you could make a case to the Range Safety Officer that it would be safe.

If you go to ThrustCurve.org and look at the data for a specific motor, many will have a peak thrust on ignition. Provided that lasts for long enough to get your rocket to the top of the rod or rail, you can use that peak figure for your 5:1 thrust to weight ratio.
If you look at the curve for the F20W single use

Although it has a peak of 40, 36 or 37 would be a more conservative number to use. It takes a fraction to get there and really doesn't hold it for long.
Where this initial start boost really shows up is in the Estes and others black powder motors. C6 for example has a massive start thrust in comparison to its average. See below. Also note that it is classified as a C6 but has an average thrust of 5 Newtons. A 20% alteration to its designation, depending on how you want to do your maths.



The trust curves are also available in OpenRocket. Once you have highlighted a specific motor in the motor list, you can select the details tab (Top Right) which will show the thrust curve plot at the bottom. If you double click on that it opens up in more detail.


Hope that helps
Norm

 

[JoePfeiffer]

The extent to which we confuse mass with weight is really unfortunate. It's bad enough that someone thinking metric will say something weighs a kilogram (it doesn't of course, it weighs 9.81 Newtons) us poor Americans working in Imperial units have to deal with pounds being the name of a unit of mass and of weight which we don't have to worry about at all until suddenly we get a result that's off by a factor of 32 and need to figure out why...

 

[OzHybrid]

The extent to which we confuse mass with weight is really unfortunate. It's bad enough that someone thinking metric will say something weighs a kilogram (it doesn't of course, it weighs 9.81 Newtons) us poor Americans working in Imperial units have to deal with pounds being the name of a unit of mass and of weight which we don't have to worry about at all until suddenly we get a result that's off by a factor of 32 and need to figure out why...

I use mixed measurements in OR.
Altitude is in Feet ( I fly in aircraft, aircraft report their height in feet, it's measureable in my head, 5000ft is a mile close enough (5280ish)
Speed in Mach (I want to know if I'm going to have to worry about it going there ish)
Dimensions in mm ( it's easier )

 

[user 30299]

The extent to which we confuse mass with weight is really unfortunate. It's bad enough that someone thinking metric will say something weighs a kilogram (it doesn't of course, it weighs 9.81 Newtons) us poor Americans working in Imperial units have to deal with pounds being the name of a unit of mass and of weight which we don't have to worry about at all until suddenly we get a result that's off by a factor of 32 and need to figure out why...

The mass appeal of the Metric system adds weight to the slow demise of the Imperial metrics Americans are so used to.

 

[OzHybrid]

The extent to which we confuse mass with weight is really unfortunate. It's bad enough that someone thinking metric will say something weighs a kilogram (it doesn't of course, it weighs 9.81 Newtons) us poor Americans working in Imperial units have to deal with pounds being the name of a unit of mass and of weight which we don't have to worry about at all until suddenly we get a result that's off by a factor of 32 and need to figure out why...

I'm unsure why everyone quotes gravity as 9.81 Where you are Joe, it would seem to be less https://www.sensorsone.com/local-gravity-calculator/
Las Cruces NM 9.79m/s^2

Looking at gravity maps for the US, few locations where it gets much above 9.8 are on the list.
I know when I was taught, 9.81 was the figure used. But I was in the UK and it is 9.81 there.
Looking at Black Rock.....

What value is OR using? Is it using gravity by location? From these values for example there would seem to be a 0.2% difference between the 9.81 and the Las Cruces value and a 0.1% for Black Rock. That's a small amount, but worth noting.

 

[rcktnut]

I don't think any of us have to worry about being an oz off one way or the other much less 1/10 or so of an oz off.

 

[kjhambrick]

The mass appeal of the Metric system adds weight to the slow demise of the Imperial metrics Americans are so used to.

Puns intended ...

Good one @QFactor ...

Fair warning: I am going to steal your work

Thanks !

-- kjh

 

[JoePfeiffer]

I'm unsure why everyone quotes gravity as 9.81 Where you are Joe, it would seem to be less https://www.sensorsone.com/local-gravity-calculator/
Las Cruces NM 9.79m/s^2

Looking at gravity maps for the US, few locations where it gets much above 9.8 are on the list.
I know when I was taught, 9.81 was the figure used. But I was in the UK and it is 9.81 there.
Looking at Black Rock.....

What value is OR using? Is it using gravity by location? From these values for example there would seem to be a 0.2% difference between the 9.81 and the Las Cruces value and a 0.1% for Black Rock. That's a small amount, but worth noting.

Yeah, I quoted what I saw upthread without reviewing.

I'm unsure why everyone quotes gravity as 9.81 Where you are Joe, it would seem to be less https://www.sensorsone.com/local-gravity-calculator/
Las Cruces NM 9.79m/s^2

Looking at gravity maps for the US, few locations where it gets much above 9.8 are on the list.
I know when I was taught, 9.81 was the figure used. But I was in the UK and it is 9.81 there.
Looking at Black Rock.....

What value is OR using? Is it using gravity by location? From these values for example there would seem to be a 0.2% difference between the 9.81 and the Las Cruces value and a 0.1% for Black Rock. That's a small amount, but worth noting.

It uses a gravity model assuming either a flat, spherical, or ellipsoidal earth (user's choice).

 

[Qwerty]

Thanks for the answers... they helped me realize what I was really trying to figure out. My research indicates that the airflow over the fins should be a minimum of 15 meters per second at launch, and that the thrust-to-weight ratio should be a minimum of 5 to 1 to achieve that speed. The math required to calculate the velocity at launch of a given rocket and a given engine is not my problem. I know how to do that and did it.

My problem is that the mass specified at the sale point for the rocket I want to evaluate does not match the mass of that same rocket at Open Rocket. And the advice I've been given regarding RockSim does not match what happens when I try to evaluate the rocket and any of the engines recommended for that rocket. RockSim, for example, does not allow me to input a length for the launch rail, nor does it display the velocity at launch for my rocket/engine combination.

Yeah, I used the same RKT file at both sites, and yeah, I'm just getting into rocketry. So I assume there's a bunch of stuff that I don't get. But it's been very difficult to discover a KISS approach to what that stuff is. I just want to compare my do-it-yourself velocity calculations with RockSim and/or Open Rocket. Any help coming my way will be much appreciated... thanks!

 

[Titan II]

RockSim, for example, does not allow me to input a length for the launch rail, nor does it display the velocity at launch for my rocket/engine combination.

It does both.

 

[user 30299]

Thanks for the answers... they helped me realize what I was really trying to figure out. My research indicates that the airflow over the fins should be a minimum of 15 meters per second at launch, and that the thrust-to-weight ratio should be a minimum of 5 to 1 to achieve that speed. The math required to calculate the velocity at launch of a given rocket and a given engine is not my problem. I know how to do that and did it.

My problem is that the mass specified at the sale point for the rocket I want to evaluate does not match the mass of that same rocket at Open Rocket. And the advice I've been given regarding RockSim does not match what happens when I try to evaluate the rocket and any of the engines recommended for that rocket. RockSim, for example, does not allow me to input a length for the launch rail, nor does it display the velocity at launch for my rocket/engine combination.

Yeah, I used the same RKT file at both sites, and yeah, I'm just getting into rocketry. So I assume there's a bunch of stuff that I don't get. But it's been very difficult to discover a KISS approach to what that stuff is. I just want to compare my do-it-yourself velocity calculations with RockSim and/or Open Rocket. Any help coming my way will be much appreciated... thanks!

Somethings in rocketry require patience and experience and a KISS method does not exist.

 

[Qwerty]

It does both.

Not for me so instead of telling me it does how about telling me how YOU get it to do that?

 

[Titan II]

Not for me so instead of telling me it does how about telling me how YOU get it to do that?

Testy? Way too many people on this forum just want answers without doing any work. Read the Help section, the Program Manual, watch the videos, etc. This is not rocket science.

 

[OzHybrid]

Not for me so instead of telling me it does how about telling me how YOU get it to do that?

OpenRocket

Edit, Preferences, Launch, set up rod, wind, angle Location, the altitude at the location. Sliimulation tab to select your flat earth preference. Or another more normal.

Or if you have an existing simulation, you can edit it either with the edit tab at the top or right click, select edit.

In OR under Help, guided tours, Introduction, information on how to do it is on the 6th and 7th slide.

As this is in the basic guide included with all versions of OR, then you should probably read both guides in the help section to make sure you've not missed anything else.

If you are going to use CAPS, add a smiley face.....

 

[OzHybrid]

Somethings in rocketry require patience and experience and a KISS method does not exist.

The KISS method in rocketry is to work in metric. 10N thrust=1Kg mass or weight (mass, but you found the mass by using kitchen scales, so you weighed it really, but the scales gave you a measurement that was a mass. Confused yet? Relax..... Deep breath....Just use the value) is close enough for a quick in your head of "will it fly and meet the 5:1 thrust to weight ratio?" So 1Kg needs 50N of thrust ish. Then if you want to go imperial 1Kg=2.2lbs. 2.2 Lbs is 2lbs +10%. So now you can do in your head calcs using easy multiply or divide by 10 or 2 and a bit of easy addition or subtraction if you want imperial. If you multiply by 2.2, multiply by 2 and add 10%. If you divide by 2.2, divide by 2 and subtract 10% Simple and close enough (divide will be 1% out but it's close enough).
There are going to be variations in motor thrust from the batch manufacturing process, "and stuff." So initially close enough is good enough.

If you're going for an altitude record or competition, you have to assume that the numbers are all accurate for thrust etc, So you would use a simulation program that calculates properly. But know in your head they might not be because of the rocket motor manufacturing tolerances allowed in NFPA1125.

Both the approximation, close enough method, and the full-blown calculation with rocket simulation method have assumptions built into them.
The simulation programs like Open Rocket, RASAero, and RockSim make life easier. But a quick approximation in your head can save time.

 

[mh9162013]

I use mixed measurements in OR.
Altitude is in Feet ( I fly in aircraft, aircraft report their height in feet, it's measureable in my head, 5000ft is a mile close enough (5280ish)
Speed in Mach (I want to know if I'm going to have to worry about it going there ish)
Dimensions in mm ( it's easier )

I hear you 100%.

I use whatever form of measurement is most convenient for me to do calculations in whole numbers.

 

[Last Ox]

Thanks for the answers... they helped me realize what I was really trying to figure out. My research indicates that the airflow over the fins should be a minimum of 15 meters per second at launch, and that the thrust-to-weight ratio should be a minimum of 5 to 1 to achieve that speed. The math required to calculate the velocity at launch of a given rocket and a given engine is not my problem. I know how to do that and did it.

My problem is that the mass specified at the sale point for the rocket I want to evaluate does not match the mass of that same rocket at Open Rocket. And the advice I've been given regarding RockSim does not match what happens when I try to evaluate the rocket and any of the engines recommended for that rocket. RockSim, for example, does not allow me to input a length for the launch rail, nor does it display the velocity at launch for my rocket/engine combination.

Yeah, I used the same RKT file at both sites, and yeah, I'm just getting into rocketry. So I assume there's a bunch of stuff that I don't get. But it's been very difficult to discover a KISS approach to what that stuff is. I just want to compare my do-it-yourself velocity calculations with RockSim and/or Open Rocket. Any help coming my way will be much appreciated... thanks!

the whole discussion of weight vs mass and precise value of g is interesting, but the nuances numerically are probably much less than expected variation in motor thrust for most commercial motors.

For basic values (you may have already done this) -

https://www.rocketryforum.com/threads/best-t-w-ratio-chart.179897/post-2433785

If Rod travel in feet times actual acceleration in g's (thrust to weight ratio minus one) exceeds 32, the rocket will exceed 45fps off the rail. Note this is the real live thrust on the rail, which will be different from published max, average, or initial thrust.

My understanding of US regulations is that 3:1 launch weight to average certified thrust is, in the US, legal requirement. 5:1 and what thrust to use is between you and your RSO.

Unfortunately, neither rail speed nor ratio of initial thrust to launch weight are strongly related to whether a rocket veers coming off the rail. They certainly help, but provide no guarantee, at least based on measurements of more than a hundred flights at a recent launch. Good rockets/motors sometimes fly bad.

https://www.rocketryforum.com/threads/velocity-off-the-rail-nrxs-data.181761/
Two factors that have at least as much of an effect are wind gusts and failure of the motor to light cleanly and rapidly come up to stable thrust.

I've got video (nearly a terabyte worth) of many more launches from this month, but real life intervenes and it will be months before I finish measurements for another set of data.

My only definite conclusion so far is that stand-off distance is a good thing.

 

[Lamp]

Not for me so instead of telling me it does how about telling me how YOU get it to do that?

Rather than just saying Rocksim cannot do it, you ask nicely how you can do it.

At the top of the program are a few icons. One of them is the "Prepare for launch" icon click it and a window pops up for motor selection. There are other tabs in this window, not just motor selection and one of them is "Starting State". You can input your guide length and angle here.

In your simulation summary, If you right click on one of the column headings you get the option to "Organize Columns", click this and you can add or delete columns for various things like "Velocity At Launch Guide Departure". You can also set up these columns by clicking Edit, then preferences then the "Simulation Summary" tab....

 

[Donnager]

All you need to do is take the motor's Newtons and divide it by 4.45 and that gives you the thrust in pounds.

The F20, that your engine link is to, shows 40.9 Newtons for maximum thrust - so 40.9/4.45= 9.19 pounds
of max. thrust. But that's just for the maximum thrust.

Some motors have a gradual thrust increase at the beginning, and some have a burst of thrust at the start.
Thrust-to-Weight ratio is just one part of the proper motor selection. So always take a look at the motor's
thrust curve. Does the motor have the thrust you need at the start of the launch.

And to play it safe, you may want to use the Average Thrust, which the manufacturer's provide, for
checking your ratio.

Or you can use my rule of thumb: Thrust Newtons/4.5=Thrust lb.

I do this for two reasons, 1) is that the thrust should be below what I estimate, and 2) It's an easy number to remember.

 

[fanga]

I use mixed measurements in OR.
Altitude is in Feet ( I fly in aircraft, aircraft report their height in feet, it's measureable in my head, 5000ft is a mile close enough (5280ish)
Speed in Mach (I want to know if I'm going to have to worry about it going there ish)
Dimensions in mm ( it's easier )

Plus 1 for this setup. However I have noticed a shift in most Aerospace reporting to talk metric when it comes to altitude.

 

[fanga]

Absolutely easiest thing is to throw it into a simulation program, like RockSim or OpenRocket. You can give it the length of your rod/rail and it’ll work the math out for you. You can even get a velocity off the rod/rail and you’ll have an even better idea of how/if you’re stable at launch.

If you want to do it by hand, just do all your units in metric, and things work out simpler. (Mass of rocket (kg) + Mass of motor (kg)) * gravity (9.81 m/s) gives the weight of the rocket.

Look at the motor data from somewhere like thrustcurve.org and you can get the thrust data. That’ll give you your T/W ratio.

Thanks for the reminder to convert mass to weight. It's easy to forget this one sometimes.

 

[Dave S.]

I thought the rules were you should have a 5:1 ratio based on the average thrust (the name of the motor), but that 3:1 is the bare minimum.

In other words, an H100 motor has an average thrust of 100 Newtons, an H283 has an average thrust of 283 newtons.

Divide 100 or 283 by 4.45 to convert to pounds.

Then divide that by 5, and your rocket must weigh less than that to have a 5:1 ratio.