# Thrust to weight Ratios of Model Rockets: ELI5

### Help Support The Rocketry Forum:

#### shockie

##### High Plains Drifter
at this website:

.

it states: [the] "ratio is determined simply by taking the rocket weight(loaded with motors) in pounds and multiplying by 22.25 to get minimum required motor thrust in Newtons (N)."

I would like to try and redo some of these charts that are more designed towards model rockets , ie less than 1lb.

If I wanted to calculate the T/W ratio for a model rocket that weighed 1 oz the calculation would be be:
1/16 oz = 0.0625 lb
so 0.0625 lb X 22.25 = 1.41 Newtons . So is this example telling me the T/W ration is 1.41?

Ok this website says: " Note this is the motor thrust off the pad, not the motor total thrust."

OK how does this "total thrust off the pad" relate to an motor's Average Thrust and its Peak/Max Thrust.

for example, lets say this 1 oz rocket has an A8-3 in it. According to the NAR motors listing , at its Peak/Max thrust is 9.6 N and the average thrust is 4.03 N

Can somebody help me understand this? Does either OpenRocket or Rocksim tell you this information?

TIA

#### Funkworks

##### Well-Known Member
That article is confusing. I’d look for another. I’m not a RSO and I use Openrocket, so I don’t really know which ratio is actually a good rule of thumb.

However, if a motor runs for 5 seconds, its thrust is different at each second (each moment really, like at each 1/10th second, it has a different thrust).

So you could say the initial thrust is the thrust at 1/10th second, and it could be 1.41 N.

The peak (maximum) thrust might happen at 2 seconds and be 9.6 N.

And the average thrust would be over the entire 5 second and could be 4.03 N.

#### Kelly

##### Usually remembers to get the pointy end up
For many motors, you can use "average thrust" as the thrust off the pad. If possible, use a published thrust curve for the motor. Estes, for example, publishes thrust curves, and designs many motors to have a higher initial thrust than the average thrust.
There is no such thing as 'motor total thrust' for a motor, so ignore that - and look skeptically at any source that uses that term!

Thrustcurve.org, which is the go-to source for motor data, has an excellent page that defines many of these terms. They define "initial thrust" as the average thrust, over the first half second. That's a bit arbitrary, but it's as good a definition as any. In fact, thrustcurve.org may have - or be able to generate - the data that you're looking for, for model rockets.

The most accurate way to determine if your thrust is enough is to run a simulation (such as OpenRocket) with the simulation files for the rocket and motor, and relevant info such as rod length entered.

#### shockie

##### High Plains Drifter
I think I may have found the answer:

I have a lot of reading and thinking to do....this is about airplanes going horizontally but I can think in terms of rockets going near verticle.....I wish I had access to a Russian math wiz that could speak English to teach me this

#### SecondRow

##### Well-Known Member
If I wanted to calculate the T/W ratio for a model rocket that weighed 1 oz the calculation would be be:
1/16 oz = 0.0625 lb
so 0.0625 lb X 22.25 = 1.41 Newtons . So is this example telling me the T/W ration is 1.41?
No. It’s telling you that you need 1.41 Newtons of thrust to achieve the requisite T/W ratio used for the conversion. In this case, they’re using a 5:1 T/W ratio.

Here‘s how they got 22.25:

weight of the rocket (lbs) x 1lb/2.2kg = mass in kg.
mass (kg)x 9.8m/s^2 = the weight of the rocket in Newtons
weight of the rocket in Newtons x 5 = Required thrust for 5:1 ratio

so 9.8 x 5 / 2.2 = 22.27 (and then they rounded to 22.25 to make it easier to remember I guess).

If you wanted a different ratio, 3:1 for example, then your factor would be 9.8 x 3 / 2.2 = 13.36.

Last edited:

#### SecondRow

##### Well-Known Member
Generally, the peak thrust will occur within the first 1/4 second or so of the burn, while the rocket is still on the rod or exiting it. Because of that, I usually use the peak thrust to check T/W. I think average thrust is too conservative sometimes.

#### Nytrunner

##### Pop lugs, not drugs
TRF Supporter
Agreed with Kelly and SecondRow.

The key to this is understanding that the Avg thrust of the motor is not necessarily the same as the initial thrust on the pad. The only way to find that out is to view the thrustcurve of the motor (like in Openrocket, Rocsim, or on thrustcurve)

The next key is to know "Why 5:1?" The answer is that's a convenient rule of thumb in order to ensure safe velocity upon exit of launch rail/rod. That's the speed at which a airflow over the fins of a conventional rocket is sufficient to keep it pointed in the direction of flight (more or less).

How fast is that? That's generally accepted to be ~30 mph, 45-50 ft/s, or ~15 m/s. However, in windy conditions, it is suggested that your exit velocity by 4x the windspeed on the ground to mitigate weathercocking. How can you determine if your rocket has sufficient exit speed? Rocsim and Openrocket tell you this in each simulation run.

Since rail exit speed requires you to know the length of the rail and there are lots of different lengths out there, I generally pick an 8' rail (or 5' and 3' rods for smaller rockets) for my calcs and keep that in mind when picking motors on the field. But 5:1 Thr/wt ratio using the initial motor thrust covers a lot of ground.

#### MetricRocketeer

##### Member of the U.S. Metric Association
TRF Supporter
No. It’s telling you that you need 1.41 Newtons of thrust to achieve the requisite T/W ratio used for the conversion. In this case, they’re using a 5:1 T/W ratio.

Here‘s how they got 22.25:

weight of the rocket (lbs) x 1lb/2.2kg = mass in kg.
mass (kg)x 9.8m/s^2 = the weight of the rocket in Newtons
weight of the rocket in Newtons x 5 = Required thrust for 5:1 ratio

so 9.8 x 5 / 2.2 = 22.27 (and then they rounded to 22.25 to make it easier to remember I guess).

If you wanted a different ratio, 3:1 for example, then your factor would be 9.8 x 3 / 2.2 = 13.36.
Hi everyone,

If we just used the metric system, we could avoid all of these conversions and get the results right away.

Stanley

#### Sooner Boomer

##### Well-Known Member
Here's something that might help, it's the new Estes catalog, page 74

It shows the thrust curve for all the styles of engines Estes currently makes. You see a big bump at the beginning of each thrust curve. That's what gets the rocket off the pad and going up. Then the thrust reduces to a lower level until the motor burns out.

The reason for the thrust curve being this shape is because all the black powder engines are a core-burner design that transitions to an end-burner. That "core", a small dimple into the black powder, gives a lot of surface area alaible for burning (and burning makes thust!). Unfortunately, the days are long gome of the B14 series of engines. These had looong core burning sections (almost all core burning). They developed a huge kick in the pants, but the burn didn't last very long. They were also drilled after the main grain had been pressed, which was a difficut and expensive process (and dangerous!)

#### dr wogz

##### Fly caster
Hi everyone,

If we just used the metric system, we could avoid all of these conversions and get the results right away.

Stanley
Most here would likely agree, but it is an uphill battle! The US, Burma & Liberia are the only countries to not have "officially" adopted the SI system of measurements..

The US did want / try back in the early 70's but, as usual, politics & lack of "push". But it does look like more & more US institutions are starting to switch

(I'm Canadian: I'm "measurement ambidextrous": I live in an SI country, but buy & sell stuff to the US. So, I work in imperial, but live in SI..)

.

#### Funkworks

##### Well-Known Member
If we just used the metric system, we could avoid all of these conversions and get the results right away.
Imagine you're a pilot near an airport and you hear the control tower say:

“Cessna Two Bravo Sierra traffic, eleven o’clock, opposite direction, altitude indicates three thousand five hundred.”

Are you safe?

Each air traffic communication must be short because near an airport, the pilots take turns using the same frequency. To keep them short, units are implied. That's why I think aviation in general is stuck in the imperial system, i.e. no one involved in live air traffic control has time to discuss "was that metric or imperial?".

Last edited:

#### MetricRocketeer

##### Member of the U.S. Metric Association
TRF Supporter
Hi Funkworks,

But if we all used the metric system, then we would never need to discuss whether we were using the metric system or the foot-pound system. We would be using metres.

In such a situation, therefore, this is what air-traffic control would transmit:
“Cessna Two Bravo Sierra traffic, eleven o’clock, opposite direction, altitude indicates one thousand zero seven zero.”

Are you safe?
Stanley

#### shockie

##### High Plains Drifter
I did the following at thrustcurve.org

#### Funkworks

##### Well-Known Member
Hi Funkworks,

But if we all used the metric system, then we would never need to discuss whether we were using the metric system or the foot-pound system. We would be using metres.

In such a situation, therefore, this is what air-traffic control would transmit:

Stanley
I fully agree but they “missed the boat” when aviation started 100 years ago. I just can’t imagine how air traffic control could possibly make the switch now.

Even if you convince every rocketeer to use the metric system, aviation will still enforce ceilings and aviation flight levels in feet, for safety’s sake.

And they’re much bigger. Our small hobby isn’t even a pixel on their radar.

#### MetricRocketeer

##### Member of the U.S. Metric Association
TRF Supporter
Hi Funkworks,

For me, I guess, hope springs eternal. I continue to advocate for the metric system. Australia and New Zealand did it. So can the United States.

Stanley

#### teepot

##### Well-Known Member
TRF Supporter
I remember when they tried the metric system many decades ago. Didn't work then won't work now. It would take generations to change over.

#### RICHARD COLARCO

##### Active Member
I teach astrodynamics and rocket propulsion.

The wild card with smaller rockets is that the propellant mass makes up a large fraction of the total rocket mass, and changes very rapidly. This is difficult to model, but can be done. Using average thrust gives you an OK approximation of your performance. It has been my experience that there is a great amount of variability in performance from engine to engine of the same type, something we would never tolerate in the real world.

If you really want to understand this, google the "rocket equation" and start studying and calculating examples. This requires only algebra. There is no easy road to knowledge.

The Saturn V had barely 1:1 thrust-to-mass when the first stage engines lit, and burned many tons of propellant over several seconds stabilizing engine performance before it actually started to rise from the pad. In contrast, our engines burn very rapidly and are usually burned out (for Estes engines) in 1/2 to 2 seconds. You have to use a fine time interval to model this properly.

#### jrap330

##### Retired Engineer, NAR # 76940
TRF Supporter
Most here would likely agree, but it is an uphill battle! The US, Burma & Liberia are the only countries to not have "officially" adopted the SI system of measurements..

The US did want / try back in the early 70's but, as usual, politics & lack of "push". But it does look like more & more US institutions are starting to switch

(I'm Canadian: I'm "measurement ambidextrous": I live in an SI country, but buy & sell stuff to the US. So, I work in imperial, but live in SI..)

.
yep,Congress or whoever ruined it. We started around 1975 to educate the public. You had your 60 seconds commercials, with a jingle.All ball parks had metric dimension underneath dimensions in feet, including Montreal Expos (reverse). I was in high school, engineering, using both systems and then...zip ..cancel. By now, 2 generations later, Americans would not know the difference. Instead poor science, engineering students do both and I am no looking comfortably in metric system.

#### RICHARD COLARCO

##### Active Member
I'm retired from a US manufacturer of high precision optical and electronic equipment. I never saw anything but SI units employed. It is only the dumb, recalcitrant US population that clings to feet, inches, and pounds.

#### Funkworks

##### Well-Known Member
For me, I guess, hope springs eternal. I continue to advocate for the metric system. Australia and New Zealand did it. So can the United States.
I do too. In that I use it whenever it doesn't offend anyone. Just a reminder as to what we're up against.

#### RICHARD COLARCO

##### Active Member
I don't talk to people who are "offended" by SI units. I just pity them.

#### shockie

##### High Plains Drifter
I'm retired from a US manufacturer of high precision optical and electronic equipment. I never saw anything but SI units employed. It is only the dumb, recalcitrant US population that clings to feet, inches, and pounds.
I will admit I'm one of the dumb recalcitrant one,( nice big word there), that clings to ft,in,lbs....but in my defense I can convert both imperial and metric back and forth in my mind so I can't be too dumb. I

Last edited:

#### Steve Shannon

##### Well-Known Member
TRF Supporter
I remember when they tried the metric system many decades ago. Didn't work then won't work now. It would take generations to change over.
That’s profoundly incorrect. You obviously don’t realize it but we did adopt it and use it in science and engineering frequently. I don’t know of any good engineer or scientist who has been formally trained in the past couple of generations who cannot move easily between the SI and English systems. Look at whiskey bottles, medicine dosages, and ammunition. The fact that other things are still being measured in English units, such as gasoline or mileage, just shows how flexible people minds are. We didn’t have to vanquish the English system to adopt the metric system.

#### CrusherDave

##### New Member
^^^^^^ This.

However, we might be sliding back at the expense of hard science......

I am a rocket scientist. When working out problems - and using metric, the issue that crops up is when you make a mistake in metric conversions - you are off by a factor of 10 or 100, or 1000, etc. So where is your mistake? Imperial is 12, 32, 386, 5280, and so on. Easy to find where you veered from the straight and narrow.

Also, we frequently get units from suppliers where something might be measured in N-mm^2 and often has a large exponent associated with it. we usually convert it to Imperial to find out where it fits in the scheme of things

#### RICHARD COLARCO

##### Active Member
^^^^^^ This.

However, we might be sliding back now that we must teach "wokeness" and social justice at the expense of hard science......

I am a rocket scientist. When working out problems - and using metric, the issue that crops up is when you make a mistake in metric conversions - you are off by a factor of 10 or 100, or 1000, etc. So where is your mistake? Imperial is 12, 32, 386, 5280, and so on. Easy to find where you veered from the straight and narrow.

Also, we frequently get units from suppliers where something might be measured in N-mm^2 and often has a large exponent associated with it. we usually convert it to Imperial to find out where it fits in the scheme of things
Geez, if you are careful, and someone checks your calculations (which is universally done), you will never have a mistake in conversions. But not really an issue in industry, where no one uses English units for anything important.

I got my BS in the 1960s, and in school we used SI units exclusively. (Called them MKS then.) I went to work on the Apollo program, and had to learn what a nautical mile is. Apollo used a mishmash of SI and MKS, and mistakes happened. But they were always caught before anything bad happened.

#### CrusherDave

##### New Member
Geez, if you are careful, and someone checks your calculations (which is universally done), you will never have a mistake in conversions. But not really an issue in industry, where no one uses English units for anything important.

I got my BS in the 1960s, and in school we used SI units exclusively. (Called them MKS then.) I went to work on the Apollo program, and had to learn what a nautical mile is. Apollo used a mishmash of SI and MKS, and mistakes happened. But they were always caught before anything bad happened.
There are pieces of a probe somewhere between here and Mars because somebody didn't bother to check the math.........

And at my company, we've built 1 4th generation fighter and 2 5th generation fighters using imperial units....... all since 1973.

Last edited:

#### b.wieting

##### Member
TRF Supporter
Metric? In the mid-70s I was in chem grad school when a well-know Soviet solid-state physicist visited to give seminars. I think his real purpose was medical consultation since I was tasked to drive him to the San Francisco medical center. (Since I had the best car, a '74 Chevy Vega).
As we drove up the Junipero Serra freeway he asked about the distance signs, kilometers or miles? I told him, "Oh no, miles. We Americans think the metric system is a communist conspiracy." He burst out laughing.

#### teepot

##### Well-Known Member
TRF Supporter
I'm not talking about engineers and scientists. I'm talking about the average person that goes to Walmart in their pajamas.

##### Lonewolf.... No Club
Apollo used a mishmash of SI and MKS, and mistakes happened. But they were always caught before anything bad happened.
and then.. fast forward a few decades and...

#### MetricRocketeer

##### Member of the U.S. Metric Association
TRF Supporter
Apollo used a mishmash of SI and MKS, and mistakes happened. But they were always caught before anything bad happened.
Hi Richard,

Here is something very bad that happened:

In 1998 and 1999, NASA launched the Mars Climate Orbiter. It crashed because one of NASA's contractors with whom it was doing business (Lockheed Martin) was using a different measurement unit from what NASA was using. NASA was using Newton-seconds -- a metric unit, so good for NASA for using it -- while Lockheed Martin was using pound-force-seconds, a non-metric unit. So shame on Lockheed Martin for not using metric units and shame on NASA for not requiring its contractors to also use metric units.

Imagine, tens of millions of dollars and thousands of labor hours all lost because of mismatched units. Thank goodness this was an unstaffed mission, so no human life was lost. Even so, however, what a waste of resources.

Stanley