# Thrust to weight Ratios of Model Rockets: ELI5

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#### shockie

nevermind

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##### Lonewolf.... No Club
It would be very easy if that chart in post #13 had units, but it doesn't and the units I try don't give those numbers. Here's something I can stand by: http://rocketrycalculator.com/rocketry-calculator/thrust-to-weight-ratio/
Thanks for posting this.
I'm running the numbers on my past builds... that are pretty much all "Jimmy Durante" designs...that need a lot of nose weight, not traditional "looks like an arrow" rockets, and I'm getting numbers like 31:1, 18:1, 14:1, 13.5:1.

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#### Funkworks

##### Well-Known Member
That calculator I linked to includes "Gravity(".
Well that "Gravity(" refers to the vertical acceleration at the Earth's surface in metric units (9.8m/s^2):
It should be: $ratio=\frac{Thrust}{Weight}$
$ratio=\frac{Trust[N]}{Mass[kg]9.8m/s^2}$

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##### Lonewolf.... No Club
That calculator I linked to includes "Gravity(".
Well that should be "Gravity(9.8m/s^2)"
So the online calculator is incorrect?

#### Funkworks

##### Well-Known Member
Well all I see in the link is "Gravity(", but you never have an open parenthesis without a closing one. I filled in the missing part in post #34. "Gravity" is shorthand for what I say specifically in post 34.

But who knows, maybe some rocket clubs define the ratio as [Thrust in Newtons / Weight in lbs], or use some other units. Could be average thrust, initial thrust, max thrust, imperial, metric. Everything is "legal". It's always "some thrust" and "some weight", but I can't garantee everyone uses the same units.

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#### Jowayen

##### Well-Known Member
All I need to know is what does the RSO use at the check-in table?
And (here's the tough part), do all RSO's use the same check using the same units?

In other words, are there TRA and NAR standards that all RSO's have to follow?
Will I get the same result (ok to go, or no) everywhere I go with the same rocket?

And if not, why not?

#### SecondRow

##### Well-Known Member
All I need to know is what does the RSO use at the check-in table?
It's up to the RSO.
And (here's the tough part), do all RSO's use the same check using the same units?
Probably not.

In other words, are there TRA and NAR standards that all RSO's have to follow?
Yes. 3:1 using the certified average thrust of the motor.
Will I get the same result (ok to go, or no) everywhere I go with the same rocket?
Probably not. But the higher your T:W, the more likely you'll get a "go".

And if not, why not?
NFPA 1127, the NAR High Power Safety Code and TRA's Safe Launch Practices require that the certified average thrust of the motor be 3 times the total weight. But this is just a minimum. In the interest of safety, the RSO is free to require a higher thrust if he chooses, based on his experience and judgment. We trust the RSO is in a better position to determine the correct T:W ratio needed for the situation than the writers of the respective codes. The correct T:W is not one-size fits all, so the RSO has the final say, so long as the minimum is met.

#### Jowayen

##### Well-Known Member
... the RSO is free to require a higher thrust if he chooses...
I have to say that's one area of this sport that I dislike the most

##### Lonewolf.... No Club
I have to say that's one area of this sport that I dislike the most
Always follow the safety code... but fly on your own...

#### SecondRow

##### Well-Known Member
I have to say that's one area of this sport that I dislike the most
It is what it is. But consider this from the RSO’s perspective. His job is to keep spectators, flyers and property as safe as he can while ensuring everyone has a good time. He has to balance between people having fun and keeping the club’s insurance intact.

As lakeroadster said, you can always fly on your own. Then you can be your own RSO. You just have to get permission from a property owner, a waiver from the FAA (for most HPR), and follow the respective safety code and the applicable regulations.

#### Jowayen

##### Well-Known Member
… As lakeroadster said, you can always fly on your own. Then you can be your own RSO. You just have to get permission from a property owner, a waiver from the FAA (for most HPR), and follow the respective safety code and the applicable regulations.
Which is a ridiculous suggestion.
My point is why so much variation from one to the next?
If it's safe, it's safe. If we have national standards, national organizations, why so arbitrary?

#### Jowayen

##### Well-Known Member
All I'm saying is I'd like to be reasonably confident after I spend weeks reading thru the standards/code, simulating, building and painting a rocket, that after I drive 3 or 4 hours to a launch, I don't get turned away because of an extra restrictive launch Nazi.

#### SecondRow

##### Well-Known Member
Which is a ridiculous suggestion.
My point is why so much variation from one to the next?
If it's safe, it's safe. If we have national standards, national organizations, why so arbitrary?
Yes. Ridiculous. No one has ever done it before. Gosh, how do the clubs do it?

The standards provide a minimum level of safety. Clubs are free to raise the standards to their level of comfort.

extra restrictive launch Nazi.

#### samb

Which is a ridiculous suggestion.
My point is why so much variation from one to the next?
If it's safe, it's safe. If we have national standards, national organizations, why so arbitrary?
I think that there is a lot less variation than you might think. The national organizations have historically depended on local expertise to administer range safety. And a relatively short list of rules.

All I'm saying is I'd like to be reasonably confident after I spend weeks reading thru the standards/code, simulating, building and painting a rocket, that after I drive 3 or 4 hours to a launch, I don't get turned away because of an extra restrictive launch Nazi.
I've met a few RSO's and I'm reasonably sure none are the "n" word. I'm extremely sure that they all want you to have a successful flight. Have you been turned away from a launch before ? Do you know someone who has ? Just trying to understand where you're coming from. Certainly, if I was going somewhere new to fly I would make an effort to get acquainted with the club notables and tell them about your rocket and what you'd like to accomplish on launch day. Yes, I believe launching with a club is different than launching by yourself. I would argue that it's safer and funer !

##### Lonewolf.... No Club
Which is a ridiculous suggestion.
My point is why so much variation from one to the next?
If it's safe, it's safe. If we have national standards, national organizations, why so arbitrary?
Why is it ridiculous?

#### icyclops

##### Well-Known Member
I don't talk to people who are "offended" by SI units. I just pity them.
You must be very lonely....

#### Ez2cDave

##### Well-Known Member
TRF Supporter
It is only the dumb, recalcitrant US population that clings to feet, inches, and pounds.
We have to "cling to feet", since today's public schools turn out young people that still have to count on their fingers ( "feet" adds another "10" to their data set ), can't tell time on a clock with hands, or "make change", in their heads !

Dave F.

#### shockie

##### High Plains Drifter
Are we talking about the soup nazi?

##### Lonewolf.... No Club
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.
Pretty wide paint brush you are using... Douche Bag.

#### dhbarr

##### Amateur Professional
In defense of the American populace, more than a quarter of us can't reasonably estimate any measurement in any system.

#### Scud-B

##### Active Member
I have a mechanical engineering degree and found working with the metric system to be a tad clunky. The unit conversion process, when doing the work by hand, took enough more time that it got annoying to work on problems in SI units. I actually subscribe to the idea that they both have real-world advantages and disadvantages and frankly don't care which someone prefers because I could see either side. When I hear/read people arguing over which system to use, it reminds me of people arguing over Coke or Pepsi, coffee or tea, etc. It's just a weird subject to get personal about.

#### MetricRocketeer

##### Well-Known Member
TRF Supporter
Hi Scud-B,

The point is to work only in the metric system. That way no one ever has to convert out of it to any other system.

Stanley

#### Scud-B

##### Active Member
Hi Scud-B,

The point is to work only in the metric system. That way no one ever has to convert out of it to any other system.

Stanley
Yeah, I got that, but even when solving entirely metric problems, beam design for bridges, bending moments, etc, I was doing conversions between metric units. I simply don't see either system as generally superior if you've been trained in both. I personally dislike some of the work using SI units, but I don't believe everyone has to agree, or even will agree. Now, since this is on my mind and the SI/Imperial (or whatever we're calling it) debate has become part of the topic, I really dislike people giving me fractional measurements with non-SI units versus decimal measurements. The need to bounce back and forth with that is a pain in the butt, but isn't a real concern except for dealing with laypeople and fasteners.

#### MetricRocketeer

##### Well-Known Member
TRF Supporter
What do you mean, please, that you were doing conversions between metric units?

#### jrap330

##### Retired Engineer, NAR # 76940
TRF Supporter
All I'm saying is I'd like to be reasonably confident after I spend weeks reading thru the standards/code, simulating, building and painting a rocket, that after I drive 3 or 4 hours to a launch, I don't get turned away because of an extra restrictive launch Nazi.
You have a good point but you have not consider another reason why.......besides the safety code and regulations ........Unless there is a Specific law stating what can be and not be done....most everything is up to discretion. So base on Thrust to weight ratio.......your SIMs, actual flight data you can provide,, discretionary decision making is normal a expectation. As others have mention in other posts...you can buy a motor on site from a vendor that RSO make agreed with.

#### Scud-B

##### Active Member
What do you mean, please, that you were doing conversions between metric units?
I'd have to dig out my old work for specific examples and that's buried in a storage unit right now.

But generally, when I say conversions among metric units, I mean something like grams*cm/s^2 to kg*m/s^2. That's not one of the specific examples from my work, it's just a conversion within a measurement system.

Again, I didn't say I was doing enough added work within SI to make it difficult, just enough for me to not enjoy the added work.

#### MetricRocketeer

##### Well-Known Member
TRF Supporter
But generally, when I say conversions among metric units, I mean something like grams*cm/s^2 to kg*m/s^2. That's not one of the specific examples from my work, it's just a conversion within a measurement system.
Hi Scud-B,

OK. That's a fine example. And solving that problem is so easy: 1 g =0.001 kg and 1 cm = 0.01 m. Thus g*cm / s^2 = (0.001)*(0.01) kg*m / s^2 = 0.00001 kg*m / s^2.

Now contrast that with converting ounces*inches / s^2 to pounds*miles / s^2. And here we have an arithmetic problem requiring a calculator.

Stanley

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#### Buckeye

##### Well-Known Member
TRF Supporter
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 am nearly retired from a rust-belt, dinosaur age, manufacturer. I never saw anything but SI units in my 35 years.