Thrust to weight Ratios of Model Rockets: ELI5

[Steve Shannon]

Hi everyone,

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

Stanley

Well, not necessarily, at least not the way things are now. Most metric scales use markings of mass (grams and kilograms) rather than weight (force - Newtons), so a person still needs to apply a factor. Probably digital scales have a setting to display Newtons but many people don't realize the difference.

 

[Steve Shannon]

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?

Maybe I can help.
The requirement in NFPA 1127 is poorly written. It ought to be corrected. Average thrust is sometimes meaningless. It's the airspeed when a rocket leaves the end of the launch platform that determines whether aerodynamic forces are sufficient to stabilize a rocket. But different rockets have different margins of stability and require more or less airspeed. Also, other factors such as wind speed may cause rockets with greater margins of stability to weathercock.
The local RSOs and Launch Directors know what has worked at their site. On a dead still day, from a long rail, a 3:1 might be okay. On a windy day or with a motor that doesn't come up to full thrust immediately the average thrust isn't helpful.
For Tripoli we actually suggest 5:1 initial thrust to weight rather than relying solely on the 3:1 average thrust to weight.
RSOs develop scars and sensitive spots from past incidents. We all have different COAs with different radiuses as well. We don't want a rocket flying out of the COA so if a rocket is predicted to fly high on a low thrust motor we might want to see a little more speed coming off the rail. If we have a long rail we might allow a rocket to fly with a lower T/W ratio. Conversely, we will want to see higher T/W on the short rails.
However, with all that said, I've never seen a 5:1 average thrust to weight ratio turned down assuming everything else looks reasonable.

 

[MetricRocketeer]

Well, not necessarily, at least not the way things are now. Most metric scales use markings of mass (grams and kilograms) rather than weight (force - Newtons), so a person still needs to apply a factor. Probably digital scales have a setting to display Newtons but many people don't realize the difference.

Hi Steve,

Thank you for your response.

I respectfully disagree. We could get the results right away -- or in any event, very quickly, and much more quickly than using the foot-pound system.

I will illustrate using the data from my L1 certification flight, which as my signature shows I flew on a Minie-Magg using an H123 motor.

So the motor designation -- in this case, 123, meaning 123 Newtons -- shows the thrust. That is the numerator, which we determine by inspection.

Now, my rocket with motor loaded had a mass of 2087 grams, which we can immediately convert to 2.087 kilograms. As you correctly point out, however, we need the weight, not the mass. To find the weight, we multiply 2.087 times 9.8, which means metres per square second. Actually, 9.8 is itself a very close approximation to one gravity. Let's just use 10, for a completely reasonable and slightly conservative result.

Thus, here's the entire calculation: \( \frac {123}{2.087 \times 10} \approx 5.9 \)

Honestly, isn't this convenient and straightforward? I believe it is.

Stanley

 

[Ez2cDave]

Hi Steve,

Thank you for your response.

I respectfully disagree. We could get the results right away -- or in any event, very quickly, and much more quickly than using the foot-pound system.

I will illustrate using the data from my L1 certification flight, which as my signature shows I flew on a Minie-Magg using an H123 motor.

So the motor designation -- in this case, 123, meaning 123 Newtons -- shows the thrust. That is the numerator, which we determine by inspection.

Now, my rocket with motor loaded had a mass of 2087 grams, which we can immediately convert to 2.087 kilograms. As you correctly point out, however, we need the weight, not the mass. To find the weight, we multiply 2.087 times 9.8, which means metres per square second. Actually, 9.8 is itself a very close approximation to one gravity. Let's just use 10, for a completely reasonable and slightly conservative result.

Thus, here's the entire calculation: \( \frac {123}{2.087 \times 10} \approx 5.9 \)

Honestly, isn't this convenient and straightforward? I believe it is.

Stanley

Hmm . . . There is a "discrepancy" in the "math" ( 5.9 : 1 vs 6.0 : 1 )

123 Newtons / 4.45 = 27.64 lb
2.087 kg = 2,087 grams
2,087 grams / 28.35 = 73.615 oz.
73.615 oz./ 16 = 4.6 lb
27.64 lb / 4.6 lb = 6.0001 : 1 ( Thrust / Weight Ratio )

Personally, I prefer "accurate results" over "conservative results", achieved through accurate calculation.

Dave F.

 

[Jowayen]

The local RSOs and Launch Directors know what has worked at their site. On a dead still day, from a long rail, a 3:1 might be okay. On a windy day or with a motor that doesn't come up to full thrust immediately the average thrust isn't helpful.
For Tripoli we actually suggest 5:1 initial thrust to weight rather than relying solely on the 3:1 average thrust to weight.
RSOs develop scars and sensitive spots from past incidents. We all have different COAs with different radiuses as well. We don't want a rocket flying out of the COA so if a rocket is predicted to fly high on a low thrust motor we might want to see a little more speed coming off the rail. If we have a long rail we might allow a rocket to fly with a lower T/W ratio. Conversely, we will want to see higher T/W on the short rails.

….everything else looks reasonable....

Way too arbitrary

 

[MetricRocketeer]

Personally, I prefer "accurate results" over "conservative results", achieved through accurate calculation.

Hi Dave F.

No problem there. If you want more accurate results, then use my exact method except do not divide my 10, and instead divide by 9.8. That will give a T:W ratio of 6.013905323.

If you want an even more accurate result, then divide by 9.81 to give a T:W ratio of 6.00777494.

If you want an even more accurate result, then divide by 9.807 to give a T:W ratio of 6.009612742.

If you want an even more accurate result, then divide by 9.8067 to give a T:W ratio of 6.009796584.

If you want an even more accurate result, then divide by 9.80665 to give a T:W ratio of 6.009827226.

The point is to forget ounces and pounds and instead use only grams and kilograms. As we know, Newtons are based upon metres, kilograms, and seconds. Thus, converting from pounds to kilograms only to reconvert kilograms back to pounds is silly and inconvenient. Work only in kilograms. One liter of water weighs one kilogram -- what could be easier than that?

Stanley

 

[Ez2cDave]

The point is to forget ounces and pounds and instead use only grams and kilograms. As we know, Newtons are based upon metres, kilograms, and seconds. Thus, converting from pounds to kilograms only to reconvert kilograms back to pounds is silly and inconvenient. Work only in kilograms. One liter of water weighs one kilogram -- what could be easier than that?

Stanley

Stanley,

It all boils down to using the system that one is most comfortable with, and proficient in using.

Dave F.

 

[Steve Shannon]

Way too arbitrary

Arbitrary implies no rationale. As I attempted to explain, the T/W needed to achieve a safe velocity off the rail varies with the length of the rail, wind speed, and other range conditions, including COA limitations. If you are not aware of the complexity then it might seem arbitrary, but it really isn’t. Our LDs and RSOs usually have reasons based in their experience for the way they do things, but if you want to avoid being challenged keep your thrust to weight above 5:1 and you’ll almost never be questioned or run RockSim or OpenRocket simulations and be ready to show the people checking in rockets the predicted velocity off the rail.

 

[Steve Shannon]

Hi Steve,

Thank you for your response.

I respectfully disagree. We could get the results right away -- or in any event, very quickly, and much more quickly than using the foot-pound system.

I will illustrate using the data from my L1 certification flight, which as my signature shows I flew on a Minie-Magg using an H123 motor.

So the motor designation -- in this case, 123, meaning 123 Newtons -- shows the thrust. That is the numerator, which we determine by inspection.

Now, my rocket with motor loaded had a mass of 2087 grams, which we can immediately convert to 2.087 kilograms. As you correctly point out, however, we need the weight, not the mass. To find the weight, we multiply 2.087 times 9.8, which means metres per square second. Actually, 9.8 is itself a very close approximation to one gravity. Let's just use 10, for a completely reasonable and slightly conservative result.

Thus, here's the entire calculation: \( \frac {123}{2.087 \times 10} \approx 5.9 \)

Honestly, isn't this convenient and straightforward? I believe it is.

Stanley

Stanley,
It is convenient and straightforward, but no more so than using a single conversion factor from an imperial weight to metric weight and then simply finding the ratio. I weigh my rockets in lbs so all I have to do is divide the thrust in Newtons by 4.45 N/lb and then divide that by the weight of the rocket in lbs to find T/W.

 

[kyle]

Way too arbitrary

RSOs know what works at their site. Launches get cancelled due to weather conditions, that's an arbitrary decision. No one is making you abide by their rules. As has been mentioned before, fly on your own if you don't like rules. Clubs want to keep their fields and will be cautious.

If you want to test the limits of the insurance coverage, have at it. Don't expect clubs to jeopardize their field because you feel restricted. Find your own field and don't mention either national organization by name if you burn something down.

Stop crying, no one here will give you cookies.

Kyle

 

[kyle]

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.

You are what makes people turn away from this hobby. Most of us (USA) are, to people like you, dumb and recalcitrant.

Why would young people want to spend their free time with people who openly disrespect them?

Some of these teams are doing things you never would have dreamed of in your supposed career.

Get off my lawn!

 

[Ez2cDave]

If you want to test the limits of the insurance coverage, have at it. Don't expect clubs to jeopardize their field because you feel restricted. Find your own field and don't mention either national organization by name if you burn something down.

Kyle

Kyle,

I don't think that either Tripoli or NAR insurance would cover anyone NOT flying with an NAR Section or a TRIPOLI prefecture . . .

Dave F.

 

[kyle]

Hi Dave F.

No problem there. If you want more accurate results, then use my exact method except do not divide my 10, and instead divide by 9.8. That will give a T:W ratio of 6.013905323.

If you want an even more accurate result, then divide by 9.81 to give a T:W ratio of 6.00777494.

If you want an even more accurate result, then divide by 9.807 to give a T:W ratio of 6.009612742.

If you want an even more accurate result, then divide by 9.8067 to give a T:W ratio of 6.009796584.

If you want an even more accurate result, then divide by 9.80665 to give a T:W ratio of 6.009827226.

The point is to forget ounces and pounds and instead use only grams and kilograms. As we know, Newtons are based upon metres, kilograms, and seconds. Thus, converting from pounds to kilograms only to reconvert kilograms back to pounds is silly and inconvenient. Work only in kilograms. One liter of water weighs one kilogram -- what could be easier than that?

Stanley

I wish I had as much free time as you do to concentrate on inconsequential problems.

 

[RICHARD COLARCO]

You are what makes people turn away from this hobby. Most of us (USA) are, to people like you, dumb and recalcitrant.

Why would young people want to spend their free time with people who openly disrespect them?

Some of these teams are doing things you never would have dreamed of in your supposed career.

Get off my lawn!

Touched a nerve, I did. Sorry if I ruined your day.

 

[DaHabes]

Way too arbitrary

Not arbitrary. Reality. Deal with it or whine somewhere else.

 

[Scud-B]

People are getting grumpy.

I'm not going to bother digging into my old school files to sort out the exact problems that I found slightly more inconvenient to do in SI units, because I was pretty clear that it wasn't inconvenient enough to stop me from solving the problems. Again, they were mostly beam design work, dealing with forces/loads, etc. That said, I didn't find solving for things in Newtons (kg*m/s^2) or pound-force (lbs*ft/s^2) to be particularly difficult either way. I was educated in both and I'm comfortable with both.

If a young person, or old person, only knows one or the other, who cares? Just seems kind of petty to denigrate someone because they choose a different measurement system than someone else.