Thrust/Weight Ratio and Flight Forces

[Nytrunner]

It's my understanding that the 5:1 rule is, in fact, based on initial thrust not average thrust. The problem with this kit seems to be though that even the recommended initial thrust is about half of what it should be to get adequate velocity off the rail. It is a pretty big rocket for the motor class just in terms of diameter (3") and length (5 ft) so maybe it's just generating a ton of drag relative to its weight? idk

I've heard several RSO's base their judgments off of average thrust unfortunately.

I think you're asking good questions. This is your rocket, and if you don't feel comfortable with a minimum 5:1, then by all means, choose a motor that simulations show will clear the rail at a sufficient speed. Also, a 5' rail sounds extremely short, so THat could explain why you're having trouble getting up to speed. Have you asked the club you're going to fly with how long their rails are?


Just for anecdote sake: My L1 was paper/ply, 4" by 52", weighed 5.2# lbs loaded (It's undergone weightloss surgery since then), and flew off an 8' rail using an H120. Perfect flight to ~1/4 mile. The RSO raised an eyebrow at the H120 on a 5 lb rocket, but he accepted the knowledge that an H120 has initial thrust ~145N and didn't predict any issue. (Which he was right about )

 

[djkingsley]

Remember when you're talking about "high thrust".. there's "high thrust" and "sustained thrust". What usually does rockets in is where the thrust continues over the length of time. That's why a I250 and J250 may sound equivalent, but the J burns for twice as long and can cause much more problems.

Watch the fins on this video here- note that it didn't break loose right at the ground- it's after some sustained force that everything went all kafloopy:

https://youtu.be/M_FCQ550770?t=69

Beautiful video, right up there with the Tacoma bridge.

 

[RomCat]

I've heard several RSO's base their judgments off of average thrust unfortunately.

I think you're asking good questions. This is your rocket, and if you don't feel comfortable with a minimum 5:1, then by all means, choose a motor that simulations show will clear the rail at a sufficient speed. Also, a 5' rail sounds extremely short, so THat could explain why you're having trouble getting up to speed. Have you asked the club you're going to fly with how long their rails are?


Just for anecdote sake: My L1 was paper/ply, 4" by 52", weighed 5.2# lbs loaded (It's undergone weightloss surgery since then), and flew off an 8' rail using an H120. Perfect flight to ~1/4 mile. The RSO raised an eyebrow at the H120 on a 5 lb rocket, but he accepted the knowledge that an H120 has initial thrust ~145N and didn't predict any issue. (Which he was right about )

The rails at the club launch looked to be 6' or 7', but I'm basing 5' on the distance between the top of the rail and the top rail button which is probably about 2' from the aft of the rocket. They have some longer ones, but I'm not sure if they're 1" rails. They might be. I'll have to ask. Thanks.

 

[AlfaBrewer]

It's my understanding that the 5:1 rule is, in fact, based on initial thrust not average thrust. The problem with this kit seems to be though that even the recommended initial thrust is about half of what it should be to get adequate velocity off the rail. It is a pretty big rocket for the motor class just in terms of diameter (3") and length (5 ft) so maybe it's just generating a ton of drag relative to its weight? idk

Your understanding is incorrect. NFPA 1127 and the NAR HP safety code specifically state that the certified average thrust is to be used for the 3:1 calculation. I didn't see a ratio in the TRA HP code.

 

[Steve Shannon]

Your understanding is incorrect. NFPA 1127 and the NAR HP safety code specifically state that the certified average thrust is to be used for the 3:1 calculation. I didn't see a ratio in the TRA HP code.

NFPA 1127 is the basis of the TRA Safety Codes. The other two documents, Tripoli HPR Safety Code and Tripoli Research Safety Code build on top of 1127, so we require 3:1 also. The 3:1 average thrust to weight ratio is a minimum and was done with an expectation that RSOs would exercise the judgement necessary to ensure a stable velocity when the rocket departs the rail.
As others have pointed out the average thrust to weight is generally useless. What's important is the speed off the rail which is the result of the thrust during the short time the rocket is on the rail.


Steve Shannon

 

[Nytrunner]

NFPA 1127 is the basis of the TRA Safety Codes. The other two documents, Tripoli HPR Safety Code and Tripoli Research Safety Code build on top of 1127, so we require 3:1 also. The 3:1 average thrust to weight ratio is a minimum and was done with an expectation that RSOs would exercise the judgement necessary to ensure a stable velocity when the rocket departs the rail.
As others have pointed out the average thrust to weight is generally useless. What's important is the speed off the rail which is the result of the thrust during the short time the rocket is on the rail.


Steve Shannon

At least they didn't require 5:1 based on average alone.... That rule still makes me shake my head. Who knows, maybe some day it will change to focus on initial thrust.

 

[Steve Shannon]

At least they didn't require 5:1 based on average alone.... That rule still makes me shake my head. Who knows, maybe some day it will change to focus on initial thrust.

That's what we expect the RSOs or rocket checkin staff to do, but the initial responsibility is actually on the flyer.


Steve Shannon

 

[markkoelsch]

For discussion sake, the vast majority of my flights are 10:1 or more off the pad.

 

[Bat-mite]

The 5:1 average thrust to weight rule sets a measurable standard that inexperienced RSOs can use to allow or deny flights. They can have a chart that shows the minimum allwable avg. thrust for the rocket's weight.

However, this should never be the final word in whether or not the rocket can launch. If an inexperienced RSO denies a flight, there should be a vehicle through which the flier can appeal. A thrust curve, a simluation result, a discussion about wind speed and direction, rail length, etc., should be had with a more experience flier who is willing to say, "This flier will be allowed to fly this rocket on that motor based on data presented."

Meanwhile, for 90% of flights, the book data is going to be adequate.

 

[Steve Shannon]

The 5:1 average thrust to weight rule sets a measurable standard that inexperienced RSOs can use to allow or deny flights. They can have a chart that shows the minimum allwable avg. thrust for the rocket's weight.

However, this should never be the final word in whether or not the rocket can launch. If an inexperienced RSO denies a flight, there should be a vehicle through which the flier can appeal. A thrust curve, a simluation result, a discussion about wind speed and direction, rail length, etc., should be had with a more experience flier who is willing to say, "This flier will be allowed to fly this rocket on that motor based on data presented."

Meanwhile, for 90% of flights, the book data is going to be adequate.

3:1 is a requirement, but not terribly useful, 5:1 is a reasonable starting guideline for some circumstances, but demands judgement in application, and although there's always the chief RSO or Launch Director to appeal to, for the most part the LD should not encourage the appeal process. Any marginal rocket/motor combinations should be discussed in advance, with simulation data exactly as you say. Lack of experience among RSOs may require an appeal process or the supervision of a more experienced RSO to assist or supervise the people working to check-in rockets. There will always be flyers who disagree with RSOs but the process must support the final authority of the head RSO, even at the risk of occasionally alienating flyers.


Steve Shannon

 

[CzTeacherMan]

Resurrecting a dead thread for a good cause ...
I'm making a table of Thrust-To-Weight ratios...
Question - wasn't there a rule of thumb that X:1 to start and then add +1 to X for every Y mph of wind? I can't remember to save my life. It was something like 3:1 then add +1 for every 5mph of wind or something like that. Anyone know?
My end goal is to make a TTW chart that will be easier to use than memory and on-the-field brain-calculations... The kind of thing you can just look at and figure a quick estimate of your TTW ratio and pick a "minimum safe column" depending on conditions. RSOs could use the chart based on average thrust or initial thrust, depending on club rules and/or current conditions.

 

[crossfire]

Eric
I think TWA had some kind of chart that the RSO could check weight with motor size and it would give one safe or not safe to fly. Maybe ask Frank if we still have this chart. It may help you with a up date to charts. Its been many years since I seen it.

 

[CzTeacherMan]

Eric
I think TWA had some kind of chart that the RSO could check weight with motor size and it would give one safe or not safe to fly. Maybe ask Frank if we still have this chart. It may help you with a up date to charts. Its been many years since I seen it.

Yeah, I found that. Ed Dewey made a thing back in 2000 (shared above). And there are a bunch out there, but they're all a bit hard to read/use because they depend on the curve of a line drawn on a graph. Definitely precise, but I'm making something a bit more "blunt-force/quick-reference"... When it's done, I'll post it.
I was looking more for a way to add something to my table that says, "3:1 is the minimum safe TTW ratio (per NFPA1127), but we use 5:1 as a minimum. Please add 1 order of TTW ratio per 5mph of wind" Etc.... After a few phone conversations, I don't think that's necessary. We can probably just instruct the RSO on the day-of the launch that, "based on the wind today, let's have the Prefect RSO any flights under 6:1 thrust" That kind of a thing.

 

[Steve Shannon]

Yeah, I found that. Ed Dewey made a thing back in 2000 (shared above). And there are a bunch out there, but they're all a bit hard to read/use because they depend on the curve of a line drawn on a graph. Definitely precise, but I'm making something a bit more "blunt-force/quick-reference"... When it's done, I'll post it.
I was looking more for a way to add something to my table that says, "3:1 is the minimum safe TTW ratio (per NFPA1127), but we use 5:1 as a minimum. Please add 1 order of TTW ratio per 5mph of wind" Etc.... After a few phone conversations, I don't think that's necessary. We can probably just instruct the RSO on the day-of the launch that, "based on the wind today, let's have the Prefect RSO any flights under 6:1 thrust" That kind of a thing.

I think that’s the perfect way to handle it!

 

[crossfire]

Yeah, I found that. Ed Dewey made a thing back in 2000 (shared above). And there are a bunch out there, but they're all a bit hard to read/use because they depend on the curve of a line drawn on a graph. Definitely precise, but I'm making something a bit more "blunt-force/quick-reference"... When it's done, I'll post it.
I was looking more for a way to add something to my table that says, "3:1 is the minimum safe TTW ratio (per NFPA1127), but we use 5:1 as a minimum. Please add 1 order of TTW ratio per 5mph of wind" Etc.... After a few phone conversations, I don't think that's necessary. We can probably just instruct the RSO on the day-of the launch that, "based on the wind today, let's have the Prefect RSO any flights under 6:1 thrust" That kind of a thing.

Yeah, I found that. Ed Dewey made a thing back in 2000 (shared above). And there are a bunch out there, but they're all a bit hard to read/use because they depend on the curve of a line drawn on a graph. Definitely precise, but I'm making something a bit more "blunt-force/quick-reference"... When it's done, I'll post it.
I was looking more for a way to add something to my table that says, "3:1 is the minimum safe TTW ratio (per NFPA1127), but we use 5:1 as a minimum. Please add 1 order of TTW ratio per 5mph of wind" Etc.... After a few phone conversations, I don't think that's necessary. We can probably just instruct the RSO on the day-of the launch that, "based on the wind today, let's have the Prefect RSO any flights under 6:1 thrust" That kind of a thing.

Eric
Where did you find Ed's info?

 

[Ez2cDave]

TRIPOLI Gerlach produced this document, a while back . . . PDF attached below !

Dave F.

 

[Ez2cDave]

I was looking more for a way to add something to my table that says, "3:1 is the minimum safe TTW ratio (per NFPA1127), but we use 5:1 as a minimum. Please add 1 order of TTW ratio per 5mph of wind" Etc.... After a few phone conversations, I don't think that's necessary. We can probably just instruct the RSO on the day-of the launch that, "based on the wind today, let's have the Prefect RSO any flights under 6:1 thrust" That kind of a thing.

How are you going to accurately measure the wind velocity, on the field ?

Dave F.

 

[markjos]

TRIPOLI Gerlach produced this document, a while back . . . PDF attached below !

Dave F.

This is nice, although it unfortunately confuses Impulse (Ns) with Thrust (N), so I'm concerned that it promotes that misunderstanding. Average Thrust is in Newtons. Impulse, or Total Impulse is in Newton-Seconds. Sorry to be picky, but this is stuff we should really get right. A couple of corrections would make this document pretty handy.

Mark

 

[Nytrunner]

...interesting pdf.

They need to clean up the unit terms, make some mention of thrust curve variation (what if the initial thrust is less than the average....), and the table can really be truncated at the higher thrust ranges (I dont thing the difference between 3200 and 3225 is that critical at that scale of thrust).

 

[Ez2cDave]

There is a problem that I would like to bring up . . . Moonburners.

They have a high initial thrust that trails off, throughout the burn, ending with a fairly long, low-thrust period . . . Unfortunately, the "Average Thrust" does not not tell the "true story" of the motor's "lifting ability".

Dave F.

 

[Nytrunner]

I find myself agreeing with Dave here.

There are motors where initial thrust are significantly more than average rating as well

 

[OverTheTop]

How are you going to accurately measure the wind velocity, on the field ?

Doesn't everyone have an anemometer? I do . You can get them for a small number of dollars on eBay. Much more accurate than my WAG.

 

[CzTeacherMan]

There is a problem that I would like to bring up . . . Moonburners.

They have a high initial thrust that trails off, throughout the burn, ending with a fairly long, low-thrust period . . . Unfortunately, the "Average Thrust" does not not tell the "true story" of the motor's "lifting ability".

Dave F.

Easy. Most flights will be RSO'd without a hitch. For the moonburners, the RSO would question the flyer who would explain it's a moonburners, and we're good to go. The idea is to initiate a conversation between the RSO and flyer to be sure both people know what is being flown and whether it's safe.

 

[CzTeacherMan]

So here's what I produced... The RSO checklist might be on the back of our flight cards this fall, depends on price to print.
The TTW chart will be useful as a handy way to approximate TTW ratio quickly without flipping through page after page of info. Something that can be taped to the table.
There may be changes, or maybe a lot of changes, but this is draft #1...
View attachment QCRS RSO Charts.pdf

 

[markjos]

So here's what I produced... The RSO checklist might be on the back of our flight cards this fall, depends on price to print.
The TTW chart will be useful as a handy way to approximate TTW ratio quickly without flipping through page after page of info. Something that can be taped to the table.
There may be changes, or maybe a lot of changes, but this is draft #1...
View attachment 385694

Eric, this is nicely done, but on the Thrust to Weight table, shouldn't you be referring to Average (or initial, or peak) Thrust (N or lb), rather than Installed Impulse (N-sec or lb-sec)? After all, we're talking about Thrust to Weight, not Impulse to Weight, right?

(https://www.tripoli.org/Portals/1/Documents/Safety Code/Range Safety Guidelines v1.3.pdf see p. 12)

Thanks for indulging me.

Mark

 

[Ez2cDave]

Easy. Most flights will be RSO'd without a hitch. For the moonburners, the RSO would question the flyer who would explain it's a moonburner, and we're good to go. The idea is to initiate a conversation between the RSO and flyer to be sure both people know what is being flown and whether it's safe.

The possible situation I may be in is my upcoming L3 Cert flight . . .

The flight goals are "low & slow", sub-transonic ( well below .8 Mach / 900 fps ), minimum speed, minimum altitude, in a 7.5" x 11 ft "sonotube" rocket ( possibly not glassed or very lightly glassed - 1 layer of 2oz. , primarily for finishing purposes ). I will be flying off my 12ft Black Sky Extreme Rail.

Both NAR & Tripoli state that a 3:1 thrust to weight ratio is acceptable and "legal". The TAP / L3CC is talking about wanting 5:1 ( which is not a problem for the first 4.5 - 5 seconds of the burn ).

The problem is the long "tail-off" of the motor bring the "average thrust" down . . .

Dave F.

 

[CzTeacherMan]

Eric, this is nicely done, but on the Thrust to Weight table, shouldn't you be referring to Average (or initial, or peak) Thrust (N or lb), rather than Installed Impulse (N-sec or lb-sec)? After all, we're talking about Thrust to Weight, not Impulse to Weight, right?

(https://www.tripoli.org/Portals/1/Documents/Safety Code/Range Safety Guidelines v1.3.pdf see p. 12)

Thanks for indulging me.

Mark

Nope.
I didn't want to put average or initial or whatever. Just impulse to weight. That way, an RSO can use the motor designation (average), but if the flyer can say that the initial is "X", then they can check that as well.
And if you're hung up on the terms... No need to be. You can just delete the words and the numbers mean the same thing. Simple.

 

[CzTeacherMan]

The possible situation I may be in is my upcoming L3 Cert flight . . .

The flight goals are "low & slow", sub-transonic ( well below .8 Mach / 900 fps ), minimum speed, minimum altitude, in a 7.5" x 11 ft "sonotube" rocket ( possibly not glassed or very lightly glassed - 1 layer of 2oz. , primarily for finishing purposes ). I will be flying off my 12ft Black Sky Extreme Rail.

Both NAR & Tripoli state that a 3:1 thrust to weight ratio is acceptable and "legal". The TAP / L3CC is talking about wanting 5:1 ( which is not a problem for the first 4.5 - 5 seconds of the burn ).

The problem is the long "tail-off" of the motor bring the "average thrust" down . . .

Dave F.

That's between you and your TAP.

If it was me... I'd make and print the data of the whole fight Sim for that discussion between me and my TAP. As long as the initial 4 seconds are good, then I'd argue for it. But I would remember that the motor test data is not the data of the motor I'm actually burning... Test data can be off by as much as 20%, so, especially for a cert flight, I would want an abundance of caution.

But if the rail velocity, acceleration, and stability is all within optimal range for the takeoff through burnout, I wouldn't be concerned at all. And if I were the TAP of a flyer who had done the work to figure out all that info for the entire flight, I'd feel safe signing off on it.

But again, it's between you and your TAP. And then, the RSO on the field has the final say.

Best of luck!

 

[JohnCoker]

I always prefer to run simulations rather than use rules of thumb. Give this a try:
thrustcurve.org/guidepage.jsp

 

[Ez2cDave]

I always prefer to run simulations rather than use rules of thumb. Give this a try:
thrustcurve.org/guidepage.jsp

Thanks, John . . . Very helpful !

Here are my "numbers", based on "Dry Weights" of 30 lb., 35 lb and 38.5 lb., respectively.

30 lb Dry Weight


35 lb Dry Weight



38.5 lb Dry Weight


Dave F.