Problems with the 5:1 thrust-to-weight ratio rule

[Nathan]

Today at the Battlepark launch the RSO would not allow me to fly my Frenzy XL on a CTI 54mm 3 grain J295 Classic motor because it did not pass the 5:1 thrust-to-weight ratio rule. According to the 5:1 rule, my 17 lb Frenzy XL requires a motor with an average thrust of at least 378 newtons. By that standard, it should be safe to fly on a CTI J380 Smokey Sam but not on a CTI J295 Classic. The problem is that the 5:1 rule does not take into account the thrust curve. Like all CTI Classic motors, the J295 has a thrust curve that starts off with lots of power and then tapers off as it burns. The J295 has more initial thrust than a J380 but the J295 burns for a relatively long 4 seconds so it has lower average thrust. OpenRocket simulations for my Frenzy XL with the J295 and J380 show that with the J295 it has higher velocity off the rod, higher maximum velocity, and higher altitude than it does with the J380. The J295 also has higher total impulse than the J380 (1196 Ns for the J295 compared to 1043 Ns for the J380).

Another consideration that needs to be factored in to the decision is the stability of the rocket. A marginally stable rocket is more likely to have problems with a low thrust-to-weight ratio motor that a very stable rocket. My Frenzy XL is very stable, with a stability of about 3.5 cal with either of these motors.

I have flown my Frenzy XL on a J295 before and it was fine. I understand the reasoning behind the 5:1 thrust-to-weight rule but without taking other factors into consideration, it is an incomplete tool for determining which motors are safe to fly with which rockets.

 

[rharshberger]

This is the reason why I have a book for each of my rockets with sims, most RSO's once the see the thrust curve for the motor would probably let you fly based on that, I know I would. That being said I may not have allowed a flight based on what at first glance is a marginal motor at 4:1. But if a flier produced a thrustcurve for the rocket motor and it was 5:1 or better I would give the go ahead. Most RSO's are busy enough to have to leave the burden of proving the motor is safe to fly with to the flier, as they dont have the time to do it themselves.

 

[rstaff3]

Yeah, what Rich said. Unless presented with data, a busy RSO has to go with a simple rule of thumb.

 

[UhClem]

What 5:1 rule? NFPA 1127 has a 3:1 thrust to weight rule (4.9.1) based on certified average thrust.

 

[rharshberger]

What 5:1 rule? NFPA 1127 has a 3:1 thrust to weight rule (4.9.1) based on certified average thrust.

Many clubs have a 5:1 thrust to weight ratio rule to guarantee enough velocity off their clubs rails.

 

[Nathan]

This is the reason why I have a book for each of my rockets with sims, most RSO's once the see the thrust curve for the motor would probably let you fly based on that, I know I would. That being said I may not have allowed a flight based on what at first glance is a marginal motor at 4:1. But if a flier produced a thrustcurve for the rocket motor and it was 5:1 or better I would give the go ahead. Most RSO's are busy enough to have to leave the burden of proving the motor is safe to fly with to the flier, as they dont have the time to do it themselves.

There was a brief discussion of thrust curves but the RSO said "It doesn't matter". I was offered the option to fly the J295 from the away cell but instead I decided to just switch motors. I flew it on a J760 White Thunder instead.

 

[DizWolf]

Another consideration that needs to be factored in to the decision is the stability of the rocket. A marginally stable rocket is more likely to have problems with a low thrust-to-weight ratio motor that a very stable rocket. My Frenzy XL is very stable, with a stability of about 3.5 cal with either of these motors.

I have flown my Frenzy XL on a J295 before and it was fine. I understand the reasoning behind the 5:1 thrust-to-weight rule but without taking other factors into consideration, it is an incomplete tool for determining which motors are safe to fly with which rockets.=

It should be used as a guide, but if a club or RSO has set 5:1, it is what it is. One thing to remember, under low thrust, overstable rockets weathercock like CRAZY on low thrust motors. It's not so much a rule for stability, as it is for getting the rocket up there.

People are always going to get a bit nervous about shooting for the low end on motors. Sure the rule sucks, but clubs tend to set their rules above the min safety margins to give themselves some padding. It's going to take a lot of proof to get past that, and even then they may still say no.

 

[Len B]

There was a brief discussion of thrust curves but the RSO said "It doesn't matter". I was offered the option to fly the J295 from the away cell but instead I decided to just switch motors. I flew it on a J760 White Thunder instead.

I don't like the "It doesn't matter" statement. However, it is not unreasonable or punitive to offer you the away cell, assuming that the away cell has a longer rail.

 

[markkoelsch]

What 5:1 rule? NFPA 1127 has a 3:1 thrust to weight rule (4.9.1) based on certified average thrust.

3:1 is a codified rule. The 5:1 is a rule of thumb- not a hard fast rule.

That said, there are factors beyond thrust to weight. Rocket static margin, rail length, wind, and yes the Thrustcurve. Average thrust is a misnomer for this application in reality. The first second or so of thrust is where it is really at, in my opinion.

 

[Zebedee]

There was a brief discussion of thrust curves but the RSO said "It doesn't matter". I was offered the option to fly the J295 from the away cell but instead I decided to just switch motors. I flew it on a J760 White Thunder instead.

What's the big deal with using the away cell?

 

[MClark]

Some people a just on a power trip.

M

 

[rharshberger]

TCR uses the rule that the rockets velocity on exiting the rail must if the winds are over 5 mph be 4 times the wind speed on H impulse or larger, and it falls on the flier to prove that their rocket does indeed meet those criteria from our rods/rails, most generally the winds are in the 10mph range for us. We don't use the 5:1 thrust:weight ratio, I usually plan on it though as a good rule, once we get our longer rails though that may change for me personally.

 

[jimzcatz]

What 5:1 rule? NFPA 1127 has a 3:1 thrust to weight rule (4.9.1) based on certified average thrust.

Yep, 5 to 1 is a suggestion, although a good one. As a long standing RSO, I go by 5to1 is best, 4to1 works, 3to1,ask alot of questions... 3to1 on an extremely stable rocket with plenty of rod/rail length. I have flown at Battlepark, I would have asked for a second opinion. As an RSO, doing so does not offend me and it shouldn't anyone else. After all, I don't know everything!

 

[cbrarick]

I like the idea of questionable rockets on the away cell. I've often said there is a rail for almost every rocket the only question being "are you willing to walk that far to use it?" After all, the longer rails (usually) and the further distance will keep most rockets from coming back to the crowd. At LDRS 31 I RSO'd most of the odd rockets. The only thing I wanted proof of was that they wouldn't make it back to the parking area.

I guess that's one of the times where a rocksim with the velocity off the rail would have helped....but that's Monday morning quarterbacking on a Sunday.

 

[Buckeye]

Of course, the 5:1 rule is incomplete and obsolete. Simulations are far better. If you had all the evidence shown in post #1 at the launch site, then a reasonable RSO should let you fly. Keep a notebook or pull up Thrustcurve Motor Guide on your smartphone.

 

[dixontj93060]

RSO's make mistakes like all of us. Next time bring the data. 4:1 initial thrust is going to be sufficient 95% of the time.

 

[robertv2]

I have only served as RSO a couple of times but I have flown with several clubs in Texas, and my first thought is anytime the simple math gives an answer that match the real world you should be ready show why. At a min the thrust curve is printed on the instructions and the flier should have presented them. I would have weighed the rocket to be sure it weighs what he thinks and ask for the curve info if seems to be to low of thrust. Offering the away cell is not punitive and it was on the flier have the data ready.

 

[michigander]

I'm not a RSO have never been a RSO

my motor picking for sims above wouldn't probably work , I use 25 avg. ns per loaded lb. , I would suspect above RSO did the same quick math


J295 = 11.8 rocket
J380 = 15.2 rocket

 

[jderimig]

It's speed off the rail. Long distance from top to rail to top button on the rocket? 3 to 1 maybe fine. Short distance from top of rail to top button? 6 to 1 may not be enough.

Good physics should trump arbitrary t/w ratio rules. But the RSO must understand physics.

 

[markkoelsch]

It's speed off the rail. Long distance from top to rail to top button on the rocket? 3 to 1 maybe fine. Short distance from top of rail to top button? 6 to 1 may not be enough.

Good physics should trump arbitrary t/w ratio rules. But the RSO must understand physics.

Exactly.

 

[Nathan]

It's speed off the rail...

What would you consider minimum acceptable speed off the rail? I think anything above 30 mph should be okay, assuming a very stable 17 lb rocket with only about 5 mph wind speed. Here is the simulation data from my Frenzy XL, sorted by speed off the rail:

 

[markkoelsch]

17 pounds with motor or without? How much wind?

That velocity is really bare minimum. Might be ok with a few mph wind by not a lot more.

For instance, my Wildman Extreme is about 16.3 pounds sans motor and stable. My first flight was in an Aerotech K805 so about a 10:1 off the pad. Velocity is your friend for velocity. I would not have done much less.

 

[jderimig]

What would you consider minimum acceptable speed off the rail? I think anything above 30 mph should be okay, assuming a very stable 17 lb rocket with only about 5 mph wind speed. Here is the simulation data from my Frenzy XL, sorted by speed off the rail:

17 pounds with motor or without? How much wind?

That velocity is really bare minimum. Might be ok with a few mph wind by not a lot more.

For instance, my Wildman Extreme is about 16.3 pounds sans motor and stable. My first flight was in an Aerotech K805 so about a 10:1 off the pad. Velocity is your friend for velocity. I would not have done much less.

40 feet per second is kind of a rule of thumb I have heard of. So those speeds (mph) is not unreasonable for low winds like Mark mentions. Another thing to consider is the polar (rotational) moment of the interia of the rocket. Newton's laws applies to rotations also. A long heavy rocket needs less launch velocity than a short rocket of the same mass (it takes more torque to rotate a long rocket than a short one). I think the FrenzyXL is in the long rocket hard to rotate category. I agree with you, methinks that rocket on that motor in a 5mph wind would fly off that pad straight as an arrow.

My Darkstar Extreme (long heavy) flys straight in a 10mph wind at a 3.7 to 1 T/W at launch. But I would never do that T/W with a short rocket.

If you notice at large launches all those big rockets at the away cells tend to go straight up while the lighter shorter rockets at the closer pads not so much. Its not the T/W alone, those heavy rockets have high rotational inertia.

 

[JohnCoker]

I agree that the 5:1 rule is measuring the wrong things when what really matters is speed at the effective end of the launch guide (when the forward guide clears the top). However, it's such an easy rule to understand and calculate that it's hard to fault a RSO for being cautious. Moreover it's so hard to get launch volunteers that we shouldn't give them a hard time even if they make a questionable call.

 

[jderimig]

In meters/s the approximate speed of a rocket leaving the rail is:

V = sqrt(20*L*T/w) where L is the distance from the top button to top of rail and T/W is the thrust to weight ratio.

For t/w=5 and s=1m V=10m/s which is pretty close to the 40ft/s ROT. So 5 to 1 has some sense to it. However T should be the thrust when its on the rail and not the average thrust of the motor.

Solving for T/W needed for 40 ft/s (12 m/s) rail speed:

Min T/W = 7.2/L where L is the distance from of rail to top of button and T is thrust of the motor when on the rail.

 

[Coop]

I agree with John's post, above, and have to add an "and furthermore..." that being:--


I try to keep in mind that it may not be the RSO I've got to convince, as much as it may be the lawyer on the other side of the club's insurance policy or deal they have with the landowner or whatever. I've not flown at Battlepark, and am not familiar with the specifics of their operations, but I do know a thing or two about keeping to a contract clause, and keep this in the back of my mind.

If they don't allow T:W of less than 5:1, it might not be the guy sitting at the table's call. Nothing wrong with bringing thrust curves and end-of-rail speed based upon sims and good solid math... but if the club has an agreement with another entity (as mentioned, insurance, landowner, local fire service, Earl of Zambeezwinot, etc.,) where the 5:1 ratio is used for simplicity (because let's face it--some fliers have difficulty figuring this out; someone not associated with the hobby is going to have even more difficulty wrapping their brain around it), it's unlikely they're going to risk the relationship with whatever entity that is for your flight.

Perhaps, in planning future flights, you could inquire with a club officer prior to going in with a lower-thrust motor. It may be that the situation was merely that someone told the guy at the table, "nothing less than 5:1," and he followed it. It might be a binding agreement they already have in place... or it may be the interpretation of a rule-of-thumb. If the latter, you might have opportunity to present your data for their consideration... then again, they may consider it and say, "no," anyway--as is their right by virtue of being the club and holder of the waiver. I would not take it personally.


Later!

--Coop

 

[Len B]

Just wondering what we do with hybrid motors? Just look at their thrust curves. 5 to 1? Some of them barely can do that with no airframe. I'm lookin' at you RATTWORKS H70
Okay, it is a bit of an exaggeration but a good example. Honestly though, some of these type of flights are why I bought a Giant Leap 10 foot rail. I haven't used it yet but that's what motivated the purchase.

 

[jderimig]

Just wondering what we do with hybrid motors? Just look at their thrust curves. 5 to 1? Some of them barely can do that with no airframe. I'm lookin' at you RATTWORKS H70
Okay, it is a bit of an exaggeration but a good example. Honestly though, some of these type of flights are why I bought a Giant Leap 10 foot rail. I haven't used it yet but that's what motivated the purchase.

View attachment 277830

The time that the rocket is on the rail is

t = sqrt( 20L/(T/W) ) (units meters for L)

For most cases that is usually less than 0.5seconds. So calculate your T/W based on the thrust during the first 0.5seconds of burn. So for the H70 that would be around 120N+.

 

[Bat-mite]

Nathan, if I were you, I would go to a club board meeting and present my case. You might get a buy in at a meeting, but on the field where the RSO is responsible for getting lots of people in the air, not so much. Just a thought.

 

[DAllen]

Just launch from the away pad. Film it and the next time you want to fly that motor/rocket combo you have proof that it flies just fine. As RSO I've told people to fly from the away pads just to see what it does and improve the margin of safety when it might not have been necessary. Nothing wrong with that IMHO if there is any question. For a J motor you walk what, another 200' out? It's kind of unfair to the RSO to expect them to memorize thrustcurve profiles of every commercial reload known to man. OTOH if I'm RSO and someone comes to me with sims and thrustcurves and has actually done a little homework with their rocket I'd be inclined to let them launch from the regular pads.

-Dave