Thrust to weight ratio

Onebadhawk
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highflyer1968

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I've been reading about thrust to wait ratio and it should be 5/1. Now does that mean there should be 5 lbs of thrust for every 1 pound the rocket weighs?
 
Look at the thrust curve. It should be 5:1 at ignition, not average as in the motor designation.
If there is significant wind or the rocket is over stable, it should be higher
 
Hah, sorry I should have mentioned that. Yes, big difference between various thrust curves, and getting up to stability is important. Everything after that is pure fun!
 
Another thing to add is the 5:1 ratio should be for the loaded weight of the rocket. For example, I have a rocket that 14 pounds but will weigh in at 20 pounds at liftoff for its first flight. When calculating the 5:1 ratio, I need to use the 20 pound liftoff weight.
 
pythonrock said:
Look at the thrust curve. It should be 5:1 at ignition, not average as in the motor designation.
If there is significant wind or the rocket is over stable, it should be higher
Click to expand...

That being said and as per another thread recently, some clubs and RSO's use the 5:1 ratio as a factor of determining safe flight, RSO's don't usually have all the thrust curves memorized so they go off the basic RTF weight of the model and the impulse rating of the motor converted to pounds of thrust. For example a G100= 22.72 pounds of thrust (fictional motor) if the rocket doesn't weigh more than 4.5 pounds its good to fly, if the rocket weighs 5lbs its not good to fly. Our fictional G100s thrust curve says it actually produces 132 newtons (30lbs of thrust) for .25 seconds after ignition, now we have a better than 6:1 ratio (actually 6.66 :eyeroll:, interesting choice of example here) so the rocket is ok to fly, but the RSO not having access to Thrustcurve.org easily or the thrust curve on the packaging still denies the flight based on the average 100 newtons of thrust and it being less than 5:1.
 
...and some clubs use a 6:1 ratio because of typical flight conditions (such as wind).

I like using flight sim software to find out exactly how the rocket will do (as much as can be estimated in a simulation), but I've noticed in some Open Rocket files posted by people that there are at times significant deviance's from actual designs, materials, weights of things, etc. It's like all computer stuff, garbage in, garbage out. And then you have to try to simulate the weather conditions as much as possible!

I'm not a big fan of "rules of thumb" when you can (and maybe should) try to simulate and find out exactly what you should expect, but the rules of thumb are used because they work! So in all cases, be careful and err on the side of safety.
 
rharshberger said:
That being said and as per another thread recently, some clubs and RSO's use the 5:1 ratio as a factor of determining safe flight, RSO's don't usually have all the thrust curves memorized so they go off the basic RTF weight of the model and the impulse rating of the motor converted to pounds of thrust. For example a G100= 22.72 pounds of thrust (fictional motor) if the rocket doesn't weigh more than 4.5 pounds its good to fly, if the rocket weighs 5lbs its not good to fly. Our fictional G100s thrust curve says it actually produces 132 newtons (30lbs of thrust) for .25 seconds after ignition, now we have a better than 6:1 ratio (actually 6.66 :eyeroll:, interesting choice of example here) so the rocket is ok to fly, but the RSO not having access to Thrustcurve.org easily or the thrust curve on the packaging still denies the flight based on the average 100 newtons of thrust and it being less than 5:1.
Click to expand...

Agree! Interesting side note Rich. Your fictional motor is nearly an exact replica of the Aerotech H100 DMS motor.
 
Since the real point is the speed off the rail, if you have a 20 foot launch rail you could actually get away with a 3:1 thrust ratio and still have safe speed off the rail.
Better have lots of documentation for the RSO if you want to try though.
 
The flier should know these numbers before he brings his rocket to safety check.......

Bob
 
bobkrech said:
The flier should know these numbers before he brings his rocket to safety check.......

Bob
Click to expand...

Exactly! And since the RSO is judge, jury, and executioner, he better have documentation if he wants to argue his case. At our club, a flyer can usually appeal to a higher court, made up of the RSO, LCO, Prefect and/or BOD members. If they happen to be handy and decide to hear the case. Of course the LCO and Prefect usually defer to the RSO on most cases, but the fliers does have a right to an appeal. We have a legal system in this country after all! ;)
 
Most longburn style motors, including BP, have an initial thrust peak. The RSO should know that, maybe not how much but if it's just little short on average thrust, should give some passes AFTER taking note of other factors such as wind and how big the rocket is.
 
Buckeye said:
View attachment 279987
Click to expand...

That only works if the RSO actually has the app, a smartphone, etc. The club I am part of requires a launch rod exit speed of 5x the wind speed, and if the flier is confident that it will do that then its allowed to fly, thrust ratios don't come into play until the flier doesn't seem to know enough of the proper answers to the RSO's questions. Last year I only denied one flight and it was an Oddroc (LPR) that not only looked and felt unstable but was, which was determined by the BOD allowing the flight from the HPR pads, which was good cuz it crashed 50' from the pad.
 
Over-stable rockets, while they will not spiral out of control like an unstable rocket, tend to weathercock more. If you think about the wind blowing against the fins of the rocket, and it rotating on its CP, then the forward location of the CG pulls it back to vertical. But the farther forward the CG is, the more it overcompensates. So you get a straight flight; you just don't get a perfectly vertical flight in strong wind.
 

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