Thrust:Weight ratio in OR

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lorstin
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Thrust:Weight ratio in OR

Postby lorstin » Sat Jul 01, 2017 4:24 pm

Hi, does anyone know a way of displaying the Thrust:Weight ratio on the simulations page of Open Rocket? The only way I seem to be able to get it is to export or print - which is a pain. Alternatively does anyone know the formula that O.R uses ?
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Re: Thrust:Weight ratio in OR

Postby SpaceManMat » Sat Jul 01, 2017 8:50 pm

The thrust to weight ratio is used as an easy quick method to ensure an adequate liftoff speed.

OR does away with that and gives actual liftoff velocity. It is superior methodology, no need to go back to the ratio method.
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Re: Thrust:Weight ratio in OR

Postby lorstin » Mon Jul 03, 2017 8:35 am

Thanks SMM, however it has only confused me more :? When looking back at some examples I had printed there didn't seem to be a correlation between T:W ratio and liftoff velocity as I would have expected given what you said. e.g. higher ratio = higher liftoff velocity. :? :? :?
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Re: Thrust:Weight ratio in OR

Postby SpaceManMat » Mon Jul 03, 2017 9:59 am

Yes this is because the ratio is just an approximation. The Thrust figure for a motor is the average thrust, most motors have a high intial thrust and then taper off, this results in more thrust at launch than the average thrust figure of the motor. One motor may have more of a peak but a lower average others only a small peak with a relative level thrust until near burnout. And there are a few oddballs out there that peak in the middle, which using the ratio rule could result issues.

Generally I'm looking for a 50kmh velocity of the rail as an acceptable speed. Lower may be ok but you are going to want to consider other aspects to make sure it flys ok, such as making sure it has a good stability margin say around about 2 and not fly if the winds are high. Also don't forget about using a longer rail.
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Re: Thrust:Weight ratio in OR

Postby lorstin » Mon Jul 03, 2017 10:14 am

Much clearer now, thanks SMM.
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Re: Thrust:Weight ratio in OR

Postby Voyager » Mon Jul 03, 2017 5:24 pm

Hi Frank
I've normally been comfortable with a preferred minimum velocity off the rail of 15 m/s (or 54 km/h). It's an easy number to remember. As it happens, this appears to be close to the value that SMM suggested of 50 km/h (approx 14 m/s)! So, a pretty good explanation and advice regarding whether to use T/W or velocity off the rail. I also consider that the velocity is more useful here because you can directly relate it to the launch site wind speeds to make a decision whether to launch or not.
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Re: Thrust:Weight ratio in OR

Postby lorstin » Tue Jul 04, 2017 5:40 pm

Thanks John, hopefully I'll be putting this into practice at the end of the month.
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Re: Thrust:Weight ratio in OR

Postby CATO » Wed Jul 05, 2017 8:54 am

SpaceManMat wrote:Yes this is because the ratio is just an approximation. The Thrust figure for a motor is the average thrust, most motors have a high intial thrust and then taper off, this results in more thrust at launch than the average thrust figure of the motor. One motor may have more of a peak but a lower average others only a small peak with a relative level thrust until near burnout. And there are a few oddballs out there that peak in the middle, which using the ratio rule could result issues.


Agreed. T(Ave):W ratio is normally accepted to be 5:1
The certified motor "Ave Thrust" value can vary by up to 20% and still be in compliance with NFPA 1125.
(NFPA 1127 4.9.1 "The maximum liftoff weight of a high power rocket shall not exceed one-third (3:1) of the certified average thrust of the high power rocket motor(s) intended to be ignited at launch.")

SpaceManMat wrote:Generally I'm looking for a 50kmh velocity of the rail as an acceptable speed. Lower may be ok but you are going to want to consider other aspects to make sure it flys ok, such as making sure it has a good stability margin say around about 2 and not fly if the winds are high. Also don't forget about using a longer rail.


Agreed. 30fps for LPR/MPR, 45fps for HPR (you can get away with 20fps for LPR in very calm conditions)
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Re: Thrust:Weight ratio in OR

Postby Lamp » Mon Jul 10, 2017 11:49 am

Hi Frank,
Some motors have a very different initial peak thrust compared to the average thrust, for example, I know the Aerotech I200W has an average thrust of around 45 pounds but the initial thrust is over 100 pounds. Different motors have different curve shapes depending on the motor length, the grain shape, the nozzle size/arrangement and the propellant type.

The 5:1 rule is a good field guide for an RSO but that is all it is, It does not take into account the initial thrust nor does it account for the length of the rod/rail and both of these can seriously affect whether the rocket is stable at liftoff. As others have said the best way to determine a safe liftoff is velocity at rod/rail departure is by using a modelling program which takes into account these factors. Both Rocksim and Open Rocket do this well...but remember to set the rod or rail length to a realistic value and don't just use the default. I personally aim for a minimum velocity of 45 fps but it is dependent on the conditions at the launch. On a calm day I might be happy with a lower impulse motor but on a windy day I will use a motor that gives me a higher liftoff velocity just to be sure. :wink:
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Re: Thrust:Weight ratio in OR

Postby lorstin » Mon Jul 10, 2017 12:15 pm

Thanks Pete(r),
Nicely put. There's more to it than I first thought - I hadn't taken into account rod length or thrust curve/shape. I'm in the habit of using "small" motors so as you and others have said I have to make allowances for some of the finer details. That being said I've had no mishaps (yet). :roll: Famous last words.
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Re: Thrust:Weight ratio in OR

Postby martymonsta » Tue Aug 22, 2017 8:02 pm

Just check your acceleration in G's if it is over 5G's then you have over 5:1
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