Is there any one source for...

[eggplant]

Is there any one source for maximum recommended liftoff weight?

 

[Larry Curcio]

Is there any one source for maximum recommended liftoff weight?

A good 6-D simulation. A 2-D simulation will do in a pinch. Nothing's perfect, but simulation is better than tabulation.

 

[eggplant]

A good 6-D simulation. A 2-D simulation will do in a pinch. Nothing's perfect, but simulation is better than tabulation.

Huh?

 

[bobkrech]

Lift-off weight is poorly defined parameter that was been assigned to a motor before simulation programs existed. Today you should be using a simple simulation program to determine that your rocket/motor combination will have sufficient velocity leaving the rod to be aerodynamically stable, and will gain enough altitude for the recovery system to deploy properly for a safe recovery.

There are a number free simulation programs, and they are used to determine how heavy a rocket you can safely launch with a given motor. OpenRocket, wrasp, etc. Webalt is one of my web favorites. https://webalt.markworld.com/webalt.html

Bob

 

[gdiscenza]

Huh?

I think that what larry is referring to is a 6-Degree of Freedom simulation, meaning full motion in all three axes.

G.D.

 

[MarkII]

Apogee Components' RockSim 9 is probably the most popular 2D (or is it a 3D?) rocket simulation program in use today. The company also has a more advanced version, RS-Pro, which is capable of full 6 DoF simulations. Unless you are planning for the type of suborbital flight that requires a launch license from NASA, the pro version is probably much more involved than you need.

Apogee has a free 30-day demo version of RockSim 9 that is available for downloading.

 

[stantonjtroy]

There is no one source. Sims aren't a bad way to go, I myself like Open Rocket (it's free). There is an easier way to go. Most everyone acknowledges a safe thrust to weight ratio is 5:1. Take the average thrust of the motor and convert to lbs (or oz.). Then figure the max weight based on the rule of thumb. 1lb/sec = 4.45newton/sec.

 

[RandyT0001]

There is an official source that can help you determine maximum lift off weight. From nar.org's Launchsafe.pdf:

Best practice would indicate that rockets to be guided by launch rods, rails, or towers until they have attained a forward velocity of at least 4 times the velocity at which the wind is blowing or gusting at the launch site (e.g. rod/rail departure of 36 m/sec (80mph) if the maximum gust is 4.2m/sec (20 mph)). Introducing a “4 x wind” requirement will require many models to use 8-foot or 10-foot rails or towers in significant crosswinds for all but the lightest rockets with high thrust/weight ratios.

Simulation software is readily available and affordable (and some manufactures have both site licenses and 30-day free trials), and can predict the velocity at which a rocket will leave the launch device of a specified length. If no such software is available, rough guidelines for thrust-to-weight limits should be used, such as the 5:1 or 6:1 rule of thumb for average thrust-to-weight used widely. Note that such a rule of thumb may not be conservative, because it is not sensitive to the initial thrust of the motor, nor to the time required to build up to that initial thrust level.

(I added some clarifying information in italics to the quote.)

 

[rocketsmith]

A simple rule of thumb is take the average thrust in newtons and divide by 25. So for a G80 take 80 divide by 25 and you get 3.2 pounds. Or for an M2500 it is 100 pounds.

 

[rockets4kids]

Another strong recommendation for OpenRocket here. While it has only a fraction of the features of RockSim, it does an *EXCELLENT* job at simulating 99% of all rockets built and is completely adequate for most users.

IMO, unless you are just launching rockets for ***** and giggles, you should be running everything you fly through a simulator before flying it.

 

[KevinDunn]

Another vote for OpenRocket. But another tool not mentioned yet is Thrustcurve.org. You enter the parameters of your rocket and it tells you which motors to use and with what delay. One feature it has that OR does not is that it tells you the speed of the rocket as it leaves the launch rod. Good for making sure the rocket has achieved a safe speed.

I routinely use both OpenRocket and Thrustcurve. For each MP or HPR flight I record the recommended delay from each, the predicted altitude, and the actual altitude if I am flying an altimeter.

 

[cwbullet]

I really liek the full features of Rocksim, but for the price, you can not beat Openrocket(free). Another couple of features and will be a good option to replace Rocksim.

Thrustcurve is a fantastic reference.

 

[David987Fox]

A thrust to weight ratio of 5:1 is a good rule of thumb. The main idea here is to ensure the rocket is on a straight flight path by the time it is off the rail. RockSim is a good source for testing different motors and observing a flight profile.

 

[accooper]

I would like to see Open Rocket expanded. Printing would be nice but a screen capture program works fine. My son has just finished a Java class and I have asked him if he could put the print feature in. He says no problem, but wants the permission from the original author.

Andrew