OpenRocket and the thrust-to-weight ratio

I’ve been relying on OpenRocket for flight safety, but I realized just last weekend that it doesn’t display the thrust-to-weight ratio, and it doesn’t flag the simulation if the value is less than 5:1 (or 4:1, or 3:1).

So to ensure safety, can I rely on OpenRocket, or should I calculate the T-to-W ratio independently?

I have a few low- and mid-power systems (<1 kg and <600 ft) that would like to fly near a 4:1 ratio, or 12 m/s off the pad, which is fine according to OR, but now I understand RSO's may not approve of them.

OpenRocket does display T/W ratio, but you need to generate the simulation summary sheet PDF to see it.

File > Print/Export PDF > Then select Design Report

A PDF is generated showing all the critical simulation parameters, including T/W on the top line over on the right.

Thrust to Weight ratio is a simple “rule of thumb” method for determining in the field if a safe launch speed will be achieved.
When you go to the bother of actually doing a simulation you should use the simulated speed to determine if the launch will be safe.

OR will not tell you if the flight is fine or not. It will simulate 1 m/s off the pad just fine and not raise any warnings.

50 ft/s or 15 m/s off the rail (no wind) usually is enough speed for the fins to be effective and stabilize the rocket.

Voyager1 said:

OpenRocket does display T/W ratio, but you need to generate the simulation summary sheet PDF to see it.

File > Print/Export PDF > Then select Design Report

A PDF is generated showing all the critical simulation parameters, including T/W on the top line over on the right.

Click to expand...

Yes, this resolves my issue.

SpaceManMat said:

Thrust to Weight ratio is a simple “rule of thumb” method for determining in the field if a safe launch speed will be achieved.
When you go to the bother of actually doing a simulation you should use the simulated speed to determine if the launch will be safe.

Click to expand...

I agree, but the local RSO's want to know the T:W ratio as a (standardized) condition for launch.

SpaceManMat said:

Thrust to Weight ratio is a simple “rule of thumb” method for determining in the field if a safe launch speed will be achieved.
When you go to the bother of actually doing a simulation you should use the simulated speed to determine if the launch will be safe.

Click to expand...

I agree with this in principle; however, when a vertical launch is at stake, I would advise also simulating a 10 degree off-vertical launch to make sure you have some leeway in the design. If the trajectory degrades of powerprangs you have some design work to do.

Funkworks said:

I agree, but the local RSO's want to know the T:W ratio as a (standardized) condition for launch.

Click to expand...

The NFPA 1127 requirement (4.9.1) is based on rocket liftoff weight and certified average thrust. No simulation required. Just make sure the ratio is greater than 3/1. But you are going to need a long rail to reach a safe velocity with a 2G liftoff.

I've always dislike the 3:1 rule. It ignores progressive motors that build up thrust and overly restricts hardhitters or dual-thrust motors that have large thrust events at the beginning of burn.

UhClem said:

The NFPA 1127 requirement (4.9.1) is based on rocket liftoff weight and certified average thrust. No simulation required. Just make sure the ratio is greater than 3/1. But you are going to need a long rail to reach a safe velocity with a 2G liftoff.

Click to expand...

In summary, if you just want to get past the RSO, you need F/T and you don't need a simulation. If you want a successful flight, OTOH, you need a simulation, and you still have to get past the RSO