Max Weight and Launch Rod Speed

[Weldo]

I've been fooling around lately with making rockets from scratch and one thing I'm struggling with is launch rod speed. Or to put it more accurately, speed of the rocket upon disengaging from the launch support.

I've heard that 50 feet per second is the standard of safety. It ensures the rocket will be adequately stable in moderate wind upon leaving the launch rod.

Here's where I get confused. Estes states that the maximum lift off weight for their D12-3 is 14 ounces. I just made a quick OpenRocket file for a wallpaper tube I got from work and it's around 12 ounces, but the lift off speed is only around 33fps.

Presumably at the max recommended weight, the launch rod speed will be even less. So do I trust Estes recommendations or go with the established norms? I can shave weight by shortening the body tube but the long tube is kinda cool. And the added overall weight keeps my apogee lower, in a "sure recovery" range.

What do you guys think? I've included the OR file if you wanna check it out. It's not 100% complete but should be complete enough for a simulation.

 

[kuririn]

Use a longer rod. Speed off the rod will go up the longer the rod or rail is.
Also 50 fps is a conservative figure. In calm conditions 33 fps might be doable.
Another option is to use a composite motor. Aerotech has 24mm motors in the E impulse class that have more kick off the pad. Try simming with those and see what you get.

 

[Weldo]

Right. I went through this all with my mailing tube build. With that one I decided to use a 6 foot rod to get as much speed as possible.

It seems to me that it's impossible to achieve 50fps on a 14oz rocket with a D12-3 motor. So do you have any idea why Estes says it's OK? Are they just working from estimations or do you think they've gone out and tested t his stuff?

Maybe I should send them an email...

 

[Nytrunner]

The "max liftoff weight" of the motors is not a very helpful metric in my experience. Is the rocket fat or thin? Is the rod 3ft or 6ft? Whats the windspeed?

All of those factor into a safe flight.

50 is definitely safe, 45 is usually safe, 40-42 only if you have zero to low wind

 

[Weldo]

So if I’m trying to design a rocket from scratch I should go by launch rod speed rather than max recommended weight. I should probably just use the max weight as the roughest of rough estimates.

 

[Andrew Brown]

14 oz on a D12 is a 3.0:1 thrust to weight ratio. A lot of people use 5:1 as acceptable, but estes may drop it down. My guess is that is where the 14 oz number came from, not the velocity off of the rod. For scratch builds, I teach the kids I work with go design for 50 ft/s off of the end of the rod as a target, similar to what others have said, rather than the thrust-to-weight ratio.

 

[Weldo]

Right on. I've heard a few different guidelines but some contradict others.

 

[caveduck]

The physics is actually pretty complicated. 50fps has been a common rule of thumb for a long time, but that makes assumptions that the crosswind is quite low, launcher is fully rigid, rotational moments of inertia are typical, rocket is not overstable, no whip due to vehicle sticking on the rod/rail, etc. Personally, having been RSO for 1000's of flights, I feel like 50fps is too low and that 75+ would eliminate a lot of hairy flights.

OpenRocket is a great way to explore launch trajectories; you can set the launcher length and wind conditions.

 

[fleafrier1]

According to thrustcurve the D12 hits pretty heavy during the first half second of burn time. I think you'd be safe figuring at least 4:1 for a 14 oz bird while it's still in the process of departing the rod. I imagine this is why estes recommends such a high max launch weight.