Minimum liftoff velocity or velocity off rod

I'm designing a completely scratch built rocket using a plastic tube my dad gave me. I want it to be a low and slow design. Or at least have a slow liftoff.

What is a minimum safe liftoff velocity? Or are there too many factors for it to be that easy? The rocket will be around 30 inches long, 2 inches diameter and weigh between 9 and 11 ounces depending on the motor I stuff in it.

Some low and slow designs I have looked at in open rocket have a Velocity off rod of 43.3 ft/s for the Big Daddy and 45.6 ft/s for the Big Bertha. My design I have got a 45 ft/s using a D12-5, which should be fine looking at the other sims, and all the way down to 32.5 ft/s or 34.7 ft/s using different E engines.

Are those two velocities too low? Will be using the standard launch rods but I plan on putting rail buttons on it and can make the rail anywhere from 4 to 8 feet tall. I will have plenty of time to make a launch pad with a rail before I find an acceptable launch site. People think my rockets will harm their cattle in some way even if the cattle are in a different field. I had a really good launch site picked out but after a few calls it turns out I can't use it. Bah.

Thanks,
Mike

Safe lift-off speed (or, rod separation speed) has been poorly defined in the past. In general, the lower the lift-off speed, the lower the local cross-wind speed should be. A longer launch rod/rail always helps. The margin of stability is going to make a difference in there too. And none of those general statements help you put an actual number to what you need.

I would not recommend that anyone deliberately attempt to design for a "slow" liftoff velocity. Quite simply, this is unsafe. IMHO, you should keep your designs in the range of 40-50 fps, or even higher.

As Powderburner said, the rule of thumb is 15m/s (49ft/s), but of course it is a lot more complex.

You might consider increasing the rod length to give the rockets more time to come up to speed. McMaster-Carr offers rods in many diameters, up to 6' in length. Remember that what matters is the effective length of the rod (the total length minus that of the rocket from aft to the forward lug).

Thanks guys. Rule of thumb was pretty much was I was going for. I wasn't necessarily going for launch speeds that low, that was just some of the results of the sims I ran and are the only ones I have had that low, which is why I was asking. Like I said, the Big Bertha is advertised as low and slow and it just over 45. So I will design with that speed in mind.

I've shopped around McMaster for lots of things which is where I will get my longer rods and rails along with other stuff for my launch pad.

Thanks again
Mike