Launch rail exit speed?

Can anybody tell me what the minimum speed for a rocket is when it leaves the rail? I have a 1010 rail 96 inches long so lets say there is 84 inches for the rocket to go up. I have read that the speed should be 50 feet/sec and some people say 40. Some of my AeroTech rockets sim at about 39 feet/sec.

I aim for about 30 miles per hour.

There's no "right" answer. 50 fps is a very safe, stable, proven speed. ThrustCurve.org fails any motors that don't leave the rail at >= 50 fps. But on a calm day, you can get away with less. I've had great flights with a launch speed of less than 50.

The thing to keep in mind (and we'll leave mach speeds out of this for now) is that the rocket is the least stable on the pad. As thew motor burns up the propellant, it gets more and more stable. So a good launch velocity helps to compensate for the lower stability. Wind affects stability, so lower wind speeds allow for lower rail speeds.

But if you stick with 50 fps, you are pretty much guaranteed a good launch in any safe (i.e., under 20 mph) wind conditions.

If you're me...100 MPH. But I like 'em fast. :kill:

My first rule of thumb is 20 mph for modrocs and 30 mph of high power. That's nominally 30 fps and 45 fps respectively.

My second rule of thumb is 4 times the crosswind velocity. It insures that the angle of attack of the fins is < 15 degrees so the fins will not stall.

Both conditions have to be met.

Bob

Just a random FYI, but stall angle is a function of Reynolds number among other things. Stall angle for Re ~1000000 may well be around 15 degrees (or higher for deltas IIRC). It is lower for lower Re. Still, if the fin is at 15 degrees at the top of the rail it will be much less soon enough.

Gerald

PS - For those not familiar with Reynolds numbers, the faster it goes, the greater the Reynolds number. The greater the length in the streamline direction, the greater the Reynolds number. So a big and fast fin would have a much greater Reynolds number than a short and slow fin. So much for the handwaiving. For a more technical answer than most would be interested in: https://en.wikipedia.org/wiki/Reynolds_number

There is no rule for this... it is very rocket dependant. Most people say 30-50 fps. However you should double check with open rocket or rocksim and check the stability of your rocket at rail clearance.

To throw another wrench(grenade) in this how do you measure the length of your rail to calculate the speed? I use the distance from the top rail button to the top of the rail. Using this will make rail distance different for each rocket, complicating everything again


Good thing we are all rocket scientists!

My finished pad may have more then 84 inches. I am going from top down to blast deflector 84 inches. The rod is 96 inches long.

I think that's what he meant about throwing a wrench in..

I usually put my top button near the CG, so I figure speed from there. Once the top button leaves the rail, the lower does nothing to stabilize it.

McKailas Dad said:

I think that's what he meant about throwing a wrench in..

I usually put my top button near the CG, so I figure speed from there. Once the top button leaves the rail, the lower does nothing to stabilize it.

Click to expand...

Exactly!!!!

highflyer1968 said:

My finished pad may have more then 84 inches. I am going from top down to blast deflector 84 inches. The rod is 96 inches long.

Click to expand...

If your rocket sits 2" above the blast plate and the upper button is 22" above the bottom, you only have 60" of rail before that upper button leaves the rail.

I have a rocket where the upper button is 34" above the bottom. That would only leave me 48" of rail.

Everyone had to calculate their own rod length base on their rocket. But they do need to know where the bottom stop is on the pad.

I never really thought about that but you are right.