Velocity leaving the rail

How does one determine what velocity a rocket needs to be going when it leaves the rail? I assume it has to do with stability and factors like wind velocity. Where to I look to find out more about this.

See ...

https://www.rocketryforum.com/showthread.php?20885-desired-velocity-at-launch-rail-departure

https://www.rocketryforum.com/showthread.php?7511-Rule-of-Thumb

https://www.rocketryforum.com/showthread.php?83180-recommended-speed-after-leaving-rail

https://www.rocketryforum.com/showthread.php?73160-Launch-Rod-Length

https://www.rocketryforum.com/showthread.php?71645-launch-rod-velocity

Greg

And, if you need help calculating the approximate speed your rocket will be traveling going up the rail you can use a simulation software such as OpenRocket or RockSim.

Best part about OpenRocket is its FREE!

Thanks all!

I have a related question. How does one determine at what velocity along the rail that the rocket achieves stability?

Rocksim seems to use 44 f/s as a baseline assumption when its determining when a rocket reaches safe speed. In playing with sim models, I've noticed that it seems like a rocket will be a lot less prone to weathercock if the speed is closer to 60 ft/s (or higher) in a mild cross wind (3-7 mph), especially if overstable. In calm conditions, it appears that you can get away with a lot lower speed. I'm looking at building a minibutton rail launcher with a 6' guide; I like the idea of keeping the speed leaving the rail as high as practical. However, I am more than happy to accept the advice of folks that have more experience with these things...

Faster is better to put it simply. The corrective force the fins can generate is greater the faster you go. Hammer the rocket and get it moving.

Working on my L3, which should be something like a 19:1 thrust to weight off the pad - put the hammer down.

The truth is that minimum speed in inversely proportional to stability and directly proportional to wind speed. The less stable your rocket, the more speed you need. The higher the wind, the more speed you need.

Having said this, most simulators use a simple minimum speed. Thrust curve considers 50 feet per second (fps) minimum. And Rocksim had a default of 44 fps. The NAR safety guidelines specify 4 times the speed of wind with 1.5 caliber stability.

I've always used 15m/s or 50ft/s, but the points about wind and stability are good ones as well.

Don't you love how these answers are always "it depends?"

1tree said:

The truth is that minimum speed in inversely proportional to stability and directly proportional to wind speed. The less stable your rocket, the more speed you need. The higher the wind, the more speed you need.

Having said this, most simulators use a simple minimum speed. Thrust curve considers 50 feet per second (fps) minimum. And Rocksim had a default of 44 fps. The NAR safety guidelines specify 4 times the speed of wind with 1.5 caliber stability.

Click to expand...

Thanks for the various numbers. I was really more interested in how to arrive at the number. Is there an equation? Are there any more variables than velocity, rocket shape (centre of pressure and centre of gravity), wind speed?

Check this out...
https://picasaweb.google.com/104803...key=Gv1sRgCIzkuf6Eq5fV1wE#6034289230167489218

Found some of my old notes, this is what I have done in the past. Take it as you will. Of course it depends on the conditions.

I will go on record as saying that I trust the research and presentation of NAR on the subject. And on page 38 of the PDFS below it says that the rocket should have 4 times the velocity of the highest ground level wind. I know that Tim from Apogee Rockets has said the same number, but not sure where to get a quote. I seem to recall him giving a bit of math background on that number.
https://www.nar.org/pdf/launchsafe.pdf

The big thing that comes to mind is to not underestimate the shape of the rocket. Many articles and research is built upon work that doesn't account for many rockets launched today. For instance you need to realize that Barrowman's equation depends on a rocket being 10 times as long as the diameter. Below that length and base drag is not properly accounted for. At some point beyond that length, (I don't know how far) then the lift generated by the body is not accounted for in Barrowman's equation. And you also have to consider the flex of the rocket itself in some cases.

Sorry to throw mud into the water, but one thing I am convinced of is that one can over simplify to one's detriment easily. But for the most part I believe the four to one rocket to wind velocity will hold up for average rockets. Then the question becomes, what is the wind speed and figure a gust could hit the rocket just as you punch the launch button. But to get the minimum of 4 times the speed of wind multiply the wind speed in mph by 5.87 and you have the minimum fps of your rocket.

1tree said:

The truth is that minimum speed in inversely proportional to stability and directly proportional to wind speed. The less stable your rocket, the more speed you need. The higher the wind, the more speed you need.

Click to expand...

Actually this is not exactly true. The purpose of Barrowman's work was to determine stability, prompted by the tendency of conservative methods to result in overstability and weathercocking. A more overstable rocket requires a higher launch velocity compared to the wind. Given the tools available, I recommend a goodly amount of angle of attack analysis. If the rocket becomes highly unstable at high angles of attack, this is bad. If the rocket becomes critically stable or slightly unstable at moderate angles of attack, this can be good, because it will not weathercock at all or may even slightly anti-weathercock. Analyzing the exact point "the nose blown downwind" becomes "unrecoverable instability" is complex, although Open Rocket and real life have tended to agree for me so far in this area.

Another consideration is what I call the size of the aerodynamic profile. A heavy rocket with small fins may need 30 fps before the aero forces even become significant. On the other hand, one way to reduce weathercocking would be to add large fins right at the center of pressure. This causes the rocket to move sideways with the wind, and more rapidly reach the phase that as far as the rocket is concerned, the ground is moving and the air is still. I had one slightly scary slow launch where the rocket weathercocked a bit, but appeared to rotate around the nose or even in front, rather than the CG. There was probably a an instant there was a 45 degree angle between the tip of the rod and the rear end of the rocket. I could actually see the rocket being blown sideways, along with a bit of weathercocking. Another of my rockets has tons of fins and a CP fairly far forward. I have yet to determine any detectable relationship between any small angling of the rocket off the rod and the wind direction, even in high winds at a ~30 fps rod velocity.

Yet another consideration is how large and dangerous your rocket ship is. Don't experiment fullsize.

Johnsfolly said:

Thanks for the various numbers. I was really more interested in how to arrive at the number. Is there an equation? Are there any more variables than velocity, rocket shape (centre of pressure and centre of gravity), wind speed?

Click to expand...

As 1tree pointed out, NAR has a rule of thumb:

"Unstable powered flights account for about 20% of all failures. In addition, underpowered rockets continue to present a stability hazard by leaving the launch guide at a low speed compared to the wind speed on launch day. Best practice would indicate that rockets should 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."
https://www.nar.org/pdf/launchsafe.pdf

Do humans notice the difference between wind of 0mph and 5mph? My guess is no, so maybe 4 x 5mph should set the absolute minimum: 9m/s (29ft/s).

The NAR rule of thumb doesn't anything to do with the minimum aerodynamically stable velocity of the rocket. It is based on not exceeding a 15 degree angle of attack on the fins to prevent an aerodynamic fin stall. The inverse tangent of (1/4) = 14 degrees! Most fins, regardless of shape or airfoil, should not aerodynamically stall at an angle of attack below 15 degrees. Also by keeping the angle of attack below 15 degrees when leaving the rod/rail, the rocket will not severely weathercock.

The conventional rule of thumb of 30 fps (20 mph) minimum rod velocity for a model rocket and 45 fps (30 mph) rod velocity for a high power rocket still hold as well.

In reality, the greater of the 2 values is the applicable rule of thumb to use if you do not want severe weather cocking. What this implies is that if the wind is blowing 20 mph (30 fps) the minimum rod velocity to prevent severe weather cocking is 80 mph (120 fps)! On those days if you want to ascend straight, If you use CTI, launch a Vmax, not Mellow in your rocket. The Vmax launch will ascent straight up while a Mellow would turn to at least 30 degrees off vertical before burnout and attain ~ 50% of the no wind apogee, and with motor ejection, deployment will occur well after apogee.

Bob