# recommended speed after leaving rail?

#### rocketsonly

##### Well-Known Member
What's the recommended speed for a rocket to be traveling after leaving a launch rail so that it would be considered safe?

#### gregtro

##### Well-Known Member
Rocketry Online's Info Central says 30 feet per second is the min. speed

#### cdma77

##### Well-Known Member
Originally posted by rocketsonly
What's the recommended speed for a rocket to be traveling after leaving a launch rail so that it would be considered safe?

45 FPS is what I use. 30 FPS would be neat to see because one can really see it leave the pad !

#### bobkrech

##### Well-Known Member
Technically speaking, there is no generic safe minimum speed. A rocket has to have enough airspeed to be aerodynamically stable when it leaves the launch rail or it won't fly safely or straight. For most rockets this occurs between 30 to 45 ft per second if there is not a strong crosswind. Using what are considered standard length rods, this is typically achieved if your rocket has a 5:1 thrust to weight ratio which means the rocket will accelerate at 4 G.

To calculate the rod length, l, required to reach a specified speed, v, for a given thrust to weight ratio, t/w, you can use the following formula.

l = V^2 /(64* (T/W-1))

for example, if v= 30 fps, then l = 30^2/(64*5-1)= 900/256=3.51 ft with a 5:1 thrust to weight ratio so you would want to have a minimum 4 foot rail for this case.

If you needed 45 fps for stability, then you would need a length l = 2025/256= 7.91 ft or an 8 ft rail to insure stability.

In a no wind situation, the fins cut through the air with a zero angle of attack. Should the rockets deviate off course, the angle of attach increases, the fins develop lift, which turns the rocket back on course and to a zero angle of attack attitude.

If the velocity leaving the rail is low, and the crosswind velocity is high, the rocket will leave the rail at a no-zero angle of attack and the rocket will turn into the wind. If the relative velocity vector angle is very large, the fins can stall and the rocket can undergo all kinds of unstable gyrations, and should it gain enough airspeed to unstall the fins, it will dart of in whatever direction it happens to be pointing in when it reached it minimum stable air speed. That's bad and unsafe, so on windy days you need to either use a longer rail, or a higher average thrust motor to get the rocket going faster before it leaves the rail. A 10:1 motor is recommended for these days.

If we use a 10:1 thrust to weight ratio motor, the rocket will reach 30 fps in only 900/(64*(10-1))= 900/(576)=1.54 ft., and at the end of a 4 ft rail, the velocity would be sqrt(4*576)= 48 fps which should be fast enough to that the fins don't stall.

Bob Krech

#### Missileman

##### Well-Known Member
the average of about 35 to 40 fps and the 5:1 ratio with a standard rail is fine for most rockets under most conditions.
The configuration of the rocket has a great deal to do with what is needed.
Think about it. If a rocket has large fins they will be effected by aerodynamic forces at slower speeds, conversely if a rocket has extremely small fins it will require more speed.

#### rocketsonly

##### Well-Known Member
Okay, thanks you all!

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