Off the rail velocity POLL

[Zeta]

What is the lowest velocity a rocket can safely leave the rail at and have a safe vertical flight?
Assume :
1) 0 - 5 mph wind
2) rocket stability between 1-2
3 length to diameter ratio between 11 and 14
4) no odd-rocks, tube fins, fin rings
5) a 3 or 4 fin design
6) 6 foot rail

 

[OverTheTop]

In relatively calm conditions I would allow a rocket to fly at a rail exit velocity of around 36fps. That is about 40 kilometers per hour.

 

[Steve Shannon]

Most frequently the number that’s quoted is 45 feet per second.

 

[Zeta]

Most frequently the number that’s quoted is 45 feet per second.

yes of course.... but ...if wind was zero what is possible

 

[rharshberger]

b

yes of course.... but ...if wind was zero what is possible

Its never zero and most RSO's are going to tell you it needs to be moving a certain velocity, most common number I hear is 3 to 5×wind speed in fps (45fps is what we typically use at our field, since wind speeds are generally close to 10mph). As an RSO minimum rail exit velocities are avoided in favor of safety, slow lift-offs look cool but have a habit of literally going sideways in a big hurry.

 

[bobbyg23]

Why are people all of the sudden worried about length to diameter ratio?

 

[Steve Shannon]

yes of course.... but ...if wind was zero what is possible

Can you also guarantee that thrust is perfectly centered on the axis of the rocket, that the center of gravity is also centered on the axis of the rocket, and that the center of pressure is also centered on the axis of the rocket?
All those things contribute to off axis flight: wind, mass distribution, drag distribution, and the thrust vector.
We all love to ask “what if” questions around hypothetical situations over beers after a launch, but when you are the person pushing the button, signing as RSO, or holding the insurance certificate it goes back up to about 45 fps pretty quickly.

 

[pathtouch]

My opinion answering the actual question: 30 fps.
This is based on the info presented in the pdf below starting on page 30 here.
https://www.nar.org/pdf/launchsafe.pdf
Essentially it’s been wind tunnel tested at MIT that an angle of attack around 15 degrees shifts the center of pressure forward 1 ish caliber. Since this is the minimum static stability margin given we want to limit angle of attack to this number. Doing some basic trig this equates to making sure rod exit speed is 4x horizontal wind velocity. 4x5mph=20mph=30 fps.


However, If I were the RSO I’d want to see 30pmh or 45 fps for some factors of safety to account for all the things Steve mentioned above.

 

[Zeta]

Why are people all of the sudden worried about length to diameter ratio?

no one is worried about this, just trying to keep a simple model for discussion

 

[bobbyg23]

no one is worried about this, just trying to keep a simple model for discussion

I've seen it come up a couple of times lately is why I was asking. I had never heard of it come into consideration before

 

[Steve Shannon]

I've seen it come up a couple of times lately is why I was asking. I had never heard of it come into consideration before

A long thin shape has a much higher moment of angular inertia, meaning it’s more difficult to turn (or restore). So it does matter.

 

[bobbyg23]

A long thin shape has a much higher moment of angular inertia, meaning it’s more difficult to turn (or restore). So it does matter.

Perfect. I didn't say it didn't matter.

 

[Steve Shannon]

Perfect. I didn't say it didn't matter.

Sorry, “it does matter” was just my way of saying “that’s why it comes into consideration.”

 

[bobbyg23]

Sorry, “it does matter” was just my way of saying “that’s why it comes into consideration.”

Understood.

 

[Steve Shannon]

What is the lowest velocity a rocket can safely leave the rail at and have a safe vertical flight?
Assume :
1) 0 - 5 mph wind
2) rocket stability between 1-2
3 length to diameter ratio between 11 and 14
4) no odd-rocks, tube fins, fin rings
5) a 3 or 4 fin design
6) 6 foot rail

Safety Code requires a three to one thrust to weight ratio. You’re assuming a 6 foot rail. Neglecting rail button friction and aerodynamic drag, the actual velocity at the end of the rail is a straightforward calculation. It’s easier to do using metric values.

 

[Zeta]

Thrust to weight ratio

 

[Steve Shannon]

Thrust to weight ratio

Thanks, I corrected it. I typed the word in my mind, it just never made it to the page.

 

[Zeta]

that would be my typical glitch ... can you share the formula you would use for a 3:1 thrust to weight ratio as it relates to the units that motors/engines are specified in

 

[FredA]

I often find myself working backwards - sifting through simulation output to find the point where the rocket is going fast enough that it won't need additional guidance -> In other words, to find out how long of a rail is needed.

I target 45fps as my goto speed - look that up in the output and find the altitude at that point which gives the rail length needed after adjustment for the penultimate rail button location.

 

[waltr]

The number in the motor designator, G80 for example, is the average Thrust in Newtons.
To convert Newton force to kilogram (kg) force divide by 9.81 (Earth's gravity).
For easy calculation divide by 10 (I can do this in my hear by moving the decimal point). This also gives a slight margin.
This gives a rocket weight in kg for 1:1 TTW.
For 3:1 divide by 3.
Therefore, For a G80, divide by 10 is 8, divide by 3 is 2.6kg rocket weight. or 80/30.

Note: At many launches the RSO will require a 5:1 Average thrust to weight. So the maximum weight of a rocket on a G80 is 80/50 = 1.6kg.

The reverse calculation is starting with rocket weight in kg.
Multiple the rocket weight in kg by 30 (for 3:1 TTW) or by 50 (5:1 TTW) to find the minimum average motor thrust.
So a 2kg rocket requires a 60N motor for 3:1 or a 100N motor for 5:1.

 

[Zeta]

Given these quick field calculations I can see how this works for a thrust curve like the Aerotech H-195

H-195 Initial thrust 231 N, average thrust 206 N

How does it work for this motor:
I - 65 Initial thrust 165N, average thrust 80N

However the I - 65 seems to be a different animal. Could one potentially fly a rocket that is heavier than what the formula would give us if we started with the average thrust of 65N, when the initial thrust is 165N?

16.5/3=5.4kg
6.5/3=2.17kg
Answer ...NO..... initial thrust is not average thrust
using open rocket
5.4kg rocket : off the rod at 25ft/sec
2.17kg rocket : off the rod at 43ft/sec
(p.s. I just figured out that my calculations did not include the weight of the motor! (smack up side the head)... remember to calculate on the pad weight not weight of the rocket.

 

[Steve Shannon]

that would be my typical glitch ... can you share the formula you would use for a 3:1 thrust to weight ratio as it relates to the units that motors/engines are specified in

The reason I said metric was easier is because the weight of a rocket is correctly reported in Newtons and our units for thrust are most commonly in Newtons. One kilogram of mass weighs 9.81 Newtons (mass times the acceleration of gravity). With a 3:1 thrust to weight ratio a rocket weighing 9.81 Newtons would require at least 29.43 Newtons of thrust.
The sum of the forces on the rocket then are thrust in the upward direction and gravity downward, or 29.43 - 9.81 = 19.62 Newtons, which is 19.62 kilogram * meters per second per second. F = mass * acceleration so one kilogram is accelerated at the rate of 19.62 meters/second squared.

When acceleration and distance are both known the formulas can be rearranged algebraically to find velocity at any given distance.

Assuming v starts at 0, acceleration ‘a’ is incremental distance ‘d’ per time ‘t’ per time -> a = d/t/t or a = d/(t^2)
Velocity ‘v’ = d/t

Express both in terms of time:
t = d/v
t^2 = d/a, so t = (d/a)^(1/2) or sqrt(d/a)
So, d/v = Sqrt(d/a)

Solve for v
v = d/(Sqrt(d/a))

A six foot rail is 1.829 meters. Plug in the numbers

v = 1.829 / Sqrt(1.829/19.62)
v = 6 meters per second or 19.65 feet per second. (I hope someone checks my algebra and basic math!!)
You can see why a 5:1 thrust to weight ratio is recommended by Tripoli in its Safety Code.

 

[Zeta]

your making my head spin

 

[Steve Shannon]

As an exercise, try the same thing using an 8 foot rail (2.438 meters). That’s the shortest we (my local club) use for most high power rockets.
And maybe with an 8 foot rail and 5:1 thrust to weight.

 

[FredA]

My rule of thumb as RSO is to ask a lot of questions if the motor designation divided by 25 is less than the pad-weight in pounds.

 

[jqavins]

What is the lowest velocity a rocket can safely leave the rail at and have a safe vertical flight?
Assume :
1) 0 - 5 mph wind
2) rocket stability between 1-2
3 length to diameter ratio between 11 and 14
4) no odd-rocks, tube fins, fin rings
5) a 3 or 4 fin design
6) 6 foot rail

I understand specifying the rocket's fineness (see below) but the rail length is completely irrelevant if you're asking about rail exit speed. For T:W it matters a lot, because it affects the exit speed.

Why are people all of the sudden worried about length to diameter ratio?

A long thin shape has a much higher moment of angular inertia, meaning it’s more difficult to turn (or restore).

True, but not the reason it matters, as I understand it.

Essentially it’s been wind tunnel tested at MIT that an angle of attack around 15 degrees shifts the center of pressure forward 1 ish caliber.

What those tests showed is that the CP moves about 10ish percent of the body tube length, which is 1ish caliber for "normal looking" rockets with 10:1 ish fineness. Long skinny rockets need a lot greater static margin as expressed in calibers, because 10% of the length is more diameters. Short rockets are OK with lower margin in calibers. I've read here that some folks use rocket length and the reference for stability instead of diameter, so 10 to15 percent is a good figure no matter the fineness.

 

[waltr]

I disagree, Rail length is VERY important for exit velocity. A longer rail allows a longer time to accelerate while guided therefore obtaining a higher exit velocity. For our club launches I add a 4 foot 1010 rail extenstion for low TTW rockets and breezy conditions. This helps by having a higher velocity off the rail and help reduce wind-cocking.

As to 'constant' thrust verse 'regressive' thrust motors. This does make a difference and the 'standard' Average trust to weight calculation under estamates rail exit velocity.
See CTI's FAQ of thrust and weight here:
http://www.pro38.com/faqs.php#liftoff
Technically, there are many factors involved of which most are covered in above posts.
However, if a club is running a launch there needs to be a simple method or calculation. to determine if a rocket is safe to fly. This is where the average thrust to weight ratio calculation is useful.

 

[pathtouch]

What those tests showed is that the CP moves about 10ish percent of the body tube length, which is 1ish caliber for "normal looking" rockets with 10:1 ish fineness. Long skinny rockets need a lot greater static margin as expressed in calibers, because 10% of the length is more diameters. Short rockets are OK with lower margin in calibers. I've read here that some folks use rocket length and the reference for stability instead of diameter, so 10 to15 percent is a good figure no matter the fineness.

Good to know. I had calibers stuck in my head based on this figure which Dahlquist published. He wasn't the MIT guy, that was Galejs and I honestly haven't read his report...BUSTED


Available from this reprinted article. It's a great read for anyone following this thread.
https://www.apogeerockets.com/Wind_Caused_Instability

 

[Steve Shannon]

A long thin shape has a much higher moment of angular inertia, meaning it’s more difficult to turn (or restore). So it does matter.

True, but not the reason it matters, as I understand it.

What those tests showed is that the CP moves about 10ish percent of the body tube length, which is 1ish caliber for "normal looking" rockets with 10:1 ish fineness. Long skinny rockets need a lot greater static margin as expressed in calibers, because 10% of the length is more diameters. Short rockets are OK with lower margin in calibers. I've read here that some folks use rocket length and the reference for stability instead of diameter, so 10 to15 percent is a good figure no matter the fineness.

I think we’re talking about the same thing. You need a greater static margin with long skinny rockets is because the greater moment of angular momentum reduces the ability of lift to quickly restore the course of a rocket.

 

[jmasterj]

I disagree, Rail length is VERY important for exit velocity. A longer rail allows a longer time to accelerate while guided therefore obtaining a higher exit velocity. For our club launches I add a 4 foot 1010 rail extenstion for low TTW rockets and breezy conditions. This helps by having a higher velocity off the rail and help reduce wind-cocking.

The OP asked about rail exit velocity, not how to get there. Rail length might determine that value for a given rocket/motor combo, but it doesn't matter to the OPs question. 45 fps off an 8ft rail is no different from 45 fps off a 6ft rail from a safety perspective.