Most people use a rail when you get to the HP but really buttons are so much easier to use and less like to break off when installing rocket on rail or road. I think the chart you found was fine but maybe stop at 1/4. anything bigger should use a rail.

just my opinion.

I'm in full agreement - if you're flying something bigger than a G (1/4" rod), you ought to be flying off a rail with either rail buttons (1010 or 1515) or Acme Rail Guides... just my opinion.

Warren

Warren, if you didn't have access to a tower, would you build a G altitude attempt rocket with a 1/4 lug? I imagine a 3/16 would suffice. There must be some science underlying what we "feel" is right to launch on. What I really need is a rocket scientist!

Some people use weight as limiting factor, while you and others suggest impulse. And it seems that initial impulse is important, not total impulse. An Apogee Medalist E-6 really launches like a B-6. Here's my thinking.. I know this may sound academic, but inquiring minds want to know:

If the rod is angled slightly, then weight and residence time (the time it takes to clear the rod) matters the most. Residence time is really rod length divided by acceleration (itself a question of weight and initial impulse).

If there is a wind, the sideward force on the rod is proportional to the sail area of the rocket. Longer residence times would amplify the bending force on the rod.

If the rocket develops a torsion about the rod (pitch), then the weight of the rocket is important, but so also seems the thrust that is driving it. So is the rod deflection and thus the required stiffness a question of weight (M), momentum (M*V), or energy (M*V*V)? The angle of pitch would seem to be pretty small, like a 1 degree departure, but the forces can be quite large, so I need some help on this.

And finally rod whip. I'm assuming this occurs when the natural resonance of the rod when converted into a wave velocity (rod length divided by frequency) is slower than the departure speed of the rocket, giving a "bullwhip" effect. If so, the resonance of a typical steel rod should be easy to figure out and totally dependent on the diameter and length, and the rocket velocity at departure is a question of Gs at initial boost.

All these relationships seem sensitive to rod length. Somewhere there is a way to compare rod strength of different dimensions- say a 3' 1/8" rod is proportionally as strong as a 4' 3/16" rod... or something like that. So any sort of nomogram would have to take rod length into account.

A lot depends on the lenght of the rod/rail needed for stable flight, not necessarily the width. UROC's guidelines are probably a great start for rules of thumb. And yes, weight to thrust ratio is critical in determining what is most appropriate (safe) for a launch guide/tower platform. Speed or velocity clearing the guidance and stability of the flight are critical.

I wouldn't build any altitude attempt rocket with launch lugs, rail buttons or guides. A launch tower is an absolute necessity to my way of thinking. You lose to much to drag.

As for towers, the club of course has the small bicycle tower that is always in the launch trailer. There are a couple other ones floating around the club that can be brought out to launches.

Warren

All these relationships seem sensitive to rod length. Somewhere there is a way to compare rod strength of different dimensions- say a 3' 1/8" rod is proportionally as strong as a 4' 3/16" rod... or something like that. So any sort of nomogram would have to take rod length into account.

I can calculate and chart the bending moment for you after I get back from the Alamogordo regional, but suffice it to say that if you double the diameter of a rod, you get 4 times the stiffness than before. ref: Moment of Inertia of a rod along the Z axis is Iz= 1/2mr^2 (m=mass, r=radius)

Iz (.125 dia) = .0039 (units ignored - we're comparing relative stiffness)
Iz (.188 dia) = .0088
Iz (.250 dia) = .0156
Iz (.375 dia) = .0351

Of course there will be more deflection as you move up the rod, and the moment arm, causing deflection, is the center of the nozzle to the axis of the rod. The farther away (distance between centers) the more torque applied to the rod.

Example, a two inch diameter rocket with 10 pounds of thrust would have a moment of 10 inch-lbs applied to the rod, that is trying to move the rod sideways, and will be at its worst at the top of the rod, when the rocket is guided only by the lower lug. Before that the rocket itself provides added stiffness to keep the rod straight.

Using a tower eliminates this torque, unless you can run the rod directly up the center of the rocket. 😀 This is weight independant and more of a function of thrust and distance.

The effects of the wind would be a minimal consideration when the rocket is on the rod, as compared to when it is in the air and weathercocking, as it spends a very little time on the rod, and most of the time in the air. The exception of course, is rocket gliders which are REALLY affected by the wind on the rod because of their large surface area 😕 .

However, if a rocket imparts a lot of sideways thrust to the rod, and even though the rod stays relatively straight, if the base attachement (tripod) is small you could actually cause the whole launch rod system to tilt over by those same moments. I think that is a bigger concern as the thrust-to-weight ratio increases significantly. 😯

When I get a chance I'll make a force deflection curve for you.

Example, a two inch diameter rocket with 10 pounds of thrust would have a moment of 10 inch-lbs applied to the rod, that is trying to move the rod sideways, and will be at its worst at the top of the rod, when the rocket is guided only by the lower lug. Before that the rocket itself provides added stiffness to keep the rod straight.

Is the force you are referring to here the torque precipitated by launch lug friction? Or is it just a random deviation that once established imparts a force on the rod? In both cases, as you indicate the diameter of the rocket is also important (which is somewhat proportional to weight I suppose).

As you and Joe both mention, thrust is important. Too little thrust and there is not enough stability at rod end OR you have to get a longer rod (which you forthcoming analysis will probably show as substantially more flexible). Too much thrust and torque increases and rod whip may be a problem.

It should be easy to relate rocket weight and thrust to distance required to reach a safe stable velocity. I'll nail down some simulator points and get that part figured out.

There is a lot of physics here! I'm always cautious of rules of thumb, in my experience they tend to be based on scant data. I picture a bunch of guys sitting in chairs with beer in hand making long leaps of logic... I love the tagline of RocketMaterials.org, "in God we trust, all others bring data."

Is the force you are referring to here the torque precipitated by launch lug friction? Or is it just a random deviation that once established imparts a force on the rod? In both cases, as you indicate the diameter of the rocket is also important (which is somewhat proportional to weight I suppose).

Chad,

The bottom lug become a 'pin' about which the rocket is trying to rotate after the upper lug leaves the rod. The stiffness of the rod reacts against the horizontal force caused by the torque of the thrust times the moment arm (typically half the radius of the rocket). I'm not taking into account any friction between the lug and rod, or the weight of the rocket, as I don't think it is relevant to the discussion. The weight would be a fraction of the thrust, as it should be, so we only need to worry about the thrust. Also a longer lug helps to keeps the lug from binding because of the torque as well. Assume the bottom lug is stuck on the rod and the rocket is trying 'turn' into the rod because of the torque. The rod stiffness tries to keep it straight, but will bend according to the horizontal force applied to it.

I need to dig up my freshman static and strengths book to so I can establish to your satisfaction the relationship of force-deflection of a round cantilever beam (mounted vertical in this case). Next week OK?

Slipstick,
Anytime is just fine. Any help is appreciated. I'll do my best to process data together with rules of thumb and produce some sort of graph.

I look forward to Mike's calculations. But in the mean time I've made some more progress on this questions and scoured just about everything available on the internet.

So as El Presidente points out, the main reason for a launch rod is to keep the pointy end up. Everything else is secondary— so, how much rod do we need to send the rocket on its way properly. The most common threshold of stability mentioned, and the one RockSim uses, is 45 feet per second. I'd be curious to see if others agree with this minimum velocity OR if the threshold velocity would vary depending on rocket mass.

But for now, lets assume that 45 FPS is a good number. The length of rod required is related to the acceleration / G force. I found a couple of incorrect formulas on the internet to deduce this, so let me get the right equation out there: Minimum Rod Length= K / (G force ^a), whereas K is a constant to make the units work out, and 'a' is a power function that is roughly -1.

Now for specifics: first, lets assume that only 80% of the rod is used for guidance. This allows leeway for the upper lug becoming free of the rod before the lower, or if the rocket has a stand-off height above the blast deflector. Next, lets wrangle our units. 4.44 Newtons equal 1 pound of thrust (and 16 oz per pound or 455 grams per pound). Thus G force is simply the converted thrust divided by rocket weight.

[note- yes I know I am interchanging units of weight with force without proper math, but since the force in question is straight up and there is no 'x' vector, I can cancel out 9.8m/sec/sec of gravitational force]

I empirically derived this equation from a test rocket and constant thrust motors in RockSim:
Rodlength (ft)= 50.66* (G force ^ -1.06)

Here is the required G force for different rod lengths:
2' rod= 24.3 Gs
3' rod= 15.8 Gs
4' rod= 11.7 Gs
5' rod= 9.2 Gs
6' rod= 7.6 Gs
8' rod= 5.6 Gs
10' rod= 4.4 Gs

A simplier rule of thumb is 50/G force= rod length in feet

These required lengths are longer than I would have imagined. I've commonly seen the figure of minimum thrust= 5G, but that requires a 9' rod! It also indicates that a typical Mean Machine on an E9 needs a 7' rod! Be cautious when computing that you base G force off the total weight of the object and that the initial thrust is the used, not the average thrust.

What is presented here will guide the rest of the analysis. As you can see heavy rockets are driven to long rods, which are proportionally weaker than short rods.