Rocket under or over rail

[rocketsam2016]

I could have sworn there was an interesting discussion a while back about the differences between putting a rocket above or below the rail when the rail is angled, but I can no longer find it.

So, I know it doesn't make a big difference, but I'm interested in hearing what reasons there might be for preferring it one way or the other.

Off the top of my head:

On top of the rail:
- Less likely to rip the rail buttons off when loading/sitting on the rail

Underneath the rail:
- Maybe will bend the rail less during takeoff? In my head it feels like the rocket will be trying to bend the rail down, but the thrust of the motor will apply a torque that will bend the rail up, and maybe these offset?​

Are there any meaningful reasons to prefer over vs under? Any existing discussion of the physics of this question?

 

[blackjack2564]

Pull the cart.......Push the cart?
Boxers.....or briefs?
Great deal of commercially launched sounding rockets are loaded & flown [ few degrees off vertical] from bottom. I did not say all!

Actually for me....it depends on size of my project.
For smaller rockets under [about] 30-35 lbs, I will work alone and use under side. Rocket hangs vertical on it's own naturally & you don't worry about sliding home at angle....binding...ripping a button off

Large rockets usually 6in. & larger I need some buddies to help, especially really big 100-300lb projects.
So first off you rarely have a choice, most pads I've seen only load from top & are hydraulic or power asset lift.
It takes 2 or more to keep rocket centered during the slide & sometimes the rocket is assembled in sections on the rail. So easier on top.

However all that being said if I had MY choice & the perfect pad if would load from underneath, where I could just "roll" rocket onto rail, but that requires a lot of support crap to go with it.

Best saved for semi or permanent install.

For other Ques. about rod bend during rocket install....
That's why many launches have pad managers to show you how to properly load & slide down base plate, with out bending rail. Support either rail in middle while rocket slides over that section. OR
if bottom push rocket up while passing through center section of rail, to keep from bend & where theres bend...it's buddy "bind' is right behind.
If need be never load with one person, where 2 are really needed.

Lift off "bend" would be same on either side of rail.
What's more of an issue.......wind direction. I want my rocket on down-wind side of rail. Breeze will push rocket Off rail and be centered downwind..Upwind or to side can cause [not always] binding and button removal by chance, during liftoff...lol

 

[caveduck]

From a pure mechanics standpoint you want the rocket under the rail. It's a stable position and gravity will tend to keep the rocket centered with the load on the rail guides being in extension. When the rocket is on top, it's mechanically unstable and any small deviation will make it diverge, with increasing lateral torque on the guides. For smaller/lighter vehicles it won't matter though. As Jim noted, design practicality makes most HPR trailer truss launchers load from the top, at the price of having to be really careful with heavy rockets.

 

[cwbullet]

I am an under type of guy.

 

[DavidMcCann]

Whichever way will put the rocket facing the flight line for better photos

 

[Eric]

At LDRS, while doing the monkey loft flights. There was a pretty steady wind. The rails would be angled into the wind a few degrees. But the larger/lighter Warlock was catching that wind while "under" the rail and it felt like it was binding the rail buttons. Placing the rocket on the "over" side of the rail with wind hitting the rail first, relieved this.

But then there was a rail in the way of your photo.

 

[blackjack2564]

. Placing the rocket on the "over" side of the rail with wind hitting the rail first, relieved this.

But then there was a rail in the way of your photo.

Most places I fly, will allow photographers to be out on opposite side of pads, if wearing a safety stripe vest.
That reason for one, & better pics another!

 

[captbk]

The pads I have flown from have the rail offset from the center of the blast plate so the exhaust hits the middle, so you really only have one way to load your rocket. If you want downwind, spin the whole pad.