Tuesday, April 21, 2009

This one is for AOG

One man's quest to honor America's Saturn V rocket, the story of the biggest scale model rocket ever built.

Interesting to see what goes into making anything more elaborate than a toy rocket fly.

(H/T Brian Appleyard)


Blogger Susan's Husband said...

Heh. If you haven't tried it, you can't imagine how much work that really is. I have a Saturn V kit that I doubt I will ever have the energy to build and it's only about 4 feet tall.

I have actually flown at the field in Danville, IL mentioned in the article.

I am currently doing an after school class, teaching 3-5 graders to build a small model rocket. Gosh, the things you've done so long they seem obvious. Of course, a couple of the kids not being comfortable with glue is a bit of an issue.

April 22, 2009 6:19 AM  
Blogger Hey Skipper said...

I wish the article had talked more about guidance and forces.

Is the model a high powered rock, or does it actively try to maintain some sort of trajectory?

Also, it never mentioned G-loading. IIRC, the Shuttle accelerates at about 3 Gs during liftoff. What does the S-V model do?

April 22, 2009 8:37 AM  
Blogger erp said...

We watched "October Sky" a couple of night ago and it brought tears to my eyes. Is/was this a great country or what?

April 22, 2009 8:40 AM  
Blogger Hey Skipper said...


Thanks for reminding me, I have been meaning to put that on my Netflix queue.

April 22, 2009 1:19 PM  
Blogger Susan's Husband said...

Purely passive guidance. A high powered rock.

It did mention a thrust to weight ratio of 5-1, which is roughly 5Gs. As noted, that's roughly a minimum G-load you want in order to have sufficient passive stability. 10-20G is more common, but I have seen models that hit 100G. Those can fail spectacularly.

April 22, 2009 9:29 PM  
Blogger Hey Skipper said...

It did mention a thrust to weight ratio of 5-1, which is roughly 5Gs.D'oh.

I was short of time when I posted this, and missed that completely.

As noted, that's roughly a minimum G-load you want in order to have sufficient passive stability.Sounds like being stuck between the devil and the deep blue sea. The rocket is more stable at higher speeds, but quicker acceleration magnifies any instability.

In order to remain stable, the center of lift -- i.e., the aerodynamic force created by the tail fins due to any non-zero angle of attack -- has to be aft of the CG. Which is why arrows, darts look the way they do.

However (full disclosure: my notion of physics could be way off here), the center of thrust is at the very tail of the rocket.

Because the motors are so powerful, any thrust asymmetry could easily create a destabilizing moment greater than the stabilizing moment of the tail fins, particularly at the beginning of the flight.

Which, I suppose, leads to a very short flight.

Even if the destabilizing moment is less than the stabilizing moment, for an unguided rocket, that would result in a trajectory other than vertical. (A competitive scoring factor, perhaps?)

What causes the spectacular 100G rocket failures -- wildly divergent instability, or splodey motors?

BTW, I waved last night as I went by on my way into Indy. You might not have noticed.

April 23, 2009 8:56 AM  
Blogger Susan's Husband said...

There will be a launch tower of some sort to hold the rocket steady until it's up to speed. That's the real reason for the lower G limit, as there's only so high you can build a tower and the rocket has to be to sufficient speed to get stabilization from the fins. So high thrust is good, because those initial instabilities are damped by the tower.

As for the CG, that's basically correctly. There several simulators out there to do the calculations for you. Plus, scale works so if a small model flies well, a scaled up will fly well too.

However, assymmetric thrust is what makes using clusters of motors a skill. It's not so bad if you can get the motor mounts to touch, but otherwise you just have to be good with tools to get everything lined up.

As for the high G failures, it can hard to build something that will hold together under that kind of stress. For instance, the motor might break the motor mount and go flying up through the rocket on its own (I've seen that happen). At 100G you get moving very fast very quickly and the air stream can rip a fin off (I've seen that too). Many of these also break mach which as you know can put some stress on an airframe which it might not be able to handle.

April 23, 2009 3:13 PM  

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