How accurate do you expect model measurements to be?

[Jeff Lassahn]

I'm thinking about doing some scale rockets, and I'm trying to decide how picky to be about accuracy.
So I'd like to hear some people's thoughts about how they decide how good is good enough. If you can get a standard size tube that's 0.020" too large do you just go with it? When you get a kit and you check a measurement how bad does it have to be before you think "Ugh, Why!?!" instead of "Oh, OK"?

I know fundamentally It's my personal decision and I can do whatever I want, but I'm curious how people who have more experience feel.

 

[mikec]

YMMV. If it "looks right" then I don't care. Anything within 10% is close enough in my book. Of course my sport-scale Argo D-4 Javelin used a standard nose cone, had no tail cone, etc, and wouldn't have won any prizes in a NAR competition, but I was satisfied with it.

 

[Ez2cDave]

Jeff,

Accuracy is a foregone conclusion in Scale model building. As you noted, the question of "how much accuracy do I want / need ?", comes into play.

If you are planning on flying in NAR Precision Scale Competition ( the model is measured and compared to the dimensions in your scale data pack ), you had better be within 1% tolerance . . .

In NAR Sport Scale, you still need to be pretty close ( the model is only "judged by eye" and not measured ).

If you are building strictly for your own pleasure and enjoyment, "close enough" is good enough.

If you get a tube that is a different diameter, just change your Scale Factor and "have at it" !

Example : You want to build a "1/12 Scale" NIKE Booster ( the real one is 16.44" in diameter )

If you use a BT-55 . . . 16.44" / 1.325" = 12.407547 . . . Your model is 1 : 12.407547 Scale.

If you use a BT-56 . . . 16.44" / 1.35" = 12.177777 . . . Your model is 1 : 12.177777 Scale.

In my opinion, Nose Cone selection / construction is one of the most critical areas, no matter what "level" you are building to.


Dave F.

 

[James Duffy]

I am certainly an outlier, but I'm looking for <1% dimensional error or less on any serious scale project. As a result I tend to build infrequently from kits, and end up producing a bunch of lightweight custom fiberglass tubes for specific applications.

James

 

[Bruiser]

If I'm trying to build to a certain scale, say 1/6 I will select the closest size tube to whatever the 1/6th scale works out to and build the rest of that rocket to whatever that scale works out to be

Let's say I want to build a 1/6th scale Nike Smoke and full scale is 16.44. 1/6th comes out to 2.74. Well the easiest tube to obtain near that is a BT80 at 2.60 so I just figure out the correct measurement for everything else by dividing by 6.32 instead of 6. 1/6.32 scale is close enough to 1/6th for me and everything stays in the correct proportions...

-Bob

 

[TheAviator]

That's typically what we do. Unless you're willing to make custom tubes, there usually isn't a tube size that makes an exact 1/N scale (where N is an integer).

 

[PeterAlway]

The traditional maximum persnicketyness is to get all dimensions within 1% of the correct value. That comes from the old Scale judging guides/pink book, where any error under 1% is considered as good as perfect. There are only a few cases where a 1% diameter error would stand out--basically if there is, say a 2% diameter change, and the model has the larger tube 1% too small and the smaller tube 1% too big (so that there is no diameter change. But those are rare. So I'd say if you are within 1% of the correct scale value, call it precise, and spend your persnickityness on other elements of the model, like smooth craftsmanship.

 

[AlphaSigOU]

I used to be absolutely hard-nosed for perfection when it came to building scale model rockets. Nowadays with advancing age, I usually go for sport scale; precision measurements aren't so important as long as the shape is close to the actual prototype. Excellent craftsmanship and a good flight are good in my book.

 

[MarkB.]

I like to build around 1/10 scale so that my rockets are comparably sized to each other. But I'll call 1/10 scale anything between 1/9.5 and 1/10.5 scale. For example, I was thinking of a 1/10 scale SA-2 Guideline. Diameter is 25.6" on the booster and 19.7" on the sustainer, so 1/10 would be 2.56" and 1.97" diameter. No readily available tubes in that size. But I noticed that that a slight upscale would get us around BT-80 and ST-20. Using ST-20 (2.042") as the sustainer yields a scale of 1/9.65. At that scale the booster should be 2.65" which is pretty darn close to a 2.635 BT-80H. It's a project for next spring, maybe.

Scale judging at my cosmodrome is really just a thumbs-up or thumbs-down from SWMBO after being shown a picture or drawing of what I was shooting for. I usually do OK. And maybe that's the key, build it with the accuracy that makes you happy; only you know at what point it stops being fun and becomes tedium. I believe you should always err on the fun side.