Finishing Questions for a Specific Project

[RocketDad]

It’s that time of year again… Science fair projects!

My twelve year-old daughter has done very well with her science fair projects the last two years using model rocketry, so she wants to do it again this year. Actually, she has aspirations to someday become an aeronautical engineer in the space program. At least she’s been saying so since she was seven.

This year, she wants to test how the apogee of a rocket is affected by the finish of the rocket. She wants to build four of the same rocket, two with the same smooth finish and two with the same rough finish. Two of each rocket will be built in case she crashes one of the two. We will also make sure all rockets weigh the same, at the time of launch using modeling clay in the payload and nose cone as needed to make sure the center of gravity is the same in all rockets too. The smooth rockets and the rough rockets will be launched three times, averaging the three and then comparing the two averages for results.

We are using a design we used last year that worked very well for us:
*29mm diameter X about 30 inches
*Plastic elliptical nose cone
*Three Clipped Delta fins to scale as directed in the Handbook of Model Rocketry
*D-12,5 engines
*Payload with a Perfect Flite Altimeter

We have two of these already built (from last year and unfinished) and want to make things easier by only having to build two more. Also, since I figured it will take more time and effort to finish the smooth rockets, we’d start finishing them while we build the other two rockets that will be finished with rough paint.

Concerns:
a. We are good with our hands, but we have finished only two rockets previously. So we are relatively inexperienced.
b. We have about a month to complete all four rockets, including the finishing.
c. Our work space is in our very hot and extremely humid garage in South Florida.

Questions:
1. Which materials are both user friendly and work best for filling in the spirals in the tubes and the balsa fins?
2. Which spray primers and paints provide the smoothest finish?
3. What techniques do you find provides the smoothest finish? Note: I’d prefer not to do any wet sanding in fear I’ll soak the cardboard rockets already built.
4. Any other info not asked here is also appreciated.

Thanks,

RocketDad

 

[NjCo]

b. We have about a month to complete all four rockets, including the finishing.

You should be able to finish a couple rockets in a month.

c. Our work space is in our very hot and extremely humid garage in South Florida.

Not the best environment for finishing rockets. Most spray paint doesn't like high temperature and high humidity. The can of Krylon I have right here says 50 - 90 degrees and <85% humidity. I'd say even 85% humidity is high. But then what do I know about humidity, I live in Colorado!

Questions:
1. Which materials are both user friendly and work best for filling in the spirals in the tubes and the balsa fins?

This depends on the spirals and how bad they are. I've filled with Elmers Fill 'n finish compound diluted about 2 parts FnF to 1 part water. Apply it to the grooves with a popsicle stick, an old paint brush or even the edge of a putty knife. Let if dry completely and then spray primer over that. Then sand it all down. The primer goes on over the FnF to protect the rest of the body tube so you don't scuff it up while sanding. This method works well if you have fairly large grooves.

The other method for more minor grooves is to just use primer. One good coat and then sand it off until all the grooves and imperfections are gone. This first sanding can be with a 180-200 grit sandpaper. Then spray another coat and this time sand it down (but not all the way off) with a 400 or so grit sandpaper. That should give you a nice surface for your coats of paint. Two coats of paint with some light sanding (600 grit or so) in between should give you good results.

2. Which spray primers and paints provide the smoothest finish?

Pretty much everything will work. Some might be easier than others but the deciding factor here might very well be your climate more than which brand of paint/primer you use. I've used Rustoleum, Krylon and another brand I picked up at Michael's hobby store. They have all worked well. The best for me was the one I got at Michael's. I forget the brand but I think they only have a few and it's not the Krylon. It sands off nicely and doesn't gunk up the sandpaper. Both Rustoleum and Krylon clog up the sandpaper and slow down the process. But they still work.

3. What techniques do you find provides the smoothest finish? Note: I’d prefer not to do any wet sanding in fear I’ll soak the cardboard rockets already built.

Two coats of primer followed by two coats of paint give you a good finish it most situations. If you want a really smooth finish you can always add a coat of acrylic floor wax (Pledge with Future shine is the brand). Put it on with a soft brush and dab off any excess. Clean up is with ammonia based window cleaner. This gives a great smooth, high gloss finish.

Don't forget about the fins though. For a project like this you might coat the fins with paper to give them a real smooth finish. I've just completed two rockets using this technique and I really like the end result. Much smoother finish than if I would have used a couple of coats of Elmers Fill 'n Finish and sanded in between. It's also a lot faster than filling and sanding. All you need to do is get some full sheet labels and stick your fins to a sheet before doing any sanding. Then cut the fins out close to the edges with a razor knife. Then you need to apply a thin CA glue around the edge of the paper, maybe about 1/8" or a little more. This will give you a good hard edge that you can sand without pulling up the paper. At this point I'd give the rocket a coat of primer so that when you do round the edges of the fins you are not sanding into the surface of the paper.

4. Any other info not asked here is also appreciated.

One concern I'd have is that the difference in the drag you will be able to see between the finished and unfinished rockets might not exceed the variance your altimeter will be able to pick up. So your data sets will not be different enough. Essentially the error bars from your multiple measurements will end up overlapping and you won't be able to detect a significant difference in your altitudes. I've never done this type of thing and am not familiar with the altimeter you are using so maybe someone else could verify if this might be a problem.

One way to get around this might be to scuff up your unfinished rockets with some rough sandpaper. You might also try rounding the edges of your fins on the finished rocket but don't round them on the unfinished one. If possible you might also go with a unfinished balsa nose cone on the unfinished rocket and use a plastic one on the finished rocket. Provided of course you can find the exact same shape for both. Basically you are trying to make the unfinished rocket as 'unfinished' as possible to ensure that any difference you see is easily measurable.

Hope that helps and good luck!

 

[NjCo]

Oh, I forgot to mention one thing. If at all possible I'd try to use four fins instead of three. Four fin rockets have more drag than three fin rockets so the difference between unfinished and finished will be slightly amplified. But I'm guessing this isn't an option since it would mean scraping two rockets you already have built and building two more.


So what were your daughters projects the previous two years and how did she do?

 

[RocketDad]

NJCO: Thanks a lot. You gave me a lot to think about.

Actually this is our fourth year working with rockets. The first year was using a water and Seltzer toy rocket with Velcro on the bottom. We dropped the tablet in the rocket, shook it, and applied it to the velcro on a platform with a hinge. We shot the rocket off at different angle to see which angle allowed the rocket to travel the farthest. 45 degrees was our result. She got a general award for this (too young to enter the contest for the county).

Two years ago, we built two Quest Superbirds exactly, but used conical and elliptical nose cones to prove that elliptical nose cones reached the higher apogee. Elementary school 5th place in the physics catagory. Each year there are more kids who do physics catagory projects than all the other categories combined.

Last year, we build four identical rockets, but two 29mm diameter and two 56mm diameter rockets to prove that the thinner rocket reached a higher apogee. Last year she was a 6th grader competing against 6th, 7th and 8th grade middle school kids. She took 1st place for the whole school (first time this science fair coordinator has seen a 6th grader take that prize in that school) and 2nd in the physics catagory in the county for middle school kids.

I am a bit concerned about how much of a difference the different finishes will make, but hey, it's part of the experiment to see if the hypothesis is true in this experiment.

 

[NjCo]

Last year, we build four identical rockets, but two 29mm diameter and two 56mm diameter rockets to prove that the thinner rocket reached a higher apogee. Last year she was a 6th grader competing against 6th, 7th and 8th grade middle school kids. She took 1st place for the whole school (first time this science fair coordinator has seen a 6th grader take that prize in that school) and 2nd in the physics catagory in the county for middle school kids.

So did you use the same altimeter for this project as you are going to use in the current project? If so then what was the smallest difference between any two data points? I'm just trying to get an idea of how much of a difference in altitude the altimeter can detect. What were the differences in altitude when you did the project looking at the difference in the type of nose cone used? Maybe the instructions for the altimeter can provide this info. I know that using a launch method that eliminates the need for a launch lug can give you about a 10% increase in altitude. I would hazard a guess that if you had a really roughed up unfinished rocket vs a nicely finished rocket you might see that much of a difference in altitude especially if using a balsa vs plastic nose cone as part of that difference. How high will the D engines take your rocket? If your overall altitude is high enough (let's say >500-600 ft) then a 10% difference would likely be easily measurable.


For more ideas for science fair projects using rockets see here:

https://www.apogeerockets.com/science_fair_book.asp


So what other types of projects are popular at these science fairs? Just curious. You said the physics category is popular, what types of projects fall into that category? And what types of projects in general have done well in the science fairs you've attended?

 

[hardinlw]

I like lacquer sanding sealer for filling the body tube grooves and grain on the fins. If the body tube grooves are really bad, I use the red spot putty from an auto shop. That's lacquer based also. My favorite brand of sanding sealer is Behlen which I get from a woodworker's shop (Woodcraft). Once most of the grain and grooves are filled, I switch to a scratch filling auto primer. Depending on brand, the names are different, but looks for the words "sandable", and "scratch filling" or "spot filling" in the name. You can spray almost any kind of paint over the lacquer primers. For a really smooth finish, you need to wet sand with 600 grit and then 1200 grit, just don't get carried away with the water because the unpainted (inside) parts of the body tube are not water resistant.

The drag difference is because the rougher surface tends to turbulate the boundary layer (the air closest to the surface of the rocket) and cause it to trip from laminar flow to turbulent flow. You can accomplish the same thing by putting a "boundary layer trip" on the nose cone, basically just a ridge that pops the air off the surface and then lets it reattach. Wrapping several turns of sewing thread or kite twine around the nose cone about 2/3 of the way from the tip to the shoulder and soaking it in CA (super glue) should do the trick. Here's the catch. If the rocket design is such that the flow is separating somewhere, this can actually make it go higher because the turbulent boundary layer stays attached better even though it has more drag than laminar. The separation can cause really high drag, and getting rid of it more than compensates for higher drag ahead of the separation. If the rocket is a simple 3FNC, separation is probably not an issue.

 

[RocketDad]

So did you use the same altimeter for this project as you are going to use in the current project? If so then what was the smallest difference between any two data points? I'm just trying to get an idea of how much of a difference in altitude the altimeter can detect. What were the differences in altitude when you did the project looking at the difference in the type of nose cone used? Maybe the instructions for the altimeter can provide this info. I know that using a launch method that eliminates the need for a launch lug can give you about a 10% increase in altitude. I would hazard a guess that if you had a really roughed up unfinished rocket vs a nicely finished rocket you might see that much of a difference in altitude especially if using a balsa vs plastic nose cone as part of that difference. How high will the D engines take your rocket? If your overall altitude is high enough (let's say >500-600 ft) then a 10% difference would likely be easily measurable.


For more ideas for science fair projects using rockets see here:

https://www.apogeerockets.com/science_fair_book.asp


So what other types of projects are popular at these science fairs? Just curious. You said the physics category is popular, what types of projects fall into that category? And what types of projects in general have done well in the science fairs you've attended?

I actually downloaded that book from Apogee Rockets. Some of their experiments seemed pretty interesting, but we seem to gravitate to other ideas. To answer your questions:
The Perfect Flite Altimeter gives you your results to the nearest foot.

I don't remember the difference from the average apogees of the nose cone experiment, but I do remember it was at least 20 or 25%. I was actually surprised how much different it was.

Last year the D-12,5 engines allowed the unfinished version of this rocket we will use to reach an average of 153.3 meters (about 503 ft). But that was with an extra 83g shared in the nose cone and payload so the rocket would match the mass of the wider rocket. I'm not sure how much the finished rocket will weigh compared to the unfinished product, but we won't be using the modeling clay this year.

The project that beat out my daugher's project last year had to do with refraction of light. I don't remember what the kid did with the light.

 

[RocketDad]

The drag difference is because the rougher surface tends to turbulate the boundary layer (the air closest to the surface of the rocket) and cause it to trip from laminar flow to turbulent flow. You can accomplish the same thing by putting a "boundary layer trip" on the nose cone, basically just a ridge that pops the air off the surface and then lets it reattach. Wrapping several turns of sewing thread or kite twine around the nose cone about 2/3 of the way from the tip to the shoulder and soaking it in CA (super glue) should do the trick. Here's the catch.

OK. This part (above) I understood.

If the rocket design is such that the flow is separating somewhere, this can actually make it go higher because the turbulent boundary layer stays attached better even though it has more drag than laminar. The separation can cause really high drag, and getting rid of it more than compensates for higher drag ahead of the separation. If the rocket is a simple 3FNC, separation is probably not an issue.

You lost me here. I think you are saying... No. I tried to explain what I thought you meant, but I have no idea. And what is 3FNC?

 

[FatBoy]

3FNC is nothing more than a shorthand way of saying "3 Fins and a Nose Cone", usually referring to a basic, simple rocket design.​