Senior Space Cadet
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- May 23, 2020
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As soon as I think I have things figured out, some new piece of information gets me all confused again.
For a free CAD program, Open Rocket is phenomenal. Whoever is responsible should get a congressional medal or something. But having said that, Open Rocket is one source of misleading information. It is invaluable for getting a good approximation of CG and CP, beyond that, I'd take a lot of the information you get from it with a grain of salt. I'm pretty sure NASA doesn't use Open Rocket.
I keep seeing photos of people, out on some dry lake, launching mid and high power rockets. I'm always struck by the fact that their rockets look nothing like what I thought they should look like, from what I thought I knew about rocket design. Namely, they were really long and skinny with itsy bitsy fins.
Either they were idiots, or I was missing something. I was pretty sure they were idiots, but I thought I should look into it .
First off, the design of a subsonic rocket is going to be somewhat different from a trans-sonic or supersonic rocket.
Here is what I currently think is correct information about the design of subsonic rockets for those of you that are just as confused as I was.
Best nose cone is Ellipsoid. Ogive is only slightly better than hemispherical. Length should be 2 to 3 time base diameter. Since a longer nose cone allows you to shorten the body tube, I think you can fudge on that.
Body tube should be ten to sixteen times the diameter. There are two types of stability, dynamic and static. Dynamic stability favors long body tubes, static favors big fins. Go for dynamic.
The best fin shapes are Elliptical or clipped delta. Tip chord should be about half of root chord (talking trapezoidal here) and sanding the tip to a sharp edge is important. All edges should be rounded, at the very least. I'm a big airfoil fan.
I'm not sure about height of the fin. I'm going for around 1.5 times the diameter. They should have a lot less sweep than I've been giving my fins.
Make the outside really smoooooooth. Dance around it while playing Paranoid, by Black Sabbath. Finnish by meditating to Misty Mountain Hop.
For a free CAD program, Open Rocket is phenomenal. Whoever is responsible should get a congressional medal or something. But having said that, Open Rocket is one source of misleading information. It is invaluable for getting a good approximation of CG and CP, beyond that, I'd take a lot of the information you get from it with a grain of salt. I'm pretty sure NASA doesn't use Open Rocket.
I keep seeing photos of people, out on some dry lake, launching mid and high power rockets. I'm always struck by the fact that their rockets look nothing like what I thought they should look like, from what I thought I knew about rocket design. Namely, they were really long and skinny with itsy bitsy fins.
Either they were idiots, or I was missing something. I was pretty sure they were idiots, but I thought I should look into it .
First off, the design of a subsonic rocket is going to be somewhat different from a trans-sonic or supersonic rocket.
Here is what I currently think is correct information about the design of subsonic rockets for those of you that are just as confused as I was.
Best nose cone is Ellipsoid. Ogive is only slightly better than hemispherical. Length should be 2 to 3 time base diameter. Since a longer nose cone allows you to shorten the body tube, I think you can fudge on that.
Body tube should be ten to sixteen times the diameter. There are two types of stability, dynamic and static. Dynamic stability favors long body tubes, static favors big fins. Go for dynamic.
The best fin shapes are Elliptical or clipped delta. Tip chord should be about half of root chord (talking trapezoidal here) and sanding the tip to a sharp edge is important. All edges should be rounded, at the very least. I'm a big airfoil fan.
I'm not sure about height of the fin. I'm going for around 1.5 times the diameter. They should have a lot less sweep than I've been giving my fins.
Make the outside really smoooooooth. Dance around it while playing Paranoid, by Black Sabbath. Finnish by meditating to Misty Mountain Hop.