On the subject of Low Hanging fins.....

[MaxQ]

I am looking at a particular design problem....many of you have probably done a project that has fins that have substantial extension below the aft end and have had to deal with recovery issues.
On lightweight rockets, may not be too much of a problem. My original Loc IV came without thru the wall fins (which the kit now has) so I had do do a lot of repairs over the years.

Once the project gets bigger, and the weight goes up, the ability of the fin to absorb the hit on landing w/o breaking off becomes a problem.
Probably why a lot of the bigger projects don't have fins that extend below the airframe.
But if you want to do a scale project..this becomes an issue (see the second picture below...Mercury Little Joe).

I've done thru the wall fins on most projects, with reinforcing internally at the the fin root to motor mount.
The fins on my Astrobee 1500 survived one recovery falling w/o the main parachute attached (ground was soft and the fin roots were heavily reinforced internally - see the first picture below)

I've heard some guys do bolt on fins for easy replacement when they are damaged, but hiding the non scale bolts would be a problem on some scale projects.
Some guys probably opt for a solid thick fin, as a hollow structure is prone to damage.
I've seen a lot of L-3 projects where the fins are locked into centering rings above and below the fin root, with threaded rod between the centering rings.

Any other ideas?

 

[dhbarr]

Round nose or v groove the trailing edge and put a CF spar on there?

 

[rharshberger]

Bigger chutes if possible, the lower the landing velocity the less likely to break the fins unless landing on a hard surface. Even reinforced fins like the first picture tend to break along the fin to airframe junction, not usually at the motor tube. Fiberglass cores with wood fins can help prevent breakage as they add a bit of toughness to go with the natural flexibility of wood.

 

[Andrew_ASC]

Okay who or what flew on the Mercury little joe?

 

[dr wogz]

A test vehicle, no one flew on her.. it was to test the escape system and the heat shield if I read it right..

https://en.wikipedia.org/wiki/Little_Joe_(rocket)

 

[MaxQ]

Okay who or what flew on the Mercury little joe?

I'll be flying in it when I finish it.
I'm going to prove the earth is flat.

 

[jd2cylman]

I never broke a fin on my LOC IV, I must have been doing something wrong... :confused2::confused2::smile:

And yes, I did fly it a lot back in the day. Now with the advent of the JL CR, I might have to dust it off and fly it some more...

 

[Handeman]

Check the team at BOR this year and what they did with their fincan. They had a line attached to the fin and run up and into the fincan. When it deployed, it was attached to the shock cord so it kept the fin can in a fin up position so they didn't break fins, the BT hit first. A very interesting solution even if it doesn't quite match the scale of the Little Joe.