Round the Leading Edge of the Fin and the Trailing Edge, or Just the Leading Edge?

[Grant_Edwards]

I also note, from flipping through the book, that Centuri once made a 1/2 A engine that was a "shorty". It was still an 18mm engine, but it was not as long as a standard 18mm engine. Estes' 1/2 A engines are mini, 13mm engines. Interesting!

Estes also used to make short 18mm engines at 1/4A, 1/2A and A impuse. They had an 'S' suffix: 1/4A3-2S, 1/2A6-2S, and A5-4S (to name a few). The old Astron Sprite used those 'S' engines.

 

[brockrwood]

Estes also used to make short 18mm engines at 1/4A, 1/2A and A impuse. They had an 'S' suffix: 1/4A3-2S, 1/2A6-2S, and A5-4S (to name a few). The old Astron Sprite used those 'S' engines.

I wonder why there was a switch to 13 mm engines for those A and sub-A engines? It seems easier to just make them 18 mm in diameter but shorter. That way you can standardize on an 18 mm engine mount.

 

[Rocketjunkie]

I wonder why there was a switch to 13 mm engines for those A and sub-A engines? It seems easier to just make them 18 mm in diameter but shorter. That way you can standardize on an 18 mm engine mount.

Cut off a spent 18 mm case to 1-3/4". Insert a 13 mm engine in it and you will have a 'shorty' motor.

 

[Rocketjunkie]

I round the leading and trailing edges. If the tip is parallel to the body I leave it square. Otherwise, I round it too.

 

[Grant_Edwards]

I wonder why there was a switch to 13 mm engines for those A and sub-A engines? It seems easier to just make them 18 mm in diameter but shorter. That way you can standardize on an 18 mm engine mount.

It's hard to put an 18mm engine mount in a 13mm rocket.

I remember the introduction of the 13mm engines being a big deal. They were cheaper: the same amount of paper route money got you 33% more engines. The "mini" versions of various rockets were also significantly cheaper. My friends and I were all big fans of the "mini-brutes" back around 1972 or so. Though the mosquito was a bit too "mini" — it didn't so much as launch as just instantly vanish. If you were lucky you could spot the delay charge smoke and then try to guess where it might land, but I don't think we ever recovered more than about 1 out of 3 mosquitoes. They were only $.49 back then.

I've still got a couple 50 year old Mini-Berthas and a Screamer that are airworthy. The almost finished Beta and Midget need some repairs.

 

[mh9162013]

I wonder why there was a switch to 13 mm engines for those A and sub-A engines? It seems easier to just make them 18 mm in diameter but shorter. That way you can standardize on an 18 mm engine mount.

If you use a shorter 18mm engine, you need a spacer so they'll still work in a standard 18mm MMT. As a result, you've got a lot of extra "dead weight" compared to an 13mm engine.

For example, an A10-3T is about 8 grams lighter than an A8-3. Even with an Estes adapter, the A10-3T will still be about 3-4 grams lighter than the A8-3. And this is 3-4 grams of less weight at the bottom of your rocket.

 

[brockrwood]

If you use a shorter 18mm engine, you need a spacer so they'll still work in a standard 18mm MMT. As a result, you've got a lot of extra "dead weight" compared to an 13mm engine.

For example, an A10-3T is about 8 grams lighter than an A8-3. Even with an Estes adapter, the A10-3T will still be about 3-4 grams lighter than the A8-3. And this is 3-4 grams of less weight at the bottom of your rocket.

Makes sense!

I have considered making a 13mm adapter out of an 18mm spent engine but it looks a lot heavier than just using the Estes plastic adapter. I need to find my Estes plastic adapters…

 

[brockrwood]

It's hard to put an 18mm engine mount in a 13mm rocket.

I remember the introduction of the 13mm engines being a big deal. They were cheaper: the same amount of paper route money got you 33% more engines. The "mini" versions of various rockets were also significantly cheaper. My friends and I were all big fans of the "mini-brutes" back around 1972 or so. Though the mosquito was a bit too "mini" — it didn't so much as launch as just instantly vanish. If you were lucky you could spot the delay charge smoke and then try to guess where it might land, but I don't think we ever recovered more than about 1 out of 3 mosquitoes. They were only $.49 back then.

I've still got a couple 50 year old Mini-Berthas and a Screamer that are airworthy. The almost finished Beta and Midget need some repairs.

$0.49 in 1972 is $3.43 today. That’s inflation for you.

 

[Hobie1dog]

just the leading edge, I'm not concerned in altitude

 

[Grant_Edwards]

$0.49 in 1972 is $3.43 today. That’s inflation for you.

It takes me far, far less time to earn $3.43 today than it took me to earn $0.49 in 1972. That's education (and experience) for you.

 

[teepot]

How do you get all sides of the foil to be exactly the same? Do you have a jig?

If they are not the same, the rocket will have some roll.

I eyeball it. I'm not looking for perfect. I'm not flying monsters to 100K'. So far I have got to 4500' at 625mph. No record attempts. A little roll is fine.

 

[Woody's Workshop]

I wonder why there was a switch to 13 mm engines for those A and sub-A engines? It seems easier to just make them 18 mm in diameter but shorter. That way you can standardize on an 18 mm engine mount.

Thrust to weight ratio I imagine is the main reason, and cost to manufacture and cost to consumer. An 18mm casing is substantially more heavier and costlier than a 13mm for somewhat close to the same amount of thrust.
Don't knock the Mini's! Besides A8-3 and some B4-4's, Mini's is about all I fly anymore. They still get up there depending upon the size/weight of the rocket. I just can't go very far to retrieve them anymore, and that's a big deal to me!
I have even built some small rockets that came with 18mm EM's with 13's just because even with an A8-3 they go dang far on the drift.

 

[fyrwrxz]

There has to be eleventeen ways to get something other than a square edge to a flying surface. It's just me, but I find a blunt edge on anything going more than 40 miles a hour is like petting the cat backwards. Prolly more about aesthetics than performance, given most flight envelopes are measured in seconds and if there is any transonic region, it's only a percentage of that. Others may know better, but even disposable munitions bother with foiling to a certain degree. Square edges seem like they will 'hunt' as airflow tries to stabilize the attached vehicle, reducing speed off the rail and the cp fights the pendulum effect. Not interested in debating aerodynamics with anybody, but we ARE building rockets, right?

 

[Paul Howard]

It's hard to put an 18mm engine mount in a 13mm rocket.

I remember the introduction of the 13mm engines being a big deal. They were cheaper: the same amount of paper route money got you 33% more engines. The "mini" versions of various rockets were also significantly cheaper. My friends and I were all big fans of the "mini-brutes" back around 1972 or so. Though the mosquito was a bit too "mini" — it didn't so much as launch as just instantly vanish. If you were lucky you could spot the delay charge smoke and then try to guess where it might land, but I don't think we ever recovered more than about 1 out of 3 mosquitoes. They were only $.49 back then.

I've still got a couple 50 year old Mini-Berthas and a Screamer that are airworthy. The almost finished Beta and Midget need some repairs.

The "Mini-Bertha" was the first rocket I had that my Dad gave me! My parents wisely in Cheyenne Wyoming(windy, dry, lots of dead grass by mid summer) refused to let us kids have anything do with fireworks but my Dad let me build rockets. I loved that thing and wore it out!

I recently scratch built a a Mini Bertha as a tribute to both of my Parents. I elongated it a smidge for better flying and it flies great with A10-3T's but now that I had an A10-3T "roman-candle" my Mini Fatboy recently, I'm now kinda hesitant to fly my Mini Bertha.

 

[Paul Howard]

There has to be eleventeen ways to get something other than a square edge to a flying surface. It's just me, but I find a blunt edge on anything going more than 40 miles a hour is like petting the cat backwards. Prolly more about aesthetics than performance, given most flight envelopes are measured in seconds and if there is any transonic region, it's only a percentage of that. Others may know better, but even disposable munitions bother with foiling to a certain degree. Square edges seem like they will 'hunt' as airflow tries to stabilize the attached vehicle, reducing speed off the rail and the cp fights the pendulum effect. Not interested in debating aerodynamics with anybody, but we ARE building rockets, right?

I definitely believe an airfoiled fin (even rounding the edges) is FAR better for straighter faster flight characteristics basically for the same reasoning you provided, I would probably have explained it in different wording, your concepts you brought up I agree with 100%. Thanks!