Scratch built Executioner--ish build questions

[Adam518]

Hello all. I've been lurking here for a bit trying to learn all I can, but this is my first post.

I'm currently building a 2.6" diameter rocket with leftover pieces from a previous semi-custom Executioner build. This particular rocket has Executioner fins, a 18" lower tube, a 14.25" stock Executioner upper tube and an ogive nose cone from Apogee.

The motor mount is 29mm attached to ttw fins and an extra centering ring at the tip of the fins for a little extra strength. It separates at the middle of the rocket and the two halves are joined with an ejection baffle.

In the upper tube I installed coupler with a bulkhead on both ends. The lower bulkhead has an eyelet for the parachute cord and the upper bulkhead serves as a platform for an internally-mounted, horizontally mounted camera. I felt two bulkheads would provide a bit more protection for the camera...I tend to overbuild.

The camera is a RunCam2 and just barely fits in the 2.6" tube. As a matter of fact, only the camera body fits...the lens had to protrude through the side of the rocket. It is still removable, however, and I mounted it as low as possible and still be able to shoe-horn it into place. I felt the lens protruding out the side would not be good for aerodynamics as well as the fact it made the lens vulnerable on landing. So, I used a camera hood from Apogee that was manufactured for an externally mounted camera, but I removed enough material that it just barely extends beyond the camera lens and protective ring and cut a round opening so that I could use it with a horizontally mounted camera.

Below are pics of where I am at:

8D65A9CF-1445-4829-801E-535C4B61175A

• Adam518
• Apr 16, 2019

ED93FF9C-F3EB-440D-BE72-3E9737C9891D

• Adam518
• Apr 16, 2019

2A7D46BA-E223-40CC-9FA0-539C293245BE

• Adam518
• Apr 16, 2019

8A61017F-AA2E-406E-8B83-1D5E1C228228

• Adam518
• Apr 16, 2019

The zip-tie around the camera is simply there to make it easier to pull the camera back out of the rocket.

Sorry to be so long winded, but this brings me to my questions:

1. Is there a best place to position the camera hood in relation to the fins? I assume that I would mount the lugs opposite the hood, but this is a 3-fin rocket. I would appreciate any advice regarding where to best position the hood & lugs in relation to the fins as well as one another.

2. I also want to install an altimeter in camera's payload bay. I've read to drill holes (usually 3) in the body of the rocket for an altimeter so that it can read external air pressure. If you look at the close-up of the camera hood you can see that there is an air-gap between the outside of the lens protector and the hood. Would this be adequate ventilation for the altimeter to read correctly? For what it's worth, I can mash the top of the rocket tight against my face around my lips with the camera in place and can comfortably breathe. I know that's not very scientific but I would think that is a fair amount of air. I'd rather not drill extra holes if not necessary.

If anyone has read this far, thanks for suffering through my post. Also, thanks in advance for any advice.

 

[Rex R]

normally folks would position the lugs centered between two fins...however if you wanted to do that with the camera in line with the lugs, they would need standoffs so the 1/4" rod would clear the camera. one could however mount the lower lug in the valley between the fin root and the body tube(and the upper lug in line with the lower lug). or rotate the upper section 180 degrees so the camera is inline with a fin.
what size altimeter? w/o a side profile of your shroud, I would guess the airflow over it would tend to pull air from the camera bay.
Rex

 

[Adam518]

Thanks for the reply Rex.

I'd rather not do standoffs to get the rod to clear the camera. I was thinking about centering the hood over a fin and mounting the lugs on the opposite side of the rocket which would center them between the opposite fins. The only thing is...I know nothing about nothing about aerodynamics. I was worried it may be a bad idea to place a sizeable disturbance directly lover a fin...it may be no big deal, but I don't know. Advice would be appreciated here.

As far as an altimeter is concerned, I was thinking about the Jolly Logic Altimeter 2. It seems like it would be rather inexpensive, easy and fun to have. Bear in mind that I'm still a novice. Quick and easy are a plus and I do have plenty of room to mount it in the payload bay.

Is it a problem if the shape of the shroud pulls air away from the bay? I would assume lower pressure inside would give a false altitude reading. It makes sense, but would the rocket not slow to a stop at apogee causing the airflow to stop and allowing the air pressure to equalize? I suppose so if it were not flying sideways due to a weather-cock. If the shroud does cause a situation that would make the camera bay unsuitable for an altimeter, would it help to make additional holes for ventilation or am I better off forgetting the altimeter all together...at least for this rocket.

Sorry for so many questions. I'm reading all I can but there is soooo much to learn.

Also, pictured below is the profile on my shroud:

20190417_182946

• Adam518
• Apr 17, 2019

 

[Rex R]

to me it looks like you will have enough distance between fin and camera for the turbulence to mostly settle down and let the bird fly w/ the camera over a fin. the odd pressure in the bay might limit the alt2 to comparing engine performance in this rocket, I don't know how the current setup would affect the accuracy. jolly logic (john beans) could probably do a better guess ). don't forget the nose cone shoulder takes up space in your av/payload bay and you'll need some way to secure the nose, I would use a pair of 2-56 screws through the side of the tube and threaded into the shoulder of the nose.
Rex

 

[dhkaiser]

I have a 2.6" DX3 with 3 fins. Camera shroud between two fins mid body and lugs between a different two fins. Flies nice and straight.

 

[Nytrunner]

On 3 fin rockets, I'll have my buttons/lugs between 2 fins, then have my camera in the next space between 2 fins.

Your low profile shroud should have very little effect on stability

 

[GlenP]

The wake behind a camera shroud might cause some tendency to spin, more so on smaller rockets like a Viking or crossfire, it will spin like crazy, but not as much on an amazon. I have flown those 808 cameras on all of those. Probably won’t be a big issue on the executioner size. You could always put a dummy shroud on the other side to keep it balanced, but it should be fine on a rocket that size.

 

[Adam518]

Thanks for your replies. I appreciate the input. This rocket on the outside is essentially an Executioner + 3.75" of extra lower body tube + another 1" +/- of nose cone length.

I think I will position the shroud centered between two fins and mount the lugs centered between the next two fins. I'll also use Rex's suggestion of 2.56 screws to secure the nose cone...but I may use 3, 1 in line with each fin...I tend to like symmetry where possible.

If it spins slowly it's no big deal...with a camera you might as well see the all of the scenery. If it's too fast I may install a 2nd shroud to help balance the rocket...we'll see.

 

[Rex R]

see if you can find button head screws
Rex

 

[BABAR]

I put 808 keychain camera on low-power rocket just taped on with no shroud at all. They fly fine. I doubt your shroud will change much regarding air flow over the airflow over the fins, at least as far as stability is concerned. I think for the view you would like to have the camera on the opposite side of the launch lug or rail buttons. You may want to consider flying it before you paint it to see if there is any significance spin. If there is significant spin consistently in One Direction you can either put a small fin tab on or even put an asymmetric taper / airfoil on the fins to counter it.

One other thing you can try is a three-point shock cord, with three shock chords each attached to two shroud lines, assuming a hexagonal parachute. It's a little tricky to load but when it does work it provides a steady descent without all the rocking and rolling.

This gives you a good video of The Descent of the rocket as well as ascent

 

[jqavins]

Remember that, in general, as a rule, with exceptions, most of the time, the details of the aerodynamics are not very important. The basics, enough to make the rocket stable, are of course absolutely critical. But a little more turbulence over one fin, a bit different drag in places amount the airframe, stuff like that, usually just don't matter. Unless you're going for best possible altitude or speed, trying to get every foot or ft/sec out of it. In which case you wouldn't be flying a camera shroud at all. So don't sweat it.

And I'd drill another hole in the altimiter bay just to be sure. Without it you're probably fine for apogee, but you might like to filter and differentiate the data for speed (though integrating an accelerometer is better for that) in which case you want accurate data for the whole ascent.

 

[Bat-mite]

Remember that, in general, as a rule, with exceptions, most of the time,

I love a good, strong, committed answer!

 

[rharshberger]

I love a good, strong, committed answer!

So...definite possibilty of a firm maybe....

 

[jqavins]

I live in fear of the day that I say "Well, that's not really important" only to have some amazingly unlikely exception blow up catastrophically and hurt someone.

So I never, no exceptions, ever state absolutes.

 

[Nytrunner]

So I never, no exceptions, ever state absolutes.

Absolutely!

 

[GlenP]

"only a Sith deals in absolutes" - Obi-Wan (but isn't that statement itself an absolute?)

 

[Bat-mite]

"only a Sith deals in absolutes" - Obi-Wan (but isn't that statement itself an absolute?)
View attachment 381059

Like the old "there is no truth."

 

[jqavins]

"only a Sith deals in absolutes" - Obi-Wan (but isn't that statement itself an absolute?)

And then it's the sithling Anikin who talks about moral relativism: "From my point of view, the Jedi are evil."

 

[Adam518]

I finally finished this rocket, though I haven't flown it yet.

In the end, I decided to center the shroud over one fin and mounted the lugs centered between fins opposite the shroud.

I followed Rex's suggestion & used button head screws to secure the nose cone. Three were used, centered between fins for symmetry.

Please don't flame me for my Black Brant themed paint. I'm well aware that it isn't an accurate reproduction. However, the shape & proportions look appropriate for a sounding rocket & the paint scheme was relatively simple to replicate...plus I think it looks good.

Here she is ready for 1st flight:

 

[mbeels]

That looks great! Looking forward to some launch photos and a video!

 

[ebruce1361]

Please don't flame me for my Black Brant themed paint. I'm well aware that it isn't an accurate reproduction. However, the shape & proportions look appropriate for a sounding rocket & the paint scheme was relatively simple to replicate...plus I think it looks good.

My #1 rule about paint jobs: If you like it, that's all that matters. Unless you're going for a scale replica that will be in a judged competition, the appearance of model rockets is all relative. At any rate, I think it looks amazing and can't wait to hear about its flight!

 

[Rex R]

if a stupid idea works...then it isn't stupid . Mine flew pretty well on E18-4/E20-4 motors. good luck and straight smoke.
Rex

 

[jqavins]

As Peter Schickele sort of says, "If it [looks] good, it is good."