Sentinel upscale 2019

[Event Horizon]

BTW, the fins all eight of them are turning out nicely, as is the nose cone.
pics coming soon

 

[Event Horizon]

Mocked up, gotta begin gluing parts together.
Not the best photo I've taken, sorry, but WOW is it ever WHITE !

 

[Event Horizon]

and yes, my work bench is in fact a card table in my living room.

 

[Bruiser]

It has a halo, like it's come from heaven

-Bob

 

[Event Horizon]

The latest progress today.
24 mm Motor mount fabbed up with homemade motor retainer, held in place with red/blue thread. I used two thrust rings.
1/'4 inch thick plywood centering ring which will also serve as the forward fin re-enforcement. The fin tab will be butted against the forward motor mount centering ring.

Sig Bond, placed in the correct spot inside the body tube.

Motor mount set in place

Centered at the front of the bottom body tube using a coupler and a bulkhead centering ring. This isn't glued yet, so I can remove it after the 1/4 thick ring has a chance for the glue to set up. I'll then pull the top ring out and place a fillet of Sig Bond against the 1/4 ring.

I drilled a bunch of holes in the fin tabs to increase adhesion, I hope it works


After the motor mount adhesive sets a bit more, I'll begin installing the rear fins.
More pics to come
Notice the baffle with an eyelet for a metal cable fishing leader with snap clips to be attached to.

 

[Event Horizon]

Fins being installed. At this point it's only one, the other is there for alignment checks. The fin that is glued in place is not visible in this photo, it's sandwiched between the two pieces of the jig. The fin alignment jig is made from a couple of 24 inch long pieces of 2 x 4, with a 45 degree bevel cut on the edge. there's about a 3/16 margin left on the edge. This jig really helps with aligning the fins 100% straight with the body tube fin line and along with the fin guide, the fins tab (TTW) end up on the centerline of the motor tube.

Another angle and more clarity of how it's going together.



One more photo, a bigger view of the fin alignment guide. The template is available at payloadbay.com

Notice I missed centering the motor retainer clip between two fins....RATS too late now, but it will work fine.

 

[Event Horizon]

Found these at the local fleet farm store. Fishing leaders 30lb test. I was thinking of doubling them up. Then the cable would be 60lb test. Unfortunately, then there will be another cable to tangle with the parachute. I suppose the two could be woven together somehow. They are 18 inches long, I'm not sure I like the snaps that close to the rocket when the chute opens. It seems to me that a cable 36 inches in length would be better.

My other option would be to find stainless steel small diameter cable, a swivel, a swivel snap link and squish cable binders and weave a loop on each end, one with the swivel and one on the eyelet end that's on the baffle.
What say you guys, how have you handled this part??

 

[Event Horizon]

I'll also be using 12 feet of 200lb test kevlar cord in the recovery system, would like to get comments on where the nose cone should be connected into all this, at the end of the kevlar or somewhere in between??

Any suggestions for a cord to use for shock absorption?

 

[Event Horizon]

Fin number three got installed before I departed for work this morning. Hoping to get fin 4 in during my lunch break.
This building rockets is somewhat addicting.....

 

[Event Horizon]

Succesful during lunch. Fourth fin has been glued in place. Now to do the fillets. I'm going to use a very fine weave of glass cloth saturated in Elmer's Carpenters Wood Glue to re-enforce the connections inside the body tube and the tabs that join to the motor mount. On the exterior I'm going to use just Elmer's and attempt to make good looking fillets. Pics are in the future keep checking back.

 

[Event Horizon]

I should have added this in on the first page, but anyway here it is. I enlarged the centering rings by c-clamping the ring to a piece of 3/4 pine board. The hole in the center of the ring was smaller than the pilot for a one inch spade bit. I chucked the spade bit in the drill press and managed to drill all the centering rings out to the spade bit size. Then I used a coarse drum shaped sanding tool to match the inner diameter to a ring that came with a 24mm heavy tube motor mount. The rings fit the 24mm tube perfectly. A success ! YAY !

 

[Event Horizon]

Last night I got the baffle/recovery anchor installed, the tube coupler and nose end coupler centering ring

which locates the nose end of the extended motor mount tube installed and joined the body tubes.

The overall length of the body tubes joined together is approximately 29 inches.

After that set for an hour or so, I managed to attached two of the strake fins before feeling too tired to stay awake and continue.

I drilled holes in the mating surface of the strakes to aid in attachment.

The glue actually does seep into those holes, hopefully they will actually add strength too

This morning before leaving for work, I got a third strake fin attached.

The jig works excellently for aligning tail fins and the strakes. I don't remember ever getting fins this perfectly aligned before.

Curiously, the bottom photo doesn't reproduce the fin alignment accurately, light and angle plays tricks on cameras I guess

.

Progress is sweet.

One more strake fin to attach and then do the fillets for 8 fins....

 

[Event Horizon]

I forgot to mention, I installed three of the 30 lb test leaders onto the recovery eyelet anchor. They are 18 inches long, so they

protrude past the end of the body tube a few inches. I didn't measure the depth of the recovery bay, but I estimate it at about 12 inches, maybe a bit more.

I hope that will give me enough room to stow the recovery system.

I'm still wondering how big a parachute it will need. I can get a more accurate weight and hopefully somewhere out there there's a rule of thumb or a chart to work with to size the parachute off of.

 

[Event Horizon]

All eight fins are attached and fillets roughed in. First coat of primer on the fillets and the body tube around the fins.
I'll use some Squadron white putty to fill the minor imperfections in the fillets that remain.
I'm not looking forward to filling spiral grooves. The Rocketarium BT80 body tube is very spongy.
Put a coat of thin CA on the inside of the tube at the top to help stiffen it a bit.

 

[Event Horizon]

It's beginning to look like a rocket.
Lots of work finishing the thinwall body tubes, gluing the centering rings creates a depression on the tube surface that must be filled, and those pesky spirals....

 

[Bruiser]

Coming along nicely!

-Bob

 

[Event Horizon]

Thanks Bob, much appreciated.
White is a problematic color to paint....
I put the first coat of gloss white on today,
more flaws appeared....
sigh......

 

[neil_w]

Looks great. You should have quite a nice gaggle of Sentinels when all done. It's a really attractive design.

 

[Event Horizon]

Thanks Neil, the praise only goes to my head and makes it swell, expect a deployment charge to go off any second now....ROFL

 

[Event Horizon]

Well folks, those of you who have been following this thread.
It's at 97% completion as of tonight.
All it needs now is the shock cord and elastic shock chord along with a parachute. Oh and a motor.
So far the rocket weighs in at a hefty 469 grams, about one pound one ounce, and that's without the parachute and a motor.
The motor mount is 24mm.
Will an E motor lift this beefcake off the launch pad??
Which E motor?
Did I build a pooch? Suitable for static display only?
What parachute diameter ?? What descent rate is a good rule of thumb?

Now I really need some good advice......

 

[Event Horizon]

I could use some help in trying to understand where the correct center of gravity should be with this rocket and the center of thrust, is that correct?? Or is it center of drag?? HELP !!!
Never worried about any of that before with kit built rockets, but I fear it's essential knowledge and physics in this case, especially if any modicum of success in launching is desired.

 

[Event Horizon]

Almost forgot to mention,
The transfer decals from stickershock are flippin awesome as the photo shows

 

[mbeels]

That looks good, those fins look like they want to fly.

Do you have any experience with Rocksim or Openrocket? That software would help answer those questions. The software can calculate where the center of pressure is, and you want to make sure that the center of gravity is comfortably in front of it. The margin is usually measured in "calibers", one caliber is equal to your body tube diameter. (for example, if your body tube is 2.6" in diameter, and your CG is 2.6" in front of your CP, your stability margin is 1 caliber).

You can also use the motor guide at thrustcurve.org.

I missed it in your build, did you add rail buttons or a launch lug? I bet you'll be able to use an E motor (like an E28), it certainly isn't a pooch! Are you planning on using reloads, or single use motors?

 

[Event Horizon]

Thanks for the kind words mbeels. Thank you for correcting me, center of pressure was the other critical factor I was looking for.

I don't have experience with any of those programs. I'll check into them. Thanks for the tip.

The plan is to use rail buttons. There are 1/4 inch thick plywood attachment points inside the airframe.

I have no experience with the E series motors. In the distant past I didn't have enough money to buy bigger rockets or motors so the C6-7 was about the most powerful motor I have been exposed to.

I looked up the specs for an E28. That motor provides 6 lbs. of thrust. I wonder what the rule of thumb is for thrust to weight ratio to achieve 750 feet apogee?

 

[neil_w]

Looks beautiful.

Here's your process:
1) Measuring CG is easy, just balance the rocket (with everything but motor) and measure from tip of nose cone.
2) Use OpenRocket to find CP.
3) In OR, override CG based on the value you measured in #1

Now you have a working model in OR, and can simulate stability, altitude, and ejection delay times with different motors.

Regarding motors: thrust/weight is only relevant for getting safely off the pad, nothing to do with altitude. OR will tell you altitude. You can also use thrustcurve.org to give you a quick and dirty (but quite useful!) idea of altitudes for different motors, *plus* a list of which motors will achieve save speed off the pad.

Off the top of my head, I'd say the E30 single-use would be perfect for this rocket, or the F44 if you want some more punch and altitude. E15 might work.

If you want me to throw together the OR model for you I'll be glad to, just send me all relevant exterior rocket dimensions, fin shapes, and fin placements.

 

[neil_w]

I just took a look at thrustcurve.org. This is certainly a heavy rocket for a 24mm mount. Thrustcurve results show all motors failing; it somewhat conservative though. The E30 shows 40 ft/s off the rod, and the F44 47 ft/s. The F32 show somewhere in the range 42-45 ft/s. I think I'd feel very comfortable flying the F44 or F32, and probably the E30 as long as it's not too windy (and with a 6' rail).

Seems like you'll need one of the F motors to get you in the range of 750'.

 

[Event Horizon]

So, I went to thrustcurve.org as suggested by mbeels and neil_w, to see what is what.

Looks like she's nearly a pooch....It could fly but only with a Cesaroni motor.

The weight of the rocket without the parachute is 469 grams, I added 76 grams for parachute and cords.

I don't know, is that enough? What does a 36" or 40" diameter ripstop parachute weigh? Is that too big or too small?

I think the idea is to limit descent rate based on how much drift and the desired speed when the rocket touches Earth.

I plan to launch in low or no wind.

Where do I find something to help sizing the parachute??

 

[mbeels]

Yeah, neil_w has better specific suggestions. Thrustcurve is also useful for getting an idea of how long a motor delay you'll need. Openrocket or Rocksim can do a bit better. I've found with Rocksim to set the finish quality to "matte" or "unfinished" gives me results that match the reality of my flights much closer than the default option of "polished". Maybe that says something of my finishing ability.

But even thrustcurve will get you pretty close. Take some of the "fails" with a grain of salt, it is very conservative. If you have 40 ft/sec off the rail (instead of 50), that is probably fine.

There a few descent rate calculators out there, such as this one. Many factors can influence your target descent rate, landing on thick grass allows for a faster descent rate than hard packed clay. Somewhere in the vicinity of 15-20 ft/sec is good starting point.

 

[mbeels]

Another useful site I only found recently is this one: "compare a motor with another motor". For example, the E30 and F44 have less than a 10% difference in total impulse, and both have a good kick off the pad. So you'll probably get similar flights with either one.

 

[Event Horizon]

Thanks mbeels.
Using descent rate calculator (such as this one.) in mbeels post.
I entered 36 ounces rocket weight, (rocket parachute and motor, although the motor will not weigh as much after it burns out the propellant, delay and ejection charges, but I thought I saw some of those motors weighing in at 16 ounces. The Rocket weighed about 20 ounces 545 grams = 19.22 ounces. 19.22 (essentially 20 ozs + 16 ozs = 36 ozs, hey I can handle simple addition problems too !
The descent rate at 36 ozs is 18.42 ft/sec. so maybe a bit less with a spent motor.
A 40 inch diameter round chute drops the descent rate to 16.58 ft/sec. Not hardly enough to worry about. Or is it?
So, cuz I hope to launch in a softer landing area, it looks like a 36 inch chute will work.