USR Sentra SRB 2.7"

[lalligood]

How are everyone's Sentras coming along??

Everything is progressing nicely with my Sentra SRB 2.7 build. Side pods are complete. I finished slotting the main airframe tube and have glued the fins (but no tthe strakes) into place. Most of the initial fin fillets are done too. I have begun tacking on the short tubes to the main airframe that the side pods will clip onto. The elegant simplicity of these side pods have me anxious to finish the build & have it all ready in time to fly sometime next month. Pics coming soon...

I suppose now is as good as any to start the discussion of motors for this beast. Jerry recommends (& it seems to make much sense) to have short burning motors in the side pods along with a high thrust central motor. Getting them to ignite simultaneously is the hard part, right? I'm thinking AT BT in the side pods & a central Pro38. I only have 1 AT 29/40-120 casing & only 1 Dr Rocket 29/180-240 system. Probably could borrow another one from a fellow rocketeer but what 24 & 29mm SU motors are available right now? Central motor will either be the H153 (2-grain) or I205 (3-grain--and I have one of those in my range box right now!)

I'll open the floor for recovery discussion too. I'm thinking of saving the small mylar chutes for another project & using plastic streamers instead. The streamers will be much less likely to have an in-flight failure since the side pods *will* be ejecting while the rocket still has quite a bit of (upward) velocity. My USR Banshee recovers by a streamer exclusively & is ~3oz heavier than a single SRB side pod. I even bought plastic that is color coordinated with my planned color scheme

 

[marvSRG]

I've really been enjoying the Sentra threads...this one and the micro. They are continually growing my intrest in getting a kit.

One thing I noticed, though, was that both the micro and 2.7 use balsa fins. I can see where that would work in the micro (even though you can use 29mm motors in it and I would think that would be pushing the speed of balsa), but how does balsa work on the 2.7? That rocket flys on 38mm motors, right? Is there any reccomendation in the instructions to stiffen the fins or does it actually work, and we are just underestimating it's strength?

Just wondering.

 

[PunkRocketScience]

marv-

The balsa on the 2.7 kit is THICK! Seemed like a little over 1/4"! I had to use a scroll saw to cut this stuff instead of my usual x-acto...

 

[lalligood]

Originally posted by RocketmanTM
marv-

The balsa on the 2.7 kit is THICK! Seemed like a little over 1/4"! I had to use a scroll saw to cut this stuff instead of my usual x-acto...

Don't underestimate the strength of 1/4" thick balsa. It should be more than strong enough to withstand flights on most if not all commercially available 38mm motors. Even the 'bigger brother' of the Sentra SRB 2.7--the 4" version--uses 1/4" balsa for the fins

My only concern about the balsa is not flight stress but damage from transporting & storing the rocket (but isn't that how most rockets get dinged up?!?) I'm currently grapling with the idea of glassing the fins with 2oz glass (super thin, light stuff) just for this reason. Part of me says "do it", while the other says "No!" USR kits are designed to be as light as possible & I don't want to mess with that philosophy (because I happen to fully agree with it!)

The fins are designed so that it is nearly impossible for the fins to take any impact on landing as long as the chute comes out. No chute = all bets are off (but again, that's true with plywood finned rockets too...)

Airfoiling the balsa takes a LOT less effort though, so that's a definite plus if you ask me...

Gotta go work on mine a little bit now... Look for more pics soon!

 

[Jerry Irvine]

Firstly use relatively low thrust motors. It is a U.S. Rockets thing.

Secondly BALSA can be reinforced quite nicely by "jap tissue" which uses laquer as the "composite" not epoxy or fiberglass.

Jerry

 

[Stymye]

Lalligood, you mentioned airfoiling the fins on your build of the sentra.. but they appear from the pictures (so far) to be beveled
just an observation

beveling the fin edges and "airfoiling" are 2 very different things.

either way,great job on the build and pictures!

 

[rstaff3]

And they look nice beveled too

 

[EMRR]

Originally posted by Jerry Irvine
Secondly BALSA can be reinforced quite nicely by "jap tissue" which uses laquer as the "composite" not epoxy or fiberglass.

Jerry

That is why mine has gone on pause... I have prepared my newspaper covers (not jap tissue) and will then use 15-minute epoxy to coat the balsa and apply the newpaper on top. Then sandwich between wax paper and stack books on top. Works great for giving a hard finish and strengthen the fin.

On pause because I have to do this when my wife is not around... she is overly sensitive to the epoxy smell.

Possibly this week.

Nick

 

[lalligood]

Slight update. I finished attaching & shaping the strakes & leading edge of the main airframe. Here is a completed pic of *just* the central rocket (no side pods). Note it is turned "sideways so that you can see the mount points for the side pods.

Completed (but unfinished) rocket pic coming soon!

 

[lalligood]

C'mon...I think I was the last one to post in this thread that I got this kit... How can I be the first one done?!? Well, if I am so be it. And check this out. Read 'em & weep boys. Like salt in an open wound. NOW GET WORKIN' & FINISH YOURS!

Complete. Finished. Finito!

 

[lalligood]

I was feeling a little artsy too. This didn't come out as well as I would have liked but you certainly get the idea. This is what it would look like coming right at you. (And I used my in-progress Mosquito 2.2 as a stand to prop it up to get the angle...)

 

[Jerry Irvine]

Originally posted by lalligood
I was feeling a little artsy too. This didn't come out as well as I would have liked but you certainly get the idea. This is what it would look like coming right at you. (And I used my in-progress Mosquito 2.2 as a stand to prop it up to get the angle...)

I hate to say it (sorry TRF censors) but that is somewhat fallic.

Maybe a view for the girls!

Jerry

 

[lalligood]

Originally posted by Jerry Irvine
I hate to say it (sorry TRF censors) but that is somewhat fallic.

Maybe a view for the girls!

Jerry

hahaha That thought genuinely didn't cross my mind until now... Any similiarities to anything or anyone is purely coincidental and unintentional. No animals were harmed in the taking of this photograph

 

[EMRR]

I have finally been able to progress.

Instead of using Epoxy to glue my newspaper to the fins... I found a new glue. Elmers' Glue Pen. I found this glue to be easy to work with, dried quickly, and I tested it for sandability and it passes.

Regards,
Nick

 

[EMRR]

A close up:

 

[bsexton]

Originally posted by Jerry Irvine
Firstly use relatively low thrust motors. It is a U.S. Rockets thing.

Secondly BALSA can be reinforced quite nicely by "jap tissue" which uses laquer as the "composite" not epoxy or fiberglass.

Jerry

I have used the "jap tissue" method (or silkspan for even greater durability) for increasing the strength of balsa , but I've allways used clear butyrate dope to apply the tissue. What kind of laquer do you use? Do you do both sides at the same time? Do you sandwich the balsa between wax paper and apply weight until it dries? Just curious...

 

[Jerry Irvine]

Originally posted by bsexton
I have used the "jap tissue" method (or silkspan for even greater durability) for increasing the strength of balsa , but I've allways used clear butyrate dope to apply the tissue. What kind of laquer do you use? Do you do both sides at the same time? Do you sandwich the balsa between wax paper and apply weight until it dries? Just curious...

I like your ideas. The last time I jap tissued anything was a NAR contest glider. I used to host several NAR contests a year and go to others as well.

That was the early 80's so my memory fades on the details now.

Jerry

 

[BobCox]

Nick,

Exactly what is the purpose of gluing newspaper to the fins? Strength or smoothness? What coating do you put over the top of the paper, and how much do you sand back off?

 

[EMRR]

Originally posted by BobCox
Nick,

Exactly what is the purpose of gluing newspaper to the fins? Strength or smoothness? What coating do you put over the top of the paper, and how much do you sand back off?

Well, I had just spent 10 minutes doing a wonderful response to you with various links to my progressive experimentation in this area. Then I tried to attach a photo which didn't pass the size requirement and ka-powie, my entire message is GONE!

I do it for both reason, but mostly for smoothness.

I don't coat the outside paper, I just finish my standard way... Plasti-Kote Filling Primer, sand, paint

I have experimented with many different ways and some day I'll rewrite that long message (perhaps in techniques).

Nick

 

[lalligood]

With some humility & embarassment, I need to confess to giving in to fiberglassing the fins on my Sentra. I have to admit that it wasn't as difficult as I thought it would be...and I had been pushing off wanting to fiberglassing anything for quite some time now.

I used 2oz cloth & as little epoxy as possible to soak the cloth. An epoxy squeegee/spreader ensured that I kept it light and smooth. Actually it's NOT smooth because I can still feel the weave of the cloth. Everything will get smoothed out with a high fill primer (probably Kilz). I tried the process out on some scrap pieces of balsa just to get familiar with laying 'glass. I'm glad I spent the time testing!

I am confident that I accomplished my goal of increasing the dent resistance to the balsa fins. I don't want to make it a habit of doing this still...

And this morning I weighed my Sentra (I kinda wish I would have done it before coating the fins though). I had to weigh it twice in pieces on my kitchen scale to make sure I was reading it properly. Yup, 35oz. Thirty-five ounces. 5 feet tall with 2 side pods and it's under two and a quarter pounds. Wow.

And I'm thinking a central Pro38 H153 with AT 29mm BT motors (exact ones are still TBD) in the side pods. It's gonna SCREAM!

 

[Jerry Irvine]

Originally posted by lalligood
With some humility & embarassment, I need to confess to giving in to fiberglassing the fins on my Sentra. I have to admit that it wasn't as difficult as I thought it would be.

One thing I actually recommend glassing is the pod hooks since they experience HIGH stresses both on boost and on eject.

Jerry

 

[lalligood]

Originally posted by Jerry Irvine
One thing I actually recommend glassing is the pod hooks since they experience HIGH stresses both on boost and on eject.

Jerry

It's not too late for me to do that...

BTW, has anyone built a Sentra in RockSim (I'm guessing you would need v7 to properly render the sidepods)? Or is this design so complex that we'll have to wait until the release of version 8? I'm really just curious about the CP location when the side pods are attached. (It shouldn't be too hard to sim the central booster by itself in most any version of RockSim though!)

 

[Jerry Irvine]

Originally posted by lalligood
It's not too late for me to do that...

BTW, has anyone built a Sentra in RockSim (I'm guessing you would need v7 to properly render the sidepods)? Or is this design so complex that we'll have to wait until the release of version 8? I'm really just curious about the CP location when the side pods are attached. (It shouldn't be too hard to sim the central booster by itself in most any version of RockSim though!)

 

[Jerry Irvine]

Originally posted by lalligood
It's not too late for me to do that...

BTW, has anyone built a Sentra in RockSim (I'm guessing you would need v7 to properly render the sidepods)? Or is this design so complex that we'll have to wait until the release of version 8? I'm really just curious about the CP location when the side pods are attached. (It shouldn't be too hard to sim the central booster by itself in most any version of RockSim though!)

The aspect ratio or finess ratio of the various sentra's is slightly different (stock tube lengths), so measure the CP from the rear and scale to the diameter.

Tech Jerry

 

[lalligood]

Jerry,

Thanks for the CP info. I quickly threw some parts data into RockSim 5 demo & grabbed some screen caps... I only simmed the central booster as the sidepods will only bring the CP (as well as the CG) further aft. However, if I use this as a baseline regardless of configuration withor without side pods, I should be safe as long as the CG is a reasonable distance forward of that point... Right?

Here's the 2D view...

 

[lalligood]

And a 3D view. I cropped out the remainder of the window & altered the color of the NC & fins. Hard to see the detail in the default all blue...

 

[EMRR]

Fins are done.
Fillets are done.
I'm done... until

1) Warmer weather for finishing
2) Jerry sends the parachutes

Overall comments in as few as words as possible: Nice and very exciting!

Here are three pics:

 

[EMRR]

This one gives you an impression of size, next to an Estes Chrome Dome.

 

[EMRR]

Last one until finished and parachuted!

 

[lalligood]

Nice pictures Nick! Mine is progressing slowly but surely. I was able to get some painting done over the past couple of days. The planned scheme is a simple 3 color design that will make it easy to identify each of the 3 pieces of the rocket. In fact, today has been so cooperative weather-wise, that I really should run to Wal-Mart to get more Krylon... Everything has been primered. One side pod nose cone, the central booster nose cone, & the payload bay have all been painted final gloss colors. The payload tube will get a couple of accent color stripes. Only drawback to my paint work is that Krylon primer had a bit of a reaction with the Kilz primer on the central booster. Never seen that happen before I'll give it a couple of days, sand it down & try again...

(And don't ask for pics...I won't be taking any until it's finished now! You'll just have to wait until then )