The design of the forward closure in the 1st illustration doesn’t really make
sense to me. Normally the whole outside (periphery) section will be inverted
from how it’s illustrated ie. the 2 protruding rings will be inverted as o-ring
grooves and the rest would be a snug fit into the inside of the tubular
section. Maybe the intention is to fill that internal section (between to 2
protruded ribs with JB Weld?) which should work, but it’s not really an optimal
method of retention. The key issue I have with your design is it relies pretty
heavily on the glue being rigid and well bonded. If there was enough thermal
transfer into that section of the tubing, there’s a chance the glue may soften
sufficiently to cause a weakness. Yes, there’s screws supporting, but there’s
also a gap where there’s a section of the screw unsupported creating a bending
stress on the screw… okay, yes, that’s unlikely to be substantial structural
weakness, but I suppose it could be an issue if the margins were tight.
The primary point I’m seeing is: if you have the capability to machine up
these parts, then why are you even bothering with glue at all? Why not rely on
threads and o-rings or screws and o-rings without the glue?
Regarding the 2nd illustration: yes, you certainly can structurally retain the
forward closure with a thread as you illustrated, but ideally you want your
o-ring seal on the pressure side of that that thread. You want this because the
axial stress on your cylindrical casing is pretty much ½ the hoop stress. So,
you really don’t want to thin your casing wall with a thread in an area exposed
to the full hoop stress from the internal chamber pressure. Okay, to load it
with the axial stress of the closure retention, but not the chamber pressure.
Okay, a hybrid design might allow for the pressure to be completely confined to
within the fuel grain, but you’ll want to be very confident of no leakage to
the casing wall if you wanted to seal the chamber after the thread.
Nozzles: I wouldn’t spend too much time on nozzle geometry for now. With a low
pressure chamber primarily working at low altitudes, there’s not much to be
gained in thrust coefficient - period. For a 300psi chamber work on something
like a x3 expansion ratio (going from memory) 45 deg half angle for the
convergent section and a 15deg half angle for the exit cone.
Troy
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Jack Hanna
Sent: Tuesday, 31 August 2021 2:07 AM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: JB Weld Usability
Okay, here's an image with current prototype for the engine design i'm going
for, it's not perfect but in hindsight I should've started with this as I've
had a hard time describing exactly what i'm trying to do and a hard time trying
to get an answer for what I want to do. Currently i'm wondering about using
JB-weld or another adhesive or some other sort of security between the hose
adapter fitting and the thread which leads into the injector bulkhead, I was
planning on having O rings surrounding the bolts, but as Troy said that doesn't
make too much sense, so I will be changing them to be next to each other with
the bolt holes more offset. The only reason i'm so concerned about the hose to
injector fitting is because I don't trust just threads to secure the pressure
of the nitrous, however from all the fittings and such i've seen it might be
perfectly fine but that's what I've been trying to ask, but failing. Regardless
if anyone has design critiques please let me know, as i'm trying to go semi
overkill with safety as hopefully i'll be able to reuse this to test out more
designs. Some questions that I currently have now that I actually have a 3d
model so I can ask better specific questions:
- Would it be worth it to thread a part of the bulkhead like in the figure
below? I am not a machinist and will be learning how to use a lathe to cut
threads while doing this, so while it makes sense to be to have it be held even
better together I don't know if the payoff would be worth time time. Also I
assume that could create other problems, with rubbing the o-rings and even just
currently looking at it I know i'm going to have trouble fitting in the o-rings
in the first place.
Above is an updated O-ring injector design
- Different profile converging diverging nozzles, worth it to test? This is
just a thrown together random shape I did to get a feel for what I want to
machine, but would it be worth it to machine multiple sizes of nozzles to swap
out their profile to see what pressures it can handle? Given how theoretically
it should be easy to swap nozzles In my mind it makes sense to be able to test
different levels of choking within the nozzle to see what level can produce the
most thrust / create mach diamonds / whatnot, I'm sorta planning on doing this
but I want to know if it's even something I should consider in this design or
if I should just look deeper into the math to figure out the optimal profile
rather than test other profiles. Also this won't be held in with only 1 screw,
it's just for simplicity's sake.
Thanks so much for all the help ya'll have given me, without it I wouldn't be
nearly as far into this project. Also, the O-rings I found should be good, but
i'll list some of the specs here to make sure I haven't made a big mistake. //
Square-Profile High-Temperature Silicone O-Rings -60 to 400f operating
temperature, Durometer 70A (Medium) and has these specifications. ASTM D2000,
SAE AS568. Once again thanks.
On Mon, Aug 30, 2021 at 10:07 AM Terry McCreary <prfesser@xxxxxxxxxxx
<mailto:prfesser@xxxxxxxxxxx> > wrote:
At TRATECH last month John DeMar showed a design for a reloadable solid motor
that was compatible with Aerotech 38 mm reload(s) but did not require machining
snap rings nor threads (well, except for tapping for some set screws). The aft
thrust ring was JBWelded to the casing as it would not get hot enough to debond.
Best -- Terry
_____
From: arocket-bounce@xxxxxxxxxxxxx <mailto:arocket-bounce@xxxxxxxxxxxxx>
<arocket-bounce@xxxxxxxxxxxxx <mailto:arocket-bounce@xxxxxxxxxxxxx> > on behalf
of Troy Prideaux <troy@xxxxxxxxxxxxxxxxxxxxx
<mailto:troy@xxxxxxxxxxxxxxxxxxxxx> >
Sent: Sunday, August 29, 2021 7:01 PM
To: arocket@xxxxxxxxxxxxx <mailto:arocket@xxxxxxxxxxxxx>
<arocket@xxxxxxxxxxxxx <mailto:arocket@xxxxxxxxxxxxx> >
Subject: [AR] Re: JB Weld Usability
Should say “… which shouldn’t really matter …”
High temp JB weld is good adhesive – it will bond to things that more regular
epoxy won’t (eg: PVC plastic) but it’s also quite brittle once fully cured –
which should really matter much for closure retention relying on the shear
strength & adhesion of any adhesive used, although watch out for potential
diameter increases from strain if the closure is directly bonded to the inside
of the tubing wall, although if also using franken-bolt retention in
combination, that should mitigate that issue substantially.
Troy
From: arocket-bounce@xxxxxxxxxxxxx <mailto:arocket-bounce@xxxxxxxxxxxxx>
[mailto:arocket-bounce@xxxxxxxxxxxxx] On Behalf Of Jack Hanna
Sent: Sunday, 29 August 2021 4:02 AM
To: arocket@xxxxxxxxxxxxx <mailto:arocket@xxxxxxxxxxxxx>
Subject: [AR] JB Weld Usability
Hello!
I've been working on a hybrid engine recently and picked up some JB weld for
cheap, and have been wondering if it's a semi-practical bulkhead adhesive that
can keep pressure. I'm planning on having two o ring grooves on either side of
bolt holes to have a hopefully overkill secure piece, however I'm wondering
especially with the hose threads if I could JB-weld it together to effectively
get one piece. Has anyone here used it and if so what are some of its
limitations or things to note when using. Thanks! Also, it says "5020 PSI" on
the packaging, i'm assuming that's the best case scenario and wondering what
practical pressures it can handle in terms of leakage.