Quick Fiberglass Question

[CameronMakesRockets]

I have designed a rocket and ran it through the simulation (Open Rocket) and with my selected motor it will pull 43.2 G and reach a max velocity of 945 mph (this is simulated and will likely be slower IRL). But this simulation data got me thinking, at what point to I need to start looking into fiberglassing the fins/fin can? I designed it as a 54mm diam body tube with a 38mm motor with through the wall fins. The fins will be around 3.175mm thick (either basswood or plywood[leaning towards plywood]). In my opinion, the amount of force and speed these fins will go through I think epoxy won't be enough. Also, which epoxy is recommended to start with fiberglass?

Thanks in advance!

 

[Hobie1dog]

Why not forget the hassle of going out and getting cardboard tubing, and plywood, then taking more time to go out and get the fiberglass, the resin,, the rollers and go through a laborious process and messy process of spreading all that resin out and ended up with something that's an irregular surface that is going to require more filling and trimming, rounding of edges, and still not end up with something with the strength that you like. If you would just make your rocket out of fiberglass tubing and fiberglass fin sheet stock, you will save yourself all that time and end up with a much stronger rocket that requires no filling, sanding, and it's ready for primer, other than your fillets. And if it suffers a hard landing it's going to survive without any problem.

 

[kjhambrick]

@CameronMakesRockets --

If you want to experience the joy of glassing a 54mm LOC Tube for it's own sake then by all means go for it.

OTOH, if you've got typical thru-the-wall 1/8 inch Plywood fins joined with decent Epoxy on LOC 54mm tubing, glassing the tube may not be necessary.

And I don't know of any rule-of-thumb to decide when you need glass ...

I flew a more-or-less stock LOC Vulcanite H97 with airfoiled fins and good epoxy filets four times on AT J570-W Motors without any issues.

Each of the J570 flights went supersonic.

These are Rho-Normalized Drag -vs- Velocity Plots for several flights on different motors of that same Vulcanite. Note the inflection in the Drag -vs- Velocity Plots around 330 m/sec:



Somewhere along the way it came in ballistic, probably because I forgot to Arm the AltAcc but the Fin Can survived and it is still flying today, 25 years later.

YMMV so go for the glass if it sounds like fun ( I enjoy getting a nice wrap on a carboard tube )...

-- kjh

These are the flight parameters from my last flight at Turkey Shoot 1999 with an Alpha Version AltAcc with a new 50G Accelerometer:

$ cat ../../altacc/prodata/pack/dat/ts991127/j570.prm

Title    =  Flight 2 at Turkey Shoot '99, Nov 28, 1999
Rocket   =  Vulcanite   # [O] Rocket Name
MMfg     =  Aerotech    # [R] Motor Manufacturer
Motor    =  J570-L      # [R] Motor Designation
Batch    =  NOV 03 1998 # [O] Motor Manufacturer Batch Id
Date     =  NOV 28 1999 # [O] Launch Date
Time     =  10:30       # [O] Launch Time
LMass    =   2.144 Kg   # [R] Launch Mass with units
DMass    =   1.232 Kg   # [O] Rocket Mass less Motor Mass with units
MMass    =   0.912 Kg   # [O] Motor Mass with units
FMass    =   0.360 Kg   # [O] Motor Mass with units
PMass    =   0.552 Kg   # [R] Propellant Mass with units
Diam     =   2.230 in   # [R] the Rocket Diameter with units
Temp     =  62     F    # [R] Site Temperature with units
Press    =   963   mb   # [O] Site Pressure with units
SiteAlt  =  1420   ft   # [O] Actual Site Altitude with units
VWind    =  3      mph  # [O] Wind Speed at launch time with units
Theta    =  0      deg  # [O] Launch angle with units
TBurn    =  2.1250 sec  # [O] Burn Time with units
TCoast   =  2.1875 sec  # [O] Coast Phase Begin Time here (default=TBurn)
TEnd     = 21.5625 sec  # [O] Time to Min Velocity with units

CalFile    = /exp/dev/altacc/prodata/pack/dat/ts991127/test7.cal
DataFile   = j570.dat  # [O] original AltAcc proread file

DragFile    = j570.eqn  # [O] file with drag vs velocity pairs

Notes

   No wind.  Straight up.  Lost Rocket until I spotted it on the
   Lakebed about one mile east.  Beauty Flight !  High Accel but
   did not ever peg the Altacc, even at 45.8 G's !!!
End Notes

And this is the AltAcc PRODUCE Report ( 16 Hz data rows skipped ; Pressure Altitude not adjusted for site temperature)

$ grep '^#'  ../../altacc/prodata/pack/dat/ts991127/j570.flt
#
# AltAcc Firmware:          AltAcc II - v2.026
# XDucer Type (1):          Motorola MPX4100
# Flight Mode:              Drogue to Main Mode
# AltAcc Data file:         j570.dat
# Calibration file:         /exp/dev/altacc/prodata/pack/dat/ts991127/test7.cal
#
# AltAcc Gain Factor:         3.2046 GHarrys / G
# AltAcc Minus One Gee:      88.3408 GHarrys
# AltAcc Zero Gee:           91.5454 GHarrys
# AltAcc Plus One Gee:       94.7500 GHarrys
# Launch Site Pressure:     224      Orvilles ( 28.46 in Hg )
# Launch Site Altitude:    1378      ft MSL
#
# Drogue Fired at Time:      21.6250 sec      (   8868 ft AGL )
# Main Fired at Time:       157.0625 sec      (    545 ft AGL )
#
#
#       Time  Accel  Press    Sum  Accelerat   Velocity   Altitude  PressAlt
#        sec  units  units  units   ft/sec^2     ft/sec       feet      feet
#  =========  =====  =====  =====  =========  =========  =========  ========
#
# MSL Pressure Altitude:   8784    ft
# AGL Pressure Altitude:   8868    ft         (  21.62500 sec )
# Max Inertial Altitude:   9955    ft         (  21.56250 sec )
# Maximum Velocity:        1353.7  ft / sec   (   1.68750 sec )
# Maximum Acceleration:    1474.12 ft / sec^2 (   0.12500 sec,  45.8 G's )
# Minimum Acceleration:    -337.55 ft / sec^2 (   2.12500 sec, -10.5 G's )

 

[OzHybrid]

There's no hard line as to where you make the decision to go full FG or CF. But if you're pulling 40+G and you have a fin that weighs 100 grams, you have to be thinking in your head "Would I hang 4Kg off the mounting point for the fins?" (2 bags of sugar or whatever in your head equivalent you want to use) At 40G things that weigh a small amount become heavy under acceleration. I think most would agree you need to consider your loadings rather than just hope for the best.
Also, you'll be going through Mach. This puts a fair bit of stress on the airframe and specifically the fins. All of it is doable with cardboard tube and plywood fins with careful consideration and very doable with FG tube and FG fins but the rocket will be heavier generally.