Weight Analysis of Composite Airframe Construction


For my L3 rocket I am planning on a HyperTEK Hybrid launch. As such I need a rocket light enough for the M-1000ish initial thrust for stable flight in "L3 Cert Conditions" (i.e. with decent wind margins).

The easiest is a wrap or two of carbon or fiberglass about the phenolic airframe to stiffen it up. It gets more complex from there. Laying up wraps of fiberglass or carbon, using sleeve fabric, etc.

Methodology

The methodology I'm considering to anchor the discusussion and evaluation is to look at
and then compare that to
This provides two stronger alternatives arguably strong enough for a L3 rocket with TTW construction.

Economy evaluation of Fiberglass vs Carbon Fiber

Carbon fiber is far more expensive than fiberglass, but the question is, for the same strength, what is the actual ratio? I.e. You'd need less equivalent thickness of carbon than fiberglass for the same stiffness and strength - so what is the actual cost ratio of fiberglass vs carbon fiber.

Note also that the fiberglass has an inherit weight penalty due to the additional epoxy that is also required.

If Carbon fiber is more brittle for equivalent strength then perhaps a wrap of kevlar would round that out.

New materials - Basalt

One new material is basalt fabric and sleeves. Soller has it. It's advantage is very high heat rating and also lighter and stronger than fiberglass (not sure of the ratio to carbon).
Right now I only see it up to 4" sleeve and no fabric.
With a high temp epoxy, could be interesting for motors and motor mounts and minimum diameter booster sections as well. High temp epoxy is elusive in hobby quantities (gallon range).

Assumptions

Data Table

Data, including weights, thicknesses, costs, are normalized to a nominal 7.5 " tube 4ft long. Cost is only of fabric and not epoxy (except for factory products). Weights in ounces.
Fabric chosen for 50" widths where possible.

PML composite products not available in 48" lengths but costs, weights are normalized to 48" ; they are also 6" products normalized to 7.5" diameter.

Soller pricing is at 30ft lengths for tubing

28.36 grams per ounce

name, fabric weight, wrap weight, layup weight, thickness, cost description
Name
 fabric weight
 wrap weight
 layup weight
 thickness
 cost
wrap
 description
PML Phenolic

48.1
 0.080
 $42
PML Pre-glassed
(27.4)
 75.5
 0.121
 $216
PML Ultralight Carbon

26.7
 0.028
 $176
PML Premium Carbon

53.6
 0.056
 $282
PML Level 3 Carbon

79.7
 0.084
 $395







Soller 8" Heavy Carbon biaxial tube
 14.2
 13.3
0.021
 $43
Soller 7" Light E Glass biaxial tube
 10.3
 7.32
0.011
 $12
Soller 7" Medium E Glass biaxial tube
 13.8
 9.81
0.015
 $22
Soller 7" Heavy E Glass biaxial tube
 22.4
 18.28
0.023
 $24
Soller T300-PW 50" Carbon fabric
 6
 4.27
0.009
 $19.0
 T300
Soller IM7-PW 50" Carbon fabric
 6
 4.27
0.009
 $25.6
 40% stronger than T300
Soller T300-TX 50" Carbon fabric
 6
 4.27
0.011
 $21.3
 T300 twill
Soller FHXHS88 38" Fiberglass H4 satin
 8.8
 6.26
0.009

Soller 6KPL50
 8.9
 6.33
0.011








Mr. Fiberglass GC0660 60"
6
 4.27
0.0093
 $3.46
 (60" Length for Hypertek :)

Mr. Fiberglass GC0750 50"

 7.5
 5.33
0.0107
 $4.25
Mr. Fiberglass GC0850 8 HS 50"
 8.9
 6.33
0.0090
 $5.24
  HS Weave E-glass
Mr. Fiberglass GC1050  50"
 10.5
 7.11
0.0154
 $4.96
 Plain E-glass







Spinal Tap Tubing (estimated)

108.4
 0.143"
 $98
 6 of 8.9, 1 of 8.9 over phenolic
Spinal Tap no phenolic

53.6
 0.063
 $56
 (estimated - subtract PML phenolic)
Spinal Tap no phenolic all Fg

53.6
 0.063
 $36
 (estimated - subtrace & sub FG)







Phenolic equivalent by thickness

114
 0.080
 $47.1
Phenolic equivalent by weight

48.1
 0.036
 $21








PML Extrapolations

If we reverse engineer PML Premium Carbon, we look at 0.056 final / 0.009 per-wrap of 6 oz fabric for 6.2 wraps. Now 6.2 x 4.27 oz/wrap = 26.4oz of fabric.
To figure fabric / epoxy, we take 26.4 oz fabric / 53.6 oz total = 50/50 or 1 to 1 fabric to epoxy.
To figure our cost :) we take 6.2 * $20 = $120 per tube which they sell for $282

Spinal Datapoint Estimations

Weight and Thickness

I am confirming with Ed. Spinal's tubing would be a solid endpoing to duplicate, with or without the phenolic. Offhand, excluding phenolic, it looks to be near "PML Premium Carbon" - at 0.056" thick we look at Ed's 0.009 * 7 = 0.063 thick. Ed's got the PML Phenolic at 0.080 thick, so I'd guess his total at 0.080 + 0.063 = 0.143" thick and I'd guess his total weight at about 0.063/0.056 * 53.6 (carbon) + 48.1 (phenolic) = 108.4 ounces or 6.78 pounds pre tube. He's also got tape bands at each end but this is estimate for "4' stock tubing" from his run.

Cost - (1 wrap carbon)

This is an estimation, esp as I don't know what CF was used, but does guess a CF I'd consider. Doesn't include epoxy.
(1 wrap carbon)
6 wraps of GC0850 and 1 wrap of (guessing wrong) Soller IM7-PW (for estimeate) = 6x 5.24 + 1x 25 = $56.44 for fabric ane $42 for phenolic.for $98 per tube.
(all FG)
7 wraps of GC0850 = 7x $5.24 = $36 for fabric.

Phenolic Equivalent

This looks at "what would cost and weight be of CF and/or Fiberglass equivalents, in thickness, and weight , to an ordinary phenolic tube. Our baseline is 0.080 thickness and 48.1 in weight as main criteria.

Fiberglass by thickness

I will use my intended "favorite pick" of Dave Muesing's fiberglass - GC0850 8 HS 50" at a nice $5.24/wrap.
Turns computation is 0.080 / 0.009 = 9 wraps
Cost computation is 9 wraps * $5.24 per wrap = $47.16 for fabric only.
Weight computation is 9 * 6.33 per wrap = 56.97 oz fabric * 2 = 113.94 oz

Fiberglass by weight

Same Fiberglass.
Turns computation 48.1oz / 2 (factor out epoxy) / 6.33=  3.8 turns (we'll make it 4)
Cost computation is 4 wraps * $5.24 per wrap = $20.96
Thickness computation is 4 wraps * 0.009 = 0.036" thick