In Defense of External Tanks By Chris Y. Taylor 42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference July 11, 2006 chrisytaylor@yahoo.com

External Tanks on Aerospace Vehicles a.k.a. drop tank, tip tank, belly tank, expendable tank

c = R Γ c = specific cost ($/lb.) = Launch Cost/Payload Mass   R = structure - payload mass ratio = Structure Mass/Payload Mass Driven by Technology & Physics Γ = structure cost ($/lb.) = Launch Cost/Structure Mass Driven by Management & Economics

+ Γrisk + Γpropellant) + RD(Γnr/a) c = R ( Γvehicle + Γops + Γrisk + Γpropellant) + RD(Γnr/a) Recurring Cost Γvehicle = Cost of Vehicle Hardware Γops = Cost of Operations Γrisk = Cost of Risk Γpropellant = Cost of Propellant Non-Recurring Cost RD = Developed Structure–Payload Mass Ratio Γnr = Non-recurring Structure Costs a = Amortization Factor

Current Specific Launch Cost Estimate

R&D costs must be lowered! Launch costs >$1000/lb. payload to LEO with current development & flight rate even if all recurring costs are zero! How can RD(Γnr/a) be lowered? a Γnr RD

Pegasus/Ithaca Launch Vehicle Concept SSTO vs. SSTO+ET Pure SSTO low hardware costs low operations costs high development costs! Adding E.T. lowers development cost a lot, for a little more hardware & ops cost Pegasus/Ithaca Launch Vehicle Concept a.k.a. stage-and-a-half, tip tanks plus reusable core, RAS, ILRV

RocketCost.xls (beta) http://www.jupiter-measurement.com/research/rocketcost.xls

SSTO+ET Specific Cost vs. ET Size Isp=450s, T/W=50, ηT=.0.96, Γcore=$100k/lb, Γet=$20k/lb, Ch,core=$2.5k/lb, Ch,et=$1k/lb, fcore=0.02, a=27

Range of SSTO+ET Tech Levels Isp=430s, T/W=40, ηT=.0.95, a=27 Baseline Isp=460s, T/W=60, ηT=.0.95 a=72 (monthly) As blue, except a=312, Ch,et=$300/lb (weekly)

SSTO+ET vs. SSTO+SRB Specific Cost Pure SSTO SSTO+SRB (5k fps) SSTO+ET (5k fps) SSTO +ET (18k fps)

SSTO+ET Conclusions Adding external tanks to an SSTO reduces development cost At existing conditions external tanks are more economical than SRBs for boosting SSTOs Conditions where pure SSTOs are cheaper than SSTO+ET aren’t likely to happen soon. If you are dreaming of an SSTO, consider adding external tanks to it.

R&D costs must be lowered! Launch costs >$1000/lb. payload to LEO with current development & flight rate even if all recurring costs are zero! How can RD(Γnr/a) be lowered? a Γnr RD

Using Identical Stages for Reduced Development Cost With Identical stages RD < R even for an entirely new launch vehicle. Identical stages increases development cost (Γnr) and has inefficient staging velocities. Bimese Image from: THE BIMESE CONCEPT: A STUDY OF MISSION AND ECONOMIC OPTIONS by Dr. John R. Olds and Jeffrey Tooley, 1999 Trimese

Reusable Bimese + ET 2STO Bimese Bimese+ET 3STO Tri-mese Amort. R&D $1,676 $1,151 $857 $1,535 $713 Hardware $36 $41 $197 $33 $39 Ops $68 $78 $94 $83 $96 Other $61 $63 $59 $60 Recurring $165 $182 $350 $176 $196 Total $1,841 $1,333 $1,207 $1,711 $909 R 0.905 0.52 0.386 0.829 0.321 RD 1.03 0.944 0.963 Adding an ET to a bimese reduces orbiter ΔV requirement substantially for small additional development cost. Isp=440s, T/W=40, ηT=.0.95, η=.0.918, Γnr,xsto=$50k/lb, Γnr,xmese=$60k/lb, Γnr,et=$15k/lb, Ch=$2k/lb, Ch,et=$500/lb, fcore=0.02, a=27

Expendable Bimese + ET 2STO Bimese Bimese+ET 3STO Tri-mese Amort. R&D $750 $468 $444 $697 $297 Hardware $694 $843 $659 $645 $803 Ops $56 $63 $80 $70 Other $52 $53 Recurring $802 $959 $791 $767 $935 Total $1,552 $1,427 $1,235 $1,464 $1,232 R 0.75 0.843 0.799 0.697 0.803 RD 0.421 0.400 0.268 Adding an ET to a bimese reduces system cost even if bimese vehicles are completely expendable! Isp=440s, T/W=40, ηT=.0.96, η=.0.928, Γnr,xsto=$27k/lb, Γnr,xmese=$30k/lb, Γnr,et=$15k/lb, Ch,xsto=$925/lb, Ch,xmese=$1k/lb, Ch,et=$500/lb, fcore=1, a=27

Reusability is for Lower Stages 2 Stage Case, subscripts indicate stage number If Γ 1 ≈ Γ 2, then changes to R1 or R2 have the same effect. Changes to Γ 1 have bigger effect than Γ 2. Therefore, 2nd stage should be expensive and light weight while 1st stage is heavier and cheaper (big&dumb or reusable).

Expendable Tank on Lower Stage Partially reusable lower stage with expendable tanks becomes economical before fully reusable lower stage. Original Boeing EELV Concept Isp=315s, T/W,reuse=87, T/W,exp =100, ηT,reuse=.0.948, ηT,exp=.0.955, Ch,engine=$1000/lb, Ch,et=$500/lb, f=1/0.05, a=27, ΔV=12,500 ft/s

Conclusions Adding ETs to SSTO designs lowers specific cost for most current and likely future design conditions. Adding ETs to a bimese design lowers the systems specific cost, even if the bimese vehicles are fully expendable. Partially reusable lower stages using expendable tanks and reusable engine pods will become economical before fully reusable stages. By any name, external tanks are still a useful feature in aerospace conceptual design.

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