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

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

3. 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

4. + 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

5. Current Specific Launch Cost Estimate

6. 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

7. 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

8. RocketCost.xls (beta)

9. 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

10. 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)

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

12. 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.

13. 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

14. 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

15. 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

16. 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

17. 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).

18. 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

19. 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.

20. Selected Bibliography
Griffin, M. D., and Claybaugh, W. R., “The Cost of Access to Space,” JBIS, Vol. 47, 1994, pp
Claybaugh, W. R., AIAA Professional Study Series Course: Economics of Space Transportation, Oct , 2002, Houston TX.
Carton, D.S., and Kalitventzeff, B., “Effect of Engine, Tank, and Propellant Specific Cost on Single-Stage Recoverable Booster Economics,” JBIS, Vol. 20, 1965,
Taylor, C.Y., “Propulsion Economic Considerations for Next Generation Space Launch,” presented at the 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, AIAA , Ft. Lauderdale, FL, 2004.
Griffin, M.D., “Heavy Lift Launch for Lunar Exploration,” presented at the U. of Wisconsin, Madison, WI, Nov. 9, 2001,
Isakowitz, S. J., Hopkins, J., and Hopkins, J. P., International Reference Guide to Space Launch Systems, 4th ed., AIAA, Reston, VA, 2004.
Ross, D.M., “Low Cost Booster Production – Technology and Management,” Reducing the Cost of Space Transportation: Proceedings of the American Astronautical Society 7th Goddard Memorial Symposium, edited by George K. Chacko, American Astronautical Society, Washington, D.C., 1969.
Kiersarsky, A. S., “Assessment of Expendable Tankage for Low Cost Transportation Systems,” NASA-CR , Nov. 5, 1969.
Rocketcost.xls spreadsheet, Rev. K., Jupiter Research and Development, Houston, TX, 2006.