In Defense of External Tanks

1
In Defense of External Tanks

• By Chris Y. Taylor
• 42nd AIAA/ASME/SAE/ASEE Joint
• Propulsion Conference
• July 11, 2006

chrisytaylor_at_yahoo.com
2
External Tanks on Aerospace Vehicles

• a.k.a. drop tank, tip tank, belly tank,
  expendable tank

3
c R G
c specific cost (/lb.) Launch
Cost/Payload Mass   R structure - payload mass
ratio Structure Mass/Payload Mass Driven by
Technology Physics   G structure cost
(/lb.) Launch Cost/Structure Mass Driven by
Management Economics
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c R ( Gvehicle Gops Grisk Gpropellant)
RD(Gnr/a)
Recurring Cost Gvehicle Cost of Vehicle
Hardware Gops Cost of Operations Grisk Cost
of Risk Gpropellant Cost of Propellant
Non-Recurring Cost RD Developed
StructurePayload Mass Ratio Gnr Non-recurring
Structure Costs a Amortization Factor
5
Current Specific Launch Cost Estimate
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RD costs must be lowered!

• Launch costs gt1000/lb. payload to LEO with
  current development flight rate even if all
  recurring costs are zero!
• How can RD(Gnr/a) be lowered?

a
Gnr
RD
7
SSTO vs. SSTOET

• 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
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RocketCost.xls (beta)
http//www.jupiter-measurement.com/research/rocket
cost.xls
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SSTOET Specific Cost vs. ET Size
Isp450s, T/W50, ?T.0.96, Gcore100k/lb,
Get20k/lb, Ch,core2.5k/lb, Ch,et1k/lb,
fcore0.02, a27
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Range of SSTOET Tech Levels
Isp430s, T/W40, ?T.0.95, a27 Baseline Isp4
60s, T/W60, ?T.0.95 a72 (monthly) As blue,
except a312, Ch,et300/lb (weekly)
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SSTOET vs. SSTOSRB Specific Cost
Pure SSTO SSTOSRB (5k fps) SSTOET (5k
fps) SSTO ET (18k fps)
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SSTOET 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
  SSTOET arent likely to happen soon.
• If you are dreaming of an SSTO, consider adding
  external tanks to it.

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RD costs must be lowered!

• Launch costs gt1000/lb. payload to LEO with
  current development flight rate even if all
  recurring costs are zero!
• How can RD(Gnr/a) be lowered?

a
Gnr
RD
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Using Identical Stages for Reduced Development
Cost

• With Identical stages RD lt R
• even for an entirely new launch vehicle.
• Identical stages increases development cost
  (Gnr) and has inefficient staging velocities.

Trimese
Bimese Image from THE BIMESE CONCEPT A STUDY OF
MISSION AND ECONOMIC OPTIONS by Dr. John R. Olds
and Jeffrey Tooley, 1999
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Reusable Bimese ET

• Adding an ET to a bimese reduces orbiter ?V
  requirement substantially for small additional
  development cost.

Isp440s, T/W40, ?T.0.95, ?.0.918,
Gnr,xsto50k/lb, Gnr,xmese60k/lb,
Gnr,et15k/lb, Ch2k/lb, Ch,et500/lb,
fcore0.02, a27
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Expendable Bimese ET

• Adding an ET to a bimese reduces system cost even
  if bimese vehicles are completely expendable!

Isp440s, T/W40, ?T.0.96, ?.0.928,
Gnr,xsto27k/lb, Gnr,xmese30k/lb,
Gnr,et15k/lb, Ch,xsto925/lb, Ch,xmese1k/lb,
Ch,et500/lb, fcore1, a27
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Reusability is for Lower Stages
2 Stage Case, subscripts indicate stage number
If G 1 G 2, then changes to R1 or R2 have
the same effect. Changes to G 1 have bigger
effect than G 2. Therefore, 2nd stage should
be expensive and light weight while 1st stage
is heavier and cheaper (bigdumb or reusable).
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Expendable Tank on Lower Stage
Partially reusable lower stage with expendable
tanks becomes economical before fully reusable
lower stage.
Original Boeing EELV Concept
Isp315s, T/W,reuse87, T/W,exp 100,
?T,reuse.0.948, ?T,exp.0.955,
Ch,engine1000/lb, Ch,et500/lb, f1/0.05,
a27, ?V12,500 ft/s
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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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• Selected Bibliography
• Griffin, M. D., and Claybaugh, W. R., The Cost
  of Access to Space, JBIS, Vol. 47, 1994, pp.
  119-122.
• Claybaugh, W. R., AIAA Professional Study Series
  Course Economics of Space Transportation, Oct.
  12-13, 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, 183-196.
• 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-2004-3561, Ft. Lauderdale, FL, 2004.
• Griffin, M.D., Heavy Lift Launch for Lunar
  Exploration, presented at the U. of Wisconsin,
  Madison, WI, Nov. 9, 2001, http//fti.neep.wisc.ed
  u/neep533/FALL2001/lecture29.pdf.
• 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-107139, Nov. 5, 1969.
• Rocketcost.xls spreadsheet, Rev. K., Jupiter
  Research and Development, Houston, TX, 2006.