When the International Space Station plans were finalized in 1993/94, NASA was very concerned about the high cost of resupplying ISS using the agency's ageing Space Shuttle fleet. The cost of seven Shuttle was then estimated to be $2.8 billion, or $90,000+ per kilogram of useful payload to ISS! Consequently NASA started a highly touted research program in 1994 to develop a radically cheaper fully reusable single-stage-to-orbit spaceplane to replace the Shuttle. Private industry was supposed to pay for and operate the SSTO after the NASA-financed subscale "X-33" program had demonstrated the required technologies by the turn of the century.

Today, the solution to the ISS transportation problem isn't as obvious, though. Lockheed-Martin's "Venturestar" X-33 design was chosen by NASA in 1996, but the company has been unable to develop the vehicle on time and on budget. At the same time as fully reusable SSTO appears to be a more challenging goal than previously thought, the cost of a Space Shuttle launch has fallen in recent years thanks to partial privatization of the day to day operations. Vehicle upgrades such as the new super-lightweight external propellant tank + a more efficient Multi-Purpose Logistics Module have also increased the Shuttle's payload capability by almost 200%. The United Space Alliance has offered to fly five annual ISS resupply missions for just $1.5 billion provided NASA agrees to certain reforms (=further privatization of the Shuttle program, standardized ISS mission format, no day-to-day NASA oversight, permission to offer 1-2 "surplus to requirements" Shuttle flights/year to commercial customers). The cost to NASA would thus be about $35,000/kg of useful payload delivered to ISS. Not cheap by any stretch of the imagination but still surprisingly competitive vs. future systems, when development costs are factored in.

It thus appears the Space Shuttle will be around for quite some time. The Shuttle does meet the basic requirements of reasonably regular delivery & return of crews, experiments and supplies. But it is not capable of increasing launch rate or launching on demand, and the Shuttle does not really provide high reliability for delivery & return of high-valued cargo and crew. Another significant shortcoming is 30-50% of all Space Station users would like to have access to their payload as late as 10 hours before launch and as early as 2 hours after landing, according to the 1993 COMMERCIAL SPACE TRANSPORTATION STUDY (CSTS) report. The Shuttle is not as affordable or flexible as ISS users would like, e.g. only 30% of the payload racks provide thermal control.

FUTURE SPACE STATION GROWTH AND NEW SPACE TRANSPORTATION OPPORTUNITIES

As noted earlier, NASA originally anticipated about seven ISS crew transfer & resupply missions per year == ~30 metric tons of cargo. The expected U.S. transportation requirement is now about 43 metric tons, in part because the Russians will be unable to launch as many Progress cargo craft as planned. The CSTS report assumed the number of flights would change little even if a low-cost launch vehicle were available; the resulting savings probably wouldn't be invested in additional ISS upgrades since the program already is over budget. NASA would, at most, be allowed to fly 1-2 additional missions per year to increase the ISS science output. This is the "high probability scenario" shown in the table below.

ANNUAL U.S. PAYLOAD TO SPACE STATION (4250kg payload/flight)
Option Scenario	1993 transp. cost (~$94K/kg)	1/10th (=$9400/kg)	1/100th (=$940/kg)
High Probability	29,750kg	31,875kg	31,875kg
Medium Probability	34,000kg	48,875kg	55,250kg
Low Probability	38,250kg	68,000kg	80,750kg
=ANNUAL U.S. ISS PAYLOAD PRODUCTION COST (@ $22,000/kg)
Option Scenario	1993 transp. cost (~$94K/kg)	1/10th (=$9400/kg)	1/100th (=$940/kg)
High Probability	$655M	$701M	$701M
Medium Probability	$748M	$1,075M	$1,216M
Low Probability	$842M	$1,496M	$1,777M
=TOTAL ANNUAL U.S. ISS TRANSPORTATION COST
Option Scenario	1993 transp. cost (~$94K/kg)	1/10th (=$9400/kg)	1/100th (=$940/kg)
High Probability	$2,800M	$300M	$30M
Medium Probability	$3,200M	$460M	$52M
Low Probability	$3,600M	$640M	$76M

These scenarios assume each kilogram of ISS payload will cost $22,000 to produce on average (=scientific equipment is ten times more expensive but food and clothes obviously cost far less). Cheaper space transportation would permit NASA to invest some of the savings in additional experiments and equipment to increase the output of the Station. Thus, the low-probability CSTS scenario assumed 80% of the funds saved thanks to cheaper space transportation would go into Space Station additions/upgrades. For the medium probability scenario, 20% of the savings would be used for Space Station growth. Since crew time is one of the most important limiting factors, any ISS expansion program probably would have to start by adding more habitation & laboratory modules. The CSTS parametric growth estimates of the U.S. hab & lab modules are:

The "high growth" scenario may well include additional developments such as assembly of large manned lunar or Mars mission spacecraft at or near the Space Station; this could increase the annual transportation requirement to 500-1,500 metric tons per year! The Station's scientific capabilities would also be enhanced if some sensitive processes were moved to separate man-tended free-flying platforms. This would also allow other ISS users to have frequent access to the main ISS complex without disturbing sensitive microgravity experiments. NASA's current plan (below) calls for two 30-day periods of undisturbed microgravity work every 92 days.

ASSESSMENT OF CURRENT SITUATION

As noted previously, NASA initially identified a new fully reusable "single stage to orbit" spaceplane as its preferred solution to the ISS transportation problem. However, in 1999 the agency was forced to expand the number of alternate systems investigated for the Space Transportation Architecture Study. The vehicle configurations were:

• The current Space Shuttle, with a minimum of upgrades but probably commercial management to reduce costs. The United Space Alliance has offered five flights for $1.5 billion
• A modified Space Shuttle plus new reusable winged "flyback" booster rockets for improved performance, safety and economy. The upgrades would cost several billion dollars and only marginally increase the Shuttle flight rate although the technologies also might be used on new modular heavy-lift launch vehicles e.g. for manned Mars missions
• The Evolved Expendable Launch Vehicle (EELV) plus "Space Taxi" mini-shuttle for manned spaceflight missions to ISS. The development cost and risk would be lower than for any option besides the existing Shuttle. But the marginal cost per mission would be as high. Since the payload carrying capability is less than the Shuttle's, about twice as many launches would be required.
• The Lockheed "Venturestar" single-stage-to-orbit spaceplane option has recently encountered enormous technological difficulties and the project is expected to be cancelled by the new Bush Administration. Like the EELV+Space Taxi, NASA would have to fly 10-12 flights per year to do the same job as five Shuttle missions. The development cost would be at least $5 billion but the marginal cost per flight could be much less than for the other options since no booster rockets or expendable components are used. The vehicle might also be applicable to emerging commercial markets such as space tourism.
• Boeing and Orbital Sciences Corp. have advocated a new partially or fully reusable "two-stage-to-orbit" spaceplane that may or may not incorporate elements such as the EELV Space Taxi or Shuttle flyback boosters. The development and operations cost would be higher than for Venturestar, but the development difficulty would be less. The vehicle would mostly serve existing customers and markets and thus replace the Shuttle and EELV.

The risk-to-reward ratio does not yet appear to be sufficiently good to warrant the $4 billion+ development cost of the reusable launch vehicle alternatives on the list. Thus, the best near term solution appears to be a combination of modest Shuttle upgrades+privatization as well as investment in an unmanned "backup" cargo transfer vehicle for the EELV. Since both EELV versions -- Boeing's Delta IV family and Lockheed-Martin's Atlas V -- will be competitors, it makes sense to outsource some unmanned Space Station resupply services on a "best bidder" basis too. Meanwhile, NASA needs to continue pursuing low cost reusable launch vehicle technologies and experimental vehicles to reduce the risk to the private sector. Eventually, a reusable equivalent of the EELV will make business sense but current launch markets are not yet sufficiently lucrative for it.