Slide 65 of 99

Lockheed-Martin's “Aeroballistic Rocket” spaceplane -- now called Venturestar -- docks with the Space Station. This 1994 illustration shows what the “Alpha” International Space Station would look like, without Russian modules. The non-Russian option was rejected in September 1993 because it would have cost more than the initial estimates if the project was to meet its 2000/2001 completion deadline. Stretching the program by two years would still have cost $1 billion more than the $10.5-11 billion FY 1994-98 limit imposed by President Clinton. The project's political appeal was also quite limited, since it was regarded as an obsolete Cold War monument. Bringing in some additional Russian capabilities thus appeared to solve several political and technological problems.

When completed, ISS will require 5-6 Shuttle flights per year for crew rotation and resupply. Four of those flights will carry pressurized cargo in the Multi-Purpose Logistics Module (MPLM). The Shuttle missions will usually take place at 90-day intervals and each will carry enough provisions to support four astronauts for three months. Three basic types of cargo are carried.

• CREW SUPPLIES, which belong to three main subcategories. Most of it is carried in stowage racks inside the MPLM, but refrigerated food and some health care supplies have to be carried in refrigerator/freezer racks. The MPLM can house up to five of these, plus 11 standard racks. Finally, up to eight aisle stowage containers can be carried in the middle, e.g. for transporting clothing soft packs for the crew. The total mass required per year is about 8,000kg plus another 6,500kg for the cargo racks, trays, containers etc.
• SPARES/MAINTENANCE PARTS are also carried in the MPLM racks. The actual upmass varies from year to year but is expected to be about 3,000kg on average (4,000kg max., 2,000kg min.) plus another 2600kg for the Passive Dry Cargo Resupply/Return Racks.
• USER PRESSURIZED CARGO. NASA has promised to set aside about 40% of the available cargo space for Space Station users. Most of it will be carried inside International Standard Payload Racks (ISPRs) in the MPLM, plus a total of 1816kg in the Shuttle's middeck lockers. The total available mass ranges from 12,000-17,500kg/year. Most of it is expected to be at the sub-rack level (i.e. lockers) but current planning within the user community indicates up to 2.5 refrigerator/freezer racks also will be used in each MPLM for scientific payloads.

PRESSURIZED CARGO
CREW SUPPLIES (4-crew,360 days)	Mass (kg)		Volume (M3)
Crew preference restraints	96		0.56
Operational & personal equipment	608		1
Clothing	2028		18.4
Workstation support	368		1.92
Ambient food	1112		3.6
Refrigerated/frozen food	2352		10.8
Non-food galley resupply	84		1.36
Personal hygiene resupplies	372		1.12
Housekeeping supplies	788		2.48
Maintenance supplies	72		0.56
Crew Health Care System (ambient)	56		0.2
Crew Health Care System (refrigerated)	25.6		0.2
TOTAL US CREW SUPPLIES		7961.6	kg	42.2
Stowage containers (trays) x 1052 units	1720	kg
Stowage racks (x 12 units @ 1.019m3)	1308	kg
Refrigerators/freezers (10 racks @ 322kg)	3220	kg
Clothing soft packs x 16	268	kg
-----------------------------------------------------------------------------------------	----------------	----------------	----------------	----------------
TOTAL US CREW SUPPLIES TARE WT.	=	6516	kg
-----------------------------------------------------------------------------------------	----------------	----------------	----------------	----------------
USOS PRESSURIZED SPARES/MAINTENANCE
US spares delivered	3080	kg
US spare tare weight (10 stowage racks @ 260kg)	2600	kg
	=	5680	kg
CARGO SPACE AVAILABLE FOR USER PRESSURIZED CARGO
US science user cargo (MPLM)+ISPR tare wt. (26 x 454kg loaded racks)	11804	kg
US science user cargo (Shuttle middeck)	1816	kg
TOTAL PRESSURIZED SCIENCE/USER	=	13620	kg
-----------------------------------------------------------------------------------------	----------------	----------------	----------------	----------------
TOTAL PRESSURIZED CARGO	=	33777.6	kg
(=4 Shuttle flights/Multi-Purpose Logistics Modules required)

Surprisingly enough, the Shuttle will transport more cargo back to Earth from ISS than it carries to the Space Station. This is because the other international partners have requested that NASA return some of their payloads launched on indigenous expendable cargo transfer vehicles (Progress, ATV, HTV). NASA estimates about 3,818kg of crew systems cargo per year is disposable while the remaining 10,680kg is to be returned. Including unpressurized payloads, the total recoverable U.S. download is about 31,150kg plus 8,050kg of trash. The net return cargo capability is about 11,074kg per pressurized logistics flight -- 10,148kg using two Unpressurized Logistics Carriers.

A single unpressurized flight per year will carry two Logistics Carriers. About 7,800kg (<60m³ volume) is available for cargo, ~5,000kg of which is required for U.S., Japanese & Canadian unpressurized spare parts. + extravehicular activity "astronaut spacewalk" equipment. This leaves about ~2000kg for unpressurized scientific payloads. The total requirement varies from year to year, and a sixth Shuttle mission may sometimes be required.

In addition to the pressurized & unpressurized cargo items listed above, the Station's life support system also needs water, oxygen & nitrogen.

• About 1000kg of water will be transported by the Shuttle to the U.S. segment every year, and the Russians will contribute another 2000kg per year. All waste water is recycled and eventually broken down into oxygen for the life support system and hydrogen which is vented overboard.
• About 350kg of oxygen and 200kg of nitrogen must also be imported to cover atmospheric losses. The Russians will transport another 200kg to the Russian ISS segment. The total gross mass of the Shuttle's high pressure O² tanks is 1,410kg per year, plus another 920kg for structural support for tanks and unpressurized spares.

ALTERNATE ACCESS TO I.S.S.?

Kelly Space & Technology "Eclipse Astroliner" + ISS logistics/crew transfer vehicle. NASA has recently started an "alternate ISS access" program to investigate complementary and/or backup space transportation systems to the Shuttle. Some cargo items could indeed be moved fairly easily to smaller commercial vehicles. The leading candidates are:

1) Light dry resupply (clothes, food, and other "upmass only" items that can be readily arranged in small packages and do not have to be returned to Earth) - this is served by Progress, and is the first area where a commercial vehicle could make an impact. Most unrefrigerated pressurized crew supplies belong to this category. The annual net mass to be transported by the Shuttle is about 5,500kg plus another 3,000kg for stowage containers & racks. Another prime candidate is the 1800kg/year of US science cargo that will be transported in Shuttle middeck lockers.

2) Reboost propellant. The Shuttle will replace some of this, both directly Shuttle reboost) and indirectly (Shuttle refueling of the Boeing prop module) but there remains a great need for an alternate American "tanker" spacecraft. This function will primarily be served by Russian Progress and European ATV spacecraft. The annual requirement is about 10,000kg.

3) Wet resupply (propellant, water, etc) - separate from dry resupply because this can be piped in automatically through Progress' tanking systems. ESA's ATV can handle some of this. The total annual requirement appears to be about 3,000kg of water plus 500kg of oxygen and nitrogen.

The following types of cargo appear to be less suitable for commercialization:

4) Emergency crew return. If NASA would accept a lifeboat type of vehicle that only has to provide basic "high risk" rescue services in emergency situations, private industry might perhaps provide a cheaper alternative to the recently postponed X-38 crew rescue vehicle. One candidate for this could be simple 8-man type capsule (e.g. BAe Multi-Role Recovery Capsule) that also might double as an unmanned spacecraft for microgravity experiments.

5) Crew rotation. NASA needs to safely transport four astronauts to ISS every 90 days while returning the previous crew to Earth. However, the requirement cannot be easily met by today's expendable launch vehicles.

6) Heavy resupply (=most pressurized & unpressurized user cargo, spares, maintenance carried in system racks & pallets) - Shuttle/MPLM + occasional Japanese HTV for the foreseeable future. These items are difficult to offload from the Shuttle since they are bulky and often require refrigeration etc. services during transportation to ISS. Most research equipment and spare parts will have to be returned to Earth which further complicates matters, if an expendable launch vehicle is used.

The best and most cost-effective option appears to be a combination of Space Shuttle and commercial "alternate access" services, with the former doing crew transfer and specialized heavy resupply (items 5,6) while small (expendable-? unmanned-?) alternative spacecraft transport most of the food, crew supplies, water & propellant (items 1,2 & 3). The total net mass is about 20,000kg/year so it could be a valuable "secondary market" to small commercial companies. The alternate access policy would also increase the cargo space available on the Shuttle for the types of ISS payloads it is ideally suited for, e.g. bulky and expensive experiments that have to be returned to Earth.