Tim Furniss/LONDON
While it acknowledges that the Space Shuttle may still be flying in 2015, NASA is looking at vehicles to complement or supplement and, eventually, to replace its current reusable space transportation system. It hopes that the replacement vehicle will do for space travel what an airliner does today for international passengers, says NASA.
The agency's Integrated Space Transportation Plan (ISTP) is intended to develop the requirements and guidelines for a future programme to meet its space operations needs through to 2030 and to support both the military and commercial space sectors.
The ISTP is designed pave the way towards development of a second-generation reusable launch vehicle (RLV) that will be operated alongside the Shuttle, at the same time laying the groundwork for a third-generation "spaceliner" that would be able to achieve safety and reliability comparable to commercial airliners.
NASA now appears to accept that a replacement for the Shuttle, such as the much-hyped Lockheed Martin VentureStar single-stage-to-orbit RLV, will take a lot longer to develop than expected. A complementary launch vehicle is required sooner, rather than later, the agency is now convinced.
Initially, NASA wants to assure a continued and improved level of safety for the Shuttle through several upgrades - many of which are already under way - while looking at the options for complementary or supplementary vehicles.
The primary objective of the ISTP is to determine those options by 2001, then to develop, flight test and validate critical technologies that would enable a significant increase in safety and reliability and a reduction in the cost of operating a second-generation launch system. One of the options is a new crew and/or cargo transportation vehicle [CCTV].
Over the next five years, NASA plans to invest heavily in technology demonstrations to prepare for "a low-risk, highly competitive selection of a new space transportation architecture" which would allow commercial development of a new launch vehicle to begin as early as 2005.
That the CCTV approach is the current favourite - but not yet the assured route - is indicated by NASA's intention to release RLV and CCTV documents together to industry. The CCTV, therefore, could be just one payload for a second-generation RLV - or it could be the new launch vehicle itself, but carried by an expendable booster. As a payload on a conventional launcher, the CCTV obviously would be unable to meet NASA's original design baseline for the next-generation RLV, which requires a payload bay at least as large as the Space Shuttle's 4.5m x 18m (15ft x 60ft) cargo compartment.
Officially, however, NASA is still preparing for the competitive selection between Shuttle-derived and "new-design" second-generation RLV concepts. Whatever type of second-generation vehicle is selected, the agency says, it will have to "demonstrate safe, reliable, simple vehicle operations, high operational utility and minimum turnaround time, [in] an economical, producible system with low costs per flight and affordable life cycle costs".
Nasa objective
NASA's objective for its new system is to reduce launch costs to $1,000/lb of payload. This compares with $14,000/lb for the Space Shuttle, based on an estimated cost per mission of $800 million. The vehicle will be required to have an ascent phase loss of launch vehicle risk of 1:1,000 and a crew loss risk of 1:10,000. The Space Shuttle's original loss risk rating of 1:100 has been increased to 1:400 by upgrades and safety improvements since the Challenger accident in 1986.
The original guidelines for the second-generation RLV indicate that the crew should be provided with a survivable abort capability throughout the mission. A launch abort would involve either recovering the vehicle intact or using a crew escape system. The feasibility of safe orbit insertion after the loss of one engine on the pad and of crew on-orbit rescue are issues to be scrutinised. Descent landing aborts will be also be studied.
Twelve reference missions have been selected by NASA to help industry define the second-generation RLV system. These include servicing the ISS and repairing and servicing spacecraft in orbit, and represent a flight rate of eight to 15 missions a year. Not all will need to be manned, however.
For example, NASA is studying satellite and spacecraft deployment missions, some using transfer stages, which could also be offered commercially and used by the military. An 18,000kg (40,000lb) to low earth orbit (LEO) capability has been baselined initially.
The reference missions include the delivery of an 11,300kg crew transfer vehicle with an 18,100kg upper stage to a 400km altitude orbit at 46¼ inclination. The vehicle would also be able to transport six crew to a high earth orbit (HEO) of between 800km and 120,500km. Missions to this altitude would require radiation protection. Missions to polar orbit are being studied and associated orbital manoeuvring vehicles may also be required.
Some future spacecraft will require on-orbit servicing, refuelling, capture, retrieval, repair and return to earth. Such missions could be used to reduce space debris.
Also baselined is a five- to seven-crew mission lasting seven days and the vehicle would have to be configured to support spacewalks. There is a requirement that non-toxic propellants be used as much as possible.
While on-orbit platform missions will be handled by the ISS, NASA is looking at separate crewed or uncrewed missions to 240km and 57¼ inclination with 18,100kg payloads. Seven missions a year will be required to exchange crews at the ISS and a complementary crew rescue reaction capability of two days will be studied.
While it may not be able to meet all baseline objectives, NASA says that a CCTV could fly as early as 2005 atop an expendable launch vehicle, to supplement the Space Shuttle fleet, but it would have to be flexible enough to meet more ambitious future objectives.
The CCTV will include all or some of the following requirements, depending on mission objectives: launch vehicle, launch escape system, orbital propulsion systems, landing systems, launch vehicle adaptor, service, payload and cargo return modules and any ground and flight adaptor systems, facilities and personnel, either crew or passengers. Mission roles may be met with a single vehicle or individualised variants of the basic vehicle.
NASA is studying a Phase III CCTV concept with the main objective of demonstrating "an early complementary or alternate [space] access system to the Shuttle for the transfer of personnel and cargo". It will have to "demonstrate safe, reliable, simple vehicle operations and high operational utility and minimum turnaround time, an economically producible system, with low cost per flight and affordable life cycle cost".
'Space taxi'
The baseline mission capability for the CCTV is to act as a "space taxi", to provide ISS crew exchange and transport four times a year for four crew, with small-priority cargo on the same flights considered.
The CCTV would provide rescue vehicle services carrying a maximum of seven crew from the ISS, replicating the capability of the planned crew rescue vehicle (CRV). The CRV is a simpler vehicle and would remain the primary rescue vehicle for the ISS.
The CCTV would have an initial operational capability of no later than 2010 to support ISS operations as part of the second-generation RLV, but, "if a crew rescue capability is required, an earlier operational capability will be needed", says NASA. In an ISS crew rescue mode, the CCTV "would be able to be flown in the Space Shuttle payload bay", the agency has specified. Its minimum design operational lifetime will be 20 years.
The CCTV design and operation must be flexible enough to accommodate future increases in personnel traffic associated with greater ISS manning and other NASA missions, including potential "HEO missions to support lunar, geostationary orbit or Mars exploration".
Also, there may be a need to support personnel transfer and servicing at the L1 and L2 Lagrangian points in solar orbit, millions of kilometres from the earth. The furthest humans have travelled is 250,000 miles (402,500km) to the earth's moon and back.
The CCTV will be studied to consider other high-potential-value civil, commercial and military missions, including orbit-to-orbit transfer, robotic or spacewalking orbital servicing and assembly, rendezvous and docking, microgravity and other missions.
With cost-effectiveness the main priority, air-launched or two-stage-to-orbit vehicles will be considered. The CCTV could be launched on an existing expendable launch vehicle (ELV), which would need to be man-rated, or on future "partially or fully reusable launch systems".
If the CCTV/ELV route is taken, it could be seen as a "back to the future" move, and also as the new, low-cost, easy Shuttle replacement gap-filler, delaying the need for a third-generation replacement.
Source: Flight International
