NASA issued its initial list of requirements Tuesday for the Orbital Space Plane program, a craft that is hoped will one day ferry crews and supplies to the International Space Station.

An artist's concept shows what the Orbital Space Plane might look like. Credit: NASA

Such a vehicle would complement NASA's space shuttle fleet by filling the role of primary crew rotation spacecraft. The space shuttles would then focus missions that require an orbital delivery of heavy payloads.

Under the new Integrated Space Transportation Plan released and amended last November, the shuttle fleet is expected to possibly fly through about 2020, with the OSP vehicle becoming available in about 2010. A full-scale next-generation reusable launch vehicle would not come until some undefined amount of time after that.

"This is an important first step in making the Integrated Space Transportation Plan a reality," said NASA Deputy Administrator Fred Gregory. "The Orbital Space Plane system will give us the flexibility needed to safely and efficiently get crew to and from orbit and to provide crew rescue and logistical support to the International Space Station. These initial requirements help to outline a comprehensive system that will significantly complement the capabilities of our existing Space Shuttle fleet."

The announcement of requirements for the OSP program comes just over two weeks after NASA lost the shuttle Columbia on February 1. It may not be known for some time exactly what affect the Columbia accident has on the long-term future of the OSP and the shuttle fleet, as well as on the full-scale development of the next-generation RLV that was put on semi-hiatus in November.

The more staggered approach of first developing a new manned spacecraft before implementing a new launch vehicle program is more realistic in terms of risk and budgetary concerns, officials say.

"It's based on existing technologies and therefore lowers risk and is more affordable. It will replace the space shuttle as the primary crew transport vehicle, freeing the orbiter fleet to focus on heavy cargo delivery," NASA Administrator Sean O'Keefe said in November.

In addition to serving as a crew transfer vehicle, the OSP would also find a niche in crew rescue, possibly augmenting or replacing the Russian Soyuz lifeboat capsule that now resides permanently at the station in case of emergency.

Guidelines set up by NASA for potential contractors to follow include the ability of an OSP system to support at least a station crew of four and to return ill or injured astronauts to Earth to seek timely medical treatment. NASA officials hope to have a live-aboard crew of six or seven astronauts by the time OSP becomes available, but multiple vehicles could be used.

The requirements also set a deadline of 2012, but urge that vehicles be ready as "soon as practical" with a crew of no less than four.

A set of safety guidelines are also set in the document, requiring the OSP to have a lower risk of loss of crew that the space shuttle or Soyuz spacecraft for transport and rescue roles, respectively.

Better on-orbit maneuvering and orbit-changing capabilities are expected of the OSP as compared to the NASA's shuttles. Less time to process and plan missions is also a fundamental goal of the OSP program.

Cost reductions are also required of OSP concepts, both in terms of development and normal operations once flying begins.

OSP spacecraft are likely to fly on one of two Evolved Expendable Launch Vehicles, or EELV's. Lockheed Martin's Atlas 5 rocket and Boeing's Delta 4 were developed with the help of Pentagon funding to support launches of Defense Department satellites in the 21st century. Both boosters also compete in the worldwide market for commercial satellite launches.

Adding the OSP into the manifest of either launcher after 2010 would bring that company not just the prestige of launching manned spacecraft, but also a list of modifications both in the rocket itself and ground infrastructure.

NASA envisions that the OSP will continue operating for at least a decade after its introduction ferrying crews and supplies to and from the space station. Officials say some concepts could include two separate vehicle designs, one dedicated to cargo transfer and one to crew rotation missions.

The OSP requirements
Below are NASA's Level 1 requirements announced Tuesday. The Level 2 requirements are expected to be released by the end of this year.

1. The system, which may include multiple vehicles, shall provide rescue* capability for no fewer than four ISS crew as soon as practical but no later than 2010. (* Rescue includes medical evacuation and emergency evacuation)

2. The system shall provide rescue capability that allows the safe return of deconditioned, ill or injured crewmembers with ongoing treatment until arrival at definitive medical care within 24 hours. Crew should not require suits in the vehicle, but the vehicle should support crew wearing suits if the situation warrants.

3. The system for rescue shall provide for rapid separation from the ISS under emergency conditions followed by return to Earth.

4. Safety requirements - system for crew rescue:

• a. The availability (defined as "a full-up vehicle able to perform its mission") for the escape mission shall be at least:
  Objective: 99%
  Minimum Threshold: 95%
  
  
  
• b. The risk of loss of crew shall be, with high confidence, lower than the Soyuz for the rescue mission.

5. The system shall provide transportation capability for no fewer than four crew to and from the ISS as soon as practical but no later than 2012.

6. Safety requirement - system for crew transport: The risk of loss of crew shall be, with high confidence, lower than the Space Shuttle for the transport mission.

7. The system shall be designed for minimum life cycle cost.

8. The system shall meet all applicable ISS requirements for visiting and attached vehicles.

9. Compared to the Space Shuttle, the system shall require less time to prepare and execute a mission and have increased launch probability.

10. Compared to the Space Shuttle, the system shall have increased on-orbit maneuverability.

Operations concepts

1. The vehicle(s) shall initially launch on an ELV.

2. The system shall be operated through at least 2020. However, the system should be designed so that it could be operated for a longer time.

3. NASA envisions that the systems for crew rescue and crew transport could be different versions of the same vehicle design.

4. The system shall provide contingency capability for cargo delivery to or from the ISS to support a minimal level of science.

5. The system shall support a nominal ISS crew rotation period of 4-6 months.