Crew Return Vehicle


The Crew Return Vehicle, sometimes referred to as the Assured Crew Return Vehicle, was a proposed dedicated lifeboat or escape module for the International Space Station. A number of different vehicles and designs were considered over two decades – with several flying as developmental test prototypes – but none became operational. Since the arrival of the first permanent crew to the ISS in 2000, the emergency return capability has been fulfilled by Soyuz spacecraft and, more recently, SpaceX's Crew Dragon – each rotated every 6 months.
In the original space station design, emergencies were intended to be dealt with by having a "safe area" on the station that the crew could evacuate to, pending a rescue from a U.S. Space Shuttle. However, the 1986 Space Shuttle Challenger disaster and the subsequent grounding of the shuttle fleet caused station planners to rethink this concept. Planners foresaw the need for a CRV to address three specific scenarios:
  • Crew return in case of unavailability of a Space Shuttle or Soyuz capsule;
  • Prompt escape from a major time-critical space station emergency;
  • Full or partial crew return in case of a medical emergency.

    Medical considerations

The ISS is equipped with a Health Maintenance Facility to handle a certain level of medical situations, which are broken into three main classifications:
  • Class I: non-life-threatening illnesses and injuries.
  • Class II: moderate to severe, possibly life-threatening.
  • Class III: severe, incapacitating, life-threatening.
However, the HMF is not designed to have general surgical capability, so a means of evacuating a crew member in case of a medical situation that is beyond the HMF's capabilities is essential.
A number of studies have attempted to assess the medical risks for long-term space station habitation, but the results are inconclusive, as epidemiological data is lacking. It is, however, understood that longer periods in space increase the risk of serious problems. The closest estimates show an illness/injury rate of 1:3 per year, with 1% estimated to require emergency evacuation by means of a CRV. For an eight-person ISS crew, this results in an expected need for a CRV flight once every 4 to 12 years. These estimates have been partially corroborated by experiences on board the Soviet Union's Mir space station. In the 1980s, the Soviets had at least three incidents where cosmonauts had to be returned under urgent medical conditions.
Because of its potential use as a medical evacuation method, the CRV design was required to address a number of issues that are not factors for a standard crewed space vehicle. Foremost of these are the g-loadings as influenced by reentry profiles and deceleration/landing methods upon patients with hemorrhagic shock issues. Patient security issues are more critical for injured astronauts than for uninjured personnel. Additionally, depending on the nature of the injury, it may be unlikely that the patient could be placed in an environmentally contained space suit or minicapsule, therefore the CRV needs to have the capability to provide a "shirt sleeve" environment. The ability to address air purity issues is included in this requirement, as air purity is especially critical in medical as well as toxic exposure situations.

Early NASA concepts

first brought up the concept of space lifeboats in a 1966 article, and then later NASA planners developed a number of early concepts for a space station lifeboat:

Capsule systems

  • The Station Crew Return Alternative Module was a capsule which could hold up to six astronauts. Reentry heat protection was provided by the use of a heat shield designed for the NASA Viking Mars probe. Costing US$600 million, the primary drawback to this design was high g-loadings on landing, which were not ideal in the case of a medically necessitated evacuation.
  • As a follow-on to the Viking-based concept, NASA considered a 1986 proposal by General Electric and NIS Space Ltd. for a commercially developed derivative of the U.S. Air Force blunt body Discoverer-type recovery capsule called MOSES, already designed for classified military projects, and initially were planned for up to four occupants, but the idea of scaling the capsule up to accommodate eight crew members was considered for a time before also being dropped. However, g-loads of up to 8-g's make this vehicle unsuitable for critical medical situations.
  • In 1989, NASA engineers patented a capsule-type ACRV concept.

    HL-20 PLS

The HL-20 Crew Rescue Vehicle was based on the Personnel Launch System concept being developed by NASA as an outgrowth of earlier lifting body research. In October 1989, Rockwell International began a year-long contracted effort managed by Langley Research Center to perform an in-depth study of PLS design and operations with the HL-20 concept as a baseline for the study. In October 1991, the Lockheed Advanced Development Company began a study to determine the feasibility of developing a prototype and operational system. A cooperative agreement between NASA, North Carolina State University and North Carolina A&T University led to the construction of a full-scale model of the HL-20 PLS for further human factors research on this concept. Of all the options, a lifting body presents the most ideal medical environment in terms of controlled environment as well as low g-loading during reentry and landing. However, the price tag for the HL-20 project was US$2 billion, and Congress cut the program from NASA's budget in 1990.

European Space Agency concepts

As a part of their wide-ranging studies of potential human spaceflight programs, the European Space Agency began a six-month, first-phase ACRV study in October 1992. Prime contractors for the study were Aérospatiale, Alenia Spazio and Deutsche Aerospace.
The ESA studied several concepts for a CRV:
  • Apollo-type capsule: This would have been a scaled-up version of the 1960s Apollo capsule capable of carrying eight astronauts. A tower that sat on top of the capsule would contain a docking tunnel as well as the capsule's rocket engines, again similar to the Apollo configuration. The tower would be jettisoned just before reentry. Landing would be via deceleration parachutes and air bags.
  • Also during Phase 1 studies, the ESA looked at a conical capsule known as the "Viking". Like the Apollo-style concept, it would have reentered base-first, but it had a more aerodynamic shape. The rocket engines for the "Viking" module were derivatives of the Ariane Transfer Vehicle. The design work continued until the end of Phase 1 in March 1995.
  • A Blunt Biconic concept was studied in 1993–1994. This design was expected to be more maneuverable, but would have been heavier and more expensive.
The ESA's US$1.7 billion ACRV program was cancelled in 1995, although French protests resulted in a two-year contract to perform further studies, which led to a scaled-down Atmospheric Reentry Demonstrator capsule, which was flown in 1997. The ESA instead elected to join NASA's X-38 CRV program in May 1996, after that program finished its Phase A study.

Lifeboat Alpha

The idea of using a Russian-built craft as a CRV dates back to March 1993, when President Bill Clinton directed NASA to redesign Space Station Freedom and consider including Russian elements. The design was revised that summer, resulting in Space Station Alpha. One of the Russian elements considered as a part of the redesign was the use of Soyuz "lifeboats." It was estimated that using the Soyuz capsules for CRV purposes would save NASA US$500 million over the cost expected for Freedom.
However, in 1995, a joint venture between Energia, Rockwell International and Khrunichev proposed the Lifeboat Alpha design, derived from the Zarya reentry vehicle. The reentry motor was a solid propellant, and maneuvering thrusters utilized cold gas, so that it would have had a five-year on-station life cycle. The design was rejected, though, in June 1996 in favor of the NASA CRV/X-38 program.

X-38

Besides referring to a generalized role within the ISS program, the name Crew Return Vehicle also refers to a specific design program initiated by NASA and joined by the ESA. The concept was to produce a spaceplane that was dedicated to the CRV role only. As such, it was to have three specific missions: medical return, crew return in case of the ISS becoming uninhabitable, and crew return if the ISS cannot be resupplied.

CRV overview and concept development

As a follow-on to the HL-20 program, the NASA intent was to apply Administrator Dan Goldin's concept of "better, faster, cheaper" to the program. The CRV design concept incorporated three main elements: the lifting-body reentry vehicle, the international berthing/docking module, and the Deorbit Propulsion Stage. The vehicle was to be designed to accommodate up to seven crew members in a shirt-sleeve environment. Because of the need to be able to operate with incapacitated crew members, flight and landing operations were to be performed autonomously. The CRV design had no space maneuvering propulsion system.
NASA and ESA agreed that the CRV would be designed to be launched on top of an expendable launch vehicle such as the Ariane 5. The program envisioned the construction of four CRV vehicles and two berthing/docking modules. The vehicles and berthing-docking modules were to be delivered to the ISS by the Space Shuttle, and each would remain docked for three years.
Depending on which mission was being operated, maximum mission duration was intended to be up to nine hours. If the mission was related to emergency medical return, the mission duration could be reduced to three hours, given optimum sequencing between ISS departure and the deorbit/reentry burn. Under normal operations, the undocking process would take up to 30 minutes, but in an emergency the CRV could separate from the ISS in as little as three minutes.
The CRV was to have a length of 29.8 ft and a cabin volume of 416.4 ft3. Maximum landing weight was to be 22,046 lb. The autonomous landing system was intended to place the vehicle on the ground within 3,000 ft of its intended target.
The Deorbit Propulsion Stage was designed by Aerojet GenCorp under contract to the Marshall Space Flight Center. The module was to be attached to the aft of the spacecraft at six points, and is 15.5 ft long and 6 ft wide. Fully fueled, the module would weigh about 6,000 lb. The module was designed with eight -thrust rocket engines fueled by hydrazine, which would burn for ten minutes to deorbit the CRV. Eight reaction control thrusters would then control the ship's attitude during deorbit. Once the burn was completed, the module was to be jettisoned, and would burn most of its mass up as it reentered the atmosphere.
The cabin of the CRV was designed to be a "windowless cockpit", as windows and windshields add considerable weight to the design and pose additional flight risks to the spacecraft. Instead, the CRV was to have a "virtual cockpit window" system that used synthetic vision tools to provide an all-weather, day or night, real-time, 3-D visual display to the occupants.