DESIGN FOR ON-ORBIT SPACECRAFT SERVICING Stephen J. Leete NASA Goddard Space Flight Center ABSTRACT Servicing to date has been in low earth orbit from the space shuttle, and in the near future will also be based from the ISS. Future servicing should occur at much more energetic orbits, such as HEO, Earth-Moon L1, Sun-Earth L1 or L2. Mass for systems at these orbits will be at a high premium. These systems will also typically include large, highly flexible components (gossamer structures, sun shades, solar sails, solar arrays, mirrors) which can sustain only limited loads once deployed. However, the current EVA interface requirements and reference designs impose considerable mass and cost penalties on spacecraft design. This is partly because of high inadvertent EVA loads (kick loads, PFR ingress loads), and massive EVA-friendly connectors and mechanisms. A new generation of requirements and design solutions is needed to enable the designer of new systems to incorporate requirements related to servicing. An assessment of the current situation and a roadmap of the technology development needed to enable future ambitious serviceable systems is presented. HISTORY OF SERVICING The servicing of spacecraft on-orbit has been one of the highlights of the space activities. The three major categories of this are servicing of space stations, such as Skylab, Salyut, Mir, and the International Space Station (ISS), engineering development of servicing capabilities, and servicing of robotic spacecraft. These activities are summarized in Table 1, History of Extravehicular Activities, and have been taken from an excellent book by Portree and Treviño for all activities up to April, 1997 [Ref 1], and various NASA web sites for the time period after that [Ref 2]. Vehicle Year(s) Activities, Accomplishments Voskhod 1965 First EVA Gemini 1965-1966 10 EVAs, 1st successful EVA, demonstration of EVA tasks Soyuz 1969 Crew transfer Apollo 1969-1972 Crew transfer, lunar surface exploration & repairs, Apollo 13 safe return Skylab 1973-1974 Deploy sunshades & solar array, repair & service science instruments Salyut 1977-1986 Install solar arrays, remove 10-m antenna, repair propellant system leak, demonstrate welding tool, erect trusses, maintain science experiments. Space Shuttle 1983 Demonstrate MMU, MFR on RMS, hydrazine transfer. Space Shuttle 1984 Solar Max Mission recovery and repair, Palapa-B and WESTAR-IV recovery and return to Earth. Space Shuttle 1985 Retrieve and repair, release Leasat-3/SYNCOM-IV satellite; test EASE & ACCESS truss assembly. Space Shuttle 1986 Challenger disaster Mir 1987-1982 Repair various Mir hardware, repair and install solar arrays, reconfigure Mir modules, install Strela boom, test Soviet MMU (SPK), install and tend science experiments, install new propulsion module. Space Shuttle 1991 Contingency GRO antenna deployment, tested EVA hardware Space Shuttle 1992 Retrieve Intelsat-VI, install new PKM & release; erect ASEM truss, test Crew Propulsive Device Page 1 of 12 Vehicle Year(s) Activities, Accomplishments Space Shuttle 1993 Gain general EVA experience, safe an appendage on EURECA payload for landing Space Shuttle 1993 Service HST (SM1): replace solar arrays, install WF/PC-2 & COSTAR with optical prescription corrections, gyros, magnetometers, and a 386 co-processor, reboost Mir 1993-1996 Install solar drive units, deploy Papana truss, extensive photography of Mir exterior, reconfigure modules, deploy and install solar arrays, install 2nd Strela boom, install and maintain science instruments, filmed a Pepsi commercial. American astronauts performed an EVA while docked to Mir. Space Shuttle 1994-1996 Test SAFR crew rescue device, improve suit performance in extreme cold, practiced various EVA tasks Mir 1997 Joint EVAs between cosmonaut and astronaut, test Orlan-M suits, inspect damage to Spektr due to severe impact in June, internal EVA to reconnect power cables from Spektr solar arrays Space Shuttle 1997 Service HST (SM2): install STIS and NICMOS axial instruments, replace FGS, install solid state recorder, replace reaction wheel, replace data interface unit, replace magnetometers, and unplanned installation of blanket patches ISS 1998 Joining of Unity and Zarya modules, first use of Space Vision System Space Shuttle 1999 Service HST (SM3A): replace gyros (3), computer, FGS, SSR/tape recorder, s-band transmitter, and install new outer blanket layers (2) Space Shuttle 1999 ORU Transfer Device and Russian Strela crane installation, Chandra deploy ISS 2000 Zvesda Module joins ISS, complete Strala and OTD installation, add Z-1 Truss & Pressurized Mating Adapter #3, re-locate S-Band Antenna, install Space to Ground Antenna, install EVA Tool Stowage Devices, Soyuz brings first expedition crew, added solar arrays (including EVA intervention to complete solar array deployment) ISS 2001 Destiny lab module installation (Curbeam sprayed with ammonia, cleaned with hydrazine brush), re-locate PMA-3 on Unity, install External Stowage Platform on Destiny, Space Station Remote Manipulator System installation, UHF antenna install/deploy, Joint Airlock Module installation using SSRMS, and install Russian Piers airlock. Table 1: History of Extravehicular Activities SPACE STATIONS If a satellite is to be serviced by humans, it must not present a hazard to those humans. It must not present unacceptable electrical hazards such as high-current connector mate/demate, thermal hazards such as extremely high or low touch temperatures, or cutting & entrapment hazards such as sharp edges that could cut a spacesuit or holes in which gloved fingers could get stuck. The spacecraft must not be a hazard when subjected to inadvertent EVA loads, also called bump or kick loads. There are provisions for allowing some hazards, as long as the crew will be adequately briefed and trained in avoiding them. This would then be mitigated by more care in other EVA accommodations, more detailed training, or other requirements on the servicer tools. However, the designer may decide to allow some or all of the satellite to be damaged to the point of loss of function. Page 2 of 12 For a space station, damage to the operation of the space hardware may also present a hazard to the occupants of the space station, so nearly all damage becomes hazardous either directly or indirectly. A space station is likely to be serviced many times over its life, sometimes on short notice, so having to brief a crew on so-called "no-touch" and "no-damage" zones can be overly risky and costly. The solution to this situation adopted for the ISS is to have a very low threshold of tolerance for potential hazards on ISS hardware, and to make the hardware very robust with regard to inadvertent astronaut damage. These practices are comparable to more mundane guidelines for ensuring safe work-sites on the ground, such as clear labels on chemicals, wearing steel-toed shoes and hardhats, etc. These choices make good sense for the ISS. Being in low earth orbit, launch costs to bring up major components are high but not unreasonable. Adding weight once that will increase the reliability, safety, ease of servicing, and other important characteristic at the cost per pound of the Space Shuttle or other Russian launch vehicle is a good decision. If servicing of the ISS is too time-consuming, costly, or dangerous, its entire mission is compromised - as was found to be the case for an earlier version of Space Station Freedom. [Ref 3, 4] SPACECRAFT SERVICING DESIGN - MMS This approach can also be taken with free-flyer satellites. They can be rugged, with many design decisions made to favor ease of servicing and simplicity. The satellites which exemplify this approach are the Multimission Modular Spacecraft (MMS) design. During the years prior to the Challenger disaster in 1986, NASA had a broad vision of satellite servicing to take place from the Space Shuttle or space station. This would be in just about any inclination low earth orbit. Even post-Challenger, the vision was that there would be space tugs (orbital maneuvering vehicles) that would bring high orbit spacecraft back down to STS orbits for servicing. For various reasons, this vision has not yet come to fruition. A graphic of the MMS design is shown in Figure 1. [Ref 5]. With the post-Challenger decision not to launch shuttles from the west coast, most of the Earth observing satellites which are in polar orbits, such as the Landsat series, were no longer candidates for servicing. The Solar Maximum Mission (SMM) launched in 1980, Landsat 4 and Landsat 5 launched in 1982 and 1984, the Upper Atmosphere Research Satellite (UARS) launched in 1991, and the Extreme Ultraviolet Explorer (EUVE) launched in 1992. All of these were built for the NASA Goddard Space Flight Center. Additional missions using the MMS design may have been built for other users. The MMS design was for a system of serviceable and re-useable satellites and satellite components. This modular design was also intended to make satellite design, assembly (integration) and test faster and less expensive. It was the implementation of a vision of regular access to space, modular and interchangeable spacecraft components, easy integration of new technology, the establishment of design standards, and other concepts. Page 3 of 12 Figure 1. MMS Components The hope was that the design, construction and servicing of satellites could be made to resemble more the current situation with, to use a current analogy, personal computers. There are a large number of these in the field, strong demand for them being kept up to date with current technology (thus a rapid turn-over, frequent up-grades), and a multiplicity of providers of components and chassis. Vendors of systems manufacture some components themselves, and purchase the rest on the open market from vendors who manufacture to standards. There is intense competition at all levels (with the possible exception of the operating system software), and customers benefit from the resulting low prices. The benefits to NASA, as well as other purchasers of satellites such as industry and military, from having a similar set of circumstances would be large.