STS-51 is a space shuttle mission currently scheduled for launch on June 30, 1993. The primary objectives of the eight day mission are to deploy ACTS/TOS (Advanced Communication Technology Satellite / Transfer Orbit Stage) and to deploy and retrieve ORFEUS/SPAS (Orbiting Retrievable Far and Extreme Ultraviolet Spectrometer / Shuttle Pallet Satellite). OV-103 (Discovery) is slated as the vehicle and the crew is composed of commander Frank Culbertson (Capt, USN), pilot William Readdy (Cmdr, USNR), and mission specialists Dan Bursch (Cmdr, USN), Carl Walz (Major, USAF), and James Newman, PhD.
All available engineering and flight information on STS-51 is based upon on February 22,1993 launch. New flight cycle data for the June launch will not be available until mid-March. The scheduled launch was set for 12:09 GMT (06:09 CST) from KSC pad 39B. Target inclination was/is 28.45 degrees with a direct insertion altitude of 160 nm. Primary abort sites were/are: KSC for a return to launch site (RTLS) abort, Banjul (Gambia) for a transoceanic abort landing (TAL), and Edwards Air Force Base for an abort once around (AOA). Launch window information was not included in the preliminary 2/22/93 flight cycle package but will be generated for the 6/30/93 pack scheduled for release in mid-March.
MISSION SEQUENCE
The planned time line for the flight calls for an OMS-2 (Orbit Maneuvering System) burn to occur at forty minutes Mission Elapsed Time (MET). This maneuver uses a delta velocity (V) of 221 fps to put Discovery into an orbit of 160 nm by 161 nm (no OMS-1 burn is needed if all goes well on ascent). ACTS/TOS is then deployed at 7 hours 58 minutes MET and is followed a minute later by a 2 fps RCS (Reaction Control System) burn. At 8 hours 15 minutes the next separation burn occurs using the OMS engines, producing an additional delta V of 24 fps which places the orbiter in a 161 nm by 173 nm orbit. This allows the orbiter to be out of harm's way when the TOS ignites twenty-eight minutes later.
After ACTS/TOS has been deployed, a circularization burn is performed at 22 hours 40 minutes to establish a 161nm by 161nm orbit. The next major event occurs when ORFEUS/SPAS is deployed via the RMS (Remote Manipulator System, i.e. the arm) at 1 day 1hour 40 minutes MET. After a few small separation burns the orbiter does a third, larger separation burn that sets up for stationkeeping ahead of ORFEUS (stationkeeping allows the orbiter to maintain a certain distance from an orbiting target). For the next four days, Discovery stays in front of ORFEUS at a range that varies from 13 nm to 20 nm. On flight day six, a burn is done which positions the orbiter behind ORFEUS, at a range of 10 nm; Discovery then stationkeeps at a range of 10nm to 17 nm. Flight day seven marks the beginning of the rendezvous sequence, during which the orbiter actually drops back further (about 30 nm) before it starts phasing in towards ORFEUS/SPAS. Grapple by the RMS occurs at 7 days 1 hour 34 minutes. All the delta Vs needed for the stationkeeping maneuvers average only about 1 fps and the rendezvous burns almost never exceed 7 fps. See figure 1 for rendezvous profile.
With ORFEUS/SPAS retrieved, the mission draws to a close with a planned deorbit burn of 279 fps occurring at 8 days 22 hours 25 minutes. Landing occurs a little over an hour later at KSC on orbit 144. Should a delay be necessary, other deorbit opportunities are available- KSC on orbits 158, 159 and 174 as well as Edwards on orbits 159 and 175.
OBJECTIVES AND CONSTRAINTS
The primary mission goals are to deploy ACTS/TOS and deploy / retrieve ORFEUS/SPAS. ACTS' main objective is to provide flight verification of advanced, high-risk communications technology in support of future systems in the Ku-band frequencies. The TOS is an upper stage that will place ACTS into a geosynchronous transfer orbit. Since it is desirable to deploy a satellite that is mated to an upper stage early in the shuttle flight (batteries and safety concerns), the launch window is affected by the desired final position of the satellite (in addition to other parameters such as abort lighting considerations, time the crew spends on their backs, etc).
ORFEUS/SPAS is a RMS deployed free-flying payload. SPAS is a structure (pallet) that can be reused- it has flown before as IBSS/SPAS. This allows the flexibility of changing scientific packages between shuttle flights without having to redo the entire payload integration process. Other packages that are on SPAS for this mission include the Interstellar Medium Absorption Profile Spectrometer (IMAPS) and a Remote IMAX Camera System (RICS). ORFEUS will study molecular hydrogen and IMAPS will observe contrasts in the chemical and physical properties of the interstellar medium. RICS will be taking IMAX photography of the earth and shuttle. Figure 2 shows the placement of ACTS/TOS and ORFEUS/SPAS in the shuttle payload bay.
An important constraint in any free-flyer mission is telemetry. In past missions, the distant at which the shuttle stationkeeps from the target has been severely restricted due to communication link capabilities of the shuttle/free-flyer system. A new piece of hardware, the Extended Range Payload Communication Link (ERPCL), is scheduled for its debut on STS-51. ERPCL will allow the orbiter to relay more data from a further distance than before. However, since this is the first flight of ERPCL, various rendezvous profiles have been planned to first check out the new hardware (the rendezvous sequence described earlier is for the case where ERPCL does not work). An additional constraint leveled on the rendezvous profile for this mission is the desire to minimize the time the payload bay is faced into the direction of the velocity vector. In order to accommodate this request and maintain communications with the free-flyer, the shuttle has to spend most of the stationkeeping time in front of the target (most stationkeeping is usually performed behind the target).
The primary payloads dictate most of the mission parameters and once those conditions have been set, secondary mission objectives can be developed. For STS-51, the two most notable secondary objectives are the Global Positioning System Detailed Test Objective (GPS DTO) and a recently approved Extra-Vehicular Activity (EVA). The GPS DTO is a simple test of a hand held GPS receiver that will be flown on-board. The receiver will be connected to a GRID laptop computer that is normally carried on shuttle missions; the GPS position data will then be compared to the shuttle's state vector, which is obtained by another connection made to the downlist telemetry. This will allow a rough check of the receiver's accuracy and potential. For the EVA, mission specialists Jim Newman and Carl Walz will be practicing certain contingency operations in order to build actual EVA experience. NASA has decided to increase the number of EVAs scheduled for future shuttle missions in order to gain experience to prepare for the construction of Space Station Freedom.
STRENGTHS AND WEAKNESSES
The strengths of this mission can be traced to the payloads and activities that are associated with the flight. ACTS is a serious attempt to advance the capabilities of communications satellites; it has been awhile since the shuttle has been involved with a R&D project that has obvious commercial applications. ORFEUS and the other equipment on the SPAS will provide good scientific data and really cool, neat-o pictures. The GPS DTO is an initial check into the usefulness of GPS for shuttle applications and the EVA allows more experience to be gained by astronauts that will be participating in the building of space station.
The weaknesses of this flight are few and involve the fact that some new pieces of hardware are being used for the first time in the shuttle program. TOS has only flown once before (it boosted Mars Observer) and when it did it had major telemetry problems. STS-51 will mark the first time TOS is used on a manned vehicle and it will be the first flight "test bed" for the telemetry changes. ERPCL is also a potential problem if it does not work as planned since it would require many real-time changes in the flight plan and profile. Even with these possible problems, the strengths far exceed the weaknesses and STS-51 should prove to be an efficient and successful flight.
Sunday, 01-Aug-2004 00:36:47 CDT
CSR/TSGC Team Web
