Tim Furniss/LONDON
Six minutes into the flight of the Space Shuttle STS87 on 19 November, the orbiter Columbia, arcing over the Atlantic Ocean heads-down, still attached to its external tank (ET) and with its main engines firing, will perform a 20s, 180íroll to a heads-up position, travelling at a speed of 3,660m/s.
This dramatic, previously untried manoeuvre is a reversal of the roll which the Shuttle undergoes after it clears the launch pad. It will be made to demonstrate the manoeuvre that the Columbia and other orbiters will make to communicate via a NASA Tracking and Data Relay Satellite (TDRS) when the space agency's Bermuda tracking station is retired from service to save NASA $5 million a year.
The manoeuvre commands form part of a package of new on-board software provided by Lockheed Martin Space Mission Systems and Services, Houston, which will also allow future Space Shuttle missions to carry more payload into orbit (Flight International, 6-12 August). This will also contribute to NASA's desire to return to the days of the early 1980s when the Shuttle was in the thick of the commercial-launcher market, a role-reversal which not everyone in the space community is happy about (see box, next page).
The latest software had its debut on the Discovery/STS85, providing constant pitch-rate at solid rocket booster (SRB) separation, and was used to command onboard changes to the trajectory on the Atlantis/STS86 ascent. The improvement will continue with a set of planned software upgrades required to launch missions to support the International Space Station (ISS).
The Operational Increment OI-26 software allows existing Space Shuttle hardware to be used in new ways. For heavyweight flights, these improvements include the use of the smaller Orbital Maneuvering System (OMS) engines during ascent in conjunction with the larger Space Shuttle main engines (SSMEs). This capability had previously been reserved for emergencies when a SSME failed: it was used on mission STS51F in July 1985, when one SSME was shut down.
The Endeavour/STS88, the first mission dedicated to assembling the ISS, will use OMS assististance, starting at T+150s into the flight.
The software contains a new routine which allows for a more optimal launch trajectory during rendezvous missions. These improvements mean the Shuttle will use less propellant to reach a particular orbit, and therefore payload weight can be increased.
On-orbit operations have also been enhanced with the new Space Shuttle software. The software can receive data from the Orbiter Space Vision System (OSVS) and change the position of television cameras in the payload bay based upon this input. The OSVS uses the output of the cameras to determine the accurate position of a payload. Previously, the cameras could only be controlled manually.
Many of the improvements can be switched off if they are not required for a particular mission. Work is under way to prepare the next set of software, OI-26B. The OI-26B software will begin to integrate global-positioning system (GPS) data into the orbiter's navigation system. The first flight to use the OI-26B will be Columbia/STS90, which will include cut-off of the SSMEs at a different altitude.
ROLL MANOEUVRE
The new Mach 11 roll manoeuvre to be tried out on 19 November will be required only for low-inclination flights, as flights to International Space Station, for example, which will operate in a higher-inclination 57í orbit, will be able to communicate via the TDRS.
The roll manoeuvre for flights to low-inclination orbits is required because the external tank blocks out signals from the orbiter to the TDRS during the ascent, and the orbiter needs to communicate via Bermuda.
Although this roll will take place in a thinner atmosphere than that for a Mach 5 return-to-launch site (RTLS) abort, full turnaround manoeuvre - as yet untried - NASA is equally as anxious about the STS87 exercise, which is scheduled to be performed by gimballing the three main engines.
The flight computer will control the manoeuvre and will decide whether it is to the left or right depending on other flight parameters during the ascent. The crew will not know which way the craft will roll until the movement starts.
The manoeuvre could be executed earlier in the ascent, but the Shuttle has to remain in a heads-down position in case of an RTLS which can only be made in this alignment. It is therefore being made after the requirement for an RTLS has passed.
In any event, no major manoeuvres are commanded until the orbiter is rid of its two solid-rocket boosters after a 2min ride in which the crew are merely passengers. Any major problems occurring during the first 120s are considered unsurvivable (Flight International, 3-9 January, 1996).
Other innovations planned to increase the Shuttle's payload capability are:
a lightweight external tank making its debut on the STS88 in July 1998; upgrades to the attitude-control system and auxiliary power units; the introduction of the GPS; the planned development of liquid fly-back boosters.SMART SOFTWARE
A further development is the introduction of smart, "intelligent" software in training aircraft used by the Shuttle commander and pilot to practise landings. Although a crew makes at least 500 practice landings in the specially converted Canadair Gulfstream II Shuttle Training Aircraft (STA), astronauts still find that the Shuttle handles slightly differently.
The new software will help the crew to familiarise with the training aircraft. The software will be introduced to flight simulators for other types of aircraft, including private craft. Tests have shown that the STA will handle about 20% better than before, which should translate to a 69% error reduction which is expected to increase pilot confidence.
The new software refines the "rules" which onboard computers use to simulate the orbiter's descent from 35,000ft (10,500m) to landing. It uses a form of "adaptive fuzzy logic" which programs a computer with words as well as with numbers, and a "neuro fuzzy logic" which enables it to learn by experience, changing the patterns it uses to make decisions.
The software is closer in behaviour to human thinking than previous software, and computers equipped with such logic can be accurately called rudimentary mechanical brains, according to NASA.
Source: Flight International
