NASA engineers are searching for ways to increase the critical payload of the orbiter fleet.


THE SPACE SHUTTLE will not be seen as being a fully operational vehicle until about 1999 when it will have flown 100 missions, says Bryan O'Connor, a former Shuttle commander and now NASA's programme director.

The Shuttle will enter its fifteenth year of service in 1996, after more than 70 missions. It will also have to undergo a 5,900kg weight reduction, mainly to the Orbiter and derived from operational changes, to increase its payload-lifting capability to a maximum of 21,300kg for flights to 51.6¡ orbits on International Space Station Alpha assembly missions, starting in 1997. By then, the Shuttle may be on the road to retirement.

Even with this weight reduction, the Shuttle will not be able to carry the 29,500kg claimed after its first few flights in 1981. As it was not then operational, but still a development test vehicle, a payload change should have been anticipated.


O'Connor says that NASA should not have declared the Shuttle operational in 1982 after only four flights. US President Ronald Reagan's 4 July declaration after the landing of the Space Shuttle STS4/Columbia was a boost for NASA, which needed to portray the Shuttle as a mature system, to cement plans such as those for the Space Station.

O'Connor was a rookie astronaut in 1982. Having left Naval Air Systems Command, he was "...amazed that we were going to declare the Shuttle operational". STS4 commander Ken Mattingly said that he would not regard the Shuttle as operational until it had made 100 flights.

The X-15 rocket-powered aircraft flew 199 missions and was always regarded as a test vehicle. "I think the Shuttle's a hybrid," says O'Connor. "It's not developmental like a prototype or an experimental test vehicle, but I don't consider the Shuttle to be operational like an [McDonnell Douglas] F-18 on a [aircraft] carrier."

The Shuttle is in the later stages of developmental tests, similar to those conducted at Edwards AFB, California, or Boscombe Down, UK, "...where you have a combination of residual development activities going on, but the operational routine is pretty well established", says O'Connor.

The F-18 had had over 2,000 flights before being handed over to the US Navy. O'Connor says: "One of the reasons why you can even compare that huge number of flight-tests to the Shuttle, when we haven't even reached 100 flights, is that they had such a variety of tasks and missions, a huge envelope to explore, and new weapons to fly with."


Portraying the Shuttle as an operational vehicle so early drew attention, to modifications and technical problems, which would have been regarded as normal in a flight test programme, but were intolerable to Congress and the public.

The flawed perception of the Shuttle as an operational vehicle - perpetuated by NASA's decision to fly passengers - was exposed cruelly on 28 January, 1986, when the Challenger was lost. "When I first showed up at NASA, the reliability was claimed to be one [loss] in 10,000 [launches]," O'Connor says.

In planning Space Station missions, NASA is aware that an Orbiter could be lost in an accident. If one were to be lost, a new craft would not be ordered until 2000 if a decision were made to continue with the programme. "We can assemble and operate the Space Station with three Shuttles to 2012, although an accident would impact on the assembly-completion date. It would also depend on what Orbiter we lost," O'Connor says.

If it were the Columbia, which is not assigned to Space Station missions, it would have no impact on completion, although "...we may have to cancel some science missions and may have to put the Hubble service missions on one of the station Shuttles". If it were a Station Orbiter - the Discovery, Atlantis or Endeavour - it would have some impact. "We can fly each Orbiter three times a year. We are talking about five to seven flights a year for the Station and four to five flights per year for logistics," he explains.

The Discovery, is the first to undergo modifications for station flights, the main task being, "to take the airlock out of the cabin, put it out in the payload bay and put the docking systems on the airlock, so there will only be one structure".

Changes before and after the Challenger accident have increased the weight - particularly that of the solid-rocket boosters (SRBs) - and decreased the Shuttle's payload capability. When conceived in 1972, the Shuttle's payload capability of 29,500kg to 28° low-Earth-orbit, was based on a flight with four crew, for four days, with two fuel cell sets and a unique ascent profile.

This was reduced to 24,950kg after the Challenger crash, for a five-crew, seven-day, mission, with four fuel-cell sets and a standardised ascent. In 1993, this fell to 24,400kg when the Shuttle's design was frozen. The heaviest payload (the Ulysses solar-polar orbiter and its twin upper-stages on the STS41) was 22,150kg.

The 24,400kg maximum payload is a nominal figure because it depends on other parameters, including orbital inclination. An extra crewmember and equipment takes 225kg off the capability. Carrying the remote manipulator system (RMS) costs around 450kg of payload. Typically, a 13,600kg-class payload can use up virtually the entire ascent capability for a mission.

The empty weight of each of the Orbiters also varies, affecting individual Orbiter capability. The empty Columbia weighs 82,265kg, the Discovery 78,700kg, the Atlantis 78,370kg and the Endeavour 78,800kg.

Using mission parameters of a 256km (160 mile), 28.45¡, orbit with five crew on a seven-day mission, with a direct insertion into orbit and with no RMS, NASA gives the Columbia a 17,690kg payload capability; the Discovery 21,275kg; the Atlantis 21,320kg; and the Endeavour 21,275kg. The Orbiters equipped for flight are now heavier because safety measures were added after the Challenger accident, including a crew bailout system and drag parachute.

Now that NASA is to fly routinely to 51.6¡ orbital-inclinations for the Shuttle-Mir missions and to assemble, maintain and operate the Alpha, the lifting capacity of the Discovery, Atlantis and Endeavour on these flights has been reduced.

NASA has ordered a "weight-watchers'" programme, to increase the payload weight by 5,900kg, to give 51.6° flights a maximum capability of 21,320kg, in the case of the Atlantis, which will also result in an increase of the maximum 28°-orbit figure to 27,220kg.

Even then "...this figure is partly theoretical as there is an arbitrary limit because of aborts", says O'Connor. There are 116,120kg and 112,500kg landing limits for fully laden orbiters making return to launch site (RTLS) and transatlantic landing (TAL) aborts respectively. The heaviest lift-off weight of an orbiter in 1995 was 117,300kg. On-board propellants would be expended during RTLS and TAL aborts, reducing weight.

NASA has been conservative in preparing for contingencies, by carrying extra reaction-control-system propellant, which can be now eliminated, O'Connor says. The external tank will be of lighter aluminum-lithium alloy to save 3,400kg - the biggest single saving. The new external tank will not be flown until 1997 at the earliest. New lightweight seats will save 140kg.


Other savings include removal of payload-bay wire harnesses and liners and some crew equipment. "Yaw steering during first stage saves 225kg. We don't need to wait until the solids are off, we do it early," says O'Connor. Engine gimbal changes during ascent will save 160kg. The orbital manoeuvring system will fire for a "few seconds" just as the SRBs are petering out "to buy 115kg". Achieving main engine cut-off at a lower altitude will gain 335kg. "We cancelled a plan to extend the nozzles on the SRBs," O'Connor says. Any modifications to the Shuttle are limited strictly to "...those to do with safety, going to the Space Station, obsolescence and environmental requirements".

Major modifications to the Shuttle may be forced on NASA if it continues to operate the system well into the next century. The Shuttle can continue to fly through its natural structural life, which "would take it up to about 2030" according to O'Connor, or it will be phased and replaced with a new system, the Re-useable Launch Vehicle. A decision is expected to be made in 2000, but a Shuttle of some kind will be required to support the Alpha, until at least 2012.

The 2030 deadline for the Shuttle coincides roughly with the 100-mission capability of each Orbiter. To allow the structure to go that far, modifications would be needed to some components such as the computers, displays and flight-control systems, and replacing hydraulic systems with electro-mechanical actuators.

Integrated avionics are for the future, but, says O'Connor, "...we are doing glass cockpits. That's an obsolescence issue. That's one of the things that's OK for us to do because the steam gauges are from the late 1960s, but we are using the same computers. If it's decided to go to 2030, then we'd have to look at the avionics".

Source: Flight International