Tim Furniss/WASHINGTON DC
BRYAN O'CONNOR, FORMER Space Shuttle commander and now director of the Shuttle programme at NASA's headquarters says, "To a pilot, it's a crazy bunch of attitudes." He is describing the procedure for a return-to-launch-site (RTLS) abort Shuttle launch. It involves "...flying backwards on your tail at Mach 4. It's all been certified by analysis, but we've never flown one of those things and there is just that little bit of doubt in the back of your mind about whether it would really work," O'Connor says.
The Shuttle has had 72 successful launches, with one catastrophic failure on the 25th flight, the STS51L Challenger, on 28 January 1986. Only one other launch encountered major trouble, forcing another type of abort called the abort-to-orbit (ATO), when one engine had to be shut down after suspected overheating.
The RTLS remains untried, however, and, as more flights take place, observers are waiting for the almost inevitable launch when a major systems failure or an engine shutdown early in flight makes the much-simulated abort a reality.
The RTLS is one of four types of "intact aborts", the objectives of which are to bring the Orbiter to a planned landing site. "Contingency" aborts are designed to permit crew survival, following more severe failures in which a ditching at sea and the sinking of the Orbiter would be the most likely result, although a landing somewhere may be attempted.
The least-serious launch-phase abort is the abort-once-around, which occurs when the Shuttle is in an initial orbit and it is impossible to achieve an operational orbit or perform viable mission. A routine entry and landing, is made at Edwards AFB, California, after one orbit.
The ATO which occurred on the STS51F Challenger/Spacelab 2 flight in August 1985 was the result of an engine being shut down at T+4min 55s, causing a slow down which had to be made up by the orbital manoeuvring systems. A limited or constrained mission can be flown after an ATO is carried out, or a premature landing can be made later. The Spacelab 2 mission was successful, although some manoeuvres had to be cancelled.
The transatlantic-landing abort was introduced later in the programme. It is designed to improve the options available if an engine fails, or if there is a systems failure such as a cabin leak, after an RTLS is no longer possible (T+4min 20s), but before an ATO can be accomplished, making a landing as quickly as possible essential. Before that , a ditching at sea would have been the most likely result, with the loss of the Orbiter and probably the crew, since a crew bail-out system was only introduced after the Challenger disaster.
Within 3-4min of the launch, either an RTLS or a transatlantic-landing abort could be made, depending on the seriousness of the malfunction.
A transatlantic-landing abort, rather than an RTLS, is always preferable. "After 4min 20sec, you cannot do an RTLS anyway. You get a 'negative return' call from capsule communications," says O'Connor. "Before negative return, he will tell the commander that he has transatlantic-landing abort capability - the preferred abort mode. We do not like the RTLS. Transatlantic-landing abort is more like a normal entry and it's not such a strange flying mode."
NASA is simulating ways in which the transatlantic-landing-abort capability can be introduced earlier for a single engine-out situation to minimise the high-risk RTLS - and later contingency-abort scenarios.
Assigned sites for transatlantic-landing aborts are at Moron, Spain; Dakar, Senegal; and Ben Guerur, Morocco, where runways are long enough to accommodate the glide landing. During a transatlantic-landing abort, the Orbiter is commanded towards the plane of the landing site, rolling the vehicle pitch-up before main engine cut-off and sending commands to burn propellant, dumping weight to increase performance, placing the vehicle centre of gravity in the correct alignment and decreasing the landing weight.
The RTLS is designed to allow an intact landing at the Kennedy Space Center (KSC), in Florida, T+25 min later, following the loss of an engine or a systems failure between lift-off and T+4min 20s, at which time a transatlantic-landing abort landing becomes the only option. Even if an engine shuts down during the first 120s of solid-rocket-booster (SRB) firing, the crew can only select an RTLS after the SRBs have been jettisoned. "You go all the way on the solids," says O'Connor.
Timing of the RTLS varies between 2min 20s to 4min 20s. For example, a three-engine RTLS can be selected 3 min 34s into the launch, in the event of a cabin leak or other systems failure, whereas an engine failure at lift-off will force an RTLS command at T+2min 20s. Then the "...crazy flying begins".
FLYING "ON YOUR BACK"
O'Connor says that the RTLS "...is a strange flying mode. You're flying out on your back, heads down and lose an engine... you punch the abort button. The computer knows how much propellant is left in the external tank and works out how long you want to continue to fly downrange. Then it turns you around and puts you on your tail."
The vehicle continues downrange to dissipate excess main propellant. The goal is to leave only enough propellant for the vehicle to be turned around, flown back towards the Kennedy Space Center and achieve proper MEC engine cut-off conditions so that a glide flight is possible after the external tank has been jettisoned.
During the downrange phase, a pitch-around manoeuvre is initiated to orientate the Orbiter/external tank configuration to a heads-up attitude, pointing towards the launch site. The vehicle is still moving away from the launch site, but the engines are thrusting to null the downrange velocity. In addition, excess orbital-manoeuvring system and reaction-control system (RCS) thrusters, fire continuously to dump weight.
The Shuttle Orbiter/external-tank stack flies downrange on its tail and its thrust-to-weight ratio of less than one causes a sink rate. "You actually begin to come down on your tail and eventually start to come back. You have been going backwards at Mach 4 on your tail."
The vehicle reaches the desired main-engine cut-off point with less than 2% excess propellant remaining in the external tank. The stack slows to Mach 1 and then starts to move back towards Florida. "The thrust-to-weight ratio due to the tank propellants going down is such that you start to climb and head back towards Florida, and you run out of propellant at a place where the external tank drops off no closer to the Florida coast than 40km [21nm]".
At engine cut-off minus 20s, a powered pitch-down manoeuvre is made to take the mated vehicle to the required external-tank separation attitude and pitch rate. After engine cut-off, the external tank is jettisoned and the RCS thrusters ensure that the Orbiter is moved far enough away to avoid a collision, and that the Orbiter is placed at the correct attitude to begin the glide into the kennedy Space Center. Even though the RTLS is certificated, Shuttle crews like the normal go-across, wings level, land normally procedure of a transatlantic-landing abort.
Shuttle commanders fly even riskier contingency aborts in the Shuttle Development Simulator at Houston, knowing that, in many cases, what they do may be possible in the simulator, but in reality would break up the vehicle. "If you lose two or three engines, you get into contingency aborts which are not certified flight modes. They may or they may not work," says O'Connor.
The simulator can sometimes fool the crew. "You can make a landing and the instructors will point out that you burned the wings off. You violated the g limits. The wings come off at 3.5g. Even though the simulator may have done it, in real life you wouldn't have survived, because some contingency aborts are not flyable," O'Connor says.
The loss of two engines during the first 4min would necessitate the contingency abort, as would a major systems failure. The immediate aim is to land at any runway available on the US east coast, for example in Bermuda. East-coast abort landings (ECAL) for higher-inclination Space Station ascents can be attempted at sites from South Carolina to Nova Scotia.
The nose has to be pitched down as low as possible without burning the wings off, with the engines throttled up to an emergency-power setting. A loaded external tank has to be jettisoned. The further up the US east coast, the Shuttle is, the more likely is a ditching at sea.
The crew would attempt to maintain the Orbiter integrity to allow a bail out using a pole extended from the Shuttle's mid-deck area, along which each crew person would slide out, using a ring attached to the parachute. This is officially possible only between 20,000ft (6,100m) and 10,000ft, before the $2 billion Orbiter is ditched in the sea. Although crews train for egress after a sea ditching, it is likely that the Orbiter would have broken up and sunk too quickly to allow escape.
If the choice was between a high-risk ECAL, or a ditching at sea, flight controllers would accept the risk of running an engine at maximum power and subjecting the wings to high stress to attempt an east-coast-abort landing.
If anything goes wrong during the first 120s while the solid-rocket boosters are firing, nothing can be attempted until they are jettisoned. "You can't punch the solids when they are going. They are attached to a ball joint, so while they are thrusting and pushing up against the joint, they can't come off until they have petered out," says O'Connor. In dire circumstances, the orbiter could break away but would be caught up in the booster exhaust plume and break apart under aerodynamic forces.
"If you lost three engines on the launch pad or in flight during firing, you may not survive the first 2min. The thrust of the solids will overstress the structure that holds them to the tanks," says O'Connor. Normally, the tank's weight is decremented by the thrust of the main engines and, if they suddenly stop, "...the structure that those things are pushing against can't handle the extra force", O'Connor says. The stack would be destroyed. There is a "negative margin for safety in that case".
If the three engines are shut down after the boosters have been jettisoned, "...there are some cases where that is survivable", in a contingency-abort bail out. "We do not have a certified abort for loss of the boosters themselves. There is nothing you can do about that," O'Connor says. The crew knows that and accepts the risk when it climbs aboard. It is little wonder that one astronaut has remarked that flying the solid-rocket boosters feels like "dicing with death".
Source: Flight International