Escape tower, ejection seat, or luck?

Though accidents and tragedy have occurred in preparations for space flight, the most likely accident scenarios occur during the mission itself, the first of which is the ascent from the launchpad into orbit. Sitting inside a spacecraft, strapped to thousands of gallons (or liters) of highly explosive fuel, the crew needs some assur­ance that if things go wrong there is at least a chance to get out, however unlikely the disaster or slim the chance of surviving it.

Crew escape systems varied with the design of each spacecraft. There were pad escape systems, incorporated into the launchpads to allow the crew either to vacate the pad inside the spacecraft or quickly exit the vehicle and clear the launch area. Slide wires and escape chutes were incorporated into the towers built for Apollo, Shuttle, and Buran, while escape from any potential explosion could be achieved by escape tower on Mercury, Apollo, Soyuz, and Shenzhou, or by ejection seats on Gemini. Each countdown process includes periods of evaluation built into the launch preparations as well as options to abort the launch before the critical time. Such safeguards continue to feature in the operational launch procedures of both Soyuz and Shenzhou.

During the Shuttle program, several launch attempts were abandoned due to the weather or over equipment concerns. Most of these were canceled long before the vehicle was committed to engine ignition. On five occasions, there was a “Redundant Set Launch Sequencer” (RSLS) abort called, which occurred between the ignition of the three main engines at 6.6 seconds before liftoff and the lighting of the solid rocket boosters at T — 0 seconds. If computers (not humans) sensed a problem in the main engines, the launch would be aborted, preventing the SRBs from igniting. The SRBs could not be turned off once ignited, thus committing the Shuttle to launch and at least 123 seconds of flight since no abort was possible prior to SRB separation, even if a main engine failed. Fortunately, the RSLS

system worked as designed on all five occasions (STS-41D in 1984, STS-51F in 1985, STS-55 in 1993, STS-51 also in 1993, and STS-68 in 1994).

Once a vehicle has left the pad, the options for escape from a pending explosion are more limited. There must at least be time to identify and react to the problem in the first place. Things happen rapidly in space flight and the journey from pad to orbit only takes eight to ten minutes. Events can occur in seconds, or even microseconds, so the technology has to be able to react quickly. Sometimes, there simply is no time to react and tragedy occurs.

The first manned spacecraft featured two methods of crew escape during launch. Vostok carried an ejection seat for its solo pilot, while Mercury incorpo­rated a launch escape tower. The tower idea was continued for Apollo and was later incorporated on Soyuz and, more recently, Shenzhou. One reason for this is simply that there was insufficient room or mass capacity to provide an ejection system for every crew member once the single-seat spacecraft were phased out. Ejection seats were retained for Gemini as it was thought more suit­able than an escape tower for that program. Like Vostok, these seats could be used for crew escape during recovery operations as well as for problems occurring during ascent.

For those programs using the escape tower, it was available for emergencies on the pad, but during ascent it was jettisoned at ballistic recovery altitude or once orbital speed was attained, which rendered the tower system unusable. For recovery, these spacecraft relied on parachutes (and, in the case of Soyuz and Shenzhou, the retro-rockets) to reduce the landing impact velocity.

These options were not available for Voskhod and Shuttle. For the amended Yostok, flying as Voskhod, crew escape was virtually impossible and the landing risky. Though promoted as an “improved”, or “upgraded” spacecraft, Voskhod was in fact nothing more than a stripped-down Vostok. It was designed to carry a crew of up to three instead of one, mainly for the purpose of achieving spectacular space firsts ahead of the Americans. It was a risky, and lucky, two-mission program.

With additional crew members, the ejection seat had to be removed, leaving the crew no method of pad or launch escape. This also affected the safe recovery of the two crews, as they could not eject prior to landing. The retro-rocket package that was added was essential for the crew to survive the landing impact. Both missions were completed without any serious incidents, but it was fortunate that nothing went wrong. This could have been a prime reason for the cancellation of the program after only two flights, moving on to the more capable Soyuz with its built-in soft-landing system.

For the American Shuttle, things were more complicated. With crews numbering up to seven, crew escape had limited options. For both the atmo­spheric landing tests using Enterprise in 1977 and the first four orbital test flights on Columbia, the two-person crews did have ejection seats for emergency escape during ascent or descent. Fortunately these were not required on the actual mis­sions. On Columbia, they were deactivated for STS-5 and removed for STS-9. No other Shuttle orbiter carried the system, as two-person Shuttle crews ceased with STS-4. Escape capsules were considered, but were deemed impractical for the spacecraft’s design.

Following the loss of Challenger in 1986, a slide pole was installed. Each of the crew wore escape pressure suits and had the capability to leave the vehicle to descend on their own parachutes—at least in theory. Crews trained for such evacuations as part of their mission preparation. Fortunately, this type of escape was never called upon. It would have required the vehicle to be in a relatively stable flight mode for the crew to avoid hitting somewhere on the orbiter as they evacuated through the side hatch. The Shuttle Orbiter was essentially a glider as it came home, capable of landing on the ground or even on the ocean, so evacuating the vehicle in stable flight seemed contrary to what it was designed to do—a controlled stable glide to an unpowered landing.

Conditions inside the orbiter as it fell through the atmosphere in an uncontrolled state would surely have made the slide pole an unlikely solution to crew escape. With the vehicle possibly breaking up in flight, the crew of up to seven would have had to leave their seats on the flight deck and mid-deck, moving around in bulky pressure suits to hook up to the slide pole. Then they would have had to hope to miss all the trailing debris as the vehicle dropped like a stone towards Earth.

In 2003, there was no time for the Columbia crew to react to the impending disaster. They were too high and traveling too fast to use the escape pole, even if they had had time to consider it.

During ascent, the Shuttle had a number of abort modes, giving the crew the option to return to the landing site, take it over the Atlantic to land at specific sites in Europe or Africa, to fly a single orbit and land on the next pass, or to abort the ascent into a low orbit. In the latter case, onboard systems would gradually have raised the orbit to at least an operational level, enabling the crew to conduct an alternative mission and probably return early. When the Shuttle launch abort modes were devised, each crew hoped they would not be called upon. They were there should the need arise and it did so during the 19th mission (STS-51F), in July 1985. On that mission, the loss of a main engine resulted in an abort to orbit and a revised, but still highly successful mission.

The abort modes were not new ideas. During Apollo, there were stages in the ascent where the mission could be aborted early to an emergency recovery in the Atlantic or to attain a lower-than-planned orbit to give the crew and Mission Control time to evaluate what to do next. The lunar missions featured points at which mission progress could be evaluated and the decision made whether to con­tinue. For most of the Apollo missions, each of these decision points was passed to allow the missions to achieve most of what was planned. The exception, of course, was Apollo 13. Here, the redundancy built into the design of the program came to the forefront and contributed to the recovery of the crew. But there were still points at which the skills and endurance of the crew and ground controllers were pushed to the limit.

It is interesting to note that, apparently, when the Astronaut Office was approached to fly a test demonstration of the Shuttle return to launch site abort mode, their response was that it could be tested when it was needed. Clearly, turning the stack around in flight and heading back to the launch site minutes after leaving it was not a favored option for the astronauts, even given their varied and very capable flying experiences. Thankfully it was never put into practice.