Boeings CST-100 Starliners four launch abort engines and several orbital maneuvering and attitude ... [+] control thrusters ignite in the companys Pad Abort Test, pushing the spacecraft away from the test stand with a combined 160,000 pounds of thrust, from Launch Complex 32 on White Sands Missile Range in New Mexico.

Early Monday morning, Boeings CST-100 Starliner spacecraft completed its first pad abort test, marking completion of an important technical milestone ahead of their uncrewed Orbital Flight Test (OFT) currently scheduled for later this year. SpaceXs Dragon 2 spacecraft, which like Starliner is being developed under NASAs Commercial Crew Program, is slated to completed a static fire test soon ahead of its fully integrated In-Flight Abort Test.

Whats the difference between these tests, and why do they matter?

Pad Abort

A pad abort test demonstrates a spacecrafts ability to transport crew and/or cargo to safety in the event of an emergency on the launch pad prior to launch. To demonstrate this capability, the spacecrafts launch abort system (sometimes referred to as a launch escape system) is activated during a trial run, during which the spacecraft must both clear the launch pad and land safely within its authorized landing zone. The setup for this trial run includes a spacecraft with a flight-like abort system, but generally does not include a launch vehicle as it would not be used during the test.

SpaceX Pad Abort Test Concept of Operations

A launch abort system can be thought of as the spacecraft equivalent of a fighter pilots ejection seat. However, instead of ejecting the pilot from the spacecraft, the launch abort system ejects the entire spacecraft away from the launch vehicle and pad. Both Commercial Crew vehicles utilize a pusher abort system, in which the spacecrafts built-in propulsion module is used to propel the vehicle to safety. Since the propulsion module is fully integrated into the spacecraft, these systems have the advantage of providing an abort capability at any point during flight.

CAPE CANAVERAL, FL - MAY 6: In this handout provided by the National Aeronautics and Space ... [+] Administration (NASA), SpaceX completes the first key flight test of its Crew Dragon spacecraft, a vehicle designed to carry astronauts to and from space, on May 6, 2015 in Cape Canaveral, Florida. (Photo by NASA via Getty Images)

Some other vehicles, including Russias Soyuz spacecraft, NASAs Apollo capsules, and more recently, NASAs Orion spacecraft, have used an extra solid-fueled rocket to achieve the same goal. This extra rocket is mounted above the capsule on a tower, and is used to tow the spacecraft away from the launch vehicle if an abort is triggered. If not used, these systems are discarded several minutes into flight, after which options for abort are limited to the vehicles remaining system capabilities.

Apollo pad abort test

Boeings test on Monday is reported to have met all of NASAs required criteria for a successful pad abort demonstration. SpaceXs Dragon 2 spacecraft successfully completed an equivalent test in May 2015.

In-Flight Abort

In contrast with a pad abort test, an in-flight abort test verifies a spacecrafts ability to keep crew and/or cargo safe during emergencies that occur after the vehicle has already lifted off the launchpad. In addition to the capability verified by a pad abort test, an in-flight abort test confirms that the spacecraft is able to abort as expected under the high dynamic pressures seen during ascent into space.

To perform this test, a spacecraft with a flight-like abort system must be integrated onto a launch vehicle. The vehicle then launches and performs a nominal ascent until it reaches its maximum dynamic pressure (often referred to by engineers as max q). At this point in the flight profile, the abort system is activated and used to separate the spacecraft from the launch vehicle. To complete the test, the separated spacecraft must be safely returned to Earth.

Of note, in-flight aborts that occur during operational flight will sometimes result in the spacecraft continuing the mission but aborting into a lower orbit than originally planned (usually referred to as an abort to orbit). The choice to return to Earth or to abort to orbit is dependent on multiple factors, including the altitude already achieved at time of abort, the objectives of the mission, and on which trajectory has the greatest chance of saving the crew.

As of November 2019, neither NASA Commercial Crew vehicle has yet completed an in-flight abort test. The last NASA-funded vehicle to complete this test was the Orion spacecraft, which did so in July 2019.

A NASA Orion test vehicle lifts off aboard a booster rocket from Space Launch Complex 46 at Cape ... [+] Canaveral Air Force Station in Cape Canaveral, Florida. The launch was a test to evaluate Orion's launch abort system designed to quickly get astronauts safely away from their launch vehicle if there is a problem during ascent to space. (Photo by Paul Hennessy/NurPhoto via Getty Images)

The hardware for SpaceXs Dragon 2 In-Flight Abort Test has already arrived at the launch site in Cape Canaveral, with NASA and industry officials stating that the test is likely to occur before the end of 2019. Since SpaceX has already completed its uncrewed demonstration mission for Dragon 2 (Demo-1), the In-Flight Abort Test will be one of the final Dragon 2 hardware demonstrations ahead of the vehicles first crewed flight in 2020.

While NASAs Commercial Crew Program requires all providers to complete verification of an in-flight abort capability prior to crewed flight, Boeing has opted to complete this verification via analysis instead of via test. SpaceXs In-Flight Abort Test will therefore be the Commercial Crew Programs only flight hardware demonstration of an in-flight abort scenario.

A Brief History of Spacecraft Aborts

Though it is rare for a spacecraft to experience an abort scenario, there are several documented instances of aborts during crewed space missions that highlight the necessity of vehicle abort capabilities.

NASAs Space Shuttle experienced its only in-flight abort on STS-51F, which launched from Kennedy Space Center on July 29, 1985. The Challenger spacecraft used for this mission experienced multiple failed sensor readings on its main engines, forcing the crew to perform an in-flight Abort To Orbit (ATO) maneuver. This maneuver required manual intervention by the missions commander to switch the cockpit abort mode switch to ATO and depress the abort switch button, which activated the flight control software sequence for an ATO abort. While the spacecraft aborted its initial flight path and did not reach its intended orbit, the mission was still carried out successfully at a slightly lowered than planned orbital altitude. Due to the Shuttles unique vehicle design, aborting to orbit was considered preferable to returning to Earth, which was considered far riskier.

STS-51F lands safely at Edwards Air Force Base after successfully completing its mission.

Russias Soyuz vehicle has experienced 3 launch aborts during its multi-decade history of flight. The first of these occurred in 1975, when the Soyuz 18-1s second stage failed to separate prior to the rockets third stage ignition. The vehicles flight computer detected an anomaly and triggered an in-flight abort, but as the vehicle had already reached an altitude of 145km, its launch abort tower had already been jettisoned. As a result, the Soyuz capsules on-board propulsion systems had to be used for the abort. Both crew members survived and were successfully recovered.

The only documented instance of a crewed pad abort occurred during Soyuz T-10-1, which was slated to launch from Baikonur Cosmodrome on September 26, 1983. The launch vehicle for this mission caught fire on the pad, triggering a pad abort. The Soyuzs launch abort system separated the spacecraft just two seconds before the launch vehicle exploded, saving the crews lives.

Soyuz T-10-1 spacecraft uses its Launch Escape System to launch away from the exploding launch ... [+] vehicle.

The most recent instance of a Soyuz abort was in October 2018, when Soyuz MS-10 experienced an in-flight anomaly during staging that caused one of the boosters to slide down the core stage and rupture the tank. The launch abort system successfully activated once the anomaly was detected, pulling the capsule away from the launch vehicle and to safety. Both crew members were recovered alive and in good health.

Every spacecraft manufacturer builds its abort systems with the hope that they will never need to be used. But when it comes to human spaceflight, you cant be too safe.

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The Importance of Spacecraft Abort Tests - Forbes