Launch abort: the helpless "trick" to save the rocket

Looking at rocket launch missions in various countries, launch aborts are not uncommon. This is both a helpless move and a "trick" to rescue rockets, avoiding the destruction of both rockets and satellites caused by flying with faults. For example, on February 17 this year, Japan's new generation rocket H-3 made its maiden flight at the Tanegashima Space Center. After the core-stage hydrogen-oxygen engine ignited normally, before the control system sent an ignition signal to the solid booster, the self-inspection found that the power supply was abnormal, so the core-stage engine was shut down in time, and the rocket launch was forced to be aborted. So, what is the difference between a launch abort and a launch failure? What are the characteristics of a launch abort? Under what conditions does it often work?

The H-3 rocket aborted its launch after ignition (Source: Japanese media)

Special "regret medicine" is effective

A launch abort means that after the rocket enters the launch countdown, the launch mission is temporarily canceled due to technical, meteorological, landing area and other reasons. This situation generally does not cause great damage to the payload and the rocket, but is an effective means of protection. Especially in the face of technical failures, the rocket can abort the launch in time, release the propellant, remove the payload, and conduct a comprehensive investigation, which is expected to achieve a higher success rate. It can be said that this rocket "regret medicine" is often effective.

According to the time point, launch aborts can be divided into two categories. The first category occurs before the rocket ignites, when personnel or systems determine that the conditions for the rocket to take off are no longer met.

For example, the launch of the US Crew-6 manned space mission was aborted on February 27 this year because the system detected an abnormal reaction in the ignition system of the Falcon 9 rocket and aborted the launch program in time. Subsequently, the astronauts in the spacecraft left the launch pad safely, avoiding possible danger.

The second type of launch abort is even more dangerous - the rocket has been ignited, but the rocket body is still "pulled" to the launch pad by the restraint release device, or because the thrust-to-weight ratio of the entire rocket is less than 1, it has not yet flown away from the launch pad. At this moment, the rocket still has hope of aborting the launch in time.

This was the case with the first flight of the H-3 rocket. After the LE-9 engine of the core stage ignited and reached 90% of its maximum thrust, the control system detected an abnormal signal. Next, the flight control software immediately stopped the countdown, shut down the LE-9 engine, and cancelled the ignition signal to the solid booster. Preliminary investigations showed that the abnormal signal was related to the ground electrical equipment of the LE-9 engine, but the final cause is still under investigation.

Intelligent self-checking is very powerful

Although launch abort is an effective means of protection, the space launch mission involves a large and complex system. In the past, it was difficult to detect potential faults in time during the pre-launch inspection of rockets, thus missing out on the "life-saving trick" of launch abort. As the intelligence of rockets increases and the self-checking performance of the system is upgraded, the number of cases where problems are discovered in time and the launch is aborted after the rocket is ignited is gradually increasing. As long as the rocket does not leave the launch pad, there is usually a chance to remedy the situation.

In August 2020, the U.S. Delta IV Heavy rocket staged a thrilling "disaster blockbuster" when it carried out the NROL-44 secret military launch mission. At that time, the three RS-68 hydrogen-oxygen engines of the rocket's core stage and booster suddenly failed 3 seconds before takeoff. The hydrogen normally discharged by the engine rose and ignited, but because the rocket failed to fly away from the launch pad, the fire quickly surrounded the rocket. Faced with the crisis, the control system automatically shut down in an emergency, the rocket launch was terminated in time, and the U.S. National Reconnaissance Office payload in the fairing was safe and sound.

The Delta 4 Heavy rocket aborted its launch after ignition (Source: US media)

It is reported that the payload of that mission was code-named "Mentor", which was the latest type of reconnaissance satellite in the United States at that time. Compared with the well-known "Keyhole" series, it is more mysterious. There are even reports that the cost of this type of satellite exceeds the same weight of gold. Faced with such payloads, the launcher must carefully check in advance, but it still cannot ensure that all hidden dangers are eliminated. More than a year before that launch, the same rocket also shut down urgently when launching the "Keyhole" satellite, which promptly prevented the rocket and payload from taking off with hidden dangers, and soon found out the sensor failure. Without the intelligent system to assist in troubleshooting and decision-making, these two reconnaissance satellite launch missions are likely to cause irreparable losses.

Emergency shutdown failures before rocket launch are not uncommon. They are often caused by abnormalities in the engine ignition process, which automatically triggers the intelligent system. For example, when the Falcon 9 rocket launched the SES-9 satellite in February 2016, the ignition system promptly discovered that the thrust of one engine had dropped abnormally, and the rocket immediately and automatically aborted the launch. Thanks to the restraint release device, the rocket was firmly "pressed" on the launch pad and did not take off. It only emitted a burst of green light due to the spontaneous combustion of the ignition agent.

Subsequent investigations showed that the temperature of the cryogenic propellant gradually increased after being left standing for a long time, and the helium used for pressurization in the rocket tank was mixed with the propellant and sucked into the turbo pump, resulting in abnormal thrust. The cause of this failure was related to some "blind spots" of the pre-launch inspectors, and the launch was terminated in time thanks to the sensitive and efficient intelligent system.

Strive to prevent problems before they occur

The timely implementation of the launch abort is due not only to technical factors but also to the overall planning, such as setting the ignition sequence of different types of engines in advance.

Take the H-3 rocket as an example. As a typical solid-liquid hybrid rocket, the core stage uses a high-specific-impulse hydrogen-oxygen liquid engine. At the same time, to make up for the lack of thrust, it is supplemented by a high-thrust solid booster. The solid booster's grain is generally formed by segmented casting and then cut and trimmed. It has the advantages of high thrust, simple structure, and reliable operation. The disadvantage is that once ignited, its combustion process will continue spontaneously, and it is difficult to shut down in time by cutting off the propellant supply like a liquid engine.

Weighing the characteristics of the two engines, when designing the ignition sequence for a solid-liquid hybrid rocket, the solid booster is without exception the "bottom runner". As the liquid engine of the rocket's core stage is ignited first, the control system will ignite the "irreversible" solid booster only after reaching sufficient thrust and working stably. Once a problem is found, the system determines that the conditions for takeoff are not met. As long as the ignition signal to the solid booster is blocked in time before the solid booster is ignited, and the core stage engine is shut down, the launch can be aborted, as is the case with the H-3 rocket.

If the system fails to prevent the solid booster from igniting in time, and the rocket flies up quickly under the instantaneous high thrust, will there be no turning back and the launch can only be considered a failure? In theory, there are two special "tricks" that may reduce the losses, but even if the mission is successful, there will be other costs.

The first "trick" is to make a good power redundancy backup. If the rocket itself is designed with a certain amount of power redundancy, once an individual engine fails, the rocket can automatically shut down the faulty engine, and the other engines in parallel can "take over" the work of the abnormal engine, and re-plan through the control and navigation system to appropriately increase the thrust of the remaining engines and extend their working time. In the past, the Saturn V heavy rocket, the space shuttle, and the Falcon 9 rocket have all used power redundancy conditions in different ways, basically achieving their goals and improving mission reliability in disguise.

The second remedy is essentially to provide redundant backups, using reserved propellant to extend the engine's operating time, but the one that "works hard" is the next stage of the faulty engine.

In March 2016, during the launch of the Cygnus spacecraft, the first-stage engine of the Atlas 5 rocket suddenly shut down 5 seconds in advance. The second-stage engine of the rocket then used the reserved propellant in the tank to extend the burning time by 1 minute, finally sending the spacecraft into orbit. It was estimated afterwards that if the first-stage engine had shut down 1 second earlier, the entire launch mission would have failed. After all, the emergency help provided by the reserved propellant is not unlimited.

However, considering factors such as cost-effectiveness and engineering difficulty, we should not have too high expectations for the power and fuel redundancy of rockets. Failure of the power system during flight can easily lead to "irreversible" launch failures. Therefore, timely use of intelligent means to troubleshoot faults, decisively terminate the launch, and eliminate hidden dangers before the rocket takes off is the real way to prevent problems before they "ignite". (Author: Wang Xin)